Editing Laser diode (section)

=== Distributed-feedback lasers ===
{{Main|Distributed-feedback laser}}

A [[distributed-feedback laser]] (DFB) is a type of single-frequency laser diode.<ref name="hecht"/> DFBs are the most common transmitter type in [[DWDM]] systems. To stabilize the lasing wavelength, a diffraction grating is etched close to the ''p''–''n'' junction of the diode. This grating acts like an optical filter, causing a single wavelength to be fed back to the gain region and lase. Since the grating provides the feedback that is required for lasing, reflection from the facets is not required. Thus, at least one facet of a DFB is [[anti-reflection coating|anti-reflection coated]]. The DFB laser has a stable wavelength that is set during manufacturing by the pitch of the grating, and can only be tuned slightly with temperature. DFB lasers are widely used in optical communication applications where a precise and stable wavelength is critical.

The threshold current of this DFB laser, based on its static characteristic, is around 11 mA. The appropriate bias current in a linear regime could be taken in the middle of the static characteristic (50 mA). Several techniques have been proposed in order to enhance the single-mode operation in these kinds of lasers by inserting a one-phase-shift (1PS) or multiple-phase-shift (MPS) in the uniform Bragg grating.<ref>{{cite journal |last1=Bouchene |first1=M.M. |first2=R. |last2=Hamdi |first3=Q. |last3=Zou |title=Theorical analysis of a monolithic all-active three-section semiconductor laser |journal=Photonics Letters of Poland |volume=9 |issue=4 |pages=131–3 |date=2017 |doi= 10.4302/plp.v9i4.785|url=http://www.photonics.pl/PLP/index.php/letters/article/download/9-47/516/0|doi-access=free }}</ref> However, multiple-phase-shift DFB lasers represent the optimal solution because they have the combination of higher side-mode suppression ratio and reduced spatial hole-burning.