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Abstract
We compare the performance of ns-pulsed mid-infrared optical parametric oscillators (OPOs) based on ZGP crystals in a simple linear cavity and two non-planar image-rotating resonators. Our 2 μm pump source is a 68 W 10 kHz diffraction limited Ho:LLF MOPA system. In a linear cavity, it allows us to generate up to 38 W of the total average power with M2<2.2 from the ZGP OPO in the 3-5 μm range. Beam quality and stability are significantly improved in both types of non-planar ring resonators (M2<1.8 in RISTRA and M2<1.4 in FIRE), while the total average power is above 21 W. Factors influencing the beam quality such as average power and pump repetition rate are studied in detail.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Demand for laser sources in the middle-infrared (mid-IR) wavelength range stems from areas like remote sensing, free space communication or optronic countermeasures in defense [1]. Many of these applications require a high-average power source with good beam quality. A mature technique to deliver such radiation is frequency down-conversion through optical parametric oscillators (OPO) and amplifiers (OPA) [2]. Indeed, the availability of other solid state sources is restricted to quantum cascade lasers [3] and lasers based on chalcogenides doped with transition metals [4], which are limited to few watts of average power. Nonlinear crystal of zinc germanium phosphide (ZGP) is known for its high nonlinear coefficient and its thermal and mechanical properties [5,6]. It is particularly suitable for high power laser sources covering the mid-IR region by nonlinear conversion from 2 $\mathrm{\mu}$m pump wavelength, where wide choice of high power pump lasers with excellent beam quality is available.
Few experimental high-power sources based on ZGP OPOs reaching more than 20 W have been demonstrated, but many suffered from limited stability or beam quality. The highest power currently reported is a two-stage ZGP OPO-OPA pumped by a 20 kHz pulse repetition rate Ho:YAG [7]. The first stage ZGP OPO delivered 78 W ($M^2 \approx 3$), while the amplifier stage had a maximum output of 161 W ($M^2 \approx 3.6$). However, since MOPA systems are relatively complex, it is valuable to investigate single stage OPOs. The highest average power demonstrated with such a setup reached 110 W [8], but beam quality at 100 W was very poor ($M^2\approx 9$), and the operation at maximum output power lasted only few seconds before crystal damage. An $\textrm{M}^2$ $\approx$4 for stable operation was reported at 30 W. All aforementioned experiments utilized 4-mirror planar ring resonators. In a similar cavity arrangement but with two separate ZGP crystals placed in series 41 W with $\textrm{M}^2$ = 4.4 was reported [9]. Other demonstrations of stable single-stage ZGP OPO include 27 W ($M^2=4$) with two ZGPs in a linear resonator [10] and 22 W ($M^2=1.4$) in a 3-mirror V-shaped planar ring cavity [11].
In principle, the mode discrimination could be improved by increasing the OPO cavity length. However, when the efficiency and compactness of the overall system are crucial, this method is not applicable. The most troublesome in above experiments are the thermal effects leading to the build-up of higher order modes in the cavities and as consequence degradation of the beam quality. The issue of thermal lens was addressed by compensation with negative lens [12] or Galilean telescope [13]. This approach was proven effective in the demonstration of a 102 W MOPA system [14].
Another technique to improve the beam quality is the use of ring resonators with a non-planar configuration, as first investigated by Smith et al. [15]. Two particular types of non-planar image-rotating ring resonators have been proposed and proved successful in beam quality improvement: the 90° Rotated Image Singly-resonant Twisted RectAngle (RISTRA) [16] and the Fractional Image Rotation Enhancement (FIRE) [17] cavities.
The type of resonator used for ns-pulsed OPO plays an important role in addressing the issues of stability and beam quality in high-power sources exceeding tens of watts in the mid-IR range, but few published studies adequately tested the performance of different types of resonators in OPO experiments. To date, the direct comparison of high-power OPO performance in different resonators has not yet been deeply examined. Most of the previous studies dealt with low-repetition pump sources and focused on increasing pulse energy in cases where high Fresnel number is required due to crystal damage constrains [18,19]. Other attempts include comparison of the RISTRA with the linear cavity at low repetition rate [20] or comparison between singly- and doubly-resonant regimes in a high-repetition rate ZGP OPO in planar ring cavity setups [21]. The FIRE cavity is of particular interest as it has never been tested before at high-power operation, although it has proven to enhance the beam quality in high-energy low-repetition rate systems [12].
The two main effects playing a role in the deterioration of the beam quality are thermal lens [22] and gain guiding [23,24]. The first depends on the heat absorbed by the crystal and is thus pump power dependent. On the contrary, gain guiding arises from the spatial intensity distribution of pump pulse energy. The influence of both effects on the beam quality has been studied in details in the RISTRA, under different pumping conditions [25].
In this article, we compare the performance of three OPO cavities: linear resonator, RISTRA and FIRE cavities. When possible, cavities with similar round trip are compared. We demonstrate a high average power ZGP OPO reaching up to 38 W of total output power in the linear cavity with an $M^2=2.2$ for the signal beam. The beam quality is then improved using the non-planar ring cavities, reaching a value of $M^2=1.4$ for 21 W of total output power in the FIRE cavity. This is the first demonstration of such cavity at high repetition rate. We investigate experimentally the scaling of the beam quality with output power and pulse repetition rate, expanding previous results of our group obtained using the RISTRA cavity [25].
2. Experimental setup
The scheme of our experimental setup is presented in Fig. 1(a). A 10 kHz pulse repetition rate high power Ho:LLF MOPA system [26] pumps a ZGP OPO. The 2 $\mathrm{\mu}$m pump source delivers 68.7 W at 2065 nm in TEM$_{00}$ operation with an optical-to-optical efficiency of 61.5 % and has a diffraction limited beam quality ($M^2 \approx 1$) at maximum pulse energy of 6.9 mJ. The pulse duration is 25 ns. The 300 mm lens focuses the pump beam from the Ho:LLF to a spot diameter of $\sim 1.1$ mm in the center of the ZGP crystal, maintaining a pump fluence < 1.5 J/cm$^2$ to avoid any risk of damage and anticipating for possible thermal effects affecting the energy density. Even though the reported laser induced damage thresholds for the ZGP crystals are $\sim$ 4 J/cm$^2$ [27,28], we operated safely below this limit. The Ho:LLF source is linearly polarized and a half-wave plate controls the polarization state. Then, a Pockels cell and a polarizer allow power adjustment and pulse picking without affecting the pulse properties. A second half-wave plate controls the polarization state at the entrance of the OPO. Behind the OPO cavity, the residual pump is reflected by a dichroic mirror, transparent for the 3-5 $\mathrm{\mu}$m output beams. Then, a CaF$_2$ wedge is used to pick off part of the two OPO beams for beam quality analysis. The pump beam power and the total OPO output power are monitored with a thermal power meter.
Fig. 1. (a) The scheme of the experimental setup. (b) The three different OPO cavities that were alternated in the experimental setup. $L$ denotes the optical round-trip length of the cavities.
The beam quality measurements are performed by recording beam profiles along the focused output beams. The profiles are monitored with an infrared pyroelectric array sensor (mks Ophir Pyrocam IV Beam Profiling Camera). The camera resolution is 320x320 pixels with a pixel pitch of 80 $\mathrm{\mu}$m. This camera is used in chopped (50 Hz) or triggered (100 Hz) mode for all beam images presented here.
2.1 Nonlinear crystals and OPO resonators
In the present study, we used two ZGP crystals with the size of 6 x 6 $\textrm{mm}^2$ (aperture) x 20 mm (length), and cut at 55° with respect to the optical axis to allow type-I phase matching. Their end faces are AR-coated for the pump and the output wavelengths (3-5 $\mathrm{\mu}$m). They were purchased from the same manufacturer (Harbin Huigong Science & Technology Co., Ltd.). Considering negligible absorption of the coating layers and taking into account their reflection, by measuring the power transmitted through the crystals, we determined different absorption coefficient at 2.06 $\mathrm{\mu}$m: $\alpha _1=0.033~\textrm{cm}^{-1}$ and $\alpha _2=0.044~\textrm{cm}^{-1}$. This corresponds to an absorption of $\approx$ 6.4% and $\approx$ 8.5% of the input power, respectively.
The ZGPs were wrapped in indium foil and mounted inside the optical cavities in copper holders, serving as heat sinks. The copper holders were then mounted in a rotational stage to allow angle tuning. For the measurements in the linear cavity, the holder was also water cooled.
The configurations of the cavities used in this study are shown in Fig. 1(b), with $L$ denoting the optical round-trip of each one. It was calculated from the geometrical length of the resonators, accounting for the length and refractive index of the ZGPs. While the linear resonator gives the agility of its length, the non-planar ones are monolithic structures and do not require any mirror adjustment.
The linear cavity is composed of two flat mirrors, it is single pump pass and doubly-resonant. The input coupler (IC) is highly transmittive for the pump wavelength (T>95%) and highly reflective in the 3-5 $\mathrm{\mu}$m range. The output coupler mirror (OC) is also highly transmitive for the pump and has $\approx 50\%$ reflectivity for both signal and idler wavelengths. For results presented here, the cavity was set to a physical length of 46 mm, corresponding to a 177 mm optical round trip length. This guarantees a short build-up time (thus good efficiency) and similar optical length with respect to the RISTRA. To avoid back reflection into the pump source, the cavity was operated with a slight misalignment.
The first non-planar cavity used in this study is the Rotated Image Singly-resonant Twisted RectAngle (RISTRA) ring resonator [16], based on 90° image rotation and composed of 4 flat mirrors in a non-planar configuration (Fig. 1(b)). Contrary to the linear cavity, it is operated in singly signal resonant regime. The pump beam enters via a dichroic input coupling mirror, which is tilted by 32.7°. It is highly transmittive for 2 $\mathrm{\mu}$m (T=86%) and highly reflective for the signal beam. The output coupler mirror is transparent for the pump and idler and has a reflectivity of $\approx 50~\%$ for the signal beam. The other two mirrors (M1 and M2) are identical to the IC. The half waveplate between OC and M1 compensates the polarization rotation in the resonator and keeps it constant after a cavity round trip. The physical length of the resonator is 130 mm, which corresponds to an optical round trip of around 172 mm.
The second non-planar cavity is the Fractional Image Rotation Enhancement cavity (FIRE), composed of 6 flat mirrors [17] (Fig. 1(b)). Likewise the RISTRA, the IC mirror (T=84%) allows the pump beam to enter and pass through the ZGP crystal while reflecting the signal beam. Again, the OC has a reflectivity of $\approx 50~\%$ for the signal beam. The other 4 mirrors that make the round trip in the resonator are all identical to the IC, and a half waveplate compensates the polarization rotation. The physical length of this cavity is 222 mm, corresponding to an optical round trip of 265 mm.
3. Results and discussion
3.1 Comparison of two ZGPs with different absoptances
The two ZGP crystals were initially tested in the linear OPO cavity. The graph in Fig. 2 shows the total OPO output power as a function of the pump power for both. The conversion thresholds are equal, but the slope efficiency is different. The crystal with less absorption (the #1) gives better results. Thus the absorption has an influence on the maximum output power achievable from the OPO.
Fig. 2. Total (signal + idler) OPO output power as a function of input pump average power for the two ZGP crystals used in linear resonator (177 mm optical round-trip, doubly resonant regime).
The beam quality of the output beams at low average power, i.e. negligible thermal effects, depends exclusively on the cavity length and geometry [24,29]. Therefore the beam quality close above threshold does not differ for the two crystals. Up-scaling the output power leads to a deterioration of the output beam quality due to the rise of a thermal gradient in the ZGP and consequently thermal lens [30]. This effect is more evident in the second crystal used, having higher absorption coefficient. That is why the first ZGP yielded the highest output power (see section 3.2). However, dust contamination caused coating damage at moderate pump power. This damage prevented us from using this crystal in the non-planar cavities. Instead, the second ZGP with higher absorption has been used for the rest of the results presented.
3.2 38 W linear cavity OPO
Using the ZGP #1, the performance of the linear cavity was studied. Operating at a repetition rate of 10 kHz, we achieved a total output power of 38 W for 66 W of input power, corresponding to a slope efficiency of 62 %. The total output power (signal + idler) as a function of the pump power is presented in Fig. 3(a).
Fig. 3. (a) Total OPO output power and conversion efficiency as a function of pump power. The top axis displays the pump pulse energy. Inset shows the OPO temporal pulse profile at maximum output power. (b) OPO and pump power stability measurement at 28 W over 10 minutes. The labels indicates the corresponding relative standard deviation.
The optical-to-optical efficiency (red circles) is calculated as the ratio between the incident pump power and the total OPO output power. Its maximum value achieved at the maximum output power was 57 %. The OPO temporal pulse width at maximum output power (38 W) is presented as an inset of Fig. 3(a). Its duration is 23 ns (FWHM), corresponding to a peak power of 165.2 kW. The power fluctuations of the OPO and the pump source are presented in Fig. 3(b). The stability of the OPO was not measured at maximum output power in order to monitor the pump source at the same time. The OPO output fluctuations follow the trend of the pump and have a slightly higher relative standard deviation ($\sigma ^*$) over a period of 10 minutes (0.74 % vs 0.45 %).
The amount of heat in the nonlinear crystal scales with the input power, inducing a positive thermal lens. As a consequence, the beam quality deteriorates as the incident power increases. Figure 4(a) shows this evolution. To measure the beam quality $\textrm{M}^2$ factor, the output beams were focused with a f = 500 mm $\textrm{CaF}_2$ lens and their diameters were measured in both X and Y directions. For clearness, only the signal beam is presented, but the idler showed similar behavior and $\textrm{M}^2$ values. The beam shapes are not symmetric, which is a consequence of walk-off and cavity misalignment, deliberately introduced to avoid back reflection and optical feedback to the pump laser.
Fig. 4. (a) The variation of the signal beam quality ($\textrm{M}^2$) with total OPO output power. The corresponding near field beam images at a distance of 25 cm from the lens are shown. (b) M$^2$ value measurement at maximum output power of 38 W.
Above threshold, where the thermal effects are low, the $\textrm{M}^2\approx$ 1.5 for an output power of 3.5 W. Then, as the thermal load into the crystal increase, the beam quality factor deteriorates, reaching a value of $\textrm{M}^2\approx$ 2.2 at maximum output power. The beam quality measurement at maximum output power is depicted in Fig. 4(b).
The output power and the beam quality reported here exceeds previous reports from linear doubly-resonant ZGP OPOs [31,32]. The demonstrations of higher output powers from single-stage ZGP OPOs used different resonant regime and planar ring resonators [8,9]. However, the reported beam quality factors were $\textrm{M}^2\approx$ 4 at 30 W of output power and $\textrm{M}^2\approx$ 4.3 at 41 W, respectively. Therefore, at comparable output powers, we recorded better beam quality, despite the use of a linear resonator. In accordance with our results are those reported by Lippert et al. [11]. They obtained 22 W of output power with $\textrm{M}^2$ of 1.4 in a V-shaped 3-mirror ring cavity. Our report presents what is, to the best of our knowledge, the up to date best beam quality at this power level from a single stage ZGP OPO. The findings of this study are only pump power limited and they are promising for further up-scaling.
3.3 Performance comparison of the three cavities
The performances of the three cavities were compared using the same ZGP crystal (ZGP #2). Fig. 5(a) shows the total (signal + idler) OPO output power as a function of average pump power for the three cavities tested. We achieved a total output power of 25 W (RISTRA) and 21 W (FIRE) in the 3-5 $\mathrm{\mu}$m range. The input power available in this case was lower due to the transmission of the ICs. The crystal was not tested at full pump power in the linear cavity to avoid any possible damage. However, from the slope and threshold it can be anticipated that it would give $\approx$ 28 W of total output power when pumped with 60 W (dashed line).
Fig. 5. (a) Performance comparison of the same ZGP crystal in different OPO cavities: a linear cavity (black squares), a RISTRA cavity (red circles), and a FIRE cavity (blue triangles). The grey squares represent the performances of the ZGP #1 in linear cavity, as shown in Fig. 3(a). (b) Variation of the signal and idler beam quality $\textrm{M}^2$ factor with input pump power for the three resonators.
The slope efficiency is around 50 % for each resonator. More specifically, their values are: 52 % (linear and RISTRA) and 50 % (FIRE). It neither depends on the cavity resonant type nor the cavity geometry. The pulses, similar to the one presented in Fig. 3(a), also present no significant difference between the three resonators. The main difference among the cavities is the conversion threshold. As the optical length per round trip of the linear and RISTRA cavities are comparable, it can be stated that the doubly-resonant regime lowers the conversion threshold. This agrees with previous theoretical [33] and experimental [21] reports. Additionally, if we compare the FIRE and RISTRA cavities, the dependence of the threshold on the cavity length can be observed. Since FIRE is singly-resonant and the longest, its threshold is the highest. Likewise the results recorded in the linear cavity, the maximum output power is limited by the pump.
The beam quality $\textrm{M}^2$ value for different output powers was measured for the three cavities. Their variation as function of pump power is presented in Fig. 5(b), averaged between X and Y directions. The evolution in the linear cavity was similar to the one presented in Fig. 4(a). Therefore, we assumed same scaling and $\textrm{M}^2$ values with average input power. The idler beam quality was measured for the linear resonator and at lower powers of the RISTRA. Since there were no differences with respect to the signal beam, such measure was not performed for the FIRE, assuming same $\textrm{M}^2$ values as the signal beam. For the RISTRA and FIRE, their beam quality measurements close to threshold and at maximum output power, corresponding respectively to the first and last point of Fig. 5(b), are presented in Fig. 6 for the resonant signal beam. Close to the threshold the thermal effects are negligible, thus the beam quality mostly depends on the geometry of the resonant cavity. This leads to a nearly diffraction-limited $\textrm{M}^2$ factor for both RISTRA and FIRE, despite their different threshold and build-up time. At higher powers levels, with the arise of thermal effects inside the ZGP, there is a deterioration of the beam quality of the output beams. For the RISTRA cavity, at a total output power of 25 W (at 58 W of input power), we obtained stable operation with the signal $\textrm{M}^2$ value around 1.8. A similar effect is observed for the FIRE, where at the maximum output power of 21 W (at 53 W of input power), we obtained an $\textrm{M}^2$ factor of 1.4.
Fig. 6. (a) $\textrm{M}^2$ value measurement at total output power of 1.6 W and 25 W in RISTRA cavity. (b) $\textrm{M}^2$ value measurement at total output power of 1.6 W and 21 W in FIRE cavity.
A direct comparison between linear and RISTRA cavity has already been done at low repetition rate [20,34], resulting in an improvement of the beam quality of the non-resonant idler beam for the RISTRA cavity. However, in the low power limit this improvement was marginal. In our study, such comparison is extended to high repetition rate regime and done considering the resonant signal beam. Moreover, we include also the FIRE cavity, never tested at high average power. Our results on the beam quality show a significant improvement on the $\textrm{M}^2$ value due to the non-planar geometry. This can be clearly seen if we compare the beam quality of the output beams close to threshold for the three cavities. While for the non-planar cavities the $\textrm{M}^2$ values are diffraction-limited, in the case of the linear resonator it is around 1.5 (see Fig. 4(a)). Although they operate in different resonant regimes, since the RISTRA and linear cavity have comparable round trip lengths, this result gives further insights that image-rotating configuration helps suppressing higher order modes and enhances the beam quality of the OPO output beams.
Regardless of the different total output power of each cavity (with the linear resonator giving the highest output), if the pump power is similar, so are the thermal effects inside the ZGP. Thus, in similar conditions (around 50 W of input power, Figure 5(b)), the beam quality factor $\textrm{M}^2$ of the output beams improves from an initial value of 2.1 in the linear cavity ($P_{in}\approx 50~W$) to a value of 1.4 in the case of the FIRE ($P_{in}\approx 53~W$). The additional improvement of the FIRE with respect to the RISTRA cavity (1.4 vs 1.8) confirms preliminary conclusions of previous works, which stated that the fractional image rotation provides a better mode suppression at low repetition rates than the 90° rotation [17].
3.4 Beam quality scaling with repetition rate
The dependence of the beam quality on the pump repetition rate was also investigated, as done previously in the case of the RISTRA [25]. Using a Pockels cell, the pump repetition rate is controlled by pulse picking without affecting the pulse properties. To isolate the dependence of the beam quality on the frequency, the pump pulse energy was kept at the constant value of 6 mJ and no other pulse properties were modified. The measured beam quality $\textrm{M}^2$ factor as a function of the pump repetition rate is presented in Fig. 7 for the RISTRA and FIRE cavities. This study could not be performed in the linear cavity: the back reflection of the IC was heating the Pockels cell, affecting the polarization of the pump beam.
Fig. 7. $\textrm{M}^2$ values in FIRE and RISTRA resonators for different repetition rates of the pump source at constant pump pulse energy of 6 mJ.
Intuitively, lowering the repetition rate, one might expect an improvement of the beam quality as the one found when the average power is lowered. However, the measured beam quality shows a minimum at different frequencies for the two cavities. For the RISTRA, starting from an initial value of 1.3 at 0.1 kHz, the $\textrm{M}^2$ drops to a minimum value of 1.2 at 1 kHz, then increases for higher repetition rates reaching the final value of 1.8 at 10 kHz. In the FIRE cavity, the initial $\textrm{M}^2$ value at 0.1 kHz is 1.2, it drops to a minimum of 1.1 at 2.5 kHz and then increases to 1.4 at 10 kHz.
This counterintuitive behavior is the result of a trade-off between the thermal lensing and the gain guiding effects, as explained in our previous paper [25], where simulations supported our interpretation. The previous results were limited only to the RISTRA cavity, here we extend these conclusions to the FIRE resonator. The different positions of the minimum might be due to the different lengths of the cavities. Further studies are needed to confirm it. Additionally, the $\textrm{M}^2$ data presented supports the improvement of the beam quality in the FIRE compared with the RISTRA. Indeed, at all repetition rates, the FIRE resonator presented a better $\textrm{M}^2$ factor.
4. Conclusions
In this paper, we compared the performances of three different types of single stage ZGP OPO in similar pumping conditions. We achieved a maximum output power of 38 W with $\textrm{M}^2$=2.2 in the 3 - 5 $\mathrm{\mu}$m spectral range using a doubly-resonant linear cavity. This is, to our knowledge, the best beam quality at this power level obtained with a linear resonator. Non-planar ring cavities have been used to improve the beam quality, obtaining $\textrm{M}^2$=1.8 (P=25 W) in RISTRA and $\textrm{M}^2$=1.4 (P=21 W) in FIRE. This is the first comparison of such cavities at high repetition rates and high average power. A low conversion threshold of the non-planar ring cavities is hampered by the fact that they are singly-resonant and for the FIRE additionally by the greater length. Therefore, a possible method to decrease the threshold and improve the performances would be to design and build a miniaturized version of these cavities. Additionally, a cooling system for the crystals, not present in current designs, could be implemented in order to improve the thermal management of the ZGP. The dependence of the $\textrm{M}^2$ with the pump repetition rate for the FIRE cavity confirms that there is a trade-off between the thermal lensing and the gain guiding and suggests some optimal condition to operate with the best beam quality.
Up to now, the highest average powers reported in the mid-IR region are provided by complex MOPA systems with limited beam quality. Our report shows the feasibility of using simpler, quasi-monolithic OPO resonators with good conversion efficiency and with a significant improvement of the output beam quality. Our results are currently pump power limited and further developments can lead to a better management of thermal effects and further up scaling of the output power from ZGP OPO while maintaining good beam quality.
Funding
Direction Générale de l’Armement.
Acknowledgments
Portions of this work were presented at the OSA Laser Congress in 2020 [35], and the 9th EPS-QEOD Europhoton Conference on Solid-State, Fibre, and Waveguide Coherent Light Sources in 2020 [36,37].
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. K. L. Vodopyanov, Laser-based mid-infrared sources and applications (John Wiley & Sons, Inc., 2020), chap. 3.
2. V. Petrov, “Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using non-oxide nonlinear crystals,” Prog. Quantum Electron. 42, 1–106 (2015). [CrossRef]
3. Y. Yao, A. J. Hoffman, and C. F. Gmachl, “Mid-infrared quantum cascade lasers,” Nat. Photonics 6(7), 432–439 (2012). [CrossRef]
4. S. Mirov, I. Moskalev, S. Vasilyev, V. Smolski, V. Federov, D. Martyshkin, J. Peppers, M. Mirov, A. Dergachev, and V. Gapontsev, “Frontiers of mid-IR lasers based on transition metal doped chalcogenides,” IEEE J. Sel. Top. Quantum Electron. 24(5), 1–29 (2018). [CrossRef]
5. 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]
6. G. Boyd, E. Buehler, F. Storz, and J. Wernick, “Linear and nonlinear optical properties of ternary $\textrm{A}^{\textrm{II}}\textrm{B}^{\textrm{IV}}\textrm{C}_{2}^{\textrm{V}}$ chalcopyrite semiconductors,” IEEE J. Quantum Electron. 8(4), 419–426 (1972). [CrossRef]
7. G. Liu, S. Mi, K. Yang, D. Wei, J. Li, B. Yao, C. Yang, T. Dai, X. Duan, L. Tian, and Y. Ju, “161 W middle infrared ZnGeP2 MOPA system pumped by 300 W-class Ho:YAG MOPA system,” Opt. Lett. 46(1), 82–85 (2021). [CrossRef]
8. B.-R. Zhao, B.-Q. Yao, C.-P. Qian, G.-Y. Liu, Y. Chen, R.-X. Wang, T.-Y. Dai, and X.-M. Duan, “231 W dual-end-pumped Ho:YAG MOPA system and its application to a mid-infrared ZGP OPO,” Opt. Lett. 43(24), 5989–5992 (2018). [CrossRef]
9. B.-Q. Yao, Y.-J. Shen, X.-M. Duan, T.-Y. Dai, Y.-L. Ju, and Y.-Z. Wang, “A 41-W ZnGeP2 optical parametric oscillator pumped by a Q-switched Ho:YAG laser,” Opt. Lett. 39(23), 6589–6592 (2014). [CrossRef]
10. A. Hemming, J. Richards, A. Davidson, N. Carmody, S. Bennetts, N. Simakov, and J. Haub, “99 W mid-IR operation of a ZGP OPO at 25% duty cycle,” Opt. Express 21(8), 10062–10069 (2013). [CrossRef]
11. E. Lippert, H. Fonnum, G. Arisholm, and K. Stenersen, “A 22-watt mid-infrared optical parametric oscillator with V-shaped 3-mirror ring resonator,” Opt. Express 18(25), 26475–26483 (2010). [CrossRef]
12. 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]
13. 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]
14. 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]
15. A. V. Smith and M. S. Bowers, “Image-rotating cavity designs for improved beam quality in nanosecond optical parametric oscillators,” J. Opt. Soc. Am. B 18(5), 706–713 (2001). [CrossRef]
16. A. V. Smith and D. J. Armstrong, “Nanosecond optical parametric oscillator with 90° image rotation: design and performance,” J. Opt. Soc. Am. B 19(8), 1801–1814 (2002). [CrossRef]
17. S. Bigotta, G. Stöppler, J. Schöner, M. Schellhorn, and M. Eichhorn, “Novel non-planar ring cavity for enhanced beam quality in high-pulse-energy optical parametric oscillators,” Opt. Mater. Express 4(3), 411–423 (2014). [CrossRef]
18. A. Dergachev, D. Armstrong, A. Smith, T. Drake, and M. Dubois, “3.4 μm ZGP RISTRA nanosecond optical parametric oscillator pumped by a 2.05 - μm Ho:YLF MOPA system,” Opt. Express 15(22), 14404–14413 (2007). [CrossRef]
19. 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]
20. G. Marchev, M. Reza, V. Badikov, A. Esteban-Martin, G. Stoppler, M. Starikova, D. Badikov, V. Panyutin, M. Eichhorn, G. Shevyrdyaeva, A. Tyazhev, S. Sheina, A. Agnesi, A. Fintisova, and V. Petrov, “Mid-infrared rotated image singly resonant twisted rectangle optical parametric oscillator based on HgGa2S4 pumped at 1064 nm,” Appl. Opt. 53(33), 7951–7962 (2014). [CrossRef]
21. Y. Chen, G. Liu, B. Yao, R. Wang, C. Yang, X. Duan, T. Dai, and Y. Wang, “Comparison of mid-infrared ZnGeP2 rectangle ring optical parametric oscillators of three types of resonant regimes,” Appl. Opt. 58(15), 4163–4169 (2019). [CrossRef]
22. J. P. Gordon, R. C. C. Leite, R. S. Moore, S. P. S. Porto, and J. R. Whinnery, “Long-transient effects in laser with inserted liquid samples,” J. Appl. Phys. 36(1), 3–8 (1965). [CrossRef]
23. F. Salin and J. Squier, “Gain guiding in solid-state lasers,” Opt. Lett. 17(19), 1352–1354 (1992). [CrossRef]
24. G. Arisholm, R. Paschotta, and T. Südmeyer, “Limits to the power scalability of high-gain optical parametric amplifiers,” J. Opt. Soc. Am. B 21(3), 578–590 (2004). [CrossRef]
25. M. Piotrowski, M. A. Medina, M. Schellhorn, G. Spindler, and A. Hildebrand-Dhollande, “Effects of pump pulse energy and repetition rate on beam quality in a high-power mid-infrared ZnGeP2 OPO,” Opt. Express 29(2), 2577–2586 (2021). [CrossRef]
26. M. Schellhorn and G. Spindler, “High-Efficient Resonantly Pumped Q-Switched Ho:LLF MOPA System,” in Laser Congress 2019 (ASSL, LAC, LS&C), (Optical Society of America, 2019), p. JTh3A.18.
27. K. T. Zawilski, S. D. setzler, P. G. Schunemannn, and T. M. Pollak, “Increasing the laser-induced damage threshold of single-crystal ZnGeP2,” J. Opt. Soc. Am. B 23(11), 2310–2316 (2006). [CrossRef]
28. K. T. Zawilski, P. G. Schunemann, S. D. Setzler, and T. M. Pollak, “Advances in single crystal ZnGeP2 processing for high energy applications,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference and Photonic Applications Systems Technologies, (Optical Society of America, 2008), p. JTuA28.
29. W. A. Neuman and S. P. Velsko, “Effect of cavity design on optical parametric oscillator performance,” in Advanced Solid State Lasers, (Optical Society of America, 1996), p. OP12.
30. W. Koechner, in Solid-State Laser Engineering, (Springer, 2006), chap. 7, sixth ed.
31. I. Elder, “Thulium fiber laser pumped mid-IR source,” in Laser Technology for Defense and Security V, vol. 7325M. Dubinskii and S. G. Post, eds., International Society for Optics and Photonics (SPIE, 2009), pp. 98–108.
32. C. Kieleck, A. Berrou, B. Donelan, B. Cadier, T. Robin, and M. Eichhorn, “6.5 W ZnGeP2 OPO directly pumped by a Q-switched Tm3+-doped single-oscillator fiber laser,” Opt. Lett. 40(6), 1101–1104 (2015). [CrossRef]
33. R. W. Boyd, in Nonlinear Optics, (Elsevier, 2007), chap. 2, 3rd ed.
34. G. Marchev, A. Tyazhev, G. Stöppler, M. Eichhorn, P. G. Schunemann, and V. Petrov, “Comparison of linear and RISTRA cavities for a 1064-nm pumped CdSiP2 OPO,” in Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XI, vol. 8240K. L. Vodopyanov, ed., International Society for Optics and Photonics (SPIE, 2012), pp. 70–76.
35. M. Piotrowski, M. A. Medina, G. Spindler, M. Schellhorn, and A. Hildenbrand-Dhollande, “Beam Quality Enhancement in High Power mid-IR ZGP OPO Sources: a Focus on Thermal Lens Build-up,” in Laser Congress 2020 (ASSL, LAC), (Optical Society of America, 2020), p. JTh2A.8.
36. M. Piotrowski, M. Medina, M. Schellhorn, and A. Hildenbrand-Dhollande, “Temporally resolved studies of thermal effects in high power ZGP OPO pumped by high-repetition Ho:LLF MOPA system,” EPJ Web Conf. 243, 18001 (2020). [CrossRef]
37. M. Medina, M. Piotrowski, M. Schellhorn, F. Wagner, A. Berrou, and A. Hildenbrand-Dhollande, “38 W high repetition rate ZGP OPO and novel approaches to improve beam quality in miniaturized non-planar cavities,” in Europhysics Conference Abstracts (9th EPS-QEOD Europhoton Conference), (European Physical Society, 2020), p. THA3.7.
