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Abstract
We report a single-frequency Q-switched Er:YAG all-solid-state laser with a pulse repetition rate of up to 10 kHz. The single-frequency feature is ensured by injecting the seed laser into a Q-switched ring cavity, and the pulse repetition rate is increased by combing the Pound-Drever-Hall method and optical feedback. Peak power of 4.12 kW with an average pulse energy of 1.35 mJ single-frequency 1645 nm laser pulses is achieved at a pulse repetition rate of 10 kHz, which matches an average power of 13.5 W.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Coherent Doppler Lidar (CDL) and Differential Absorption Lidar (DIAL) offer wide application possibilities in the field of meteorological monitoring, aviation safety, wind energy, disaster alert, and so on [1–7]. Real-time and three-dimensional atmospheric remote sensing requires high data availability. To meet this requirement, a high pulse repetition rate (PRR) with high peak power is desired for the single-frequency pulse laser transmitter.
Erbium-doped single-frequency pulse lasers in the 1.5 µm to 1.7 µm spectral region are attractive for the advantages of high atmospheric transmittance, high detection efficiency and eye safety. Erbium-doped fiber amplifiers (EDFAs), planar waveguide amplifiers, and injection-seeded solid-state oscillators are three main ways widely used as the transmitters of wind CDLs and DIALs.
EDFAs with single-mode optical fibers have the advantages of high efficiency, good beam quality, and compactness. However, limited by nonlinear effects, especially the stimulated Brillouin scattering (SBS), it is still difficult to obtain single-frequency pulses with high peak power by using fiber amplifiers [8–13].
The above problems can be overcome by using a waveguide structure [14–19]. In 2019, Takeshi Sakimura et al. reported a two-stage 1.55 µm laser amplifier by using EDFA and Er, Yb:glass planar waveguide with a peak power of 5.5 kW (pulse energy of 3.2 mJ) at a PRR of 4 kHz [7]. However, pulse shaping is necessary because of the gain saturation in the amplifiers, and the amplified spontaneous emission (ASE) suppression and avoiding beam quality degradation are also tricky. In addition, Er3+ doped gain materials have relatively small emission cross-section (10−21 cm2) and more serious energy transfer up-conversion (ETU), which cannot provide a large unity gain for amplifiers [20].
Injecting a single-frequency seed beam into a solid-state oscillator has been proved to be an effective method to obtain single-frequency lasers with higher energy. Tens of thousands or more oscillations in the cavity can provide enough gain, even when the unity gain is small. In 2019, Shi Yang et al. achieved a maximum 20.3 mJ single-frequency pulse of 1645 nm at the PRR of 200 Hz [21]. In 2021, Zhang Zhenguo et al. reported an injection-seeded Q-switched laser based on a double corner cube retroreflector ring cavity, which achieved a maximum 1.8 mJ single-frequency 1645 nm pulse output at the PRR of 143 Hz [22]. Compared with the EDFA and planar waveguide amplifiers, although injection-seeded solid-state oscillators can obtain higher energy single-frequency pulses, it is difficult to run at high PRRs. This is mainly because the seed laser cannot get sufficient gain to suppress the self-excited oscillation in the oscillators at high PRRs. Another reason is that the commonly used ramp-fire injection seeding method needs the cavity length of the oscillator to scan a wide range (at least one free spectral region), but this is challenging for cavity length control units (such as a piezoelectric transducer, PZT, or a Rubidium Titanyl Phosphate crystal, RTP), especially at high PRRs. Some efforts have been made to improve the PRR of the injection-seeded solid-state oscillators. In 2019, Zhang Meng et al. used a double crystal pumped by two pump sources to increase the gain in the cavity, and they achieved a 1645 nm single-frequency pulse output of 12.84 mJ at the PRR of 1 kHz [23]. In 2020, Huang Shuai et al. adopted the ramp-fire method and a ‘triple-reflection’ configuration on a piezoelectric actuator to improve the PRR, and eventually, they increased the PRR to 3 kHz [24]. Besides, Pound-Drever-Hall (PDH) injection seeding method is another potential way to improve the PRRs which can reduce the performance requirements for PZT or RTP [25]. In 2019, Moran Chen et al. reported a PRR of 6 kHz 1645 nm single-frequency pulse output with the PDH injection seeding method and a high-speed PZT mirror, while the pulse energy was 650 µJ with a linewidth of about 37 MHz [26]. The frequency and energy stability of the single-frequency can also be improved by using the PDH injection seeding method was proven by us in 2020 [27]. Factors that limit the PRR in the previous work are larger laser spot, longer slave cavity, and bidirectional emission.
In this paper, we firstly demonstrate that the PRR of more than 10 kHz and pulse energy of more than 1 mJ of an injection-seeded single-frequency 1645 nm pulse laser can be realized. A ring oscillator with a single-frequency seed laser injection is adopted. The PRRs are improved by using the PDH injection seeding method and optical feedback. The single-frequency PRRs can be set over a wide range between 100 Hz and 12 kHz. The average pulse energy reaches 1.35 mJ at the PRR of 10 kHz. The pulse width is 327.3 ns. It can be calculated that the peak pulse power is 4.12 kW and the average power is 13.5 W.
2. Experimental considerations and setup
According to the above discussion, we fully consider increasing the gain of seed light, suppressing self-excited oscillation, and adopting a suitable injection seeding method. The schematic diagram of the 10 kHz PRR single-frequency Er:YAG solid-state pulse laser is shown in Fig. 1. It mainly consists of three parts: a single-frequency continue-wave (CW) seed laser, a Q-switched slave laser, and a detector and control system.
Fig. 1. Schematic diagram of the single-frequency 1645 nm injection seeded laser with 10 kHz pulse repetition rate. PZT, piezoelectric transducer. AOM, acoustic-optical modulator. NPRO, nonplanar ring oscillator. IS, isolator. EOM, electro-optical modulator. PBS, polarized beam splitter. RPD, resonant photodetector. BPD, broadband photodetector.
The setup of the seed laser is important to the injection-seeded single-frequency pulse laser, especially at high PRRs. The power, linewidth, beam quality of the seed light, the mode-matching efficiency between the seed light and the slave cavity, and the detuning between the seed light and the slave cavity are important factors that affect the success of injection seeding [28]. A stable 1645 nm Er:YAG ceramic nonplanar ring oscillator (NPRO) seed laser with the power of 220 mW and the linewidth of 8 kHz is used in our experiment. It is resonantly pumped by a 1470 nm laser diode (LD). The seed light (about 100 mW) is injected into the slave oscillator through the first diffraction order of the acoustic-optical modulator (AOM), and mode matching with the slave oscillator by the lens of L3 (f = 200 mm) and L4 (f = 500 mm). The isolator (IS) is used to prevent the feedback light from entering the NPRO. The injected seed light is p-polarized. The injection angle of the AOM is about 2 degrees, and the diffraction efficiency of the AOM to the seed light is about 50%.
Our core purpose is to obtain single-frequency pulsed lasers at high PRRs. The structure of the slave oscillator and the pump source are two important factors to be fully considered. Firstly, a ring cavity consisting of six mirrors (M2 to M7) is used. Single-frequency laser output can be obtained more easily in ring cavity than in standing wave cavity because there is no space hole burning effect, and the unwanted double pulses phenomenon can be avoided [23]. Mirrors M2 to M6 all have a high reflectance of more than 99.5% at 1645 nm. M3 has a radius of curvature of 200 mm. M7 is the output mirror with a radius of curvature of 500 mm and a transmission of 15%. Besides, M2 is a dichroic mirror, which also has a high transmission of more than 95% at the pump wavelength 1532 nm. The active medium is a ϕ4 mm ✕ 60 mm Er:YAG crystal doped with 0.25at.% erbium. Low doped concentration is beneficial to reducing the ETU effect and obtaining bigger laser gain at 1.6 µm. The total cavity length is 1.21 m. The designed diameter of the laser beam at the crystal is about 430 µm. Secondly, the slave laser is resonantly pumped by a CW 1532 nm fiber laser. The absorption cross-section of the Er:YAG crystal at 1532 nm is larger than that at 1470 nm. The pump beam is mode matched with the slave cavity by the lens of L1 with a focal length of 200 mm. The combination of L5 and M12 is an optical feedback device. The functions of this optical feedback device will be explained in detail in section 3.
PDH injection seeding method is adopted in our experiment. The detuning between the seed light and the slave cavity is controlled by the modified PDH scheme [25,29]. The seed laser is phase modulated by an electro-optical modulator (EOM), and symmetrical sidebands will appear at both sides of the central spectrum of the seed laser. The dispersion effect of the slave cavity to sidebands can be used to identify cavity detuning. The detuning signal is detected by a resonant photodetector (RPD) [30], which is used to receive the leak light behind M6. The core equipment of the control system is a proportional-integral (PI) controller and a signal source. The PI controller receives and processes the detuning signal detected by the RPD, and then stabilizes the cavity length by controlling the piezoelectric transducer (PZT). After that, the signal source controls AOM to realize the Q-switch operation. The single-frequency characteristic of the output pulse is monitored by the method of heterodyne coherence (consists of a half-wave plate λ/2, beam splitter mirror M11, broadband photodetector (BPD), and an oscilloscope). The CW laser output is detected by a power meter, and the Q-switched laser output is detected by an energy meter.
3. Results and analysis
3.1 CW operation
Continuous pumping is a good choice for high PRR Q-switched lasers because no timing control of pumping light is required. A 1532 nm fiber laser with a maximum pump power of 29.1 W is adopted. The output power under continuous operation is an important output index that needs to be considered in the design of a high PRR pulsed solid-state laser. This is because it represents the maximum average power that the laser can obtain under Q-switched operation. Under the CW operation, when the AOM is closed, the seed beam is not injected into the salve cavity. In this case, the slave laser has a bidirectional CW laser output. Figure 2 shows the relationship between the continuous output power and the incident pump power. The pump threshold of the slave laser is 3.01 W. The maximum power of continuous output is 13.6 W with the pump power of 29.1 W, corresponding to the optical-to-optical conversion efficiency of 46.74%. The laser output power has a linear relationship with the pump power, and the slope efficiency is 51.13%. There is no saturation phenomenon at the maximum pump power, indicating that the output power of the laser has a large room for improvement with the increase of the pump power.
3.2 Bidirectional single-frequency pulse laser output
Normally, the ring cavity slave laser is unidirectional and single-frequency output when the seed laser is injected [22,23,26,27]. If there is a bidirectional laser output, the output laser is usually not in single-frequency, which means loss of lock. While in our experiment, a new phenomenon, that is the bidirectional single-frequency pulse output with the PDH injection seeding method, has been developed.
At the beginning of the experiment, there has no optical feedback device which consists of L5 and M12 in the experimental setup, shown in Fig. 1. The slave cavity can generate unidirectional single-frequency Q-switched pulses at lower PRR (below 5 kHz), and the laser output direction is consistent with the injected seed light, that is the slanting laser beam on the right side of the M7 (we call it the slant laser). With the decrease of pump power, the lower the PRR of unidirectional single-frequency pulsed laser can be obtained. When the PRR exceeds 6 kHz, the slave laser has a stable bidirectional pulse laser output, one is the slant laser and the other is the straight laser. We changed the PRRs from 6 kHz to 10 kHz in the experiment. The PRRs of the two directions are consistent with the setting. The average pulse energy of the two directions is almost equal, but the pulse energy instability is greater than that of unidirectional operation. What’s more, heterodyne beat frequency monitoring shows that both directions pulses are single-frequency pulses. Figure 3 shows the heterodyne beat note signals and their spectrums in two directions at the PRR of 10 kHz. The center frequencies of the frequency spectrums in the two directions are 62.79 MHz and 62.86 MHz, respectively. The difference between them is small and within the frequency fluctuation range (about 1 MHz), so the laser frequencies in the two directions can be considered to be consistent.
Unlike the ramp-fire injection seeding method, there are two distinct processes in the PDH injection seeding method: a) slave cavity length stabilization process, b) seed laser pulling single-frequency pulse laser establishment process. In the case of continuous pumping, with the increase of the PRR, the pump energy available for each pulse gradually decreases, and the inverted population density decreases, which results in the seed laser cannot obtain enough gain to suppress the self-exited oscillation of the straight laser during very short establishment time of Q-switched pulses. So the laser has bidirectional output. There are three main reasons for bidirectional single-frequency in our opinion. Firstly, Since the length of the slave cavity is always locked, the salve cavity can be regarded as a filter whose longitudinal mode is determined. Secondly, the short Q-switched pulse establishing time allows only a small number of longitudinal mode initiation. Third, the seed laser can still suppress most of the oscillated longitudinal modes, except for the one which is closest to the seed laser’s frequency. Therefore, only the longitudinal mode closest to the seed laser is allowed to oscillate preferentially in both directions of the slave cavity, so that the output lasers in two directions are in single-frequency.
3.3 Optical feedback unidirectional single-frequency high PRRs pulse laser output
Although we obtain bidirectional single-frequency pulse laser output at high PRRs, this reduces the pulse energy of each laser path. To achieve unidirectional single-frequency pulse laser output with higher PRRs, optical feedback is adopted to suppress the unwanted laser path (the straight laser). The optical feedback consists of a lens L5 with a focal length of 500 mm and a planar reflector M12 with a reflectivity of 70% at 1645 nm, as shown in Fig. 1. The straight laser reflected by M12 is focused by L5 so that it is mode matched with the laser inside the crystal. The optical feedback makes the small-signal gain of the slant laser larger than that of the straight laser so that the slant laser starts preferentially and the straight laser is suppressed. We choose the M12 with a reflectivity of 70% not only because it reflects the straight laser, but also, we can monitor the effect of the suppression behind it. The adjustment of the optical feedback device can be operated in CW operation mode. Fine-tuning of L5 and M12 maximizes the suppressing effect, the partial reflector M12 has almost no laser behind. At this point, the slant laser power reaches its maximum, and its power is the same as the sum of the power of the two paths in the case of the bidirectional laser output. On this basis, we inject the seed laser into the slave cavity, stabilize its cavity length by using the PDH injection seeding method, and then perform Q-switching to achieve a stable unidirectional single-frequency pulse output.
We realized unidirectional single-frequency 1645 nm pulse output with the PRR from 100 Hz to12 kHz by using the optical feedback. To avoid gain saturation and optical element damage, pulse output characteristics with the PRRs of less than 1 kHz were measured at 50% pump power. The single-frequency pulse energies at the PRR of 100 Hz, 200 Hz, and 500 Hz are 5.13 mJ, 5.01 mJ, and 4.62 mJ, respectively. The output characteristics of a single-frequency pulse with the PRR of 1 kHz to12 kHz were measured at 100% (29.1 W) pump power. Figure 4 shows the variation curves of unidirectional single-frequency pulse energy, pulse width, and peak pulse power with the PRRs.
Fig. 4. Relationship between the pulse energy, pulse width, peak power of the single-frequency pulse and the pulse repetition rate.
The properties of the 10 kHz PRR pulse laser are given below. 500,000 pulse energy values were recorded at the PRR of 10 kHz, as shown in Fig. 5. The average energy of the single-frequency pulse is 1.35 mJ with a standard deviation of 0.036 mJ. The pulse width is 327.3 ns which is measured by a high-speed photodetector with a bandwidth of 10 GHz, as shown in Fig. 6. The single-frequency laser pulse sequence is tested by the direct-current (DC) output terminal of the RPD. The peak power is 4.12 kW, and the average power is 13.5 W.
The heterodyne beat note signal and spectral analysis of single-frequency pulse and seed laser are given in Fig. 7. A fast Fourier transform (FFT) of the beat note signal shows an asymmetric spectrum with a full width at half maximum (FWHM) of 1.63 MHz.
Fig. 7. Beat-note signal and spectral analysis of the unidirectional laser pulse at the PRR of 10 kHz.
When the PRR exceeds 12 kHz, a stable unidirectional single-frequency pulse cannot be obtained. The specific phenomenon is that the PRR is inconsistent with the set Q-switching frequency, and occasional pulse spot flickering can be seen behind the reflector M12. This is because with the increase of PRR, the pump energy available for each pulse gradually decreases, and the number of upper-level particles cannot be effectively accumulated. This indicates that the current design can achieve a maximum PRR of 12 kHz, and the PRR can be further improved by increasing the pump power and optimizing the cavity structure.
4. Conclusion
A single-frequency Q-switched Er:YAG all-solid-state laser is introduced in this paper. PDH injection seeding method and optical feedback method are used to increase the PRRs. PDH injection seeding method stabilizes the cavity length of the slave cavity all the time reducing the performance requirements for PZT or RTP. The optical feedback can effectively suppress unwanted laser, reduce its consumption of the inverted population, and increase the gain of the required laser, thus realizing the stable operation of a unidirectional laser. The output of 1645 nm single-frequency laser pulses with the PRR greater than 10 kHz is realized for the first time. The PRR can be set in the wide range of 100 Hz to 12 kHz. The single-frequency pulse laser energy is 1.35 mJ and the pulse width is 327.3 ns at the PRR of 10 kHz, corresponding to the pulse peak power of 4.12 kW and average power of 13.5 W. Laser transmitters with this performance are very attractive for eye-safe CDLs.
Funding
National Natural Science Foundation of China (61627821).
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 may be obtained from the authors upon reasonable request.
References
1. T. Wei, H. Xia, B. Yue, Y. Wu, and Q. Liu, “Remote sensing of raindrop size distribution using the coherent Doppler lidar,” Opt. Express 29(11), 17246–17257 (2021). [CrossRef]
2. F. Shen, Z. Wang, Y. Xia, B. Wang, P. Zhuang, and C. Qiu, “Quad-Fabry-Perot etalon based Rayleigh Doppler lidar for 0.2-60 km altitude wind, temperature and aerosol accurate measurement,” Optik 236, 166668 (2021). [CrossRef]
3. Y. Zhu, J. Yang, X. Chen, X. Zhu, J. Zhang, S. Li, Y. Sun, X. Hou, D. Bi, L. Bu, Y. Zhang, J. Liu, and W. Chen, “Airborne Validation Experiment of 1.57-µm Double-Pulse IPDA LIDAR for Atmospheric Carbon Dioxide Measurement,” Remote Sens. 12(12), 1999 (2020). [CrossRef]
4. K. Wang, C. Gao, Z. Lin, Q. Wang, M. Gao, S. Huang, and C. Chen, “ nm coherent Doppler wind lidar with a single-frequency Er:YAG laser,” Opt. Express 28(10), 14694–14704 (2020). [CrossRef]
5. D. T. Michel, A. Dolfi-Bouteyre, D. Goular, B. Augère, C. Planchat, D. Fleury, L. Lombard, M. Valla, and C. Besson, “Onboard wake vortex localization with a coherent 1.5 µm Doppler LIDAR for aircraft in formation flight configuration,” Opt. Express 28(10), 14374–14385 (2020). [CrossRef]
6. N. Cezard, S. Le Mehaute, J. Le Gouet, M. Valla, D. Goular, D. Fleury, C. Planchat, and A. Dolfi-Bouteyre, “Performance assessment of a coherent DIAL-Doppler fiber lidar at 1645 nm for remote sensing of methane and wind,” Opt. Express 28(15), 22345–22357 (2020). [CrossRef]
7. T. Sakimura, K. Hirosawa, Y. Watanabe, T. Ando, S. Kameyama, K. Asaka, H. Tanaka, M. Furuta, M. Hagio, Y. Hirano, H. Inokuchi, and T. Yanagisawa, “1.55-µm high-peak, high-average-power laser amplifier using an Er,Yb:glass planar waveguide for wind sensing coherent Doppler lidar,” Opt. Express 27(17), 24175–24187 (2019). [CrossRef]
8. L. Lombard, M. Valla, C. Planchat, D. Goular, B. Augère, P. Bourdon, and G. Canat, “Eyesafe coherent detection wind lidar based on a beam-combined pulsed laser source,” Opt. Lett. 40(6), 1030–1033 (2015). [CrossRef]
9. L. Lombard, A. Azarian, K. Cadoret, P. Bourdon, D. Goular, G. Canat, V. Jolivet, Y. Jaouën, and O. Vasseur, “Coherent beam combination of narrow-linewidth 1.5 µm fiber amplifiers in a long-pulse regime,” Opt. Lett. 36(4), 523–525 (2011). [CrossRef]
10. S. Kameyama, T. Ando, K. Asaka, Y. Hirano, and S. Wadaka, “Compact all-fiber pulsed coherent Doppler lidar system for wind sensing,” Appl. Opt. 46(11), 1953–1962 (2007). [CrossRef]
11. X. Zhang, W. Diao, Y. Liu, X. Zhu, Y. Yang, J. Liu, X. Hou, and W. Chen, “Eye-safe single-frequency single-mode polarized all-fiber pulsed laser with peak power of 361 W,” Appl. Opt. 53(11), 2465–2469 (2014). [CrossRef]
12. V. Philippov, C. Codemard, Y. Jeong, C. Alegria, J. K. Sahu, J. Nilsson, and G. N. Pearson, “High-energy in-fiber pulse amplification for coherent lidar applications,” Opt. Lett. 29(22), 2590–2592 (2004). [CrossRef]
13. W. Diao, X. Zhang, J. Liu, X. Zhu, Y. Liu, D. Bi, and W. Chen, “All fiber pulsed coherent lidar development for wind profiles measurements in boundary layers,” Chin. Opt. Lett. 12(7), 072801 (2014). [CrossRef]
14. 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]
15. N. Ter-Gabrielyan, V. Fromzel, X. Mu, H. Meissner, and M. Dubinskii, “High efficiency, resonantly diode pumped, double-clad, Er:YAG-core, waveguide laser,” Opt. Express 20(23), 25554–25561 (2012). [CrossRef]
16. Y. Tan, L. Ma, S. Akhmadaliev, S. Zhou, and F. Chen, “Ion irradiated Er:YAG ceramic cladding waveguide amplifier in C and L bands,” Opt. Mater. Express 6(3), 711–716 (2016). [CrossRef]
17. J. Mackenzie, J. Grant-Jacob, S. Beecher, H. Riris, A. Yu, D. Shepherd, and R. Eason, “Er:YGG planar waveguides grown by pulsed laser deposition for LIDAR applications,” Proc. SPIE 10082, 100820A (2017). [CrossRef]
18. J. Mackenzie, S. Kurilchik, J. P. Jake, J. Grant-Jacob, and R. Eason, “1.6 µm Er:YGG waveguide amplifiers,” Proc. SPIE 10896, 3 (2019). [CrossRef]
19. V. Fromzel, N. Ter-Gabrielyan, and M. Dubinskii, “Efficient resonantly-clad-pumped laser based on a Er:YAG-core planar waveguide,” Opt. Express 26(4), 3932–3937 (2018). [CrossRef]
20. S. Li, C. Gao, R. Song, F. Hou, J. Guo, Q. Wang, and M. Gao, “Er:YAG MOPA system based on a polarization-multiplexing 4-pass structure,” Opt. Express 28(10), 15424–15431 (2020). [CrossRef]
21. Y. Shi, C. Gao, S. Wang, S. Li, R. Song, M. Zhang, M. Gao, and Q. Wang, “High-energy, single-frequency, Q-switched Er:YAG laser with a double-crystals-end-pumping architecture,” Opt. Express 27(3), 2671–2680 (2019). [CrossRef]
22. Z. Zhang and Y. Ju, “Injection-seeded Q-switched laser based on a double corner cube retroreflector ring cavity,” Opt. Express 29(25), 41954–41963 (2021). [CrossRef]
23. M. Zhang, C. Gao, Q. Wang, M. Gao, S. Huang, and K. Wang, “High-repetition rate, single-frequency laser with a double Er:YAG ceramics ring cavity,” Opt. Express 27(16), 23197–23203 (2019). [CrossRef]
24. S. Huang, Q. Wang, M. Zhang, C. Chen, K. Wang, M. Gao, and C. Gao, “3 kHz Er:YAG single-frequency laser with a ‘Triple-Reflection’ configuration on a piezoelectric actuator,” Chin. Phys. B 29(8), 084204 (2020). [CrossRef]
25. V. Wulfmeyer, M. Randall, A. Brewer, and R. M. Hardesty, “2-µm Doppler lidar transmitter with high frequency stability and low chirp,” Opt. Lett. 25(17), 1228–1230 (2000). [CrossRef]
26. M. Chen, W. J. Rudd, J. Hansell, D. Pachowicz, S. Litvinovitch, P. Burns, N. W. Sawruk, M. D. Turner, and G. W. Kamerman, “Er:YAG methane lidar laser technology,” Proc. SPIE 11005, 25 (2019). [CrossRef]
27. C. Chen, Q. Wang, S. Huang, X. Zhang, K. Wang, M. Gao, and C. Gao, “Single-frequency Q-switched Er:YAG laser with high frequency and energy stability via the Pound–Drever–Hall locking method,” Opt. Lett. 45(13), 3745–3748 (2020). [CrossRef]
28. N. P. Barnes and J. C. Barnes, “Injection seeding I: Theory,” IEEE J. Quantum Electron. 29(10), 2670–2683 (1993). [CrossRef]
29. R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, “Laser phase and frequency stabilization using an optical resonator,” Appl. Phys. B 31(2), 97–105 (1983). [CrossRef]
30. C. Chen, Z. Li, X. Jin, and Y. Zheng, “Resonant photodetector for cavity- and phase-locking of squeezed state generation,” Rev. Sci. Instrum. 87(10), 103114 (2016). [CrossRef]
