In optics, a supercontinuum is formed when a collection of nonlinear processes act together upon a pump beam in order to cause severe spectral broadening of the original pump beam, for example using a microstructured optical fiber. The result is a smooth spectral continuum (see figure 1 for a typical example). There is no consensus on how much broadening constitutes a supercontinuum; however researchers have published work claiming as little as 60 nm of broadening as a supercontinuum. There is also no agreement on the spectral flatness required to define the bandwidth of the source, with authors using anything from 5 dB to 40 dB or more. In addition the term supercontinuum itself did not gain widespread acceptance until this century, with many authors using alternative phrases to describe their continua during the 1970s, 1980s and 1990s.
In the decade leading up to 2014, the development of supercontinua sources emerged as a research field. This is largely due to technological developments, which have allowed more controlled and accessible generation of supercontinua. This renewed research has created a variety of new light sources which are finding applications in a diverse range of fields, including optical coherence tomography, frequency metrology, fluorescence lifetime imaging, optical communications, gas sensing,H. Delbarre and M. Tassou, Atmospheric gas trace detection with ultrashort pulses or white light continuum, in Conference on Lasers and Electro-Optics Europe, (2000), p. CWF104.M. Ere-Tassou, C. Przygodzki, E. Fertein, and H. Delbarre, Femtosecond laser source for real-time atmospheric gas sensing in the UV - visible, Opt. Commun. 220, 215–221 (2003). and many others. The application of these sources has created a feedback loop whereby the scientists utilising the supercontinua are demanding better customisable continua to suit their particular applications. This has driven researchers to develop novel methods to produce these continua and to develop theories to understand their formation and aid future development. As a result, rapid progress has been made in developing these sources since 2000.
While supercontinuum generation has for long been the preserve of fibers, in the years leading up to 2012, integrated waveguides came of age to produce extremely broad spectra, opening the door to more economical, compact, robust, scalable, and mass-producible supercontinuum sources.
Historical overview
The 1960s and 1970s In 1964 Jones and Stoicheff reported using a continua generated by a maser to study induced Raman absorption in liquids at optical frequencies. It had been noted by Stoicheff in an early publication that "when the maser emission was in a single sharp spectral line, all the Raman emission lines were sharp; whenever the maser emission contained additional components, all of the Raman emission lines, with the exception of the first Stokes line, were considerably broadened, sometimes up to several hundred cm−1." These weak continua, as they were described, allowed the first Raman absorption spectroscopy measurements to be made.
In 1970 Alfano and Shapiro reported the first measurements of frequency broadening in crystals and glasses using a frequency doubled Nd:Glass mode-locked laser. The output pulses were approximately 4 ps and had a pulse energy of 5 mJ. 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. The filaments themselves were of no real use as a source; nevertheless the authors suggested that the crystals might prove useful as ultrafast light gates. Alfano is the discoverer and inventor of the supercontinuum in 1970 with three seminal articles in same issue of Phy Rev Letters (24, 592,584,1217(1970)) on ultimate white light source now called supercontinuum.
The study of atomic vapours, organic vapours, and liquids by Raman absorption spectroscopy through the 1960s and 1970s drove the development of continua sources. By the early 1970s, continua formed by nanosecond duration flash lamps and laser-triggered breakdown spark in gases, along with laser-excited fluorescence continua from scintillator dyes, were being used to study the excited states. These sources all had problems; what was required was a source that produced broad continua at high power levels with a reasonable efficiency. In 1976 Lin and Stolen reported a new nanosecond source that produced continua with a bandwidth of 110-180 nm centred on 530 nm at output powers of around a kW. The system used a 10-20 kW dye laser producing 10 ns pulses with 15-20 nm of bandwidth to pump a , 7 μm core diameter silica fibre . They could only manage a coupling efficiency in the region of 5-10%.
By 1978 Lin and Nguyen reported several continua, most notably one stretching from 0.7-1.6 μm using a GeO-doped silica fibre with a 33 μm core. 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. Indeed, they had so much power available to them that two thirds was attenuated away to prevent damage to the fibre. The 50 kW coupled into the fibre emerged as a 12 kW continuum . Stokes lines were clearly visible up to 1.3 μm, at which point the continuum began to smooth out, except for a large loss due to water absorption at 1.38 μm. As they increased the launch power beyond 50 kW they noted that the continuum extends down into the green part of the visible spectrum. However, the higher power levels quickly damaged their fibre. In the same paper they also pumped a single mode fibre with a 6 μm core diameter and "a few 100 m in length." It generated a similar continuum spanning from 0.9 μm to 1.7 μm with reduced launch and output powers. Without realising it, they had also generated optical solitons for the first time.
The 1980s In 1980 Fujii et al. repeated Lin's 1978 setup with a mode-locked Nd:YAG. 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. Unusually, they did not report their pulse duration. Their spectrum spanned the entire spectral window in silica from 300 nm to 2100 nm. The authors concerned themselves with the visible side of the spectrum and identified the main mechanism for generation to be four-wave mixing of the pump and Raman generated Stokes. However, there were some higher order modes, which were attributed to sum-frequency generation between the pump and Stokes lines. The phase-matching condition was met by coupling of the up-converted light and the quasi-continuum of cladding modes.
A further advance was reported by Washio et al. in 1980 when they pumped of single-mode fibre with a 1.34 μm Q-switched Nd:YAG laser. This was just inside the anomalous dispersion regime for their fibre. The result was a continuum which stretched from 1.15 to 1.6 μm and showed no discrete Stokes lines.
Up to this point no one had really provided a suitable explanation why the continuum smoothed out between the Stokes lines at longer wavelengths in fibres. In the majority of cases this is explained by soliton mechanisms; however, solitons were not reported in fibres until 1985.E. Golovchenko, E. M. Dianov, A. Prokhorov, and V. Serkin, Decay of optical solitons, JETP Lett. 42, 87–91 (1985). It was realised that self-phase modulation could not account for the broad continua seen, but for the most part little else was offered as an explanation.
In 1982 Smirnov et al.V. Grigor'yants, V. I. Smirnov, and Y. Chamorovski, Generation of wide-band optical continuum in fiber waveguides, Sov. J. Quant. Elect. 12, 841–847 (1982). reported similar results to that achieved by Lin in 1978. 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. 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 continua mostly in the visible region.R. R. Alfano, The Supercontinuum Laser Source: Fundamentals With Updated References (Springer, 2006), 2nd ed. Self-phase modulation was normally used to explain the processes although from the mid-1980s other explanations were offered, including second harmonic generation cross-phase modulation and induced phase modulation. 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.