Introduction to Optical Transmitters

The opto-electronic device that converts the electrical signal in to optical signal is called optical transmitter. An optical transmitter received the incoming electrical signals, processes it and converts it to optical signal. Thus the function of an optical transmitter is to convert the electrical signal into optical form and to launch the resulting optical signal into the optical fiber. A diagram of an optical transmitter is shown below;


Optical transmitter consists of an optical source, a modulator, and a channel coupler. Semiconductor lasers (LDs) or Light emitting diodes (LEDs) are used as optical sources. LDs and LEDs are compatible with the optical fiber communication channel. Optical signal is generated by modulating the optical carrier wave. Although an external modulator is sometimes used, it is not necessary in some cases, since the output of a semiconductor optical source can be modulated directly by varying the injection current. Such a scheme simplifies the transmitter design and is generally cost-effective. The coupler is typically a micro-lens that focuses the optical signal onto the entrance plane of an optical fiber with the maximum possible efficiency.

One of the important parameters in transmitter design is the launched power. We can increase the amplifier/repeater spacing by increasing the launch power. Launch power can not be increased as we like since it has influence on non-linear effects. The onset of various nonlinear effects limits quantity of input power. The launched optical power is expressed in “dBm” units with 1 mW as the reference level (1 mW is 0 dBm).

The optical source converts electrical energy into optical energy.  The optical energy that is light emitted by an optical source is launched into an optical fiber for transmission. Fiber optic data link performance depends on the amount of optical power launched into the optical fiber.

In short, an optical transmitter consists of the following components:

  • Optical Source
  • Electrical Pulse Generator
  • Optical Modulator

An optical source to be used in fiber optic transmitters need to be at the accurate wavelength as much as possible. Also it must be able to be modulated fast enough to transmit data and be efficiently coupled into optical fiber. Four commonly used optical sources are:

– Light Emitting Diodes
– Fabry-perot (FP) lasers
– Distributed Feedback (DFB) lasers
– Vertical Cavity Surface-Emitting Lasers (VCSELs)

The common thing among the above optical sources is thal all of them convert electrical signals into optical signals, but are otherwise quite different devices. LEDs and VCSELs are fabricated on semiconductor wafers such that they emit light from the surface of the chip. Fabry-Perot lasers emit light from the side of the chip from a laser cavity created in the middle of the chip.


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