BEYOND 3 µM: HIGH-INTENSITY FEMTOSECOND PULSES FROM OPCPA AND ROTATIONAL SRS

Sources of intense femtosecond laser radiation in the wavelength range of 1.8 – 3 µm are in demand for generating THz and attosecond X-ray pulses [1], particle acceleration and remote sensing, biological imaging, medicine, and material processing by direct excitation of phonon modes. Several approaches to delivering intense ultrashort laser pulses in the wavelength range greater than  2 μm have been explored. The most advanced sources have been shown to be primarily based on optical parametric chirped pulse amplification (OPCPA), operating in the degeneracy range when pumped by a single-picosecond lasers near 1 µm.

We demonstrate a compact and cost-effective mid-IR laser source driven by 1.3 ps pulses from a Yb-rod laser operating at 1030 nm. Broadband supercontinuum (SC) seed pulses in the wavelength range 1050 – 2400 nm obtained in a 130 mm long YAG crystal were amplified in a three-stage collinear OPCPA, and then both the signal and idler pulses were focused into a hydrogen cell to extend the OPCPA coverage beyond 3 µm by the high-order cascade rotational stimulated rotational Raman scattering (SRS). During the experiments, the pressure, cell length, spectral mismatch between the signal and idler pulses, and the chirp induced in the SC pulses were optimized. A modified two-stage four-pass Yb-rod chirped pulse amplifier [2] with a record gain of 10 million was used as the pump source for both SC and BiBO-based OPCPA (Fig. 1 left). Compared to the previous implementation [3] the dispersion of SC seed pulse has been varied using an acousto-optic programmable dispersive filter (AOPDF).

With a total energy contained in the incident signal and idler pulses of 1.5 mJ and a hydrogen pressure of 6 MPa, a third-order rotational Stokes pulse was observed at a wavelength beyond 3 μm (Fig. 1). The transient cascade rotational SRS in compressed hydrogen provides a Stokes pulse energy of over 70 μJ at 2.6 μm with a conversion efficiency of 11% after optimization of the signal and idler spectra, the chirp induced in the signal pulses, and the cell length. Positively chirped broadband SC seed pulses contribute to the extension of the SRS spectral bandwidth to longer wavelengths compared to negatively chirped ones. Although the possibility of third-order Stokes generation at a wavelength of 3 μm has been demonstrated, to improve the SRS conversion efficiency it is necessary to use another SC seed covering the range of 2.5 – 3.5 μm. This possibility was also demonstrated when pumping the YAG rod near 2 μm.

Thus, the generation of high-order Stokes pulses via transient rotational SRS in compressed hydrogen was investigated using sub-picosecond signal and idler pulses in the short-wave infrared spectral range. This new approach to generating intense femtosecond pulses, combining OPCPA with transient rotational SRS, now already provides spectral extension from 2 μm to 3.1 μm.