FOA Guide

Fiber Optic Transmitters and Receivers (Transceivers)

fiber optic link 

Fiber Optic Datalink

Fiber optic transmission systems (datalinks) all work similar to the diagram shown above. They consist of a transmitter on one end of a fiber and a receiver on the other end. Most systems operate by transmitting in one direction on one fiber and in the reverse direction on another fiber for full duplex operation.

fiber optic transceiver
Fiber Optic Transceiver

Most systems use a "transceiver" which includes both transmission and receiver in a single module. 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.

As the use of links at 100Gb/s or more become common, datalinks become more complex. Above  about 25Gb/s, the average limit for direct modulation of typical laser sources, wavelength division multiplexing, parallel optics and coherent fiber optic systems are used. In addition coherent systems can be more effective to overcome dispersion in long links. Read more about coherent fiber optic systems.

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). All convert electrical signals into optical signals, but are otherwise quite different devices. All three are tiny semiconductor devices (chips). LEDs and VCSELs are fabricated on semiconductor wafers such that they emit light from the surface of the chip, while f-p lasers emit from the side of the chip from a laser cavity created in the middle of the chip.  

fiber opitc sources

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 multimode fibers. Laser have smaller tighter light outputs and are easily coupled to singlemode fibers, making them ideal for long distance high speed links. 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.
Because of their fabrication methods, LEDs and VCSELs are cheap to make. Lasers are more expensive because creating the laser cavity inside the device is more difficult, the chip must be separated from the semiconductor wafer and each end coated before the laser can even be tested to see if its good.

Typical Fiber Optic Source Specifications

Device Type Wavelength (nm) Power into
Fiber (dBm)
Bandwidth Fiber Types
LED 850, 1300
-30 to -10
<250 MHz MM
Fabry-Perot Laser 850,
1310 (1280-1330) 1550 (1480-1650)
0 to +10 >10 GHz MM, SM
DFB Laser  1550 (1480-1650) 0 to +25 >10 GHz SM
VCSEL 850 -10 to 0 >10 GHz MM

LEDs have a limited bandwidth while all types of lasers are very fast. Another big difference between LEDs and both types of lasers is the spectral output. LEDs have a very broad spectral output which causes them to suffer chromatic dispersion in fiber, while lasers have a narrow spectral output that suffers very little chromatic dispersion. DFB lasers, which are used in long distance and DWDM systems, have the narrowest spectral width which minimizes chromatic dispersion on the longest links. DFB lasers are also highly linear (that is the light output directly follows the electrical input) so they can be used as sources in AM CATV systems.

LEDs and lasers

The choice of these devices is determined mainly by speed and fiber compatibility issues.  As many premises systems using multimode 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.

launch into multimode fiber

The electronics for a transmitter are simple. They convert an incoming pulse (voltage) into a precise current pulse to drive the source. Lasers generally are biased with a low DC current and modulated above that bias current to maximize speed.

More technical information on LEDs and Lasers used as fiber optic sources.  

Detectors for Fiber Optic Receivers

Receivers use semiconductor detectors (photodiodes or photodetectors) to convert optical signals to electrical signals. Silicon photodiodes are used for short wavelength links (650 for POF and 850 for glass MM fiber). Long wavelength systems usually use InGaAs (indium gallium arsenide) detectors as they have lower noise than germanium which allows for more sensitive receivers.

detector sensitivity

Very high speed systems sometimes use avalanche photodiodes (APDs) that are biased at high voltage to create gain in the photodiode. These devices are more expensive and more complicated to use but offer significant gains in performance.

Data Protocols And Modulation Methods

In general, the demand is for transceivers with higher bit rate capability but with low cost. Below 10 Gb/s transceivers can be simpler, with one laser modulated with binary encoding, where the laser transmits light for a "1" and is turned off for a "0". But the limit for laser modulation is ~25-50 Gb/s. If one needs faster speeds like 100Gb/s, there are several schemes used.

Parallel transmission: Multimode fiber with limited bandwidth uses 4 or 10 lasers transmitting at 10G or 25G over an equal number of fibers. It requires the use of array connectors (12 or 16 fibers in a connector) and many fibers. 

WDM (wavelength division multiplexing): Transmitters have several lasers transmitting over a single fiber. Receivers split out the wavelengths to separate detectors. This has been used since the 1990s for long distance telecom and more recently has become widely used in data centers to reduce the numbers of fibers needed. A scheme for WDM on multimode fiber has not gained much use.

PAM (pulse amplitude modulation): By using the height of the pulse to also encode data, a transceiver using PAM4 can double the data rate on a single fiber. A single laser can achiever 100Gb/s using PAM4 modulation. Higher data rates can use combined PAM4 channels with WDM.

Coherent transmission: Coherent transmission uses a very complex transceiver that can achieve speeds up to terabits per second over very long distances. It has primarily been used in long distance and submarine links but cost reductions have led to some use in high speed links for data centers.
PAM4 vs binary

Binary vs PAM4 Modulation
Binary transmission simply turns the laser on for a 1 or 0. In a PAM4 encoded signal, the bit has information in the pulse height, which means the bit can carry two bits of information - "0, 1,2 or 3," twice as much information in the same bit, doubling the information carrying capacity of the link. A PAM4 transceiver needs only one laser to achieve 100Gb/s data rates, making it less expensive than a transceiver that has 4 lasers and a WDM. The same technology can be leveraged for 400G also. PAM4 is not a fiber innovation. It has been used for years in electrical communication circuits to increase bandwidth. But the implementation requires some signal processing in the receiver which took some time to implement in fiber optic transceivers. 



Just as with copper wire or radio transmission, the performance of the fiber optic 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 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.


The operating range of a data link will look like this figure of BER vs received optical power for a typical fiber optic transceiver. There must be a minimum power at the receiver to provide an acceptable S/N or BER. As the power increases, the BER or S/N improves until the signal becomes so high it overloads the receiver and receiver performance degrades rapidly. More on power budgets and the similar "loss budget" which is the estimate of fiber optic cable plant loss.


Transceivers are usually packaged in industry standard packages like these XFP modules for gigabit datalinks(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.



Test Your Comprehension
After you study this page and "More on fiber optic datalinks", you should test your comprehension here.

Table of Contents: The FOA Reference Guide To Fiber Optics


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