Supercontinuum

[14][15] In 1964 Jones and Stoicheff[16] reported using a continua generated by a maser to study induced Raman absorption in liquids at optical frequencies.

The filaments formed produced the first white light spectra in the range from 400-700 nm and the authors explained their formation through self-phase modulation and four-wave mixing.

[20] The system used a 10-20 kW dye laser producing 10 ns pulses with 15-20 nm of bandwidth to pump a 19.5 m long, 7 μm core diameter silica fibre .

[21] The optical setup was similar to Lin's previous work with Stolen, except in this instance the pump source was a 150 kW, 20 ns, Q-switched Nd:YAG laser.

[22] The peak power of the pulses was reported as being greater than 100 kW and they achieved better than 70% coupling efficiency into a 10 μm core single-mode Ge-doped fibre.

Using multimode phosphosilicate fibres pumped at 0.53 and 1.06 μm, they saw the normal Stokes components and a spectrum which extended from the ultraviolet to the near infrared.

However they also noted that their calculations showed that the continuum remained much larger than self-phase modulation would allow, suggesting that four-wave mixing processes must also be present.

[30] In the same year Nakazawa and Tokuda reported using the two transitions in Nd:YAG at 1.32 and 1.34 μm to pump a multimode fibre simultaneously at these wavelengths.

They attributed the continuum spectrum to a combination of forced four-wave mixing and a superposition of sequential stimulated Raman scattering.

[31] During the early to late 1980s Alfano, Ho, Corkum, Manassah and others carried out a wide variety of experiments, though very little of it involved fibres.

The majority of the work centred on using faster sources (10 ps and below) to pump various crystals, liquids, gases, and semiconductors in order to generate continua mostly in the visible region.

[34] Indeed, efforts were made to explain why self-phase modulation might well result in much broader continua, mostly through modifications to theory by including factors such as a slowly varying amplitude envelope among others.

A year later Gouveia-Neto et al.[38] from the same group published a paper describing the formation and propagation of soliton waves from modulation instability.

They noted pulses emerging with durations of less than 500 fs (solitons) and as they increased the pump power a continuum was formed stretching from 1.3 to 1.5 μm.

Gross et al. in 1992 published a paper modelling the formation of supercontinua (in the anomalous group velocity dispersion region) when generated by femtosecond pulses in fibre.

In 1993 Morioka et al.[9] reported a 100 wavelength channel multiplexing scheme which simultaneously produced one hundred 10 ps pulses in the 1.224-1.394 μm spectra region with a 1.9 nm spectral spacing.

Advances made during the 1980s meant that it had become clear that to get the broadest continua in fibre, it was most efficient to pump in the anomalous dispersion regime.

This work was followed by others pumping short lengths of PCF with zero-dispersions around 800 nm with high-power femtosecond Ti:sapphire lasers.

Lehtonen et al.[43] studied the effect of polarization on the formation of the continua in a birefringent PCF, as well as varying the pump wavelength (728-810 nm) and pulse duration (70-300 fs).

Herrmann et al. provided a convincing explanation of the development of femtosecond supercontinua, specifically the reduction of solitons from high orders down to the fundamental and the production of dispersive waves during this process.

These chip-scale platforms promise to miniaturize supercontinuum sources into devices that are compact, robust, scalable, mass producible and more economical.

The breakthroughs in recent years have involved understanding and modelling how all these processes interact together to generate supercontinua and how parameters can be engineered to enhance and control continuum formation.

In the soliton fission regime a short, high-power, femtosecond pulse is launched into the PCF or other highly nonlinear fiber.

Generally these dispersive waves will undergo no further shifting[49] and thus the extension short of the pump is dependent on how broadly the soliton expands as it breathes.

The first supercontinuum generated in PCF operated in this regime[5] and many of the subsequent experiments also made use of ultra-short pulsed femtosecond systems as a pump source.

[49] One of the main advantages of this regime is that the continuum often exhibits a high degree of temporal coherence,[49] in addition it is possible to generate broad supercontinua in very short lengths of PCF.

Disadvantages include an inability to scale to very high average powers in the continuum, although the limiting factor here is the available pump sources; and typically the spectrum is not smooth due to the localised nature of the spectral components which generate it.

As the MI process is noise driven, a distribution of solitons with different energies are created, resulting in different rates of self-frequency shifting.

In this case soliton trapping has been shown to play a role in short wavelength generation in the MI driven regime.

However, if the pulses are not ultra-short then stimulated-Raman scattering tends to dominate and typically a series of cascaded discrete Stokes lines will appear until the zero-dispersion wavelength is reached.