Supercontinuum generation
in photonic crystal fibers

Stéphane Coen*, Alvin Hing Lun Chau, Rainer Leonhardt, and John D. Harvey

Physics Department
The University of Auckland
Private Bag 92019, Auckland, New Zealand

Jonathan C. Knight, William J. Wadsworth, and Philip St.J. Russell

Optoelectronics group, Department of Physics
University of Bath
Bath, BA2 7AY, United Kingdom





ABSTRACT:
Supercontinuum (SC) generation is a complex nonlinear phenomenon that is characterized by the dramatic spectral broadening of intense light pulses passing through a nonlinear material [1]. It was first observed in 1970[2]. Since then, it has been shown to occur in various nonlinear media and has been extensively used in numerous applications ranging from spectroscopy to ultrashort pulse generation [1]. The spectral slicing of SC laser sources has also been proposed as a means to make a multiwavelength source for wavelength-multiplexed optical telecommunications [3] and, more recently, an ultra-broadband SC spanning more than an octave from the ultraviolet to the infrared has been applied to high precision optical frequency metrology [4].

The development of photonic crystal fibers (PCF) has recently led to the demonstration of white-light SC generation directly from unamplified femtosecond Ti:Sapphire oscillators [5]. PCF are made up of a pure silica core surrounded by an array of microscopic air holes running along their entire length (see the scanning-electron micrographs to the right). The large refractive-index step between silica and air allows light to be concentrated into a very small area, resulting in enhanced nonlinear effects. Moreover, because of the large waveguide contribution to their group-velocity-dispersion, PCF can exhibit very unusual chromatic dispersion characteristics. These two properties are the key for efficient supercontinuum generation.

In contrast to most previous experiments that were relying on femtosecond pump pulses [4,5], we have studied SC generation in PCF with much longer picosecond pump pulses (~ 60 ps). The figure at the top of this page, that shows the spectrum of the light leaving a 10m-long PCF excited with 700 W peak power pulses, demonstrates that SC generation is also possible in those pumping conditions, therefore revealing that ultra-broadband white-light SC generation does not require a complex ultrafast laser.

In our experiments, the spectral broadening has been identified has being due to stimulated Raman scattering and parametric four-wave mixing generation, with a negligible contribution of the self-phase-modulation of the pump pulses [6]. The observation of a strong anti-Stokes Raman component has also revealed the importance of the coupling between stimulated Raman scattering and parametric four-wave-mixing in highly nonlinear photonic crystal fibers, and has also indicated that non-phase-matched processes contribute to the continuum. Additionally, the pump input polarization affects the generated continuum through the influence of polarization modulational instability. To complement our experimental study, a detailed numerical model of SC generation in PCF has also been developed. The numerical results are in good agreement with the experiments. These findings demonstrate the importance of index-guiding photonic crystal fibers for the design of picosecond or nanosecond supercontinuum light sources.

In our talk, we will give a review of past and present supercontinuum experiments and we will describe the different mechanisms and techniques that can be used to generate a supercontinuum spectrum. We will then present our most recent experimental and numerical results.

* Permanent address: Service d'Optique et d'Acoustique, Université Libre de Bruxelles, Av. F.D. Roosevelt 50, CP 194/5, B-1050 Brussels, Belgium

The figure to the right illustrates the broadening of the spectrum and the formation of the supercontinuum for increasing peak input powers. The PCF used here was 3 m long.
REFERENCES:

  1. The Supercontinuum Laser Source, R. R. Alfano, ed. (Springer-Verlag, New-York, 1989).
  2. R.R. Alfano and S.L. Shapiro, "Emission in the region 4000 to 7000 angstrom via four-photon coupling in glass,'' Phys. Rev. Lett. 24, 584-587 (1970).
  3. T. Morioka, K. Mori, and M. Saruwatari, "More than 100-wavelength-channel picosecond optical pulse generation from single laser source using
    supercontinuum in optical fibres," Electron. Lett. 29, 862-864 (1993).
  4. D.J. Jones, S.A. Diddams, J.K. Ranka, A. Stentz, R.S. Windeler, J.L. Hall, and S. T. Cundiff, "Carrier-envelope phase control of femtosecond mode-locked lasers and direct optical frequency synthesis," Science 288, 635-639 (2000).
  5. J.K. Ranka, R.S. Windeler, and A.J. Stentz, "Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,'' Opt. Lett. 25, 25-27 (2000).
  6. S. Coen, A.H.L. Chau, R. Leonhardt, J.D. Harvey, J.C. Knight, W.J. Wadsworth, and P.St.J. Russell, "White light supercontinuum generation with 60-ps pump pulses in a photonic crystal fiber," Opt. Lett. 26, (2001) 1356-1358.