What Is Optical Transceiver

What Is Optical Transceiver

As a central component of optical communication, optical transceiver is a kind of optical-electric/electric-optical converter. It is also called fiber optic transmitter and receiver. An optical transceiver consists of a transmitter on one end of a fiber and a receiver on the other end. The transmitter end takes in and converts the electrical signal into light, after the optical fiber transmission in the fiber cable plant, the receiver end again converts the light signal into electrical signal. Both the receiver and the transmitter ends have their own circuitry and can handle transmissions in both directions.

The structure of optical transceiver

Optical transmitter: since the sources used for fiber optic transmitters need to be at the correct wavelength, be able to be modulated fast enough to transmit data and be efficiently coupled into fiber, LEDs, fabry-perot (FP) lasers, distributed feedback (DFB) lasers and vertical cavity surface-emitting lasers (VCSELs) are the four commonly used types of sources. Thought these decvices all convert electrical signals into optical signals, they are quite different from each other.
Optical receiver: the optical receiver uses semiconductor detectors (photodiodes or phodetectors) to convert optical signals to electrical signals. Silicon photodiodes are used for short wavelength links (650 for plastic optical fiber and 850 for glass MM fiber). Long wavelength systems usually use indium gallium arsenide (InGaAs) detectors as they have lower noise than germanium which allows for more sensitive receivers. Very high speed systems sometimes use avalanche photodiodes (APDs) that are biased at high voltage to create gain in the photodiode.

The performance of optical transceiver

Just as with copper wire or radio transmission, the performance of the optical data link can be determined by how well the reconverted electrical signal out of the receiver matches the input to the transmitter. The discussion of performance on datalinks applies directly to transceivers which supply the optical to electrical conversion.

Every manufacturer of optical transceivers specifies their product for receiver sensitivity (perhaps a minimum power required) and minimum power coupled into the fiber from the source. Those specifications will end up being the datalink specifications on the final product used in the field.

All datalinks are limited by the power budget of the link. The power budget is the difference between the output power of the transmitter and the input power requirements of the receiver. The receiver has an operating range determined by the signal-to-noise ratio (S/N) in the receiver. The S/N ratio is generally quoted for analog links while the bit-error-rate (BER) is used for digital links. BER is practically an inverse function of S/N.

Three primary types of optical transceiver

There are many different kinds of optical transceivers that can be used in telecommunications applications. The different specs and designs are widely used to meet the changing needs of designers. The small form-factor (SFP), the small form-factor pluggable (SFP+) and the XFP transceiver are the three primary kinds. SFP is available for designers to use in various applications. SFP+ , which is an enhanced version of an SFP, is designed to support data rates up to 10 Gbit/s. While the XFP transceiver can operating at wavelengths of 850nm, 1310nm, and 1550nm, which are capable of operating at a single wavelength.