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Acousto-optic glossary

In acousto-optics, many technical terms are used to describe how components work. This page provides definitions of the main terms used, for greater clarity.

Bragg cell:

A device using a bulk acousto-optic interaction (eg. deflectors, modulators, etc…).

“Zero” order,”1st” order:

The zero order is the beam directly transmitted through the cell. The first order is the diffracted beam generated when the laser beam interacts with the acoustic wave.

Bragg angle (⊖B):

The particular angle of incidence (between the incident beam and the acoustic wave) which gives efficient diffraction into a single diffracted order. This angle will depend on the wavelength and the RF frequency.

Separation angle (⊖):

The angle between the zero order and the first order.

RF bandwidth: (∆F)

For a given orientation and optical wavelength there is a particular RF frequency which matches the Bragg criteria. However, there will be a range of frequencies for which the situation is still close enough to optimum for diffraction still to be efficient. This RF bandwidth determines, for instance, the scan angle of a deflector or the tuning range of an AOTF.

Maximum deflection angle: (∆⊖)

The angle through which the first order beam will scan when the RF frequency is varied across
the full RF bandwidth.

Rise time: (TR)

Proportional to the time the acoustic wave takes to cross the laser beam and, therefore, the time it takes the beam to respond to a change in the RF signal. The rise time can be reduced by reducing the beam’s width.

3dB Analog modulation bandwidth: (∆Fmod)

The maximum frequency at which the light beam can be amplitude modulated. It is related to the rise time – and can be increased by reducing the diameter of the laser beam.

Diffraction efficiency: (η)

The fraction of the zeroth order beam which can be diffracted into the “1st” order beam.

Static extinction ratio: (SER)

The ratio between maximum and minimum light intensity in the “1st” order beam, when the acoustic wave is “on” and “off” respectively.

Frequency shift: (F)

The difference in frequency between the diffracted and incident light beams. This shift is equal to the acoustic frequency and can be a shift up or down depending on orientation.

Resolution: (N)

The number of resolvable points, which a deflector can generate – corresponding to the maximum number of separate positions of the diffracted light beam – as defined by the Rayleigh criterion.

RF power: (PRF)

The electrical power delivered by the driver, which will be converted into acoustic power by the transducer.

Acoustic power: (Pac)

The acoustic power generated in the crystal by the piezo-electric transducer. This will be lower than the RF power as the electro-mechanical conversion ratio is lower than 1.