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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-06

(Acronym: ASE)

Definition: a process where spontaneously emitted radiation (luminescence) is amplified

In a laser medium with large gain, the luminescence from spontaneous emission can be amplified to high power levels. This amplified fluorescence may be used in applications where light with low temporal coherence but good spatial coherence is required. It also occurs in lasers, even when operated below the laser threshold.

Whereas fluorescence originally goes in all spatial directions, ASE can be strongly directional for gain media with a large aspect ratio. As an extreme case, consider a fiber laser or fiber amplifier, where ASE propagating along the fiber can be much more powerful than the omnidirectional fluorescence emission.

In lasers and particularly in high-gain amplifiers, amplified spontaneous emission is usually an unwanted effect. It tends to limit the achievable gain in a single stage of a fiber amplifier to the order of 40-50 dB. Higher gain values are possible e.g. for amplification of pulses, if several amplifier stages are used, which are separated by filters, isolators, and/or optical modulators (switches). Particularly in some fiber lasers, ASE can prevent lasing at extreme wavelengths, if the gain at other wavelengths is high enough for generating strong ASE. Such problems can often be overcome by optimizing the overall laser design, with special attention to fiber length, doping level and the like. Similar challenges arise in the context of some bulk lasers, e.g., Nd:YAG lasers operating at 946 nm, where strong ASE at 1064 nm can even prevent 946-nm lasing.

Even if amplified spontaneous emission in an amplifier is not strong enough to extract significant power, it can contribute significantly to the noise of the amplified signal. The noise figure of a laser amplifier can be considered to be limited by ASE. Note that for quasi-three-level gain media this ASE effect is stronger than for four-level media.

Figure 1: ASE spectra of forward and backward ASE in a forward-pumped ytterbium-doped fiber amplifier, calculated with the RP Fiber Power software of RP Photonics.

As shown in Figure 1, the spectrum of ASE in the output of a rare-earth-doped fiber amplifier can differ strongly from that of the side fluorescence. This is because of wavelength-dependent amplification and reabsorption. (The latter occurs only for quasi-three-level gain media.) Furthermore, ASE powers emitted in forward and backward direction in a fiber amplifier can differ. Usually, ASE is stronger in the direction opposite to that of pumping. Finally, the spectral shape of ASE can depend on the pump intensity level (see Figure 2).

Figure 2: Spectra of backward ASE in the same fiber amplifier as above, calculated for different pump power levels. With increasing power, the spectrum shifts toward shorter wavelengths (where the gain grows more quickly) and becomes narrower.

Interesting aspects come into play when ASE occurs in waveguide structures such as fibers. For example, the amount of ASE into a single-mode core does not depend on the detailed waveguide parameters (e.g., on the core diameter and numerical aperture). Contrary to a common belief, the ASE power depends only on the number of modes, not on particular mode properties. Considerations involving some "capture fraction" for spontaneous emission, which is limited by the condition of total internal reflection, are only approximate – appropriate for strongly multimode waveguides, but not for few-mode or single-mode situations. (See the Photonics Spotlight of 2007-08-06 for details.) Some fiber amplifier models are wrong in this respect.

ASE is sometimes also called superluminescence, and correspondingly there is the term superluminescent sources (also ASE sources or white light sources) for light sources emitting superluminescent radiation. Such broadband (but usually spatially coherent) sources are useful for a number of applications.

A phenomenon related to ASE, but with important physical differences, is superfluorescence.

Bibliography

See also: spontaneous emission, amplifiers, laser noise, amplifier noise, superluminescent sources, superluminescence, white light sources, Spotlight article 2007-07-30, Spotlight article 2007-08-06

Categories: amplifiers, fluctuations and noise, lasers, quantum optics