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We reported a compact self-cascaded KTA-OPO source for 2.6 μm coherent light generation. The OPO is driven in a diode end-pumped and Q-switched Nd:YVO4 laser cavity. Two OPO processes occurred in the same KTA crystal with non-critical phase matching. At an incident diode pump power of 8.7 W and a pulse repetition frequency of 60 kHz, the OPO can generate a maximum average output power of 445 mW at 2.59 μm. The slope efficiency was about 12.7%, and the power fluctuation was less than 8%. Therefore, the self-cascade OPO based on KTA offers a promise scheme for the rugged and compact mid-infrared 2.6 μm laser generation.
© 2016 Optical Society of America
1. Introduction
2.6 micrometer band lasers own characteristics of eye safety, large water molecules absorption, and atmospheric penetration. There are a plenty of potential applications in fields such as laser therapy, biological engineering, remote sensing, and industrial pollution sources monitoring [1,2]. High power lasers operating in the wavelength band are the desired pump source for the far-infrared band optical parametric oscillators (OPO). In recent years different methods have been used to generate laser emission in the wavelength range, such as using the quantum well semiconductor lasers [3], the direct generation from the erbium ions doped lasers [4,5], or the erbium-doped laser pumped OPO [6]. OPO technique has the advantage of producing pulsed mid-infrared radiation with broad wavelength coverage, where either no laser emission is available or laser performance is poor. In 2012 Wang et al. reported 2.7 μm laser output from a 1.645 μm Er:YAG laser pumped periodically poled lithium niobate (PPLN) crystal [6]. 0.7 W output was obtained under 2.8 W pumping. Baudisch et al. compared the infrared frequency conversion performance of PPLN, KNbO3 and KTA crystals [7]. It was shown that PPLN would be restricted from the high power laser application due to its low damage threshold and thin crystal thickness. KTA has the advantages of high damage threshold and broad transparency range (0.35–5.3 μm), which attracted a great deal of interest for producing high power mid-infrared laser emission with the optical parameter amplification or oscillation method.
Recently, non-critical phase matching (NCPM) OPOs based on KTA and its isomorphs KTP and RTP which own the merits of no walk-off and large acceptance angle with compare to that of normal phase matching, are attracted increasing attentions for frequency conversion. However, most reports were focused on the situation where the generated signal light is around 1.5 μm and the idler light around 3.5 μm, pumped by 1.0 μm lasers [8–13]. When the fundamental wavelength is changed, the NCPM KTA-OPO will result in new wavelength output. In 2011 Bai et al. proposed a NCPM KTA-OPO output at 1810 nm, pumped by an acousto-optic Q-switched Nd:YAG/SrWO4 Raman laser at 1180 nm [14]. An average power output of 485 mW was obtained under a pump power of 7.2 W, and the conversion efficiency was 6.75%. In 2013 our group reported a 2.1 μm emission NCPM KTA-OPO, intra-cavity pumped by an acousto-optic Q-switched Nd:YVO4 laser at 1.3 μm [15]. An average power of 1.01 W was obtained under a pump power of 9.8 W, and the conversion efficiency was 10%. In this paper, we proposed an intra-cavity pumped self-cascaded OPO based on the NCPM KTA-OPO for 2.6 μm laser generation. Conventional Q-switched neodymium doped crystal was used for fundamental laser generation. In our experiment, average output power of 445 mW at 2.59 μm was achieved under the pump power of 8.7 W. Similar cascaded KTA-OPOs could also be realized with other neodymium-doped crystal lasers with different fundamental wavelengths. We show that the KTA-OPO can generate mid-infrared pulses with high peak power and pure laser spectrum line.
2. Experiments setup design
Making full use of the NCPM that possesses the highest effective nonlinear optical coefficient (deff) and no walk-off angle, efficient KTA-OPO frequency conversion could be achieved, especially for the intra-cavity pumped system by employing high power intensity of the fundamental laser in the cavity. By tuning the pump laser wavelength, different laser output wavelength could also be obtained. Figure 1 shows calculated both the signal and idler wavelengths with responding to the pump wavelength for KTA-OPO with the NCPM angle of (θ = 90°, ϕ = 0°). Based on the NCPM KTA-OPO and pumped at the wavelengths of 1.06 and 1.53 μm, the wavelength of the signal light is 1.53 and 2.59 μm, respectively, which are in the transparency range of the KTA crystal and where the deff for both OPO processes are about 3.19 and 2.97 pm/V according to the calculation by SNLO software [16]. Therefore, if 1.53 μm signal light (named as first-order signal light) is produced by a 1.06 μm fundamental light and it could be confined in the OPO cavity, it could further generate 2.59 μm signal light (named as second-order signal light) based on the cascaded OPO process in the same KTA crystal.
Fig. 1 Calculated signal and idler wavelengths versus pump wavelength for KTA-OPO with the NCPM (θ = 90°, ϕ = 0°).
A schematic of the experimental setup used for the mid-infrared 2.59 μm laser generation based on the self-cascaded KTA-OPO is shown in Fig. 2. An X-axis cut KTA crystal (Crystech Inc) with a dimension of 4 × 4 × 25 mm3, was used as the nonlinear optical crystal for the cascaded OPO conversion. Both end faces of the KTA crystal were anti-reflection (AR) coated for the wavelengths of fundamental light at 1.06 μm, first-order signal light at 1.53μm and second-order signal light at 2.59 μm. The surface temperature of the crystal was kept at about 25 °C through mounting the crystal in a thermoelectric cooled copper block. The OPO cavity comprised two BK7 glass mirrors: an intra-cavity plane mirror (M2) and an output coupler (M1). The transmittances of both the OPO cavity mirrors were displayed on Fig. 3. M2 was AR coated at 1.06 μm (R<1%) and high-reflection (HR) coated at 1.53 and 2.59 μm (R > 99%). M1 has a radius of curvature of 320 mm. It was HR coated at 1.06 and 1.53 μm (R > 99.9%), and partial-reflection (PR) coated (T = 9.5%) at 2.59 μm. The OPO set was driven by an acousto-optic Q-switched Nd:YVO4 crystal laser. Therefore, the fundamental light at 1.06 μm was oscillated in the cavity formed by S1 and M1, and OPO cavity comprised by M1 and M2 were shared for oscillating of two signals at 1.53 and 2.59 μm.
Fig. 2 Schematic diagram of self-cascaded KTA-OPO derived by laser diode end-pumped Q-switched Nd:YVO4 laser.
A a-cut, 0.3 at.% Nd3+ doped Nd:YVO4 crystal with a dimension of 3 × 3 × 8 mm3 was used as the laser crystal. The crystal was wrapped with indium foil and mounted in a water-cooled copper block with temperature kept at 20 °C. The Nd:YVO4 crystal was end-pumped by a fiber (core diameter of 200μm and numerical aperture of 0.22) coupled commercial laser diode module (LIMO Lissotschenko Mikrooptik GmbH) operating at the wavelength range from 803 nm to 808 nm depending on the current level with the line-width of about 3 nm. The pump light was re-imaged into the laser crystal with a spot size of about 320 μm in diameter using a pair of achromatic lenses. A 30-mm-long acousto-optic Q-switch module (AOM, Gooch & Housego Co.) was inserted between the Nd:YVO4 and OPO for Q-switching operation. The pump incident face (S1) of the Nd:YVO4 crystal and the M2 forms the fundamental cavity for 1.06 μm laser oscillation with a total cavity length of 90 mm.
3. Experimental results
Accompanying the second-order signal light output, there are also low power fundamental and first-order signal light outputs. During the experiment, two filter mirrors were used to filter the fundamental light at 1.06 μm and the first-order signal light at 1.53 μm. The idler lights for the two OPO processes are around 3.5 and 3.7 μm, respectively, which are strongly absorbed by the cavity mirrors and the filter mirrors (The transmittance of 5 nm thickness BK7 glass is lower than 3% for the wavelength longer than 3.5 μm). Only the 2.59 μm laser was received by a thermal sensor power meter (Model: PM310D, Thorlabs Inc). The average output power of the second-order signal light was optimized by changing the Q-switching pulse repetition frequency (PRF) and the position of the pump beam waist in the Nd:YVO4 crystal.
Either a plane or a curved (320 mm radius of curvature) mirror was used as the OPO output mirror. When a plane output coupler was used, the output power of the OPO was instable. The output power varied form 150 to 370 mW at an incident diode pump power of about 8.0 W. The output power became much stable when the curved output mirror was used. Therefore, the cascaded OPO was rather sensitive to misalignment of the cavity. Average output powers of 2.59 μm signal light with respect to the incident diode pump powers at the PRF of 60 kHz, as well as the maximum average output powers at the pump power of 8.0 W versus the PRFs are shown in Fig. 4. The cascade OPO threshold was about 3.2 W. Its average output power near linearly increased with the incident diode pump power below 8.0 W, corresponding to a slope efficiency of 12.7%. The maximum average output power of 445 mW was achieved at a Q-switching PRF of 60 kHz and an incident diode pump power of 8.7 W. The highest diode to 2.59 μm signal conversion efficiency achieved was about 5.3%, where the output power was 420 mW and the incident diode pump power was 8.0 W. The output power fluctuation was less than 8% in ten minutes at the incident diode pump power of 8.0 W. The output power became rollover while continue to increase the pump power, which maybe caused by enlarged mode mismatching between OPO cavity and fundamental cavity. The radius of fundamental cavity mode is related to the thermal focal length of the Nd:YVO4 crystal, while that of the OPO cavity is not. With the increasing of pump power, the mode radius difference between the fundamental cavity and OPO cavity enlarged as the thermal focal length of Nd:YVO4 crystal reduced [17].
Fig. 4 Average output power versus incident diode pump power at the pulse repetition frequency(PRF)of 60 kHz. Inset was average output power versus PRF at the incident diode pump power of 8.0 W.
The cascaded OPO laser spectra and the temporal pulse profile were measured and shown in Fig. 5 and Fig. 6. The laser spectra directly output from the M1 was measured with a grating monochromater (model Omni-λ500). Both the first-order signal light at 1.53 μm and the second-order signal light at 2.59 μm were detected with multi-longitudinal mode. The line-width was narrower than 45 GHz, which are much narrower than the directly output by the erbium ions doped solid-state lasers [4]. The temporal pulse profile received by an InGaAs free-space photo detector, and displayed on a 500 MHz oscilloscope (Model DPO3052B). Because the response range of the high speed detector available in our lab are below 2.1μm, only the temporal pulse profiles of the fundamental light at 1.06 μm and the first-order signal light at 1.53μm can be detected. Figure 7 shows the temporal pulse profiles and pulse trains at the PRF of 60 kHz and the incident diode pump power of 8.7 W. Pulse widths for the first-order signal light and the fundamental light were about 7.0 and 14.5 ns. It is expected that the pulse width for the second-order signal light at 2.59 μm could be much narrower than the first-order signal light.
Fig. 6 Temporal pulse profiles and pulse trains for fundamental light at 1.06 μm and first-order signal light at the PRF of 60 kHz and an incident diode pump power of 8.7 W.
4. Discussion
The cascaded PPLN-OPO extra-cavity pumping by Q-switched Nd:YAG laser also had been demonstrated by Vaidyanathan et al. [18] Recently, synchronous OPO and OPA based on a single PPLN crystal also have been reported by Wei et al. [19] PPLN owns the merit of large nonlinear coefficient, but its characters of low damage threshold and thin crystal thickness [7] may limit it usage in high power, especially for intra-cavity pumped OPO system. Therefore, most cascaded OPO based on PPLN was introduced by extra-cavity pumped. For the 2.59 μm generation, it also can be realized in single parametric process pumped by 1064 nm laser under critical phase-matching with the angle of (θ = 56°, ϕ = 0°). Its deff (about 2.57 pm/V) is lower than that for cascaded OPO based on NCPM, and its walk-off angle reaches 40 mrad [16] which also is unfavorable for intra-cavity pump OPO system. Therefore, self-cascaded NCPM KTA-OPO with no walk-off angle and high damage threshold could be intra-cavity pumped and uses long KTA crystal to improve its conversion efficiency. The 2.59 μm light generated by such a self-cascaded KTA-OPO also owns the merits of being rugged and compact.
Although in our experiment the self-cascaded OPO in single KTA crystal was realized, the output power and the conversion efficiency were still low, which might be improved by optimizing the transmittance of the output mirror and the mode match between the fundamental cavity and the OPO cavity at a higher pump power. Moreover, the output wavelength can be changed using different fundamental wavelengths. As shown in Table 1, the output wavelength around 2.5 and 2.7μm also could be obtained by replacing the laser crystal with Nd:YLF or Nd:YAP whose fundamental wavelength are around 1.05 and 1.08 μm, respectively.We believe that the self-cascade OPO based on KTA could be a promise scheme for the mid-infrared 2.5-2.7 μm laser generation.
Table 1. Calculated signal and idler wavelengths for self-cascade KTA-OPO using different laser crystals
5. Conclusion
In conclusion, we have demonstrated a self-cascade NCPM KTA-OPO driven by a LD end-pumped and Q-switched Nd:YVO4 laser. A maximum average output power of 445 mW at 2.59 μm and a line-width narrower than 45 GHz were achieved at an incident diode pump power of 8.7 W and a PRF of 60 kHz. The OPO has an output power fluctuation of less than 8%. Similar cascaded NCPM KTA-OPOs could also be realized with other neodymium-doped crystal lasers with different fundamental wavelengths, e.g. the fundamental wavelength of the Nd:YLF and Nd:YAP lasers are at 1.05 and 1.08 μm, respectively, using them as the pump source laser emission at around 2.5 and 2.7 μm also could be produced. Therefore, we believe that the self-cascade OPO based on KTA could be a promise scheme for the generation of mid-infrared 2.5-2.7 μm radiations.
Funding
National Natural Science Foundation of China (NSFC) (61505147, 61405126 and 61377021); Public Welfare Projects of Zhejiang Province (2017C34008 and 2015C34017); Public welfare projects of Wenzhou City (G20140057); Science and Technology Planning Project of Guangdong Province (2016B050501005)
Acknowledgment
The authors thank H.Y.Wang (Crystech Inc.) for providing the KTA crystal.
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