near infrared. They calculated that the spectral broadening due to self-phase modulation should have been 910 cm−1, but their continuum was greater than 3000 cm−1. They concluded that "an optical continuum cannot be explained by self-phase modulation alone." They continued by pointing out the difficulties of phase-matching over long lengths of fibre to maintain four wave mixing, and reported an unusual damage mechanism (with hindsight this would probably be considered a very short fibre fuse). They note a much earlier suggestion by Loy and Shen that if the nanosecond pulses consisted of sub-nanosecond spikes in a nanosecond envelope, it would explain the broad continuum. This idea of very short pulses resulting in the broad continuum was studied a year later when Fork et al. reported using 80 fs pulses from a colliding mode-locked laser. The laser's wavelength was 627 nm and they used it to pump a jet of ethylene glycol. They collimated the resulting continuum and measured the pulse duration at different wavelengths, noting that the red part of the continuum was at the front of the pulse and the blue at the rear. They reported very small chirps across the continuum. These observations and others led them to state that self-phase modulation was the dominant effect by some margin. 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. They stated that it was much easier to produce a reliable, repeatable continuum using a femtosecond source. Over the ensuing years this source was developed further and used to examine other liquids. 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. The main advantage of this was that they were able to generate a continuum at the relatively low pump powers of a few kW, compared to previous work. 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