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Abstract
We report a self-Q-switched mode-locked Nd:LGGG waveguide laser with tunable dual-wavelength emission fabricated by femtosecond laser direct writing (FsLDW). The waveguide laser delivers pulses with a minimum temporal duration of approximately 30 ps at a fundamental repetition rate of up to 8.03 GHz, without using any modulation element. The maximum output power is determined to be 226 mW with a slope efficiency of 25.38%. This work indicates the promising applications of waveguide lasers based on mixed crystals for integrated optics.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The tremendous development of micro-/nano-machining technology in recent years allows for fabrication of compact laser sources with flexible geometries, small footprints, and good tunability based on diverse laser gain media [1–5]. These compact laser sources are potential candidates for development of high-performance on-chip light sources compatible to multifunctional photonic integrated circuits, which are of significance importance for high-speed information processing [6–11]. Multi-wavelength and self-Q-switching (SQS)/self-mode-locking (SML) are two intriguing interesting operation regimes of solid-state lasers, both of them are highly advantageous for boosting the integration level of laser sources with less or even without optical modulation elements. And because of this unique feature, intensive research has been carried out on multi-wavelength and self-modulated lasers in recent years, fascinating their applications in terahertz technology, biomedicine, range finding, remote sensing, etc [12–17].
The SQS regime was originally obtained with a nonlinear loss mechanism created by population inversion induced refractive index changes and time-dependent lens occurring inside the gain medium. Such a phenomenon can be easily obtained in disordered media (crystals with random distribution of cations with different valences on the same lattice points, or crystals with compositional disorder, also called mixed crystals) [14–18]. Disordered crystalline hosts feature inhomogeneous broadening and split gain spectral lines because they possess random distribution of cations, making them promising laser gain media for realization of simultaneous multi-wavelength emission in Q-switching and mode-locking regimes [19–23]. In this sense, disordered crystal based solid-state lasers are expected to be a game changer for construction of advanced laser systems with compact packages. This also explains why a great deal of research has taken place on growth and functionalization of rare-earth-doped, such as Nd3+-doped, disordered crystals including Nd:GAGG [19], Nd:CLTGG [24], Nd:CLNGG [25], Nd:CTGG [26], and Nd:LGGG [19,25,28–33] in recent years.
Combining compact geometries of waveguide structures and lasing properties of disordered crystals, multi-wavelength SQS laser sources with small footprints can be realized. In bulk disordered crystal lasers, SQS lasers with kHz or MHz repetition rate have been obtained, such as 50.2-kHz pulsed laser based on Nd:GYSGG [22] and 96.5-MHz pulsed lasers in Nd:YSAG [27]. By using the very short cavity length of waveguide structure, GHz-repetition-rate pulsed lasers can be realized [4,37]. Together with its multi-wavelength emission properties, disordered-crystal-based waveguide lasers are very promising laser sources for high-speed electro-optical sampling technology and optical frequency clock applications. Furthermore, the broader emission spectra of disordered crystals are very beneficial for generating much shorter laser pulses via mode-locking technique. Particularly, harmonically self-mode-locked operation for generating sub-picosecond pulses in a Nd:Sr3Y2(BO3)4 disordered crystal laser has been experimentally demonstrated and numerically verified recently, suggesting the great potential of disordered crystals for applications in SML lasers [35]. In case of waveguide laser configuration, however, the very first experimental demonstration of SQS or SML laser based on disordered crystal waveguide is still missing.
In this work, we well combine the concepts of waveguide technology and mixed crystals, demonstrating self-Q-switched mode-locked lasers based on femtosecond-laser-direct-writing (FsLDW) of waveguides. FsLDW technique is one of the most successful micro-/nano-machining methods for fabrication of waveguide structures with flexible geometries in a variety of optical materials [5], while Nd:LGGG is one of the most promising mixed laser crystals (compositional disorder) with broadened absorption and emission spectra caused by the Lu3+-doping in the GGG matrix [21,25,28,34]. The preservation of the fluorescence properties in waveguide and FsLDW induced damage areas are firstly investigated. Then the laser performance of the fabricated Nd:LGGG waveguides is experimentally studied.
2. Nd:LGGG cladding waveguide fabrication
We use FsLDW technique for fabrication of cladding waveguides in a Nd:LGGG crystal wafer (with doping concentrations of 1.2 at.% Nd3+ ions and 1.4 at.% Lu3+ ions, dimensions of 10(x) mm × 9.5(y) mm × 2(z) mm, polished to the optical-grade quality). The crystal wafer is placed on a PC-controlled XYZ micro-position stage for precise translation with a constant velocity of 1 mm/s (to minimize the stress effect induced by laser pulses in this work). The 800-nm Fs-laser is delivered by a femtosecond laser system (Astrella, Coherent Inc., USA) with a pulse width of 60 fs and a repetition rate of 1 kHz and focused by a microscope objective lens (50×, N.A. = 0.47) with a maximum depth of 100 μm beneath the largest crystal surface. A pulsed energy of 0.56 μJ is set for producing laser-damage tracks while avoiding crystal cracking. For each scan, a damage track induced by Fs-laser with a vertical length of 10 μm is produced, and a cladding waveguide with a diameter of around 50 μm in the Nd:LGGG crystal is obtained after multiple scans (with a constant separation of approximately 3 µm between two adjacent laser-induced tracks). The main intention of choosing this FsLDW parameter combination is realizing low-loss waveguides with optimized guiding and lasing performance. As a result, the refractive index change Δn in the irradiated regions is determined to be about 2.8 × 10−3 by measuring the maximum incident angular deflection at which no transmitted power change occurs, and the numerical aperture of the waveguide is determined to be about 0.104.
3. Waveguide characterization and laser performance
3.1 Confocal μ-PL characterization of Nd:LGGG crystal
To study the FsLDW-induced crystalline lattice changes and the preservation of fluorescence properties within waveguide volume, μ-spectroscopic analysis employing a fiber confocal microscope (Alpha300 R, WITec GmBH) at room temperature is performed [36]. In the experiment, a CW 488-nm laser source for luminescence excitation is focused through the cladding waveguide cross-section with a depth of 10 μm by a microscope objective (100×, N.A. = 0.9), the emitted signal is detected via a spectrometer (UHTS 300 SMFC VIS). The WITec software is used to analyze the μ-PL spectra. Figure 1(a) shows the confocal μ-PL emission spectra collected from three representative regions, namely, waveguide, bulk, and filament regions. Figure 1(b) exhibits the optical transmission photograph of the Nd:LGGG cladding waveguide cross section. The μ-PL intensity collected from the waveguide and the bulk material are nearly identical but with a 3% difference, resulting in a very slight fluorescence quenching in the guiding area due to the lattice damage caused by Fs-laser pulses. This nearly identical intensity indicates the good preservation of original luminescence property of Nd:LGGG crystal in the guiding area. While there is a noticeable intensity reduction of μ-PL emission in the filament region compared to the bulk region, suggesting the partial lattice distortion and damage in the laser-modified region. From the spatial 2D distribution of μ-PL response of the 869.73-nm emission line, a distinct reduction of the intensity of the μ-PL emission intensity (see Fig. 1(c)) and a slight red-shift (see Fig. 1(d)) as well as broadening (see Fig. 1(e)) of the μ-PL emission line can be identified. These results confirm the above argument that the FsLDW induces partial lattice distortion and damage [37–40]. Compared with Nd:GGG cladding waveguides we fabricated in the previous work [37], the observed change trends of the μ-PL intensity and width broadening of femtosecond laser induced damages are identical while the shift trend is different. This difference is mainly due to the choice of fabrication parameters with the consideration of uneven lattice field caused by Lu3+ ions doping. Detailed and systematical studies on the specific influence of ion doping on fabrication parameters are required in future work.
Fig. 1. (a) Confocal μ-PL emission spectra collected from the waveguide, the bulk material, and the filament regions, respectively. (b) Microscopic photograph of the fabricated cladding waveguide end face. The spatial 2D distributions of μ-PL (c) intensity, (d) shift, and (e) bandwidth of 869.73-nm emission line.
3.2 Waveguiding properties under passive regime
To estimate the waveguiding properties of the fabricated Nd:LGGG waveguide under passive regime, we implement a typical end-coupling system with a solid-state laser with a working wavelength of 1064 nm. A linear half-wave plate is used to adjust the polarization of the laser source. Then, the input laser is coupled into the waveguide by a microscope objective (20×, N.A. = 0.4) and the transmitted laser from the waveguide is collected by another identical objective. Light transmittance under all-angle polarizations and modal profiles under TE and TM polarizations are summarized in Fig. 2 (a). The results show that this fabricated Nd:LGGG waveguide can guide light efficiently along different polarizations with a transmittance of approximately 65%, corresponding to a total insertion loss of 1.87 dB. The modal profiles of the fabricated waveguide are imaged by a CCD camera (Fig. 2 (b), left, under TE polarization, right, under TM polarization). It is noted that 50-μm diameter waveguides in many other crystalline materials support multiple guided modes. In present work, in contrast, the fabricated waveguide shows a near-single-modal operation.
Fig. 2. (a) Polar plot of all-angle optical transmittance of Nd:LGGG waveguide at 1064 nm. (b) Modal profiles of the fabricated Nd:LGGG waveguide under TE (left) and TM (right) polarizations.
3.3 Laser performance under active regime
The laser performance of fabricated cladding waveguide is explored also by the end-face coupling arrangement similar to the mentioned above (see Fig. 3 the schematic illustration of the experimental setup). While, the laser source is replaced by a tunable CW Ti:Sapphire laser (Coherent MBR), delivering an 808-nm laser to serve as the optical pumping source. A plano-convex lens (f = 25 mm) is used for coupling the pump laser into the waveguide, and a microscope objective (20×, N.A. = 0.4) is employed for collecting the generated laser. The laser cavity is a compact Fabry-Pérot cavity constituted by a pump mirror (M1 with a transmittance of 99.8% at 808 nm and a reflectivity of >99.9% at 1064 nm) and an output mirror (M2 with a reflectivity of approximately 60% at 1064 nm) butt-adhered to the two end-facets of Nd:LGGG crystal. The output laser is detected by a fiber-coupled high-speed InGaAs photodetector (New focus, 1414 model) and analyzed by a digital oscilloscope (Tektronix, MSO 72504DX, 25 GHz bandwidth) after a long-pass filter to filter the residual pumping laser.
Fig. 3. Schematic illustration of an experimental setup for dual-self-Q-switched mode-locked waveguide laser based on FsLDW Nd:LGGG cladding waveguide.
The characteristics of the SQSML laser are summarized in Fig. 4. The maximum output power under TE polarization is determined to be 226 mW with a slope efficiency of 25.38%, as shown in Fig. 4(a). The laser threshold of Nd:LGGG waveguide is 195 mW, which is a relatively high value compared to Nd:GGG waveguides. This is mainly because the disorder crystals are in structural closer to laser glasses, generally possessing higher lasing thresholds than standard crystals, such as Nd:GGG. The near-field modal profile is exhibited in the inset of Fig. 4(a), exhibiting a multi-mode operation under high power operation. Figure 4(b) shows the Q-switched envelope of the output laser (the inset of Fig. 4(b) demonstrates the corresponding successive Q-switched envelopes). The pulse duration is determined to be approximately 30 ps based on mode-locked pulse trains (see Fig. 4(c)). The SQSML laser gives a fundamental repetition rate up to 8.03 GHz with a signal-to-noise ratio (SNR) of 40 dB (see Fig. 4(d)). According to the formula frep = c/2nl (where c is the light speed, n ≈ 1.94 is the refractive index of Nd:LGGG [28], and l ≈ 9.5 mm is the cavity length), the theoretical fundamental repetition rate of the waveguide cavity used in this work can be calculated to be approximately 8.13 GHz, which is in fairly good agreement to the experimental results in this work. The slightly difference is most likely due to the refractive index difference between Nd:LGGG and Nd:GGG. It is the highest repetition rate and the narrowest pulse duration of SQSML lasers ever reported, to the best of our knowledge [14]. When increasing the pump power up to 1 W, the output Q-switched mode-locked waveforms are more volatile than that at low-power operation regime (with >20% fluctuation on the signal intensity), which is mainly due to the thermal effect and the multi-mode operation under high pump power. The maximum output power present here is limited by the available pump power. The Q-switched and the mode-locked pulse train waveforms have been testified for around 3-hour time scale, and only <5% difference in pulse intensity can be identified at low-power operation regime. It is worth mentioning that the lasing performance is rather robust and is not very sensitive to the mirror alignment, which is benefitted from the compact geometries and good optical confinement of waveguide structures. In particular, if the optical gain is sufficiently high, an individual single-pass may also result in efficient lasing oscillation, even without using any reflective coatings or mirrors [5].
Fig. 4. (a) Output power as a function of launched power under self Q-switched mode-locked operation in the Nd:LGGG cladding waveguide. The insert is the corresponding near-field modal profile of the output waveguide laser. (b) Single Q-switched envelope and the inset is successive Q-switched envelopes. (c) Mode-locked pulse trains. (d) RF spectrum.
Furthermore, we also implement Z-scan and I-scan arrangements using a 1035-nm femtosecond laser to investigate the nonlinear optical responses of the Nd:LGGG crystal sample. The experimental results show that the sample has no identical saturable absorption property at 1-μm wavelength. Thus, it can be concluded that the generation of pulse laser is attributed to the properties of the disordered crystal itself. In the disordered gain medium, there exists changes of refractive index caused by population inversion and time-dependent lens, which leads to the SQS state through a nonlinear loss mechanism [15,18]. Besides, this performance is believed to be significantly enhanced and stabilized in the waveguide structure since we do not obtain stable SQS or SQSML laser in the bulk material.
Moreover, dual-wavelength operation is observed due to the splitting of spectra lines, emitting 1061-nm and 1063-nm lasers. The spectra of output lasers are measured by a spectrometer (FX2000, with a resolution of 1 nm). The relative intensity ratios of 1061-nm to 1063-nm laser emission can be controlled by adjusting polarizations of the pump laser. When the pump polarization angles are adjusted from 0°-90° (TM to TE), the corresponding ratios are determined to be 7.37% (0°, TM), 28.93% (30°), 43.17% (60°), 62.94% (90°, TE), respectively (see Fig. 5). We suppose such a polarization-dependent effect is attributed to the cladding waveguide geometries fabricated by FsLDW. The optical confinement capability of every single laser-induced track under TE and TM polarization is different, resulting in polarization-dependent waveguide losses and optical gains. Thus, polarization-dependent dual-wavelength generation is obtained. This tuning of the dual-wavelength emission intensity is very promising for tunable nonlinear optical frequency conversion for, e.g., THz generation (the two bands have a center frequency difference of 0.53 THz). The wavelength difference of 2 nm in this work is comparable with the previously demonstrated result of 1.8 nm obtained in a Nd:LGGG bulk laser [21]. Therein, the dual-wavelength passively mode-locked Nd:LGGG laser with a pulse duration of 6.3 ps was obtained, which is shorter than our result of 30 ps. Nevertheless, the performance of this compact SQS dual-wavelength lasers still has room for improvement. With the optimization of coupling condition, further reduction of propagation losses of waveguides, and efficient dispersion management, multi-wavelength self-pulsed lasers with shorter pulses can be obtained. Particularly, in order to achieve CW mode-locking while maintaining the compactness of waveguide lasers, a thin air-filled gap between the waveguide end-facet and a cavity mirror can be utilized for dispersion management [7]. Besides, dual-wavelength laser at 1.3 μm in Nd:LGGG bulk crystal with a repetition rate of 32 MHz and a pulsed duration of 4.55 ps modulated by a SESAM has been demonstrated [31]. Due to the well-confined optical intensity in waveguide structures, it is also possible to realize dual-wavelength self-pulsed lasers at 1.3 μm in Nd:LGGG waveguides in the future work.
Fig. 5. Wavelength tunings of Nd:LGGG cladding waveguide laser emissions under different polarization angles. (a) 0°, (b) 30°, (c) 60°, (d) 90°, respectively.
4. Conclusion
In summary, a SQSML waveguide laser with dual-wavelength operation is obtained in Nd:LGGG cladding waveguide fabricated by FsLDW technique. The pulse duration is determined to be approximately 30 ps and the fundamental repetition rate is up to 8.03 GHz. The relative intensity ratios of 1061-nm laser emission to 1063-nm laser emission can be well controlled by adjusting the pump polarizations. This work shows the promising applications of Nd:LGGG waveguides in multifunctional photonics integrated devices.
Funding
National Natural Science Foundation of China (12074223); Taishan Scholar Foundation of Shandong Province; China Postdoctoral Science Foundation (2020M682155).
Acknowledgments
Y. Jia acknowledges the support from “Taishan Scholars Youth Expert Program” of Shandong Province and “Qilu Young Scholar Program” of Shandong University, China. The authors gratefully acknowledge Mr. Q. Lu from Shandong University, Prof. H. Yu from Shandong University, and Ms. L. Sun from Shandong Normal University for their kind help on crystal processing, optical characterization, and µ-PL analysis
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
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