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.