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Kelly Sidebands

Definition: sidebands in the spectrum of certain mode-locked lasers, related to a periodic disturbance of soliton pulses in the laser resonator

German: Kelly-Seitenbänder

Category: article belongs to category light pulses light pulses


Cite the article using its DOI: https://doi.org/10.61835/opu

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In a soliton mode-locked laser, a quasi-soliton pulse is circulating in the laser resonator. If the effects of chromatic dispersion and nonlinearities per resonator round trip are weak, the pulse essentially experiences their average values plus some periodic disturbance due to the discrete nature of dispersion and nonlinearity. Additional disturbances can result from the periodically occurring losses and amplification in the laser resonator.

The mentioned periodically occurring disturbances couple the soliton to a copropagating dispersive wave. This often does not have strong effects, since the relative phase of soliton and dispersive wave is constantly changing due to the nonlinearity which is experienced only by the soliton. However, particularly in fiber lasers a kind of resonant coupling (or quasi-phase matching) can occur, when for some optical frequencies the relative phase of soliton and dispersive wave changes by an integer multiple of 2<$\pi$> per resonator round trip. This results in the formation of narrow peaks superimposed on the soliton spectrum (see Figure 1). Such peaks are called Kelly sidebands. From their positions in the optical spectrum, information on the spectral profile of the chromatic dispersion of the fiber can be retrieved.

Kelly sidebands
Figure 1: Optical spectrum of the output of a soliton fiber laser, exhibiting Kelly sidebands.

Kelly sidebands are usually not observed in mode-locked bulk lasers, but often in soliton fiber lasers. This is because the intracavity nonlinearity and dispersion is much larger in fiber lasers (→ mode-locked fiber lasers). The nonlinear phase shift per resonator round trip in a soliton fiber laser is inversely proportional to the pulse duration. Therefore, Kelly sidebands become much more pronounced as the pulse duration is decreased e.g. by increasing the pulse energy. When the laser is driven too far into this regime, the soliton pulses become unstable. Strong Kelly sidebands thus indicate that the pulse duration is near the minimum possible value.

For mitigating problems with excessive Kelly sidebands, one should try to reduce the nonlinear phase shift per resonator round-trip, so that the disturbance of the circulating soliton is reduced. For example, one may operate the laser with a longer pulse duration by increasing the amount of anomalous chromatic dispersion (but without increasing higher-order dispersion), or reduce the pulse energy by pumping the laser less strongly.

More to Learn

Encyclopedia articles:


[1]S. M. Kelly, “Characteristic sideband instability of periodically amplified average soliton”, Electron. Lett. 28 (8), 806 (1992); https://doi.org/10.1049/el:19920508
[2]N. J. Smith, K. J. Blow, and I. Andonovic, “Sideband generation through perturbations to the average soliton model”, IEEE J. Lightwave Technol. 10 (10), 1329 (1992); https://doi.org/10.1109/50.166771
[3]J. P. Gordon, “Dispersive perturbations of solitons of the nonlinear Schroedinger equation”, J. Opt. Soc. Am. B 9 (1), 91 (1992); https://doi.org/10.1364/JOSAB.9.000091
[4]M. L. Dennis and I. N. Duling III, “Experimental study of sideband generation in femtosecond fiber lasers”, IEEE J. Quantum Electron. 30 (6), 1469 (1994); https://doi.org/10.1109/3.299472
[5]D. J. Jones, Y. Chen, H. A. Haus, and E. P. Ippen, “Resonant sideband generation in stretched-pulse fiber lasers”, Opt. Lett. 23 (19), 1535 (1998); https://doi.org/10.1364/OL.23.001535

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Questions and Comments from Users


Why are Kelly sidebands not observed on dispersion-managed solitons (near zero dispersion)?

The author's answer:

That's a difficult question, which immediately raises further questions, e.g. how about higher-order dispersion, what dispersion map to you mean, etc.? I am not sure whether the statement behind your question is even generally true. So this question would need to be addressed with a quite sophisticated study.

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