Acousto-Optic Resource Center

Learn More About Our Acousto-Optic Devices

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SOLUTION NOTES

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FREQUENTLY AKSED QUESTIONS

Acousto Optics (AO) is using the phase grating set up by sound waves inside a material to affect a light beam passing through the material. The sound wave is created by a piezeo-electric transducer bonded to the material (think: 'joy buzzer.'). The transducer is excited by a RF source.

An AOTF is a device, where putting a mixture of wavelengths into it, one wavelength can be separated (diffracted) out by electronic means. To clarify, except for minor transmission losses, all the light input into one side will be output to the other side; However, the chosen wavelength will be output at an angle different from that of the rest of the light. By means of an aperture, the 'rest of the light' (Zero Order) can be blocked, and the chosen wavelength will be the only light emitted. All the 'chosen light' (diffracted order, First Order) will be output at roughly the same angle. The 'electronic means' is a source of a specific RF frequency for a specific wavelength. The aperture, wavelength range, spectral resolution, and other possible desire data of a given AOTF depend on the details of its construction, and to some degree can be selected.

A Modulator is used to change the amplitude of the light signal passing through it. A Frequency Shifter gives a 'Doppler Shift' to the frequency of the beam passing through it. That said, a Modulator also gives the signal a small frequency shift, unless a No Frequency Shift model is purchased. Similarly, the amplitude to a Frequency Shifter can often be varied to amplitude modulate the beam passing through it. So to some extent they are interchangeable.

The carrier frequency (reading the model number left to right, generally the first number in the model number,) is the frequency shift given to the light beam. The frequency of light is so high (~ 10^15 Hz) that the shift imparted ( ~ 10^8 Hz) is generally negligible. It is the frequency of the grating the acoustics creates in the crystal.

Modulating is creating and collapsing the sound grating in the crystal; this must be done fast enough to support the modulation rates you want to achieve. In terms of specifications, the carrier frequency should be approximately four times the digital modulation bandwidth; the RF bandwidth quoted should be twice the digital modulation rate wanted, and the RF bandwidth should be approximately four times the analog bandwidth wanted. In terms of functional devices, the smaller the active aperture/beam size, the higher the modulation rates possible.

Except for Acousto-Optic Tunable Filters, the incoming light beam needs to be at the Bragg Angle for the light-acoustic interaction to occur in the crystal. The Bragg angle should be specified in the manual or can be calculated from: sin (Bragg angle) = (Wavelength x Frequency)/(2 x Acoustic velocity in crystal). You can set the device at the Bragg angle and then try translating, but generally the angle is small and set up imprecise, so the empirical way to do it is to put a slow (to the eye, 3 Hz works well) modulation on the carrier frequency, and watch for a blinking spot as the device is slowly rotated around the Bragg angle. The blinking is the First Order (diffracted spot) appearing, or the Zero Order dimming as energy is taken from it to the First Order. Once the angle is found at which the blinking occurs, (the 'Bragg angle',) the alignment can be optimized up/down, in/out/ and around.

The light beam inside the device should be parallel to the electrode to feel the full affect of the acoustic beam. Except in Brewster Cut devices, the electrode is in a normal (perpendicular) direction to the optical faces. Thus to start aligning, the incoming beam should retro-reflect from the front optical. If you have a sensitive source, you may not want the retro-reflection back into a source that could cause run away gain and burn out your source; if this is the case, have the reflection from the AO device to reflect to the side of the source. By the way, acousto-optics is by and large reversible, and it doesn’t matter which side of the AO device is 'In' and which side is used as 'Out.' You may see 2 retro-reflected spots, one from the front face and one from the back face; the faces are not parallel so the device can not act as an etalon.

Not necessarily. The Ohmmeter is meant to work with DC voltages; AO devices work at frequencies generally in the MHz or higher frequencies. Inside the AO device is a circuit that enables the RF power supplied to better excite the transducer; in the circuit may be inductors that show 'short' at low frequencies, and capacitors that show 'open' at low frequencies. The resistance/impedance is frequency dependent, and requires a RF Vector- or Scalar- Network Analyzer to measure.