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Abstract
In this paper we present a high power long-wave infrared ZnGeP2 (ZGP) optical parametric amplifier (OPA) pumped by a 2097-nm Q-switched Ho:YAG laser with pulse repetition frequency of 20 kHz. When the incident Ho pump power was 116.0 W, the maximum average output power of 11.4 W at 8.3 μm was achieved in the ZGP OPA. The optical conversion efficiency from Ho to long-wave infrared was about 9.8%. The ZGP OPA produced 30.4 ns long-wave infrared laser pulse. The beam quality factor (M2) of ZGP OPA was measured to be about 2.9 at the maximum average output power.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High power long-wave infrared solid-state lasers are attractive for many applications such as chemical remote sensing, spectroscopy and military countermeasure [1]. Conventional discrete-wavelength CO2 laser technology cannot cover the long-wave infrared spectral regions [2]. In contrast, nonlinear frequency conversion technology, such as optical parametric oscillator (OPO) and optical parametric amplifier (OPA), is an efficient approach to obtain a wide infrared spectral range. Among several kinds of nonlinear optical materials, zinc germanium phosphide (ZGP) is an attractive material due to its high nonlinear coefficient (d14 = 75 pm/V), high thermal conductivity, acceptable damage threshold, and high transmission in the infrared range (2~10 μm) [3–5]. ZGP has a high absorption below 2 μm [6], so the pump wavelength longer than 2 μm is required for efficient ZGP OPO or OPA. With above advantages, ZGP OPO was used to generate long-wave infrared (8-12 μm) laser radiation in the past two decades.
In 2000, Vodopyanov et al reported a ZGP OPO pumped at 2.9 μm, which can be tuned to 12.4 μm. Its maximum pulse energy of 1 mJ at 8.1 μm was achieved [7]. In 2003, Rustad et al demonstrated a 2-μm laser pumped single resonant ZGP OPO with tunable range of 8~11 μm. They obtained the maximum pulse of 180 μJ at 8 μm under the pulse repetition frequency (PRF) of 20 Hz [8]. In 2007, Lippert et al obtained 0.95 W at 8 μm with beam quality factor (M2) value of 2.7 from a long-wave infrared ZGP OPO with two walk-off compensating crystals [9]. In 2010, same group improved the average output power to 1.5 W at 8 μm using a V-shaped 3-mirror ring resonator [10]. In 2011, they increased the average output power to 2.6 W at 8 μm using similar long-wave infrared ZGP OPO setup [11]. However, despite considerable efforts, high power long-wave infrared ZGP OPO was no further reported up to present. The output power of long-wave ZGP OPO was limited by low coating damage threshold of ZGP crystal, high quantum defect, walk-off effect and lack of pump power.
The master-oscillator/power-amplifier (MOPA) is another approach to achieve high power and good beam quality long-wave infrared laser. In 2008, Haakestad et al obtained more than 8 mJ at 8 μm by a ZGP OPA with a M2~3.6 [12]. In 2013, Clement et al demonstrated a ZGP OPA seeded by a single-frequency continuous-wave quantum cascade laser. The maximum pulse energy of 7 μJ at 8μm was obtained with the pump energy of 6 mJ [13]. In above works, an additional laser was used to pump the OPA. Actually, we can use the idler beam from OPO as the seed light of OPA and the signal beam from OPO as the pump laser of OPA rather than an additional pump laser, which was called subsequent OPA. Bakkland et al improved the pulse energy of ZGP OPA from 30 mJ to 45 mJ at 8 μm by using this method [14,15]. However, the conversion efficiency of the OPA was only about 12%.
In this paper, we have demonstrated a combination of ZGP OPO and subsequent ZGP OPA laser pumped by a Q-switched Ho:YAG laser at 2097 nm. The maximum average output power of the long-wave infrared ZGP OPO was up to 8.2 W for the idler and 32.7 W for the signal. Considering the walk-off effect, and using a separation and combination system between the OPO and OPA, we make the idler laser and signal laser from the OPO overlap before injected into the OPA system. The long-wave infrared ZGP OPA produced maximum average output power of 11.4 W at 8.3 μm and 5.8 W at 4.3 μm. The M2 of about 2.9 was observed at 8.3 μm under the maximum output level. To the best of our knowledge, this is the highest average output power in reported long-wave infrared ZGP OPAs. In addition, the optical conversion efficiency of the OPA reached ~20%.
2. Experimental setup
The experimental setup of the long-wave infrared ZGP MOPA system was shown in Fig. 1 schematically. The pump source was a 2097-nm acousto-optical Q-switched Ho:YAG laser with a M2 of ~2.1 and PRF of 20 kHz [16]. In this work, four volume Bragg grating (VBG)-locked diode-pumped Tm:YLF lasers at 1908nm were used to pump the two Ho:YAG rods. Two Ho:YAG rods with dopant concentration of 0.8 at % and dimension of 5 mm (in diameter) × 40 mm (in length) were inserted in one cavity in order to achieve high output power. The maximum average output power of the Q-switched Ho:YAG laser was 116 W at 2097 nm, while the minimum pulse duration was approximately 34 ns. In this experiment, the first part was a long-wave infrared ZGP OPO. The pump beam was focused into ZGP1 crystal with a beam diameter of approximately 1 mm. The ZGP OPO had a ring cavity which consisted of three plane mirrors M1 with antireflection (AR) for the p-polarized pump light and high-reflection (HR) for s-polarized long-wave infrared light, and an output coupler M2 (plane mirror) with transmittance of about 27% around 8 μm and AR for the p-polarized pump light. The physical length of the OPO ring cavity was about 102 mm. A ZGP1 crystal (School of Chemical Engineering & Technology, HIT) was cut for Type-I phase matching (θ = 51.5° and φ = 90° relative to the optical axis) with dimensions of 6 × 6 mm2 (in cross section) × 25.0 mm (in length). Both end surfaces of ZGP crystal were coated with high transmission (HT) for the pump light, signal light (2.8 μm) and idler light (8.3 μm). The absorption coefficient of ZGP crystal at 2.1 μm was measured to be α = 0.03 cm−1.
To achieve high output power, we introduced the subsequent ZGP OPA system. In general, the signal beam (2.8 μm) from the ZGP OPO would be filtered out before the idler beam (8.3 μm) was injected into the ZGP OPA. However, in this subsequent ZGP OPA system, we used the idler beam (8.3μm) from OPO as the seed laser and the signal beam (2.8μm) from OPO as the pump laser. Because of the walk-off effect, the propagations of signal and idler beams did not overlap. Therefore, a separation and combination system was used in this experiment in order to solve this problem, as shown in Fig. 1. This system was composed of four 45° flat mirrors, two mirrors (M3) were the principal which was HT for the signal light and HR for the idler light, and the other two mirrors were HR for the idler (M1) and signal laser (M4). For the two mirrors M3, due to the problems of the coating process, the transmittance of the single mirror was only about 80% for the signal light. The separation and combination system had relatively high light loss for signal laser. After passing through the system, the idler and the signal were overlapped and focused into the ZGP2 by a plano-convex ZnSe lens with focal length of 50 mm. The beam diameters of the idler and signal beams were about 0.4~0.5 mm and 0.4~0.6 mm, respectively. The ZGP2 crystal was cuted for Type II phase-matching (θ = 68.4° and φ = 45° relative to the optical axis) with a dimension of 6 × 6 × 25.0 mm3.Its end surfaces were AR-coated for the pump light (2.8 μm), signal light (4.3 μm) and idler light (8.3 μm). A 45° flat mirror (M3) was utilized to separate the 8.3 μm from the OPA output laser. The mirror M3 was HR for the 8.3 μm and HT for the 2.8 µm and 4.3 µm. The 45° flat mirror (M5) with HR at the 4.3 μm and HT at the 2.8 μm was used to separate the 4.3 μm and 2.8 μm laser. Both ZGP crystals were wrapped in indium foil and installed into two same copper blocks which was water-cooled at approximately 16°C.
3. Results and discussion
As shown in Fig. 2, the average output power of ZGP OPO was measured as a function of the incident Ho:YAG pump power. In order to avoid the water absorption around 2.8 μm signal light damage the coating of optical elements, we employed idler light singly resonant cavity to address this problem. The threshold pump power was about 38.0 W, which was slightly more than the doubly resonant cavity. To compensate this difference caused by the singly resonant cavity, the synchronously pumping technique was applied to improve the efficiency of ZGP OPO. The optical length ratio of the Ho resonator and OPO cavity was set to about 2 in order to achieve the good length matching [17]. The maximum average output power of the ZGP OPO was about 8.2 W at 8.3μm, corresponding to the slope efficiency of about 9.0%. As authors’ knowledge, this is the highest average output power in the long-wave infrared 8.3μm ZGP OPO. The maximum average output power of the ZGP OPO was about 32.7 W at 2.8 μm, corresponding to the slope efficiency of about 35.1%. In addition, the energy density of Ho pump was approximately 1.4 J/cm2 on the fore end surface coating of ZGP crystal, which nearly reached its damage threshold of 1.5 J/cm2. Therefore, increasing the energy density of Ho pump is not a feasible method to improve output power of long-wave infrared ZGP OPO. In addition, the beam profiles of 8.3 µm and 2.8µm were taken with a camera (Cinogy CR 200HP), as shown in the inset of Fig. 2.
We had designed a subsequent ZGP OPA system to further increase the long-wave infrared laser power. In order to promote the output performance of ZGP OPA, the output characteristics of ZGP OPO should satisfy two requirements, higher power and better beam quality of the idler beam which will be as the seed light of ZGP OPA. Compared with the doubly resonant OPO, the singly resonant OPO is suitable for achieving higher power because of its better stability. If 2.8-μm light oscillates in the cavity, water molecules will absorb it and vaporize. Evaporation of water will lead to the damage of end surface coating of the ZGP crystal. For solving this problem, we employed 8.3-μm idler light as the resonating light in the singly resonant cavity. In this case, the 2.8-μm signal light just propagated through rear end surface of the crystal and output coupler and did not oscillate in the cavity, so the water absorption could be avoided on the fore end surface coating and then the probability of damage would decline. As shown in Fig. 3, when the 2.8-μm pump power was 18.6 W, ZGP OPA produced maximum average output power of 11.4 W at 8.3 μm, while the 4.3-μm laser power reached to 5.8 W. As a result, the average output power of the long-wave infrared ZGP OPA system was increased from 7.7 W to 11.4 W, which means the extraction efficiency was approximately 19.9%. In contrast, the maximum extraction efficiency for an OPA pumped at 2.8 μm and injected at 8.3 μm is calculated to be approximately 34%. The experimental result is less than the theoretical value. We think that the reason is the parametric up-conversion of the signal and idler. Apparently, the extraction efficiency is far greater than the slope efficiency of OPO, which can be attributed to the lower quantum defect from 2.8 μm to 8.3 μm. No additional pump laser was used to pump the OPA, so OPO and OPA can be seen as an entirety. The optical conversion efficiency from 2097 nm to 8.3 μm was about 9.8%. In this experiment the transmittance of the 2.8 μm laser through the separation and combination system was only 57% because of low coating quality, which limited the pump power of the ZGP OPA. With the improvement of transmittance at 2.8 μm, the power loss resulted from M3 at 2.8 μm could be reduced and the output performance of the ZGP OPA would be better.
A Lecroy digital oscilloscope (Wavesurfer 64 Xs, 2.5 GS/s, 600 MHz bandwidth) and an HgCdTe detector (VIGO System S.A., PVM-10.6) was employed to measure the pulse profile of 8.3-μm idler light. At maximum output level, the shortest FWHM pulse duration of 30.4 ns (see Fig. 4.) was achieved, corresponding with a peak power of 18 kW. The spectrum of the idler light was measured by a 150 mm WDG30-Z monochrometer. The HgCdTe detector was located at output of monochrometer. A lock-in amplifier (Stanford, SR 830) was used to improve the signal-to-noise ratio. The output spectrum of long-wave infrared OPA is shown in Fig. 5. The central wavelength of the idler light was around 8.3 μm, corresponding to a broad output spectrum envelop with a FWHM of 182 nm. We measured the spectrum several times during 15 minutes, and all the experimental results were nearly the same, which proved the spectrum of the ZGP MOPA system was stable.
Utilizing shorter resonator could reduce the building time of the signal and idler lights and the OPO threshold. However, it could not effectively suppress high-order transverse modes. Therefore, it may be desirable to use longer cavity for the suppression of high-order transverse modes so as to obtain better beam quality. To produce good beam quality, synchronously pumping and long cavity were used to suppress high-order transverse modes and reduce the thermal load in ZGP crystal. With a ZnSe lens with focal length of 200 mm, the 90/10 knife-edge technique was used to evaluate the M2 of the ZGP MOPA system at the maximum output level. Figure. 6 showed the propagation characteristics of the 8.3-μm idler beam. The M2 factor was calculated to be 2.9 by fitting Gaussian beam standard expression, which indicating TEM00 mode operation of the long-wave infrared laser. This was verified by far-field beam profile, which was shown in the inset of Fig. 6.
4. Summary
In conclusion, a high power long-wave infrared ZGP MOPA laser system was demonstrated. A Q-switched Ho:YAG laser at 2097 nm was used to pump source, the ZGP MOPA laser system produced the maximum average output power of 11.4 W at 8.3 μm and 5.8 W at 4.3 μm under the incident Ho pump power of 116W. The optical conversion efficiency of the ZGP MOPA system was about 9.8%. A M2 factor of about 2.9 at 8.3 μm was measured with the maximum output level. To the best of our knowledge, this is the highest output power in long-wave infrared solid-state lasers. By increasing the transmission of the separation and combination system, the power loss of 2.8 μm could be reduced and then the performance of the OPA system would be better, which will be the future work.
Funding
National Natural Science Foundation of China (NSFC) (61405047, 51472251, 51572053, and 61805209); Science Fund for Outstanding Youths of Heilongjiang Province (JQ201310), and Fundamental Research funds for the Central Universities (HIT.NSRIF.2014044, and 2015042), and Fundamental Research funds for the Provincial Universities (WL17B14).
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