The pump source is a home-made 1064 nm Nd∶YAG amplifier. A half-wave plate is inserted behind the 1064 nm pump laser to align the pump beam laser polarization direction to match the <111> direction of the diamond crystal to maximize the Raman gain. A focus lens with a focal length f of 200 mm is used to focus the pump beam onto the diamond. The waist radius of the pump beam in the diamond is ~200 μm.
Compared to other Raman laser materials, high-quality diamonds have high Raman gain, high thermal conductivity, high damage threshold, and wide spectral transmission. A low-nitrogen, low-birefringence diamond crystal with a size of 2 mm×2 mm×7 mm is used as the Raman gain medium. The propagation direction in the diamond crystal is along the <110> direction.
The intracavity nonlinear crystal is a LiB3O5 (LBO) crystal with a size of 4 mm×4 mm×10 mm. It is coated with antireflection films at wavelengths of 1064, 1240, and 620 nm. The LBO crystal is wrapped in indium foil and mounted on a copped heat sink using a temperature controller. The temperature is maintained at 37.1 ℃ by thermoelectric cooler (TEC) for type I noncritical phase matching. M1 and M2 mirrors are used to separate the residual 1064 nm pump laser, 1240 nm Raman laser, and 620 nm red laser.
An extracavity frequency-doubled 310 nm ultraviolet laser is demonstrated using a BaB2O4 (BBO) crystal with a size of 4 mm×4 mm×7 mm. A half-wave plate is used to align the fundamental-frequency laser polarization. A focal lens with a focal length of 200 mm is used to focus the 620 nm laser onto the BBO crystal. A prism is used to separate the 620 nm laser and 310 nm ultraviolet laser.
An output power of 48 mW at 310 nm is achieved when the fundamental-frequency laser power is 550 mW at 620 nm (Fig.5). The frequency-doubling efficiency is 8.7%. The central wavelength is 309.8 nm (Fig.6). The pulse width is 762 ps, with an output power of 48 mW at 310 nm (Fig.7).
Ozone is one of the most important gaseous components in the Earth’s atmosphere. Atmospheric ozone includes stratospheric and tropospheric ozone. Stratospheric ozone absorbs most of the ultraviolet rays from the sun to prevent damage to life. Differential absorption Lidar (DIAL) has been widely used to measure ozone concentrations. As transmitters are key components of a DIAL system, several groups have demonstrated their work on transmitters. However, compared to transmitters with wavelengths below 300 nm, ultraviolet transmitters with wavelengths of 300?320 nm can transmit laser through a high ozone concentration in the atmosphere. Compared to optical parameter oscillators (OPO) and second harmonic generation (SHG) from 1.3 μm laser, Raman lasers do not require phase matching management. Solid-state Raman lasers offer the advantages of compactness and high beam quality. In this study, we investigate a pulsed ultraviolet 310 nm laser with stimulated Raman scattering and frequency doubling, aiming at the demand for transmitters for ozone DIAL.
The 310 nm ultraviolet solid-state Raman laser includes three parts: a pump source for the diamond Raman laser, a 620 nm intracavity frequency-doubled diamond Raman laser, and a 310 nm ultraviolet laser based on extracavity doubling. The experimental setup of the 310 nm ultraviolet laser is shown in Fig. 1.
The output power of the 620 nm laser versus that of the 1064 nm laser is shown in Fig.2. An output power of 550 mW is achieved with a 1064 nm pump power of 4 W. The conversion efficiency is 13.7%. The central wavelength is 620.1 nm (Fig.3). Because of the beam clean-up effect in solid-state Raman lasers, the beam quality of the 620 nm laser is apparently better than that of the pump laser (Fig.4).
A high-repetition-frequency pulsed ultraviolet laser is designed using a frequency-doubled diamond Raman laser pumped using a 1064 nm laser. An intravacity-frequency-doubled diamond Raman laser with a 620 nm output laser is demonstrated. A laser output power of 550 mW is achieved using a 1064 nm pump power of 4.0 W. The conversion efficiency is 13.7%. With extracavity doubling, an average output power of 48 mW is achieved at 310 nm using a BBO crystal. The repetition frequency is 2 kHz, and the pulse width is 762 ps. The conversion efficiency is approximately 8.7%. By improving the power of the 620 nm laser, the power of the 310 nm ultraviolet laser can be further improved to satisfy the requirements for ozone DIAL transmitters.
