Abstract
The advancement of contemporary ultrafast science requires reliable sources to provide high-energy few-cycle light pulses. Nonlinear pulse compression is one of the most common methods to generate intense few- (or single-) cycle pulses. The basic concept is based on a nonlinear interaction to broaden the spectral bandwidth first and then compensate the residual pulse chirp to shorten the pulse duration. Currently, three mainstream methods can produce millijoule level few-cycle pulses: hollow-core fibers (HCF) [1], multiple-plate continuum [2], and multi-pass cell [3]. Hollow-core fiber uses a hollow waveguide filled with gas as the nonlinear medium to broaden the spectral bandwidth of driving lasers. The output beam quality is quite well, and the throughput is typically 70%. This technique can achieve a large compression ratio through meter-long fiber or multi-stages arrangements. Multiple-plate continuum uses a sequence of solid thin plates as the nonlinear medium. By the self-focusing effect in plates, periodic beam divergence and focusing is formed and which makes an identical beam size on all the thin plates. This method can be low-cost and less sensitive to beam pointing. However, applying in millijoule level laser, damage of plates could be an issue. The technique of multi-pass cell is based on a Herriott cell cavity which filled with gas as the nonlinear medium. This geometry can handle tens of millijoule pulses and achieve a large compression ratio. Few- to single-cycle pulse generation through the multi-pass cell is promising once the mirror coating is available for high damage threshold, high reflectivity, and wide spectral bandwidth.