Fiber Optic Transmitters

A fiber optic transmitter is a hybrid opto-electronic device used in optical communication. Fiber optic transmitter converts electrical signals to optical signals and launches the optical signals into an optical fiber. This is the process that makes the optical transmission happen. A fiber optic transmitter consists of the following essential parts;

– Interface circuit
– Source drive circuit
– Optical source

Interface circuit accepts the incoming electrical signals and processeses them to make it compatible with the Source drive circuit. With respect to the electric current flowing throug the source drive circuit unit, modulates the intensity of optical source. In other words, a source drive circuit intensity modulates the optical source by varying the electricity flowing through it. Optical source produces light pulses known as optical signal. This optical signal is then coupled into an optical fiber through the transmitter output interface.

Semiconductor LEDs (Light emitting diodes) and LDs (Laser diodes) are used as transmitters in fiber optic communication. Semiconductor LEDs and LDs share many common characteristics. Note that a unique transmitter design is a result of the differences between LED and LD sources. Transmitter designs compensate for differences in optical output power, response time, linearity, and thermal behavior between LEDs and LDs to ensure proper system operation. Junction heating in LEDs lead to nonlinearities. This non-linearities and and mode instabilities in LDs necessitate the use of linearizing circuits within the transmitter in some cases.

Fiber optic transmitters based on Laser diodes require more complex circuitry than transmitters using Light emitting diodes. The basic requirement for digital systems is for drive circuitry to switch the optical output on and off at high speeds in response to logic voltage levels at the input of the source drive circuit. Laser diodes are threshold devices and hence they are supplied with a bias just below the threshold in the off state. This bias is referred to as prebias. Prebiasing is done to the Laser diode for reducing the turn-on delay in digital systems.

Most Laser diode transmitters contain output power control circuitry to compensate the temperature sensitivity. Output power control circuitry maintains the Laser diode output at a constant average value by adjusting the bias current of the laser. In contrast, most LED transmitters do not contain output power control circuitry. Laser diode and Light emitting diode transmitters may also contain cooling devices to maintain the source at a relatively constant temperature. Most Laser diode transmitters either have an internal thermo electric cooler. Otherwise they need a controlled external temperature.

Fiber optic transmitters using Laser diodes are comparatively expensive than fiber optic transmitters using LEDs. The high cost for Laser diode based fiber optic transmitters is due to their complex circuitry. LED based fiber optic transmitters have simple circuitry.

Fiber optic transmitter output interfaces are of two categories generally. First category use optical connectors and the second category use optical fiber pigtails. Optical pigtails are attached to the transmitter optical source. This pigtail is generally routed out of the transmitter package as a coated fiber in a loose buffer tube or a simplex cable. The pigtail is either soldered or epoxied to the transmitter package to provide fiber strain relief. The buffer tube or simplex cable is also attached to the transmitter package to provide additional strain relief.

In some cases, fiber optic transmitter output interface consists of a fiber optic connector. The optical source can be coupled to the output optical connector through an intermediate optical fiber. One end of the optical fiber is attached to the source and the other end terminates in the transmitter’s optical output connector. There are cases where the optical source couples to the output optical connector directly without using an intermediate optical fiber. The optical source is placed within the transmitter package to launch power directly into the fiber of the mating optical connector. Some transmitter design make use of lenses to couple light from the source into the mating optical connector more efficiently.

Author: Prakash

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