Sub-MHz-linewidth 200-mW actively stabilized 2.3-µm semiconductor disk laser

We report on the realization of an actively stabilized single-frequency vertical-external-cavity surface-emitting semiconductor laser (VECSEL) at a wavelength of 2.3 µm. The laser was locked to a high-finesse Fabry-Pérot interferometer with a free-spectral range of 1 GHz. From the error signal of the control feedback loop a laser linewidth of 390 kHz could be derived (1-s sampling time). Reducing the sampling time to 100 µs, a much narrower linewidth of 55 kHz was obtained, indicating the dominant effect of low-frequency technical noise. The output power exceeded 200 mW. By rotating the intracavity birefringent filter, a wide tuning range of 62 nm could be achieved. Changing the resonator length with the help of a piezoelectric transducer mounted to a cavity mirror, a modehop-free fine tuning range of 5.5 GHz was achieved.