Fiber Optic Transceivers

Fiber optic transceivers consist of a transmitter on one end of a fiber and a receiver on the other end. The transmitter takes an electrical input and converts it to an optical output from a laser diode or LED. The light from the transmitter is coupled into the fiber with a connector and is transmitted through the fiber optic cable plant. The light from the end of the fiber is coupled to a receiver where a detector converts the light into an electrical signal which is then conditioned properly for use by the receiving equipment.

Sources For Fiber Optic Transmitters

The sources used for fiber optic transmitters need to meet several criteria: it has to be at the correct wavelength, be able to be modulated fast enough to transmit data and be efficiently coupled into fiber. Four types of sources are commonly used, LEDs, fabry-perot (FP) lasers, distributed feedback (DFB) lasers and vertical cavity surface-emitting lasers (VCSELs).

There are many differences between LEDs and lasers. Firstly, LEDs have much lower power outputs than lasers and their larger, diverging light output pattern makes them harder to couple into fibers, limiting them to use with multi-mode fibers, while lasers are on the contrary. Secondly, LEDs have much less bandwidth than lasers and are limited to systems operating up to about 250 MHz or around 200 Mb/s. Lasers have very high bandwidth capability, most being useful to well over 10 GHz or 10 Gb/s. Thirdly, because of their fabrication methods, LEDs and VCSELs are cheap to make. Lasers are more expensive. Fourthly, LEDs have a limited bandwidth while all types of lasers are very fast. Moreover, LEDs have a very broad spectral output, while lasers have a narrow spectral output that suffers very little chromatic dispersion.

The choice of these devices is determined mainly by speed and fiber compatibility issues. As many premises systems using multi-mode fiber have exceeded bit rates of 1 Gb/s, lasers (mostly VCSELs) have replaced LEDs. The output of the LED is very broad but lasers are very focused, and the sources will have very different modal fill in the fibers. The restricted launch of the VCSEL (or any laser) makes the effective bandwidth of the fiber higher, but laser-optimized fiber, usually OM3, is the choice for lasers.

Detectors For Fiber Optic Receivers

The electronics for a transmitter are simple. They convert an incoming pulse (voltage) into a precise current pulse to drive the source, while receivers use semiconductor detectors to convert optical signals to electrical signals. Silicon photodiodes are used for short wavelength links. Long wavelength systems usually use InGaAs detectors as they have lower noise than germanium which allows for more sensitive receivers.

Transceivers Packaging

As to the packaging, transceivers are usually packaged in industry standard packages like these XFP modules (as the picture below) for gigabit data links(L) and Xenpak (R). The XFP modules connect to a duplex LC connector on the optical end and a standard electrical interface on the other end. The Xenpak are for 10 gigabit networks but use SC duplex connection. Both are similar to media converters but are powered from the equipment they are built into.

From this article, we can know better of fiber optic transceivers from the following aspects: sources for fiber optic transmitters, detectors for fiber optic receivers, and transceivers packaging. As the rapid development of fiber optic communication industry, fiber optic transceivers are playing an increasingly important role in modern communication.