Technology Basics

There are two main types of laser diode structures, Fabry-Perot (FP) and distributed feedback (DFB) used in fiber optic communication systems. DFB lasers offer the highest performance levels and also the highest cost of the two types. They are nearly monochromatic (i.e. they emit a very pure single color of light) while FP lasers emit light at a number of discrete wavelengths. DFB lasers tend to be used for the highest speed digital applications and for most analog applications because of their faster speed, lower noise, and superior linearity. DFB lasers are the choice for high-performance fiber optic telecommunication systems.

DWDM systems require that the optical emission wavelength stability of the DFB lasers be controlled to within ±5 GHz of the ideal channel center. At a wavelength of 1550 nm, ±5 GHz corresponds to a wavelength variation of ±0.04 nm. The main factor that affects the wavelength of a DFB laser, other than the way it is manufactured, is the temperature of the laser. Most high-performance DFB lasers incorporate a TEC (theremoelectric cooler) and a thermistor into the laser package. The thermistor provides a highly accurate measure of the temperature of the laser chip and the TEC allows the laser chip to be heated or cooled by sending a DC current through the TEC. Laser manufacturers usually specify the exact resistance of the internal thermistor resistance to achieve the desired DWDM optical wavelength.