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Abstract
A high-efficiency, tunable 8-9 μm optical parametric oscillator with a type-II phase-matching BaGa4Se7 crystal pumped by a 2090.7 nm acousto-optical Q-switched Ho:YAG laser was demonstrated for the first time. A maximum average idler output power of 314 mW at 8925.7 nm and 1 kHz was achieved with a pulse duration of 16 ns and a spectral bandwidth of 67.8 nm under the incident pump power of 9.58 W, corresponding to a slope efficiency of 7.44% and quantum slope efficiency of 32%. Furthermore, the idler tuning range of 8-9 μm was demonstrated. The results indicate the potential of BaGa4Se7 pumped at 2.1 μm as a candidate for realizing high-efficiency long-wavelength infrared laser radiation.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Widely tunable long-wavelength infrared (8-12 μm) laser sources are in great demand in many applications, such as laser spectroscopy, remote sensing and biomedical applications. The main way to obtain long-wavelength infrared laser radiation is using nonlinear optical frequency conversion technology. The high-quality nonlinear crystals for producing the wavelength of 8-12 μm mainly include ZnGeP2 (ZGP), CdSe, GaSe, AgGaSe2 and BaGa4Se7 (BGSe) [1–3]. ZnGeP2 has a large nonlinear optical coefficient (deff = 75 pm/V) and a wide transparency range of 0.75-12 μm. However, the strong absorption of ZnGeP2 near 9 μm and 10-12 μm limits its application to the long-wavelength infrared laser radiation. Compared with GaSe and AgGaSe2, CdSe and BGSe have higher laser damage threshold which makes them possible to generate high power and high energy laser output [1,4]. Due to the space group 6mm, CdSe can only be matched with the type-II phase matching, which means that CdSe is applicable to 9-18 μm laser radiation with the pump laser of 2.1 μm. BGSe is a newly developed chalcogenide crystal with large nonlinear optical coefficient (d11 = 24.3 pm/V and d13 = 20.4 pm/V) and wide transparency range of 0.47-18 μm [5,6]. BGSe crystal can be pumped by both conventional 1 μm lasers and 2 μm lasers. With a wide band gap of about 2.64 eV, BGSe has large damage threshold of 100 MW/cm2 at 1.064 μm and 122.2 MW/cm2 at 2.1 μm respectively [4,7]. According to the conservation of energy, however, more of the idler photons can be obtained for 2 μm pump source than 1 μm pump source at the same pump conversion energy. Taking into account the damage threshold and the idler conversion efficiency, 2 μm pump source is a promising driver for BGSe to produce long-wavelength infrared laser radiation.
The long-wavelength infrared laser radiation based on BGSe was achieved for the first time by a picosecond type-I BGSe optical parametric amplifier (OPA) with tunable range of 3-5 μm and 6.4-11 μm [8,9]. The output energy of ~125 μJ at 7.8 μm was obtained under the pump energy of ~9 mJ. In 2016, a widely tunable 2.7-17 μm BGSe optical parametric oscillator (OPO) pumped at 1.064 μm was demonstrated [7]. The output energy of ~3.7 mJ at 7.2 μm was obtained under the pump energy of ~63 mJ, corresponding to a slope efficiency of ~6.5%. Recently, only one type-I BGSe OPO pumped at 2.1 μm was demonstrated [4]. The tunable range of the BGSe OPO was 3-5 μm, and the maximum output power was up to 1.55 W. Nevertheless, the experimental results show no obvious advantages to the ZGP OPO at 3-5 μm. The advantages for BGSe are mainly concentrated at the tunable range of ~9 μm and 10~18 μm compared with ZGP because of ZGP’s transparency range. So far, the study of long-wavelength infrared BGSe OPOs is insufficient, especially for the pump source of 2 μm.
In this paper, we present a high-efficiency long-wavelength infrared OPO based on a type-II BGSe crystal pumped by a 2090.7 nm acousto-optical Q-switched Ho:YAG laser. The idler tuning range of 8-9 μm was demonstrated, and the published Sellmeier equations were discussed. This work demonstrates that the BGSe OPO pumped at 2.1 μm is an effective way to obtain high-power and high-efficiency long-wavelength infrared laser output.
2. Experimental setup
The experimental setup is shown in Fig. 1. The acousto-optical Q-switched Ho:YAG laser, using U-shaped configuration, consisted of a convex mirror (M1, R>99.8% at 2.1 μm) with a curvature radius of 200 mm, two 45° dichroic mirrors (M2 and M3, T>97.7% at 1.9 μm and R>99.8% at 2.1 μm) and an output coupler (OC1, T~80% at 2.1 μm) with a curvature radius of −5000 mm. The Ho:YAG crystal with a nominal Ho3+ doping concentration of 0.3 at.% and length 100 mm was longitudinally pumped by two 60 W orthogonally polarized Tm:YLF lasers. The Tm pump beam was focused into the Ho:YAG crystal with a 1/e2 spot diameter of 1.4 mm. The residual Tm pump laser power was isolated by two thin-film polarizers (TFP1). A fused silica acoustic optical modulator (AOM) with radio frequency (RF) of 41 MHz and RF power of 100 W was employed in the cavity, which was 48 mm long with transmission of 99.7% at 2.1 μm. The physical cavity length of the Ho:YAG laser was about 520 mm. The Ho:YAG laser was operated at a pulse repetition frequency (PRF) of 1 kHz with a wavelength of 2090.7 nm and a beam quality factor M2<1.2. The Ho pump beam was focused into the OPO crystal with 1/e2 spot diameter of 1.17 mm × 1.14 mm in x- and y-direction, respectively. A half-wave plate (HWP) and a polarizer (TFP2) were used in OPO experiments to change the Ho pump power without altering the pulse duration and pump beam profile.
Fig. 1 Schematic outline of the BGSe OPO setup. AOM: acousto-optic modulator; M1: convex mirror with curvature radius of 200 mm; M2 and M3: 45° dichroic mirrors; OC1: 2.1 μm output coupler with curvature radius of −5000 mm; TFP1: 1.9 μm thin-film polarizer; TFP2: 2.1 μm thin-film polarizer; HWP: half-wave plate; M4: input coupler; OC2: OPO output coupler; M6: signal filter; M7: idler filter.
The BGSe crystal was grown by the Bridgman-Stockbarger method with seed crystal, and had a clear aperture of 6 mm × 6 mm and 30 mm long cut at an angle of θ = 6° with respect to the y-z plane for type-II phase matching. The BGSe crystal was anti-reflection (AR) coated at pump, signal and idler wavelengths. Figure 2 shows the transmission spectrum of the AR-coated BGSe sample in the range of 1.8-20 μm. The BGSe sample has a better transmission in the range of 2.1-12 μm, and can transmit up to 17.8 μm. The absorption coefficient of the BGSe crystal at 2.1 μm, 2.73 μm and 8.93 μm were measured to be 0.144 cm−1, 0.082 cm−1 and 0.062 cm−1, respectively.
The OPO cavity had a physical length of ~45 mm consisting of two flat mirrors. The input coupler (M4) was high-reflection (HR) coated at 2.6-3 μm (R>99.5%) and AR coated at 2.1 μm (R<0.05%). Two kinds of output couplers (OC2) were used in this experiment, one with approximately 100% reflection at 2.6-3 μm and 93.9% transmission at 8-10 μm, and the other with 27% transmission at 2.6-3 μm and 95.7% transmission at 8-10 μm. The two output couplers were AR coated at 2.1 μm (R<0.05%). The signal was isolated by a dichroic mirror (M6) which was highly reflective at 2.6-3 μm (R>99.9%) and highly transmissive at 2.1 μm (T>99%) and 8-10 μm (T = 93.1%). The residual Ho pump was filtered out by another dichroic mirror (M7) which was highly reflective at 2.1 μm (R>99.9%) and highly transmissive at 8-10 μm (T = 86.2%). All measured values in our experiment were corrected for these transmission losses.
3. Results and discussion
Figure 3(a) shows the output characteristics of the acousto-optical Q-switched Ho:YAG pump laser at PRF of 1 kHz and wavelength of 2090.7 nm. The Ho:YAG laser delivered 28.2 ns long pulses with maximum average output power of 38.6 W at the incident pump power of 114.9 W, corresponding to a slope efficiency of 54.4%. Taking into account the surface damage threshold of the BGSe crystal and the pulse width of Ho pump laser, the Ho:YAG laser worked at the pump power of 97 W, corresponding to an output power of 26 W and a pulse width of ~35 ns.
The idler output powers of BGSe OPO at 8.93 μm as a function of the incident pump power are shown in Fig. 3(b), corresponding to the signal wavelength of 2.73 μm. The threshold of BGSe OPO was 5.6 W, corresponding to a pulse energy fluence of 1.07 J/cm2, which was slightly larger than CdSe OPO (0.84 J/cm2 at 8.9 μm [1]). The reason might be that the transmission loss of BGSe crystal at 2.73 μm is too large. At the incident pump power of 9.58 W, the maximum idler output powers of 314 mW and 241 mW were obtained for T = 0% and T = 27% respectively, corresponding to slope efficiencies of 7.44% and 5.86%. Assuming the same number of signal photons generated, the quantum slope efficiencies above threshold were ~32% for T = 0% and ~25% for T = 27%. Benefiting from the PRF of 1 kHz, the idler output powers were much higher than the maximum output power (<0.5 mJ at 10 Hz and ~9 μm under the 1.064 μm pump energy of 27 mJ, corresponding to the idler output power of <5 mW, presented in the Fig. 4 from [7]) reported in [7]. However, the output energies (~0.31 mJ) were slightly lower than the output (~0.5 mJ) reported in [7].
Fig. 4 (a) Output characteristics of BGSe OPO obtained at different PRFs for T = 0% under the same pump pulse energy of 8.5 mJ. (b) Beam quality of BGSe OPO at PRF of 1 kHz and pump power of 8.5 W.
Further improvements of the slope efficiency and output power at 8.93 μm were restricted by the absorbing losses at 2.1 μm, 2.73 μm and 8.93 μm. Better performance can be expected as the BGSe crystal has low absorption coefficient at pump, signal and idler wavelengths. Another effective method to improve the slope efficiency of BGSe OPO might be to use the ring OPO for its reduction of round-trip loss at signal wavelength. The BGSe crystal surface near output coupler was damaged at the pump power of ~11 W, corresponding to a surface damage threshold of 2.1 J/cm2. The surface damage threshold was less than 3.3 J/cm2 from [4], which was because the absorption of the BGSe crystal at 2.1 μm made the pump laser slightly convergent. Figure 4(a) shows the output characteristics of BGSe OPO obtained at different PRFs for T = 0% under the same pump pulse energy of 8.5 mJ. The output power of the BGSe OPO was not increased linearly with the increase of PRF, which was attributed to the broadening of pump pulse width with the increase of PRF. The 1/e2 idler beam radii (shown in Fig. 4(b)) at different positions after passing a 75 mm focal length lens were measured when the BGSe OPO worked at PRF of 1 kHz and pump power of 8.5 W. The beam quality factors were calculated to be 5.3 and 4.5 in x and y directions, respectively.
The temporal profiles of the incident and depleted pump and idler were measured by an HgCdTe detector and a LeCroy digital oscilloscope. As shown in Fig. 5(a), a ~16 ns idler pulse was obtained at PRF of 1 kHz and phase-matching (PM) angle (internal angle) of θ = 4.71° under the pump power of 9.58 W. As a result of depletion, the temporal profile of depleted pump was reshaped. Measurements of the spectrum of BGSe OPO were performed by using a WDG30-Z grating monochromator and an HgCdTe detector. As shown in Fig. 5(b), the idler central wavelength is about 8925.7 nm at the PM angle of 4.71°, corresponding to spectrum bandwidth (FWHM) of 67.8 nm and full bandwidth of 172.4 nm. Table 1 lists the achieved spectrum parameters of BGSe OPO at the PM angles of 4.71°, 5.48°, 6.16° and 6.65°. The FWHM of BGSe OPO at the PM angle of 4.71° is wider than others, which might because that the PM angle of 4.71° is relatively near to the relative rotating pole of the PM curve. We estimated the gain bandwidth with the following equation:
where l is crystal length, ns is the refractive index of signal, ni is the refractive index of idler, λs is signal wavelength, and λi is idler wavelength.Fig. 5 (a) Temporal profiles of the incident and depleted pump and idler at pump power of 9.58 W. (b) BGSe OPO spectrum at phase-matching angle of θ = 4.71°.
Table 1. Spectrum parameters of the type-II BGSe OPO
We determined ns and ni by using the Sellmeier equations given in [10]. As shown in Table 1, the theoretical gain bandwidth (TGB) is slightly larger than the full bandwidth because the TGB shows the maximum bandwidth based on the theoretical phase mismatch. However, the losses of both cavity and crystal reduced the spectrum bandwidth in the experiment.
Meanwhile, the tuning performance of type-II BGSe OPO in y-z principal plane was measured as shown in Fig. 6. The signal wavelength can be tunable from 7.91 to 9.00 μm, corresponding to the PM angle from 7.64° to 4.58°. Based on the Sellmeier equations given in [9–12], we also performed four theoretical PM curves in Fig. 6. Note that the imaginary part of theoretical PM curve from [11] was ignored. Compared to the theoretical calculations from [9,11,12], the calculations from [10] had a much better agreement with the experimental data at the tuning range of 7.91-9.00 μm. The average deviation value for the calculations from [10] and the experimental data was about 154.24 nm at same PM angles. This discrepancy was most likely due to the errors of both cutting angle and Sellmeier equations.
In conclusion, a tunable 8-9 μm type-II BGSe OPO pumped at 2.1 μm was demonstrated. The maximum idler output power of 314 mW at 8925.7 nm was obtained experimentally at the incident pump power of 9.58 W, corresponding to slope efficiency of 7.44% and quantum slope efficiency of 32%. Benefiting from the type-II PM operation, the BGSe OPO generated narrow-band pulses with spectrum bandwidth of 67.8 nm and pulse duration of ~16 ns at 8925.7 nm. Further, the tuning range of 8-9 μm was demonstrated. This work illustrates that the BGSe OPO pumped at 2.1 μm is an effective way to obtain high-efficiency long-wavelength infrared laser radiation.
Funding
National Natural Science Foundation of China (NSFC) (51472251, 51572053); the Innovation Fund of the Chinese Academy of Sciences (CXJJ-17-M164).
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