Amplified spontaneous emission

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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.

1 Production of ASE
2 Unwanted ASE
3 References
4 See also

[edit] Production of ASE

ASE is produced when a laser 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 disipates some of the laser's power. In optical amplifiers, ASE limits the achievable gain of the amplifier and increases its noise level.

Devices which produce ASE are called superluminescent sources or superluminescent lasers. ASE sources produce light that has a wide bandwidth, low temporal coherence, but high spatial coherence. Use of ASE seems to be the only way to generate highly collimated beams of high frequency radiation, for which there exist no efficient mirrors. X-ray lasers are actually ASE sources rather than true lasers.

[edit] Unwanted ASE

ASE (together with overheating and background scattering loss) limits the maximal size (and maximal power) of lasers, especially if the main laser field travels across the medium, as it takes place in a laser with short wide resonator. ^[1]

ASE becomes important issue in the disk lasers (active mirrors); whde acrive mirror should have low gain. This requires tha thr round-trip loss scales down at the power scaling of disk lasers. To avoid overheating, the size should be increased at the power scaling. The product of gain and the size of the pumped region should not be much larger than unity; so the background round-trip loss should be scaled down at the power scaling of such lasers ^[2]

ASE creates serious problems in any laser with high gain and/or large size. In particular, edges of large slab of pumped medium should provide the effective extraction or absorption of the ASE; otherwise, the excitation of the gain medium will be depleted by the incoherent ASE rather than by the desired coherent laser radiation.