LASERs (also known as laser diodes or LD) and LEDs (light emitting diode) have different characteristics in the way in which they emit light. While a LASER emits converged light, the output of an LED is highly diverged.
Spectral width is defined as the 3dB optical power width and it is measured in nanometers (nm) or microns.
The spectral width of an LED is bigger than that of a LD. A bigger spectral width enables higher link bandwidth on the FOC. For an LED the spectral width is about 80 nm when it operates at 1310 nm and 40 nm at 850 nm. The spectral width of a LD is 3 nm for operation at 1310 nm and 1 nm at 850 nm.
Figure 1: Spectral Characteristics of LED and Laser
Two types of laser diodes are commonly used:
- Distributed Feedback (DFB)
- Fabry—Perot (FP)
Fabry-Perot lasers are highly economical and have a high signal to noise (S / N) ratio. They are however slower than Distributed Feedback lasers. FP lasers also emit light at a number of discrete wavelengths.
Multi-Quantum Well (MQW) and Buried Hetero (BH) are two FP laser types commonly used.
BH lasers were used extensively in the early days, while MQW eventually took over and they are used more in present day applications. MQW lasers have several advantages of over BH lasers. These include:
- High Efﬁciency
- Low threshold current
- Enhanced linearity
- Low Noise
- Lower operating and manufacturing costs.
- Stability over a wide range of temperatures
One big disadvantage of the MQW Laser is that they cause back reﬂections.
DFB lasers are monochromatic, i.e. they emit light at a single color. They are also faster and quieter than what MQW lasers are. Used extensively in high-speed applications, MQW lasers offer enhanced linearity, high performance and have narrow spectral widths. MQW lasers are used in both digital and analog applications.
Vertical Cavity Surface Emitting Lasers (VCSEL) have large output apertures that emit a circular low-divergence beam. This enables them to achieve high coupling efﬁciencies with optical ﬁbers. This efﬁciency is in fact higher than that of either LD or LED. VCSELs have low threshold currents that give them high intrinsic modulation bandwidths and low power consumption.
The main disadvantages of VCSELs include complex manufacturing processes, high cost and limited application support (only 1310 nm and 850 nm). The types of VCSEL commonly used are the 10Gbps MM VCSEL and the pigtailed 850 nm VCSEL.
Applications that commonly use VCSELs include spectroscopy, ﬁber optic communication, laser printers, broadband transmission (Analog) and optical mouse. Some models of Multimode Fiber Media Converters use MM VCSCEL lasers.