Acousto-optic modulator

An acousto-optic modulator AOM abbreviated, is an optical component that affects the incident light and the direction of propagation in frequency or intensity. For this purpose, an optical grating is formed in a transparent solid with sound waves. In this mesh of the light beam is bent and displaced in frequency at the same time. Acousto-optical modulators, which are used for deflection of the light are also known as Bragg cells.

Operation of an acousto-optic modulator

An acousto-optic modulator consists of a transparent solid body, glass or a crystal, in which a piezoelectric element is attached to the generation of sound waves at one end. At the other end there is a sound absorber, to avoid reflections and standing waves.

The deflection of light into an acousto-optic modulator works on the principle of the diffraction of light by a diffraction grating. The optical grating consists in the density fluctuations of the crystal continuous sound wave.

The acoustic wave with the signal frequencies, typically in the MHz crystal causes a periodic change in the density, and thus a periodic modulation of the refractive index. The spacing of the "lines" of this grating is equal to the wavelength of the ultrasonic wave and is calculated over the speed of sound and the sound frequency f, by

Usually LiNbO3 or PbMoO4 is used for the crystal. Typical speeds of sound in such crystals amounted to 3700-4300 m / s At a frequency of 200 MHz, the lattice constants yield of 18.5 to 21.5 microns. These are typical values ​​, but depend on the used ultrasonic frequency, the medium used and the speed of sound.

Typically, the cross section of the incident light beam is significantly larger than the period of the refractive index modulation, and the light speed is very much greater than the speed of sound.

Then it can be assumed approximately such that the light provides a static refractive index modulation and a constructive interference of light to the Bragg angle

Receives, wherein the wavelength of light in the crystal, and the period of the refractive index modulation is.

The scattered light undergoes a Doppler frequency shift of the frequency of the ultrasound. The process is similar to the reflection on a moving mirror.

Another, equivalent thereto approach considers the sound wave in the solid state as phonons, which interact with the photons of light. The deflection of the light results from the fact that the pulse of the phonons is added to the momentum of the photons vectorially:

Here is Planck 's constant divided by and the wave vector of the photons or phonons. In this approach follows from the conservation of energy that changes the frequency of the light by the frequency of the sound wave by the interaction:

Here is the Planck constant and the frequency of the light, ie photons. The term refers to the frequency of the sound wave. The frequency of the light will be moved to the frequency of the sound wave.

The frequency shift of the light is very small relative to the frequency of the light waves, because the ultrasonic frequency ( ≈ 107 ... 109 Hz ) is substantially smaller than the frequency of the light ( > 1014 Hz). However, it is essential for some applications.

Applications

The applications can be divided into the areas of fast switching, deflection, amplitude modulation (periodic losses ), frequency modulation ( frequency shift ) and frequency selection:

  • Fast switching: Q-switching in pulsed lasers.
  • Blanking on scanning methods, such as certain methods of photolithography or laser shows.
  • Periodic losses: Introducing periodic losses in a laser resonator, active mode locking of the laser, provides pulsed operation.
  • Distraction: Electrically controllable deflection of a laser beam, for example, for automatic adjustment of the beam position. In the best case, the maximum used for the first order () about 90% of the beam intensity; the rest is lost (not distracted or higher orders )
  • Intensity variation of the laser beam deflection in an absorber. This is due to the fact that low intensity of the acoustic wave at the deflected portion of the beam is proportional to the sound intensity.
  • Frequency shift: of the laser light for heterodyne interferometer,
  • Of the laser light for the generation of current interference pattern between the original laser beam and the frequency-shifted by the AOM the laser beam,
  • Of the laser light for applications in high resolution spectroscopy and manipulation of atoms in ion traps,
  • Of the laser light in the laser Doppler velocimetry to determine the direction of movement.
  • Frequency selection: Selection of the color for color laser shows.
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