Stimulated emissions amplified spontaneous emission

ASE is what happens when a long medium with inverted atoms starts amplifying by stimulated emission the spontaneous emission noise of some atoms at the medium extremity. In such a case all atoms of the medium behave individually and there is no self-building of a macroscopic dipole. Then the intensity remains proportional to N or p, instead of N2 and ffi for SF. Also there is practically no delay, since there is no need for building up the dipole. 

In the literature there are extensive reports on the observation of amplified spontaneous emission (ASE) of polyfluorene [73-76]. ASE occurs at high excitation intensities when it is possible to create a transient excited state population which is greater than the population of the lower lying state to which it radiatively decays, i.e. forming a population inversion. If there is some feedback mechanism of emitted photons, stimulated emission can build... 

The above treatment is valid quite generally, even if f2 — fi > huj. when the denominator in (4.17) is negative. Under the same conditions, the absorption coefficient in (4.15) is also negative, and the spontaneous emission rate in (4.17) remains positive. When the absorption coefficient is negative, stimulated emission overcompensates the rate of upward transitions and the 2-level system amplifies the incident light exactly as in a laser. The condition f2 — fi > tko is also known as the lasing condition and is called inversion. 

When spontaneous emission is amplified by stimulated emission, the spectrum is narrowed because the highest amplification occurs at the wavelength where the gain is the highest. Fig. Vll-8 shows a series of emission spectra from the... 

Spontaneous emission initiates a chain reaction in which a series of stimulated emissions amplifies the number of photons enormously. 

Fig. 3.30. Schematic representation of ci absorption (A absorbs radiation and reaches the excited state A which is not shown in n) b spontaneous emission from two species A (the two emissions arc not correlated) c stimulated emission (incoming radiation on the left-hand side forces two A species to return to the ground state the outcoming radiation on the right-hand side is amplified by a factor 3 as far as the amplitude is concerned)...

Nonlaser light sources emit radiation in all directions as a result of the spontaneous emission of photons by thermally excited solids (filament lamps) or electronically excited atoms, ions, or molecules (fluorescent lamps, etc.). The emission accompanies the spontaneous return of the excited species to the ground state and occurs randomly, Le. the radiation is not coherent, in a laser, the atoms, ions, or molecules are first pumped to an excited state and then stimulated to emit photons by collision of a photon of the same energy. This is called stimulated emission. In order to use it, it is first necessary to create a condition in the amplifying medium, called popidatlon inversion, in which the majority of the relevant entitles are excited. Random emission firom one entity can then trigger coherent emission firom the others that it passes. In this way amplification is achieved. 

The onset of stimulated emission can be detected by a collapse of the broad emission spectrum to a narrow line. This line narrowing due to optical gain in a waveguide without resonator is commonly referred to as amplified spontaneous emission (ASE) [49], or travelling wave lasing, hi the case of additional optical resonators like gratings or microcavities, different modes with much narrower linewidths can be resolved. 

Semiconductor optical amplifiers (SOA), also referred to as semiconductor laser amplifiers (SLA), are devices very similar to semiconductor lasers, which amplify light that is injected into the device. The principle behind the operation of a semiconductor optical amplifier is identical to that of other semiconductor lasers. It is the creation of a population inversion that allows stimulated emission and optical irr. As in a conventional semiconductor laser, the population inversion is achieved by injecting carriers by an electrical current into the active region, which subsequently recombine through spontaneous and stimulated emissioa... 

Auzel s chapter on coherent emission is different from many reviews on the subject, which are concerned with the laser effect itself, in that he concentrates on the broader issues. The emphasis of chapter 151 is on superradiance, superfluorescence, amplification of spontaneous emission by other stimulated emission than the laser effect, and coherent spontaneous emission. Also discussed are up-conversion by energy transfer, up-conversion by the avalanche effect, and recent advances in lanthanide lasers and amplifiers. 

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