Superluminescent diodes are diodes that, when biased in the forward direction, become optically active and generate amplified spontaneous emission over a wide range of wavelengths. In contrast to laser diodes, there is not sufficient feedback to achieve lasing action. This is usually achieved by the joint action of a tilted waveguide in which the generated radiation is guided and anti-reflection coated end facets. Superluminescent diodes are made by the same processes as laser diodes, and like laser diodes, can be fabricated to operate at various wavelengths such as 835 nm, 960 nm, 1300 nm, and 1550 nm. Superluminescent diodes offer higher power output as compared to conventional light emitting diodes, and offer broader spectral range (lower coherence) than semiconductor lasers. The broadband width is similar to but not as wide as an LED, but the output power is as high as that of a semiconductor laser. Superluminescent diodes can emit low coherent light at high output power with good directionality. The SLD emits in a broad spectral band which reduces phase noise in the sensor caused by the Kerr effect and by coherent backscatter in the fiber. In the superluminescent diode the optical feedback present in laser diodes is suppressed by placing the current stripe at an angle with respect to the diode facets so that the reflected light is away from the stripe. Superluminescent diodes utilize stimulated emission as their primary radiative mechanism, but do not exceed the threshold for oscillation. They do not form hot spots and do not catastrophically degrade. Superluminescent diodes provide a high power output of broad band low coherent radiation, that being radiation having a coherence length of less than about 200 micrometers and typically about 50 micrometers. As the power of a superluminescent diode is increased and its spectral width is consequently decreased, the coherence length of light from the device is increased. The coherence length is another measure of the spectral purity of light, and is inversely proportional to spectral width. As the spectral width becomes narrower, the coherence length increases. As an SLD has a broad spectrum similar to the light emitting diode, and can emit light having high output power similarly to the semiconductor laser. Thus, the SLD has an advantage that light having high output power and low coherency is taken out with good directionality.

The superluminescent diode is a light source with several important functions. A superluminescent diode is an optical amplifier which can be used to amplify an optical signal by several orders of magnitude. It is a broad band source which is used in several applications such as a light source for fiber optic gyroscopes and low coherence imaging. Superluminescent diodes can be inserted into an external cavity (a space between two reflectors) to make lasers, including tunable lasers that are free from undesirable lasing modes that would be caused by facet reflection. Superluminescent diodes can be tailored to the requirements of a wide variety of optical systems. High-power superluminescent diodes are useful as low-coherent light sources for optical sensing, fiber-optic gyroscopes, and medical instrumentation, and also as gain media for mode-locked lasers and broad-band tunable lasers. Superluminescent diodes are candidates for use as the light source in interferometric fiber optic gyroscopes (IFOG), where a relatively broadband, low-coherence, and high optical power fiber-coupled source is desired. Superluminescent diodes are attractive for applications in which a higher intensity than the one emitted by conventional LEDs is required, but where an even distribution of the emitted wavelength over a broad spectral range is desired. A superluminescent diode is the key element in several applications such as a tunable external cavity semiconductor laser which provides a tunable, stable, and ultra-narrow linewidth (less than 100kHz) laser source, and for optical intensity enhancement using high Q optical cavity. Fiber optic communication systems which utilize direct detection of information signals that are modulated on a carrier do not require a coherent light source such as that produced by a laser. Lasers are used, however, because they are readily available and provide a convenient source of high intensity light. The light source most suitable for the fiber-optic rotation sensor is the superluminescent laser diode. The superluminescent diode emits in a broad spectral band which reduces phase noise in the sensor caused by the Kerr effect and by coherent backscatter in the fiber.