MODULATORS

An acousto-optic modulator (AOM) uses sound waves within a crystal to create a diffraction grating. As the power of the applied RF signal is varied, the amount of diffracted light varies proportionally. Acousto-optic modulators can be used like a shutter (cycling light on and off at a set frequency), or as a variable attenuator (controlling the intensity of transmitted light dynamically).

The most important factor in selecting an acousto-optic modulator (AOM) is the required speed. This influences the choice of material, AOM design, and RF driver to be used. The speed of an acousto-optic modulator is described by the rise time, which determines how quickly the acousto-optic modulator can respond to the applied RF driver, and limits the modulation rate. Rise time is proportional to the time required for the acoustic wave to traverse the optical beam and for this reason is influenced by the beam diameter within the AOM. 

Acousto-optic modulators fall into two general categories as regards speed. Very fast modulators can provide modulation frequencies up to ~70 MHz and can have a rise time as low as 4 ns. The input beam must be focused very tightly focused into the acousto-optic modulator to achieve this speed. Lower frequency modulators do not have this constraint, however, and can accept larger input beams. Their rise time is usually specified relative to the input beam diameter, in ns/mm.  

In addition to speed, we also consider other selection criteria when identifying the right acousto-optic modulator and RF driver:

• wavelength of operation
• optical power
• type of modulation needed (analog or digital)
• beam diameter
• desired contrast ratio
• light polarization

Most applications require high contrast between the “on” and “off” states of the modulator, and thus make use of the first order diffracted beam. This results in extinction ratios of 40 dB and higher, but results in lower throughput of the deflected beam (typically 85-90%). In some applications such as intensity leveling, transmission is more important and a contrast ratio of ~10 dB is acceptable. This allows the undiffracted 0th order beam to be used, typically resulting in > 99% light throughput.

Applications:

• heterodyne interferometry
• intensity modulation
• laser cooling
• laser Doppler velocimetry
• laser Doppler vibrometry (LDV)
• laser linewidth measurements
• printing
• pulse picking
• marking
• material processing
• micromachining
• fiber amplifier noise measurements
• fiber transmission testing
• intensity leveling
• LIDAR