First and Second Order Stokes Generation by SRS in Methane: Influence of Rep-Rate, Beam Quality and Astigmatism (Opwekking van Eerste en Tweede Orde Stokes d.m.v. SRS in Methaan: Invloed van de Pulsfrequentie, de Bundelkwaliteit en Astigmatisme)

摘要

Experiments were performed to obtain a high efficiency for conversion of 1064 nmto eye-safe 1543 nm radiation by stimulated Raman scattering (SRS) in methane. A first order Stokes photon conversion efficiency of about 75 percent is obtained for an input energy of 280 mJ, using combination of cylindrical lenses for focusing. The highest obtainable efficiency appears to be limited by generation of second order Stokes radiation, at a wavelength of 2805 nm. The energy of the second order Stokes component could be increased 10 17.9 mJ, which is sufficient to be useful for medical purposes. Without circulation of the gas in the Raman cell, thermal blooming effects decreased the efficiency and caused deflection of the beam at pulse repetition rates above 3 Hz. With a gas circulating device, which replaced the gas in the focal area after each laser pulse, we succeeded in maintaining a high efficiency up to 10 Hz (the highest pulse repetition rate of the laser). This report also includes threshold and beam profile measurements which were carried out for the evaluation of a numerical model of the stimulated Raman scattering process that is being developed at this laboratory.