What is a Fiber-Optic Transmitter?
Fiber optic transmitters are devices or modules used to produce modulated optical signals for the data to be transmitted through optical fibers in optical communication systems. They have both optical signal-producing elements and the electronics that drive them. They convert the input electrical data into equivalent data-carrying optical signals and help to launch them into the optical fibers. To establish a complete two-way communication system or network, commercial packages containing a fiber optic transmitter module will also have a fiber optic receiver counterpart, a module that receives the incoming optical signals and convert them into equivalent electronic signals.
Fiber optic transmitters have semiconductor modules that supply power and drive the optical signal source for proper modulation of the optical signals. An Integrated Circuit (IC) package can perform both functions. They convert the incoming voltage pulse into a precise current pulse that drives the optical source, i.e., an LED or a Laser. Once this drive current is applied to it, an equivalent optical signal carrying the data can be generated.
LED or Laser is an essential component to produce optical signals in fiber optical transmitters and is employed depending on the application requirements. Certain factors that can affect the choice of LED-based or Laser-based fiber optic transmitter are the required power levels & data rates, transmission distance, fiber compatibility, and deployment cost for the communication network. LED-based fiber optic transmitter would be sufficient for short-distance (up to a kilometer) communication with lower data rate (in Mbps regime) requirements while a laser-based fiber optic transmitter would be needed for long-distance (several kilometers) communication or a high data rate (in Gbps regime) communication. Also, the wavelength or the wavelength range of the optical emission is important as the optical fiber will have some optimum operating frequency bands.
LEDs are based on the spontaneous emission process and hence emit incoherent light with wider spectral content or bandwidth of over 40 nm. Hence, the optical signal produced by LED will suffer chromatic dispersion as the fiber length increase which limits the overall transmission distance. LEDs are cheaper, reliable, and have a longer life which makes them a better choice for short-distance communications as the effects of chromatic dispersion will be negligible for shorter fiber lengths. Also, the light output of an LED is not unidirectional and hence the optical network should use multimode fibers for coupling the LED output.
On the other hand, lasers work on the principle of the stimulated emission process and emit highly coherent optical signals with a higher output intensity and a very narrowband spectral profile which reduces dispersion effects. Apart from that, they are also highly directional which makes laser-based fiber optic transmitters suitable for their application in long-distance high data-rate optical communication systems where the higher cost of deploying laser can be justified. Also, the high directionality of laser output makes it possible to be coupled with both single-mode and multimode optical fibers with high coupling efficiency. Lasers can also be modulated for high data rate signals.
Different types of lasers such as Fabry Perot cavity (FP) lasers, Distributed Feedback (DFB) lasers, and Vertical Cavity Surface Emitting Lasers (VCSELs) are usually used in fiber optic transmitters. VCSELs emit laser output from the surface of the chip, while FP lasers emit from the side of the chip from a laser cavity created in the middle of the chip. VCSELs are cheaper among these lasers due to their relatively simpler fabrication process and can be used to replace LEDs in systems where an LED-based fiber optic transmitter is used.
DFB lasers are more suitable for long-distance and DWDM systems, as they are single-frequency lasers (having the narrowest spectral width) and hence minimize the chromatic dispersion for long-distance communication. They also feature highly linear output characteristics.