Amplified spontaneous emission (ASE) or super-luminescence is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a gain medium. It is inherent in the field of random lasers. Spontaneous emission is the process in which a quantum mechanical system (such as an atom, molecule or subatomic particle) transitions from an excited energy state to a lower energy state (e.g., its ground state) and emits a quantum in the form of a photon. Spontaneous emission is ultimately responsible for most of the light we see all around us, it is so ubiquitous that there are many names given to what is essentially the same process. If atoms (or molecules) are excited by some means other than heating, the spontaneous emission is called luminescence.
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed.
Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to the electromagnetic field, creating a new photon with a phase, frequency, polarization, and direction of travel that are all identical to the photons of the incident wave. This is in contrast to spontaneous emission, which occurs at random intervals without regard to the ambient electromagnetic field.
The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source. In order to fire a laser, the active gain medium must be in a non-thermal energy distribution known as a population inversion. The preparation of this state requires an external energy source and is known as laser pumping. Pumping may be achieved with electrical currents (e.g. semiconductors, or gases via high-voltage discharges) or with light, generated by discharge lamps or by other lasers (semiconductor lasers).
Principles of operation
A super-luminescent light emitting diode is, similar to a laser diode, based on an electrically driven pn-junction that, when biased in forward direction, becomes optically active and generates amplified spontaneous emission over a wide range of wavelengths. The peak wavelength and the intensity of the SLED depend on the active material composition and on the injection current level. SLEDs are designed to have high single pass amplification for the spontaneous emission generated along the waveguide but, unlike laser diodes, insufficient feedback to achieve lasing action. This is obtained very successfully through the joint action of a tilted waveguide and anti-reflection coated (ARC) facets.
When an electrical forward voltage is applied an injection current across the active region of the SLED is generated. Like most semiconductor devices, a SLED consists of a positive (p-doped) section and a negative (n-doped) section. Electric current will flow from the p-section to the n-section and across the active region that is sandwiched in between the p- and n-section. During this process, light is generated through spontaneous and random recombination of positive (holes) and negative (electrons) electrical carriers and then amplified when travelling along the waveguide of a SLED.
The pn-junction of the semiconductor material of a SLED is designed in such a way that electrons and holes feature a multitude of possible states (energy bands) with different energies. Therefore, the recombination of electron and holes generates light with a broad range of optical frequencies, i.e. broadband light.
Applications of SLEDs
SLEDs find application in situations demanding high intensity and spatial coherence but where a need for a broad, smooth optical output spectrum makes laser diodes unsuitable. Some examples include optical coherence tomography, white light interferometry, optical sensing and fiber optic gyroscopes.