Accuracy of Waveform Spectrum Analysis for Ultrashort Optical Pulses

We investigate the waveform power spectrum (WPS) measurement of ultrashort pulses using the nonlinear Kerr effect in an optical waveguide. Our study focuses on a recent experiment reporting the WPS measurement of 260-fs pulses using a 6-cm-long, highly nonlinear chalcogenide (ChG) planar waveguide. By numerical simulation of the underpinning nonlinear propagation, we show the importance of low chromatic dispersion in the waveguide for avoiding measurement inaccuracy due to bandwidth narrowing and asymmetric distortion of the WPS. We also show the distortion effect of excessive input power on broadening the WPS bandwidth. In comparison to using conventional nonlinear fibers, highly nonlinear ChG waveguides are shown to generally enable a more accurate and broadband measurement of shorter pulses over a multiterahertz frequency range, and for a wider ranging signal wavelength. Furthermore, higher order dispersion effects are also avoided. Experiments with nonlinear fiber show its capability to measure the WPS of high-speed 640-Gb/s data signals, albeit for broader pulses and with less wavelength flexibility. Furthermore, the WPS is used to effectively retrieve the signal autocorrelation waveform. The factors impacting the measurement accuracy is compared to other recent experiments using ChG and silicon waveguides. Analysis shows that extending the technique to pulses shorter than 260 fs requires further optimizing the ChG waveguide dispersion, which would benefit broadband signal processing in general.