Spectro-Temporal Characterization of Tunable Supercontinuum Using X-FROG Measurements

Optical wave-packet control and optimization is highly sought-after in numerous demanding applications. Recent advances have shown the potential of programable photonic integrated chips (PICs) [1] for enabling new optical functionalities. For instance, programmable delay lines (PDL) can be used for autonomous picosecond pulse shaping [2] as well as supercontinuum (SC) spectral shaping [3]. Recent work also demonstrated the potential of such approaches for gaining access to advanced spectro-temporal control [4] which would directly benefit applications such as nonlinear imaging and multiphoton microscopy [5]. Here, we demonstrate the spectro-temporal characterization and adaptive adjustment of broadband supercontinua (SC). The experimental setup shown in Fig. 1(a) comprises a femtosecond laser at 1550 nm, injected into an on-chip PDL [2]–[4] and used to generate a train of femtosecond pulses with a tunable pattern. This train of pulses is then amplified and sent into a highly nonlinear fiber (HNLF) to experience nonlinear spectral broadening. The broadband output SC spectrum is finally characterized via an X-FROG [6] setup that mixes the SC with an asynchronous femtosecond laser source (at 1040 nm) resulting in a sum-frequency generation (SFG) signal measured with a fast spectrometer. Here, the two lasers exhibit different yet stable repetition rates (i.e. locked but asynchronous), such that there is a constant frequency detuning yielding a fast scanning effect for the acquisition of the X-FROG spectrograms.