371-385 nm tunable alexandrite continuous-wave laser

  Objective  Compared with all-solid-state UV lasers which use nonlinear crystals to perform tertiary or quadruple harmonic conversion of infrared light, the alexandrite lasers can obtain ultraviolet lasers by single frequency doubling. It has the characteristics of superior broadband emission spectrum which enables it to achieve tunable ultraviolet laser output. Therefore, tunable continuous ultraviolet laser plays an important role in applications. The ultraviolet 371-385 nm laser can be used in many fields, such as the ultra-precision material processing, laser Doppler cooling, entangled photon pair generation and quantum communication. Therefore, developing the tunable continuous ultraviolet laser has a great research value.

  Methods  In order to achieve the laser output in the wavelength range of 371-385 nm, a tunable alexandrite continuous laser was developed. The experimental structure is a V-folded cavity and the length of the cavity is about 13 cm. A fiber-coupled 635 nm LD (Changchun New Industry Optoelectronic Technology Co., Ltd) with the polarization ratio of greater than 100:1 is used as the pump source. The maximum pump power is 17 W. A c-axis-cut alexandrite crystal with Brewster angle, a size of 3 mm×3 mm×10 mm and Cr³⁺ doping concentration of 0.2 at.%, was used as the gain medium. It is wrapped with an indium foil and mounted in a water-cooled heat sink. A type-I phase-matched β-BaB₂O₄ (BBO, θ=31°, φ=0°) with the size of 3 mm×3 mm×7 mm was used as the frequency doubling crystal. In the experiment, the polarization direction of the pump beam was adjusted by a half-wave plate to match the maximum absorption direction (the b axis) of the crystal. The optimization of the resonator mirror coating and the theoretical simulation cavity length reduce the loss of resonator and improve the conversion efficiency. The continuous tunable ultraviolet laser output at 371-385 nm is realized by adjusting the BBO angle.

  Results and Discussions   The experimental schematic of the ultraviolet tunable alexandrite laser is shown (Fig.1). The maximum laser output power with the center wavelength of 378 nm is obtained by turning the half-wave plate and adjusting the resonator mirror and BBO crystal angle. The 378 nm laser light threshold is 4 W. As the pump power increases, the output power increases first rapidly and then becomes slowly. When the 635 nm LD output power is 17 W, the 378 nm laser power is 1.25 W (Fig.5), corresponding to the optical-to-optical conversion efficiency of 7.3% from 635 nm pump laser to 378 nm UV laser. In the experiment, the angle of BBO (θ=31°) is cut according to λ=756 nm/378 nm as the center wavelength. Therefore, the farther the fundamental frequency optical wavelength deviates from the central wavelength, the greater the angle of incidence is, the greater the reflection loss is, and the smaller the corresponding frequency doubling light energy is. Due to the limited angle of the BBO crystal, the continuous tunable wavelength range of the output ultraviolet is only from 371 nm to 385 nm. When the pumping power is 17 W, the farther the wavelength deviates from the central wavelength of 378 nm, the smaller the laser output power is (Fig.7). The laser beam quality along the x and y axis at 378 nm is 1.13 & 1.12, respectively (Fig.8).

  Conclusions  In summary, through theoretical and experimental research, a tunable 371-385 nm continuous ultraviolet laser was realized. The UV laser is characterized by simple structure, adjustable wavelength, high efficiency and high beam quality. And a series of its key parameters were tested, which has certain application value.