Phase Modulators

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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.

Applications

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 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.

More to Learn

Suppliers

The RP Photonics Buyer's Guide contains 17 suppliers for phase modulators. Among them:

ALPHALAS

Using KTP, RTP and LiNbO₃ crystals, ALPHALAS offers phase modulators for various applications like active mode locking, side-frequency generation and laser resonator cavity control. In addition to the standard products designed for 1064 nm, other customer-specific wavelengths can be offered as well.

Exail

Designed using state-of-the-art and proven lithium niobate technology, the MPZ-LN phase modulators are easy to operate and to integrate. Available at both 1550 nm and 1310 nm, with low insertion loss and high polarization extinction ratio, either AC and DC coupled for RAM and pyroelectrical effects control, these modulators offer the optimum for industrial Quantum Key Distribution (QKD) systems.

Exail near-infrared phase modulators NIR-MPX are a modulator family ranging from DC-coupled low-frequency modulators to wide bandwidths up to 30 GHz, requiring very low driving voltages and offering incomparable 300 mW optical powers handling capabilities. These products benefit from the technological mastery of the "Annealed Proton Exchange" (APE) waveguide manufacturing process. These modulators find a lot of interest in the laser application, such as Pound–Drever–Hall cavity locking as well as Spectral Beam Combining (SBC) and directed and high energy material processing lasers.

The optical spectral broadening ModBox-OSB can broaden an optical signal by modulating its phase with a very efficient LiNbO₃ phase modulator. A number of sidebands are created over a spectral width that can reach several hundred GHz. The ModBox-OSB contains a single tone RF generator, a white noise source or a PRBS source. It well integrates into laser chains such as High Energy Lasers (HEL) lasers or Spectral Beam Combining lasers architectures (SBC).

Hamamatsu Photonics

Hamamatsu Photonics LCOS-SLMs are reflective spatial light phase modulators that freely modulate optical phases and optical phase of laser is modulated by the liquid crystal. Wavefront control of the light can be applied to optical beam photolithography, aberration correction.

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