Definition: devices for manipulating the phase of a laser beam
More general term: optical modulators
Author: Dr. Rüdiger Paschotta
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A phase modulator is an optical modulator which can be used to control the optical phase of a laser beam.
Frequently used types of phase modulators are electro-optic modulators based on Pockels cells, and liquid crystal modulators, but it is also possible e.g. to exploit thermally induced refractive index changes or length changes e.g. of an optical fiber, or induce length changes by stretching. Various kinds of phase modulators are used within the area of integrated optics, where the modulated light propagates in waveguides.
Important properties of phase modulators are:
- the amount of phase modulation which can be achieved (determining the possible modulation index and relative power in optical sidebands)
- the required drive voltage (see the article on Pockels cell drivers)
- the modulation bandwidth (range of modulation frequencies), which can e.g. be many gigahertz for electro-optic modulators, and far less for devices based on thermal effects or using liquid crystal materials
- the optical bandwidth in which the device can be used
- the device aperture, limiting the beam radius of the modulated beam
- the outer dimensions of the device
For different types of phase modulators, such properties vary in huge ranges. Therefore, different kinds of phase modulators are appropriate for different applications.
In special cases, additional properties of a phase modulator may be relevant. For example, a phase modulator may also exhibit some small amount of residual amplitude modulation, which can be disturbing e.g. in certain ultrasensitive measurements, and may need to be minimized with additional measures.
Phase Modulators with Spatial Control
A special class of phase modulators can not only modulate the optical phase of a beam as a whole, but apply controlled phase changes with a certain spatial resolution. Such devices are needed in particular for adaptive optics. More clear terms for such devices are spatial light modulator and wavefront modulator.
Some examples of applications of phase modulators are:
- A phase modulator within a laser resonator of a single-frequency laser can be used for wavelength tuning, or for active mode locking (FM mode locking) of a laser.
- A phase modulation of a beam with moderate modulation strength is often used in laser frequency stabilization schemes, e.g. with the Pound–Drever–Hall method.
- Various kinds of interferometers and setups for spectroscopic measurements require phase modulators, often with a periodic drive signal.
- Some optical metrology applications require frequency combs, generated by sending a single-frequency beam into a phase modulator. In that case, the phase modulation often needs to be strong in order to obtain a large number of optical sidebands.
- In data transmitters of optical fiber communication systems, phase modulators can be used for encoding the transmitted information. An example is the method of phase shift keying.
The RP Photonics Buyer's Guide contains 16 suppliers for phase modulators. Among them:
See also: optical phase, optical modulators, electro-optic modulators, liquid crystal modulators, integrated optics
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