Open Access
Add to BibSonomy
Abstract
This paper reviews recent progress in the field of optically pumped rare-earth-doped channel waveguide lasers, with a focus on operation utilizing distributed-feedback resonators on silicon wafers. Rare-earth-doped amorphous aluminum oxide thin films have been deposited onto silicon wafers by RF reactive co-sputtering from metallic Al and rare-earth targets, the spectroscopy and optical gain of Er3+, Yb3+, Nd3+, and Tm3+ ions has been investigated, and the near-infrared laser transitions near 1 μm in Yb3+, 1.5 μm in Er3+, and 2 μm in Tm3+ and Ho3+ have been demonstrated. Output power between a few μW and hundreds of mW have been achieved in different waveguide geometries, and ultranarrow-linewidth laser operation has been demonstrated.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For numerous applications, the spectral-coherence properties of lasers are of crucial importance, and single-longitudinal-mode operation is often required. Whereas this condition can be matched rather easily by gas lasers because of their spectrally narrow emission lines, many other types of lasers, such as dye lasers, semiconductor lasers, or transition-metal- and rare-earth-doped dielectric lasers exhibit strongly broadened emission lines, and their spectral gain bandwidth typically extends over hundreds to thousands of longitudinal resonator modes. Gain saturation in homogeneously broadened laser media should nevertheless ensure single-longitudinal-mode laser operation, because only the mode with the highest gain would reach the threshold condition. However, spatial-hole burning in homogeneously broadened laser media due to the standing-wave pattern occurring in linear resonators as well as spectral-hole burning in inhomogeneously broadened laser media due to a variation in the center frequencies of emission lines will, in most cases, allow adjacent longitudinal modes to retrieve enough gain to reach laser threshold as well, typically leading to simultaneous laser operation on several to tens or even hundreds of longitudinal modes [1, 2].
Various strategies to circumvent these problems and achieve single-longitudinal-mode operation have been invented. The standing-wave pattern can be avoided by use of a ring resonator, in combination with an optical isolator, to enforce travelling-wave operation. A homogeneously broadened laser medium then ensures single-mode emission. The most successful implementation of this idea is probably the monolithic, non-planar ring-oscillator laser design [3]. Another approach aims at significantly reducing the resonator length, thereby increasing the resonator’s mode spacing, i.e., its free spectral range, beyond the gain bandwidth. Such micro-chip lasers [4] allow one to exploit inhomogeneously broadened laser media, but may suffer from insufficient optical pump absorption owing to the accordingly reduced length of the laser medium. Alternatively, the spectral selectivity of the resonator mirrors can be exploited by narrowing their reflection band to less than the free spectral range of the resonator, thereby simultaneously allowing for long resonator lengths and inhomogeneously broadened laser media. Such lasers may utilize the distributed feedback (DFB) in a periodic refractive-index structure [5–11], created by a spectrally narrow-band, highly reflective Bragg grating that often extends over hundreds of micrometers or even millimeters.
Among DFB lasers, electrically pumped semiconductor materials in waveguide geometry are the usual choice. Electrical pumping ensures high laser efficiency, and the large emission cross sections of electron-hole pairs [12] allow for high gain and accordingly short device lengths of less than a millimeter. However, strong heating, resulting in significant refractive-index variations, as well as rather high propagation losses, resulting in low-Q passive resonators, both tend to increase the laser linewidth to levels typically in the range of hundreds of kHz to several MHz [13, 14]. Recently, Yariv and associates demonstrated a laser design [15], in which the oscillating laser light propagates mostly outside the semiconductor gain material. This configuration reduces significantly the gain, but simultaneously the losses per unit length and improves the Q-factor of the passive resonator, resulting in a laser linewidth in the range of a kHz. Also sophisticated grating designs can reduce the laser linewidth to similar values.
Alternatively, rare-earth- or transition-metal-doped dielectric materials in planar-waveguide geometry can be utilized. Since the 1990s various laser transitions in different ions, materials, and operational regimes have been demonstrated [16–74]. These lasers require optical pumping, hence are less efficient than their semiconductor counterparts. Moreover, the weak transition cross sections of rare-earth ions [12] necessitate either long interaction lengths [75] or high dopant concentrations [76] in planar waveguides to achieve significant gain, thus often leading to undesired energy-transfer processes between neighboring active ions [77–87]. However, these disadvantages can be outmatched by a lower heat generation, smaller refractive-index variations, and lower propagation losses in dielectric DFB lasers, thus naturally leading to laser linewidths in the range of a few kHz. Also the compatibility of amorphous dielectric materials with, e.g., silicon chips and silicon-on-insulator waveguides [88] [Fig. 1] or polymer waveguides [90] can provide a distinct advantage over semiconductor materials.
Fig. 1 (a) Schematic of a silicon-on-insulator to Al2O3:Er3+ inverted-taper waveguide coupler. (b) Illustration of advanced integrated photonic circuit with amplification of existing signal light and additional signal light generated by miniature active Er-doped waveguide amplifiers and lasers. (Figures taken from [88] and [89]).
Silicon photonics offers the promise of low-cost, energy-efficient, and highly compact microsystems on a silicon chip [91,92]. Silicon-based photonic microsystems have recently matured to the point that several standard foundry processes are available for low-cost mass production of photonic components [93–96]. Nevertheless, one of the key missing elements in silicon photonic microsystems are integrated amplifiers and lasers. Progress has been made in a number of approaches, including III-V hybrid, silicon Raman, monolithic germanium, organic, and rare-earth-doped waveguide devices [97–99]. Of these, rare-earth-doped lasers offer a favorable combination of monolithic integration, wavelength flexibility, low temperature sensitivity, high-power handling, and straightforward design. This paper reviews recent progress in optically pumped rare-earth-doped DFB lasers on silicon.
2. Rare-earth-doped amorphous Al2O3 on silicon
Amorphous aluminum oxide (Al2O3) can be deposited on thermally oxidized silicon wafers, hence is compatible with silicon waveguide technology and has been integrated with silicon-on-insulator waveguides [88]. A number of integrated optical structures were realized already in the 1990s [100]. A large refractive-index contrast between waveguide and SiO2 cladding is obtained because of the high refractive index of 1.65 in Al2O3, allowing for small waveguide cross sections and compact devices. Also, polymer-based optical backplanes have been combined with Al2O3 [90].
Initial work on Er3+-implanted Al2O3 waveguides on silicon included fabrication [101], spectroscopic characterization [102–104], and the demonstration of 0.6 dB/cm gain per unit length and 2.3 dB total gain [105]. However, implanted Al2O3 requires high-temperature annealing, leading to Er3+ clusters. Reactive co-sputtering reduces the clustering [106].
2.1 Deposition and micro-structuring of rare-earth-doped amorphous Al2O3 on silicon
We deposit [107] rare-earth-doped, 1-μm-thick amorphous Al2O3 layers by RF reactive co-sputtering from metallic targets onto thermally oxidized silicon wafers [Fig. 2(a)] at a deposition rate of 5 nm/min. The thickness non-uniformity is within 1% over an area of 50 mm × 50 mm. Propagation losses are 0.11 dB/cm at 1523 nm in undoped layers. Er3+ doping up to 5 × 1020 cm−3 was demonstrated with propagation losses of 0.21 dB/cm and 0.16 dB/cm at 633 nm and 1320 nm, respectively.
Fig. 2 (a) Schematic illustration of the reactive co-sputtering system used for the Al2O3:RE3+ deposition. (b) SEM micrograph profile of a 1.3-μm-wide and 530-nm-deep channel waveguide in Al2O3. (Figures taken from [108] and [109]).
There exist numerous ways to produce optical channel waveguides [110], including a number of micro-structuring methods. Al2O3 has been micro-structured in various ways, e.g., by focused-ion-beam etching [111], Ar+ milling [112], or reactive ion etching [113]. We employed inductively coupled reactive ion etching using a BCl3/HBr plasma [109], resulting in optimized channel waveguides [Fig. 2(b)]. Straight sidewalls with low roughness and etch depths of 530 nm were obtained. Typical channel waveguides were 10 mm long, 2.5−3.0 μm wide, and etched to a depth of 0.1 μm for single-transverse-mode operation at the pump and laser wavelengths. A SiO2 cladding layer was deposited on top of the ridge waveguides by plasma enhanced chemical vapor deposition. Low optical propagation losses of 0.2 dB/cm around 1550 nm were obtained. Bent waveguides, Y-splitters, and directional couplers were also fabricated.
2.2 Spectroscopy and gain in rare-earth-doped amorphous Al2O3 on silicon
The spectroscopy of Er3+ in Al2O3 was investigated [85, 86, 114]. An internal net gain per unit length over a wavelength range of 80 nm, covering the entire telecommunication C band (1525-1565 nm), was achieved. A peak gain of 2 dB/cm at 1533 nm was demonstrated for Er3+ concentrations of 1−2 × 1020 cm−3 [114]. It is limited by migration-accelerated energy-transfer upconversion (ETU) and a fast quenching process [85] [Figs. 3(a) and 3(b)].
Fig. 3 (a) Emission cross section determined from the luminescence spectra of Al2O3:Er3+. The absorption spectrum calculated using McCumber theory and measured at single wavelengths in the range 1480-1580 nm are indicated by the dashed line and the plotted points, respectively. (b) Internal net gain per unit length at 1533 nm versus Er3+ concentration, for a launched pump power of 100 mW at 976 nm and signal power of 1 μW at 1533 nm: measured data (dots) and calculations (lines) without quenching, only ETU quenching, as well as ETU and fast quenching. Two different values τ1q = 50 ns and 1 µs of the fast quenching process were tested; the two resulting curves are almost identical. (c) Photograph of a pumped (λP = 976 nm) Al2O3:Er3+ spiral amplifier on a silicon chip. The green light emitted from the spiral is the usual upconversion luminescence on the Er3+ transition 4S3/2 → 4I15/2. (d) Internal net gain in Al2O3:Er3+ spiral waveguide amplifiers for different waveguide lengths and an Er3+ concentration of 0.95 × 1020 cm−3. (Figures taken from [114], [85], and based on information from [115]).
By use of the improved fabrication processes, rare-earth-doped Al2O3 amplifiers [75, 114, 116, 117, 118] have been integrated on a silicon chip. In order to achieve a useful total gain in Al2O3:Er3+ for on-chip applications, spiral-waveguide amplifiers were fabricated and characterized [75]. Small-signal internal net gain of 20 dB was measured at 1532 nm for waveguides with 12.9 cm length and 1.92 × 1020 cm−3 Er3+ concentration, as well as 24.4 cm length and 0.95 × 1020 cm−3 concentration [Figs. 3(c) and 3(d)]. Spectroscopic and gain data were also measured for Al2O3:Nd3+ [118], Al2O3:Tm3+ [87], and Al2O3:Yb3+ [119–121]. In Al2O3:Nd3+ with Nd3+ concentrations between 0.65−1.68 × 1020 cm−3, 1.57 dB/cm, 6.3 dB/cm, and 1.93 dB/cm of internal net gain at 880 nm, 1064 nm, and 1330 nm, respectively, was achieved [118].
2.3 Lasing in rare-earth-doped amorphous Al2O3 on silicon
Although amorphous Al2O3 suffers from inhomogeneous linewidth broadening and non-radiative quenching processes [85–87], in these rare-earth-doped Al2O3 channel waveguides the gain exceeds the propagation losses and the fast quenching process affects only the laser threshold, but is much slower than the stimulated-emission process [119]. Consequently, integrated lasers at various wavelengths have been realized on silicon substrates [122–125]. The first laser demonstrated in a rare-earth-doped Al2O3 channel waveguide on a silicon substrate was an Al2O3:Er3+ ring-resonator laser with an Er3+ concentration of 1 × 1020 cm−3 [122]. This laser was diode-pumped at 980 nm and exhibited a threshold as low as 6.4 mW of launched pump power, but the obtained slope efficiency was only 0.11% and an output power of only 9.5 μW was achieved. Nevertheless, by varying the wavelength-dependent element of the ring resonator, namely its coupler to the access waveguide through which the pump power was launched and the laser power was coupled out, we were able to vary the laser wavelength [Fig. 4] over most of the telecom C-band ranging from 1525 to 1565 nm.
Fig. 4 (a) Schematic and (b) photograph of Al2O3:Er3+ ring laser. (c) Laser output spectra of the ring-resonator laser for coupler lengths LC of 550, 450, and 400 μm and resonator lengths LR of 3.0 and 5.5 cm. (Figures taken from [122]).
3. Distributed-feedback resonators in rare-earth-doped Al2O3 on silicon
Periodic corrugated structures have been extensively investigated since the 1970s. A variety of compact monolithic optical devices have been realized for a wide range of applications, being employed as spectral filters [126, 127], couplers [128], beam splitters, add-drop multiplexers [129], and dispersion compensators [130], temperature and strain sensors [131] and bio-medical sensors [132]. Such structures have also been utilized as resonators in DFB and distributed-Bragg-reflector (DBR) lasers in glass fibers [133], semiconductors [134], and dielectric crystalline [135–138], glass [139–142], and amorphous [124,143] waveguides, allowing for the development of ultranarrow-linewidth lasers [124, 133, 139, 143–145], dual-wavelength lasers and microwave beat-signal generation [125,146], as well as intra-laser-cavity optical sensing [147]. The grating period determines the longitudinal-mode pattern [148–150]. For DFB lasers, chirped gratings [151–153] have been designed to diminish spatial-hole burning [154–156].
3.1 Bragg-grating-based resonators
UV-photoinduced writing [157], femtosecond laser writing [158], and physical corrugation of surface-relief Bragg gratings [159] have been exploited to create Bragg gratings. We fabricated by laser interference lithography surface-relief Bragg gratings [143] [Fig. 5], using a negative resist layer on top of the SiO2 cladding of Al2O3 channel waveguides and etching the pattern into the SiO2 layer. A grating period of 316 nm or 507 nm for the ytterbium- or erbium-doped waveguides resulted in Bragg wavelength of 1020 nm or 1590 nm. The propagation and grating-induced scattering losses were 0.14 ± 0.07 dB/cm. The obtained coupling coefficient κ was 6.5 cm−1, allowing for reflectivities up to 99% for TE polarization in 1-cm-long gratings.
Fig. 5 (a) A transverse cross-sectional view of the waveguide layer structure showing the calculated TE mode profile. (b) An axial cross-sectional view of the waveguide structure showing the thickness D of each layer. (c) Scanning electron microscope (SEM) image of the cross-sectional layer structure showing the developed grating in the resist mask layer before it was etched into the SiO2 cladding. (d) Grating reflectivity at the Bragg wavelength for TE polarization as a function of grating length. The blue dots represent the measured reflectivity, while the dashed line is the predicted reflectivity. (Figures taken from [143]).
The spectral response of a uniform Bragg grating is symmetrical with respect to the Bragg wavelength. At the Bragg wavelength the round trip optical phase is π. Since this round trip phase is not an integer multiple of 2π, as required for constructive interference in a cavity, no cavity resonance is possible at the Bragg wavelength. However, the resonance condition is satisfied for a number of wavelengths on either side of the stopband. As a result, such a device has two lowest order resonances on either side of the stopband, which will reach laser threshold simultaneously if the structure has sufficient optical gain.
In order to achieve a single resonance exactly at the Bragg wavelength, an additional round trip phase of π must be added to the structure to fulfill the resonance condition. The additional required phase shift corresponds to exactly one quarter of the Bragg wavelength, which is equivalent to a change in the grating structure of half a period. Consequently, the optical response of such a quarter-wavelength-shifted Bragg grating is altered such that there appears an extremely narrow resonance inside the center of the stopband. This narrow resonance is confined around the phase-shift region from where the power decays exponentially into the Bragg grating on either side. Since the resonant mode at the Bragg wavelength experiences the strongest optical feedback, i.e., the highest reflectivity, it has the lowest required threshold gain when this structure is used as a laser.
Therefore, a quarter-wavelength phase shift was induced by adiabatic widening of the waveguide, resulting in a resonance with a passive Q-factor up to 1.35 × 106 [Fig. 6]. The performance of the fabricated Bragg gratings and DFB or DBR cavities in Al2O3 show good agreement with theoretical models based on coupled-mode theory (CMT) [9].
Fig. 6 (a) Schematic of the distributed phase shift region via a local widening of the waveguide. (b), (c) Resonance of a distributed-phase-shift DFB resonator with a passive Q-factor of 1.35 × 106. (Figures or information taken from [160]).
3.2 Thermal chirp
Optically pumped DFB lasers exhibit non-uniform heating due to absorbed pump power being partially converted into heat [161–163], which causes a chirp in the Bragg grating [164]. The resulting changes in refractive index and material expansion change the resonance frequency and linewidth. Thermally induced chirped gratings have been reported with respect to power stability and spectral response [165–171].
We investigated an Al2O3:Yb3+ waveguide with an Yb3+ concentration of 4.37 × 1020 cm−3, a length of ℓ = 1 cm, 2.5 µm × 1.0 µm cross-sectional area [164], and a 350 nm thick SiO2 top cladding. The refractive index of the layer depends on the dopant concentration [173]. A corrugated Bragg grating with a coupling coefficient κ equaling 8.33 cm−1 [143] was inscribed into the top cladding [124, 143] for single-mode lasing at the Bragg wavelength λB [122]. The propagation losses were 0.20 dB/cm and the Yb3+ absorption losses were 0.33 dB/cm [120], resulting in a sum of propagation and absorption losses of 0.53 dB/cm. The λB/4 phase shift was obtained by adiabatically widening the waveguide [174, 124] with a sin2 function from 2.5 to 2.85 µm over a length of 2 mm. Consequently, the refractive index modulation and, thus, the Bragg grating became non-periodic. The phase-shift region was positioned closer to the pumped end of the waveguide to ensure unidirectional out-coupling [175, 124].
A linear temperature gradient was generated along the waveguide direction. The resonance near 1028 nm was measured with a scanning narrow-linewidth probe laser in the unpumped sample, and its peak wavelength and full-width-at-half-maximum (FWHM) linewidth were characterized, thereby understanding the situation inside the resonator [176]. The temperature dependence of transition cross sections in rare-earth ions is large [177, 178]. However, the investigated temperature range was rather small, such that the absorption and propagation losses remained approximately constant. In simulations [164] using the characteristic-matrix approach by Born and Wolf [172], see also [8, 179–182,9], the grating reflectivity profiles were calculated for different linear chirp coefficients δlin [164]. Experimental and simulated results showed good agreement.
Naturally, a Fabry-Perot resonator should be investigated in frequency space [183]. Nevertheless, here we used wavelength space, because the relevant wavelength range was small, resulting in insignificant asymmetry of the resonance. When uniformly heating the waveguide, the resonance wavelength shifted linearly, 12.0 pm/K near 1028 nm, resulting in a relative wavelength shift of 1.2 × 10−5 K−1, comparable to 19 pm/K [143] and 20 pm/K [184] at 1560−1590 nm. The resulting change dn/dT of refractive index with temperature was 1.86 × 10−5 K−1, representing an upper limit to the refractive index change, because the shift is partially a result of waveguide expansion. In Y3Al5O12 and YLiF4 the refractive index change with temperature is smaller [161], whereas a larger amount of 4.58 × 10−5 K−1 was reported for amorphous Al2O3 in an earlier investigation [185].
The resonance wavelength increased linearly with temperature at the phase-shift center [dots in Fig. 7(a)], because it depends mostly on the grating period at the phase-shift center [164]. The latter increases proportionally to δlin, hence the resonance wavelength also shifts linearly with δlin [solid line in Fig. 7(a)]. The mirror reflectivities decrease drastically with increasing temperature, thereby increasing the linewidth [164]. With a linear chirp, the experimental [dots in Fig. 7(b)] and simulated [blue line in Fig. 7(b)] linewidth increased non-linearly [red line in Fig. 7(b)].
Fig. 7 (a) Experimental shift of resonance wavelength (dots) versus temperature at phase-shift center (bottom x-axis). Simulation shift (line) versus δlin (top x-axis). (b) Experimental (dots) and simulated (lines) increase ΔνL − Δν0 in resonance linewidth Δν0 versus shift of resonance wavelength. Blue curve: resonance linewidth calculated from the characteristic-matrix approach; red curve: simulation exploiting the calculated grating reflectivities R1 and R2. (Figures taken from [164].)
4. Wafer-scale Al2O3:RE3+ DFB Lasers on Silicon
Based on the developed waveguide and resonator technology, Al2O3:Er3+ DFB lasers [124], Al2O3:Yb3+ DBR [123] and DFB [186] lasers, and dual-wavelength Al2O3:Yb3+ DFB lasers [125] were demonstrated.
4.1 Al2O3:Er3+ DFB laser
Erbium-doped integrated lasers, particularly narrow-linewidth DFB lasers and arrays thereof with different laser wavelengths, are of high interest for telecom applications [89].
We realized the first DFB laser in Al2O3 on silicon and investigated its performance [124]. The 1-μm-thick channel waveguides were fabricated in an Al2O3:Er3+ layer with an erbium concentration of 3 × 1020 cm–3. The ridge waveguides were 1 cm long, 3 μm wide, and were etched 0.1 μm deep. A 650-nm-thick SiO2 cladding layer was deposited on top of the ridge waveguides by use of plasma-enhanced chemical vapor deposition. This waveguide geometry was designed to support only single transverse-mode operation at the 1480 nm pump and 1545.2 nm laser wavelengths for both TE and TM-polarized modes. A surface-relief Bragg grating was defined with an etch depth of ~150 nm, a period of 488 nm, and a duty cycle of ~50%. A quarter-wave phase shift with a 2-mm-long adiabatic sinusoidal tapering of the waveguide width, widened adiabatically from 3.0 μm to a maximum of 3.45 μm, was placed 1 mm from the center of the cavity. Consequently, by pumping the cavity from the end-facet nearer to the phase shift, the majority of the laser power was emitted in the direction of the pumped facet where the gain is highest.
The Al2O3:Er3+ DFB waveguide was pumped by a 1480 nm laser diode. The continuous-wave Al2O3:Er3+ laser demonstrated a threshold of 2.2 mW absorbed pump power and a maximum output power at a wavelength of 1545.2 nm of more than 3 mW with a slope efficiency of 41.3% versus absorbed pump power [Fig. 8(a)]. Single-longitudinal-mode, TE-polarized operation was achieved, because the TE polarization has lower propagation losses than the TM polarization.
Fig. 8 (a) Laser output power of the Al2O3:ER3+ DFB laser as a function of absorbed pump power (circles). The slope efficiency is 41.3%. (b) Self-heterodyne set-up for measuring the laser linewidth. (c) Measured RF beat signal (circles) of the Al2O3:Er3+ DFB laser, along with the best fitted theoretical RF power spectrum of a 1.70 kHz Lorentzian linewidth (solid line). The dashed/dashed-dotted curves are calculated for Lorentzian linewidths of 1.70 ± 0.58 kHz. (Figures and information taken from [124] and [160].)
A delayed self-heterodyne interferometer [Fig. 8(b)] [187] was implemented to measure the laser linewidth. The setup was constructed with an 8.9 km fiber delay. The measured RF beat signal [Fig. 8(c)] exhibited interference fringes, indicating that the coherence length of the DFB laser was considerably longer than the 8.9 km fiber. The laser linewidth was extracted by a fit of the theoretical RF power spectrum, resulting in an emission linewidth of 1.70 ± 0.58 kHz, corresponding to a coherence length of more than 55 km and a Q-factor of 1.14 × 1011. This linewidth is approximately twice the Schawlow-Townes linewidth [188].
4.2 Al2O3:Yb3+ DFB laser
Ytterbium-doped waveguides are suitable for realizing compact and highly efficient lasers due to the relatively large absorption and emission cross-sections and high quantum efficiency of ytterbium. Because of the absence of energy-transfer upconversion and excited-state absorption, waveguides can be highly doped with Yb3+ ions in order to realize compact devices in which efficient absorption of the pump light is achieved within cavity lengths of a few millimeters.
Channel waveguides with a width of 2.5 μm and an etch depth of 90 nm were fabricated in an Al2O3:Yb3+ layer with an ytterbium concentration of 5.8 × 1020 cm–3 These ridge waveguides supported single-transverse-mode operation at the the 976 nm pump and ~1020 nm laser wavelengths. A SiO2 cladding layer was deposited on top of the ridge waveguides. A 1-cm-long surface corrugated Bragg grating with a period of 316 nm was etched into the SiO2 cladding. A 2-mm-long distributed quarter-wave phase shift, tapered to a maximum width of 2.85 μm and centered 4 mm away from the pumped end, was introduced to the resonator.
The laser was pumped with a 976 nm laser diode. Above a threshold of 5 mW, the TE-polarized, single-longitudinal-mode emission at a wavelength of 1022.2 nm produced up to 55 mW of output power, which resulted in a slope efficiency of 67% versus launched pump power [Fig. 9].
Fig. 9 Measured input-output power characteristics of the Al2O3:Yb3+ DFB channel waveguide laser. (Figure modified from [186].)
4.3 Dual-wavelength DFB laser and microwave beat-signal generation
The photonic generation of microwave or millimeter-wave signals has attracted interest due to its potential applications in satellite communication [189] and phased-array-antenna systems [190], as well as broad-band wireless and radio-over-fiber networks, radar, and sensor devices [191]. Microwave signals can be generated in the optical domain by a dual-wavelength laser. An electrical beat signal is generated at the output of a photodetector, with a frequency corresponding to the wavelength spacing of the two optical waves. Compared with electronic solutions, photonic generation of microwaves has many advantages, such as high-speed operation, low power consumption, low cost, and the distribution of the optical carrier signals via low-loss, inexpensive optical fibers over large distances [192]. Dual-wavelength lasers have been demonstrated in rare-earth-ion-doped fibers [193] and semiconductor-based devices [194].
We demonstrated a dual-wavelength DFB channel waveguide laser in Al2O3:Yb3+, which represents the first dual-wavelength laser on silicon. Operation of the laser is based on two localized quarter-wavelength phase shifts in a DFB cavity, thus generating two resonance peaks in the reflection band of the device [195].
When both phase shifts are varied symmetrically from a quarter-wavelength phase-shift, the two resonance wavelengths separate symmetrically from each other with respect to the Bragg wavelength [Figs. 10(a) and 10(b)]. Since both phase shifts are located close to each other on the same chip, their temperature dependence is almost identical, thus providing a very stable beat frequency [Fig. 10(c)].
Fig. 10 (a) Schematic of the dual-wavelength Al2O3:Yb3+ DFB resonator, along with the calculated longitudinal field distribution of the two respective laser wavelengths. (Figure taken from [125]). (b) Measured TE-polarized transmission spectrum in a 10-mm-long DFB cavity with two distributed quarter-wavelength phase shifts. (c) Measured temperature dependence of the two resonances. (d) Measured output power (blue dots) of the dual-wavelength Al2O3:Yb3+ DFB waveguide laser as a function of launched pump power. The slope efficiency is 41%. (Figures taken from [160]).
Laser oscillation commenced at a launched pump power of 5 mW, after which the laser output power increased linearly with a slope efficiency of 41% to produce a pump-power-limited output power of 28 mW [Fig. 10(d)]. The free-running laser was used to generate a 9 kHz wide microwave beat signal at ~15 GHz with a long-term frequency stability of ± 2.5 MHz measured over a period of 45 minutes, while the power of the microwave signal was stable within ± 0.35 dB.
5. Wafer-scale silicon-nitride-based Al2O3:RE3+ DFB lasers on silicon
Silicon integration enables leveraging of standardized CMOS processing and co-integration with electronic control circuitry. However, in order to integrate rare-earth-doped DFB lasers onto a silicon photonic platform, fabrication challenges had to be overcome. The key challenge was developing a process compatible with CMOS processing, since rare-earth-doped materials are not allowed within the CMOS foundry, and complex post-processing steps are generally to be avoided. With those considerations in mind, researchers at MIT [144, 196–198] and UCSB [145, 199–202] recently developed a platform for integration of rare-earth-doped lasers using CMOS-compatible and wafer-scale processing of silicon nitride waveguides and cavities, followed by deposition of the gain medium outside the silicon foundry. The work builds on that of Bradley et al. who demonstrated the first Al2O3:RE3+ waveguide lasers on silicon. A number of lasers have been demonstrated using the Al2O3:RE3+/Si3N4 platform with various cavity structures and dopants. These include continuous-wave DBR [144, 203, 196, 199], DFB [203–205, 184, 206–209, 196, 145, 199], and microcavity [210, 211] devices and pulsed lasers [197,198]. While microcavity lasers offer lower thresholds and DBR lasers exhibit high efficiencies and output powers, the DFB lasers have consistently single-mode emission, high output powers and narrow linewidths. In this section we review the current status of DFB lasers that have emerged on this promising platform.
5.1 CMOS-compatible design and fabrication
Silicon-nitride-based Al2O3:RE3+ laser waveguide designs are shown in Figs. 11(a) and 11(b). Both designs build on the work by Bernhardi et al. [124] and lead to similar DFB laser waveguide properties, including mode size and intensity overlap with the Al2O3:RE3+ gain medium. The basic design in Fig. 11(a), simultaneously developed by groups at UCSB and MIT includes a SiO2 lower-cladding, thin nitride strip with a SiO2 spacer layer and Al2O3:RE3+ gain layer on top, with air or SiO2 top-cladding. The thin SiO2 layer reduces the influence of the higher index Si3N4 layer on the optical mode and improves wavelength insensitivity, particularly for widely separated pump and laser wavelengths (e.g. 980 nm and 1550 nm for erbium). The sub-wavelength design shown in Fig. 11(b) uses the same principle but extends the design to enable integration on thicker Si3N4 layers. Such thicker layers are useful for high-confinement passive devices on the silicon nitride platform and common in silicon photonic wafer-scale processes [95]. To avoid the mode being pulled strongly into the nitride layer, a segmented nitride design is used which is enabled by high-resolution and wafer-scale immersion lithography processes. This results in a highly wavelength-insensitive design and even enables pump and signal separation by more than an octave [207]. Typical mode profiles for both designs are shown in Figs. 11(c) and 11(d).
Fig. 11 Integrated Al2O3:RE3+ DFB laser design utilizing silicon nitride cavities: (a) and (b) Al2O3:Er3+ laser waveguide cross-sections using flat and segmented Si3N4 designs, respectively; (c) and (d) pump (top) and laser (bottom) mode profiles showing high-intensity overlap with the Al2O3:Er3+ gain medium and good pump/laser mode overlap; (e) 3D illustration showing a Al2O3:Tm3+ DFB structure; (f) SEM top-view image showing sub-micrometer Si3N4 segmented waveguide and periodic side-grating features. (Figures taken from [144], [208], and [203].)
Using these waveguide designs, DFB cavities are formed by introducing periodic structures, typically along the side of the waveguide, to provide a weak perturbation. The period for such DFBs, determined by the grating condition, is typically around 500 nm for erbium lasers, therefore high-resolution lithography is required to realize feature sizes of 250 nm or less. Both quarter-wave-shifted and distributed phase shift cavity designs have been investigated [207,145], with the latter offering better performance as in [207]. Further considerations for the cavity design include the cavity length, grating strength and doping concentration. The cavity length is limited by the lithography reticle size of ~2 cm, which in turn determines the required grating strength on the order of 1−10 cm−1 and doping concentrations for sufficient gain on the order of 1 × 1020 cm−3 or greater. However, curved designs [209] can enable longer cavities on the chip, relaxing the latter two parameters. A 3D sketch of a typical cavity and a top-view image of the Si3N4 waveguide and grating features using the segmented waveguide design are shown in Figs. 11(e) and 11(f), respectively.
The DFB laser fabrication steps are presented in detail in several references including [207, 145]. A several-µm-thick SiO2 lower cladding is either thermally grown or deposited by PECVD on a silicon wafer. A ~80−200-nm-thick Si3N4 layer is then grown by PECVD or LPCVD and the waveguide and cavity features patterned into the Si3N4 layer by high resolution stepper lithography. A thin SiO2 layer is then deposited by sputtering, or PECVD. The Al2O3:Er3+ layer is then deposited by reactive co-sputtering, based on the process developed by University of Twente researchers and presented in [107]. For the top cladding, either the laser is left uncladded (air top cladding) or a SiO2 layer is deposited on top. Individual chips and laser facets are formed by dicing and polishing or deep reactive ion etching and dicing. All fabrication steps prior to Al2O3:RE3+ deposition are CMOS compatible, thus enabling the laser chips to be mass-produced in a foundry, while the gain medium is deposited outside the foundry in a single, straightforward post-processing step.
5.2 DFB laser results
A selection of DFB lasers that have been demonstrated by using the Al2O3:RE3+/Si3N4 platform are summarized in Table 1.
Table 1. Summary of silicon-nitride-based Al2O3:RE3+ DFB laser resultsa
Figure 12(a) shows a typical characterization setup applied for double-sided 980-nm pumping of an on-chip erbium-doped DFB laser (similar setups were used to characterize the various lasers, substituting the appropriate pump sources and components for the various rare earth ions).
Fig. 12 Demonstration of high power and single-mode Al2O3:RE3+ lasers: (a) schematic of typical double-pumped measurement setup; (b) and (c) emission spectrum and laser curve for a Al2O3:Er3+ laser (inset in (b) shows suppression of self-pulsing at high pump power); (d) and (e) emission spectra and laser curve for Al2O3:Tm3+ lasers; (f) and (g) emission spectra and laser curve for Al2O3:Ho3+ lasers. (Figures taken from [208], [204], [203], and [196].)
The first erbium DFB laser on silicon nitride was reported by researchers at UCSB [145]. The device exhibited relatively high threshold and low slope efficiency compared to [124], due to a short cavity. Because of the lower output power a linewidth of 501 kHz was measured [145]. Improvements were made in [199], including extension of the cavity length to allow for greater pump absorption and optical gain, leading to a reduced threshold pump power of 20 mW and a slope efficiency close to 1% (close to that demonstrated in [124] by Bernhardi et al for 980nm pumping). Further, an array of lasers emitting across the C-band from 1534−1570 nm was shown by varying the grating period from 478−490 nm. In [204], researchers at MIT demonstrated record-high output power for an optically pumped laser on silicon using a high-power in-band 1480-nm pump [Figs. 12(b) and 13(c)]. Up to 1.1 W pump power was coupled onto the chip, demonstrating the high power handling capability of the Al2O3:Er3+ waveguides. The motivation for pumping at high power was to remove self-pulsing, due to quenching [185], a common challenge in erbium lasers. At highest pump power the self-pulsations were suppressed. In that same work a high slope efficiency of 7% was demonstrated. In [207], the MIT group demonstrated improved performance under 980-nm pumping, with up to 2.9% slope efficiency, a threshold of only 13 mW, and a free-running linewidth of only 5.3 kHz the latter close to the performance demonstrated by Bernhardi et al. Further, widely separated emission wavelengths (1536−1596 nm) in the C- and L-bands were shown. In addition to the erbium DFB lasers in the table, improved designs and fabrication methods have been reported. In [209], the dependence of cavity Q factor and laser performance on Al2O3:Er3+ thickness non-uniformity was investigated, and it was shown that even nanoscale thickness variations across cm-long cavities can significantly reduce the device performance. Compensation via a curved cavity to match the circular geometry of the sputtering chamber resulted in a high-quality DFB transmission spectrum and improved performance. A lower temperature (250°C vs. ≥ 500°C) Al2O3:Er3+ CMOS-compatible deposition process was demonstrated in [208], enabling integration of erbium lasers with active devices such as modulators and photodetectors on the same chip. In [201], it was shown that such Al2O3:Er3+ lasers can operate in extreme temperatures (up to 400C), several times higher than semiconductor lasers. Their straightforward and low cost fabrication, high temperature and temperature-insensitive operation and high power output mean integrated erbium lasers have significant potential for eye-safe free space and optical communications applications around 1550 nm.
Fig. 13 Advanced Al2O3:Er3+ laser designs enabled by wafer-scale integration: (a) a resonantly-pumped DFB laser with DBR pump cavity showing (b) improved slope efficiency vs. non-resonant pumping; (c) a WDM source based on cascaded DFB lasers demonstrating (d) single-mode emission at 4 wavelengths; (e) a DFB laser integrated with a silicon nitride micro-ring filter bank demonstrating (f) temperature-control free operation via simultaneous shift of laser output and channel 1 ring filter resonance wavelengths. (Figures taken from [205], [184], and [206].)
Recently, the silicon nitride platform has been extended to realize thulium and holmium waveguide lasers emitting near 2 µm. This is a particularly promising wavelength region for on-chip DFB lasers because of the reduced efficiency of semiconductor lasers at these wavelengths and emerging applications around 2µm, including free-space and fiber-optic communications and sensing. Thulium and holmium lasers emitting around 2 µm are of particular interest for nonlinear silicon photonic applications, due to silicon’s lower two-photon absorption in this wavelength range. As shown in Figs. 12(d) and 12(e), in [203], using a tunable laser and high power L-band EDFA up to 267 mW output power was demonstrated in a thulium-doped DFB laser (in the same work up to 367 mW was demonstrated in a DBR device). More than 70 dB side mode suppression was demonstrated, as well as laser output in the range 1820−1950 nm. In [196], emission was extended above 2 µm in holmium-doped DFB lasers [Figs. 12(f) and 12(g)]. The minimum threshold observed was 130 mW while a slope efficiency of up to 2% and 15 mW output power were obtained under 1900-nm pumping. The available emission bandwidth of > 300 nm provides significant motivation for future development of optically pumped thulium and holmium waveguide lasers on silicon.
5.3 Integration of optically-pumped DFB lasers in photonic microsystems
Based on the initial laser demonstrations on the silicon nitride platform described above, several advanced cavity designs and applications have been explored. Examples of devices which exploit the ability to fabricate high-resolution features and active and passive designs on the same platform are shown in Fig. 13. In Fig. 13(a) a double cavity design is shown based on integrated DBR mirrors for resonant pumping at 980 nm. By pumping on resonance the slope efficiency was almost doubled, as displayed in Fig. 13(b). Application of resonant pumping to in-band 1480-nm pumping (which exhibits higher slope efficiency for erbium) and perhaps other laser ions and wavelengths is a promising direction. In [184], a multi-wavelength source was demonstrated by cascading 4 DFBs with input from a single 980-nm pump [Fig. 13(c)]. Similar output power was observed across the different emission lines with an average of 38 dB side-mode suppression ratio per channel, as shown in Fig. 13(d). Such a device is promising as a simple WDM light source. Another promising result illustrating the potential of monolithic rare-earth-doped DFB lasers as scalable components for microsystems applications is the demonstration of a DFB laser co-integrated with silicon nitride micro-ring filters in [206], displayed in Fig. 13(e). The advantage of such a design is that the ring filter can be aligned to the DFB output without temperature control, based on the similar thermo-optic coefficients of the materials [Fig. 13(f)].
These initial demonstrations illustrate the potential of optically-pumped DFB lasers in integrated photonic microsystems. Several technological challenges remain, including effective pump laser packaging and control, optical isolation on the chip, improving laser stability by reducing self-pulsations, and improving device efficiencies. Besides their integration in silicon nitride platforms, the potential for integration of rare-earth-doped waveguide lasers on a full silicon-on-insulator platform is a highly promising direction for advanced microsystems applications [197].
6. Conclusion
Rare-earth-doped amorphous aluminum oxide thin films, in combination with different technologies and waveguide geometries, have enabled various lasers and, particularly, distributed-feedback, ultranarrow-linewidth lasers on silicon chips. For several reasons, this approach has turned out to be a viable alternative to hybrid integration of semiconductor materials on silicon chips. Further improvement in output power and efficiency and additional laser wavelengths can be expected to emerge in the future. Special interest may arise in applications exploiting the ultranarrow-linewidth features of these devices on silicon, in combination with silicon-on-insulator technology.
Funding
European Research Council (ERC) Advanced Grant “Optical Ultra-Sensor” (No. 341206); Natural Sciences and Engineering Research Council of Canada (NSERC) (No. RGPIN-2017-06423).
Acknowledgments
The authors thank their colleagues—from the University of Twente: Edward H. Bernhardi, Laura Agazzi, Jing Yang, Feridun Ay, Sergio A. Vázquez-Córdova, Meindert Dijkstra, Anton J. F. Hollink, Henk A. G. M. van Wolferen, Sonia M. García-Blanco, Kerstin Wörhoff, and René M. de Ridder; from KTH−Royal Institute of Technology: Cristine C. Kores, Nur Ismail, Dimitri Geskus, and Pavel Loiko, and from the Massachusetts Institute of Technology, Ehsan Shah Hosseini, Purnawirman, Zhan Su, Nanxi Li, Emir Salih Magden, Jie Sun, Gurpreet Singh, Michele Moresco, Mathew Byrd, Christopher V. Poulton, Alfonso Ruocco, Neetesh Singh, Patrick T. Callahan, Katia Shtyrkova, Ming Xin, Christopher Baiocco, Diedrik Vermeulen, Gale S. Petrich, Leslie A. Kolodziejski, Franz X. Kärtner, Erich P. Ippen, and Michael R. Watts— for their contributions to the experiments and many fruitful discussions.
References and links
1. A. E. Siegman, Lasers (University Science Books, 1986).
2. O. Svelto, Principles of Lasers, 5th ed. (Springer, 2010).
3. T. J. Kane and R. L. Byer, “Monolithic, unidirectional single-mode Nd:YAG ring laser,” Opt. Lett. 10(2), 65–67 (1985). [CrossRef] [PubMed]
4. J. J. Zayhowski, “Microchip lasers,” Linc. Lab. J. 3(3), 427–446 (1990).
5. H. Kogelnik and C. V. Shank, “Stimulated emission in a periodic structure,” Appl. Phys. Lett. 18(4), 152–154 (1971). [CrossRef]
6. C. V. Shank, J. E. Bjorkholm, and H. Kogelnik, “Tunable distributed-feedback dye laser,” Appl. Phys. Lett. 18(9), 395–396 (1971). [CrossRef]
7. I. P. Kaminov, H. P. Weber, and E. A. Chandross, “Poly(methyl methacrylate) dye laser with internal diffraction grating resonator,” Appl. Phys. Lett. 18(11), 497–499 (1971). [CrossRef]
8. H. Kogelnik and C. V. Shank, “Coupled-wave theory of distributed feedback lasers,” J. Appl. Phys. 43(5), 2327–2335 (1972). [CrossRef]
9. A. Yariv, “Coupled-mode theory for guided-wave optics,” IEEE J. Quantum Electron. 9(9), 919–933 (1973). [CrossRef]
10. S. R. Chinn and P. L. Kelley, “Analysis of the transmission, reflection and noise properties of distributed feedback laser amplifiers,” Opt. Commun. 10(2), 123–126 (1974). [CrossRef]
11. T.-Y. Chung, A. Rapaport, V. Smirnov, L. B. Glebov, M. C. Richardson, and M. Bass, “Solid-state laser spectral narrowing using a volumetric photothermal refractive Bragg grating cavity mirror,” Opt. Lett. 31(2), 229–231 (2006). [CrossRef] [PubMed]
12. M. Eichhorn and M. Pollnau, “Spectroscopic foundations of lasers: Spontaneous emission into a resonator mode,” IEEE J. Sel. Top. Quantum Electron. 21(1), 9000216 (2015). [CrossRef]
13. J. Seufert, M. Fischer, M. Legge, J. Koeth, R. Werner, M. Kamp, and A. Forchel, “DFB laser diodes in the wavelength range from 760 nm to 2.5 µm,” Spectroc. Acta Pt. A-Molec. Biomolec. Spectr. 60(14), 3243–3247 (2004). [CrossRef] [PubMed]
14. T. Kunii and Y. Matsui, “Narrow spectral linewidth semiconductor lasers,” Opt. Quantum Electron. 24(7), 719–735 (1992). [CrossRef]
15. C. T. Santis, S. T. Steger, Y. Vilenchik, A. Vasilyev, and A. Yariv, “High-coherence semiconductor lasers based on integral high-Q resonators in hybrid Si/III-V platforms,” Proc. Natl. Acad. Sci. U.S.A. 111(8), 2879–2884 (2014). [CrossRef] [PubMed]
16. J. I. Mackenzie, “Dielectric solid-state planar waveguide lasers: A review,” IEEE J. Sel. Top. Quantum Electron. 13(3), 626–637 (2007). [CrossRef]
17. C. Grivas, “Optically pumped planar waveguide lasers, Part I: Fundamentals and fabrication techniques,” Prog. Quantum Electron. 35(6), 159–239 (2011). [CrossRef]
18. C. Grivas, “Optically pumped planar waveguide lasers: Part II: Gain media, laser systems, and applications,” Prog. Quantum Electron. 45–46, 3–160 (2016). [CrossRef]
19. P. J. Chandler, S. J. Field, D. C. Hanna, D. P. Shepherd, P. D. Townsend, A. C. Tropper, and L. Zhang, “Ion-implated Nd:YAG planar waveguide laser,” Electron. Lett. 25(15), 985–986 (1989). [CrossRef]
20. D. Pelenc, B. Chambaz, I. Chartier, B. Ferrand, C. Wyon, D. P. Shepherd, D. C. Hanna, A. C. Large, and A. C. Tropper, “High slope efficiency and low threshold in a diode-pumped epitaxially grown Yb:YAG waveguide laser,” Opt. Commun. 115(5−6), 491–497 (1995). [CrossRef]
21. A. A. Anderson, R. W. Eason, L. M. B. Hickey, M. Jelínek, C. Grivas, D. S. Gill, and N. A. Vainos, “Ti:sapphire planar waveguide laser grown by pulsed laser deposition,” Opt. Lett. 22(20), 1556–1558 (1997). [CrossRef] [PubMed]
22. C. L. Bonner, A. A. Anderson, R. W. Eason, D. P. Shepherd, D. S. Gill, C. Grivas, and N. Vainos, “Performance of a low-loss pulsed-laser-deposited Nd:Gd3 Ga5O12 waveguide laser at 1.06 and 0.94 µm,” Opt. Lett. 22(13), 988–990 (1997). [CrossRef] [PubMed]
23. A. Rameix, C. Borel, B. Chambaz, B. Ferrand, D. P. Shepherd, T. J. Warburton, D. C. Hanna, and A. C. Tropper, “An efficient, diode-pumped, 2 µm Tm:YAG waveguide laser,” Opt. Commun. 142(4–6), 239–243 (1997). [CrossRef]
24. C. Becker, T. Oesselke, J. Pandavenes, R. Ricken, K. Rochhausen, G. Schreiber, W. Sohler, H. Suche, R. Wessel, S. Balsamo, I. Montrosset, and D. Sciancalepore, “Advanced Ti:Er:LiNbO3 waveguide lasers,” IEEE J. Sel. Top. Quantum Electron. 6(1), 101–113 (2000). [CrossRef]
25. D. L. Veasey, D. S. Funk, P. M. Peters, N. A. Sanford, G. E. Obarski, N. Fontaine, M. Young, A. P. Peskin, W. C. Liu, S. N. Houde-Walter, and J. S. Hayden, “Yb/Er-codoped and Yb-doped waveguide lasers in phosphate glass,” J. Non-Cryst. Solids 263(1−4), 369–381 (2000). [CrossRef]
26. E. R. Thoen, E. M. Koontz, D. J. Jones, D. Barbier, F. X. Kärtner, E. P. Ippen, and L. A. Kolodziejski, “Erbium-ytterbium waveguide laser mode-locked with a semiconductor saturable absorber mirror,” IEEE Photonics Technol. Lett. 12(2), 149–151 (2000). [CrossRef]
27. G. Sorbello, S. Taccheo, P. Laporta, O. Svelto, E. Cianci, V. Foglietti, S. Jiang, and N. Peyghambarian, “Singlemode Er:Yb waveguide laser array at 1.5 µm,” Electron. Lett. 37(16), 1014–1015 (2001). [CrossRef]
28. J. I. Mackenzie, S. C. Mitchell, R. J. Beach, H. E. Meissner, and D. P. Shepherd, “15 W diode-side-pumped TmYAG waveguide laser at 2 µm,” Electron. Lett. 37(14), 898–899 (2001). [CrossRef]
29. R. J. Beach, S. C. Mitchell, H. E. Meissner, O. R. Meissner, W. F. Krupke, J. M. McMahon, W. J. Bennett, and D. P. Shepherd, “Continuous-wave and passively Q-switched cladding-pumped planar waveguide lasers,” Opt. Lett. 26(12), 881–883 (2001). [CrossRef] [PubMed]
30. J. R. Lee, H. J. Baker, G. J. Friel, G. J. Hilton, and D. R. Hall, “High-average-power Nd:YAG planar waveguide laser that is face pumped by 10 laser diode bars,” Opt. Lett. 27(7), 524–526 (2002). [CrossRef] [PubMed]
31. J. I. Mackenzie and D. P. Shepherd, “End-pumped, passively Q-switched Yb:YAG double-clad waveguide laser,” Opt. Lett. 27(24), 2161–2163 (2002). [CrossRef] [PubMed]
32. C. B. E. Gawith, A. Fu, T. Bhutta, P. Hua, D. P. Shepherd, E. R. Taylor, P. G. R. Smith, D. Milanese, and M. Ferraris, “Direct-UV-written buried channel waveguide lasers in direct-bonded intersubstrate ion-exchanged neodymium-doped germano-borosilicate glass,” Appl. Phys. Lett. 81(19), 3522–3524 (2002). [CrossRef]
33. G. Jose, S. Taccheo, G. Sorbello, D. Migliorati, V. Foglietti, E. Cianci, S. Jiang, N. Peyghambarian, and P. Laporta, “Multiwavelength waveguide laser array in C-band,” Electron. Lett. 38(21), 1275–1276 (2002). [CrossRef]
34. J. J. Mackenzie, C. Li, and D. P. Shepherd, “Multi-Watt, high efficiency, diffraction-limited Nd:YAG planar waveguide laser,” IEEE J. Quantum Electron. 39(3), 493–500 (2003). [CrossRef]
35. S. Taccheo, G. D. Valle, R. Osellame, G. Cerullo, N. Chiodo, P. Laporta, O. Svelto, A. Killi, U. Morgner, M. Lederer, and D. Kopf, “Er:Yb-doped waveguide laser fabricated by femtosecond laser pulses,” Opt. Lett. 29(22), 2626–2628 (2004). [CrossRef] [PubMed]
36. L. M. B. Hickey, V. Apostolopoulos, R. W. Eason, J. S. Wilkinson, and A. A. Anderson, “Diffused Ti:sapphire channel-waveguide lasers,” J. Opt. Soc. Am. B 21(8), 1452–1462 (2004). [CrossRef]
37. W. Sohler, B. K. Das, D. Dey, S. Reza, H. Suche, and R. Ricken, “Erbium-doped lithium niobate waveguide lasers,” IEICE Trans. Electron. E88–C(5), 990–997 (2005). [CrossRef]
38. B. Unal, M. C. Netti, M. A. Hassan, P. J. Ayliffe, M. D. B. Charlton, F. Lahoz, N. M. B. Perney, D. P. Shepherd, C. Y. Tai, J. S. Wilkinson, and G. J. Parker, “Neodymium-doped tantalum pentoxide waveguide lasers,” IEEE J. Quantum Electron. 41(12), 1565–1573 (2005). [CrossRef]
39. M. Fujimura, H. Tsuchimoto, and T. Suhara, “Yb-diffused LiNbO3 annealed/proton exchanged waveguide lasers,” IEEE Photonics Technol. Lett. 17(1), 130–132 (2005). [CrossRef]
40. C. Grivas, D. Shepherd, T. May-Smith, R. Eason, and M. Pollnau, “Single-transverse-mode Ti:sapphire rib waveguide laser,” Opt. Express 13(1), 210–215 (2005). [CrossRef] [PubMed]
41. C. Grivas, D. P. Shepherd, R. W. Eason, L. Laversenne, P. Moretti, C. N. Borca, and M. Pollnau, “Room-temperature continuous-wave operation of Ti:sapphire buried channel-waveguide lasers fabricated via proton implantation,” Opt. Lett. 31(23), 3450–3452 (2006). [CrossRef] [PubMed]
42. Y. E. Romanyuk, C. N. Borca, M. Pollnau, S. Rivier, V. Petrov, and U. Griebner, “Yb-doped KY(WO4)2 planar waveguide laser,” Opt. Lett. 31(1), 53–55 (2006). [CrossRef] [PubMed]
43. R. Osellame, N. Chiodo, G. Della Valle, G. Cerullo, R. Ramponi, P. Laporta, A. Killi, U. Morgner, and O. Svelto, “Waveguide lasers in the C-band fabricated by laser inscription with a compact femtosecond oscillator,” IEEE J. Sel. Top. Quantum Electron. 12(2), 277–285 (2006). [CrossRef]
44. G. Della Valle, R. Osellame, G. Galzerano, N. Chiodo, G. Cerullo, P. Laporta, O. Svelto, U. Morgner, A. G. Rozhin, V. Scardaci, and A. C. Ferrari, “Passive mode locking by carbon nanotubes in a femtosecond laser written waveguide laser,” Appl. Phys. Lett. 89(23), 231115 (2006). [CrossRef]
45. S. Rivier, X. Mateos, V. Petrov, U. Griebner, Y. E. Romanyuk, C. N. Borca, F. Gardillou, and M. Pollnau, “Tm:KY(WO4)2 waveguide laser,” Opt. Express 15(9), 5885–5892 (2007). [CrossRef] [PubMed]
46. G. Della Valle, S. Taccheo, R. Osellame, A. Festa, G. Cerullo, and P. Laporta, “1.5 µm single longitudinal mode waveguide laser fabricated by femtosecond laser writing,” Opt. Express 15(6), 3190–3194 (2007). [CrossRef] [PubMed]
47. G. Della Valle, A. Festa, G. Sorbello, K. Ennser, C. Cassagnetes, D. Barbier, and S. Taccheo, “Single-mode and high power waveguide lasers fabricated by ion-exchange,” Opt. Express 16(16), 12334–12341 (2008). [CrossRef] [PubMed]
48. D. Pudo, H. Byun, J. Chen, J. Sickler, F. X. Kärtner, and E. P. Ippen, “Scaling of passively mode-locked soliton erbium waveguide lasers based on slow saturable absorbers,” Opt. Express 16(23), 19221–19231 (2008). [CrossRef] [PubMed]
49. H. X. Kang, H. Zhang, P. Yan, D. S. Wang, and M. Gong, “An end-pumped Nd:YAG planar waveguide laser with an optical to optical conversion efficiency of 58%,” Laser Phys. Lett. 5(12), 879–881 (2008). [CrossRef]
50. M. Ams, P. Dekker, G. D. Marshall, and M. J. Withford, “Monolithic 100 mW Yb waveguide laser fabricated using the femtosecond-laser direct-write technique,” Opt. Lett. 34(3), 247–249 (2009). [CrossRef] [PubMed]
51. F. M. Bain, A. A. Lagatsky, S. V. Kurilchick, V. E. Kisel, S. A. Guretsky, A. M. Luginets, N. A. Kalanda, I. M. Kolesova, N. V. Kuleshov, W. Sibbett, and C. T. A. Brown, “Continuous-wave and Q-switched operation of a compact, diode-pumped Yb3+:KY(WO4)2 planar waveguide laser,” Opt. Express 17(3), 1666–1670 (2009). [CrossRef] [PubMed]
52. D. Geskus, S. Aravazhi, E. Bernhardi, C. Grivas, S. Harkema, K. Hametner, D. Günther, K. Wörhoff, and M. Pollnau, “Low-threshold, highly efficient Gd3+, Lu3+ co-doped KY(WO4)2:Yb3+ planar waveguide lasers,” Laser Phys. Lett. 6(11), 800–805 (2009). [CrossRef]
53. A. Kahn, S. Heinrich, H. Kühn, K. Petermann, J. D. B. Bradley, K. Wörhoff, M. Pollnau, and G. Huber, “Low threshold monocrystalline Nd:(Gd, Lu)2O3 channel waveguide laser,” Opt. Express 17(6), 4412–4418 (2009). [CrossRef] [PubMed]
54. H. Kühn, S. Heinrich, A. Kahn, K. Petermann, J. D. B. Bradley, K. Wörhoff, M. Pollnau, and G. Huber, “Monocrystalline Yb3+:(Gd,Lu)2O3 channel waveguide laser at 976.8 nm,” Opt. Lett. 34(18), 2718–2720 (2009). [PubMed]
55. J. Siebenmorgen, T. Calmano, K. Petermann, and G. Huber, “Highly efficient Yb:YAG channel waveguide laser written with a femtosecond-laser,” Opt. Express 18(15), 16035–16041 (2010). [CrossRef] [PubMed]
56. T. Calmano, J. Siebenmorgen, O. Hellmig, K. Petermann, and G. Huber, “Nd:YAG waveguide laser with 1.3 W output power, fabricated by direct femtosecond laser writing,” Appl. Phys. B 100(1), 131–135 (2010). [CrossRef]
57. J. Yang, M. B. J. Diemeer, C. Grivas, G. Sengo, A. Driessen, and M. Pollnau, “Steady-state lasing in a solid polymer,” Laser Phys. Lett. 7(9), 650–656 (2010). [CrossRef]
58. C. Grivas, J. Yang, M. B. J. Diemeer, A. Driessen, and M. Pollnau, “Continuous-wave Nd-doped polymer lasers,” Opt. Lett. 35(12), 1983–1985 (2010). [CrossRef] [PubMed]
59. D. Geskus, S. Aravazhi, C. Grivas, K. Wörhoff, and M. Pollnau, “Microstructured KY(WO4)2:Gd3+, Lu3+, Yb3+ channel waveguide laser,” Opt. Express 18(9), 8853–8858 (2010). [CrossRef] [PubMed]
60. D. Geskus, S. Aravazhi, K. Wörhoff, and M. Pollnau, “High-power, broadly tunable, and low-quantum-defect KGd1-xLux(WO4)2:Yb3+ channel waveguide lasers,” Opt. Express 18(25), 26107–26112 (2010). [CrossRef] [PubMed]
61. A. Z. Subramaniam, C. J. Oton, D. P. Shepherd, and J. S. Wilkinson, “Erbium-doped waveguide laser in tantalum pentoxide,” IEEE Photonics Technol. Lett. 22(21), 1571–1573 (2010). [CrossRef]
62. D. G. Lancaster, S. Gross, H. Ebendorff-Heidepriem, K. Kuan, T. M. Monro, M. Ams, A. Fuerbach, and M. J. Withford, “Fifty percent internal slope efficiency femtosecond direct-written Tm3+:ZBLAN waveguide laser,” Opt. Lett. 36(9), 1587–1589 (2011). [CrossRef] [PubMed]
63. B. Charlet, L. Bastard, and J. E. Broquin, “1 kW peak power passively Q-switched Nd3+-doped glass integrated waveguide laser,” Opt. Lett. 36(11), 1987–1989 (2011). [CrossRef] [PubMed]
64. A. Choudhary, A. A. Lagatsky, P. Kannan, W. Sibbett, C. T. A. Brown, and D. P. Shepherd, “Diode-pumped femtosecond solid-state waveguide laser with a 4.9 GHz pulse repetition rate,” Opt. Lett. 37(21), 4416–4418 (2012). [CrossRef] [PubMed]
65. K. van Dalfsen, S. Aravazhi, C. Grivas, S. M. García-Blanco, and M. Pollnau, “Thulium channel waveguide laser in a monoclinic double tungstate with 70% slope efficiency,” Opt. Lett. 37(5), 887–889 (2012). [CrossRef] [PubMed]
66. W. Bolanos, F. Starecki, A. Benayad, G. Brasse, V. Ménard, J.-L. Doualan, A. Braud, R. Moncorgé, and P. Camy, “Tm:LiYF4 planar waveguide laser at 1.9 μm,” Opt. Lett. 37(19), 4032–4034 (2012). [CrossRef] [PubMed]
67. F. Starecki, W. Bolaños, A. Braud, J. L. Doualan, G. Brasse, A. Benayad, V. Nazabal, B. Xu, R. Moncorgé, and P. Camy, “Red and orange Pr3+:LiYF4 planar waveguide laser,” Opt. Lett. 38(4), 455–457 (2013). [CrossRef] [PubMed]
68. N. Ter-Gabrielyan, V. Fromzel, X. Mu, H. Meissner, and M. Dubinskii, “Resonantly pumped single-mode channel waveguide Er:YAG laser with nearly quantum defect limited efficiency,” Opt. Lett. 38(14), 2431–2433 (2013). [CrossRef] [PubMed]
69. W. Bolaños, F. Starecki, A. Braud, J.-L. Doualan, R. Moncorgé, and P. Camy, “2.8 W end-pumped Yb3+:LiYF4 waveguide laser,” Opt. Lett. 38(24), 5377–5380 (2013). [CrossRef] [PubMed]
70. D. Geskus, E. H. Bernhardi, K. van Dalfsen, S. Aravazhi, and M. Pollnau, “Highly efficient Yb3+-doped channel waveguide laser at 981 nm,” Opt. Express 21(11), 13773–13778 (2013). [CrossRef] [PubMed]
71. J. W. Kim, S. Y. Choi, D. I. Yeom, S. Aravazhi, M. Pollnau, U. Griebner, V. Petrov, and F. Rotermund, “Yb:KYW planar waveguide laser Q-switched by evanescent-field interaction with carbon nanotubes,” Opt. Lett. 38(23), 5090–5093 (2013). [CrossRef] [PubMed]
72. K. van Dalfsen, S. Aravazhi, C. Grivas, S. M. García-Blanco, and M. Pollnau, “Thulium channel waveguide laser with 1.6 W of output power and ∼80% slope efficiency,” Opt. Lett. 39(15), 4380–4383 (2014). [CrossRef] [PubMed]
73. Y. Tan, C. Cheng, S. Akhmadaliev, S. Zhou, and F. Chen, “Nd:YAG waveguide laser Q-switched by evanescent-field interaction with graphene,” Opt. Express 22(8), 9101–9106 (2014). [CrossRef] [PubMed]
74. J. W. Kim, S. Y. Choi, S. Aravazhi, M. Pollnau, U. Griebner, V. Petrov, S. Bae, K. J. Ahn, D. I. Yeom, and F. Rotermund, “Graphene Q-switched Yb:KYW planar waveguide laser,” AIP Adv. 5(1), 017110 (2015). [CrossRef]
75. S. A. Vázquez-Córdova, M. Dijkstra, E. H. Bernhardi, F. Ay, K. Wörhoff, J. L. Herek, S. M. García-Blanco, and M. Pollnau, “Erbium-doped spiral amplifiers with 20 dB of net gain on silicon,” Opt. Express 22(21), 25993–26004 (2014). [CrossRef] [PubMed]
76. D. Geskus, S. Aravazhi, S. M. García-Blanco, and M. Pollnau, “Giant optical gain in a rare-earth-ion-doped microstructure,” Adv. Mater. 24(10), OP19–OP22 (2012). [CrossRef] [PubMed]
77. F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev. 104(1), 139–174 (2004). [CrossRef] [PubMed]
78. R. I. Laming, S. B. Poole, and E. J. Tarbox, “Pump excited-state absorption in erbium-doped fibers,” Opt. Lett. 13(12), 1084–1086 (1988). [CrossRef] [PubMed]
79. S. A. Payne, G. D. Wilke, L. K. Smith, and W. F. Krupke, “Auger upconversion losses in Nd-doped laser glasses,” Opt. Commun. 111(3−4), 263–268 (1994). [CrossRef]
80. Y. Guyot, H. Manaa, J. Y. Rivoire, R. Moncorgé, N. Garnier, E. Descroix, M. Bon, and P. Laporte, “Excited-state-absorption and upconversion studies of Nd3+-doped single crystals Y3Al5O12, YLiF4, and LaMgAl11O19.,” Phys. Rev. B Condens. Matter 51(2), 784–799 (1995). [CrossRef] [PubMed]
81. G. Rustad and K. Stenersen, “Modeling of laser-pumped Tm and Ho lasers accounting for upconversion and ground-state depletion,” IEEE J. Quantum Electron. 32(9), 1645–1656 (1996). [CrossRef]
82. M. Pollnau, P. J. Hardman, W. A. Clarkson, and D. C. Hanna, “Upconversion, lifetime quenching, and ground-state bleaching in Nd3+:LiYF4,” Opt. Commun. 147(1−3), 203–211 (1998). [CrossRef]
83. M. Pollnau, D. R. Gamelin, S. R. Lüthi, H. U. Güdel, and M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61(5), 3337–3346 (2000). [CrossRef]
84. P. S. Golding, S. D. Jackson, T. A. King, and M. Pollnau, “Energy-transfer processes in Er3+-doped and Er3+,Pr3+-codoped ZBLAN glasses,” Phys. Rev. B 62(2), 856–864 (2000). [CrossRef]
85. L. Agazzi, K. Wörhoff, and M. Pollnau, “Energy-transfer-upconversion models, their applicability and breakdown in the presence of spectroscopically distinct ion classes: A case study in amorphous Al2O3:Er3+,” J. Phys. Chem. C 117(13), 6759–6776 (2013). [CrossRef]
86. L. Agazzi, K. Wörhoff, A. Kahn, M. Fechner, G. Huber, and M. Pollnau, “Spectroscopy of upper energy levels in an Er3+-doped amorphous oxide,” J. Opt. Soc. Am. B 30(3), 663–677 (2013). [CrossRef]
87. P. Loiko and M. Pollnau, “Stochastic model of energy-transfer processes among rare-earth ions. Example of Al2O3:Tm3+,” J. Phys. Chem. C 120(46), 26480–26489 (2016). [CrossRef]
88. L. Agazzi, J. D. B. Bradley, M. Dijkstra, F. Ay, G. Roelkens, R. Baets, K. Wörhoff, and M. Pollnau, “Monolithic integration of erbium-doped amplifiers with silicon-on-insulator waveguides,” Opt. Express 18(26), 27703–27711 (2010). [CrossRef] [PubMed]
89. J. D. B. Bradley and M. Pollnau, “Erbium-doped integrated waveguide amplifiers and lasers,” Laser Photonics Rev. 5(3), 368–403 (2011). [CrossRef]
90. J. Yang, T. Lamprecht, K. Wörhoff, A. Driessen, F. Horst, B. J. Offrein, F. Ay, and M. Pollnau, “Integrated optical backplane amplifier,” IEEE J. Sel. Top. Quantum Electron. 17(3), 609–616 (2011). [CrossRef]
91. G. T. Reed, Silicon Photonics: The State of the Art (Wiley, 2008).
92. L. Chrostowski and M. Hochberg, Silicon Photonics Design (Cambridge University, 2015).
93. P. Dumon, W. Bogaerts, R. Baets, J.-M. Fedeli, and L. Fulbert, “Towards foundry approach for silicon photonics: silicon photonics platform ePIXfab,” Electron. Lett. 45(12), 581–582 (2009). [CrossRef]
94. M. Streshinsky, R. Ding, Y. Liu, A. Novack, C. Galland, A. E.-J. Lim, P. Guo-Qiang Lo, T. Baehr-Jones, and M. Hochberg, “The road to affordable, large-scale silicon photonics,” Opt. Photonics News 24(9), 32–39 (2013). [CrossRef]
95. M. R. Watts, J. Sun, E. Timurdogan, E. Shah Hosseini, C. Sorace-Agaskar, A. Yaacobi, Z. Su, M. Moresco, J. D. B., Bradley, N.A. Purnawirman G. Leake, T. N. Adam, and D. Coolbaugh, “Very large scale integrated photonics,” in CLEO: Science and Innovations Conference, 2014 OSA Technical Digest Series (Optical Society of America, 2014), paper SM4O.4.
96. D. Thomson, A. Zilkie, J. E. Bowers, T. Komljenovic, G. T. Reed, L. Vivien, D. Marris-Morini, E. Cassan, L. Virot, J.-M. Fédéli, J.-M. Hartmann, J. H. Schmid, D.-X. Xu, F. Boeuf, P. O’Brien, G. Z. Mashanovich, and M. Nedeljkovic, “Roadmap on silicon photonics,” J. Opt. 18(7), 073003 (2016). [CrossRef]
97. D. Liang and J. E. Bowers, “Recent progress in lasers on silicon,” Nat. Photonics 4(8), 511–517 (2010). [CrossRef]
98. Z. Fang, Q. Y. Chen, and C. Z. Zhao, “A review of recent progress in lasers on silicon,” Opt. Laser Technol. 46(3), 103–110 (2013). [CrossRef]
99. M. Pollnau, “Rare-earth-ion-doped channel waveguide lasers on silicon,” IEEE J. Sel. Top. Quantum Electron. 21(1), 1602512 (2015). [CrossRef]
100. A. Polman, “Erbium implanted thin film photonic materials,” J. Appl. Phys. 82(1), 1–39 (1997). [CrossRef] [PubMed]
101. M. K. Smit, G. A. Acket, and C. J. van der Laan, “Al2O3 films for integrated optics,” Thin Solid Films 138(2), 171–181 (1986). [CrossRef]
102. G. N. van den Hoven, E. Snoeks, A. Polman, J. W. M. van Uffelen, Y. S. Oei, and M. K. Smit, “Photoluminescence characterization of Er-implanted Al2O3 films,” Appl. Phys. Lett. 62(24), 3065–3067 (1993). [CrossRef]
103. G. N. van den Hoven, E. Snoeks, A. Polman, C. van Dam, J. W. M. van Uffelen, and M. K. Smit, “Upconversion in Er-implanted Al2O3 waveguides,” J. Appl. Phys. 79(3), 1258–1266 (1996). [CrossRef]
104. G. N. van den Hoven, J. A. van der Elsken, A. Polman, C. van Dam, K. W. M. van Uffelen, and M. K. Smit, “Absorption and emission cross sections of Er3+ in Al2O3 waveguides,” Appl. Opt. 36(15), 3338–3441 (1997). [CrossRef] [PubMed]
105. G. N. van den Hoven, R. J. I. M. Koper, A. Polman, C. van Dam, J. W. M. van Uffelen, and M. K. Smit, “Net optical gain at 1.53 μm in Er-doped Al2O3 waveguides on silicon,” Appl. Phys. Lett. 68(14), 1886–1888 (1996). [CrossRef]
106. S. Musa, H. J. van Weerden, T. H. Yau, and P. V. Lambeck, “Characteristics of Er-doped Al2O3 thin films deposited by reactive co-sputtering,” IEEE J. Quantum Electron. 36(9), 1089–1097 (2000). [CrossRef]
107. K. Wörhoff, J. D. B. Bradley, F. Ay, D. Geskus, T. P. Blauwendraat, and M. Pollnau, “Reliable low-cost fabrication of low-loss Al2O3:Er3+ waveguides with 5.4-dB optical gain,” IEEE J. Quantum Electron. 45(5), 454–461 (2009). [CrossRef]
108. J. D. B. Bradley, “Al2O3:Er3+ as a gain platform for integrated optics,” Ph.D. Thesis, University of Twente, The Netherlands (2009).
109. J. D. B. Bradley, F. Ay, K. Wörhoff, and M. Pollnau, “Fabrication of low-loss channel waveguides in Al2O3 and Y2O3 layers by inductively coupled plasma reactive ion etching,” Appl. Phys. B 89(2–3), 311–318 (2007). [CrossRef]
110. M. Pollnau and Y. E. Romanyuk, “Optical waveguides in laser crystals,” C. R. Phys. 8(2), 123–137 (2007). [CrossRef]
111. F. Ay, K. Wörhoff, R. M. de Ridder, and M. Pollnau, “Focused-ion-beam nano-structuring of Al2O3 dielectric layers for photonic applications,” J. Micromech. Microeng. 22(10), 105008 (2012). [CrossRef]
112. C. Grivas, D. P. Shepherd, T. C. May-Smith, R. W. Eason, M. Pollnau, A. Crunteanu, and M. Jelinek, “Performance of Ar+-milled Ti:Sapphire rib waveguides as single transverse-mode broadband fluorescence sources,” IEEE J. Quantum Electron. 39(3), 501–507 (2003). [CrossRef]
113. A. Crunteanu, M. Pollnau, G. Jänchen, C. Hibert, P. Hoffmann, R. P. Salathé, R. W. Eason, C. Grivas, and D. P. Shepherd, “Ti:sapphire rib channel waveguide fabricated by reactive ion etching of a planar waveguide,” Appl. Phys. B 75(1), 15–17 (2002). [CrossRef]
114. J. D. B. Bradley, L. Agazzi, D. Geskus, F. Ay, K. Wörhoff, and M. Pollnau, “Gain bandwidth of 80 nm and 2 dB/cm peak gain in Al2O3:Er3+ optical amplifiers on silicon,” J. Opt. Soc. Am. B 27(2), 187–196 (2010). [CrossRef]
115. S. A. Vázquez-Córdova, “Erbium-doped channel waveguide amplifiers in amorphous aluminum oxide and crystalline potassium double tungstate,” Ph.D. Thesis, University of Twente, The Netherlands (2017).
116. J. D. B. Bradley, M. Costa e Silva, M. Gay, L. Bramerie, A. Driessen, K. Wörhoff, J. C. Simon, and M. Pollnau, “170 Gbit/s transmission in an erbium-doped waveguide amplifier on silicon,” Opt. Express 17(24), 22201–22208 (2009). [CrossRef] [PubMed]
117. J. D. B. Bradley, R. Stoffer, A. Bakker, L. Agazzi, F. Ay, K. Wörhoff, and M. Pollnau, “Integrated Al2O3:Er3+ zero-loss optical amplifier and power splitter with 40-nm bandwidth,” IEEE Photonics Technol. Lett. 22(5), 278–280 (2010). [CrossRef]
118. J. Yang, K. van Dalfsen, K. Wörhoff, F. Ay, and M. Pollnau, “High-gain Al2O3:Nd3+ channel waveguide amplifiers at 880 nm, 1060 nm, and 1330 nm,” Appl. Phys. B 101(1–2), 119–127 (2010). [CrossRef]
119. L. Agazzi, E. H. Bernhardi, K. Wörhoff, and M. Pollnau, “Impact of luminescence quenching on relaxation-oscillation frequency in solid-state lasers,” Appl. Phys. Lett. 100(1), 011109 (2012). [CrossRef]
120. L. Agazzi, “Spectroscopic excitation and quenching processes in rare-earth-ion-doped Al2O3 and their impact on amplifier and laser performance,” Ph.D. thesis, University of Twente, The Netherlands (2012).
121. P. Loiko and M. Pollnau are preparing a manuscript titled “Quantitative analysis of cooperative upconversion in Al2O3:Yb3+”.
122. J. D. B. Bradley, R. Stoffer, L. Agazzi, F. Ay, K. Wörhoff, and M. Pollnau, “Integrated Al2O3:Er3+ ring lasers on silicon with wide wavelength selectivity,” Opt. Lett. 35(1), 73–75 (2010). [CrossRef] [PubMed]
123. E. H. Bernhardi, H. A. G. M. van Wolferen, K. Wörhoff, R. M. de Ridder, and M. Pollnau, “Highly efficient, low-threshold monolithic distributed-Bragg-reflector channel waveguide laser in Al2O3:Yb3+.,” Opt. Lett. 36(5), 603–605 (2011). [CrossRef] [PubMed]
124. E. H. Bernhardi, H. A. G. M. van Wolferen, L. Agazzi, M. R. H. Khan, C. G. H. Roeloffzen, K. Wörhoff, M. Pollnau, and R. M. de Ridder, “Ultra-narrow-linewidth, single-frequency distributed feedback waveguide laser in Al2O3:Er3+ on silicon,” Opt. Lett. 35(14), 2394–2396 (2010). [CrossRef] [PubMed]
125. E. H. Bernhardi, M. R. H. Khan, C. G. H. Roeloffzen, H. A. G. M. van Wolferen, K. Wörhoff, R. M. de Ridder, and M. Pollnau, “Photonic generation of stable microwave signals from a dual-wavelength Al2O3:Yb3+ distributed-feedback waveguide laser,” Opt. Lett. 37(2), 181–183 (2012). [CrossRef] [PubMed]
126. H. G. Limberger, N. H. Ky, D. M. Costantini, R. P. Salathé, C. A. P. Muller, and G. R. Fox, “Efficient miniature fiber optic tunable filter based on intra-core Bragg grating and electrically resistive coating,” IEEE Photonics Technol. Lett. 10(3), 361–363 (1998). [CrossRef]
127. D. M. Costantini, H. G. Limberger, R. P. Salathé, C. A. P. Muller, and S. A. Vasiliev, “Tunable loss filter based on metal coated long period fiber grating,” IEEE Photonics Technol. Lett. 11(11), 1458–1460 (1999). [CrossRef]
128. M. L. Dakss, L. Kuhn, P. F. Heidrich, and B. A. Scott, “Grating coupler for efficient excitation of optical guided waves in thin films,” Appl. Phys. Lett. 16(12), 523–525 (1970). [CrossRef]
129. J. M. Castro, D. F. Geraghty, S. Honkanen, C. M. Greiner, D. Iazikov, and T. W. Mossberg, “Optical add-drop multiplexers based on the antisymmetric waveguide Bragg grating,” Appl. Opt. 45(6), 1236–1243 (2006). [CrossRef] [PubMed]
130. C. J. Brooks, G. L. Vossler, and K. A. Winick, “Integrated-optic dispersion compensator that uses chirped gratings,” Opt. Lett. 20(4), 368–370 (1995). [CrossRef] [PubMed]
131. G. Meltz, J. R. Dunphy, W. H. Glenn, J. D. Farina, and F. J. Leonberger, “Fiber optic temperature and strain sensors,” Proc. SPIE 798, 104–114 (1987). [CrossRef]
132. S. V. Pham, M. Dijkstra, A. J. F. Hollink, L. J. Kauppinen, R. M. de Ridder, M. Pollnau, P. V. Lambeck, and H. J. W. M. Hoekstra, “On-chip bulk-index concentration and direct, label-free protein sensing utilizing an optical grated-waveguide cavity,” Sens. Actuator B 174, 602–608 (2012). [CrossRef]
133. G. A. Ball, W. W. Morey, and W. H. Glenn, “Standing-wave monomode erbium fiber laser,” IEEE Photonics Technol. Lett. 3(7), 613–615 (1991). [CrossRef]
134. S. Oku, S. Kondo, Y. Noguchi, T. Hirono, M. Nakao, and T. Tamamura, “Fabrication and performance of LnGaAlAs MQW distributed Bragg reflector lasers with grooved surface gratings formed by reactive beam etching,” Proc. Inst. Elect. Eng. Optoelectron. 146(1), 39–43 (1999). [CrossRef]
135. C. Becker, A. Greiner, T. Oesselke, A. Pape, W. Sohler, and H. Suche, “Integrated optical Ti:Er:LiNbO3 distributed Bragg reflector laser with a fixed photorefractive grating,” Opt. Lett. 23(15), 1194–1196 (1998). [CrossRef] [PubMed]
136. B. K. Das, H. Suche, and W. Sohler, “Single-frequency Ti:Er:LiNbO3 distributed Bragg reflector waveguide laser with thermally fixed photorefractive cavity,” Appl. Phys. B 73(5−6), 439–442 (2001). [CrossRef]
137. B. K. Das, R. Ricken, and W. Sohler, “Integrated optical distributed feedback laser with Ti:Fe:Er:LiNbO3 waveguide,” Appl. Phys. Lett. 82(10), 1515–1517 (2003). [CrossRef]
138. B. K. Das, R. Ricken, V. Quiring, H. Suche, and W. Sohler, “Distributed feedback-distributed Bragg reflector coupled cavity laser with a Ti:(Fe:)Er:LiNbO3 waveguide,” Opt. Lett. 29(2), 165–167 (2004). [CrossRef] [PubMed]
139. S. Blaize, L. Bastard, C. Cassagnetes, and J. E. Broquin, “Multiwavelengths DFB waveguide laser arrays in Yb-Er codoped phosphate glass substrate,” IEEE Photonics Technol. Lett. 15(4), 516–518 (2003). [CrossRef]
140. L. Bastard, S. Blaize, and J. E. Broquin, “Glass integrated optics ultranarrow linewidth distributed feedback laser matrix for dense wavelength division multiplexing applications,” Opt. Eng. 42(10), 2800–2804 (2003). [CrossRef]
141. G. D. Marshall, P. Dekker, M. Ams, J. A. Piper, and M. J. Withford, “Directly written monolithic waveguide laser incorporating a distributed feedback waveguide-Bragg grating,” Opt. Lett. 33(9), 956–958 (2008). [CrossRef] [PubMed]
142. T. Kitagawa, F. Bilodeau, B. Malo, S. Thériault, J. Albert, D. C. Jihnson, K. O. Hill, K. Hattori, and Y. Hibino, “Single-frequency Er3+-doped silica-based planar waveguide laser with integrated photo-imprinted Bragg reflectors,” Electron. Lett. 30(16), 1311–1312 (1994). [CrossRef]
143. E. H. Bernhardi, Q. Lu, H. A. G. M. van Wolferen, K. Wörhoff, R. M. de Ridder, and M. Pollnau, “Monolithic distributed Bragg reflector cavities in Al2O3 with quality factors exceeding 106,” Photon. Nanostructures 9(3), 225–234 (2011). [CrossRef]
144. J. Purnawirman, J. Sun, T. N. Adam, G. Leake, D. Coolbaugh, J. D. Bradley, E. Shah Hosseini, and M. R. Watts, “C- and L-band erbium-doped waveguide lasers with wafer-scale silicon nitride cavities,” Opt. Lett. 38(11), 1760–1762 (2013). [CrossRef] [PubMed]
145. M. Belt, T. Huffman, M. L. Davenport, W. Li, J. S. Barton, and D. J. Blumenthal, “Arrayed narrow linewidth erbium-doped waveguide-distributed feedback lasers on an ultra-low-loss silicon-nitride platform,” Opt. Lett. 38(22), 4825–4828 (2013). [CrossRef] [PubMed]
146. M. R. H. Khan, E. H. Bernhardi, D. A. I. Marpaung, M. Burla, R. M. de Ridder, K. Wörhoff, M. Pollnau, and C. G. H. Roeloffzen, “Dual-frequency distributed feedback laser with optical frequency locked loop for stable microwave signal generation,” IEEE Photonics Technol. Lett. 24(16), 1431–1433 (2012). [CrossRef]
147. E. H. Bernhardi, K. O. van der Werf, A. J. F. Hollink, K. Wörhoff, R. M. de Ridder, V. Subramaniam, and M. Pollnau, “Intra-laser-cavity microparticle sensing with a dual-wavelength distributed-feedback laser,” Laser Photonics Rev. 7(4), 589–598 (2013). [CrossRef]
148. A. Suzuki and K. Tada, “Theory and experiment on distributed feedback lasers with chirped grating,” Proc. SPIE 0239, 10–18 (1981). [CrossRef]
149. P. Zhou and G. S. Lee, “Mode selection and spatial hole burning suppression of a chirped grating distributed feedback laser,” Appl. Phys. Lett. 56(15), 1400–1402 (1990). [CrossRef]
150. H. Hillmer, A. Grabmeier, S. Hansmann, and H. Burkhard, “Continuously distributed phase shifts in chirped DFB lasers using bent waveguides,” Electron. Lett. 30(18), 1483–1484 (1994). [CrossRef]
151. Y. A. C. Livanos, A. Katzir, and A. Yariv, “Fabrication of grating structures with variable period,” Opt. Commun. 20(1), 179–182 (1977). [CrossRef]
152. A. Katzir, A. Livanos, J. Shellan, and A. Yariv, “Chirped gratings in integrated optics,” IEEE J. Quantum Electron. 13(4), 296–304 (1977). [CrossRef]
153. N. Chen, Y. Nakano, K. Okamoto, K. Tada, G. I. Morthier, and R. G. Baets, “Analysis, fabrication, and characterization of tunable DFB lasers with chirped gratings,” IEEE J. Sel. Top. Quantum Electron. 3(2), 541–546 (1997). [CrossRef]
154. G. P. Agrawal, J. E. Geusic, and P. J. Anthony, “Distributed feedback lasers with multiple phase-shift regions,” Appl. Phys. Lett. 53(3), 178–179 (1988). [CrossRef]
155. P. Zhou and G. S. Lee, “Chirped grating λ/4 shifted DFB laser with uniform longitudinal field distribution,” Electron. Lett. 26(20), 1660–1661 (1990). [CrossRef]
156. M. M. Okai, T. Tsuchiya, K. Uomi, N. Chinone, and T. Harada, “Corrugation-pitch modulated MQW-DFB lasers with narrow spectral linewidth,” IEEE J. Quantum Electron. 27(6), 1767–1772 (1991). [CrossRef]
157. S. Yliniemi, J. Albert, Q. Wang, and S. Honkanen, “UV-exposed Bragg gratings for laser applications in silver-sodium ion-exchanged phosphate glass waveguides,” Opt. Express 14(7), 2898–2903 (2006). [CrossRef] [PubMed]
158. G. D. Marshall, M. Ams, and M. J. Withford, “Direct laser written waveguide-Bragg gratings in bulk fused silica,” Opt. Lett. 31(18), 2690–2691 (2006). [CrossRef] [PubMed]
159. T. E. Murphy, J. T. Hastings, and H. I. Smith, “Fabrication and characterization of narrow-band Bragg-reflection filters in silicon-on-insulator ridge waveguides,” J. Lightwave Technol. 19(12), 1938–1942 (2001). [CrossRef]
160. E. H. Bernhardi, “Bragg-grating-based rare-earth-ion-doped channel waveguide lasers and their applications,” Ph.D. Thesis, University of Twente, The Netherlands (2012).
161. M. Pollnau, P. J. Hardman, M. A. Kern, W. A. Clarkson, and D. C. Hanna, “Upconversion-induced heat generation and thermal lensing in Nd:YLF and Nd:YAG,” Phys. Rev. B 58(24), 16076–16092 (1998). [CrossRef]
162. P. J. Hardman, W. A. Clarkson, G. J. Friel, M. Pollnau, and D. C. Hanna, “Energy-transfer upconversion and thermal lensing in high-power end-pumped Nd:YLF laser crystals,” IEEE J. Quantum Electron. 35(4), 647–655 (1999). [CrossRef]
163. M. Pollnau, “Analysis of heat generation and thermal lensing in erbium 3-µm lasers,” IEEE J. Quantum Electron. 39(2), 350–357 (2003). [CrossRef]
164. C. C. Kores, N. Ismail, D. Geskus, M. Dijkstra, E. H. Bernhardi, and M. Pollnau, “Temperature dependence of the spectral characteristics of distributed-feedback resonators,” Opt. Express 26(4), 4892–4905 (2018). [CrossRef] [PubMed]
165. J. Lauzon, S. Thibault, J. Martin, and F. Ouellette, “Implementation and characterization of fiber Bragg gratings linearly chirped by a temperature gradient,” Opt. Lett. 19(23), 2027–2029 (1994). [CrossRef] [PubMed]
166. J. A. Rogers, B. J. Eggleton, R. J. Jackman, G. R. Kowach, and T. A. Strasser, “Dual on-fiber thin-film heaters for fiber gratings with independently adjustable chirp and wavelength,” Opt. Lett. 24(19), 1328–1330 (1999). [CrossRef] [PubMed]
167. N. Q. Ngo, D. Liu, S. C. Tjin, X. Dong, and P. Shum, “Thermally switchable and discretely tunable comb filter with a linearly chirped fiber Bragg grating,” Opt. Lett. 30(22), 2994–2996 (2005). [CrossRef] [PubMed]
168. Y. T. Dai and J. P. Yao, “Numerical study of a DFB semiconductor laser and laser array with chirped structure based on the equivalent chirp technology,” IEEE J. Quantum Electron. 44(10), 938–945 (2008). [CrossRef]
169. G. D. Marshall, R. J. Williams, N. Jovanovic, M. J. Steel, and M. J. Withford, “Point-by-point written fiber-Bragg gratings and their application in complex grating designs,” Opt. Express 18(19), 19844–19859 (2010). [CrossRef] [PubMed]
170. G. A. Miller, J. R. Peele, C. G. Askins, and G. A. Cranch, “Characterization of strong fiber Bragg gratings using an applied thermal chirp and iterative algorithm,” Appl. Opt. 50(36), 6617–6626 (2011). [CrossRef] [PubMed]
171. S. Tjörnhammar, B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Thermal limitations of volume Bragg gratings used in lasers for spectral control,” J. Opt. Soc. Am. B 30(6), 1402–1409 (2013). [CrossRef]
172. M. Born and E. Wolf, Principles of Optics (Pergamon, 1975), Chap. 1.
173. P. Loiko, N. Ismail, J. D. B. Bradley, M. Götelid, and M. Pollnau, “Refractive-index variation with rare-earth incorporation in amorphous Al2O3 thin films,” J. Non-Cryst. Solids 476, 95–99 (2017). [CrossRef]
174. K. Tada, Y. Nakano, and A. Ushirokawa, “Proposal of a distributed feedback laser with nonuniform stripe width for complete single-mode oscillation,” Electron. Lett. 20(2), 82–84 (1984). [CrossRef]
175. M. Usami, S. Akiba, and K. Utaka, “Asymmetric λ/4-shifted InGaAsP/InP DFB lasers,” IEEE J. Quantum Electron. 23(6), 815–821 (1987). [CrossRef]
176. H. Soda and H. Imai, “Analysis of the spectrum behavior below the threshold in DFB lasers,” IEEE J. Quantum Electron. 22(5), 637–641 (1986). [CrossRef]
177. M. Eichhorn, S. T. Fredrich-Thornton, E. Heumann, and G. Huber, “Spectroscopic properties of Er3+:YAG at 300−550 K and their effects on the 1.6 µm laser transition,” Appl. Phys. B 91(2), 249–256 (2008). [CrossRef]
178. Y. S. Yong, S. Aravazhi, S. A. Vázquez-Córdova, J. J. Carjaval, F. Díaz, J. L. Herek, S. M. García-Blanco, and M. Pollnau, “Temperature-dependent absorption and emission of potassium double tungstates with high ytterbium content,” Opt. Express 24(23), 26825–26837 (2016). [CrossRef] [PubMed]
179. C. J. Wu, B. H. Chu, and M. T. Weng, “Analysis of optical reflection in a chirped distributed Bragg reflector,” J. Electromagn. Waves Appl. 23(1), 129–138 (2009). [CrossRef]
180. J. Hong, W. Huang, and T. Makino, “On the transfer matrix method for distributed-feedback waveguide devices,” J. Lightwave Technol. 10(12), 1860–1868 (1992). [CrossRef]
181. T. Makino, “Transfer matrix method with applications to distributed feedback optical devices,” Prog. Electromagn. Res. PIER 10, 271–319 (1995).
182. G. Björk and O. Nilsson, “A new exact and efficient numerical matrix theory of complicated laser structures: properties of asymmetric phase-shifted DFB lasers,” J. Lightwave Technol. 5(1), 140–146 (1987). [CrossRef]
183. N. Ismail, C. C. Kores, D. Geskus, and M. Pollnau, “Fabry-Pérot resonator: spectral line shapes, generic and related Airy distributions, linewidths, finesses, and performance at low or frequency-dependent reflectivity,” Opt. Express 24(15), 16366–16389 (2016). [CrossRef] [PubMed]
184. N. X. Purnawirman, N. Li, E. S. Magden, G. Singh, M. Moresco, T. N. Adam, G. Leake, D. Coolbaugh, J. D. B. Bradley, and M. R. Watts, “Wavelength division multiplexed light source monolithically integrated on a silicon photonics platform,” Opt. Lett. 42(9), 1772–1775 (2017). [CrossRef] [PubMed]
185. M. R. Saleem, R. Ali, S. Honkanen, and J. Turunen, “Thermal properties of thin Al2O3 films and their barrier layer effect on thermo-optic properties of TiO2 films grown by atomic layer deposition,” Thin Solid Films 542, 257–262 (2013). [CrossRef]
186. E. H. Bernhardi, H. A. G. M. van Wolferen, K. Wörhoff, R. M. de Ridder, and M. Pollnau, “Distributed feedback channel waveguide lasers in erbium- and ytterbium-doped Al2O3 on silicon,” Conference on Lasers and Electro-Optics Europe, Munich, Germany, 2011, Conference Digest, paper CJ10.4.
187. T. Okoshi, K. Kikuchi, and A. Nakayama, “Novel method for high resolution measurement of laser output spectrum,” Electron. Lett. 16(16), 630–631 (1980). [CrossRef]
188. A. L. Schawlow and C. H. Townes, “Infrared and optical masers,” Phys. Rev. 112(6), 1940–1949 (1958). [CrossRef]
189. A. Meijerink, C. G. H. Roeloffzen, R. Meijerink, L. Zhuang, D. A. I. Marpaung, M. J. Bentum, M. Burla, J. Verpoorte, P. Jorna, A. Hulzinga, and W. van Etten, “Novel ring resonantor-based integrated photonic beamformer for broadband phased array receive antennas-Part I: Design and performance analysis,” J. Lightwave Technol. 28(1), 3–18 (2010). [CrossRef]
190. D. Marpaung, C. Roeloffzen, W. Beeker, B. Noharet, J. Verpoorte, and R. Baggen, “Development of a Broadband and Squint-Free Ku-Band Phased Array Antenna System for Airborne Satellite Communications,” in Future Aeronautical Communications, S. Plass, ed. (InTech, 2011), pp. 201–222.
191. J. Yao, “Microwave photonics,” J. Lightwave Technol. 27(3), 314–335 (2009). [CrossRef]
192. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]
193. J. Sun, Y. Dai, Y. Zhang, X. Chen, and S. Xie, “Dual-wavelength DFB fiber laser based on unequalized phase shifts,” IEEE Photonics Technol. Lett. 18(23), 2493–2495 (2006). [CrossRef]
194. R. K. Price, V. B. Verma, K. E. Tobin, V. C. Elarde, and J. J. Coleman, “Y-branch surface-etched distributed bragg reflector lasers at 850 nm for optical heterodyning,” IEEE Photonics Technol. Lett. 19(20), 1610–1612 (2007). [CrossRef]
195. G. E. Villanueva, P. Pérez-Millán, J. Palací, J. L. Cruz, M. V. Andrés, and J. Martí, “Dual-wavelength DFB erbium-doped fiber laser with tunable wavelength spacing,” IEEE Photonics Technol. Lett. 22(4), 254–256 (2010). [CrossRef]
196. N. X. Li, E. S. Magden, Z. Su, N. Singh, A. Ruocco, M. Xin, M. Byrd, P. T. Callahan, J. D. B. Bradley, C. Baiocco, D. Vermeulen, and M. R. Watts, “Broadband 2-mu m emission on silicon chips: monolithically integrated Holmium lasers,” Opt. Express 26(3), 2220–2230 (2018). [CrossRef] [PubMed]
197. C. M. Sorace-Agaskar, P. T. Callahan, K. Shtyrkova, A. Baldycheva, M. Moresco, J. D. B. Bradley, M. Y. Peng, N. Li, E. S. Magden, N.A. Purnawirman, M. Y., Sander, G. Leake, D. D. Coolbaugh, M. R. Watts, and F. Kärtner, “Integrated mode-locked lasers in a CMOS-compatible silicon photonic platform,” in CLEO: Science and Innovations Conference, 2015 OSA Technical Digest Series (Optical Society of America, 2015), paper SM2I.5.
198. P. T. Callahan, K. Shtyrkova, N. Li, E. S. Magden, and C. Purnawirman, Baiocco, D. Coolbaugh, E. P. Ippen, M. Watts, and F. X. Kärtner, “Fully-integrated CMOS-compatible Q-switched laser at 1.9µm using thulium-doped Al2O3,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optical Society of America, 2017), paper STh3N.2. [CrossRef]
199. M. Belt and D. J. Blumenthal, “Erbium-doped waveguide DBR and DFB laser arrays integrated within an ultra-low-loss Si3N4 platform,” Opt. Express 22(9), 10655–10660 (2014). [CrossRef] [PubMed]
200. J. F. Bauters, M. J. R. Heck, D. D. John, J. S. Barton, C. M. Bruinink, A. Leinse, R. G. Heideman, D. J. Blumenthal, and J. E. Bowers, “Planar waveguides with less than 0.1 dB/m propagation loss fabricated with wafer bonding,” Opt. Express 19(24), 24090–24101 (2011). [CrossRef] [PubMed]
201. M. Belt and D. J. Blumenthal, “High temperature operation of an integrated erbium-doped DBR laser on an ultra-low-loss Si3N4 platform,” in Optical Fiber Communication Conference, 2015 OSA Technical Digest Series (Optical Society of America, 2015), paper Tu2C.7. [CrossRef]
202. M. Belt, J. Bovington, R. Moreira, J. F. Bauters, M. J. R. Heck, J. S. Barton, J. E. Bowers, and D. J. Blumenthal, “Sidewall gratings in ultra-low-loss Si3N4 planar waveguides,” Opt. Express 21(1), 1181–1188 (2013). [CrossRef] [PubMed]
203. N. Li, P. Purnawirman, Z. Su, E. Salih Magden, P. T. Callahan, K. Shtyrkova, M. Xin, A. Ruocco, C. Baiocco, E. P. Ippen, F. X. Kärtner, J. D. Bradley, D. Vermeulen, and M. R. Watts, “High-power thulium lasers on a silicon photonics platform,” Opt. Lett. 42(6), 1181–1184 (2017). [CrossRef] [PubMed]
204. E. S. Hosseini, J. D. B. Purnawirman, J. D. Bradley, J. Sun, G. Leake, T. N. Adam, D. D. Coolbaugh, and M. R. Watts, “CMOS-compatible 75 mW erbium-doped distributed feedback laser,” Opt. Lett. 39(11), 3106–3109 (2014). [CrossRef] [PubMed]
205. G. Singh, P. Purnawirman, J. D. Bradley, N. Li, E. S. Magden, M. Moresco, T. N. Adam, G. Leake, D. Coolbaugh, and M. R. Watts, “Resonant pumped erbium-doped waveguide lasers using distributed Bragg reflector cavities,” Opt. Lett. 41(6), 1189–1192 (2016). [CrossRef] [PubMed]
206. N. Li, Z. Su, E. S. Purnawirman, E. Salih Magden, C. V. Poulton, A. Ruocco, N. Singh, M. J. Byrd, J. D. B. Bradley, G. Leake, and M. R. Watts, “Athermal synchronization of laser source with WDM filter in a silicon photonics platform,” Appl. Phys. Lett. 110(21), 211105 (2017). [CrossRef] [PubMed]
207. N. X. Purnawirman, N. Li, E. S. Magden, G. Singh, N. Singh, A. Baldycheva, E. S. Hosseini, J. Sun, M. Moresco, T. N. Adam, G. Leake, D. Coolbaugh, J. D. B. Bradley, and M. R. Watts, “Ultra-narrow-linewidth Al2O3:Er3+ lasers with a wavelength-insensitive waveguide design on a wafer-scale silicon nitride platform,” Opt. Express 25(12), 13705–13713 (2017). [CrossRef] [PubMed]
208. E. S. Magden, N. Li, J. D. B. Purnawirman, N. Bradley, A. Singh, G. S. Ruocco, G. Petrich, D. D. Leake, E. P. Coolbaugh, M. R. Ippen, Watts, and L. A. Kolodziejski, “Monolithically-integrated distributed feedback laser compatible with CMOS processing,” Opt. Express 25(15), 18058–18065 (2017). [CrossRef] [PubMed]
209. N. X. Li, N.A. Purnawirman, G. Singh, E. S. Magden, Z. Su, N. Singh, M. Moresco, G. Leake, J. D. B. Bradley, and M. R. Watts, “Reliable integrated photonic light sources using curved Al2O3:Er3+ distributed feedback lasers,” IEEE Photonics J. 9(4), 1504708 (2017).
210. J. D. B. Bradley, E. S. Hosseini, Z. Purnawirman, Z. Su, T. N. Adam, G. Leake, D. Coolbaugh, and M. R. Watts, “Monolithic erbium- and ytterbium-doped microring lasers on silicon chips,” Opt. Express 22(10), 12226–12237 (2014). [CrossRef] [PubMed]
211. Z. Su, N. Li, E. Salih Magden, M. Byrd, P. Purnawirman, T. N. Adam, G. Leake, D. Coolbaugh, J. D. Bradley, and M. R. Watts, “Ultra-compact and low-threshold thulium microcavity laser monolithically integrated on silicon,” Opt. Lett. 41(24), 5708–5711 (2016). [CrossRef] [PubMed]
