Distributed feedback laser (DFB)

A distributed feedback laser (DFB) is a type of laser diode, quantum cascade laser or optical fiber 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. The reflection of the coating can be varied to make laser oscillate near the Bragg Wavelength.

DFB laser diodes do not use two discrete mirrors 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 wavelengths, 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.

分布反馈激光器（DFB）是一种激光二极管、量子级联激光器或光纤激光器，其器件的有源区周期性地构成衍射光栅。该结构形成一维干涉光栅（布拉格散射），光栅为激光器提供光反馈。涂层的反射可以改变，使激光在布拉格波长附近振荡。DFB激光器不使用两个分立的反射镜形成光腔（因为它们用于常规激光器设计）。光栅用作至少一个反射镜的波长选择元件，并提供反馈，将光反射回空腔形成谐振器。光栅被构造成只反射窄波段的波长，从而产生单纵模激射模式。这与法布里-佩罗特激光器形成对比，在这种情况下，芯片的各个面形成两个反射镜，并提供反馈。在这种情况下，反射镜是宽带的，要么激光器在多个纵向模式同时工作，要么在纵向模式之间容易跳跃。改变器件的温度会导致光栅的间距改变，因为折射率对温度的依赖关系。这种依赖性是由于半导体激光器的带隙随温度和热膨胀而改变的。在折射率的变化会改变光栅结构的波长选择和使激光器的输出波长，波长可调谐激光器或TDL生产（可调谐二极管激光）。调谐范围通常是一个~ 6 nm为50 K（90°F）温度的变化，而一个DFB激光器的线宽是几兆赫。改变电流的激光也将调整设备，因为电流的变化引起的温度变化内的设备。集成DFB激光器通常用于光通信应用，如可调谐激光信号，以及在需要窄线宽的传感中，或者在气体传感应用中，在波长调谐的DFB激光器中检测吸收气体信号。有传统的DFB激光器的替代品。传统上，DFB涂腔涂高反射率的另一侧的抗反射（AR /小时）。在这种情况下，光栅在减反射涂层侧形成分布镜，而高反射率侧的半导体小面形成另一面镜子。这些激光器通常具有较高的输出功率，因为光是从Ar侧提取的，而HR端则防止电源从背面丢失。不幸的是，该激光器的制造和切割面时，它在这一点上，在光栅的激光切割成小的控制几乎是不可能的。所以有时激光HR小面在光栅的波峰处形成，有时出现在斜坡上。根据光栅相位和光学模式，激光输出光谱可以变化。通常，高反射的相位发生在两个纵向模具有相同的腔增益的点上，因此激光同时工作在两种模式下。因此，AR /小时激光器进行筛选，多模或较差的边模抑制比制造及零部件（SMSR）必须报废。此外，劈裂的相位影响波长，因此在制造过程中控制一批激光器的输出波长是一个挑战。另一种方法是相移DFB激光器。在这种情况下，两个面都是反反射涂层，并且在腔中存在相移。这可能是一个单一的1/4波位移在中心的空腔，或多个较小的位移分布在空腔中。这种装置具有更好的重现性和理论上的所有波长的激光在单模式。在DFB光纤激光器中，布拉格光栅（在这种情况下也形成激光器的腔）在反射带中集中了一个相移，类似于一个非常窄的法布里-珀干涉仪的窄透射槽。当配置正确时，这些激光器工作在一个单一的纵向模式，相干长度超过几十公里，基本上受限于由相干相干检测技术引起的时间噪声，用于测量相干性。这些DFB光纤激光器通常用于需要极窄线宽的传感应用中。