Open Access
Add to BibSonomy
Abstract
We report an idler-resonant, continuous-wave (CW) seed injected, optical parametric oscillator (OPO) based on cadmium selenide (CdSe). The CdSe OPO was pumped by a 2.09 µm ns-pulsed laser and injection-seeded by a 2.58 µm CW laser. The idler-resonant oscillator was designed to maximize the optical-to-optical conversion efficiency and optimize the beam quality. The injected seed laser was designed to reduce the pump threshold. With this setup, the average idler output power of 802 mW was obtained corresponding to a pulse energy of 0.8 mJ at the wavelength of 11.01 µm and linewidth (FWHM) of 0.6 cm−1, optical-to-optical conversion efficiency of 4.4%, quantum conversion efficiency of 23.3%, beam quality of M2x = 1.23, M2y = 1.12, and pulse width of 21 ns. In addition, by turning the angle of the CdSe, wavelength tuning of 10.55-11.98 µm was achieved.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The long-wave infrared lasers in the spectral range of 8-12 µm have advantages of significant application in the fields such as high-resolution molecular fingerprint spectroscopy, remote sensing, and optoelectronic countermeasure. An optical parametric oscillator (OPO) is one of the technologies used for generating 8-12 µm lasers. At present, limited by the development of nonlinear crystals, the output power of OPO in separate wavelengths of 8-12 µm was quite different. In the 8-10 µm waveband, benefiting from the ZnGeP2 crystal which has large nonlinear coefficient, large size, low absorption coefficient, and the output power reaches the ten-watt level [1,2]. However, in the waveband of 10-12 µm, due to lack of nonlinear crystals which simultaneously satisfies large nonlinear coefficient, large size, and low absorption coefficient, the output power only reaches the watt level [3].
Currently, nonlinear crystals suitable for generating 10-12 µm lasers mainly include OP-GaAs, GaSe, AgGaSe2, BaGa4Se7, and CdSe. OP-GaAs has a very large nonlinear coefficient (d14 = 94 pm/V [4]), so it was used to demonstrate an efficient long-wave infrared laser with an average output power of 812 mW at 10.6 µm [5]. GaSe has a relatively large nonlinear coefficient (d22 = 54 pm/V [6]), but its layered structure leads to poor mechanical properties that make it difficult to be processed into a designed phase-matching angle. AgGaSe2 has a moderate nonlinear coefficient (d36 = 33 pm/V [6]), but its damage threshold is only 18 MW/cm2 @ 2.09 µm [7], which limits the generation of high-power laser radiation. BaGa4Se7 is a newly developed crystal, and its growing technique is immature. The damage threshold of CdSe is 56 MW/cm2 @ 2.09 µm [7], and its nonlinear coefficient is the smallest (d31 = 18 pm/V [6]), but it has a small walk-off angle, so the downside of the small nonlinear coefficient can be compensated by increasing the crystal length. Our previous work demonstrated a CdSe-OPO with an output power of 1.05 W at 10.1 µm [3]. In summary, OP-GaAs and CdSe are the most potential nonlinear crystals in the 10-12 µm waveband. The fabricating process of OP-GaAs is epitaxial growth, and the usable thickness is currently limited to about 3.5 mm [8]. Since higher power lasers need crystals with larger apertures, the limited aperture limits further power scaling. In contrast, the mature growing process and the large size of the producible crystals of CdSe offer the potential for power scaling. In 2016, our group reported a 140 mW 10.28 µm CdSe signal-resonate OPO (SROPO) with optical-to-optical conversion efficiency of 2% [9]. In 2017, a 12.07 µm CdSe SROPO with average power of 170 mW and optical-to-optical conversion efficiency of 1.16% was investigated [10]. In 2018, J. Wang et al., reported a 320 mW 10.20 µm CdSe SROPO with optical-to-optical conversion efficiency of 1.8% [11]. In 2020, our group demonstrated a 1.05 W, 10.1 µm CdSe SROPO with seed-injection and intracavity beam expansion, which has an optical-to-optical conversion efficiency of 4.69% and beam quality M2 of ≤ 2.25 [3]. The currently reported 10-12 µm CdSe OPOs are all based on the structure of SROPO. Although this structure is beneficial to increase the power of idler (due to the reduction of loss of idler), but with the increase of output power, the deterioration of beam quality becomes obvious, and it is necessary to insert a compensation lens with an appropriate focal length in the cavity [12–14], which poses challenges to the design and adjustment of the laser system. Moreover, since the single-photon energy of the signal is large, the energy density in the cavity is pretty large, which is not conducive to high-power long-term operation.
Idler-resonant OPO (IROPO) is a simple solution to improve the beam quality of idler and to facilitate high-power long-term operation of the device. On the one hand, compared with SROPO, under the premise of the same beam radius, IROPO has a smaller Fresnel number [15], and is subjected to multiple round-trip spatial filtering in the cavity [16,17], thus bringing a better idler beam quality. On the other hand, the single-photon energy of the resonant-laser of IROPO is smaller than that of SROPO, so the energy density in the cavity is lower (under the premise of the same number of resonant-laser photons and the same beam radius), which is more favorable for the operation of high power and long time. For example, for idler wavelength of 11.01 µm and signal wavelength of 2.58 µm, with the same beam radius, the effective Fresnel number of IROPO is only about a quarter of that of SROPO, and the single-photon energy of idler is only about a quarter of that of signal. However, as far as we know, there are no reports of IROPO in the ∼11 µm waveband at present. This is because IROPO at such a long wavelength will bring a very high pump threshold, making it difficult to achieve high power idler output. To solve this problem, we injected a continuous-wave (CW) seed laser into the conventional IROPO cavity and successfully reduced the pump threshold. The idler output at 11.01 µm wavelength with average power of 802 mW, quantum conversion efficiency of 23.2%, and beam quality M2 of ≤1.23 was obtained. To the best of our knowledge, this is the first time to achieve ∼11 µm IROPO. Compared with the best CdSe SROPO [3] at present, we achieved a higher quantum conversion efficiency and a better beam quality under the premise of a simpler cavity. Our work demonstrates that seed-injected CdSe IROPO can simultaneously ensure high efficiency and high beam quality of output idler, and is a more durable scheme due to the lower energy density in the cavity (compared to SROPO).
2. Experimental setup and cavity design
The experimental setup is shown in Fig. 1. The pump source was a home-made Q-switched Ho: YAG laser [18] (wavelength λ = 2090.6 nm, linewidth Δλ (FWHM) < 1 nm, pulse repetition frequency (PRF) = 1 kHz, pulse width ≈ 35 ns, M2 < 1.2). Using a combination of a half-wave plate and a thin film polarizer, the pump power injected into the CdSe was changed while the pump beam radius and pulse width were kept constant [18]. The seed laser was a home-made CW Cr2+: ZnSe laser (λ = 2580.2nm, Δλ (FWHM) < 0.2nm, M2 < 5) pumped by a 1908 nm CW fiber laser, which has a similar structure to [19]. The wavelength of the Cr2+: ZnSe laser was fixed at 2580.2 nm by a volume Bragg grating, and its laser polarization-state was fixed to p-polarization by a 0.05 mm thick YAG etalon placed at the Brewster angle.
Fig. 1. Experimental setup of CdSe IROPO with CW seed injection. CW, continuous-wave; s, s-polarization; p, p-polarization.
The flat mirror M1 was highly reflective at 1.9-2.2 µm. The flat mirror M2 was highly reflective at 1.9-2.2 µm and highly transmittance at 2.5-2.8 µm. The flat mirror M3 had a transmittance of 91.9% at 2.09 µm for s-pol. light, a transmittance of 99.0% at 2.58 µm for p-pol. light, and a reflectance of 98.2% at 11.01 µm for s-pol. light. The idler output coupler (OC) M4 had a transmittance of 40.0% at 11.01 µm for s-pol. light. The measured absorption coefficient of the CdSe crystal (49 × 6 × 8 mm3, θ = 70 ± 0.5°, φ = 0°, type-II phase matching) was 0.025 (2.09 µm), 0.043 (2.63 µm), and 0.006 (10.22 µm) cm−1, the end surfaces were anti-reflection coated for pump (2.09 µm), signal (2.58 µm), and idler (11.01 µm). The temperature of the CdSe was kept at room temperature by a water-cooling machine. The beam radii of the pump and seed at the front-end of the CdSe were about 1.19 mm and 1.50 mm, respectively. The pump and seed were divergently injected into the CdSe, and the divergence half-angles in the air were 5 mrad and 7 mrad, respectively. The physical size of the OPO cavity was 76 × 23 mm2.
By calculating the beam radius of self-reproducing mode in the cavity, the beam radius of the resonant-laser at front-end of the CdSe was captured, the results are shown in Fig. 2(a). The resonant-laser beam radii of SROPO and IROPO are about 0.4 mm and 0.9 mm, respectively, while that of the pump is about 1.2 mm. The beam radius of the SROPO resonant-laser is only 1/3 size of the pump, so the mode-matching is poor. In contrast, the mode-matching between the IROPO resonant-laser and pump is better, which can extract the pump energy more efficiently. Numerical simulations were carried out using SNLO software. Some selections of the simulation results are shown in Figs. 2(b)–2(c), where the performance of an IROPO and a SROPO configuration are compared for different crystal lengths. The transmittance of the OCs are set to 40%, and the power of the pump and seed are set to 18.2 W and 90 mW, respectively. It can be seen from Fig. 2(b) that the seed injection significantly increases the output power of idler, which is more obvious for IROPO. The idler power of IROPO is highest when the crystal length is about 48 mm, and decreases when the crystal length is larger, because the back conversion [20] becomes obvious as crystal length increases. In the experiment, we selected a 49 mm long CdSe crystal according to the available conditions. Figure 2(c) shows the change in the beam quality of the idler of SROPO and IROPO with the length of the CdSe crystal. It can be seen that IROPO has better beam quality than SROPO, because the back conversion in an idler-resonant design is minimized.
Fig. 2. (a) Calculated beam radii of resonant-lasers at front-end of the CdSe crystal versus CdSe thermal lens focal length. IROPO, idler-resonant OPO; SROPO, signal-resonant OPO. (b) Simulated idler output average power and (c) simulated idler beam quality factor M2 versus length of the CdSe crystal. The power of seed laser is 90 mW.
3. Results and discussion
Due to the transmission losses of the mirrors, the maximum available average pump power into the CdSe was 18.2 W. As shown in Fig. 3(a), when the seed is not injected or the seed is injected at 90 mW, the maximum average output power of idler is 397 mW and 802 mW, respectively. The slope efficiency is 6.0% and 8.8%, the optical-to-optical conversion efficiency is 2.2% and 4.4%, the quantum conversion efficiency is 11.5% and 23.2%, and the pump thresholds is 0.54 J/cm2 and 0.41 J/cm2, respectively. The seed injection reduced the pump threshold by 24.1%, increased the slope efficiency by 46.4%, and increased the idler average power by 102.0%. It can be seen that the injected seed significantly improves the output power of idler, which is consistent with the simulated results as shown in Fig. 2(b). Compared to Ref. [3], the pump threshold in this paper is lower, which is due to the use of a longer CdSe crystal. Figure 3(b) is the optical-to-optical conversion efficiency of the idler corresponding to Fig. 3(a). It can be seen that up to the maximum pump power, there is no gain saturation, which implies that the output power can be increased by simply increasing the pump power. Figure 3(c) shows the power stabilities under the maximum average power of idler and pump within 10 minutes recorded by a Coherent PM30 USB power meter. The standard deviation of the normalized power of idler and pump is 2.0% and 1.3%, respectively.
Fig. 3. (a) Idler output average power and (b) idler optical-to-optical conversion efficiency under different seed power. (c) Normalized power stabilities of idler and pump in 10 minutes. The average power of idler and pump is 802 mW and 18.2 W, respectively. SD: standard deviation.
Under different pump power and 90 mW seed, the pulse time-domain waveforms of signal, idler and depleted pump were recorded by a HgCdTe detector (Vigo, pvm-10.6) and an oscilloscope (Tektronix, DPO5204B). Figure 4 shows the evolution of the temporal pulse profiles. When the output laser starts to build up, the top of the pump pulse is weakened (Fig. 4(a)). Subsequently, as the pump power increases, the weakened portion gradually increases. When the pump power is reaching its maximum, the middle part of the depleted pump bulges slightly (about 32 ns in Fig. 4(d)), which reflects a slight back conversion, i.e., the signal and idler are recombined into the pump. So, the transmission of the OC M4 needs to be slightly increased. It can also be seen from Fig. 4 that as the pump power increases, the initial build-up time of the idler gradually decreases, from about 24 ns in Fig. 4(a) to 16 ns in Fig. 4(d). How to further reduce the initial build-up time will be the key to increasing the output power. Under the maximum pump power and 90 mW seed, the pulse widths of idler and signal are 21 ns and 19 ns, respectively.
Fig. 4. Temporal pulse profiles of the depleted pump, pump, signal, and idler at the pump average power of (a) 11.8W, (b) 14.2 W, (c) 15.9 W, and (d) 18.2 W and 90 mW seed.
At 90 mW seed and maximum pump power, the idler spectrum was recorded by a 50 lines/mm monochromator (WDG30P, ∼2 nm resolution for 11 µm light), a HgCdTe detector, and an oscilloscope. The results, which are illustrated in Fig. 5(a), shows that the center wavelength is 11014 nm, and the linewidth (FWHM) is 7 nm (0.6 cm−1). We measured the spectrum of the idler for 7 times, each measurement took about 5 minutes. The measured center wavelengths of the idler are shown in the inset of Fig. 5(a). Most of the time, the center wavelength of idler is stable at 11014 nm, and the minimum center wavelength is 11011 nm, which is basically within the resolution of the monochromator. At maximum output power, the 90/10 knife-edge technology was used to measure the idler beam radius and fit the beam quality M2. As shown in Fig. 5(b), the beam quality factor M2 in horizontal and vertical direction is 1.23 and 1.12, respectively. It is worth noting that the signal has a walk-off in the x direction within the crystal (walk-off angle 5.13 mrad). Generally, the walk-off restricts the phase-matching acceptance angle, thereby improving the beam quality in the walk-off direction [21], i.e., the beam quality in the x direction in Fig. 5(b) should be better than that in the y direction, but the experimental results are the opposite. This is because the upper position of the crystal end face was used in the experiment, so the restriction of the crystal aperture in the y direction brings a greater diffraction loss.
Fig. 5. (a) Idler spectrum at 802 mW output power. Inset, the central wavelength of the idler measured multiple times. (b) Beam propagation and M2 factors of idler at 802 mW output power.
When the seed was not injected, the idler wavelength was tuned by rotating the angle of the CdSe. The tuning range was 10.55-11.98 µm. As shown in Fig. 6(a), the red curve is the angle tuning curve calculated by the Sellmeier equation in Ref. [22]. The measured values agree with the trend of the calculated curve, but the measured values are about 0.13-0.20 µm greater than the calculated curve, the reason of which may be the deviation of crystal cutting angle and errors of the Sellmeier equation. Because the wavelength of the seed laser was single and non-tunable, we only achieved high power output at the wavelength of 11.01 µm, but with wavelength tunable seed laser [23–25], we believe that in the whole wavelength tuning range of 10.55-11.98 µm, we can achieve high power and high beam quality idler output.
Fig. 6. (a) Calculated CdSe angle tuning curve and measured wavelength value, (b) H2O absorption lines and corresponding sound intensity. PAE, photoacoustic effect.
Since the wavelength range of the corresponding signal is 2.61-2.53 µm, and part of it is located in the H2O absorption lines, when the signal wavelength is located in the H2O absorption peak, a strong water absorption sound will be generated, which is caused by the photoacoustic effect (PAE) [26], so the presence or absence of sound was recorded during the experiment. In Fig. 6(b), the green square dots represent no PAE, the red round dots represent PAE, and the blue H2O absorption lines are plotted using HITRAN data [27]. It can be seen that the signal wavelength is in good agreement with the H2O absorption lines. The negative significance brought by H2O absorption is that when H2O vapors are attached to the cavity mirror or the crystal surface, the H2O absorption causes the H2O molecules to vibrate and cause damage to the cavity mirror or the crystal coatings. IROPO is idler-resonant in the cavity, and the signal is directly output from the cavity. So compared with SROPO, IROPO is less susceptible to H2O absorption. Of course, the influence of H2O absorption can be eliminated by filling with inert gas.
Table 1 summarizes the results of the 10-12 µm CdSe OPOs reported so far and this paper. The reported 10-12 µm CdSe OPOs are all based on signal-resonant structure, among which, the output power and quantum conversion efficiency of Ref. [3] are the highest. The core difference between this paper and the Ref. [3] is the resonant wavelength, i.e., the resonant wavelength of the Ref. [3] is 2.64 µm, while the resonant wavelength of this paper is 11.01 µm, which is a 4 times difference in wavelength. Since the wavelength difference of the resonant-laser is large enough, the results obtained in this paper are sufficiently different from those of the Ref. [3]. Compared with Ref. [3], the highlights of this paper are the higher quantum conversion efficiency, better beam quality, and simpler cavity (without the use of an intracavity beam expander). Among them, the better beam quality and simpler cavity are the advantages of IROPO. This is because compared with SROPO, IROPO has a smaller effective Fresnel number [15] and intracavity multi-pass spatial filtering [16,17,21], which are conducive to ensuring good beam quality. The beam radius of IROPO resonant-laser is larger, which is conducive to a better mode-matching, so there is no need to use an additional intracavity beam expander. In addition, by injecting a CW seed to reduce the pump threshold and increase the output power, the idler quantum conversion efficiency in this paper is even slightly higher than that in Ref. [3], which shows that IROPO can also achieve a high conversion efficiency. In general, IROPO with seed injection can simultaneously ensure high efficiency and high beam quality of output idler, and is a more durable scheme due to the lower energy density in the cavity (compared to SROPO).
Table 1. Comparison of performances of reported 10-12 µm CdSe OPOs
4. Conclusion
In summary, injection-seeding by a CW 2.58-µm laser, an 11-µm CdSe-IROPO was demonstrated with average output power of 0.8 W and pulse width of 21 ns at PRF of 1 kHz. The IROPO type ensures output idler with high beam qualities (M2x=1.23, M2y=1.12). Seed injection significantly reduced the pump threshold and improved the output power. Besides, a single CdSe crystal achieved a wavelength tuning of 10.55-11.98 µm. By using a larger size of CdSe, a broader range of wavelength tuning is expected. Our work demonstrates that seed-injected CdSe IROPO can simultaneously ensure high efficiency and high beam quality of output idler, and is a more durable scheme (compared to SROPO).
Acknowledgments
We thank Dr. Arlee V. Smith and Mr. Jesse Smith for their SNLO software, which brought very useful inspiration and effective references to our experiments.
Disclosures
The authors declare no conflicts of interest.
References
1. G. Liu, Y. Chen, B. Yao, K. Yang, C. Qian, T. Dai, and X. Duan, “Study on long-wave infrared ZnGeP2 subsequent optical parametric amplifiers with different types of phase matching of ZnGeP2 crystals,” Appl. Phys. B 125(12), 233 (2019). [CrossRef]
2. C. Qian, X. Duan, B. Yao, Y. Shen, Y. Zhang, B. Zhao, J. Yuan, T. Dai, Y. Ju, and Y. Wang, “11.4 W long-wave infrared source based on ZnGeP2 optical parametric amplifier,” Opt. Express 26(23), 30195–30201 (2018). [CrossRef]
3. Y. Chen, G. Liu, C. Yang, B. Yao, R. Wang, S. Mi, K. Yang, T. Dai, X. Duan, and Y. Ju, “1 W, 10.1 µm, CdSe optical parametric oscillator with continuous-wave seed injection,” Opt. Lett. 45(7), 2119–2122 (2020). [CrossRef]
4. T. Skauli, K. Vodopyanov, T. Pinguet, A. Schober, O. Levi, L. Eyres, M. Fejer, J. Harris, B. Gerard, and L. Becouarn, “Measurement of the nonlinear coefficient of orientation-patterned GaAs and demonstration of highly efficient second-harmonic generation,” Opt. Lett. 27(8), 628–630 (2002). [CrossRef]
5. J. Wueppen, S. Nyga, B. Jungbluth, and D. Hoffmann, “1.95 µm-pumped OP-GaAs optical parametric oscillator with 10.6 µm idler wavelength,” Opt. Lett. 41(18), 4225–4228 (2016). [CrossRef]
6. V. G. Dmitriev, G. G. Gurzadyan, and D. N. Nikogosyan, Handbook of Nonlinear Optical Crystals, Springer Series in Optical Sciences (Springer, 1999), pp. XVIII, 414.
7. Y. Ni, H. Wu, M. Mao, W. Li, Z. Wang, J. Ma, S. Chen, and C. Huang, “Growth and characterization of mid-far infrared optical material CdSe crystal,” Opt. Mater. Express 8(7), 1796–1805 (2018). [CrossRef]
8. P. G. Schunemann, K. T. Zawilski, L. A. Pomeranz, D. J. Creeden, and P. A. Budni, “Advances in nonlinear optical crystals for mid-infrared coherent sources,” J. Opt. Soc. Am. B 33(11), D36–D43 (2016). [CrossRef]
9. J. Yuan, X. Duan, B. Yao, Z. Cui, Y. Li, T. Dai, Y. Shen, and Y. Ju, “Tunable 10-to 11-µm CdSe optical parametric oscillator pumped by a 2.1-µm Ho: YAG laser,” Appl. Phys. B 122(7), 202 (2016). [CrossRef]
10. J. Yuan, Y. Chen, X. Duan, B. Yao, T. Dai, and Y. Ju, “CdSe optical parametric oscillator operating at 12.07 µm with 170 mW Output,” Opt. Laser Technol. 92, 1–4 (2017). [CrossRef]
11. J. Wang, L. Yuan, Y. Zhang, G. Chen, H. Cheng, and Y. Gao, “Generation of 320 mW at 10.20 µm based on CdSe long-wave infrared crystal,” J. Cryst. Growth 491, 16–19 (2018). [CrossRef]
12. C.-P. Qian, B.-Q. Yao, B.-R. Zhao, G.-Y. Liu, X.-M. Duan, T.-Y. Dai, Y.-L. Ju, and Y.-Z. Wang, “High repetition rate 102 W middle infrared ZnGeP2 master oscillator power amplifier system with thermal lens compensation,” Opt. Lett. 44(3), 715–718 (2019). [CrossRef]
13. M. Schellhorn, G. Spindler, and M. Eichhorn, “Improvement of the beam quality of a high-pulse-energy mid-infrared fractional-image-rotation-enhancement ZnGeP2 optical parametric oscillator,” Opt. Lett. 42(6), 1185–1188 (2017). [CrossRef]
14. M. Schellhorn, G. Spindler, and M. Eichhorn, “Mid-infrared ZGP OPO with divergence compensation and high beam quality,” Opt. Express 26(2), 1402–1410 (2018). [CrossRef]
15. M. Eichhorn, M. Schellhorn, M. W. Haakestad, H. Fonnum, and E. Lippert, “High-pulse-energy mid-infrared fractional-image-rotation-enhancement ZnGeP2 optical parametric oscillator,” Opt. Lett. 41(11), 2596–2599 (2016). [CrossRef]
16. G. Rustad, G. Arisholm, and Ø Farsund, “Effect of idler absorption in pulsed optical parametric oscillators,” Opt. Express 19(3), 2815–2830 (2011). [CrossRef]
17. S. Pearl, Y. Ehrlich, S. Fastig, and S. Rosenwaks, “Nearly diffraction-limited signal generated by a lower beam-quality pump in an optical parametric oscillator,” Appl. Opt. 42(6), 1048–1051 (2003). [CrossRef]
18. B. Zhao, Y. Chen, B. Yao, J. Yao, Y. Guo, R. Wang, T. Dai, and X. Duan, “High-efficiency, tunable 8-9 µm BaGa4Se7 optical parametric oscillator pumped at 2.1 µm,” Opt. Mater. Express 8(11), 3332–3337 (2018). [CrossRef]
19. J.-H. Yuan, Y. Chen, H.-Y. Yang, B.-Q. Yao, X.-M. Duan, T.-Y. Dai, and Y.-L. Ju, “Investigation of a gain-switched Cr2+: ZnSe laser pumped by an acousto-optic Q-switched Ho: YAG laser,” Quantum Electron. 46(9), 772–776 (2016). [CrossRef]
20. G. Baxter, J. Haub, and B. Orr, “Backconversion in a pulsed optical parametric oscillator: evidence from injection-seeded sidebands,” J. Opt. Soc. Am. B 14(10), 2723–2730 (1997). [CrossRef]
21. M. W. Haakestad, H. Fonnum, and E. Lippert, “Mid-infrared source with 0.2 J pulse energy based on nonlinear conversion of Q-switched pulses in ZnGeP2,” Opt. Express 22(7), 8556–8564 (2014). [CrossRef]
22. G. C. Bhar, “Refractive index interpolation in phase-matching,” Appl. Opt. 15(2), 305–307 (1976). [CrossRef]
23. E. J. Turner, S. A. McDaniel, N. Tabiryan, and G. Cook, “Rapidly tunable HIP treated Cr:ZnSe narrow-linewidth laser,” Opt. Express 27(9), 12282–12288 (2019). [CrossRef]
24. M. Yumoto, N. Saito, U. Takagi, and S. Wada, “Electronically tuned Cr:ZnSe laser pumped with Q-switched Tm:YAG laser,” Opt. Express 23(19), 25009–25016 (2015). [CrossRef]
25. M. Yumoto, N. Saito, and S. Wada, “50 mJ/pulse, electronically tuned Cr:ZnSe master oscillator power amplifier,” Opt. Express 25(26), 32948–32956 (2017). [CrossRef]
26. R. M. Sullenberger, S. Kaushik, and C. M. Wynn, “Photoacoustic communications: delivering audible signals via absorption of light by atmospheric H2O,” Opt. Lett. 44(3), 622–625 (2019). [CrossRef]
27. I. E. Gordon, L. S. Rothman, C. Hill, R. V. Kochanov, Y. Tan, P. F. Bernath, M. Birk, V. Boudon, A. Campargue, and K. Chance, “The HITRAN2016 molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transfer 203, 3–69 (2017). [CrossRef]
