Investigation Of Picosecond Pulses Pumped Supercontinuum Generation In SOI Waveguides

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. Supercontinuum generation has attracted a great deal of attention in recent years because of its broad applications in frequency metrology, spectroscopy, bio-photonic, and wavelength-division multiplexing(WDM) communications. The generation of broadband light in fiber has been experimentally and theoretically investigated, and broad spectrum has been achieved. However, the smaller nonlinear coefficient of fiber means long transmission distance when supercontinuum generation occurred in fiber, which is hard to be integrated. Huge effective nonlinearity, the ability for dispersion engineering and CMOS(complementary metal-oxide-semiconductor) compatibility mean that SOI waveguides are promising in the spectrum generation field. However, spectral broadening in silicon is restricted by two-photon absorption(TPA), free-carrier absorption(FCA) and narrow Raman gain spectrum with a picosecond or longer input pulse. This leads to much smaller broadening factors in silicon than those in silica glass.In this paper, we discuss the nonlinear processes that occur in silicon waveguide. We provide a theoretical background and develop the description of pulse propagation in silicon waveguide in the time domain and frequency domain. We also introduce a fourth order Runge-kutta algorithm to solve the nonlinear propagation equation. Using this model, we investigate the dynamics of continuum generation in silicon waveguide. We compare the different influence of self-phase modulation and free-carrier refraction on pulse propagation. We consider the relative importance of the two-photon absorption and free-carrier absorption. We also give a brief description of the effect of modulation instability. At last, we discuss the limiting nature of the spectrum broadening in silicon waveguide and find that spectral broadening in silicon waveguide is mainly restricted by two-photon absorption and free-carrier absorption. On this basis, we propose the improved pulse shaping method, which is achieved by adding chirp and cubic spectral phase to the pump pulse. Adding cubic spectral phase can change the shape of the input pulse, which can decrease the effect of FCA, and the chirp will improve the SPM simultaneously. Properly pre-shaping the input chirped pulse not only extends the broadening limits, but also improves the stability of the output spectrum. The simulation results show that the broadening factor can be greatly increased using the pulse shaping method. The output spectrum exhibits good coherence in the spectral range of 100 nm and the signal to noise ratio(SNR) can be up to 15 dB in this region. Compared with the seeded supercontinuum generation, there is no requirement for the external seeding light. Thus makes the overall system more compact and provides an alternative method for the development of on-chip ultra-broadband light sources. Such stable silicon supercontinuum generation light sources are of great importance to many practical and commercial applications, such as frequency metrology and optical communication.