Frequency doubling of a fibre-amplified 1083 nm DBR laser

A good frequency standard is required at 1083 nm for measurements on the fine structure of helium and of the fine structure constant. Several milliwatts of CW frequency-doubled light offers the prospect of stabilisation to a Doppler-free hyperfine transition in molecular iodine. The 1083 nm emission of an extended-cavity DBR diode laser has been amplified using an ytterbium-doped fibre amplifier, and applied to a type-I phase matched Mg:LiNTbO3 crystal in a high-Q fundamental-resonant cavity for frequency doubling. The amplifier gain at 1083 nm under typical operating conditions was 13.8 dB, with a coherent output power up to 63 mW, limited by the maximum signal input power. The doubling cavity Q was similar to 2 x 10(7), and about 70% of the incident power was coupled in. The maximum 2nd-harmonic conversion efficiency exceeded 20% and provided an output power of 3.7 mW for making continous frequency scans of up to 600 MHz in the green. All optical spectrum analyser at 541.5 nm showed fringes of 4.6 MHz full width half maximum, close to the instrumental width.