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How Do Bragg Cells Work in Acoustic-Optic Applications?

An acousto-optic modulator uses sound waves to modify the frequency, wavelength, polarization and direction of an optical beam. AOMs have many uses including regenerative amplifiers, Q-switching and mode locking ultrafast lasers.

Opposite of nonsectioned transducers, where light diffraction only takes place at one incidence angle, the transfer function of an AO cell has several maxima symmetrically situated around its Bragg angle.

How Bragg Cells Work

An acousto-optic (AO) device works on the principle of light being scattered by acoustic waves, producing an optically transparent medium onto which piezoelectric transducers have been attached. When powered up, these transducers convert an input radio-frequency (RF) signal into traveling acoustic waves within this medium – by altering frequency or amplitude, an optical beam can be deflected or modulated using this device.

As the frequency changes, so too does the direction of the diffracted beam due to changes in refractive index of acoustic waves in medium. An optimal interaction geometry is key in an AO system and various techniques have been devised to maximize performance of this device. An anisotropic acousto-optic diffraction employs a phased array transducer with antiphase excitation of individual sections – this approach produces normalized angular characteristics symmetrical around Bragg angles as compared to conventional non-sectioned transducer versions.

Acoustic waves produced by an array of interdigital transducers are guided along a spatially periodic acoustic waveguide with phase spacing and step size that are optimized to form blazed phase gratings at Bragg angle, satisfying both NPM and TPM requirements while permitting large angular aperture and high modulation bandwidth. This arrangement simultaneously fulfills NPM and TPM conditions and facilitates large modulation bandwidths.

An ideal acousto-optic system relies on a monoclinic KY(WO4)2 crystal substrate with high directional selectivity and wide bandpass transmission capabilities. Autocollimation of the acoustic wave helps achieve high directional selectivity for incident and output acoustic waves and supports TPM conditions with large input/output apertures.

As part of its acousto-optic deflection and modulation applications, this invention uses an acousto-optic Bragg cell’s acoustic waveguide to direct incident acoustic waves towards an appropriate mode in its crystal. These modes could either be Shear-mode or Raman-Nath modes; typically Shear mode waves interact with crystal surfaces directly for routing purposes.

Optical Waves

The acousto-optic effect is an interaction between light and mechanical vibrations, and can be most readily observed through fiber Bragg gratings which respond to strain by shifting their period and thus index of refraction, with laser beams producing visible optical signals; other applications produce subtler responses. As vibrations in crystals have different frequencies depending on frequency of vibrations produced through an acousto-optic effect this allows users to control or modulate optical beams using this mechanism.

An AO Bragg cell utilizes the acousto-optic effect to diffract and shift an optical beam using sound waves (usually at radio-frequency). These devices are widely utilized across industries including lasers for Q-switching, telecommuting for signal modulation, and spectroscopy for frequency control. Warsash Scientific offers an assortment of AO modulators designed to fit different uses.

When laser light is diffracted using an acousto-optic Bragg cell, its frequency will be altered by the sound waves in the crystal due to scattering process in which light rays from moving planes scatter light – unlike regular Bragg diffraction where only two conditions must be fulfilled: wavelength and refractive index are met. Acoustic modulators require all these conditions be met for effective operation.

At an acousto-optic modulator, wide angular aperture is of paramount importance; as more tightly collimated the incident laser beam becomes, so greater will be its capacity to produce distinct resolvable spots by an acousto-optic modulator.

There are some restrictions associated with this device. For instance, sound waves must have frequencies greater than their wavelength in order to produce enough energy at the resonator for Doppler shifting to occur. Furthermore, its size must allow sufficient room for accommodating acoustic vibrations without decreasing efficiency.

However, these are minor problems that can be mitigated through careful design of acousto-optic crystals. Particularly important is matching resonator size and shape with that of the crystal in order to minimize losses and maximize diffraction efficiency, along with selecting materials and orientation that fulfill these criteria.

Acoustic Waves

Interaction between an acoustic wave and a Bragg cell can occur in one of two distinct ways. One approach sees longitudinal acoustic waves traveling along the plane (001) of the crystal; this form of acousto-optic interaction is also known as isotropic. Here, both waves and incident laser beam experience identical refractive index values for maximum performance with low power dissipation.

A second acousto-optic mode occurs when shear acoustic waves are excited at nonzero angles to the crystal plane, leading to different refractive indices for them and incident laser beam. As a result, shear acoustic waves shift relative to each other by an integer multiple of their frequency relative to incident laser beam; this phenomenon is known as Raman-Nath diffraction; it typically takes place when angles between incident laser beam and shear acoustic waves do not match those associated with Bragg angles.

Acoustic-optic devices tend to be more effective when shear acoustic waves occur in isotropic mode due to their slower velocity resulting in reduced energy dissipation.

An integrated acousto-optic Bragg cell consists of an optical waveguide 41 attached to an optically birefringent crystal substrate 42 such as lithium niobate, as well as a phased array 43 of interdigital piezoelectric transducers deposited on its surface. All individual transducer elements in this array are linked together via phase delays of various lengths to produce spatially periodic output waves with an acoustic wavevector called “ka”.

This acoustic wave interacts with laser light through a periodical acousto-optic grating. More specifically, all transducer outputs interact constructively to form an acoustic field with an inclined phase grating as a result of combined phase shifts from all transducers’ outputs; this phase grating fulfills Bragg conditions and achieves very high efficiency and large time aperture.


Piezoelectric transducers generate the acoustic waves in a cavity by converting electrical energy to mechanical energy that then leads to shear acoustic modes with frequencies typically in the range of 0.1-50MHz depending on their geometry and crystal thickness.

Transducer geometry also impacts the frequency characteristics of an acousto-optic interaction. A nonsectioned transducer only exhibits limited angles at which acousto-optic interactions take place; however, antiphase excitation between adjacent sections may help increase this number for given incident angles.

An excitation point for shear modes in paratellurite (TeO2) Bragg cells is at an intersection between 9deg to the crystal plane and optical waveguide plane, where nine antiphase sections form an array that produces spatially periodic wavefields with frequency characteristics that depend on where their intersection occurs with diffraction orders, as well as angular characteristics governed by interconnected sections.

FIG. 5 depicts a preferred configuration for a multi-resolution acousto-optic (AO) Bragg cell. This arrangement includes an acoustic medium 31 and planar acoustic phased array 32 with multiple transducer elements 33 mounted closely against it on one surface 34; these transducers release an acoustic wave 35 along their wavevector ka.

Acoustical phased arrays are typically configured so that individual acoustic wave vectors of transducer element groups and of the array’s vector kb are closely aligned. This ensures that an acoustic wavevector of order m is tangent to its locus in an acoustically porous medium and parallel with any diffracted light ray, and also ensures the alignment between an array’s locus (kc) and that of an ordered diffracted light beam.

At its heart is a wide spectrum of incident angles for which acoustic mode interaction with diffraction order occurs, providing a large modulation bandwidth with many resolvable spots at particular incident angles depending on frequency of the RF signal that must meet input acoustic group velocity constraints as well as time aperture limitations caused by walkoff acoustic walkoff.
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