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Acousto optic modulator working | Acousto optic modulator basics

This page covers Acousto optic modulator basics including its construction and describes Acousto optic modulator working operation.

Introduction:
Acousto optic modulator works based on Acousto optic effect. This effect deals with modulation of optical property through variation of refractive index of the medium under the influence of acoustic wave.

What is Acousto Optic Modulator?

Acousto Optic Modulator

• It works based on acousto-optic effect which deals with modulation of optical property by varying refractive index of medium under influence of acoustic wave.
• The refractive index variation under photoelastic effect is observed in all the materials. But materials such as LiNbO3, PbMoO4 etc. have shown potential applications.
• Fiber-coupled acousto optic modulators have become more popular. It has three terminals viz. laser input, laser output and RF Input.

Bragg condition Q-factor

• The Q-factor determines interaction regime between acoustic wave and laser beam. The condition of Q >> 1 is known as Bragg regime. In this condition, at one particular incidence angle ΘB, only one diffraction order is produced and rest of the orders are annihilated by destructive interference.
• Most of the acousto-optic devices operate in this Bragg regime.

Acousto optic modulator working operation

Acousto Optic Modulator and Driver

Following steps describes acousto optic modulator (AOM) working operation:
➨It works based on photo-elastic effect or interaction of acoustic sound waves and light signal in a crystalline material. An acoustic wave is generated in the crystal by application of RF drive signal into a piezo-electric transducer which is used at "RF IN" port.
➨Due to compression and refraction, periodic changes occur to refractive index. As a result light input signal is diffracted into number of orders at the output. AO modulators are travelling wave devices unlike diffraction grating.
➨Absorber is used at the opposite end of the "RF IN" terminal or port. It is placed to direct all the acoustic wave towards the absorbing surface. It is designed such that remaining reflections do not create any secondary diffraction.
➨Acousto optic modulators are designed to maximize diffraction of input laser beam into single 1st order position. In this condition, AO modulator device is said to be operating in the Bragg regime.
➨It requires correct alignment of the AO device and drive power is adjusted to be near or below the RF saturation level.
➨In the figure-2, Bragg angle is expressed as follows.
ΘBragg = (λ * f ) / (2 * V),
This angle is in the plane of deflection and relative to the normal from the input optical surface.
➨The output separation angle between the zeroth order and first order is expressed as follows.
Θsep = λ * f /V
Where,
f =RF frequency
λ = wavelength
V = Acoustic velocity
➨Frequency of laser output vary based on RF frequency ( 'f' )and laser frequency ( 'fL'). The frequency shift is equal to fL +/- f.



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