Octave spanning supercontinuum generation in dispersion- managed silicon waveguides
Junxiong Wei 1,3,* , Charles Ciret 2, Maximilien Billet 1,3, François Leo 1, Bart Kuyken 3, and Simon-Pierre Gorza 1
1 OPERA-Photonique, Université libre de Bruxelles (ULB), 50 av. F.D. Roosevelt, CP194/5, B-1050, Bruxelles, Belgium.
2 Laboratoire de Photonique d’Angers EA 4464, Université d’Angers, 2 Bd. Lavoisier, 49000 Angers, France
3 Photonics Research Group, Department of Information Technology, Ghent University-IMEC, B-9000, Ghent, Belgium.
Author e-mail address: Junxiong.wei@ulb.ac.be
Abstract: Coherent supercontinuum spanning [1.2-2.7] μm is experimentally demonstrated in dispersion-managed silicon waveguides. We show that part of the spectrum is broadened by the trapping of dispersive waves, lowering the power to get octave-spanning spectra.
1. Introduction
The integration of supercontinuum (SC) sources on a chip, based on planar waveguide circuits rather than optical fibers, can provide a compact, robust, and power efficient platform that can be used in many applications. The silicon-on-insulator (SOI) platform, showing low propagation loss and high index contrast is one of the leading candidate to achieve this purpose. Silicon also has a large nonlinear coefficient, similar to that of chalcogenide glass and about 200 times higher than the nonlinear index of silica, facilitating ultra-compact nonlinear devices for SC generation at low power. The first demonstrations of octave-spanning SC on SOI chips were demonstrated in dispersion engineered straight waveguides pumped with sub 100 femtosecond pulses near 2.5 μm [1], and 1.9 μm [2]. Following works on SC generation in optical fibres, cascaded waveguides, tapers and more complex dispersion managed integrated structures are investigated numerically and experimentally (see e.g. [3-5]). The interest of varying the dispersion along the propagation lies in the possibility to locally adapt the phase matching conditions required for the generation of new frequencies through the four-wave mixing process. This includes the generation of dispersive waves at a number of different detuning wavelengths of the pump, but also the frequency shifting of these waves by collisions and trapping at event horizons. In our work, we demonstrate how the management of the dispersion along the propagation allows to improve supercontinuum generation compared to more conventional straight and tapered waveguides. We observe more than one octave-spanning supercontinua in 3 mm-long dispersion managed (DM) waveguide, and achieve maximum bandwidth from the edge of the silicon transmission window, ~ 1.17 μm, up to 2.76 μm by pumping at 2260 nm.
2. Experimental Setup and Results
The DM structure consists in 220-nm thick silicon waveguide with six taper sections as is depicted in Fig 1. (a). The positions where the slope changes as well as the waveguide width at those points are given in the Table in Fig 1.
These widths where optimized by numerical simulations following the same methodology as in [3]. The pump source is a mode-locked Cr:ZnSe laser delivering 70 fs and is tunable in the wavelength range [2130-2260] nm, i.e.
close to the two-photon absorption silicon band-edge. At the input, the laser beam is butt-coupled with a high NA microscope objective to excite the TE mode of the waveguide. The light is collected at the output by a lensed fiber and sent to an optical spectrum analyzer.
Fig.1. (a) Schematic geometry of the dispersion managed structures and the nominal width at the adjustment points along the waveguide. (b) Spectra recorded at the DM waveguide output for various input peak powers. Each of the spectra has been shifted by 40 dB for clarity. (c) Comparison between the SC generated in optimal straight (red), single tapered (blue) and DM (cyan) waveguides at 11W input peak power.
Thespectrum at the DM waveguide output is reported in Fig. 1 (b) for seed pulses at 2260 nm and for an on-chip input peak power ranging from 5 to 50 W. When the input power increases, the spectrum first broadens by self- phase modulation. At 11 W peak power, we already observe an octave spanning supercontinuum. The comparison with the SC generated in fixed width and single tapered waveguides shows that the management of the dispersion decreases to power required for reaching octave-spanning spectral broadening (see Fig.1 (c)), demonstrating the interest of the DM design for low energy applications. Increasing further the input power does not significantly change the output spectrum, probably because the spectrum is limited by the energy of the bandgap for short wavelengths and by the losses encountered in the waveguides beyond 2.7 μm.
Fig.2 (a) Numerical simulation of the spectral broadening dynamics. The picture only shows the part of the spectrum below 1500 nm, between z = 1.5 and 3 mm. The wavenumber D at z = 1.5 mm and at z = 2 is plotted in (b).
The numerical simulation of the dynamics in the DM waveguide reveals that, at short wavelengths, the dispersive wave, generated close to 1400 nm after 1.5 mm propagation distance is continuously blue shifted between z = 1.5 mm and z = 2 mm. From the analysis of the dynamics in the time domain and the evolution of the wavenumber D in the corresponding taper section (see Fig.2(b)), we infer that the dispersive wave remains group velocity matched with the temporally compressed pulse. This wave trapping mechanism of the dispersive wave [6] is responsible for its continuous blue shift, as highlighted by the arrows in Fig. 2.
Ideally, the supercontinuum would perform as a broad frequency comb that inherits the spectral coherence from the pump source. Numerical simulations of the first order coherence predict that it remains above 0.99 over the full bandwidth of the spectrally broadened pulse in the DM waveguide, starting from a 70 fs, 47 W-peak input pulse (see Fig. 3(a)). A very high coherence is thus expected over the whole bandwidth of the supercontinuum. As seen in Fig.
3 (b) this is experimentally confirmed in the telecom C-band through the measurement of the beating note between one tooth of the on-chip broadened frequency comb and a narrowband CW laser close to 1520 nm.
Fig.3. (a) Numerically simulated first-order coherence of the SC generated in the DM waveguide at 47 W input power and (b) experimentally recorded beat note with a narrow band laser at 1520 nm.
3. Conclusion
We experimentally demonstrate supercontinuum generation in dispersion managed waveguides pump by sub-100 fs input pulse at 2260 nm. We show that the spectral broadening involves a trapping mechanism of the dispersive waves leading to output spectra that span from 1.1 up to 2.76 μm. These spectra are robust against variations of the input power or pump wavelength and shows excellent coherence properties over their whole bandwidth.
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