optical transmitter (rus. оптический передатчик) — a device converting input electric signals into output optical signals transmitted in an optical transmission medium.

Description

Optical transmitters consist of optical sources and devices modulating optical radiation in accordance with an electric input signal. Depending on the modulation method used, optical transmitters are classified into direct (internal) and external modulation transmitters.

In optical transmitters with direct modulation, the radiant power of the light source is modulated by the feed current. The main advantage of such modulators is their simple design. Their disadvantages include limited response time (rate of data transmission in digital communication systems) and the use of only one parameter (power) of the light wave for modulation purposes. Light sources in direct modulation transmitters are represented by light emitting diodes or direct modulation lasers.

In optical transmitters with external modulation, continuous optical radiation is modulated by an external modulator controlled by an electric data signal. In such transmitters, light sources are usually narrow-band monomode continuous-wave semiconductor lasers, such as distributed feedback lasers or distributed Bragg reflector lasers.

These lasers help to create an optical signal with minimum spectrum width. Besides, in addition to modulation of amplitude (power), external modulation transmitters encode information using the modulation of other light wave parameters, such as phase, frequency, polarisation or their combinations. External modulation transmitters are used in long-distance communication systems requiring premium quality of optical signal. The most commonly-used modulator types in communication systems are Mach-Zehnder modulators and electroabsorption modulators.

Dense wavelength division multiplexing (DWDM) communication systems use transmitters with a tunable wavelength [2]. Optical transmitters are sometimes enhanced with optical amplifiers to increase the power of the optical signal.

Transmitters for fibre-optic communication systems are often manufactured in one case with receivers, and are called sender-receiver optical modules or transponders.

Illustrations

Diagram of an integrated monoblock optical transmitter based on variable-wavelength laser with ex

Diagram of an integrated monoblock optical transmitter based on variable-wavelength laser with external modulator and amplifier [2].


Author

  • Nanii Oleg E.

Sources

  1. Nanijj O. E. Optical transmitters (in Russian) // Lightwave Russian Edition. 2003. №2. 48–51 pp.
  2. Nanijj O. E. A wavelength-tunable optical transmitters for DWDM communication systems (in Russian). // Part 1 Lightwave Russian Edition, 2006, № 1, 51–56 pp.; Part 2 Lightwave Russian Edition — № 3, 2006. — 53–56 pp.

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