The general function of lasers is based on optical feedback. Photons emitted from the active region are back-reflected by the mirrors into the gain medium. In this way, optical feedback is realized and finally controls the process of stimulated emission. But when speaking of optical feedback one usually refers to external optical feedback. Here is a review of optical feedback effects and consequences on a laser spectrum.
Optical feedback is often associated to an undesirable optical feedback in the optical chain. Feedback is introduced into a laser when some portion of the optical output is back into the device. Every inhomogeneity of the system, every connection between different parts of the system sends a small amount of light back into the laser. It comes from optical elements like micro-lenses in fiber-coupled modules, fiber ends, fiber combiners, and also radiation from other sources. Even very small portions of the light reflected can destabilize the laser and produce different kinds of regular or irregular, reversible or irreversible effects. Optical feedback has various effects on the operating characteristics of a laser. It can be disadvantageous, as it may cause unwanted instabilities in the laser output, or advantageous, as under certain conditions it can improve several features of the laser, such as increasing the side mode suppression and narrowing the linewidth.
But external optical feedback can be intentionally implemented. An example of intentionally implemented optical feedback is when external gratings and mirrors are used for stabilization and controlled tuning of the emission wavelength or for improving the brightness of device emission .
Regimes of optical feedback
These optical feedback effects impact all semiconductor lasers: Fabry-Perot (FP), DFB, DBR lasers and so on. The diagram of Tkach and Chraplyvy (T–C)  has been the reference for describing and classifying feedback effects in semiconductor laser. Since its publication it has become a milestone reference.
The T-C diagram points out five types of regimes of optical feedback. These regimes rely on three factors:
- Feedback power ratio
- Distance to reflection
- Phase of the incoming power
Here is a figure showing how those five regimes are determined by the feedback power ratio and the distance to reflection:
Figure 1. Regimes of optical feedback according to the description of Tkach and Chraplyvy.
Each regime determines how a semiconductor laser operates under external optical feedback .
Regime 1: Stable regime where the laser linewidth is broadened or narrowed. This is the lowest level of optical feedback. The laser spectrum shows that the laser linewidth is broadened or narrowed based on the distance to the reflector which determines the phase of the optical feedback signal.
Regime 2: Conditionally stable. The larger the distance to the feedback reflector, the more sensitive the laser will be to lower levels of feedback. In this regime, changes in the phase of the optical feedback lead to mode hopping of the laser spectrum. These hops correspond to external cavity modes between the laser output facet and the feedback reflector.
Regime 3: Stable single mode operation with linewidth reduction. This very narrow regime from feedback levels of -39 dB to -45 dB causes the linewidth to narrow. In this regime, the laser operates on a single line and length of the reflector and the optical feedback phase are inconsequential. Feedback reflections of greater magnitude can still cause problems.
Regime 4: Unstable operation with coherence collapse. In this regime, the laser spectrum develops side modes separated by the main mode by the relaxation oscillation frequency. As optical feedback increases, the laser enters “coherence collapse”. In coherence collapse, the laser line broadens to several tens of GHz with several peaks. The laser is insensitive to changes in feedback phase.
Regime 5: Stable operation with significant linewidth reduction. This regime typically requires an anti-reflection coating on the laser output facet. In this regime the laser is a short active section in a long cavity laser. The back facet of the laser and the feedback reflector serve as cavity mirrors. With sufficient wavelength selectivity, the laser operates on a single longitudinal mode with a narrow linewidth for all phases of feedback.
At the time the T–C diagram was developed, in 1986, optical feedback was considered mainly as a disturbance affecting the performance of linewidth and noise of a laser, and impairing the use of the source in optical fiber communication systems. But since, the T–C diagram of feedback effects has been enriched by a number of phenomena . That opened the door to important new and groundbreaking applications. Some applications are now well established as chaos cryptography and self-mixing interferometry. But some applications are more recent as random number, microwave tone generation and ranging. And new applications will most probably keep appearing in the years to come.
 Britta Leonhäuser, Heiko Kissel, Andreas Unger and Jens Biesenbach from DILAS Diodenlaser GmbH, and Jens W. Tomm and Martin Hempel from Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Germany, “High-Power Diode Lasers under External Optical Feedback”, Photonics West 2015.
 R. W. Tkach and A. R. Chraplyvy, “Regimes of feedback effects in 1.5-um distributed feedback laser,” IEEE J. Lightw. Technol., vol. JLT-4, no. 11, pp. 1655–1661, Nov. 1986.
 Photodigm application note “Optical feedback in laser diodes”, http://www.photodigm.com/literature/applications-notes/optical-feedback-in-laser-diodes
 Silvano Donati, Life Fellow and Ray-Hua Horng, “The Diagram of Feedback Regimes Revisited”, IEEE Journal of selected topics in quantum electronics, vol.19, no.4, July/August 2013.