What is an Acousto-Optic Modulator?

An Acousto-Optic Modulator (AOM) is a device that uses sound waves to control the transmission of light through a material. It is an intensity modulator that works on the principle of the acousto-optic effect i.e. when an acoustic wave is applied to a crystal or glass material, a change in the refractive index of the material is caused. This change in the refractive index causes a change in the direction and intensity of the light passing through the material. 

Working of an Acousto-Optic Modulator

Figure 1: Internal diagram of an acousto-optic modulator

In AOM, a piezoelectric transducer is attached inside the acousto optic modulator that produces sound waves in a material like glass or quartz. When a radio frequency (RF) drive signal is applied to the transducer, an acoustic wave or sound wave is generated. The AOM transducer stimulates a sound wave with an order of frequency up to 100 MHz, acoustic wavelengths of 10 μm -100 μm, and acoustic power of the order of 1 W - 10 W. These sound waves create periodic planes of compression and rarefaction that change the index of refraction as shown in figure 2. As a result, the incoming light beam is diffracted into a number of orders at the output. An absorber is used at the opposite end of the transducer to direct all the acoustic waves toward the absorbing surface. The absorber prevents reflections that create secondary waves. A typical working of an AOM is shown in figure 1 where the transducer produces a sound wave and the light ray is partly diffracted.

An AOM, also known as bragg cell operates under Bragg’s condition i.e. the incident light comes at a Bragg angle from the direction perpendicular to the direction of propagation of the sound wave. Diffraction orders of -1 (low intensity), 0 (no change in light intensity), and +1 (high intensity) are obtained when there is an occurrence of diffraction from a sinusoidal modulation in a thin crystal. 

The diffracted beam is shifted in frequency with the incident laser beam by an amount equal to RF frequency. If f is the RF frequency and fL is the laser frequency, then the diffracted light beam has frequencies fL+f or fL-f and this frequency shift depends upon the orientation of the acousto optic modulator.

Most AOMs operate in the Bragg condition, where there is a significant diffraction efficiency for the first order diffraction and hardly any diffraction into other orders. Therefore, the deflection is typically in the range of tens of milliradians.

Parameters of an Acousto Optic Modulator

• A typical AOM has an aperture size, of millimeters. It has rise time, which denotes the overall speed of the device. This is the amount of time required for a signal to change to full power. 
• An AOM has insertion loss, which signifies the loss caused by the insertion of an optical component. 
• The insertion modulation bandwidth is an important specification of AOM sometimes called tuning bandwidth or video bandwidth is the input (sinewave) modulation frequency at which frequency deviation reduces to -3 dB of its direct current (DC) value.

AOMs are generally rack-mounted and have a fiber pigtail attached and the devices attached to them are self-supporting (does require an additional system) and reserve the incoming signal polarization.

A continuously varying amplitude modulation signal and pulsed digital amplitude modulation signal carried by a traveling acoustic wave is shown in figure 2.

Figure 2: (a) Continuously varying amplitude modulation signal and (b) pulsed digital amplitude modulation signal

Applications of Acousto-Optic Modulator

One of the most common applications of acousto optic modulators is laser frequency modulation. Acousto optic modulators can be used to rapidly and accurately modulate the frequency of a laser beam. This is useful in many applications, such as in laser spectroscopy, where precise control over the frequency of the laser is essential. They are also used to create frequency combs, which are used in a wide range of applications, including precision metrology, telecommunications, and high-speed data transmission.

Another important application of acousto optic modulators is in laser beam steering. They can be used to deflect laser beams in a controlled manner, which is useful in applications such as laser machining and material processing, where precise control over the position of the laser beam is essential. They are also used in laser scanning microscopy, where they are used to rapidly scan the laser beam across a sample.

Acousto optic modulators are used in optical signal processing applications. They can be used to modulate the intensity and phase of optical signals, which is useful in applications such as optical communications, where precise control over the signal properties is essential. AOMs can also be used in optical switching and routing applications, where they are used to direct optical signals to specific locations.

They are also used in acousto-optic imaging, a technique that combines ultrasound and optical imaging to produce high-resolution images of biological tissues. In acousto-optic imaging, an ultrasound wave is used to generate a small displacement in the tissue, which is then detected by an optical probe. AOMs are used to modulate the optical signal, which allows for precise control over the depth of the image and the resolution.

These modulators are also used in optical trapping and manipulation applications, where they are used to create optical tweezers that can trap and manipulate small particles, such as cells and bacteria. In optical trapping, a tightly focused laser beam is used to trap a particle, while an AOM is used to modulate the intensity and phase of the laser beam, which allows for precise control over the position and movement of the particle.

Acousto optic modulators are also used in a variety of applications like laser printers, video recorders, and video projection systems. They are used in lasers for Q-switching i.e, to vary and control laser beam intensity.