Acousto-optical modulators (AOM)

 

An acousto-optic modulator (AOM) is a device which can be used for controlling the power, frequency or spatial direction of a laser beam with an electrical drive signal.

Acousto-optic modulators find many applications:

- Q switching of solid-state lasers. The AOM, called Q-switch, serves to block the laser resonator before the pulse is generated. In most cases, the zero-order (not diffracted) beam is used under lasing conditions, and the AOM is turned on when lasing should be prohibited.

- AOMs for cavity dumping of solid-state lasers, generating either nanosecond or ultra-short pulses.

- AOM as a pulse picker for reducing the pulse repetition rate of a pulse train, e.g. in order to allow for subsequent amplification of pulses to high pulse energies.

- In laser printers and other devices, an AOM can be used for modulating the power of a laser beam. The modulation may be continuous or digital (on/off).

Main Acousto-optic modulation principles

AOM is based on the acousto-optic effect, i.e. the modification of the refractive index by the oscillating mechanical pressure of a sound wave.

 

The acousto-optic interaction has also been used to modulate light. In order to match the Bragg condition over the modulator bandwidth, the acoustic beam should be made narrow, as in the case of deflectors. Unlike the case of deflectors, however, the optical beam should also have a divergence approximately equal to that of the acoustic beam, so that the carrier and the sidebands in the diffracted light will mix collinearly at the detector to give the intensity modulation. The actual value of the divergence ratio depends on the tradeoff between desired efficiency and modulation bandwidth. Acousto-optic modulation can broaden the dimension of the diffracted beam with respect to the incident beam because of the finite sound field depth L (transducer length) and the oblique propagation angle needed for maximum diffraction in the Bragg regime. 

 

 

In practice it is appeared by the crystal (and transducer) length variation to select the best compromise to solve targeted matters.

One more variant - using focusing optics to divergent optical beam. Diffraction geometry of a focused-beam AO modulator is shown below:

 

 

The focused-beam-type AO modulator has certain disadvantages. The diffraction spread associated with the narrow optical beam tends to lower the diffraction efficiency. More importantly, the focusing of the incident beam results in a high peak intensity that can cause optical damage for even relatively low laser power levels. For these reasons, it is desirable to open up the optical aperture. Due to the basic issue of acoustic transit time, the temporal bandwidth of the wide-beam AO modulator will be severely degraded. In certain applications, such as the laser display system, it is possible to use a much broader optical beam in the modulator than that which would be allowed by the transit time limitation.

 

АОM main characteristics

Typical values for TeO2 modulators

Optical Wavelength Range

514nm, 633nm, 1064nm, 1330nm

Optical Aperture

0.3 mm - 3 mm

Operating Mode

Longitudinal, axis (001)

Optical Rise Time

9-200 nsec on beam diametr

Beam Separation (633 nm)

10-30 mrad

Diffraction Efficiency

70-85 %

Modulation Frequency (-3db)

6-50 MHz

 

We offer cells’ blanks for modulators with various interaction length according to your particular requirements.

 

 

 Every blank contain 5-6 elements to arrange optimum processing (polishing, AR coating, transducer bounding).  

 Also we’d like to offer elements with AR coating and one golden electrode ready for bounding.

 

Note

- Upon your request we can supply blanks with any linear dimensions up to 70mm

- Upon your request we can offer AR or protective coating.

- Blanks for AOD & AOTF are also available

- Blanks for transducers from LiNbO3 crystals with different orientations are also available

 

Reference list:

[1] Xu J and Stroud R 1992 Acousto-Optic Devices (New York:Wiley)

[2] Handbook of optics. CHAPTER 12 ACOUSTO-OPTIC DEVICES AND APPLICATIONS I. C. Chang

[3] Goutzoulis A and Pape D 1994 Design and Fabrication of Acousto-Optic Devices (New York: Dekker)

Events/News

14th - 16th March 2017 - visit us at "Laser World of Photonics China 2017" fair in Shanghai, China. Hall 1 Booth 1735.

22 - 25 June 2015

June 2015 - visit us at "Laser World of Photonics 2015" fair in Munich, Germany.

Booth B1.439.

 

January 2015 - new edition of OptoIndex catalogue was published. You can find our company page here.