Acousto-Optic Modulators

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Principle

The acousto-optic interaction

An AOM usually looks like a small box with two holes (typically around 50 x 22 x 15 mm), one for the incoming beam and one that projects the diffraction pattern out of the device.

Free space power modulation and shuttering using AOMs

An acousto-optic modulator (AOM) can advantageously replace other types of shuttering mechanisms (electronic shuttering of the laser, mechanical shutters, chopper wheels) and power modulation systems (analog modulation of the laser, filters and filter wheels). Basically an AOM will do both shuttering and power modulation, giving you any power (up to the theoretical limit of generally 80% of the incoming laser power) you want at any time. When the AOM is "OFF", that means no acoustic power is travelling through the crystal of the AOM, some light is still deflected. This means that the shuttering is not absolutely perfect. But the extinction ratio (ratio between maximum and minimum light intensity) is usually very high (meaning that very few light is deflected into the first order when the device is off). Typical extinction ratio are around 2000/1 (0.5‰ of the incoming light goes in the first order in the off mode). In practice, the power transmitted through the first order of an AOM in the off mode will be around 1 microW for an incoming laser beam around 80-100 mW.

This schematics is a proposal for an optical setup feeding light into 3 rigs using the same laser beam and 3 independent AOMs. AOMs become interesting when scaling up an optogenetic setup. You can buy a cheap powerful laser that will operate in constant emission (it needs to be stable enough but many affordable DPSS laser are, with typical RMS values of < 3% over hours), and the AOMs will do the job of modulating the shape and power of light pulses injected into the optical fibers. Such setups are used in several labs (Janelia Farm in Ashburn VA, Champalimaud Foundation in Lisbon Portugal).

Modulation of multiple wavelengths using AOTFSs

Glossary

  • 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: 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: 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: 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.
  • Modulation bandwidth: 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.
  • Efficiency: The fraction of the zero order beam which can be diffracted into the “1st” order beam.
  • Extinction ratio: The ratio between maximum and minimum light intensity in the “1st” order beam, when the acoustic wave is “on” and “off” respectively.
  • Frequency shift: 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: 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: The electrical power delivered by the driver.
  • Acoustic power: 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.

Links

References