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Abstract
High power and high brightness InP based lasers around 2 µm are attractive for many applications due to their intrinsic compatibility with photonic integrated circuits. However, high output power and low lateral divergence are difficult to be realized simultaneously with the traditional ridge waveguide structure. In this paper, we demonstrate significantly enhanced performance of 1.96 µm InP based InGaAs quantum well lasers by tapered waveguide structures. The double-channel waveguide laser with a straight waveguide section and a small-angle tapered optical amplifier section showed fundamental transverse mode lasing with an excellent beam quality. In agreement with our designed waveguide structures, the devices’ lateral divergence is remarkably reduced. For the device with a 3° tapered angle, the narrowest full width at half maximum (FWHM) of 8.2° of the far-field distribution is realized and the continuous-wave output power, measured at 25 °C, is increased up to 40 mW, which is much higher than the 12 mW of the laser with only a straight ridge waveguide.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Short wave infrared semiconductor lasers emitting around 2 µm is very attractive for free space communications, trace gas sensing, medical application and so on [1–5]. A major requirement for these applications is the availability of laser sources which have high power emission, high brightness and potential for low-cost, high-volume production. To date, high-performance lasers have been developed based on strained GaInAsSb/AlGaAsSb quantum wells (QWs) on GaSb substrate. The laser devices with a threshold current density of 50 A/cm2 [6], a continuous wave (CW) output power close to 200 mW in single-spatial mode [7] have been reported. On the other hand, InP based material system is another attractive approach to achieve this wavelength range. In addition to high-quality, low-cost and large diameter substrate, InP-based lasers are beneficial for their compatibility with the mature device fabrication process developed for optical fiber communications [8]. What’s more, for several applications like VCSELs or Silicon integrated devices, InP-based lasers are favorable since the InP material platform is monolithically compatible with many photonics integrated circuits.
In terms of the InP material system, InGaAs multiple QWs (MQWs) were first investigated, and buried heterostructure lasers were successfully fabricated two decades ago [9–11]. However, for a long time, these reported room temperature (RT) CW maximal output power of InP based lasers have generally been limited to a few milliwatts [8,12–15] due to the difficulties of large strain epitaxy and the fabrication of narrow ridge waveguide structure used to guarantee the fundamental transverse mode lasing. Recently we have reported the realization of hundreds of milliwatts output power emission by broad-area (BA) structure [16]. Nevertheless, the penalty for BA lasers is the deteriorative beam quality due to the allowance of multimode operation. Based on these traditional ridge waveguides, some other special methods were reported to control the lateral divergence, such as fishbone-shape microstructure used in GaSb-based 2 µm laser [17]. In spite of their positive effect on reducing lateral divergence, fundamental transverse mode still can’t be guaranteed only with these microstructures. On the contrary, the tapered waveguide structure has achieved the diffraction limited beam quality [18] with high output power and has been extensively applied to many kinds of lasers such as near-infrared QWs laser [19–21], mid-infrared interband cascade laser [22], and quantum cascade laser [23]. The narrow ridge section used as mode filtering and the tapered optical amplifier section providing enough material gain, thus the tapered waveguide structure can reach high power with the fundamental transverse mode. However, the beam propagation in the tapered waveguide section would be enormously influenced by different wavelengths, leading to difficulty of utilizing the already reported structures directly. So the theoretical calculation and experimental data should be investigated systematically to acquire the most suitable structure for InP-based QWs laser near 2 µm. Considering the current situation about InP based short wave infrared laser which needs further optimization in many aspects, we devote to develop the lasers utilizing the tapered waveguide for its easy fabrication and capability of maintaining a good beam quality at high output power.
In this paper, we report several InP type-I high-performance lasers emitting around 2 µm. The lasing spectra, power-current-voltage (P-I-V) characteristics, and lateral far-field (FF) distribution were measured and analyzed. The simulation and experimental results demonstrated that this tapered waveguide structure was able to effectively reduce the number of lateral modes, improve the lateral divergence and output power simultaneously. Comparing with 12 mW for the laser only with straight ridge waveguide, a remarkably increased CW output power of 40 mW at 25 °C was achieved. The lateral divergence was reduced for different taper angles of 1°, 2°, 3° by 45.9%, 57.6% and 68.1% at 100 mA respectively. Moreover, the narrowest full width at half maximum (FWHM) of 8.2° of near diffraction limited FF beam divergence was realized for 3° tapered devices. To the best of our knowledge, these performances are among the best ever achieved for the InP based lasers emitting in this wavelength.
2. Simulation
In order to analyze the behaviors of lateral far-field divergence of the tapered structures, we simulate the optical mode propagation with the mode coupling theory and the finite-element method [24]. In the calculation, the tapered region of the waveguide is divided into a series of triangle subregions with gradually increased width. Within each triangle region, the field distributions are calculated from the Helmholtz equation$\nabla \times ({{\varepsilon_r}^{ - 1}\nabla \times \boldsymbol{H}} )- {k_0}^2{\mu _r}\boldsymbol{H} = 0$, where ${\varepsilon _r}$ and ${\mu _r}$ are conductivity and permeability of solved regions, which include the active region, substrate, Au, Ge/Au/Ni/Au layer as well as an air layer. ${\varepsilon _r} = {n^2}$, where n is the refractive index. ${k_0} = \frac{{2\pi }}{\lambda }$ is the wave number in free space. According to the continuity condition of the tangential electric and magnetic field at the interface, the optical mode distribution at the output facet of the tapered region can be deduced from the optical mode imported from the ridged region. The far-field distribution of the laser beam can be obtained from the optical mode distribution at the output facet according to the Fourier transform formula [25] as ${|{{E_{far}}({{\theta_x},{\theta_y}} )} |^2} = \frac{{{{\cos }^2}{\theta _x}{{\cos }^2}{\theta _y}}}{{1 - {{\sin }^2}{\theta _x}{{\sin }^2}{\theta _y}}}{\left|{\int\!\!\!\int {E({x,y} ){e^{i{k_0}\sin {\theta_x}x}}{e^{i{k_0}\sin {\theta_y}y}}dxdy} } \right|^2}$ where $E({x,y} )$ are the optical field at the output facet, ${E_{far}}({{\theta_x},{\theta_y}} )$ are the far-field distribution.
The simulation is performed by the COMSOL software and the results are respectively shown in Fig. 1. The narrow straight waveguide region effectively suppresses the high-order transverse modes, and the tapered angles are much smaller than the diffractive angle of the fundamental transverse mode (about 17°), so as shown in Fig. 1(a)(b)(c), only the propagation of the fundamental transverse mode within the tapered devices need to be considered. In Fig. 1(d), the black curve with squares presents the relationship between the far-field divergence angle and different taper angles of the waveguide structure. Note that the far-field divergence angle used here reflecting the intrinsic characteristics of lateral FF is defined with 95% power content [17]. Comparing with the straight waveguide structure with 0° taper angle, the far-field divergence dropped drastically with the increase of taper angle and the improvement of lateral divergence seem to be stable near 3° taper angle. The inset shows simulated angular distributions of the far-field beam intensity of five representative structures with 0°, 1°, 2°, 3° and 4°-tapered angles. For the larger taper angle, i.e. 3°, 4°, the simulated results show a small side peak which may be caused by high-order transverse modes within tapered optical amplifier region.
Fig. 1. (a) The calculated magnetic-field intensity |B|2 distribution of the tapered waveguide structure, (b) the details in straight waveguide section and (c) tapered optical amplifier section. (d) The black curve with squares is the relationship between the far-field divergence angle and different taper angles of the waveguide structure. The inset shows simulated angular distributions of the far-field beam intensity of different taper angle waveguide structures.
3. Materials and fabrication
The laser structures were grown on 2-in. S-doped InP substrates by means of an Aixtron 3×2 FT close-coupled showerhead (CCS) metal organic chemical vapor deposition (MOCVD) system that was equipped with a Laytec EpiTT to monitor the in-situ growth temperature of the wafer surface. Trimethylindium (TMIn), triethylgallium (TEGa), arsine (AsH3), and phosphine (PH3) were used as the precursors, and hydrogen was used as the carrier gas. The reactor pressure was fixed at 100 mbar. A 300 nm Si-doped n+-InP buffer, a 1000 nm n-InP lower cladding layer, and a 100 nm lattice-matched InGaAsP quaternary alloy were deposited on an InP substrate at 645 °C. Then the substrate temperature was ramped down for the growth of the active region. The active region deposited at 520 °C was a separate confinement heterostructure (SCH) MQWs with three periods, which contained 5-nm-thick In0.61Ga0.39As wells and 20-nm-thick In0.53Ga0.47As barriers, emitting at 2 µm. After the growth of the active region, the wafer temperature was returned to 645 °C for the growth of the 100 nm InGaAsP and 1200 nm p-InP upper cladding layer. Finally, a 200 nm Zn-doped p+-InGaAs was grown as the contact layer.
Figure 2 shows the double-channel ridge waveguide, which was processed by photolithography and wet-etching. According to our simulation, we fabricated a total of four different structures consisting of a 1 mm-long and 3.4 µm-wide ridge section at back and a 0.3 mm-long tapered section at front. As plotted in Fig. 2(a), the width of output facet varied from 3.4 µm to 19.1 µm, corresponding to the tapered angle changing from 0° to 3°, labeled as devices 0#, 1#, 2# and 3# respectively. Then a 450 nm thick SiO2 layer was deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) for insulation around the ridges. After evaporation of the top non-alloyed Ti/Au contacts, an additional 5-µm-thick gold layer was subsequently electroplated around the ridges to further improve heat dissipation. After being thinned down to about 120 µm, a Ge/Au/Ni/Au metal contact layer was deposited on the substrate side of the wafer. Figure 2(b), (c) and (d) respectively show the top view, back facet and output facet of 3# tapered waveguide structure by an optical microscope. In our tapered lasers, enough long straight section ensures the fundamental transverse mode operation and the tapered section improves the output power. In addition, the tapered section can also significantly narrow the beam divergence angle and reduce the heat density at front facet. A high reflectivity (HR) coating consisting of Al2O3/Ti/Au/Ti/Al2O3 (200/5/100/10/200 nm) was deposited on the back facet. Finally, the cleaved chips were epilayer-down bonded on diamond submounts and then indium soldered to copper heat sinks for effective heat dissipation.
Fig. 2. (a) The 3D schematic of the tapered waveguide structure lasers with detailed parameters, (b) the top view of tapered waveguide structure, (c) the optical microscope image of back facet before coating, and (d) the output facet.
4. Results and discussion
4.1 P-I-V characteristics
The measured power-current-voltage (P-I-V) curves for fabricated lasers at 25 °C are shown in Fig. 3. The devices operate under CW mode. It can be seen that the voltages for different structures are almost the same, but the tapered device shows an evident improvement in the output power. The output power of straight ridge waveguide laser 0# and small-angle tapered structure 1# are at the same level due to little difference between their active region areas. However, for larger taper angle 2# and 3#, the output powers were greatly elevated to 25 mW and 40 mW respectively due to the increased pumped volume of the active region [23]. Considering the compact cavity of 1.3 mm, such high power is enough for practical application and is the best result which has been reported until now around this wavelength based on the InP material system. With the aid of tapered section, reducing the heat density may be another reason to improve output power because the thermal rollover limits the maximum output power. In addition, these devices have a similar threshold current ∼40 mA which indicates a little decline of threshold current density with a larger tapered angle. It’s related to the interface between ridge section and tapered section which would provide a mode reflectivity. The mode reflectivity increases with the tapered angle and the larger tapered angle structure would have low mirror mode loss. The slope efficiency ∼0.11 W/A at a low injection current and the maximum wall-plug efficiency ∼ 7.2% can be deduced from the above P-I-V curves.
4.2 Far-field characteristics
To further verify the influence of the tapered waveguide structures on the beam quality, the lateral far-field characteristics are measured and shown in Fig. 4. To measure the FF profiles of these devices, the laser was mounted on a rotation stage with a step resolution of 0.1° controlled by the computer. An HgCdTe detector operating at 77 K was located 30 cm away from the laser device to collect the light. The FWHM of the lateral far-field profile and the corresponding injection current are shown in Fig. 4. Although the tapered angle was as small as 1° for 1# device, the lateral FF FWHM can be sharply reduced to 45.9% at 100 mA comparing with the straight waveguide laser 0#. When the injected current increases from 100 mA to 200 mA, the lateral FF FWHM increases from 13.9° to 17.2° for tapered 1# device. The corresponding values vary from 25.7° to 30.1° for the straight 0# device. For larger taper angle devices as 2# and 3#, the lateral divergence was reduced by 57.6% and 68.1% respectively. The narrowest FWHM of 8.2° of near diffraction limited FF beam divergence was realized for 3° tapered devices at 100 mA. The inset in Fig. 4(d) shows a typical lateral FF distribution of BA laser with 30 µm ridge. Suffering from the multiple-lobe far-field pattern, the brightness of BA laser is still very low even though the total output power is up to 50 mW. It’s also worth noting that although the fundamental transverse mode dominates the lasing process, a small side peak begins to appear from 300 mA for 3° taper angle, which matches well with our calculation. Predictably, the lateral divergence with a fundamental transverse mode can’t be improved through using a larger taper angle >3°. Double-tapered waveguide structure [26] may be another promising way to further enhance the laser performance. In addition, vertical FF characteristics were also measured and these different structures showed similar single-lobe profiles with the FWHM ∼54° due to the same material thickness in SCH.
Fig. 4. Lateral far-field distribution measured in pulsed mode (30 kHz and 3 µs) for four different waveguide structures. The FWHM of the far-field profiles and injection current were listed in the figure. The inset in (d) shows a typical lateral far-field distribution of BA laser with 30 µm ridge.
4.3 Spectrum property
The spectral properties of these four kinds of laser devices were measured under CW operation, as shown in Fig. 5. The measurements were performed using a NICOLET 8700 FTIR spectrometer with 0.125 cm-1 resolution in a rapid-scan mode. Their wavelengths are around 1.95 µm at 20 °C and nearly located in the center of electroluminescence (EL) spectrum from Fig. 5(a). Figure 5(b) shows more detailed spectra about 3# devices along with 0# at different heat sink temperatures. Multi-longitudinal lasing modes with a 1.1 cm-1 cavity mode interval were observed because of the Fabry–Perot cavity that we fabricated without any grating. The maximum operating temperature of devices is 70 °C and the temperature tuning coefficients Δν/ΔT = −1.284 cm−1 K−1 is extracted from 20 °C to 70 °C based on the main peak. In fact, these spectra behaviors coming from different devices are similar under the same conditions during our measurement. This is because the tapered region has little effect on mode modulation considering the same underlying F-P cavity of different laser structures.
Fig. 5. (a) Room-temperature EL spectrum of the MQWs wafer and CW lasing spectra of the four different waveguide structure lasers at 20 °C. (b) CW lasing spectra of 3° taper angle laser at different heat sink temperatures. The black spectra are coming from 0# structure laser for contrast.
5. Conclusion
In this paper, we have demonstrated the improved performances of InP based type-I lasers using a simple tapered waveguide structure. It was shown that the fundamental transverse mode lasing was guaranteed by enough long straight waveguide section and the lateral divergence was reduced by the tapered optical amplifier section. The narrowest FWHM of 8.2° of the lateral FF beam divergence was realized for 3° tapered devices. The tapered waveguide structure could effectively increase CW output power up to 40 mW, compared with 12 mW for the laser only with straight ridge waveguide. Considering the current situation of short wave infrared InP based MQWs lasers emitting in this wavelength range, our results show remarkable progress in high power, high brightness operation and compatibility for low-cost fabrication. We believe these results will contribute to the development of InP based diode lasers, especially high-power single-mode distributed feedback (DFB) devices which is the prerequisite to many applications.
Funding
National Key Research and Development Program (2016YFB0402303, 2018YFA0209103); National Natural Science Foundation of China (NSFC) (61574136, 61627822, 61774146, 61774150, 61790583).
Acknowledgment
The authors would like to thank Ping Liang and Ying Hu for their help in device processing.
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