Fiber delivery of sub-30 fs pulses at 830 nm using a grism-based dispersive line

Abstract

International audienceFemtosecond pulse delivery with fiber is desirable for many applications (e.g. non linear endomicroscopy). We present the direct delivery of compressed ultrashort (sub-30 fs) nanojoule (1.2 nJ) pulses at 830 nm through a standard single mode fiber (SMF) and also through a large mode area (LMA) fiber. For delivery with the SMF, the pulse coming from a standard femtosecond oscillator (10 nm bandwidth) is first spectrally and temporally broadened in a first SMF under the combined effects of self-phase modulation (SPM) and positive second order dispersion (SOD). Then it passes through an anomalous dispersive line in which it is largely negatively stretched. During its final propagation inside the delivery fiber the signal is temporally and spectrally compressed [1]. This is obtained when the anomalous dispersive line is adjusted to compensate for the positive SOD and for the positive third order dispersion (TOD) of the two fiber pieces (Fig. 1 up). For delivery with the LMA, the setup was composed of a large bandwidth oscillator (80 nm bandwidth), of the same grism-based stretcher and of the fiber delivery (the first fiber was removed) (Fig. 1 down). For simultaneous SOD and TOD compensation, conventional dispersive lines (e.g. prism line [3], grating line [4]) were previously combined with a multi-pass chirped mirror pair [2] resulting in a cumbersome setup involving expensive elements. We worked with a home-built dispersive line made of two grisms (association of a diffraction grating in contact with a prism) [5]. Playing with the distance d between the two grisms and with the incidence angle θ permitted simultaneous adjustments of SOD and TOD. It allows us to obtain what is to the best of our knowledge, the shortest pulses ever delivered directly by an optical fiber. We demonstrated the delivery of 55kW sub-30fs pulses directly at the output of LMA fiber. SMF setup gave rather close results. For the two setups, we used the same home-built grism-based stretcher which was made of usual components (40° BK7 prisms, 600 g/mm gratings). Our grism-based stretcher efficiency amounted to 34%