Objective Lasers operated at 3.5 μm stretch the absorption peaks in the C—H bonds and exhibit high transparency in the atmosphere windows, making the lasers that generate beams with frequencies of 3.5 μm useful in applications using spectroscopy, remote sensing, environmental monitoring, and infrared (IR) countermeasures. Optical parametric oscillation (OPO) is an effective method to radiate mid-IR laser beams that can transform near-IR lasers into mid- and far-IR radiation. The KTiOAsO₄ (KTA) crystal is a member of the KTiOPO₄ (KTP) crystal family and an excellent nonlinear material. The spectroscopy transmittance range of KTA is 0.35--5.30 μm. The absorption loss in KTA crystals in the 3--5 μm bandwidth is much lower than that of the KTP crystal. The KTA crystal has a large nonlinear coefficient, a wide-angle and temperature-matching bandwidth, a high damage threshold, and stable physical and chemical properties. Experiments on noncritical phase matching (NCPM) demonstrated that the KTA optical parametric oscillator (OPO) pumped by a Nd∶YAG 1064-nm laser can radiate a 3.5-μm laser beam. Due to the influence of temperature dispersion, the wavelength value of KTA-OPO changes as a function of temperature. The detailed study of KTA-OPO temperature tuning properties presented herein can guide the application of KTA-OPO lasers. To date, there are few theoretical or experimental studies concerning the temperature tuning performance of KTA-OPO.

Methods Temperature tuning properties of KTA-OPO are studied theoretically and experimentally. Based on the normal temperature dispersion equation and the temperature dispersion equation of the KTP crystal, the temperature tuning properties of KTA-OPO at different cutting angles (θ) are calculated. The wavelength of idle light (λ_i) at T=25 ℃ and the temperature tuning slope (Δλ_i/ΔT) of KTA-OPO at different cutting angles are calculated with an accuracy of 1° when the wavelength of the pump laser is set at 1064 nm. An experimental study using a 3.5-μm NCPM KTA-OPO laser pumped by a Nd∶YAG 1064 nm laser is conducted. The pump source is an SL800 series pulsed Nd∶YAG laser with a pulse width of 13 ns, a spot diameter of 8 mm, and a repetition rate of 1 Hz. A small hole is placed behind the laser for dimming, and the spot diameter is compressed to 4 mm using a telescope system to increase the energy density of the pump laser. The temperature of the KTA crystal is controlled in a temperature-controlled furnace with an accuracy of 0.1 ℃. The wavelengths of the KTA-OPO idlers at 30, 80, 130 and 180 ℃ are measured using an Omni-300λ spectrometer (Zolix Instruments Co., Ltd) with an accuracy of 1 nm. At the back end of the grating spectrometer, a DEC-M204-InSb detector and a ZAMP amplifier (Zolix Instruments Co., Ltd) are used to detect and amplified the laser signal at the specified transmission wavelength. A DSOX3054T oscilloscope is used to display and measure the signal amplitude from the ZAMP amplifier. The value of the maximum wavelength of the laser signal is set equal to the peak wavelength of the output idler by the grating spectrometer.

Results and Discussions The temperature tuning properties of KTA at different θ angles and the OPO pumped by a 1064-nm laser are studied (Fig. 1). Comparison of the laser wavelength and Δλ_i/ΔT of the idler at different θ angles revealed that the wavelength of the idler increased monotonically as the value of θ increased, while Δλ_i/ΔT decreased monotonically (Fig. 2). When the temperature is 30, 80, 130, and 180 ℃, the measured values of λ_i are 3463, 3466, 3469, and 3474 nm, respectively, and Δλ_i/ΔT is 0.073 nm/℃. The experimental results show that the wavelength of the output idler of KTA-OPO (θ=90° and ?=0°) is less affected by changes in temperature under conditions of type II phase matching, which confirms the theoretical conclusions (Fig. 4). The results demonstrate that the temperature dispersion equation (Table 1) can be extended from near-IR to mid-IR band spectroscopy.

Conclusions The laser wavelength value of KTA-OPO is relatively stable at different temperatures. In type I and II phase matching, with the increase of θ, the idler wavelength value increased monotonically, but Δλ_i/ΔT decreased monotonically and ranged from to 0.0774 to 1.4968 nm/℃. The Δλ_i/ΔT value of the type I phase matching KTA-OPO is generally larger than the Δλ_i/ΔT value of the type II phase match. Among all phase matching points for angle θ, the temperature tuning range is smallest when θ=90° under type II phase matching, and the theoretical value of Δλ_i/ΔT is 0.0774 nm/℃. The experimental results are consistent with the theoretical calculations. Both theoretical and experimental studies show that the wavelength of the mid-IR laser beam of the NCPM KTA-OPO is less affected by temperature. This results of this study demonstrate the potential application of KTA-OPO pumped by a 1064-nm laser.