Investigation of serial coherent laser beam combination based on Brillouin amplification

Based on transferring energy from multiple pump beams into one Stokes beam using Brillouin amplification, a serial coherent laser beam combination scheme is presented, which has many advantages, such as, simple structure, low cost, ease of adjustment, higher load capability, scalable easily, etc. Furthermore, it has been demonstrated that the combination of several beams using this method is theoretically possible. But in practice, the amplification of high power Stokes beam is a key problem to solve. In this paper, the amplification of Stokes beam whose power is higher than the pump beam is first studied and proved experimentally. Coupling the two laser beams by this method is proved experimentally, and the coupling efficiency reaches more than 80%. Then the feasibility of multiple beams combination based on Brillouin amplification is analyzed and tested theoretically.

STIMULATED RAMAN SCATTERING IN MULTI-MODE FIBERS AND ITS APPLICATION TO BEAM CLEANUP AND COMBINING

Laser beam cleanup properties of stimulated Raman scattering (SRS) in a multi-mode fiber have been characterized. It is shown that a fiber, which supports propagation of a highly aberrated pump beam, can generate a Gaussian-like Stokes beam consisting of one or more orders of Stokes components. The beam cleanup properties are found to be similar to those of stimulated Brillouin scattering, which are briefly reviewed herein. The dramatic enhancement in beam quality achievable through SRS clearly demonstrates the potential utility of fiber-based Raman beam cleanup/combining devices for laser brightness scaling.

Measuring the Concentration of Minor Species from the Modulation Dip of the Nonresonant Background of Broad-Band CARS Spectra

A new technique for measuring the concentration of species from the modulation dip of the nonresonant background of broad-band CARS spectra has been proposed. To reduce the mode noise superimposed on the CARS spectra, we used a modeless laser generating amplified spontaneous emission as the Stokes beam of the broad-band CARS. The modulation dip induced by a minor species, CO in Ar, was numerically calculated and fitted as a function of CO concentration and temperature. We applied this technique in measuring CO concentration in a static cell and also the profile of CO concentration in a CH4/air premixed flame of a counterflow burner.