Interferometric autocorrelation measurements of supercontinuum based on two-photon absorption

Abstract

We report on interferometric autocorrelation measurements of broadband supercontinuum light in the anomalous dispersion regime using two-photon absorption in a GaP photodetector. The method is simple, low-cost, and provides a direct measure of the second-order coherence properties, including quantitative information on the coherence time and average duration of the supercontinuum pulses as well as on the presence of temporally coherent sub-structures. We report measurements in regimes where the supercontinuum is coherent and incoherent. In the former case, the interferometric measurements are similar to what is observed for mode-locked laser pulses while in the latter case the interferometric measurements and coherence properties are shown to have characteristics similar to that of a stationary chaotic light source.

Optical Autocorrelation Measurement for Ultrafast Pulses at NIR Wavelengths Using GaP, GaAsP, and Si Photoconductive Detectors

“…Inside the femtosecond laser (Insight X3), there was an extra-cavity GDD setup outside the oscillator to control the pulse width and deliver the shortest pulses at a particular center wavelength, to meet the demands of specific applications. The AC function for a coherent light source yields a peak-tobackground ratio of 2:0, while the ratio can be increased to 8:1 for the IAC function [23,29]. Most semiconductor PDs have intrinsic sharp bandgaps; however, owing to their material inhomogeneity, their linear absorption bands exhibit smooth response curves over a limited wavelength range across their inherent bandgaps-such as the spectral response curves over the 300-680 nm range of GaAsP PDs.…”

“…These techniques still require nonlinear crystals or media to obtain spectrograms or sonograms for retrieving the amplitude and phase of ultrashort pulses. Although IAC does not provide a direct measure of the phase structure of an input pulse, it nonetheless provides useful information about coherent artifacts and the presence of coherent sub-structures, and provides a clear measure of second-order coherence characteristics for determining the pulse characteristics [23]. Stable ultrafast laser systems can be challenging to develop and maintain; this is because multiple scatter pulses with variable separations and relative phases with respect to the main pulses can be easily induced by over-pumping the power and due to the improper choice of output coupler transmittance.…”

Towards a monolithic, multi-gigahertz mode-locked Ti:Sa laser

“…With an incident pump power of up to 8.5 W and 60% absorption, output powers up to 1.8 W with power stability of ≤ 0.04% rms over one hour and 10 nm bandwidth are produced. The interferometric autocorrelation shows a contrast of just 3:1 instead of 8:1 which indicates that we are not seeing mode-locked pulses [4]. The laser beam was then sent into a pulse stretcher introducing 10 4 fs² and later 10 6 fs² of group delay dispersion and then measured again by the autocorrelator.…”