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Superluminescent Sources

Acronym: SFS

Definition: optical sources based on superluminescence

German: superlumineszente Lichtquellen

Categories: photonic devices, non-laser light sources

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URL: https://www.rp-photonics.com/superluminescent_sources.html

Superluminescent sources (also called ASE sources) are broadband light sources (white light sources) based on superluminescence. (They are sometimes erroneously called superfluorescent sources, which would be based on the substantially different phenomenon of superfluorescence.) Essentially, a superluminescent source contains a laser gain medium which is excited in order to emit and then amplify luminescent light.

A superluminescent source has a very low temporal coherence, resulting from the large emission bandwidth (compared with that of, e.g., a laser). That greatly reduces the tendency for laser speckle, as are often observed with laser beams, e.g. from laser diodes. On the other hand, the spatial coherence is usually very high: the output of a superluminescent source can be very well focused (similar to a laser beam) and is thus suitable for obtaining by far higher optical intensities than with an incandescent lamp, for example. This combination of low temporal and high spatial coherence makes such devices interesting for applications such as optical coherence tomography (OCT) (e.g. in the medical sector), device characterization (e.g. in optical fiber communications), gyroscopes, and fiber-optic sensors. See the article on superluminescent diodes for more details on applications.

The main kinds of superluminescent sources are superluminescent diodes (SLDs) and fiber amplifiers. Fiber-based sources can provide much higher output powers, whereas SLDs are much more compact and also cheaper. In both cases, the emission bandwidth is at least several nanometers and often tens of nanometers, sometimes even well above 100 nm.

For all high-gain ASE sources, it is very important carefully to suppress any optical feedback, e.g. via reflections from fiber ends, because this can lead to parasitic lasing. (Using a Faraday isolator may not be sufficient.) For fiber-based devices, Rayleigh scattering from within the fiber may introduce the final performance limitations.

ASE spectra of ytterbium-doped fiber amplifier
Figure 1: Spectra of ASE from a fiber amplifier, calculated for different pump power levels. With increasing power, the optical spectrum shifts toward shorter wavelengths (where the gain grows more quickly) and becomes narrower. The former effect is typical for sources with quasi-three-level laser gain media, whereas the latter occurs in essentially all types of superluminescent sources. The simulation was done with the RP Fiber Power software.

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Bibliography

[1]M. J. F. Digonnet, “Theory of superfluorescent fiber lasers”, IEEE J. Lightwave Technol. 4 (11), 1631 (1986), doi:10.1109/JLT.1986.1074661
[2]P. F. Wysocki et al., “Broadband fiber sources for gyros”, Proc. SPIE 1585, 371 (1992), doi:10.1117/12.135068
[3]P. F. Wysocki et al., “Characteristics of erbium-doped superfluorescent fiber sources for interferometric sensor applications”, IEEE J. Lightwave Technol. 12 (3), 550 (1994), doi:10.1109/50.285318
[4]R. Paschotta et al., “Efficient superfluorescent light sources with broad bandwidth”, J. Sel. Top. Quantum Electron. 3 (4), 1097 (1997), doi:10.1109/2944.649547
[5]P. Wang and W. A. Clarkson, “High-power, single-mode, linearly polarized, ytterbium-doped fiber superfluorescent source”, Opt. Lett. 32 (17), 2605 (2007), doi:10.1364/OL.32.002605
[6]G. Smith et al., “High-power near-diffraction-limited solid-state amplified spontaneous emission laser devices”, Opt. Lett. 32 (13), 1911 (2007), doi:10.1364/OL.32.001911
[7]D. Y. Shen et al., “Broadband Tm-doped superfluorescent fiber source with 11 W single-ended output power”, Opt. Express 16 (15), 11021 (2008), doi:10.1364/OE.16.011021
[8]M. Blazek et al., “Unifying intensity noise and second-order coherence properties of amplified spontaneous emission sources”, Opt. Lett. 36 (17), 3455 (2011), doi:10.1364/OL.36.003455
[9]R. Paschotta, case study on a fiber-based ASE source

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See also: superluminescence, amplified spontaneous emission, superluminescent diodes, white light sources, coherence
and other articles in the categories photonic devices, non-laser light sources

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