The 2‒3 μm short-wave infrared band exhibits unique properties during interactions with gas molecules and biological tissues. Thus, it finds applications in various domains such as remote sensing of greenhouse gases, atmospheric pollution, and medical treatments. Coherent sources in this band play a key role in the above fields and the tunability of the coherent sources is an important factor, especially in multiple molecule spectroscopy. OPOs have proved to be effective light sources with desired wavelengths, owing to the following merits: the commercial availability of well-developed lasers and nonlinear crystals, coupled with their agile frequency tunability. Bulk KTiOPO₄ (KTP) serves as an excellent OPO medium owing to its high optical damage threshold, moderate nonlinearity, wide phase-matching (PM) wavelength region, and low cost. However, most previous studies focused on doubly resonant operation at wavelengths below 2.4 μm. In this study, we perform an experimental analysis of nearly singly resonant KTP-OPO with a wide tuning range of 2.05‒2.97 μm under a small rotation angle.

A commonly used PM geometry of Nd∶YAG laser pumping KTP-OPO (type-II, θ-tuning in the x-z plane) is presented in Fig.1. In the range of θ<50°, the output wavelength changes rapidly with the PM angle. Thus, a KTP cut at θ≈48° incorporating properly coated cavity mirrors can theoretically perform a singly resonant OPO, covering 2.2‒3.0 μm under a rotation (external) angle of ±4°. The experimental setup of the OPO is illustrated in Fig.2. A Q-switched Nd∶YAG laser is used as the pump source. The pump pulses pass through a variable attenuator (comprising a half-wave plate and a Brewster polarizer) and an isolator (ISO) and are incident on a plane-parallel OPO cavity. Two configurations are tested: single-pass OPO (SP-OPO) and double-pass OPO (DP-OPO). In the first case, the cavity mirrors M3 and M4 are identical (CaF₂ coated with high-reflection films at 1.65‒2.00 μm and anti-reflection films at 1.06 μm and 2.2‒3.0 μm) with a spacing of 28 mm. Two long-pass filters (CaF₂ coated with high-reflection films at 1.06 μm and anti-reflection films at 1.65‒3.00 μm) are used to block the residual pump. In the second case, M4 is changed into a total reflection mirror M4' at 1.06 μm and 1.65‒3.00 μm. The backward idler beam is transmitted through M2 (CaF₂) and a long-pass filter and is detected by a pyroelectric sensor. The KTP crystal has cut angles of θ=47.8° and φ=0°, and dimensions of 10 mm×8 mm×20 mm, and is mounted on a rotation stage.

The output wavelength of the KTP-OPO at normal incidence is measured by a spectrometer. In Fig.3, the main peak at 2.43 μm is corresponding to the idler light, and the secondary peak at 1.89 μm is corresponding to the resonant signal light. The values at different KTP incidence angles, plotted as scatters in Fig.1, almost follow the calculated curves. By adjusting the half wavelength plate and changing the pump attenuation, the relationship between the OPO output and pump input is observed at 2.43 μm (Fig.4). The SP-OPO has an extremely high threshold of 209.3 mJ, which is approximately 3 times that (70.46 mJ) of the DP-OPO. The idler pulse energies of SP- and DP-OPO rise above 18 mJ under pump energies of 328 mJ and 148.3 mJ with slope efficiencies of 24.9% and 27.9%, respectively. The input and output pulse envelopes are captured with an InGaAs photodiode. Figure 5 shows that the idler pulse of DP-OPO arises earlier than that of SP-OPO. The tuning curves are measured under given pump energies. Here, the outputs of the two configurations are set to be approximately equal at normal incidence. The wavelength band extends from 2.05 μm to 2.97 μm (Fig.6) by rotating the PM angle of KTP from -6° to 4.6°. In the region below 2.2 μm, both the idler and signal lights are emitted from the OPO cavities because the transmittance of M3/M4 increases gradually from less than 1% at 2.0 μm to more than 98% at 2.2 μm. The signal energies are excluded by separating the two orthogonally polarized wavelengths with a Glan prism. The curve of DP-OPO is mostly higher than that of SP-OPO. Under a pump energy of 148.3 mJ, the output at 2.05‒2.57 μm exceeds 10 mJ (56.5% coverage), and that at 2.77 μm exceeds 5 mJ (78.3% coverage).

We present a tunable short-wave infrared source based on a Nd∶YAG laser pumping KTP-OPO. A wide tuning range from 2.05 μm to 2.97 μm is obtained under a small KTP crystal rotation angle. The highest output energy exceeds 18 mJ. The advantages of the pump-reflected double-pass OPO over the single-pass OPO are demonstrated, including the reduced threshold, enhanced efficiency, and decreased build-up time. The wavelength coverage with an output above 5 mJ is 78.3%. The continuous and wide tunability, as well as the high peak power, can find application in the fields such as multiple gas analysis and DP-OPO can be used as the pump source of semiconductor crystal based nonlinear long-wave generation.