|
|
|
|
|
Silicon Optical Modulator
When you use a laser to produce light, the output is a continuous wave. So you need an
optical modulator to encode data for use in an optical communications link. An optical
modulator is a device in which a signal-controlled element displaying electro-optic
effect is used to modulate a beam of light. The modulation may be imposed on the phase,
frequency, amplitude, or direction of the modulated beam. Modulation bandwidths extending
into the gigahertz range are possible with the use of laser-controlled modulators.
It modulates or varies the amplitude of an optical signal in a controlled manner.
An optical modulator generates desired intensity, color and the like in the passing light
by changing optical parameters such as the transmission factor, refractive index,
reflection factor, degree of deflection and coherency of light in the optical system
according to the modulating signal. Optical modulators are one of the key devices for
realizing such high-performance optical networks. In recent years, enormous volumes of
data in communications have been transmitted through high-performance information and
communication instruments so that it becomes essential to popularize widespread optical
communication networks using optical fibers. In the optical communication networks,
high-speed semiconductor lasers or the like are used as key devices thereof, and
semiconductor optical modulators are also used for modulating input light beams
generated by the semiconductor lasers.
Digital communications employing laser beams as optical carriers generally having carrier
frequencies in the terahertz range that permit extremely high modulation bandwidths.
Much of the optical communications network in place utilizes optical fibers. Optical fiber
transmission has played a key role in increasing the bandwidth of telecommunications
networks. Optical fiber offers much higher bandwidths than copper cable and is less
susceptible to various types of electromagnetic interferences and other undesirable
effects. As a result, optical fiber is the preferred medium for transmission of data at
high data rates and over long distances. One property of optical fibers that is of concern
is dispersion. Dispersion in optical fiber occurs as a result of variation in the refractive
index of the optical fiber with wavelength. Modulation of an optical signal results in
optical harmonics of the modulation frequency about the carrier frequency. The optical
carrier signal is modulated by an RF information signal or many information signals on
respective sub-carriers.
One common method of modulation uses an optical modulator consisting of complementary
electrically responsive optical phase shifters in each of two branches of an interferometer
such as a Mach-Zehnder interferometer. The optical modulator is interposed between the
optical carrier source (a laser) and the communication channel (a fiber optic cable).
Optical modulators have many different uses in optical communication systems. For example,
high-speed optical modulators are used to encode information into an optical signal
generated by an optical source such as an optical laser, where the information is
represented by changes in the amplitude of the optical signal. A low-speed optical modulator,
also referred to as an optical attenuator, may be used in conjunction with an optical
amplifier in the amplifier stage of an optical communication system. The optical attenuator
controls the overall gain of the amplifier stage to account for gradual changes in the
received optical signal, for example, as the optical source ages.
|
|
|
|
Semiconductor Materials Research Laboratory, Department of Materials Science and Engineering, University of California at Los Angeles
Box 951595, Los Angeles, CA 90095-1595 (Tel) +1 310 825 2971 UCLA SMRL © 2006 | All Rights Reserved
|
|
|
|
|
|
