Optical modulator

A modulator is an optical component to direct light to impart a defined characteristic. This may for example be a temporal or spatial variation of amplitude or phase.

Application Examples

Applications are modulators in optical spectrometers, to impart a characteristic of the measurement light so that it can be better separated after passage through the spectrometer of the ambient light. The ambient light usually means a certain constant light component plus a share out of the room lighting, which is modulated with the 50 Hz (or 60 Hz) of the power system and hence in intensity at 100 Hz (or 120 Hz). The latter effect can also be easily couple to electrical means in the receiver signal, and can be eliminated in this way. Also disruptive pure direct light can be eliminated by AC evaluation of the sensor signal. For example, help lock-in amplifier to amplify only the signal component at the frequency of modulation and filter out contributions from direct light or with other frequencies.

Further applications are in laser material processing or laser distance measurement.

When it comes to news technical applications, are modulators to a support beam to modulate an information or data signal - analog or digital. The modulated light is typically passed through a fiber optic cable to the receiver.

Modulator types

The following optical modulators are commonly used:

• Acousto-optic modulator (AOM ): the laser beam is diffracted by ultrasonic waves

• Electro-optic modulators (EOM ) - also known as electro-optical switch called (EOS) or polarization modulators - change phase or polarization: Kerr cells
• Pockels cells
• Liquid crystal display

• Wavelength modulators for example consist of oscillating scanning mirrors mounted in the beam path within a monochromator, for example in dispersive optical spectrometers

• Chopper (rotating sector shutter for intensity modulation )

• Polarization modulators

• Interferometric optical modulators: Mach-Zehnder modulator,
• For modulation of laser beams within or outside the resonator (the principle of the Fabry -Perot interferometer: partially transparent plane-parallel mirrors, that are moved to each other)

• Piezo- modulation (see below)

Chopper

In optics, a chopper ( dt pulse shaper ) is usually a rotating mirror shutter for intensity modulation. It modulates the light in its intensity rectangular between 0% and 100 %. Similar modulations can be achieved by rotating mirrors, angle mirrors or prisms.

Piezo - modulation

Wherein the piezo - modulation, in contrast to other types of modulation is not the measurement light is affected, but the sample is mounted on a piezoelectric crystal and mechanically stretched and compressed, whereby it will, if necessary, to adjust the direction of the mechanical stress in a plurality of orientations on a single-crystal sample. Characterized the degree of ( pre-existing ) birefringence, and therefore the Reflexions-/Transmissionverhalten the sample is modulated and thus the measurement signal. The tension load on the sample by the piezoelectric modulator is accurately applied in parallel or perpendicular to the optical axis of the material. This method is mainly used for measurements where the birefringence is only caused by an external magnetic field, ie in the magneto-optics.

The piezo - modulation provides, as partly also the polarization modulation, sinusoidal instead of rectangular modulations, which of course must be considered in the evaluation of the receiver signal.

See also photoelasticity

Evaluation

For evaluating the receiver signal is usually used a lock-in amplifier, which also receives the modulation signal adjacent to the signal receiver, and therefore the useful signal can be separated from the substrate.

Applications

Laser beam modulators are used in both message transmission as well as to research and materials processing purposes. The modulation can within the laser cavity (so-called Q-switch (English q- switch) ) take place in or outside the laser beam.

Diode laser ( Telecommunications, Printing, CD burners ) usually do not have a separate modulator, but are modulated by the modulation of the operating current.

In liquid crystal displays ( LCD) are also located electro-optic modulators: they rotate the plane of polarization of light, the backlight and work together with polarizing filters.