cw, 325nM, 100mW semiconductor laser system as potential substitute for HeCd gas lasers

摘要

In the last decades, diode laser systems conquered the spectral range step-by-step from conventional gas lasers, wherever they can match or outperform in optical specifications. Although highly anticipated in the ultraviolet wavelength range, for instance in high-resolution lithography, biological and medical fluorescence applications or holography, cw single frequency operation of sufficient power has been a challenge for diode or other solid state laser systems. Currently this scope is still dominated by the HeCd gas laser, emitting at 325 run with powers of up to 100 mW. In this paper we present a diode laser system emitting at 325 nm offering the same output power by efficient second harmonic generation (SHG) of a master oscillator power amplifier (MOPA) at 650 nm.For the master oscillator a ridge waveguide diode is anti-reflection coated and used in an external cavity diode laser (ECDL) with grating feedback in Littrow configuration. This setup features a MHz line width (coherence length of 100m), a coarse tuning range from 649 nm to 657 nm and a mode hope free tuning of 20 GHz. In a second step, we use a tapered amplifier to boost the output from the ECDL to a level of 400 mW, for powering an efficient second harmonic generation process in an enhancement cavity. Faraday isolators on both ends of the amplifier stage prevent back reflection and stabilize the single mode operation of the system. Together with astigmatism compensation this yields to a high spatial quality (m~2<1.5) of the amplified beam. The frequency doubling is achieved by using a four mirror bow-tie enhancement resonator fitted with a Beta-Barium Borate (BBO) crystal. The cavity length is actively locked to the laser frequency using the Pound-Drever-Hall method.With this set-up, stable and reliable laser operation is achieved. After a few minutes warm-up time, fixed frequency and tunable UV output power of more than 100 mW could be generated, opening this important wavelength range for future product development.