Supercontinuum generation

When an intense pulse propagates through a medium with a strong third-order nonlinearity, several processes broaden its spectrum at once, and together they can turn a pulse a few nanometers wide into a continuum hundreds of nanometers or more across. The usual medium is a fiber, because it holds the light at high intensity over a long length, and the most dramatic results come from microstructured (photonic crystal) fibers, whose small silica core surrounded by air holes gives a large nonlinearity and a zero-dispersion wavelength that can be placed near the pump, around 800 nm or 1 µm.

Which processes dominate depends on the pulse and on the sign of the group-velocity dispersion. With femtosecond pulses in the anomalous-dispersion regime, the pulse first compresses as a higher-order soliton and then breaks into fundamental solitons, a step called soliton fission. The soliton order is $N = \sqrt{\gamma P_0 T_0^2/|\beta_2|}$; for a nonlinear coefficient of 0.1 W⁻¹m⁻¹, 10 kW peak power, $T_0$ = 50 fs (88 fs full width) and $\beta_2$ = −10 ps²/km, $N$ = 15.8, and fission occurs after roughly $L_D/N$, 1.6 cm of fiber. The ejected solitons shift toward longer wavelengths through the Raman self-frequency shift, each shedding dispersive waves on the normal-dispersion side, and four-wave mixing and cross-phase modulation between them fill in the gaps. With longer pulses or pumping in normal dispersion, self-phase modulation and optical wave breaking give a smoother but narrower spectrum.

Coherence is the specification that distinguishes applications. With short pulses and a modest soliton order, the continuum is reproducible from shot to shot, and its spectral phase is fixed, so it can serve a frequency comb: an octave-spanning continuum allows the f-to-2f comparison, in which a comb line at $f_n$ is doubled and beaten against the line at $f_{2n}$, reading out the carrier-envelope offset frequency. Long pulses and high soliton orders instead amplify noise through modulation instability, so each shot's spectrum differs, which suits applications that need broad average spectra but not phase stability.

Commercial supercontinuum sources, a picosecond ytterbium fiber laser feeding a photonic crystal fiber, provide a few watts from the visible to beyond 2 µm and serve as tunable sources for fluorescence microscopy, broadband device testing and spectroscopy. The same broad spectrum gives fine axial resolution in optical coherence tomography. On chips, silicon nitride waveguides produce octave spans from much lower pulse energies, and microcombs reach similar spans with continuous-wave pumping.

References: J. M. Dudley, G. Genty, S. Coen, Rev. Mod. Phys. 78, 1135 (2006); J. K. Ranka, R. S. Windeler, A. J. Stentz, Opt. Lett. 25, 25 (2000).