Richard C. Powell

Raman spectroscopy of crystals for stimulated Raman scattering

Abstract Raman frequency shift, line width, integral and peak Raman scattering cross sections were measured in various crystals using spontaneous Raman spectroscopy. The highest Raman gain coefficient in steady state Stimulated Raman Scattering (SRS) regime was proved to be in barium nitrate crystal; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium molybdate crystal is proposed as a new highly efficient Raman material. OCIS : 300.6450; 290.5910; 190.2640

Comparative spontaneous Raman spectroscopy of crystals for Raman lasers

A comparison of the spectroscopic parameters of Raman-active vibronic modes in various crystalline materials with a view to the use of these crystals for stimulated Raman scattering (SRS) is presented. It includes data on the Raman frequency shift, linewidth, integral, and peak Raman scattering cross sections. For steady-state SRS the highest Raman gain coefficient has been proved to be in barium nitrate and sodium nitrate crystals; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium tungstate and strontium tungstate are proposed as new highly efficient Raman materials for both SRS cases.

Generation of 1.5-μm radiation through intracavity solid-state Raman shifting in Ba(NO 3 ) 2 nonlinear crystals

An intracavity solid-state Ba(NO3)2 Raman shifter produces 1.535–1.556-μm radiation when pumped by a Nd3+:YAG laser operating at 1.318–1.338 μm. Raman beam cleanup and 48% energy conversion efficiency have been observed. Second Stokes lines at 1.82–186 μm were observed in the output at higher pump input energy levels. Diffraction-limited pulses of 0.25 J at 1.535–1.56 μm have been achieved.

Intracavity Raman conversion and Raman beam cleanup

Abstract Transient solutions of the transverse depleted-pump and Stokes field distributions are obtained for an externally-pumped solid-state Raman laser. The results are shown to agree well with experiment measurements made on a Q-switched Nd:YAG pumped solid-state barium nitrate Raman laser. The theory of intracavity Raman conversion of real optical beams is presented, and the mechanisms responsible for Raman beam cleanup and mode confinement are qualitatively discussed and modeled by computer simulations.

Intensity scan and two photon absorption and nonlinear refraction of C60 in toluene

A single beam technique is introduced to determine the magnitude and sign of third order nonlinear optical coefficients of materials using intensity dependence of the complex refractive index. It is applicable to a large range of materials including those with a low damage threshold such as polymers and absorbing glasses. It is used to determine the two photon absorption and nonlinear refraction coefficients of C60 in toluene. Concentration dependence of the nonlinear optical coefficients results in second hyperpolarizabilities of 10−31 (esu) for C60.

Gain characteristics of a high-concentration Er3+ doped phosphate glass waveguide

A channel waveguide on an erbium doped phosphate laser glass (Kigre Q89) was fabricated by a dry silver-film ion exchange technique, and its gain properties were studied experimentally. The propagation loss of the fabricated waveguide was 0.6 dB/cm at 1.3 micrometers . Er3+ concentration of 1 X 1020 ions/cm3 was chosen so that no concentration quenching occurred. This was confirmed by measuring a fluorescence lifetime of 1.54 micrometers (8.4 ms). Gain of the fabricated waveguide was measured by using a Ti:sapphire laser at a wavelength of 977 nm and a laser diode of 1.530 micrometers producing pump and signal beams respectively. The signal wavelength used for the experiments was shorter than the emission peak, and the measured gain of the 1.8 cm waveguide was comparable to the total loss. However, the model which adopted experimental conditions showed that lasing is expected at the emission peak with a 3.6 cm long waveguide fabricated by current method. Calculations results showed that the 6.5 dB gain can be realized at 300 mW pump power with a 5.4 cm long waveguide, provided that mode overlap can be increased by 25% and propagation loss can be reduced by 50%. No significant upconversion effect was observed up to 1.1 X 106 W/cm2 pump intensity.

PROPERTIES OF STIMULATED RAMAN SCATTERING IN CRYSTALS

Abstract This paper summarizes our current research on the development of solid-state Raman lasers based on new materials and novel cavity designs. Criteria for selecting good materials for stimulated Raman scattering are reviewed and a theoretical model is presented to demonstrate the origin of Raman beam cleanup.

Picosecond laser pulse induced effects in bismuth-tellurite, Bi2TeO5

Abstract The interaction of short laser pulses were investigated with a new nonlinear optical material, bismuth tellurite. The time evolution of the four-wave mixing (FWM) signal was followed and analyzed with 18 ps laser pulse excitation out to the ms time scale. The effect of Cr and Fe dopants to the FWM signal characteristics were also studied.

Solid state Raman laser for MMT sodium guide star

Generation of sodium guide stars for adaptive optics requires very precise control of the frequency and bandwidth of the laser to maximize the brightness of the generated guide star. The ruggedness, efficiency and ease of use of a solid state system has great potential for improving the reliability and power of the laser guide star over the dye laser system currently used. The dearth of solid state transitions at the precise wavelength required for exciting resonance scattering in sodium drives us toward Raman shifting to downshift a nearby solid-state transition line tuned to work with the Raman-shifting material. The system being developed for the 6.5 meter multiple mirror telescope (MMT) takes two approaches to creating the sodium guide star: one uses YGAG to maximize the Raman-shifted output at the sodium D2 resonance. The second approach is to thermally tune the output of YAG to reach the appropriate wavelength for Raman shifting to 589 nm. Initial results from YGAG indicate that it will not be a suitable material for creating the sodium guide star laser. Initial results from the YGAG laser is presented, along with a discussion of the potential of the technology.

Theory of Raman gain spectrum transformations

Abstract A theory describing the spectral characteristics of Raman lasers is presented. The foundation of the theory is anchored in the probabilistic nature of light–matter interactions. The Raman gain spectrum is predicted for the cases when the pump laser is oscillating in both a single longitudinal mode and multi-longitudinal mode configuration. The gain profile predicted in the two previous cases are shown to be equivalent to homogeneously and inhomogeneously broadened laser transitions, respectively. This theory is necessary to predict the behavior of Raman lasers when injection-seeding, mode-locking, or multi-frequency operation is desired. Detailed calculations are performed for the case where the pump laser medium in Nd:YAG and the Raman medium is Ba(NO 3 ) 2 .