The 1.5 μm waveband laser has attracted remarkable attention in various fields, including radar, remote sensing, laser vibration measurement, and material microprocessing, because of its low atmospheric-transmission loss and safety for human eyes. The 3?5 μm waveband laser has a typical “atmospheric infrared window” and fingerprint band, and it can be used for molecular-content detection, environmental detection, imaging, telemetry, and biomedicine. Owing to its high spectral purity, long coherence length, and low phase noise, a narrow-linewidth, high-beam-quality infrared laser can be better applied in gravitational-wave detection, cold atomic physics, coherent optical communication, and optical-precision detection. An optical parametric oscillator (OPO) is an effective device for frequency conversion and can directly convert the wavelength of a solid-state laser into near- and mid-infrared wavelengths. Compared to the femtosecond OPO, the picosecond OPO not only has a high average power output but also exhibits a good balance between the pulse width and narrow spectral bandwidth.

A schematic of the synchronous-pumping picosecond KTiOAsO₄ (KTA)-OPO, used for high-power, high-beam-quality, narrow-bandwidth laser generation in the near- and mid-infrared wavelengths, is presented in Figure 1. The pump source is an all-solid-state picosecond laser based on a diode-pumped Nd∶YVO₄ crystal, which provides up to 18.5 W of maximum output power at 1064 nm in 15 ps pulses at a 120 MHz repetition frequency. A half-wave plate was used to rotate the polarization of the pump beam to excellently match the phase in the KTA crystal. Using a plano-concave lens with a focal length of 100 mm, the output of the pump beam was focused to ~63 mm at the center of the nonlinear crystal. The cavity parameters and spacing were theoretically estimated using LASCAD software.

To achieve the best mode coupling of the pump and signal beams in the nonlinear crystal, the signal beam had a spot radius of 64 μm at the center of the KTA. The 90°-cut KTA crystal (5 mm×5 mm×30 mm) was selected as the nonlinear-gain medium, and the two end surfaces were antireflection coated for the pump (1.064 μm), signal (1.535 μm), and idler (3.468 μm) beams. To achieve synchronous pumping, the OPO was configured into a signal-resonant Z-shaped standing-wave cavity using five reflective mirrors. Reflector M₁ is a plane. Mirrors M₂ and M₃ are plane-concave mirrors with a 150 mm radius of curvature; the distance between them is 167 mm. Reflector M₄ is a concave mirror with a 500 mm radius of curvature. The output coupling mirror M₅ is a concave mirror with a 1000 mm radius of curvature.

M₁?M₄ are highly reflective (99.9%) to the signal beam and highly transmissive to the pump and idler beams. The output coupling mirror (OC) M₅ has a 20% transmittance and 80% reflection to the signal beam. This design not only ensures a single-resonance signal but also enables the signal and idler beam to simultaneously be stably output from the cavity.

By adjusting various cavity parameters, a standing-wave cavity with high stability is established, successfully achieving high-beam-quality, high-power, narrow-linewidth, and highly stable signal and idler outputs. The spatial-profile distributions of the signal and idler beams are near-Gaussian modes (Fig. 2). The beam quality factors (M²) of the output beam are measured using the knife-edge method. The M² values of the signal beam in the two orthogonal directions are 1.11 and 1.12 [Fig. 3(a)], whereas those of the idler beam are 1.16 and 1.17 [Fig. 3(b)]. At the maximum pump energy of 18.5 W, 3.55 W of signal and 1.75 W of idler beams are output, with conversion efficiencies of 27.8% and 13.6% (Fig. 4), respectively. The extracted signal and idler beams exhibit passive power stabilities of better than 1.6% (RMS) and 1.5% (RMS) over 6 h (Fig. 5), respectively. The full width at half maximum (FWHM) of the signal and idler outputs are measured to be Δλ_s=0.27 nm and Δλ_i=0.75 nm (Fig. 6), respectively.

This study demonstrates the generation of high-beam-quality, narrow-linewidth, high-stability, and high-power ultrafast lasers in the near- and mid-infrared wavelengths by building a single-resonant signal KTA-OPO that is synchronously pumped by a 1 μm picosecond pulsed laser. At a maximum pump energy of 18.5 W, a signal output in the near-infrared region and an idler output in the mid-infrared region are obtained, corresponding to slope efficiencies of 27.8% and 13.6%, respectively. By combining the excellent nonlinear characteristics of the KTA crystal and the stable resonant cavity, M² values of 1.12 and 1.17 are achieved for the signal and idle outputs, respectively. The signal and idler beams exhibit high power stability over 6 h.