Adiabatic Soliton Compression
Adiabatic soliton compression is a technique for the temporal compression of ultrashort pulses in a fiber. The principle of operation is described in the following. For a fundamental soliton pulse in a fiber, the product of pulse energy and pulse duration is proportional to the group velocity dispersion divided by the nonlinearity of the fiber. Thus, the pulse duration must be reduced if the dispersion is reduced while keeping constant the pulse energy. Significant pulse compression can therefore be obtained by propagating the pulses through a dispersion-decreasing fiber. However, the following conditions must be satisfied:
- The initial pulses must fulfill the soliton condition at the input fiber end.
- The fiber dispersion must be varied sufficiently slowly to allow adiabatic adaptation of the pulses to the fiber parameters (otherwise, the pulses can become distorted). More precisely stated, the dispersion must not vary significantly over a length scale of a soliton period. As the latter scales with the square of the pulse duration, rather long fibers are required if the initial pulses are longer than e.g. 1 ps.
- The fiber dispersion must stay sufficiently constant over the whole spectral range of the compressed pulses. In other words, higher-order dispersion must be sufficiently weak. However, it has been shown that slightly normal dispersion in the wings of the generated pulse spectrum can be beneficial.
Interestingly, there are situations where Raman scattering and higher-order dispersion combine in such a way that the pulse compression stays adiabatic, even though each of the mentioned effects separately would lead to severe pulse distortion .
Even though the method is elegant and powerful, it suffers from the need to use a dispersion-decreasing fiber. The latter requirement is eliminated by a variant of the method, where the fiber has constant dispersion but contains a laser-active dopant which allows the amplification of the pulses. Here, an increasing pulse energy for constant dispersion also results in temporal compression.
Instead of using a dispersion-decreasing fiber, it is also possible to concatenate (fusion-splice) different fibers with different dispersion values. This may lead to more reproducible results, but as the dispersion does not vary continuously, the compression factor and/or the pulse quality can be compromised.
Generally, adiabatic soliton compression is limited to fairly low pulse energies, since the soliton energies of pulses in fibers cannot be made very high. Therefore, the technique is mainly applied to high repetition rate pulse trains, e.g. in the context of optical fiber communications.
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