Amplified spontaneous emission
Encyclopedia
Amplified spontaneous emission (ASE) or superluminescence 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 laser
s.
gain medium is pumped
to produce a population inversion
. Feedback
of the ASE by the laser's optical cavity
may produce laser operation if the lasing threshold
is reached. Excess ASE is an unwanted effect in lasers, since it limits the maximum gain
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
of the gain medium will be depleted by the incoherent ASE rather than by the desired coherent
laser radiation
. ASE is especially problematic in lasers with short and wide optical cavities, such as disk laser
s (active mirrors)..
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.
power scaling
of disk laser
s, the round-trip gain
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 power scaling
can be achieved without modification of existing laser materials
.
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
, produced by spontaneous emission
Spontaneous emission
Spontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy, e.g., the ground state and emits a photon...
, that has been optically amplified
Optical amplifier
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...
by the process of stimulated emission
Stimulated emission
In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...
in a gain medium. It is inherent in the field of random laser
Random laser
A random laser is a laser that uses a highly disordered gain medium. A random laser uses no optical cavity but the remaining principles of operation remain the same as for a conventional laser...
s.
Origins and problems
ASE is produced when a laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
gain medium is pumped
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...
to produce a population inversion
Population inversion
In physics, specifically statistical mechanics, a population inversion occurs when a system exists in state with more members in an excited state than in lower energy states...
. Feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
of the ASE by the laser's optical cavity
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
may produce laser operation if the lasing threshold
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 threshold, the slope of power vs. excitation is orders...
is reached. Excess ASE is an unwanted effect in lasers, since it limits the maximum gain
Gain
In electronics, gain is a measure of the ability of a circuit to increase the power or amplitude of a signal from the input to the output. It is usually defined as the mean ratio of the signal output of a system to the signal input of the same system. It may also be defined on a logarithmic scale,...
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
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
of the gain medium will be depleted by the incoherent ASE rather than by the desired coherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
laser radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
. ASE is especially problematic in lasers with short and wide optical cavities, such as disk laser
Disk laser
A disk laser or active mirror is a type of 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...
s (active mirrors)..
ASE in organic dye lasers
ASE in pulsed organic dye laserDye laser
A dye laser is a laser which 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. The wide bandwidth makes them particularly suitable for tunable lasers and...
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 thepower scaling
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....
of disk laser
Disk laser
A disk laser or active mirror is a type of 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...
s, the round-trip gain
Round-trip gain
Round-trip gain refers to the laser physics, and laser cavitys .It is gain, integrated along a ray, which makes a round-trip in the cavity....
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 power scaling
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....
can be achieved without modification of existing laser materials
.