Noise cancellation of white pulsed light with pulse-to-pulse observation of probe and reference pulses in spectral pump/probe measurement

In pump/probe spectroscopy employing supercontinuum (SC) probe light (Pr), multi-wavelength information is provided at the same time. The SC probe generated by a photonic crystal fiber is advantageous owing to the broad spectrum and synchronized timings of the SC Pr pulse and pump pulse. However, SC light accompanies formidable wavelength dependent noise due to the various contributions of generation mechanisms. We previously developed two noise cancellation methodologies suitable for multiwavelength measurements with SC light, that is, phase detection noise cancellation (PDNC) and time division noise cancellation (TDNC), where the pulse timings for Pr and reference (Rf) are time-shifted and detected by a single photodiode (PD) after passing through the common optical system, providing an identical spectral dispersion to conserve the wavelength-dependent noise correlation. In PDNC, Pr and Rf are discriminated based on the pulse repetition phase, and the Pr is then divided by Rf, whereas in TDNC they are distinguished by the detection time, and Rf is subtracted from Pr. However, a shot noise limited performance with high-power input has yet to be realized. In this study, the deterioration mechanisms for PDNC and TDNC are investigated. The nonlinear effect of the photodetector was found to have a non-negligible impact on the phase noise and intensity noise correlation between Pr and Rf. Adequate decay of the photocurrent of Pr at the detection of the Rf optical pulse is effective for avoiding nonlinearity and maintaining the noise correlation of Pr and Rf for the best noise cancellation. Based on this fact, a near-shot-noise-limited performance is demonstrated by the TDNC, where the Rf optical pulse is incident on the PD after sufficient relaxation of the Pr photocurrent. In addition, the coincidence of the timing of Pr and Rf for the noise subtraction was found to be essential for wideband noise cancellation.