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Abstract
A room-temperature highly efficient Tm:YAP laser pumped MgO:PPLN optical parametric oscillator operating at 3.87 µm near degeneracy is demonstrated. The pump source is an acousto-optical (AO) Q-switched Tm:YAP laser, which delivers a maximum output power of 6.17 W with a pulse duration of 45 ns and a repetition rate of 6 kHz. The temperature dependent wavelength tuning characteristics of the PPLN-OPO is investigated, and a maximum OPO output power of 1.2 W at around 3.87 µm is achieved at 35°C, corresponding to an optical-optical conversion efficiency of 19.4%. To the best of our knowledge, this is the maximum output power ever reported from 2 µm waveband laser pumped 3-5 µm MgO:PPLN OPOs.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared (MIR) 3-5 µm pulsed laser sources have been widely used in medicine, remote sensing, environmental monitoring, military countermeasures, and so on [1–4]. Especially the laser sources with emission spectra ranging from 3.8 to 4 µm is of special interest for laser directional jamming and countermeasure infrared (IR) imaging seeker, due to its excellent atmospheric transmittance [5]. In addition, 3.8 µm lasers are exactly located in specific ‘fingerprint’ absorption peaks of harmful gas to humans, such as H2S, thus have become important IR sources for dangerous gas detections [6]. Due to the lack of direct emission laser sources within wavebands over 3 µm, optical parametric oscillators (OPOs) provide a powerful and efficient way to generate 3-5 µm coherent radiations. Apart from typical non-oxide nonlinear crystals suitable for MIR wavebands, such as ZnGeP2 (ZGP) and CdSiP2 (CSP), periodically poled MgO-doped LiNbO3 (MgO:PPLN) is one of the most developed and widely applied crystals due to its advantages of high effective nonlinear coefficient (d33=27.4 pm/V), broad wavelength tuning range [7,8] and quasi phase matching (QPM), which can significantly increase the nonlinear conversion efficiency within the entire transmission spectral range without walk-off effect in space [9,10]. Even under CW condition, a high conversion efficiency of 26% was reported in 1996 with the idler wave output at 3.25 µm from a singly resonant four-mirror ring cavity PPLN-OPO [11].
In recent years, high performance MgO:PPLN OPOs emitting at 3.8 µm have been reported mainly under 1 µm laser pumping. In 2010, Peng et al. reported an output power of 22.6 W at 3.86 µm from a MgO:PPLN OPO system by focusing the pump laser beam with an elliptical spatial shape to match the rectangular incident surface of MgO:PPLN with the temperature controlled up to 90°C, and the conversion efficiency of the idler wave output was about 15% [12]. In the meantime, Wu et al. reported a high-efficiency MgO:PPLN OPO with semi-external-cavity structure at room temperature, from which a maximum output power of 9.23 W was obtained at 3.82 µm with a corresponding conversion efficiency of 19% [13]. In 2012, Lin et al. employed a large aperture MgO:PPLN OPO to generate 5.5 W idler output at 3.82 µm with a conversion efficiency of about 9.48% [14]. In 2014, a high repetition rate (200 kHz) PPLN OPO with an idler wavelength up to 4 µm was demonstrated with a delivered output power of 2.75 W and a conversion efficiency of 12.03%, which was pumped by a master oscillator power amplifier fiber laser with a pulse shape having a steep leading edge [15]. Subsequently, many efforts have been paid on improving conversion efficiency, beam quality and miniaturization of MgO:PPLN OPOs with the aids of fiber laser pumping and the idler-resonant cavity configurations [16,17]. However, in all these approaches, it is quite difficult to obtain a high-efficiency conversion due to the serious quantum defect induced by the Manley-Rowe relationship between the pump light and idler wave.
One efficient method to break through the limit of Manley-Rowe relationship is utilizing pump sources with long wavelength, for example, by using 2 µm lasers as pump sources, while both signal and idler waves are located in the spectral range of 3-5 µm. For 2 µm laser pumping OPOs, although the cost is currently larger and the OPO gain is decreased by several times than the case of 1 µm laser pumping according to the defined OPO gain coefficient [18], the OPO could work near degeneracy, which would cause the expected high conversion efficiency and somehow compensate the decrease of OPO gain. Unfortunately, only a few works have been found concentrating on 2 µm laser pumped MIR PPLN-OPOs. In 1997, Hansson et al. first reported a 2.013 µm laser pumped PPLN-OPO yielding 3.61 µm signal and 4.55 µm idler radiations at a controlling temperature of 180°C [19]. Subsequently, they demonstrated a PPLN OPO pumped by a Tm:YAG laser at 2.0124 µm with an average output power of 30 mW for signal wave at 3.64 µm, corresponding to a conversion efficiency of 16.7% [20]. In 2007, Zhang et al. reported a Tm,Ho:GdVO4 laser pumped PPLN-OPO with output signal wave at 3.88 µm and idler wave at 4.34 µm, and a total maximum output power of 195 mW was achieved, corresponding to an entire optical-optical conversion efficiency of 6.5% [21]. By tuning the temperature of the PPLN crystal, the PPLN-OPO worked within a spectral range of 3.87-4.34 µm. In 2012, Xu et al. realized a highly efficient MgO:PPLN OPO pumped by a 1905nm Tm:YLF laser at room temperature, generating a signal wave at 3.6 µm and a idler wave at 4 µm, respectively, and the total MIR output power of 900 mW was obtained with an overall optical conversion efficiency as high as 31% [22]. In 2017, Antipov et al. reported a 1966nm Tm3+:Lu2O3 ceramic laser pumped MgO:PPLN-OPO [23], which delivered a total average output power of ∼530 mW including signal and idler waves within ∼3.7-4.1 µm at an optimal temperature of 147°C, corresponding to an optical-optical conversion efficiency of 6.6%. Unfortunately, the output power of the 2 µm laser pumped obtained OPOs are still below 1 W up to now, to the best of our knowledge. Forcing the OPOs working at degeneracy is a good way to enhance the output power and the conversion efficiency, however, there is no report on the 2 µm laser pumped obtained PPLN-OPOs at around room temperature.
In this paper, an AO Q-switched 1937nm Tm:YAP laser pumped MgO:PPLN OPO working at degeneracy point of 3.87 µm around room temperature of 35°C has been demonstrated, which consists of a double-pass doubly resonator (DPDR) cavity configuration. At a pulse repetition frequency (PRF) of 6 kHz, a maximum output power of 1.2 W is achieved, corresponding to an optical-optical conversion efficiency of 19.4%.
2. Experimental setup
Figure 1 shows the experimental setup of the room temperature AO Q-switched Tm:YAP laser pumped MgO:PPLN OPO. The c-cut Tm:YAP crystal has a Tm3+ ions doping concentration of 3 at.% with dimensions of 3 × 3 × 10 mm3, which is pumped by a 792 nm laser diode (LD) with fiber core diameter of 200 µm and numerical aperture of 0.22. Both ends of the Tm:YAP crystal are antireflection (AR) coated at 790 nm and 1950nm. The pump light is focused into Tm:YAP crystal with a beam radius of nearly 200 µm by using a 1:2 coupling lens. A 70 mm-length Tm laser cavity is employed, which consists of a plane input mirror M1 and a concave output mirror M2 (R=200 mm). M1 is AR coated at 750-850 nm and high reflection (HR) coated at 1800-2100 nm. To avoid optical damages to the intracavity elements, an output coupler M2 with a large output coupling rate of 40% at 1850-2150 nm is employed. The employed AO modulator (The 26th Electronics Institute, Chinese Ministry of Information Industry) is made of fused quartz with a physical length of 50 mm and an aperture of 3×3 mm. Both faces of the fused quartz are also AR coated around 2 µm to lower the insertion losses. In order to efficiently remove the pump light induced heat, both the Tm:YAP crystal and the AO modulator are mounted in copper blocks and cooled to 18°C with water. To prevent the output Tm laser from returning into the Tm:YAP laser system, an optical isolator (ISO) combined with a λ/2 wave plate (M3) is employed. After the ISO, M4 is a λ/2 wave plate to rotate the polarization state of the light to satisfy the required phase matching condition e→e + e. M5 (R > 99.5% at 1.9 µm and T ∼ 95% at 792 nm) is a flat 45° dichroic mirror, which reflects the Tm laser into the PPLN-OPO. Through a plano-convex lens (M6) with a focal length of 150 mm, the pump laser beam is focused into the PPLN crystal with a beam diameter of about 330 µm. The uncoated 25 × 3× 1 mm3 PPLN crystal is doped with 5 mol.% MgO, which is placed in an oven with a temperature controlling accuracy of 0.1°C. A grating period of 30.2 µm is employed for all the results presented herein. The absorbance of the PPLN crystal within the spectral range from 1 µm to 3 µm is measured by a UH4150 Spectrophotometer (Unpolarized light from 200 nm to 3000 nm), which shows a transmission ratio of only about 73% at 1937nm due to the influence of Fresnel reflectivity of uncoated end faces. A DPDR structure is formed with a cavity length of 60 mm by two plane mirrors. M7 is an input mirror with AR coated at 2-2.3 µm and HR coated at 3.6-5.3 µm, and M8 is the output coupler mirror with partial reflectivity ∼80% coated at 3-5 µm and HR coated at 2 µm for double passing of the pump beam.
3. Results and discussions
Firstly, the AO Q-switched Tm:YAP laser for the OPO pump source is built. The output Tm:YAP laser characteristics are demonstrated at PRFs of 6 kHz, 8 kHz and 10 kHz as shown in Fig. 2. When the incident pump powers increase from 10 W to 26 W, the average output powers increase linearly as shown in Fig. 2(a). And a maximum output power of 6.7 W is obtained under a PRF of 6 kHz, corresponding to a slope efficiency of 40.8%. In addition, the average output powers are not obviously changed under high PRFs of 8 kHz and 10 kHz. With the increase of the incident pump powers, the minimum pulse widths are decreased to be 45 ns, 63 ns and 73 ns under PRFs of 6 kHz, 8 kHz and 10 kHz, respectively, as shown in Fig. 2(b). The corresponding single pulse energies are increased to be 1.1 mJ, 0.8 mJ, 0.7 mJ as shown in Fig. 2(c), and the peak powers are increased to be 24.4 kW, 13.6 kW and 9.2 kW as shown in Fig. 2(d), respectively. The output spectrum of the Tm:YAP laser is recorded by a NIR spectrometer (APE WaveScan, APE Inc.) with a spectral resolution of 1 nm. The center wavelength is located at 1937nm with a full width at half-maximum (FWHM) of 2.8 nm.
Fig. 2. (a) Output powers, (b) pulse widths, (c) pulse energies and (d) peak powers versus the incident pump powers of Tm:YAP laser at PRFs of 6 kHz, 8 kHz and 10 kHz.
Subsequently, the characteristics of the DPDR PPLN-based OPO are recorded under different PRFs at 35°C. With the pump power slowly increased to be near 1 W, a faint red light begins to flash on PPLN crystal. Then an optimal phase matching is achieved when the output power reaches maximum by rotating the λ/2 wave plate. As shown in Fig. 3, it is found that the OPO output powers and conversion efficiencies increase with the decrease of PRFs. Under the maximum pump power of 6.17 W, the maximum output powers are 1.2 W, 905 mW, and 750 mW, corresponding to optical-optical conversion efficiencies of 19.4%, 14.7%, and 12.2% for PRFs of 6 kHz, 8 kHz and 10 kHz, respectively. The output power characteristics show a lower slope efficiency than that of [22], which is attributed to the narrower acceptable linewidth of only about 1.17 nm for the PPLN OPO than the fundamental pump laser linewidth of about 2.8 nm, although the PPLN crystal length in this work is only half of that in [22]. In addition, the conversion efficiency of 19.4% is still lower than that of other nonlinear crystals pumped at 2 µm, such as ZGP (75.6%) [24], CSP (65%) [25], and OP-GaAs (46.5%) [26]. However, considering the growth difficulty and cost of the above nonlinear crystals, 2 µm laser pumped PPLN-OPO is a promising way to obtain mid-infrared radiations below 4 µm.
Fig. 3. Output powers and conversion efficiencies of Tm:YAP laser pumped PPLN-OPO as a function of incident pump powers at 35°C.
To record the emission wavelength of the OPO, a Monochromator (Omni-λ300, Zolix) with a spectral resolution of 1 nm is used to measure the output spectra, which are shown in Fig. 4. It is well found that the center wavelength of the output OPO spectrum is located at approximately 3872 nm with a FWHM of 20 nm, indicating the OPO works exactly at degeneracy point. In addition, the visible red and blue waves arise simultaneously accompanying with the MIR OPO oscillations as shown in Fig. 4 (inset). The red wave at ∼645.52 nm is identified as the sum frequency generation (SFG) in PPLN crystal between the pump light at 1937nm and its second harmonic generation (SHG) at 968.45 nm, while the blue wave at 484.52 nm is coming from the fourth harmonic generation (FHG) of the pump light. The simultaneously existing SHG, SFG and FHG processes are competing effects with OPO as described in Ref. [27], which result in not only the decrease of OPO efficiency, but the instabilities of the OPO operation shown later.
Fig. 4. Output spectrum of Tm:YAP laser pumped PPLN-OPO obtained at 35°C. Inset: the corresponding spectra of sum frequency and second harmonic generations.
The dependences of the OPO output powers and emission spectra on PPLN temperatures are also investigated by tuning the temperature of the oven holding PPLN crystal from 25°C to 50°C. However, the PPLN OPO remains working at degeneracy point within the total temperature tuning range, which may be caused by small tunable temperature range around the degeneracy temperature of 35°C. Figure 5 shows the relationship between PPLN OPO output powers and PPLN temperatures at the RFP of 6 kHz, which demonstrates strong temperature dependent tendency. At the typical room temperature of 25°C, the total maximum output power of the OPO containing signal and idler waves is 600 mW. As the controlling temperature increases, the optimal temperature is measured to be ∼35°C for generating the maximum output power of 1.2 W, exactly corresponding to the degeneracy point of the OPO. At the maximum pump power, the temporal pulse characteristics of the Q-switched 1.94 µm pump laser and the emitted 3.87 µm laser from the OPO are measured by a fast InGaAs photodetector (EOT, ET-5000, USA) and a slow mid-infrared detector (PVI-2TE-5, VIGO System S.A.) with a response time of 20 ns, respectively. The temporal laser pulse train and single pulse profiles is recorded by a digital oscilloscope (1 GHz band width, Tektronix DPO 7102, USA). Figures 6(a) and (b) show the temporal pulse trains from the Tm:YAP laser and the synchronously pumped PPLN-OPO. From Fig. 6(b), an up to 20% pulse to pulse amplitude fluctuation is found in the OPO output pulse train, which not only comes from the instabilities of the self-built pump laser source, but the intrinsic characteristics of OPOs operating near degeneracy point. Single pulse profiles of the incoming pump light, depleted pump light and generated OPO output laser are also recorded and shown in Fig. 6(c). When the pulse width of pump laser is 45 ns, the corresponding OPO pulse width is found to be about 75 ns, which is longer than the pump laser pulse due to the relatively slow response time (20 ns) of the mid-infrared photodetector working at 3.8 µm. The beam quality factors of the OPO output laser at 3.87 µm are measured to be Mx2=2.3 and My2=2 by using the 90/10 knife edge method as shown in Fig. 7. The corresponding far-field divergence half-angles are calculated to be about 8.1 mrad and 7.5 mrad in x and y directions, respectively. The corresponding far field spatial beam shape of the generated 3.87 µm laser is shot with a pyroelectric camera (Win CamD, DataRay) as illustrated in Fig. 7. Due to the rectangular shape of incident surface of PPLN crystal, the recorded spatial beam form shows a little ellipticity and asymmetry.
Fig. 6. Temporal pulse trains of (a) pump laser and (b) OPO at 6 kHz; (c) Single temporal pulse shapes of pump laser, depleted pump laser, and PPLN-OPO.
Fig. 7. Beam quality factors M2 and far field spatial beam shape of the 3.87 µm PPLN-OPO output laser.
4. Conclusion
In conclusion, a highly efficient MgO:PPLN-based OPO emitting at the degeneracy point of 3.87 µm is demonstrated around room temperature. An AO Q-switched Tm:YAP laser at 1937nm is built and used as pump source with a maximum output power of 6.17 W and a pulse duration of 45 ns at the repetition rate of 6 kHz. At the typical room temperature of 25°C, the total maximum output power of the OPO containing signal and idler waves is 600 mW. An optimal temperature for the Tm:YAP laser pumped PPLN-OPO in generating 3.87 µm laser is found to be ∼35°C, under which a maximum MIR output power of 1.2 W is obtained, corresponding to a maximum optical-optical conversion efficiency of 19.4%. If a narrow linewidth pumping source is employed, higher optical conversion efficiency is expected. The experimental results well verify that 2 µm laser pumped PPLN-OPO is a promising approach to achieve highly efficient MIR laser output.
Funding
National Key Research and Development Program of China (2017YFB0405204); National Natural Science Foundation of China (61775119); Key Technology Research and Development Program of Shandong (2017CXCC0808); Shenzhen Science and Technology Research and Development Funds (JCYJ 20180305163932273); Program of State Key Laboratory of Quantum Optics and Quantum Optics Devices (KF201908).
Disclosures
The authors declare no conflicts of interest.
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