High-energy nanosecond lasers operating at
Chinese Optics Letters, Volume. 23, Issue 2, 021401(2025)
162.6 mJ high-energy and high-efficiency KTP optical parametric oscillator at 2 µm
A high-energy and high-efficiency 2 µm nanosecond optical parametric oscillator (OPO) with excellent energy stability is reported. The cavity adopts a plane–plane configuration with two potassium titanyl phosphate (KTP) crystals inserted using a spatial walk-off compensated orientation. The KTP-OPO is pumped by a 1064 nm Nd:YAG Q-switched laser at a repetition rate of 10 Hz and produces a maximum pulse energy of 162.6 mJ at a pump energy of 431 mJ, corresponding to an optical conversion efficiency of 37.7% and a slope efficiency of 45.2%. The energy stability shows a record root mean square (RMS) of 0.4% over 30 min. To our knowledge, this represents the highest 2 µm pulse energy achieved via the 1 µm laser-pumped KTP-OPO scheme, which could be an excellent laser source for driving extreme ultraviolet (EUV) radiations in the subsequent demonstration experiments.
1. Introduction
High-energy nanosecond lasers operating at
For the 2 µm laser-driven EUV experiments, a high-energy drive source is generally required. In 2021, Behnke et al. scattered tin droplet targets utilizing a 2 µm laser with a total pulse energy of several 100 mJ and produced a high EUV CE of 3.1% and a spectral purity (SP; the proportion of in-band EUV energy to the total energy in the range of approximately 5.5–25.5 nm) of 7.4%, surpassing the CE (1.7%) and SP (4.2%) obtained with the use of 1-µm-drive lasers with the same conditions[16]. They combined the KTP-OPO and OPA (KTP-MOPA) to generate 2 µm lasers employing a total of four KTP crystals. The 2 µm pulse energy of the OPO stage is about 3.6 mJ, and the optical CE of the oscillator and amplifier are both less than 25%. In 2023, Lars Behnke et al. reported a high-energy 2 µm KTP-MOPA light source for EUV plasma applications, and they obtained 2 µm pulse energy of 17 mJ in the OPO stage with an optical CE of 24% and a root mean square (RMS) energy stability of 2%[17].
In this work, we developed a 2 µm nanosecond laser with a total pulse energy of 162.6 mJ and an optical CE of 37.9% using a plane–plane cavity OPO, which employed two KTP crystals positioned opposite to compensate for the walk-off effect. The 2 µm OPO features an excellent RMS energy stability of
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2. Experimental Setup
The schematic of the experimental setup is shown in Fig. 1. The pump source is an electro-optically
Figure 1.Layout of the 2 µm optical parametric oscillator system. HWP, half-wave plate; FR, Faraday rotator; TFP, thin-film polarizer; L, lens; M, 45° high reflector for 1064 nm; M1, input mirror; M2, output coupling mirror; DM, dichroic mirror.
3. Results and Analysis
We first investigated whether the compensation of spatial walk-off has a notable impact on the output performance of 2 µm OPO, and the results are illustrated in Fig. 2. The total energy of the signal and idler are measured behind the DM (detected by Ophir-PE50BF). It is found that the pulse energy and efficiency with compensation are higher than those without compensation at a high pump energy. In the absence of compensation, there exists a distinct spatial separation between the signal and the idler; consequently, the pulse energy cannot be spatially concentrated and sufficiently exploited, as shown in the inset of Fig. 2. The pump intensity is
Figure 2.The total 2 µm output pulse energy and efficiency of KTP-OPO with and without walk-off compensation. Inset: the spatial separation signal and idler observed without compensation.
The effect of the pump trips, crystal length, and OC transmittance on the 2 µm output performance is investigated with the OPO cavity length maintained at 62 mm in all cases. As shown in Fig. 3, apparently, higher 2 µm pulse energy can be obtained when using an OC with higher transmittance. The comparison between Figs. 3(a) and 3(b) demonstrates that the pump threshold of OPO can be reduced by increasing the pump trips at the same crystal length of 20 mm. Higher energy can be obtained in the double-pass pump configuration over that of the single-pass case at the OC transmittances of 10% and 20%. The results are consistent with the qualitative analysis of enhancing the nanosecond OPO CE and reducing the OPO threshold in Ref. [21]. However, when using the OC of
Figure 3.The total 2 µm pulse energy curves with OCs of T = 10%, 20%, and 30% at KTP crystal lengths and pump trips of (a) 20 mm, double-pass pump, (b) 20 mm, single-pass pump, (c) 25 mm, double-pass pump, and (d) 25 mm, single-pass pump.
In addition, the optical CE and pump threshold exhibit negligible alteration with the extension of the crystal length in the double-pass pump configuration. For 20 and 25 mm KTP crystals, the maximum optical CEs of 29.16% and 29.86% and the minimum pump thresholds of
Although extending the crystal length can increase the effective gain in the cavity, it simultaneously leads to a larger total intrinsic loss introduced by the KTP crystals. We then utilized a 15-mm-long KTP crystal to exemplify this phenomenon observed in the aforementioned circumstance, as shown in Figs. 4(a) and 4(b). With the same cavity length of 62 mm, it is evident that the 2 µm pulse energy obtained using the double-pass pump configuration is higher than that of the single-pass case in each OC transmittance. In addition, the maximum pulse energy of the double-pass case is 135.9 mJ, surpassing the pulse energy of 124.8 and 125.8 mJ when employing the 20 and 25 mm crystals in the double-pass case, which implies that the intrinsic loss effect can be mitigated by appropriately decreasing the gain length. Different pump passes correspond to different pump optical pathlengths in the crystals. Table 1 summarizes the maximum output energy under various pump pathlengths. It is obvious that the output energy first increases and then decreases with the pump pathlength. To summarize the above-mentioned experiments, the highest 2 µm pulse energy was obtained in the single-pass pump configuration utilizing the 25-mm-long KTP crystal with the OC of
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Figure 4.The effects of pump trips, OC transmittances, and cavity lengths on the total 2 µm energy output performance. Energy curves for the (a) double-pass pumped and (b) single-pass pumped 15 mm KTP-OPO. (c) Total 2 µm pulse output performance at different cavity lengths.
Furthermore, we also optimized the output performance of the KTP-OPO by adjusting the cavity length, and the results are shown in Fig. 4(c). The cavity lengths of 115, 87, 75, and 58 mm were evaluated, and it is evident that the shorter cavity length embodies the higher pulse energy and lower OPO pump threshold. As the cavity length increases, the diffraction loss in the OPO also increases, resulting in a decrease in the total output energy. The shortest cavity length is 58 mm due to the limitation of the mechanical structure. The maximum 2 µm pulse energy reaches 162.6 mJ, with an optical CE of 37.7% and a slope efficiency of 45.2%. For specific application scenarios, the cavity length can be adjusted appropriately to improve the beam quality.
The signal (o-light) and the idler (e-light) feature disparate polarizations owing to type II phase matching and thus can be separated by TFP3. The maximum pulse energy of the signal is 81.9 mJ, and the slope efficiency is 26.3%, as shown in Fig. 5(a), which is similar to the idler (79.5 mJ, 25.4%). The central wavelengths of the signal and the idler are 2109 and 2149 nm, respectively, as shown in Fig. 5(b). The spectral full widths at half-maximum (FWHMs) of the signal and the idler are 10.6 and 10.8 nm, respectively, and the actual value should be smaller than the measured value due to the limited resolution of the spectrometer, which is only 3 nm. Typical pulse profiles of the signal and idler are measured by a fast photodetector (rise time
Figure 5.(a) The pulse energy of the signal and idler. (b) The spectra of the signal and the idler, with central wavelengths of 2109 and 2149 nm, and spectral FWHMs of 10.6 and 10.8 nm, respectively.
Figure 6.Waveforms of 2 µm pulse. (a) Signal pulse width and (b) idler pulse width.
Figure 7.RMS energy stabilities of the KTP-OPO over 30 min measurement with the (a) pump, (b) signal, (c) idler, and (d) total 2 µm laser. The inset of (d) shows the far-field beam spot of the combined signal and idler.
4. Conclusion
In conclusion, we have demonstrated a high-energy and high-efficiency 2 µm nanosecond laser based on a dual-KTP-OPO. The KTP-OPO is pumped by a 1064 nm Nd:YAG laser with a repetition rate of 10 Hz and can produce a maximum pulse energy of 162.6 mJ, which is the highest 2 µm pulse energy achieved solely with a single KTP-OPO without any additional amplification stages. A maximum optical CE of 37.7% and a slope efficiency of 45.2%, as well as an RMS energy stability of 0.4%, are achieved. Additionally, we characterized the effects of various factors on the output performance of KTP-OPO and found that higher pulse energy could be obtained with the compensation of spatial walk-off and the use of relatively larger OC transmission and shorter cavity lengths. The total intrinsic in the KTP crystals cannot be negligible; thus, appropriately decreasing the crystal length and pump trips can facilitate the improvement of 2 µm pulse energy. In the future, with further optimization of cavity mode matching and mirror coatings, we believe that higher pulse energy and better beam quality should be achievable, which can be exploited as an excellent laser source for demonstration experiments to drive EUV radiations.
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Yuchun Liu, Jiajun Song, Yujie Peng, Enhao Li, Yingbin Long, Jianyu Sun, Liya Shen, Yinfei Liu, Junze Zhu, Yuxin Leng, Zhizhan Xu, "162.6 mJ high-energy and high-efficiency KTP optical parametric oscillator at 2 µm," Chin. Opt. Lett. 23, 021401 (2025)
Category: Lasers, Optical Amplifiers, and Laser Optics
Received: Jul. 11, 2024
Accepted: Aug. 19, 2024
Published Online: Mar. 6, 2025
The Author Email: Jiajun Song (songjiajun@siom.ac.cn), Yujie Peng (yjpeng@siom.ac.cn), Yuxin Leng (lengyuxin@mail.siom.ac.cn)