Picosecond optical parametric amplification of stimulated Raman as high peak-power source and ultra-sensitive preamplifier

We report the characteristics of the amplified stimulated Raman scattering (SRS) pulses generated in liquid benzene by a picosecond (ps) β-barium borate (BBO) optical parametric amplifier (OPA). When the OPA system was used as an energy amplifier for SRS pulses, with a pump energy of 2.4 mJ at 355 nm for the OPA, the maximum output energy of the amplified SRS was about 0.73 mJ for the signal and 0.18 mJ for the idler, the energy conversion efficiency was 30.4% from the pump beam to the amplified third order Stokes component at 635.1 nm. The total efficiency would be as high as 37.9% if the output of the idler is also included. The corresponding spectral line width of the amplified Raman pulse was 11.8 cm^-1 with a pulse width of ∼10.9 ps and a peak power of 67 MW. The OPA system was also used as an amplifier for very weak Raman signal, the slope gain factor of this amplifier was found to be as high as ∼4.1 × 10⁷ and the energy detection limit was as low as 14.8 aJ per pulse, or 48 photons at 635.1 nm, in particular. Such a detection limit corresponds to approximately 0.5 photons per pulse if the time-gate of the OPA is reduced to 150 fs and it is about the same as or even better than a recently report on the 0.75 photons detection limit for a 150-fs OPA of coherent signal at 800 nm.