Won B. Roh

Laser beam combining and cleanup by stimulated Brillouin scattering in a multimode optical fiber

A new technique for combining low-power laser beams has been demonstrated by use of semiconductor diode lasers. The technique, which is appropriate for any single-longitudinal-mode laser, is based on stimulated Brillouin scattering (SBS) in long multimode optical fibers. It produces a clean Gaussian-like beam that corresponds to the fundamental fiber mode, irrespective of the profile of the pump. Coherent as well as incoherent combining was demonstrated, and conversion slope efficiencies as high as 67% and 83% were shown to be achievable for the single-pass and ring-cavity SBS geometries, respectively.

Single-mode Raman fiber laser based on a multimode fiber.

Single-mode operation of a Raman fiber laser based on a multimode fiber has been demonstrated for what is to our knowledge the first time. Single-mode operation was achieved through the beam cleanup effect of stimulated Raman scattering in multimode fibers. The Raman laser, consisting of a 50-microm-core multimode fiber and a multimode Nd:YAG laser pump, produced an output beam corresponding to the fundamental mode of the fiber.

Pressure dependence of integrated CARS power

The pressure dependence of the integrated power of coherent anti-Stokes Raman scattering (CARS) has been investigated experimentally as well as theoretically. The integrated CARS power has been found to be inversely proportional to linewidth, in contrast to the spontaneous Raman scattering for which the integrated power is independent of linewidth. Because of the pressure broadening of the Raman lines and the interference among them, the integrated power deviates from a simple square-law dependence, the degree of deviation being a function of molecular species. Experimental results are presented for CH(4), CO, N(2), and H(2), with collisional narrowing being demonstrated for H(2).

Incoherent beam combining using stimulated Brillouin scattering in multimode fibers

A beam combining technique for producing a single, spatially coherent beam from two mutually incoherent (temporally and spatially) lasers is demonstrated and the spatial coherence properties of the resulting beam are characterized. The technique is based on simultaneous excitation of stimulated Brillouin scattering by two independent lasers operating at two different wavelengths in a long multimode optical fiber. Though spectrally independent, the resulting Stokes beams produce essentially identical intensity distributions corresponding to the fundamental fiber mode.

Phasing of a two-channel continuous-wave master oscillator–power amplifier by use of a fiber phase-conjugate mirror

Phasing of two-channel cw master-oscillator-power-amplifier beams by using a phase-conjugate mirror has been demonstrated for the first time to the best of our knowledge. Phasing was achieved with two fiber amplifiers in conjunction with a single-frequency Nd:YAG master oscillator by using a fiber phase-conjugate mirror in a double-pass geometry. The successful demonstration of phasing of two cw amplifier beams with a fiber phase-conjugate mirror greatly enhances the prospects for phasing of multiple laser amplifiers without complex servo-loop control systems.

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.

Multi-port beam combination and cleanup in large multimode fiber using stimulated Raman scattering.

We demonstrated the successful combination and cleanup of four laser beams via stimulated Raman scattering (SRS) using a multi-port fiber combiner and a large multimode fiber. Multiple Stokes orders were observed in the output, but loss at longer wavelengths reduced the transmission of the higher Stokes orders and limited the SRS conversion efficiency. SRS beam cleanup was also investigated using a single laser beam. The output beam had a measured M(2) better than 2 for fiber lengths from 400-1400 meters.

High-resolutions cars measurement of Raman linewidths of H2

Abstract High-resolution coherent anti-Stokes Raman spectroscopy (CARS) has been perfomed in hydrogen gas using a cw system. Accurate measurements of the Raman linewidths of the Q(0) through Q(3) transitions of molecular hydrogen have been made at pressures ranging from 0.75 to 40 atm at room temperature. The pressure-broadening coefficients determined for these lines are important in analytic and diagnostic applications of CARS in high-pressure media.

Passive combination of multiple beams in an optical fiber via stimulated Brillouin scattering.

We demonstrate passive combination of multiple beams in an optical fiber via stimulated Brillouin scattering (SBS). Four off-axis beams are combined in a long multimode optical fiber using a novel all-optical mount. The combined beam has the high spatial coherence properties of the LP(01) mode owing to the beam cleanup properties of SBS. The threshold for SBS when off-axis pumps are used is shown to be a factor of 13 times higher than for on-axis pump beams. We propose this method would improve the brightness of an array of fiber amplifiers.

SPIN-ORBIT RELAXATION KINETICS OF BR(4 2P1/2)

Pulsed and steady‐state photolysis techniques have been employed to measure the rate coefficients for collisional deactivation of the spin–orbit excited state of atomic bromine, Br(4 2P1/2). Pulsed lifetime studies for quenching by Br2 and CO2 yielded absolute rate coefficients at room temperature of kBr2=1.2±0.1×10−12 and kCO2=1.5±0.3×10−11 cm3/molecule s. The rate coefficients for quenching by rare gases, N2, O2, NO, NO2, N2O, CO, CO2, COS, SO2, SF6, CF4, CH4, H2S, H2, D2, HBr, HCl, and HI, relative to that for Br2 were determined in a steady‐state photolysis experiment. Correlation of the deactivation probabilities with energy defect for the case of electronic‐to‐vibrational energy transfer is demonstrated.