Ultraviolet and deep ultraviolet ultrafast lasers have important applications in the development of ultraviolet semiconductor devices, ultrafast spectroscopy, and extreme ultraviolet laser generation owing to their advantages, such as ultrahigh temporal and spatial resolution. Continuous wavelength tuning in the ultraviolet and deep-ultraviolet ranges can expand the applicability of such lasers to satisfy the application requirements of different materials and research fields. However, many technical challenges remain for continuously tunable ultrashort pulse lasers that cover ultraviolet and deep-ultraviolet wavelengths below 200 nm to 300 nm. This study reports the development of a high-repetition-rate continuously tunable ultraviolet/deep ultraviolet ultrashort pulse laser with a tuning range covering the 192?300 nm wavelength band.
A commercial tunable Ti∶sapphire mode-locked laser with a continuously tunable output wavelength in the range of 690?1040 nm, a pulse width of 120 fs, and a repetition rate of 80 MHz is used as the fundamental laser. The BBSAG crystal, an improved BBO crystal, is used as the frequency conversion medium. In theory, a fundamental laser wavelength can be converted into a wide range of ultraviolet/deep-ultraviolet wavelengths of 190?300 nm through cascaded frequency doubling and sum-frequency mixing. However, tuning the crystal angle over a wide range can significantly decrease the frequency-doubling efficiency. In addition, owing to the limitations of the phase-matching conditions of BBSAG crystals, laser wavelengths below 205 nm can be generated only through multistage sum-frequency mixing. Based on these factors, the laser is divided into three wavelength bands to generate ultrashort pulse lasers covering a wide range of 192?300 nm by optimizing the design of the multilevel frequency-doubling/sum-frequency mixing and nonlinear crystal angle.
The laser system consists of three laser modules. In Module 1, a laser in the range of 210?250 nm is generated by direct frequency doubling (ω0+ω0→2ω0) and frequency quadrupling (2ω0+2ω0→4ω0) . The 250?300 nm wavelength band is generated using the second frequency-conversion module, which contains a frequency doubling unit (ω0+ω0→2ω0) and a sum-frequency mixing unit (2ω0+ω0→3ω0) to obtain the third harmonic of the fundamental laser. A delay line is introduced before the sum frequency of the fundamental laser and second harmonic to compensate for the group-velocity delay between the fundamental and second harmonics. Similar to Module 1, the output wavelength is tuned by adjusting the fundamental wavelength and crystal angle. The third harmonic is generated by directing the beam into different optical paths using a beam switcher, where one path is sent to a beam combiner and combined with the other two to form one output, and the other path is switched to the third laser frequency-conversion module to continue sum-frequency mixing to generate a shorter wavelength. The shortest wavelength that a BBSAG crystal can support for direct frequency doubling is 205 nm, and the phase-matching angle is approximately 90° at a wavelength of approximately 210 nm. The effective nonlinear absorption (deff) is close to zero at this phase-matching angle, which results in inefficient frequency doubling. Therefore, sum-frequency mixing is adopted to generate wavelengths of approximately 210 nm and lower. The fourth harmonic generation of the wavelength range 192?210 nm (ω0+3ω0→4ω0) is achieved through the sum-frequency mixing of the remaining fundamental laser and the generated third harmonic from Module 2 after the generation of the third harmonic. Similar to Module 2, a delay line is required to compensate for the group-velocity delay between the fundamental laser and third harmonic.
After the three bands are generated, they are directed to an optical path combiner controlled using an electric mobile platform. By adjusting the position of the reflector on the platform, the three bands are directed towards a single output, facilitating subsequent laser applications. After they merge into one path, the beam is introduced into a beam-pointing stabilization system (BPS) to compensate for the position deviation generated by the beam during the switching, frequency doubling, wavelength tuning, and beam combination. When the three bands are generated at 192?210 nm, 210?250 nm, and 250?300 nm, respectively, they are combined into one optical path to output. Finally, a continuous tunable ultrashort pulse laser covering the 192?300 nm wavelength range is achieved (Figs.2 and 4). Through the electronic control design of beam switching, crystal-angle adjustment, group-velocity compensation, and beam-pointing stabilization, the entire tuning process of the laser can be controlled using a program that does not require complex manual adjustment, which would provide a single laser with high controllability and practicality.
