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Abstract
A high-energy and tunable mid-infrared source based on BaGa4Se7 crystal was demonstrated by single-pass difference-frequency generation (DFG). Orthogonally polarized wave at 1064 nm (λ1) and tunable idler wave (λ2) generated by KTP-OPO, which could be tuned in the wavelength range of 1360–1600 nm, were used as the DFG dual-wavelength pump. The pump parameters including total pump energy and energy ratio were studied. Maximum pulse energy of 5.72 mJ at 3.58 μm was obtained at the dual-wavelength pump energy of 58.4 mJ/pulse. The wavelength tuning range was 3.36–4.27 μm with a flat tunability. Moreover, a saturation phenomenon of DFG output was observed and experimentally inferred to be related to the input energy of λ2 in the BaGa4Se7 crystal.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared waves in the 3–5 μm wavelength range have a variety of applications, such as remote sensing, chemical detection, environmental monitoring, and national defense [1,2]. Compared with the mid-infrared laser, nonlinear optical conversion techniques are the ideal methods to generate tunable and high-energy mid-infrared waves. Among the general methods, difference-frequency generation (DFG) under proper phase-matching conditions has been proven to be a simpler approach to obtain coherent waves with the advantages of wide tunability, no threshold, and narrow linewidth [3,4].
Nonlinear optical (NLO) crystals with wide transparent regions, large NLO coefficients, and high laser damage threshold have been identified as the crucial materials for high-performance mid-infrared sources [5]. Many oxide and non-oxide crystals have been used to generate mid-infrared waves. Compared with oxide crystals [6–9], the non-oxide crystals, including arsenides, phosphides, and chalcogenides, have much wider transparency spectrum. Therefore, they are suitable for the generation of mid-infrared waves with the wavelengths above 4 μm. AgGaS2 crystal has a wide tunability up to 11.3 μm in optical parametric oscillator (OPO), but the crystal is vulnerable to low damage threshold [10]. HgGa2S4 crystal has a high output energy, but it is very difficult to grow in large size [11]. Crystals such as LiGaS2 and LiInSe2 were also used in OPO to generate tunable mid-infrared waves, but their small nonlinearity limited their conversion efficiency [12–14]. ZnGeP2 crystal is another good substitute for mid-infrared wave generation based on OPO and DFG. However, it is necessary to use a longer-wavelength wave to pump the ZnGeP2 crystal to avoid two-photon absorption at 1 μm [15–17]. Other crystals such as CdGeAs2, BaGa4S7, orientation-patterned GaP (OP-GaP), and OP-GaAs have also been reported to be promising materials for mid-infrared wave generation [18–22]. Recently, the chalcogenide NLO crystal BaGa4Se7 was reported to be very promising for mid-infrared radiation generation [23,24]. The BaGa4Se7 crystal belongs to the monoclinic class, point group m, space group Pc. The transparency spectrum was 0.47–14 μm. The large nonlinear tensor elements d16 and d23 were reported to be 20–30 pm/V and 11.3 ± 0.8 pm/V, respectively. Further, the high damage threshold of BaGa4Se7 crystal (over 2 J/cm2 for ns pulse) due to its intrinsic wide bandgap [24–26], makes it suitable for generating high-energy mid-infrared waves under an intense pump condition.
The tunable mid-infrared picosecond optical parametric amplifier (ps-OPA) source based on BaGa4Se7 crystal can be tuned in wavelength ranges of 3–5 μm and 6.1–11 μm [27,28]. High-energy and widely tunable mid-infrared OPO sources pumped at 1064 nm and 2.1 μm under type I and type II phase-matching conditions have been realized in 2016 [26,29,30]. Recently, a difference-frequency mixing mid-infrared source based on BaGa4Se7 crystal has been reported [31]. The BaGa4Se7 crystal was intra-cavity pumped by a Rb:PPKTP OPO to improve the pump intensity. However, the tuning range and output energy were limited by the dual-wavelength pump source. Single-pass DFG is a much more convenient approach to a widely tunable, high-energy and compact mid-infrared source, due to advantages such as cavity-free structure and no threshold.
In this paper, we demonstrated a high-energy and widely tunable mid-infrared source with much more convenient single-pass DFG method based on BaGa4Se7 crystal. The BaGa4Se7 crystal was pumped by orthogonally polarized wave at 1064 nm and tunable idler wave generated by KTP-OPO. The DFG dual-wavelength pump parameters of total pump energy and energy ratio were studied experimentally by controlling the energies of dual-wavelength pump and theoretically analyzed based on coupling wave equation beyond the small-signal limit, respectively. Due to advantages such as cavity-free structure and no threshold in DFG method, maximum pulse energy of 5.72 mJ at 3.58 μm was obtained under the dual-wavelength pump energy of 58.4 mJ/pulse, corresponding to the conversion efficiency of 9.8%. And the wavelength tuning range of 3.36-4.27 μm was realized in our experiment, with a flat tuning characteristic in the wavelength range of 3.36-4.0 μm without beam cutting. Moreover, the saturation phenomenon was observed under intense pump condition, which was experimentally inferred to be related to the input energy of λ2 in the BaGa4Se7 crystal.
2. Experimental setup
The schematic diagram of BaGa4Se7-DFG is shown in Fig. 1. A 10 Hz Nd:YAG laser (Spectra-Physics, 1064 nm, 200 mJ, 14 ns) was employed as the fundamental pump source. The 1064 nm beam with a 4 mm diameter from the pump source passed through a half-wave plate (HWP1). The HWP1 was used to adjust the polarization of the 1064 nm pump beam to control the phase-matching condition of second harmonic generation (SHG) in KTP (7 × 7 × 10 mm3, θ = 90°, φ = 23.5°) crystal. The energy controllable frequency doubled 532 nm green light and the residual 1064 nm pump were separated by M3 (532 nm HR incident at 45°, 1064 nm & 1300–1600 nm HT incident at 45°). Reflected by M3, the 532 nm green light was adopted as the pump for the double-pass OPO, consisting of cavity mirrors (M1 and M2), and KTP crystal (7 × 10 × 15 mm3, θ = 65°, φ = 0°) rotated by galvano-optical beam scanner (Cambridge Technology, 6230H). The wavelength tuning range of λ2 from the KTP-OPO was measured to be 1360–1600 nm by spectroscopy (Yokogawa, AQ6375) and calibrated by the voltage on the galvano-optical scanner. λ2 with vertical polarization was filtered by the dichroic mirror (DM1, 532 nm HR and 1300–1600 nm HT). A convex lens (M4) with a long focal length of 300 mm was utilized to collimate the beam shape of λ2. Due to the elliptical beam shape and different divergence angles, the positions of the beam waists of λ2 were different in vertical and horizontal direction after passing through M4. At a certain position away from the waists of λ2, the diameter of λ2 maintained to be 2.5 mm within a distance interval of more than 2 cm both in vertical and horizontal direction. And the shape of λ2 was almost a circle, showing a uniform intensity distribution at this position. The pulse energy of λ2 was controlled in the range of 0-10 mJ by adjusting the energy of 532 nm green light. A Brewster window (BW1) was used to choose the polarization of residual 1064 nm pump transmitted from M3. An attenuator with the combination of HWP2 and BW2 was used to control the intensity of λ1, while keeping the horizontal polarization of λ1. The coupling mirror M7, used to combine λ1 and λ2 with orthonormal polarizations in space as the dual-wavelength pump for the DFG, was high-transmission (HT) coated at 1300–1600 nm and high-reflection (HR) coated at 1064 nm, both incident at 45°. The distances from M6 to M7 and M3 to M5 were adjusted to be equal to the distance from M1 to M3, which guaranteed the overlapping of λ1 and λ2 in time domain. To achieve tunable mid-infrared output in 3–5 μm, the BaGa4Se7 crystal was cut at θ = 55.8° based on the Sellmeier equation [27] for oe-e phase-matching condition, interaction in the x-z plane. Its dimensions were 6 mm × 6 mm × 17.54 mm (length), as shown in the inset of Fig. 1. The crystal was placed at position described above to improve the spatial overlapping of λ1 and λ2 as much as possible. The calcium fluoride dichroic mirror (DM2), placed after the BaGa4Se7 crystal to filter the dual-wavelength DFG pump, was HT coated at 3–5 μm and HR coated at 1064 nm and 1300–1600 nm. The generated tunable mid-infrared waves were collected by an off-axis parabolic mirror (OAP, f = 50.8 mm) and measured with an energy meter (Newport Corporation, 842-PE). Wavelength of the generated mid-infrared wave was calculated by the wavelength difference of λ1 and λ2, while other wavelength components would not exist based on the phase-matching condition. The wavelength tuning of the mid-infrared wave was achieved by changing the phase-matching angle of KTP crystal in the OPO cavity and tilting the BaGa4Se7 crystal in the critical plane at the same time.
3. Results and discussion
λ1 and λ2 generated by KTP-OPO were adopted as the pump of the single-pass DFG. Compared with the signal wave generated in BaGa4Se7 OPO, the intensity of λ2 generated in KTP-OPO was improved to obtain a second-order nonlinear process with relatively high conversion efficiency in BaGa4Se7 crystal. The time overlappings of λ1 and λ2 are the crucial factors during the DFG process. Figure 2 illustrates the temporal profiles of λ1, λ2, and their combination, measured as 14.48 ns, 11.30 ns, and 13.42 ns, respectively, by a photodiode (Thorlabs, DET08C). A good overlapping of λ1 and λ2 was achieved in our experiment.
In our experiment, the DFG process in the BaGa4Se7 crystal was described using the coupling wave equations (given below) beyond a small-signal limit and were numerically solved by the Runge-Kutta algorithm.
Here, A1, A2, Am and ω1, ω2, ωm are the amplitudes and frequencies of λ1, λ2 and mid-infrared wave, respectively. A2*, Am* are conjugation of A2 and Am. α1, α2, and αm are the absorption coefficients of λ1, λ2, and mid-infrared wave in BaGa4Se7 crystal, respectively. n1, n2, and nm are the refractive indices in the crystal. deff is the effective nonlinear coefficient, ∆k is the phase mismatch, z is the crystal length, and c is the velocity of light. Due to the small absorption and phase-matching condition, the absorption coefficients and phase mismatch are estimated to be zero in our calculation, and the deff was set to be 27.5 pm/V in BaGa4Se7 crystal based on the experimental results.
The dual-wavelength pump energy and energy ratio of λ1 and λ2 (E(λ1)/ E(λ2)) were changed together by the rotation of HWP1 and measured by an energy detector (Newport Corporation, 919E-10-20-250). The red squares in Figs. 3(a) and 3(b) represent the output energy and conversion efficiency of the generated 3.81 μm mid-infrared wave under different DFG pump energies of 1064 nm and 1476 nm. The theoretical results of the mid-infrared wave energy and conversion efficiency are plotted as blue lines in Figs. 3(a) and 3(b) based on the coupling wave equations beyond the small-signal limit. The theoretical calculations was performed in the assumption of ideal conditions of crystal quality and spatial overlapping, etc. Therefore, the theoretical results were normalized just to compare the trends with the experimental results.
As the total energy of λ1 and λ2 increased, the energy ratio of the dual-wavelength pump gradually decreased. The 3.81 μm output energy and conversion efficiency increased with the improvement in DFG dual-wavelength pump energy at a relatively low pumping level. A saturation effect occurred at the dual-wavelength pump energy of 48.8 mJ/pulse, corresponding to the maximum conversion efficiency of 9.8%. However, with further increase in the dual-wavelength pump energy, a significant roll-over in the conversion efficiency was observed. The increase in total pump energy led to the improvement in conversion efficiency, while the deteriorating energy ratio resulted in its roll-over. First, the improvement in conversion efficiency was dominantly induced by the increase in dual-wavelength pump intensity. Then, with the increase in the total pump energy, the gradually deteriorated energy ratio of the dual-wavelength pump became the dominant factor to affect the DFG process, resulting in the decrease of DFG output.
The relationship between the generated mid-infrared wave and dual-wavelength pump energy ratio was studied by pumping the BaGa4Se7 crystal at a fixed total pump energy of 35 mJ/pulse. The energy of 1476 nm was changed from 3.5 mJ/pulse to 8.02 mJ/pulse, corresponding to the dual-wavelength pump energy ratio of 9:1–3.36:1. The theoretical and experimental results are shown in Figs. 4(a) and 4(b). As the energy ratio approaching, the output energy and conversion efficiency of the generated 3.81 μm increased. This proves that the DFG nonlinear process would be influenced or even broken down by the huge difference in the energy ratio of dual-wavelength pump energy. And the improvement of the total dual-wavelength pump energy and the optimized dual-wavelength energy ratio would lead to a DFG process with relatively high output energy and conversion efficiency.
Fig. 4 Theoretical and experimental results of (a) output energy, and (b) conversion efficiency at the total pump energy of 35 mJ/pulse with different energy ratio of 1064 nm and 1476 nm.
Figure 5(a) shows the generated mid-infrared energy at 3.81 μm versus the energy of λ1 under different fixed pump energies of λ2 by adjusting HWP1 and HWP2. It could be observed that the DFG output energy at 3.81 μm increased monotonously with the energy of λ1 at every fixed energy of λ2. No output saturation phenomenon of λ1 was observed. Similarly, the input-output characteristics of λ2 and the generated mid-infrared wave at 3.81 μm were extracted and analyzed at λ1 energy of 40.8 mJ/pulse, as shown in Fig. 5(b). Saturation phenomenon was observed in the output energy and conversion efficiency. Similar phenomenon was inferred to be related to the back conversion and nonlinear absorption in the BaGa4Se7 crystal [26,27]. Based on the coupling wave equation, the theoretical calculation results had a saturation trend with the improvement in the energy of 1476 nm, as shown in Fig. 5(b). It confirmed the back conversion in the BaGa4Se7 crystal. But it was still unclear for the existence of nonlinear absorption. To study the absorption characteristics of BaGa4Se7 crystal under different input intensities, the transmissions of λ1 and λ2 at different input intensities were measured as shown in Fig. 5(c). It is clearly seen that only the transmission of λ2 was influenced by the input intensity, decreasing with the improvement in input energy, while the transmission of λ1 was relatively stable. The additional absorption of λ2 in BaGa4Se7 crystal had some similarities to the nonlinear absorption, such as the wavelength-sensitive and intensity-sensitive characteristics. Defect levels associated with impurities in BaGa4Se7 crystal might result in the experimental phenomenon of additional absorption of λ2, considering the non-ideal quality of our crystal. But the origins of the additional absorption should be more complex and need further study. The existence of back conversion and additional absorption of λ2 would limit the energy transformation process in BaGa4Se7 crystal under an intense pump condition. Therefore, the pump parameters should be chosen carefully to realize the balance between total pump energy, pump energy ratio, and crystal absorption in the high-performance DFG mid-infrared source based on BaGa4Se7 crystal.
Fig. 5 (a) Input-output characteristics of λ1 at 3.81 μm at different pump energies. (b) Experimental and theoretical results of saturation phenomenon at fixed energy of λ1 at 40.8 mJ. The inset shows the experimental and theoretical results of conversion efficiency. (c) Transmission of λ1 (blue) and λ2 (red) at different input energies.
In our experimental setup, the BaGa4Se7 crystal was optimally pumped at the energy of 58.4 mJ/pulse, corresponding to 53.6 mJ/pulse of λ1 and 4.8 mJ/pulse of λ2. The temporal pulse width of the generated mid-infrared wave could be conjectured to be about 10 ns, due to the pulse width of the dual-wavelength pump. The square solid line in Fig. 6(a) shows the mid-infrared DFG tuning characteristic of BaGa4Se7 crystal. A tuning range of 3.36–4.27 μm was obtained. The maximum output energy was 5.72 mJ/pulse at 3.58 μm, corresponding to the optical-to-optical conversion efficiency of 9.8%. In the mid-infrared OPO source, the intensities of the generated mid-infrared wave and signal wave oscillated in the cavity are much sensitively are both affected by the cavity loss induced the coating of the cavity and the Fresnel reflection while rotating the crystal to meet the phase-matching condition, as well as the intrinsic decrease for parametric down-conversion gain. λ2 was generated by KTP-OPO, which was independent of the parametric process in mid-infrared OPO. And the intensity of λ2 stayed stable during entire tuning range in our experiment. Due to the cavity-free structure, the mid-infrared source based on single-pass DFG process have the unique advantage of no threshold. And the cavity-free structure makes it much easier to get a flatter tuning characteristics in single-pass DFG. The energy of the generated mid-infrared wave should be related to deff = d16 cos2(θ) + d23 sin2(θ), derived from the symmetry of BaGa4Se7 crystal [5,28]. Our DFG output shows a much flatter tuning characteristic with a wider tuning range of 3.36–3.95 μm, fitting well with the estimated oe-e DFG result shown by the green line in Fig. 6(a). The output energy decay above 3.95 μm was caused by the cutting pump limited by the crystal size while rotating the crystal to realize the phase-matching condition, estimated as the blue line in Fig. 6(a). Therefore, the flat tuning range would be extended for a larger crystal. To evaluate the stability of the DFG mid-infrared source, the mid-infrared output at 3.58 μm was recorded for 28 min at an interval of 30 s, as shown in Fig. 6(b). The root mean square (RMS) fluctuation in the output energy was 1.4%.
Fig. 6 (a) Tuning curve of the generated mid-infrared wave at the pump energy of 58.4 mJ/pulse. (b) Stability of the mid-infrared wave output for a 28 min duration.
The diameters of the DFG generated mid-infrared beam (3.81 μm) were measured by the knife-edge method at different distances from the rear surface of the BaGa4Se7 crystal in the horizontal (X) and vertical (Y) directions, as shown in Fig. 7. The divergence angles of the mid-infrared beam in the X and Y directions were found to be 21.1 mrad and 16.8 mrad respectively by quadratic fitting method. The spatial distributions of the mid-infrared wave were measured and estimated by knife-edge method at the distance of 175 mm from BaGa4Se7 crystal, shown as the inset of Fig. 7. The results showed that the beam of mid-infrared wave had a good shape of Gaussian distributions in X and Y directions.
Fig. 7 Diameters of generated mid-infrared beam at 3.81 μm in X and Y directions measured by the knife-edge method. The inset shows the beam distribution by knife-edge method.
4. Conclusion
A high-energy and widely tunable DFG mid-infrared source based on BaGa4Se7 crystal was presented in this paper. The mid-infrared source was pumped by orthogonally polarized wave at 1064 nm and tunable idler wave generated by KTP-OPO. The different pump conditions such as pump intensity and energy ratio were experimentally and theoretically studied. Maximum pulse energy of 5.72 mJ at 3.58 μm was obtained at the dual-wavelength pump energy of 58.4 mJ/pulse, corresponding to the conversion efficiency of 9.8%. Moreover, the generated mid-infrared wave could be tuned continuously in the 3.36–4.27 μm wavelength range with an RMS of 1.4%. Flat energy output was obtained in the 3.36–3.95 μm wavelength range without beam cutting. The divergence angles of the generated mid-infrared wave were measured to be 21.1 mrad and 16.8 mrad in the horizontal and vertical directions, respectively. Moreover, the saturation phenomenon of DFG output was observed and experimentally inferred to be related to the intense input energy of λ2.
Further optimal improvement in the crystal length, crystal quality and mode coupling of pump could lead to a mid-infrared wave based on DFG method with much better performance in output energy, conversion efficiency, and divergence.
Funding
The National Basic Research Program of China (973) (2015CB755403); The National Key Research and Development Projects (2016YFC0101001); National Natural Science Foundation of China (61775160, 61771332, 61705162, 51472251); China Postdoctoral Science Foundation (2016M602954); Postdoctoral Science Foundation of Chongqing (Xm2016021); Innovation Fund of the Chinese Academy of Sciences (CXJJ-17-M164).
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