High power single-mode delivery of mid-infrared sources through chalcogenide fiber.

Mechanically robust and low loss single-mode arsenic sulfide fibers are used to deliver high power mid-infrared sources. Anti-reflection coatings were deposited on the fiber facets, enabling 90% transmission through 20 cm length fibers. 10.3 W was transmitted through an anti-reflection coated fiber at 2053 nm, and uncoated fibers sustained 12 MW/cm2 intensities on the facet without failure. A Cr:ZnSe laser transmitted >1 W at 2520 nm, and a Fe:ZnSe laser transmitted 0.5 W at 4102 nm. These results indicate that by improving the anti-reflection coatings and using a high beam quality mid-infrared source, chalcogenide fibers can reliably deliver ≥10 W in a single mode, potentially out to 6.5 µm.

High-power, widely-tunable Cr(2+):ZnSemaster oscillator power amplifier systems.

We demonstrate high-power Cr(2+):ZnSe master oscillator power amplifier (MOPA) pure continuous wave (CW) laser systems with output power of 14 W and amplifier gain greater than 2X. In addition, we develop a theoretical model for this type of amplification and show single-knob tunability at high powers over 400 nm.

Aperture scaling effects with monolithic periodically poled lithium niobate optical parametric oscillators and generators.

We used elliptical beams to demonstrate aperture scaling effects in nanosecond single-grating and multigrating periodically poled lithium niobate (PPLN) monolithic optical parametric oscillators and generators. Increasing the cavity Fresnel number in single-grating crystals broadened both the beam divergence and the spectral bandwidth. Both effects are explained in terms of the phase-matching geometry. These effects are suppressed when a multigrating PPLN crystal is used because the individual gratings provide small effective subapertures. A flood-pumped multigrating optical parametric generator displayed a low output beam divergence and contained 19 pairs of signal and idler frequencies.

Periodically poled lithium niobate monolithic nanosecond optical parametric oscillators and generators.

We fabricated and characterized periodically poled lithium niobate monolithic optical parametric oscillators (OPO's) and generators. The compact monolithic devices were trivial to align and operate and provided widely tunable, nearly diffraction-limited, stable output pulses. Low thresholds and high conversion efficiencies were obtained when the devices were pumped with 3.5-ns 1.064-mum pulses. In addition, the monolithic OPO devices exhibited broad tuning by crystal rotation through noncollinear phase matching. The bandwidth-broadening effects exhibited in the noncollinear phase-matching geometry were measured and explained.

Efficient grating-tuned mid-infrared Cr2+:CdSe laser.

Room-temperature, all-solid-state, broadly tunable laser operation of Cr(2+) -doped CdSe has been demonstrated. Pumping with a Q -switched Tm, Ho:YLF laser running at a 1-kHz repetition rate achieved broadband output of 500 mW at 2.6 microm with 48% absorbed power slope efficiency. With reduced efficiency, as much as 815 mW of power was obtained. With a diffraction grating, the Cr(2+): CdSe laser was tuned from 2.3 to 2.9 microm with 10-nm bandwidth (FWHM) and output power up to 350 mW.

Electro-optic spectral tuning in a continuous-wave, asymmetric-duty-cycle, periodically poled LiNbO(3) optical parametric oscillator.

We demonstrate electro-optic spectral tuning in a continuous-wave periodically poled LiNbO(3) (PPLN) optical parametric oscillator (OPO). We achieve 8.91 cm(-1) of rapid spectral tuning, with a linear tuning rate of 2.89 cm(-1) /(kV/mm), by applying electric fields up to +/-1.5 kV/mm across the crystal while it is operating within the OPO. Intentionally poling the PPLN crystal with an asymmetric domain structure enables tuning, and numerical predictions closely match the experimental observations. The tuning is considerably larger than the typical operational bandwidth of the OPO, indicating that we are in fact shifting the gain curve of the PPLN crystal.

Difference-frequency generation of 4-microm radiation from Nd:YAG lasers.

More than 30 mJ of mid-IR 4-microm energy was generated with difference-frequency mixing of 1.064- and 1.444-microm Nd:YAG laser pulses in a lithium niobate crystal. The energy conversion efficiency in converting from the pump at 1.064-micro to the mid-IR at 4.043-micro was ~5.5%. The amount of 4-micro output produced was limited by optical damage to the lithium niobate crystal. In this experiment, the optical damage threshold occurred at a peak intensity of 300 MW/cm(2).

Thermal lensing in silver gallium selenide parametric oscillator crystals.

We performed an experimental investigation of thermal lensing in silver gallium selenide (AgGaSe(2)) optical parametric oscillator crystals pumped by a 2-µm laser at ambient temperature. We determined an empirical expression for the effective thermal focusing power in terms of the pump power, beam diameter, crystal length, and absorption coefficient. This relation may be used to estimate average power limitations in designing AgGaSe(2) optical parametric oscillators. We also demonstrated an 18% slope efficiency from a 2-µm pumped AgGaSe(2) optical parametric oscillator operated at 77 K, at which temperature thermal lensing is substantially reduced because of an increase in the thermal conductivity and a decrease in the thermal index gradient dn/dT. Cryogenic cooling may provide an additional option for scaling up the average power capability of a 2-µm pumped AgGaSe(2) optical parametric oscillator.

Transmission and damage-threshold measurements in AgGaSe(2) at 2.1 microm.

Measurements of transmission loss coefficients in AgGaSe(2) at 2.1 microm ranged from 0.012 to 0.072 cm(-1). Energy density, not peak-power density, proved to be the determining factor in AgGaSe(2) damage threshold. Coated-surface damage thresholds originally were found to be in the 1.1-2.5-J/cm(2) region by using 180-nsec laser pulses, but damage thresholds as high as 3.5 J/cm(2), with a 3 J/cm(2) average, were measured in coated samples with improved surface quality.

Raman spectra of normal and ultraviolet-induced cataractous rabbit lens.

The Raman spectra of a normal rabbit lens have been recorded over the range of 0 to 4000 cm-1. Spectral results indicate that protein conformation is predominantly beta-pleated sheet. Polarization measurements and assignments for most of the bands have been made; the spectral region for water indicates extensive hydrogen bonding. An N2 laser operating at 337 nm was used to produce cataracts in a rabbit lens; the Raman spectra of the cataractous lens were also recorded. Spectral changes are observable in the water region, and these are correlated with a mechanism of ultraviolet cataractogenesis.

Implications of the corneal temperature range in the prediction of laser thermal damage.

Corneal temperatures of the rhesus monkeys have been measured under conditions that may exist during laser experiments. The minimum and maximum temperatures found for all experimental conditions were 29.54 degrees C and 39.16 degrees C, respectively, a range of 9.62 degrees C. A computer model of thermal damage due to laser irradiation was used to determine the effect varying initial temperature could have on corneal damage thresholds (ED50's). The range of 9.62 degrees C found in monkeys for all experimental conditions corresponded to a 39% difference in threshold power. The dependence of damage thresholds on initial temperature could be an important factor to consider when basing laser safety standards on damage threshold data.

A physical explanation of the EPR spectrum observed during catalysis by enzymes utilizing coenzyme B12.

We have proposed that the "doublet" EPR spectra observed during catalysis by a number of coenzyme B12-requiring enzymes arises from a weak electrostatic exchange interaction between an organic free radical and low spin Co(II), B12r. By varying the magnitude of the exchange of coupling we have quite accurately simulated the published EPR spectra from the enzyme systems: diol dehydrase, glycerol dehydrase, ribonucleotide reductase, and ethanolamine ammon-ia lyase. A dipolar model was shown to be incompatible with the observed properties of these systems.

Interaction of ubisemiquinone with a paramagnetic component in heart tissue.

The origin of most of the electron paramagnetic resonances obtained at low temperature and low microwave power from heart tissue and subcellular fractions derived therefrom is now understood. A signal that emerges on partial reduction with characteristic lines at 3227 G (0.3227 tesla) and 3309 G (0.3309 tesla) (at 9.2 GHz) and disappears again on full reduction has remained unidentified. According to its behavior on oxidation-reduction, the substance giving rise to this signal has the properties of a two-electron acceptor. The signal is strongly dependent on temperature and can only be well resolved at less than 20 degrees K. It is readily elicited in submitochondrial particles by partial reduction, but has not been observed in submitochondrial particles from which ubiquinone has been removed by pentane extraction. When ubiquinone is reincorporated into extracted submitochondrial particles, the signal is again easily produced by partial reduction. Electron paramagnetic resonance spectra of partially reduced submitochondrial particles recorded at 34 GHz show lines centered about g approximately 2 with the same separation (approximately 82 G; approximately 0.0082 tesla) as do 9.2 GHz spectra, whereas no lines are detected with a separation of approximately 82 X 34/9.2 G (0.0082 X 34/9.2 tesla). We suggest, on the basis of these observations, that the unidentified signal arises from an interaction of ubisemiquinone and a second paramagnetic species. Three obvious choices exist concerning this second species: ubisemiquinone, flavin semiquinone, or an iorn-sulfur center. It is not possible without much additional information to decide between these possibilities. Since we have never observed the signal in the absence of the membrane-bound, high-potential type iron-sulfur protein, we have considered involvement of this species in the interaction. However, according to computer simulations of the observed electron paramagnetic resonance spectra, which yield best fits for semiquinone-semiquinone interaction, the possibility that ubi- or flavin semiquinone is the interaction partner appears more likely at this time. The interaction appears to be of the magnetic dipole-dipole type, but it is not certain whether there is also a contribution from spin exchange coupling. If it is assumed that the signal is due to magnetic dipole-dipole interaction, the distance of the partners is less than or equal to 7.7 A.