Acousto-optic Modulator (AOM)

An acousto-optic modulator is a device which can pulse the laser or alter the transmitted power at a very high frequency. Basically, they function by sending the laser beam through a crystal which is subjected to acoustic vibrations. These vibrations alter the refractive index of the material, so the beam can be redirected from the output aperture to a beam dump in an incredibly small amount of time. The rise/fall time is limited only by the amount of time it takes for the acoustic wave to traverse the width of the crystal, so this is usually on the order of nanoseconds. An AOM can modulate a laser with essentially any arbitrary input waveform, with a response frequency in the MHz domain.

Advantages

• Laser operates continuously, so pulse shape is clean and consistent.
• Maximum modulation frequency is very high (many MHz).
• Rise/fall time is very low (ns).
• Most feature full extinction.
• Works well with low-noise or SLM lasers.
• Pulses and transmitted power are individually controlled with fully customizable timing.

Disadvantages

• Costly: The least expensive systems start in the thousands.
• Transmission efficiency is less than 100%. Average figures are around 80% or so, but this varies widely by type of AOM and laser being used.
• High-frequency acoustic noise is not detectable to humans but may affect animals and sensitive equipment.
• Because the laser is passing through a material, damage threshold can be an issue with very powerful lasers.
• Input beam diameter is limited to a few mm.