An optical DPSK demodulator is a device that provides a method for converting an optical
differential phase-shift keying (DPSK) signal to an intensity-keyed signal at the receiving end in
fiber-optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pr ...
networks. It is also known as
delay line interferometer A delay line interferometer (DLI) can be a Mach–Zehnder interferometer or Michelson interferometer based on two-beam interference, in which one beam is time-delayed to the other by a desired interval.
Delay line interferometers are also known as ...
(DLI), or simply called DPSK demodulator.
The DPSK decoding method is achieved by comparing the
phase of two sequential bits. An incoming DPSK optical signal is first split into two beams with equal intensities, in which one beam is delayed in space by an
optical path difference that introduces a
time delay corresponding to ''one bit''. The two beams in the two paths are then coherently recombined to
interfere each other constructively or destructively. The interference intensity is measured and becomes the intensity-keyed signal. A typical optical system for such a purpose is
Mach–Zehnder interferometer
The Mach–Zehnder interferometer is a device used to determine the relative phase shift variations between two collimated beams derived by splitting light from a single source. The interferometer has been used, among other things, to measure pha ...
or
Michelson interferometer
The Michelson interferometer is a common configuration for optical interferometry and was invented by the 19/20th-century American physicist Albert Abraham Michelson. Using a beam splitter, a light source is split into two arms. Each of those ...
, forming an optical DPSK demodulator.
Delay time depends on the
data rate Data rate and data transfer rate can refer to several related and overlapping concepts in communications networks:
Achieved rate
* Bit rate, the number of bits that are conveyed or processed per unit of time
** Data signaling rate or gross bit rate ...
. For instance, in a 40 Gbit/s system, one bit corresponds to 25 picoseconds, and light travels 5 mm in a
fiber optics
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means ...
or 7.5 mm in
free space
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
within that period. Thus the optical path difference between the two beams is 5 mm or 7.5 mm depending on the type of interferometer used.
DQPSK
Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at ...
is the four-level version of DPSK. DQPSK transmits two bits for every symbol (bit combinations being 00, 01, 11 and 10) and has an additional advantage over conventional binary DPSK. DQPSK has a narrower optical spectrum, which tolerates more dispersion (both chromatic and polarization-mode), allows for stronger optical filtering, and enables closer channel spacing. As a result, DQPSK allows processing of 40 Gbit/s data-rate in a 50 GHz channel spacing system. A demodulator for optical DQPSK signals can be constructed using two matched DPSK demodulators with phase off-set at
.
References
{{DEFAULTSORT:DPSK demodulator, optical
Fiber-optic communications
Photonics