The Electro-Optic Effect: Fundamentals of Electro-Optic Modulators

Electro-optic modulators are crystals with special optical properties that allow an electrical signal to control a beam of light. This example demonstrates how this can be used in practice.

An applied voltage is increased by a factor of approximately 100 across the electro-optic crystal by a tuned rf cavity. This voltage change induces a change in the refractive index of the crystal, thereby varying the phase of a linearly polarized laser beam traversing it.

This modulation is described in the formation of sidebands at +/- the drive frequency on a monochromatic laser because a time-varying phase is analogous to a frequency. Several laser technology applications such as frequency locking depend on such a well-defined triplet of frequencies wherein the two sidebands are +/-90° out of phase with the carrier.

Bessel functions provide the amplitude of these sidebands as the unitless modulation index, which is proportional to the drive voltage. When the electro-optic modulator (EOM) is driven harder, higher-order sidebands can be created and the carrier can be completely suppressed. This occurs at a modulation index of roughly 2.4.

The maximum possible transfer of intensity into each first sideband is around 33% of the total at a modulation index of roughly 1.8. Hence, these EOMs can also be used as frequency shifters, particularly in frequency ranges that are not conveniently accessible by acousto-optic modulators (AOM) beyond 700MHz.

In addition, unlike AOMs, the additionally generated frequencies are perfectly in the range of the same spatial mode of the laser beam, making the EOMs suitable for applications requiring multiple frequencies, for instance, for repumping in atomic physics.

This information has been sourced, reviewed and adapted from materials provided by Qubig.

For more information on this source, please visit Qubig.

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