Hi everyone!
Today I will finish writing about light sources for optical transmitters – LEDs and lasers. Let's get started.
Lasers have high output power, but smaller tighter light outputs than LEDs and are much easier to pair with fibers. They are ideal for systems with high speeds and long distances. They are mainly used for systems over 10 GHz or 10 Gbps.
Inexpensive transmitters are LEDs and VCSELs, on the other side, lasers are expensive devices. The main reason is the process of making the laser cavity inside the device. For lasers, it is very difficult.
Another very important difference between LEDs and lasers is the spectral output. LEDs have a wide spectral output, while lasers have a narrow spectral output. Therefore, a large chromatic dispersion is a problem with LEDs.
There is also a difference between FB and DFB lasers. The laser cavity in the chip is very important. It can reduce the spectral width. DFB lasers have a narrower spectral width. Thanks to this, DFBs can be used in DWDM systems and over long distances. Chromatic dispersion is lower than with FB lasers.
Figure 1. The spectral widths for different sources
DFB lasers have a linear characteristic, which means that the level of output light follows the input current. Therefore, these lasers are used in AM CATV systems.
FB lasers can be used for different wavelengths. DFB lasers are usually in the range of 1490 nm to 1550 nm.
Figure 2. Characteristics of laser and LED
Which optical source we will use depends on the speed, distance, application and fiber compatibility.
LEDs, VCSELs and FB lasers are used in MM fibers. Specifically, LED is used in POF (Plastic Optical Fiber) with wavelengths from 1270 nm to 1330 nm. For MM fibers, VCSEL at 850 nm is usually used, rarely FB at 1300 nm. OM2, OM3 and OM4 are commonly used as MM fibers. FB and DFB lasers are used for SM systems. DFB is especially used for long distances, for systems with CATV services, as well as for DWDM systems.
Table 1. The main features of light sources
This is the end of this topic. I hope you have learned something new.
Thank you!
