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Pound-Drever-Hall Technique Laser/Cavity Lock
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FM Spectroscopy Laser Lock
Operating principle
A tuned rf cavity (tank circuit) boosts the applied voltage by a factor of ~100 across the electro-optic crystal. The alternating voltage induces a change in the crystals refractive index, thereby modulating the phase of a linearly polarized laser field passing through it.
Since a time-varying phase if equivalent to a frequency, this modulation is expressed in the formation of sidebands on a monochromatic laser at +/- the drive frequency. Many applications in laser technology, like frequency locking rely on such a well defined triplet of frequencies, where the two sidebands are +/-90 degrees out of phase with the carrier.
The amplitude of these sidebands is given by Bessel functions in terms of the unitless modulation index which is essentially proportional to the drive voltage. By driving the EOM harder it is possible to create higher-order sidebands and completely suppress the carrier, which happens at a modulation index ~ 2.4.
The maximum intensity that can be transferred into each first sideband is about 33% of the total (modulation index ~ 1.8), therefore these EOMs are also suitable as frequency shifters, especially in frequency ranges not easily accessible by acousto-optic modulators (AOM) beyond about 700MHz.
Moreover, in contrast to AOMs the additionally generated frequencies are all perfectly within the same spatial mode of the laser beam, which is especially useful when multiple frequencies are needed, e.g. for repumping in atomic physics.