Distributed feedback laser
Encyclopedia
A distributed feedback laser (DFB) is a type of laser diode
, quantum cascade laser
or optical fibre laser where the active region of the device is periodically structured as a diffraction grating
. The structure builds a one dimensional interference grating (Bragg scattering) and the grating provides optical feedback
for the laser.
DFB laser diodes do not use two discrete mirror
s to form the optical cavity
(as they are used in conventional laser
designs). The grating acts as the wavelength selective element for at least one of the mirrors and provides the feedback, reflecting light back into the cavity to form the resonator. The grating is constructed so as to reflect only a narrow band of wavelength
s, and thus produce a single longitudinal lasing mode. This is in contrast to a Fabry-Perot Laser, where the facets of the chip form the two mirrors and provide the feedback. In that case, the mirrors are broadband and either the laser functions at multiple longitudinal modes simultaneously or easily jumps between longitudinal modes. Altering the temperature
of the device causes the pitch
of the grating to change due to the dependence of refractive index on temperature. This dependence is caused by a change in the semiconductor laser's bandgap with temperature and thermal expansion. A change in the refractive index alters the wavelength selection of the grating structure and thus the wavelength of the laser output, producing a wavelength tunable laser
or TDL (Tunable Diode Laser). The tuning range is usually of the order of 6 nm for a ~50 K
(90 °F) change in temperature, while the linewidth of a DFB laser is a few megahertz. Altering of the current
powering the laser will also tune the device, as a current change causes a temperature change inside the device. Integrated DFB lasers are often used in optical communication
applications, such as DWDM where a tunable laser signal is desired as well as in sensing where extreme narrow line width is required, or in gas sensing applications, where the signal of the absorbing gas is detected while wavelength tuning the DFB laser.
There are alternatives to traditional types of DFB lasers. Traditionally, DFBs are antireflection coated on one side of the cavity and coated for high reflectivity on the other side (AR/HR). In this case the grating forms the distributed mirror on the antireflection coated side, while the semiconductor facet on the high reflectivity side forms the other mirror. These lasers generally have higher output power since the light is taken from the AR side, and the HR side prevents power being lost from the back side. Unfortunately, during the manufacturing of the laser and the cleaving of the facets, it is virtually impossible to control at which point in the grating the laser cleaves to form the facet. So sometimes the laser HR facet forms at the crest of the grating, sometimes on the slope. Depending on the phase of the grating and the optical mode, the laser output spectrum can vary. Frequently, the phase of the highly reflective side occurs at a point where two longitudinal modes have the same cavity gain, and thus the laser operates at two modes simultaneously. Thus such AR/HR lasers have to be screened at manufacturing and parts that are multimode or have poor side mode suppression ratio (SMSR) have to be scrapped. Additionally, the phase of the cleave affects the wavelength, and thus controlling the output wavelength of a batch of lasers in manufacturing can be a challenge.
An alternative approach is a phase-shifted DFB laser. In this case both facets are anti-reflection coated and there is a phase shift in the cavity. This could be a single 1/4 wave shift at the center of the cavity, or multiple smaller shifts distributed in the cavity. Such devices have much better reproducibility in wavelength and theoretically all lase in single mode.
In DFB fibre lasers the Bragg grating (which in this case forms also the cavity of the laser) has a phase-shift centered in the reflection band akin to a single very narrow transmission notch of a Fabry–Pérot interferometer. When configured properly, these lasers operate on a single longitudinal mode with coherence lengths in excess of tens of kilometres, essentially limited by the temporal noise induced by the self-heterodyne coherence detection technique used to measure the coherence.
These DFB fibre lasers are often used in sensing applications where extreme narrow line width is required.
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
, quantum cascade laser
Quantum cascade laser
Quantum cascade lasers are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.Unlike typical...
or optical fibre laser where the active region of the device is periodically structured as a diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
. The structure builds a one dimensional interference grating (Bragg scattering) and the grating provides optical feedback
Optical feedback
Optical feedback is the optical equivalent of acoustic feedback. A simple example is the feedback that occurs when a loop exists between an optical input, e.g., a video camera, and an optical output, e.g., a television screen or monitor...
for the laser.
DFB laser diodes do not use two discrete mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
s to form the optical cavity
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
(as they are used in conventional laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
designs). The grating acts as the wavelength selective element for at least one of the mirrors and provides the feedback, reflecting light back into the cavity to form the resonator. The grating is constructed so as to reflect only a narrow band of wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s, and thus produce a single longitudinal lasing mode. This is in contrast to a Fabry-Perot Laser, where the facets of the chip form the two mirrors and provide the feedback. In that case, the mirrors are broadband and either the laser functions at multiple longitudinal modes simultaneously or easily jumps between longitudinal modes. Altering the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
of the device causes the pitch
Pitch
Pitch may refer to:* Pitch , a viscous substance produced by plants or formed from petroleum* Pitch * sales pitch** elevator pitch, a very short sales pitch such as that made during an elevator ride* Pitch accent-Music and acoustics:...
of the grating to change due to the dependence of refractive index on temperature. This dependence is caused by a change in the semiconductor laser's bandgap with temperature and thermal expansion. A change in the refractive index alters the wavelength selection of the grating structure and thus the wavelength of the laser output, producing a wavelength tunable laser
Tunable laser
A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range....
or TDL (Tunable Diode Laser). The tuning range is usually of the order of 6 nm for a ~50 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
(90 °F) change in temperature, while the linewidth of a DFB laser is a few megahertz. Altering of the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
powering the laser will also tune the device, as a current change causes a temperature change inside the device. Integrated DFB lasers are often used in optical communication
Optical communication
Optical communication is any form of telecommunication that uses light as the transmission medium.An optical communication system consists of a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the...
applications, such as DWDM where a tunable laser signal is desired as well as in sensing where extreme narrow line width is required, or in gas sensing applications, where the signal of the absorbing gas is detected while wavelength tuning the DFB laser.
There are alternatives to traditional types of DFB lasers. Traditionally, DFBs are antireflection coated on one side of the cavity and coated for high reflectivity on the other side (AR/HR). In this case the grating forms the distributed mirror on the antireflection coated side, while the semiconductor facet on the high reflectivity side forms the other mirror. These lasers generally have higher output power since the light is taken from the AR side, and the HR side prevents power being lost from the back side. Unfortunately, during the manufacturing of the laser and the cleaving of the facets, it is virtually impossible to control at which point in the grating the laser cleaves to form the facet. So sometimes the laser HR facet forms at the crest of the grating, sometimes on the slope. Depending on the phase of the grating and the optical mode, the laser output spectrum can vary. Frequently, the phase of the highly reflective side occurs at a point where two longitudinal modes have the same cavity gain, and thus the laser operates at two modes simultaneously. Thus such AR/HR lasers have to be screened at manufacturing and parts that are multimode or have poor side mode suppression ratio (SMSR) have to be scrapped. Additionally, the phase of the cleave affects the wavelength, and thus controlling the output wavelength of a batch of lasers in manufacturing can be a challenge.
An alternative approach is a phase-shifted DFB laser. In this case both facets are anti-reflection coated and there is a phase shift in the cavity. This could be a single 1/4 wave shift at the center of the cavity, or multiple smaller shifts distributed in the cavity. Such devices have much better reproducibility in wavelength and theoretically all lase in single mode.
In DFB fibre lasers the Bragg grating (which in this case forms also the cavity of the laser) has a phase-shift centered in the reflection band akin to a single very narrow transmission notch of a Fabry–Pérot interferometer. When configured properly, these lasers operate on a single longitudinal mode with coherence lengths in excess of tens of kilometres, essentially limited by the temporal noise induced by the self-heterodyne coherence detection technique used to measure the coherence.
These DFB fibre lasers are often used in sensing applications where extreme narrow line width is required.