Optical frequency shifting of a mode-locked laser beam

Further progress in the development of an optical frequency shifter will be reported. In this device, light pulses emerging from a mode-locked He-Ne laser at a repetition rate of 100 MHz, are passed through an electro-optic phase modulator (LiNbO3) driven by a 100 MHz sine wave phase-locked to the light pulse train. By properly adjusting the rf phase, the optical frequency of the light pulses can be up-shifted or down-shifted by an amount which is proportional to the maximum rate of change of the refractive index. Shifts as large as ±5 GHz have been observed with a scanning Fabry-Perot interferometer. Interferograms showing clearly resolved modes were recorded. The Gaussian-shaped mode spectrum of the mode-locked laser beam, about 1.5 GHz wide at half height, was shifted up or down along the frequency scale without any visible change in shape, in agreement with theory. For these measurements the beam made a single pass through a 3-cm long crystal of LiNbO3. The shifts are cumulative with the number of passes. Work on larger shifts produced after several passes through the crystal will be reported.