Acousto-Optic Modulation And Q-Switching - Spectra-Physics Pulseo User Manual

Acousto-Optic Modulation and Q-Switching

An acousto-optic modulator (AOM) is a block of fused silica that acts as an
optical phase grating when vibrated by an ultrasonic wave. The photoelas-
tic effect describes how the strain field produced in the material by the
ultrasonic wave changes the optical index of refraction in the block. A
standing wave of ultrasound will result in an optical grating has a period
and an amplitude set by the acoustic (ultrasonic) wavelength.
When a light beam is incident upon this grating, a portion of its intensity is
diffracted out of the beam. Placing the AOM inside of a laser cavity pro-
duces a type of light switch that can either allow laser action to proceed or
halt laser activity.
By choosing beam parameters properly, any laser beam that attempts to cir-
culate within the resonator experiences a diffraction loss that is sufficient to
prevent lasing (i.e. there is no circulating beam). The otherwise low loss
("high Q") design of the resonator has been switched to a high loss ("low
Q") condition.
With no circulating laser light available to pass through the laser gain
medium, the pump energy boosts the gain to a much higher level than
would otherwise be present. A long lifetime for the upper-state laser level
is beneficial to the Q-switch process.
The ultrasonic wave is impressed on the AOM by a piezo-electric trans-
ducer. Switching off the driving voltage to the transducer returns the AOM
to its passive state of high optical transmission, and the laser resonator is
returned to its high Q state. The internal beam is no longer deflected, but
instead is amplified by the high gain now available in the laser rod, and a
powerful "Q-switched" laser pulse is emitted.
Voltage is then re-applied to the AOM transducer, which again spoils the
cavity Q and allows the gain to rebuild to a high level. This process is
repeated at the frequency at which pulsed laser output is desired, taking
into consideration the characteristics of the laser. The result is a concentra-
tion of the otherwise continuos laser beam into pulses of extremely high
peak power.
Beam directed
out of cavity
Beam directed
to other cavity
end mirror
Figure B-7: An Intracavity Acousto-Optic Modulator
A Brief Review of Laser Theory
AOM on
AOM off
RF Drive
Transducer
Cavity
End
Mirror
B-9