Ultrafast pulse characterization using XPM in silicon

Due to the high-index contrast between the silicon core and silica cladding, the silicon waveguide allows strong optical confinement and large effective nonlinearity, which facilitates low cost chip scale demonstration of all-optical nonlinear functional devices at relatively low pump powers. One of the challenges in ultrafast science is the full characterization of optical pulses in real time. The time-wavelength mapping is proven to be a powerful technique for real time characterization of fast analog signals. Here we demonstrated a technique based on the cross-phase modulation (XPM) between the short pulse and the chirped supercontinuum (SC) pulse in the silicon chip to map fast varying optical signals into spectral domain. In the experiment, when 30 nm linearly chirped supercontinuum pulses generated in a 5 km dispersion-shifted fiber at the normal regime and 2.4 ps pulse are launched into a 1.7 cm silicon chip with 5 μm modal area, a time-wavelength mapped pattern of the short pulses is observed on the optical spectrum analyzer. From the measured spectral mapping the actual 2.4ps temporal pulse profile is reconstructed in a computer. This phenomenon can be extended to full characterization of amplitude and phase information of short pulses. Due to time wavelength mapping this approach can also be used in real time amplitude and phase measurement of ultrafast optical signals with arbitrary temporal width. The high nonlinearity and negligible distortions due to walk off make silicon an ideal candidate for XPM based measurements.