Frequency quadrupling of a continuous-wave laser is carried out in coupled cavities with the stabilization to the resonance. The fourth harmonic generation in the coupled cavities is expected to be more efficient than in the two build-up cavity configuration in tandem, because input couplers, which increase the optical loss of the cavity, are not required. The cavity length is stabilized by the feedback of the error signal obtained by the cavity length modulation. In the present demonstration, the ultraviolet beam of 6μW at 238 nm is obtained by the fourth harmonic generation of the near infrared laser of 180 mW at 952 nm. This output power is expected to become higher (∼1.7mW) when the cavity setup is optimized.
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