FOA Guide 

Sources For Fiber Optic Transmitters - LEDs And Lasers

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.

fiber optic transceiver

Fiber Optic Transceiver

The source used for a fiber optic transmitter needs 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.


3 types of semiconductor lasers used as fiber optic sources

The types of sources used include LEDs, lasers, fabry-perot (F-P) 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) really the size of grains of sand. LEDs and VCSELs are fabricated on semiconductor wafers such that they emit light from the surface of the chip, while f-p and DFB lasers emit from the side of the chip from a laser cavity created in the middle of the chip. 

L-I curve
Light-current (L-I) curves for LEDs and lasers

Lasers and LEDs are quite different devices as you can see from this diagram of their light output as a function of drive current. LEDs are simple emitters that generate more light output as the drive current increases until higher currents heats them up and their ligh output decreases, limiting the total power output. Lasers start off like LEDs, generating more light with more drive current, but the light is confined in a small areas in the semiconductor chip called the laser cavity, horizontally inside the chip for most lasers but vertically in a VCSEL Like all lasers, once a certain amount of light is generated inside the laser cavity, the device becomes a "laser" - an acronym for "light amplification by stimulated emission of radiation." Once the device reaches a certain current level, it passes the laser threshold and the light output becomes much higher with little increase in current.

The L-I curves help show why lasers have higher bandwidth than LEDs. LEDs are modulated over higher current ranges to pulse the light output on and off. Lasers are biased at the threshold then modulated with small current changes to get large changes in light output. The smaller size of of lasers also makes them easier to modulate faster. Generally LEDs are limited to several hundred megabits/second links while lasers are good for 25-50 gigabits per second links when direct modulated. (Higher bit rates are possible by having the laser on all the time (CW) and modulating it externally.

Lasers and LEDs
Coupling power into the core of a fiber. Generally LEDs and VCSELs are used with multimode fiber and lasers with singlemode fiber.

LEDs have much lower power outputs than lasers and their larger, diverging light output beam pattern makes them harder to couple into fibers, generally limiting them to use with multimode fibers. 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 smaller tighter light outputs and are easily coupled to singlemode fibers, making them ideal for long distance high speed links. Lasers have very high bandwidth capability, most being useful to well over 10 GHz or 10 Gb/s.

VCSELs are a strange device. They use semiconductor fabrication tricks to create a vertical laser cavity in the chip so the light comes out the top, making it easy to couple into fiber. But the device structure has only been feasible for ~850nm sources, the wavelength used for multimode fiber.

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.

Comparison of spectral output of a LED and a VCSEL, both with a center wavelengh around 850nm.

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. In multimode fiber, the bandwidth of LEDs is highly limited by chromatic dispersion because of its large spectral width (light at longer wavelengths travels faster than light at shorter wavelengths causing dispersion). This adds to VCSELs advantage for higher speed networks.

F-P lasers have a variety of wavelengths, but DFB lasers are generally in the range of 1490 to 1500nm wavelength

There is also a big difference in F-P and DFB lasers. Modification of the laser cavity in the chip can reduce the spectral width of DFB lasers considerably.  DFB lasers, which are used in long distance and DWDM systems, have the narrowest spectral width which allows much denser wavelength division multiplexing with more channels in a single fiber. The narrower spectral width of DFB lasers minimizes chromatic dispersion for use 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 analog transmitter sources in AM CATV systems. In fact, it was the invention of the DFB laser that made hybrid fiber-coax CATV networks - and the subsequent invention of the cable modem for broadband - possible.

launch into multimode fiber

Differences in multimode launches with LEDs and alsers

The choice of these devices is determined mainly by application, speed, distance 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 or OM4, is the choice for lasers. OSP networks that are short and not so high speed generally use 1310nm F-P lasers while longer, faster links, especially those using DWDM will use DFB lasers at ~1550nm.

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

Note/Trivia: At around 1300nm wavelength, LEDs are generally referred to as 1300nm and lasers as 1310nm. LEDs, as you see above, have broad spectral outputs so the wavelength is harder to define precisely. LEDs vary trom ~1270 to 1330nm. Lasers have always been called 1310nm because that is what AT&T called them in the early 80s when they first started making them commercially. In fact lasers vary from ~1280-1330nm. This has also become the reason that multimode systems, which were mostly LEDs until VCSELs were invented, are called 1300nm and singlemode systems which always used lasers were called 1310nm.

Table of Contents: The FOA Reference Guide To Fiber Optics


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