Objective Laser sources at 1.5 μm, providing high pulse energies and short pulse durations are used in various applications, such as electro-optical countermeasures and high precision ranging. Lasers at 3--5 μm are used as lighting sources for active remote sensing and gas detection, which show important potential applications. Therefore, lasers with high energy at both wavelength bands have become research hotspots. The optical parametrical oscillators (OPO) are effective ways to generate lasers with wavelengths at 1.5 μm and interval 3--5 μm due to their compactness, wavelength-tunable property, and potential for generating high energy and short pulse width. Presently, the major nonlinear crystals with high-quality include biaxial crystals such as KTiOAsO4 (KTA), KTiOPO4 (KTP), ZnGeP2 (ZGP), and periodically poled crystals such as PPKTP, PPLN, PPLT, etc. KTP crystals are used to obtain lasers at 1.5 μm, which is affected by severe absorption in the mid-infrared region. To obtain lasers at 3--5 μm, ZGP crystals have been under investigation for a long time. However, 2 μm pump sources are more in need, which is technically more difficult than their 1-μm counterpart. PPLN crystals are used to obtain mid-infrared lasers. Compared with crystals such as KTP, the damage threshold of PPLN crystals is lower. KTA and KTP crystals belong to the same crystal group and have a high damage threshold (>600 MW/cm 2), large nonlinear coefficient (d24=3.2 pm/V), large acceptance angle, a wide temperature range, and stable physical and chemical properties. The transmission performance of KTA crystals in the mid-infrared band (3--5 μm) must be better than that of KTP crystals. These characteristics make KTA crystals suitable for high energy mid-infrared laser applications. In this study, we report a 100 Hz high energy KTA crystal-based OPO system.
Methods The 100 Hz high energy KTA-OPO system is composed of 1064 nm Nd: YAG main oscillator power amplifier (MOPA) and KTA crystal-based OPO. The Nd∶YAG MOPA laser at 1064 nm served as the pump source. To obtain high beam quality, the Nd∶YAG MOPA system adopted the “unstable cavity oscillator + two-stage amplifiers” scheme. Both the oscillator and the two-stage amplifier used a double rod structure connected in series, and a 90° quartz rotator between the two Nd∶YAG crystal rods to compensate for the thermal depolarization effect. To prevent self-excited oscillation and spontaneous radiation between the stages while protecting the optical components of each stage, isolators are placed between each stage. The X-cut KTA crystal is used in the experiment, and the dimension of the KTA crystal is 10 mm×10 mm×33 mm. The cavity is designed as a signal resonant oscillator with a cavity length of 65 mm. The input mirror is coated to be highly reflective for the signal and high transmittance for the pump light. The output mirror is coated with a partial reflectivity of 50% for the signal and high transmittance for the idler. The pump light passed the OPO twice. An isolator protects the pump laser from the remaining pump light that comes back from the OPO cavity.
Results and Discussions A homemade 1064 nm Nd∶YAG MOPA with a pulse energy of 580 mJ at 100 Hz repetition rate is employed as the pumping source. After two-stage amplification, 580 mJ of 1064 nm laser is obtained with the extraction efficiency of the primary amplifier and secondary amplifier at 6.7% and 10.8%, respectively (Fig. 3). The beam quality factor of the 1064 nm laser is Mx2=4.6 and My2=3.7 (Fig.4). The pulse width of the laser from the oscillator and primary amplifier and secondary amplifier are 15.3, 16.9, and 18.0 ns, respectively (Fig.5). In the OPO experiment, the optical-to-optical conversion efficiency is optimized by increasing the cavity length and KTA crystals length. The output energy and conversion efficiency of the KTA crystal with a length of 33 mm are higher than that of the KTA crystal with a length of 38 mm (Fig.6). Then, experiments with different OPO cavity lengths are performed on the 33-mm KTA crystal. The results indicated that the output energy and conversion efficiency are higher for short cavity length (Fig.6). The threshold of the OPO is about 20 mJ. When the pump energy is 580 mJ, 64 mJ idler is obtained at 3.47 μm and associated signal at 1.54 μm is 178 mJ (Fig.7). The OPO efficiency is 46.3% high, and OPO output stability is 1.2% rms (Fig.7). The pulse width of the output laser at 1.54 and 3.47 μm is 13.7 and 11.8 ns, respectively (Fig.8). The beam quality factor of the 1.54 μm laser is Mx2=30.5 and My2=28.2 (Fig.9). In addition, the center wavelength of the signal laser is 1.535 μm (Fig.10).
Conclusions A 100-Hz, high-energy KTA crystal-based OPO system is reported. A homemade 1064 nm Nd∶YAG MOPA with a pulse energy of 580 mJ at a 100 Hz repetition rate is used as the pumping source. We adopted plane-plane cavity configuration for the OPO, and an X-cut KTA crystal as the nonlinear crystal. The obtained pulse energies at a signal wavelength of 1.53 μm and idler wavelength of 3.47 μm are 178 and 64 mJ at a pulse repetition rate of 100 Hz, respectively. Furthermore, the pulse durations are 13.7 and 11.8 ns, respectively, and the optical-to-optical conversion efficiency is 43.6%.
