Monolithic single-shot dispersion-scan: A new tool for real-time measurement and optimization of femtosecond pulses

Summary form only given. The precise characterization of femtosecond laser pulses is as challenging as their generation and a topic of intense research. Dispersion-scan (d-scan) [1] is a recently established technique where the spectrum of a nonlinear signal, e.g., second-harmonic generation (SHG), is measured as a function of dispersion applied to the pulse. The spectral phase of the pulse can then be retrieved from the resulting 2D trace using an iterative algorithm. An important implementation of d-scan, based on a chirped mirror and wedge compressor, involves progressively moving one of the wedges around the maximum compression point and acquiring the resulting SHG spectrum for each insertion with a standard spectrometer. This robust and fully inline approach, which does not require any beamsplitting or temporal delays, has enabled the simultaneous compression and measurement of pulses down to single-cycle durations [2-4], but its scanning nature precludes single-shot operation. A single-shot d-scan variant that explores the spatially dependent dispersion of a glass prism was successfully demonstrated with 3.2 fs pulses [5], but the relatively small amount of dispersion that can be introduced by a single prism limits its use to few-cycle pulses.Here we present a novel single-shot d-scan architecture, based on automatically phase-matched transverse SHG (TSHG) [6] in a random nonlinear crystal. The crystal itself serves as a highly dispersive and ultra-broadband nonlinear medium simultaneously, enabling the live single-shot measurement of a two-dimensional SHG d-scan trace with an imaging spectrometer placed perpendicularly to the propagation direction. The setup is fully inline and extremely compact, without any moving parts. Owing to the large transparency range of the crystal and the ultra-broadband phase-matching characteristics of TSHG, one is in principle able to measure pulses with Fourierlimited durations down to 4 fs and longer than 100 fs, with spectra from 400 to 2200 nm, as long as the central wavelength is higher than 750 nm. Figure 1 shows a measurement example of pulses from a sub-30-fs 1 kHz Ti:Sapphire CPA laser system equipped with an acousto-optic programmable dispersive filter (AOPDF). Imparting a given amount of third-order dispersion (TOD) on the pulse results in a clearly tilted d-scan trace, as expected [1]. For example, by feeding back the d-scan measured phase into the AOPDF, Fourier-limited pulses trace are obtained. We should point out that the repetition rate of the measurement is only limited by the speed of the 2D detector and available laser energy, and is in principle scalable to higher repetition rates. In summary, this new monolithic single-shot d-scan architecture based on TSHG in a nonlinear random crystal provides a compact but scalable tool for convenient and intuitive real-time visualization, measurement and control of ultrashort light pulses over a broad wavelength range.