Gerald M. Bonner

Continuous-wave diamond Raman laser

Continuous-wave operation of a diamond Raman laser is demonstrated. Low-birefringence synthetic single-crystal diamond is used and is intracavity pumped by a Nd:YVO(4) laser. A cw output power of 200 mW is achieved at the Raman wavelength (1240 nm), and 1.6 W of on-time output power is obtained in quasi-cw mode. Losses in the diamond (approximately 1% per pass) and thermal effects in the Nd:YVO(4) limit the efficiency.

1.6 W continuous-wave Raman laser using low-loss synthetic diamond.

Low-birefringence (Δn<2x10(-6)), low-loss (absorption coefficient <0.006 cm(-1) at 1064 nm), single-crystal, synthetic diamond has been exploited in a CW Raman laser. The diamond Raman laser was intracavity pumped within a Nd:YVO4 laser. At the Raman laser wavelength of 1240 nm, CW output powers of 1.6 W and a slope efficiency with respect to the absorbed diode-laser pump power (at 808 nm) of ~18% were measured. In quasi-CW operation, maximum on-time output powers of 2.8 W (slope efficiency ~24%) were observed, resulting in an absorbed diode-laser pump power to the Raman laser output power conversion efficiency of 13%.

An intra-cavity Raman laser using synthetic single-crystal diamond.

Low birefringence synthetic single-crystal diamond was used as a Raman laser medium inside a Q-switched Nd:YVO(4) laser. A maximum average output power of 375 mW was achieved at a wavelength of 1240 nm and a repetition rate of 6.3 kHz. This equates to a conversion efficiency of 4% from the diode laser to the first Stokes component at 1240 nm. Optical losses within the diamond (approximately 1% per single pass) limited the performance and are currently the main barrier to the demonstration of an efficient CW diamond Raman laser.

Spectral broadening in continuous-wave intracavity Raman lasers

Spectral broadening of the fundamental field in intracavity Raman lasers is investigated. The mechanism for the spectral broadening is discussed and the effect is compared in two lasers using Raman crystals with different Raman linewidths. The impact of the spectral broadening on the effective Raman gain is analyzed, and the use of etalons to limit the fundamental spectral width is explored. It was found that an improvement in output power could be obtained by using etalons to limit the fundamental spectrum to a single narrow peak.

Measurement of thermal lensing in a CW BaWO4 intracavity Raman laser.

The thermal lens induced in an a-cut BaWO(4) crystal by stimulated Raman scattering is measured using lateral shearing interferometry. The strength of the lens is proportional to the Stokes output power. For light polarized parallel to the a-axis, and a Stokes mode radius of 120 μm, the lens is negative and highly astigmatic: -0.8 D W(-1) in the plane parallel to the a-axis and -7.7 D W(-1) in the plane parallel to the c-axis. The implications of this thermal lens for Raman laser design are discussed.

Probing the effect of the solution environment on the vibrational dynamics of an enzyme model system with ultrafast 2D-IR spectroscopy

Ultrafast 2D-IR spectroscopy has been applied to study the structure and vibrational dynamics of (μ-C(CH3)(CH2S)2(CH2S(CH2)2Ph)Fe2(CO)5, an organometallic model of the active site of the FeFe[hydrogenase] enzyme. 2D-IR spectra have been obtained in solvents ranging from non-polar to polar and protic. The influence of the solvent bath on vibrational relaxation, including rapid intramolecular population transfer, has been characterized. In addition, the temporal dependence of the 2D-IR lineshape has been used to extract information relating to hydrogen bond-mediated spectral diffusion via the frequency–frequency correlation function. Comparisons with previous 2D-IR studies of hydrogenase model systems offer insights into the dependence of the rate of population transfer upon vibrational mode separation and solvent environment, with important implications for the composition and reactivity of the active site of the enzyme.

Optical parametric oscillator-based trace detection of gases in the mid-infrared region using phase-fluctuation optical heterodyne spectroscopy

Laser absorption spectroscopy utilizes a tunable infrared source, providing the necessary selectivity, to detect the characteristic fingerprint spectral absorption of an abundant gas. In a simple embodiment such as single-pass absorption, sensitivity is limited as attenuation becomes minuscule for trace level concentrations; a problem exacerbated in the midinfrared region due to significant detector noise. Sensitivity can be improved by increasing interaction between the optical field and molecular ensemble with methods such as a multiple-pass Herriot cell or resonant cavity ring-down spectroscopy but these techniques have a substantial overhead in instrumentation. An alternative approach to this problem is Phase Fluctuation Optical Heterodyne (PFLOH) spectroscopy. Here, interferometric effects are used to detect the minute heating of the sample gas when incident laser light of the appropriate wavelength is absorbed. More specifically, by placing the absorption chamber within one arm of a Mach-Zehnder interferometer, heat-induced changes in the optical path length can be detected with great sensitivity through the resulting fringe modulation. A secondary benefit is that although excitation occurs in the infrared, its effects can be detected using visible lasers and silicon detectors, thereby obviating the need for cooled, infrared detectors. We will present our results used to detect ethane using absorption in the 3.33-3.37 μm region. The Mach-Zehnder interferometer used a Helium Neon laser for the probe laser, and a broadly tunable Optical Parametric Oscillator (OPO) for spectroscopic excitation. We have demonstrated detection levels at parts per billion with further sensitivity possible by implementing several identified improvements.

Continuous-wave SrMoO 4 intracavity Raman laser pumped using a disk laser

The use of disk lasers to pump continuous-wave (cw) intracavity Raman lasers is discussed with a view to power scaling these devices via improved thermal management. Preliminary results for a Nd:YVO4/SrMoO4 Raman laser in this geometry are reported. An output power of 295 mW at the 1st Stokes wavelength of 1175 nm was obtained for 13.2 W absorbed pump power at 808 nm.

1.6W Continuous-wave Diamond Raman Laser

Low-birefringence, single-crystal, synthetic diamond is used as a Raman medium in a Nd:YVO4 laser. CW output powers of 1.6W at the Raman wavelength were recorded. In quasi-CW operation, on-time output powers of 2.8W were obtained.