FM-laser operation of the Nd:YAG laser

The FM-laser or frequency-sweeping mode of laser oscillation has been demonstrated in a Nd :YAG 1.06-μ laser with an intracavity LiNbO 3 phase modulator. The experimental results are in excellent agreement with the theoretical expression \Delta= (\Delta\Omega/\Delta\nu) (\delta/\pi) where δ= peak single-pass phase retardation in the modulator, \Delta\Omega = axial mode spacing, \Delta\nu =modulator detuning, and Δ=resulting FM index of the laser output. Modulation indices as large as \Delta \approx 230 rad have been obtained, in which case the instantaneous laser frequency is sweeping over a full spectral range of 2\Delta \cdot f_{m} \approx 120 GHz (≈ 4 cm-1) at a repetition frequency f_{m} \approx 260 MHz, with a time-bandwidth product per period \approx 2\Delta \approx 460 . The coherently mode-locked spectral bandwidth thus obtained in the FM-laser case is very much wider than can be achieved in the pulsed mode-locked case with the same Nd:YAG laser. Some possible ways of using this broad-band coherent FM spectrum are suggested.

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