The mid-infrared band of 2.5–5 μm is in the transmission window of the atmosphere and covers the absorption spectrum of most molecules, playing a wide range of roles in the environmental, medical, communication, and defense fields (1), (2). In particular, the high repetition rate (kHz and above) and widely tunable mid-infrared picosecond laser has important applications in trace gas analysis, material characterization, minimally invasive laser therapy, and optical frequency comb due to its short pulse interval and high peak power (3), (4), (5). At present, different methods have been proposed to obtain mid-infrared lasers, including gas lasers (6), fiber lasers (7, 8), quantum cascade lasers (9), nonlinear frequency conversion (10), (11), (12), etc. The nonlinear frequency conversion is the main means to produce mid-infrared picosecond and femtosecond pulses, such as optical parametric oscillator (OPO), optical parametric generator (OPG), and difference frequency generator (DFG). OPG has a single-pass structure, which not need to introduce pulse-synchronized signal light, and build a complex resonant cavity. With the features of a simple structure, wide tuning range, and low cost, it has attracted much attention in the preparation of current mid-infrared light sources.

The common mid-infrared nonlinear crystals include KTP (13), KTA (14), ZGP (15), BGSe (16), and PPLN (17), (18). Among these crystals, PPLN, as a widely used and mature quasi-phase-matching crystal, overcomes the optical walk-off effect in the frequency conversion process, has a high nonlinear coefficient (d₃₃ = 27.2 p.m./V), a wide transparency range (0.5–5 μm), and no two-photon absorption for the conventional near-IR pumping sources. In particular, MgO: PPLN, by adding 5% MgO to PPLN, can significantly improve the optical refractive threshold and optical damage of the crystal while retaining a high nonlinear coefficient. The wide tuning and high efficiency mid-infrared output can be achieved in the crystal by different grating period designs as well as temperature tuning.

Recently, there have been many studies on mid-infrared generation based on MgO: PPLN crystal. J. Wueppen’s group presented the OPG system using MgO: PPLN pumped by a 1,064 nm Nd: YVO₄ laser, a wide tuning range of 2.3–2.8 µm laser output was achieved with a repetition rate of 20 MHz (19). S. Parsa et al. proposed a Yb-fiber pumped picosecond OPO at 80 MHz based on MgO: PPLN, an average power of 2 W was achieved at a continuous tuning range of 2,198–4,028 nm, and measured the power stability better than 0.31–0.46% rms over 1 h (20). Jin Guangyong et al. reported a single resonant MgO: PPLN-OPO and single-pass MgO: PPLN-OPA pumped by fiber laser respectively, and they obtained output power of 3.02 W at 3.8 μm with a repetition rate of 140 kHz (21). Chen Fei et al. presented a MgO: PPLN-based OPO pumped by an all-fiber Yb-doped fiber laser. A maximum power of 79.1 W at 1,064.1 nm was obtained with a repetition rate of 300 kHz and pulse duration of 200 ns (22). However, most of these studies focus on the OPO approach and pumping with solid-state laser. The single-pass OPG system using fiber laser pumping requires a higher pumping threshold, but the structure is simpler, and the pump source could be constructed with all-PM components, which greatly reduces the size of the system and provides a more stable operation.

In this paper, we demonstrated a mid-infrared OPG regime pumped by an all-PM mode-locked Yb-fiber laser. A 2.76–3.98 μm widely tunable mid-infrared output was obtained and a maximum output power of 500 mW at 3.22 μm was achieved. Moreover, the relative power fluctuation was measured as 0.37% over 30 min. The results illustrated a simple method to obtained wide-tunable mid-infrared source with a stable output.

Figure 1 illustrates the experimental setup of mid-infrared single-pass OPG pumped by Yb-fiber laser. The pump source was composed of a fiber oscillator and amplifier. As shown in Figure 1A, the oscillator was based on the principle of NALM, using the interference of the Sagnac loop in the resonant cavity to act as a saturable absorber to achieve mode-locking (23). In contrast to the real saturable absorbers, the NALM would benefit passive mode-locking with considerably high damage threshold and extremely short response time (24), (25). The Yb-doped fiber (PM-YDF-6/125, Nufern) with a length of 1.5 m was used for the gain medium to provide enough nonlinear and pulse amplification. The fiber Bragg grating (FBG) was used for end-face reflection in the laser cavity, with a bandwidth of 5 nm and a reflectivity of 99%. The 50:50 output coupler (OC) was used to make the laser propagating in the coupler interference with the maximum contrast, so that NALM showed better strength permeability. The Phase shifter (PS) was used to provide π/2 non-reciprocal phase shift in the cavity, which could effectively reduce the mode-locking threshold.

Then the pulse from the oscillator was steered into the amplifier for power boosting, which would serve as a pump source in the subsequent OPG. As shown in Figure 1B, the amplifier adopted a cascaded fiber amplification structure which was composed of the pre-amplifier and main amplifier. The pre-amplifier was pumped by a single-mode laser diode (LD) at 976 nm, with a maximum output power of 680 mW, and 100 mW output power was achieved in 1.2 m YDF. The main amplifier was pumped by two identical multi-mode LDs at 976 nm, each with a maximum output power of 9 W, and 6.2 W output power was obtained in 2 m photonic crystal fiber (PM-PCF-14/135, NKT Photonics).

Finally, the pump beam was focused into the MgO: PPLN crystal by a spherical lens (L1, f = 75 mm). As shown in Figure 1C, the crystal was housed in an oven with a tuning range of 25–200°C and an accuracy of 0.1°C. The dimensions of the MgO: PPLN crystal were 10 × 1 × 25 mm³ (Width × Height × Length), and the grating periods were 28.3–30.5 μm with a step of 0.5 μm. Both end surfaces of the crystal were antireflection coated for three bands from 1,000 to 1,100 nm, 1,300–1700 nm, and 2,600–5,000 nm. The half-wave plate (HWP1) and polarization prism (PBS) form a power attenuator to change the pump power. The polarization was adjusted to be perpendicular by the HWP2 to satisfy the type-0 phase-matching condition (26). The generated signal light and idler light were collimated by a calcium fluoride Lens (L2, f = 75 mm), and then separated by a dichroic mirror (DM) to obtain a pure mid-infrared laser. The DM has a high transmittance (>95%) at 3–4 μm, and high reflectivity (>99.7%) at both 1.03μm and 1.4–1.7 μm. The mid-infrared output power was detected by a thermal power sensor (Thorlabs S401C).

In conclusion, we proposed a single-pass mid-infrared OPG regime pumped by an all-PM Yb-fiber laser at 1,032.5 nm based on multi-grating MgO: PPLN. The Yb-fiber laser was mode-locked by NALM with a nine-figure configuration, followed by a cascaded fiber amplification system to achieve an average power of 6.2 W pulse output, with a repetition rate of 15.8 MHz and pulse duration of 18 ps. By varying the crystal grating periods (28.3–30.5 μm) and temperature (40–130°C), the widely tunable (2.76–3.98 μm), picosecond mid-infrared laser at 15.8 MHz was obtained. Corresponding to a maximum output power of 500 mW at 3.22 μm and a quantum conversion efficiency of 25%. The whole system has high stability due to the all-PM fiber structure of the pump source, and the relative power fluctuation was 0.37% over 30 min. We believe that our results are meaningful in terms of mid-infrared generation in an all-PM fiber laser-pumped OPG system.