Amplified spontaneous emission (ASE) or superluminescence is

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a

gain medium 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 photons through electronic or molecular transitions to a lower energy state from a ...

. It is inherent in the field of

random laser A random laser (RL) is a laser in which optical feedback is provided by scattering particles. As in conventional lasers, a gain medium is required for optical amplification. However, in contrast to Fabry–Pérot cavities and distributed feedback ...

Origins

ASE is produced when a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

gain medium is pumped to produce a

population inversion In science, specifically statistical mechanics, a population inversion occurs while a system (such as a group of atoms or molecules) exists in a state in which more members of the system are in higher, excited states than in lower, unexcited energy ...

Feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...

of the ASE by the laser's optical cavity may produce laser operation if the

lasing threshold The lasing threshold is the lowest excitation level at which a laser's output is dominated by stimulated emission rather than by spontaneous emission. Below the threshold, the laser's output power rises slowly with increasing excitation. Above t ...

is reached. Excess ASE is an unwanted effect in lasers, since it is not coherent, and limits the maximum

gain Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...

that can be achieved in the gain medium. ASE creates serious problems in any laser with high gain and/or large size. In this case, a mechanism to absorb or extract the incoherent ASE must be provided, otherwise the

excitation Excitation, excite, exciting, or excitement may refer to: * Excitation (magnetic), provided with an electrical generator or alternator * Excite Ballpark, located in San Jose, California * Excite (web portal), web portal owned by IAC * Electron exc ...

of the

will be depleted by the incoherent ASE rather than by the desired

coherent Coherence, coherency, or coherent may refer to the following: Physics * Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference * Coherence (units of measurement), a deri ...

laser

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

. ASE is especially problematic in lasers with short and wide optical cavities, such as

disk laser A disk laser or active mirror (Fig.1) is a type of diode pumped solid-state laser characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium. Despite their name, disk lasers do not hav ...

s (active mirrors). ASE can also be a desirable effect, finding use in broadband light sources. If the cavity has no optical feedback, lasing will be inhibited, resulting in a broad emission bandwidth due to the bandwidth of the gain medium. This results in low temporal coherence, offering reduced

speckle noise Speckle, speckle pattern, or speckle noise is a granular noise texture degrading the quality as a consequence of interference among wavefronts in coherent imaging systems, such as radar, synthetic aperture radar (SAR), medical ultrasound and op ...

when compared with a laser. Spatial coherence can be high, however, allowing for tight focusing of the radiation. These characteristics make such sources useful for fiber optic systems and

optical coherence tomography Optical coherence tomography (OCT) is an imaging technique that uses low-coherence light to capture micrometer-resolution, two- and three-dimensional images from within optical scattering media (e.g., biological tissue). It is used for medical ...

. Examples of such sources include superluminescent diodes and doped fiber amplifiers.

ASE in organic dye lasers

ASE in pulsed organic

dye laser A dye laser is a laser that uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths, often spanning 50 to 100 ...

s can have very broad spectral characteristics (as much as 40-50 nm wide) and presents, as such, a serious challenge in the design and operation of tunable narrow-linewidth dye lasers. In order to suppress ASE, in favor of pure laser emission, researchers use various approaches including optimized laser cavity designs.

ASE in disk lasers: Controversy

According to some publications, at the

power scaling Power scaling of a laser is increasing its output power without changing the geometry, shape, or principle of operation. Power scalability is considered an important advantage in a laser design. this means it can increase power without changing outs ...

s, the

round-trip gain Round-trip gain refers to the laser physics, and laser cavities (or laser resonators). It is gain, integrated along a ray, which makes a round-trip in the cavity. At the continuous-wave operation, the round-trip gain exactly compensates both the ...

should be reduced, which means hardening of requirement on the background loss. Other researchers believe the existing disk lasers work far from such a limit, and the

can be achieved without modification of existing laser materials .

ASE in self healing dye doped polymers

In 2008, a group at Washington state university observed reversible

photodegradation Photodegradation is the alteration of materials by light. Commonly, the term is used loosely to refer to the combined action of sunlight and air, which cause oxidation and hydrolysis. Often photodegradation is intentionally avoided, since it destro ...

or simply, self healing in organic dyes like Disperse orange 11 when doped in polymers. They used amplified spontaneous emission as a probe to study self healing properties.

ASE in high-power short-pulse laser systems

In high-power CPA-laser systems with a peak power of several terawatt or petawatt, e.g. the POLARIS laser system, the ASE limits the temporal intensity contrast. After the compression of the laser pulse, which is temporally stretched during the amplification, the ASE causes a quasi-continuous pedestal which is partly located at times before the compressed laser pulse. Due to the high intensities within the focal spot of up to 10^22 W/cm² the ASE is often sufficient to significantly disturb the experiment or even make the desired laser-target interaction impossible.

References