1. Introduction

Mid-infrared (MIR) optical parameter oscillators (OPOs) around 3 to 5 µm have been widely used in many application fields in recent years, such as environmental monitoring, medical diagnostics, and infrared counter-measures [1–3]. For MIR OPO systems, the periodically poled magnesium-oxide doped lithium niobate (PPMgLN) is one of the most popular nonlinear crystals due to its large effective nonlinear coefficient, high damage threshold, design flexibility and wide spectral transmission range [4–6]. To obtain MIR laser output from PPMgLN –based OPO systems, different types of pump sources, including solid state lasers or fiber lasers operating in either pulsed or continuous wave (CW) mode, have been employed. Among these lasers, fiber lasers have obvious advantages under high average power operation including compactness, robustness, excellent beam quality, simple thermal management and ultra-high electric-optic conversion efficiencies. As a result, fiber-laser-pumped OPOs have been extensively investigated both in CW and pulsed regimes in the past decade [7–14].

The reported highest output power from a CW fiber laser pumped OPO was 34.2 W operating at 3.35 μm, which was pumped by a quasi-single-frequency (SF) Yb-doped fiber laser [15]. Compared to these cw fiber lasers, the pulsed fiber lasers posses much higher peak power to reach the OPO threshold easily even under the circumstance of large cavity mode sizes and are thus more suitable to be the pump sources of the OPO. As well known, the master oscillator power amplifier (MOPA) structured fiber lasers are the main technical approaches for the development of high average power nanosecond, picosecond and even femtosecond pulsed fiber lasers and have been applied as the pump sources for MIR OPO operation. In this aspect, we have previously reported several ns and ps OPO systems with high average power output in MIR region by using the pulsed fiber MOPAs to pump homemade PPMgLN crystals in a simple linear cavity structure [8–10, 12]. However, as the MOPA systems normally consist of a seed laser and one or several amplification stages, the structures are comparatively complicated and thus result in high cost. More severely, the MOPA systems still suffer the damage risk due to the accidental seed instability and the back reflection from the output end. Comparatively, the gain-switched pulsed fiber lasers have much a simpler structure with only one oscillation stage. The drawback of the gain-switched fiber lasers is the reduced peak power, which might lead to less nonlinear conversion efficiency. However, by modulating the pulse width of the pump laser diodes, the “figure-of-h” pulse shape can be achieved from the gain-switched fiber laser, which has been theoretically and experimentally proved to improve the OPO’s performance [11, 16, 17]. The peak power is moderate but high enough to pump a PPMgLN based OPO. By optimizing the parameters of the gain switched fiber laser, the repetition rate and the pulse temporal shape can be controlled to support high average power output with moderate peak power for nonlinear conversion. As a result, the gain-switched high power fiber lasers are interesting candidates, showing high level of compactness and robustness, to serve as the pump sources of MIR OPOs.

In this paper, we report our recent experimental results of a gain-switched Yb fiber laser pumped compact high power MIR OPO system. The gain-switched fiber laser was constructed with a piece of Yb doped polarization maintaining(PM) fiber (about 7 meters), a pair of fiber Bragg gratings written into the matched passive PM fiber and 6 pump laser diodes working at 915 nm each with 30 W output from a pigtailed 105/125 micron multimode fiber. By adjusting the pulse width of the pump laser diode, simple pedestal-free pulse or “figure-of-h” pulse was achieved from the gain-switched fiber laser. A single-pass singly resonant (SPSR) cavity configuration was applied in the OPO experiment to reduce the back-reflection of the residual pump light from the OPO and thus to deter the unstable oscillation or even the damage of the gain switched fiber laser. By using such fiber laser to pump a home-made PPMgLN-based OPO system, high power MIR laser was generated with average output power of 5.15 W at 3.8 μm and 8.54 W at 3.3 μm, corresponding to optical to optical conversion efficiency of 11.7% and 19.1% respectively. It is, to the best of our knowledge, the first report on a gain switched pulse fiber laser to pump a mid-infrared OPO system with high MIR power output.

2. Experimental setup

The experimental setup of the gain-switched fiber laser pumped OPO system is illustrated in Fig. 1. A specially designed laser diode (LD) electronic driver, featuring fast current modulation with rise time and fall time both less than 200 ns, was used to drive the LDs in pulsed operation through an arbitrary waveform generator (AWG). The pump source was composed of six fiber-pigtailed LDs working around 915 nm with a maximum output power up to 30 W each. The pigtailed fiber from each LD was 105/125 µm in diameter with numerical aperture (NA) of 0.15. A (6 + 1) multi-mode fiber combiner was used to couple the pump into the cavity. The fiber laser cavity was formed by a piece of 7-m-long Yb-doped double-clad PM fiber (PLMA-YDF-10/125 from Nufern) with NA of 0.075 and cladding absorption of 1.6 dB/m at 915 nm and a pair of fiber Bragg gratings (FBGs) written into the matched passive PM fiber. The Bragg wavelengths of the FBGs were carefully specified and the axes of the two FBG associated PM fibers were aligned with 90° rotation during splicing (as illustrated in Fig. 1) to enable linearly polarized laser oscillation. In this way, a simple linear resonant cavity was constructed with high reflectivity (HR) of 99% and low reflectivity (LR) of 4% respectively. The slow axis of HR FBG was aligned with the fast axis of LR FBG. The bandwidth of the HR FBG and LR FBG was 0.35 nm and 0.05 nm respectively. The aligned center wavelength of the FBGs was about 1062.8 nm. The LR FBG served as the output coupler and the linearly polarized pulse laser emitted through a PM fiber-coupled isolator, which was placed between the gain-switched fiber laser and the OPO to deter the back reflection. The laser beam output from the isolator was focused onto the PPMgLN with a waist of about 300 µm and polarization direction identical to the Z axis of the PPMgLN crystal.

 

Fig. 1 Experimental setup of the gain-switched fiber laser pumped OPO system.

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The OPO system was designed to be a single-pass singly resonant (SPSR) cavity structure. The input mirror (M1) was antireflection (AR) coated for 1.04-1.07 µm, and high reflection (HR) coated for 1.4-1.7 µm and 3-5 µm with a 500 mm radius of curvature. The output mirror (M2) was also AR coated for 1.04-1.07 µm and 3-5 µm, HR for 1.4-1.7µm but with a 200 mm radius of curvature. A homemade PPMgLN crystal, which was fabricated using high voltage poling technique, with size of 50 × 10 × 1 mm was used as the nonlinear medium. The input and output mirrors were closely placed to the PPMgLN crystal and the total cavity length was about 52 mm. The PPMgLN contained two channels with poling periods of 29.4 µm and 30 µm respectively. Both end surfaces of the crystal were finely polished and AR coated with reflectivity lower than 2% around 1.04-1.07 µm, 1.4-1.7 µm and 3-4 µm. Two other filters, M3 and M4, were used to enable the measurement of the depleted pump, output signal and idler independently, among which M3 was HR for 1.04-1.07 µm and 1.4-1.7 µm, AR for 3-4 µm while M4 was HR for 1.4-1.7 µm and 3-4 µm, AR for 1.04-1.07 µm.

3. Experimental results

To realize effective gain-switching pulse operation, the pump sources are generally modulated in an appropriate pulse repetition rate with the corresponding duty cycle so that single powerful oscillation spike trains could be obtained without the following small spikes [18–20]. In our experiment, stable gain-switched pulses were obtained when the pulse repetition rate was continuously adjusted between 20 kHz and 200 kHz with suitable pulse duration from AWG. Figure 2 shows the pulse train output from the gain-switched fiber laser under repetition rate of 200 kHz with 1 µs and 2 µs pump pulse duration respectively. Because the peak power of the pump source was fixed, the output pulse shape was almost unchanged under the different pump repetition rate if we kept the pulse duration of the pump source unchanged. However, it is possible for us to increase the average output power of the fiber laser by increasing the repetition rate of the pump laser. Under the maximum pump peak power of 160 W, we can clearly see that, for the 1 µs pump pulse, the fiber laser emitted pulse train with single peak simple pedestal-free pulse. And the pedestal-free trailing pulse shape, “figure-of-h” as we call it, was obviously observed when the pump pulse duration was beyond 1 µs. After passing the isolator, the maximum output average power of about 16.5 W and 45 W was achieved with 1 µs and 2 µs pump pulse duration respectively corresponding to the pulse energy of about 82.5 µJ and 225 µJ under repetition rate of 200 kHz. The peak power of the output pedestal-free pulse and “figure-of-h” pulse was about 835 W and 480 W respectively. The spectral center wavelength of the fiber laser was at 1062.88 nm and the full width at half maximum (FWHM) was about 0.13 nm. The stimulated Raman scattering (SRS) at 1117 nm was 40 dB lower than the laser signal peak at 1062.88 nm under 45 W output power.

 

Fig. 2 The fiber laser output pulse train under pump of 200 kHz and pump pulse width of (a)1 μs and (b) 2 μs. Inset, the single pulse shapes.

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To obtain the high nonlinear parametric conversion efficiency and the high average power output in the MIR OPO system, we hope the pump fiber laser source could output the moderate high peak power and the high average power simultaneously. As mentioned before, when the pump pulse duration was beyond 1 µs, the output pulse obtained from the gain switched fiber laser had a “figure-of-h” shape, which was composed of a higher front pedestal-free pulse shape and a relatively lower platform in the following part. Generally, gain switching is applied to select the first pulse of the relaxation oscillations for nice shape and short pulse duration output in many applications [18–20]. However, it has been theoretically pointed out [11] and experimentally confirmed [8,11,17] that by using a steep leading edge pulse or double rectangular pulse to pump the OPO, the pump energy during the OPO buildup time can be minimized and the back-conversion be eliminated. Thus the higher front pedestal-free pulse shape here should be helpful to reduce the buildup time and the platform in the following part should have a constant lower intensity, favoring high conversion efficiency by avoiding back-conversion. Clearly, the “figure-of-h” shape obtained here was favorable to the OPO operation because it offered much a higher average output power from the gain switched fiber laser.

The output pulse train from the gain-switched fiber laser was then directed to pump the PPMgLN-based MIR OPO system thereafter. For the OPO, signal band spectral shift under different pump powers were measured using an optical spectrum analyzer (ANDO, AQ6375). Figure 3(a) and 3(b) give out the output signal spectra for 3.8 μm and 3.3 μm band MIR output respectively. Apparently, OPO generating longer idler wavelength suffered more from the thermal effect as indicated by the corresponding signal wavelength shifts.

 

Fig. 3 The spectra of signal wavelength under different pump power corresponding to MIR output at (a)3.8 μm and (b) 3.3 μm respectively.

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Figure 4 shows the idler output power and the calculated pump-to-idler conversion efficiency as a function of the pump power. The highest idler output powers of 5.15 W and 8.45 W were obtained at wavelength of 3.8 μm and 3.3 μm with calculated pump-to-idler conversion efficiency of about 11.7% and 19.1% respectively. At high pump level, the OPO cavity tended to be unstable. We attributed this instability to the idler absorption induced thermal effect in PPMgLN crystal. In order to solve this problem, a temperature control system was used to set the PPMgLN crystal with more uniform temperature distribution and keep the cavity stable. It was noted that the nonlinear conversion efficiency was almost the same when the pump power increased. This is mainly because the pump pulse energy and the pulse shape under different repetition frequency were almost the same, and then with the same peak power.

 

Fig. 4 Idler output power and pump-to-idler conversion efficiency dependence on the pump power.

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The pulse shapes of the pump, depleted pump and the signal at 1479 nm with different pump pulse shape were measured and are plotted in Fig. 5. Figure 5(a) shows the simple pedestal-free pump pulse evolution. It is clear that the front of the pump pulse was not depleted due to lack of signal and idler intensity. After the signal pulse was built up, the depleted pump pulse behaved an oscillatory behavior due to the back conversion. The pump to idler conversion efficiency was about 8%. The “figure-of-h” pulse provided a significant improvement to the conversion efficiency, as shown in Fig. 5(b). The overall pump to idler conversion efficiency was about 11.7%, about 50% increase in the conversion efficiency between the “figure-of-h” pulse and the pedestal-free pulse. From Fig. 5(b) it can also be found that the back conversion phenomenon was reduced with the pedestal-free pump pulse.

 

Fig. 5 Different temporal profiles of the pump, depleted pump and the signal wave from the OPO system (a) simple pedestal-free pulse (b) “figure-of-h” pulse shape

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4. Conclusion

In summary, we have demonstrated, to the best of our knowledge, the first experimental realization of a gain-switched Yb fiber laser pumped high power mid-infrared OPO system. High average power MIR lasers were generated with output power of 5.15 W at 3.8 μm and 8.45 W at 3.3 μm respectively. The pump to idler conversion efficiencies of the OPO were about 11.7% and 19.1% respectively. Considering the high level of compactness and robustness, we believe the gain-switched fiber laser pumped OPO system will find great practical applications in many fields.