Robert G. Wenzel

Phonon dispersion curves in bismuth

Dispersion curves for phonons propagating in the trigonal direction in bismuth at room temperature and at 75°K have been obtained in a neutron inelastic scattering experiment. Observed frequencies (units 1013 rad/sec) at 75°K are as follows: at the zone center, ωTO = 1.40 ± 0.02, ωLO = 1.89 ± 0.02; at the zone boundary in the trigonal direction, ωTA = 0.73 ± 0.01, ωLA = 1.12 ± 0.02, ωTO = 1.91 ± 0.02, ωLO = 2.03 ± 0.02. At room temperature, the observed frequencies were about 1.5 percent lower. Data were also obtained for longitudinal phonons propagating in the binary direction at 75°K. It is interesting to note that the splitting between the zone boundary frequencies for the optical and acoustic branches for each polarization is quite large. This splitting is difficult to understand if bismuth is thought of as a slightly distorted simple cubic lattice. The experimental results may be qualitatively understood if bismuth is considered to be made up of a series of double layers normal to the trigonal axis. The atoms in each double layer form a crinkled hexagonal net with strong, probably covalent, bonds between atoms. The forces between atoms on adjacent double layers are relatively weak. This model is consistent with the easy cleavage of bismuth normal to the trigonal axis. Analysis of the trigonal dispersion curves in terms of a linear chain model indicates that there are significant forces connecting a given atom with atoms situated on the four planes on either side of it.

Parametric oscillator: HF oscillator–amplifier pumped CdSe parametric oscillator tunable from 14.1 μm to 16.4 μm

A singly resonant CdSe parametric oscillator with tunable output from 14.1 microm to 16.4 microm is described. The oscillator, pumped by the 2.87-microm line from an HF oscillator-amplifier, is resonant on the signal near 3.5 microm and has produced idler outputs of 100 microJ at 16 microm. Bandwidths varied over the tuning range from 9 cm(-1) to 1.8 cm(-1). Techniques for obtaining a highly coherent HF laser pump beam are presented. The uniform beam from the gain-saturated amplifier allows the use of Fresnel diffraction theory to produce a nearly uniform pump beam at the oscillator cavity.

Efficient stimulated Raman scattering due to absence of second Stokes growth

Stimulated Raman scattering of a XeCl laser at 308 nm in a high-pressure H(2) cell shows high conversion into first Stokes (S1) because of an unexpected holdoff of the second Stokes (S2) component. Specifically, a photon efficiency of 88% is obtained into S1. Comparison with a plane-wave model indicates that a theory including a spatially nonuniform gain and higher-order mode generation may be necessary to understand the holdoff of the S2 growth.

Rapid recording of infrared spectra from pulsed chemical lasers

A method is described for rapidly recording permanent ir spectrograms of the output of a pulsed chemical laser. The method also lends itself to the simultaneous observation of the time behavior of several spectral lines. Spectra from an HF laser under different operating conditions are reported to demonstrate the utility of the method.

Effect of the aperture–lens separation on the focal shift in large-F-number systems

We treat the problem of a coherent spherical wave passing through a circular aperture and subsequently focused by a lens. Using the Huygens–Fresnel integral representation, an expression is derived for the axial intensity beyond the lens. It predicts that the large-F-number focal shift may be reversed or eliminated, depending on the aperture–lens separation.

Fresnel diffraction in an optical system containing lenses

A convenient expression describing the location of Fresnel diffraction patterns when a diffracted spherical wave is focused by a lens is given. Experimental results confirm the predicted positions of 26 axial extrema corresponding to integer numbers of Fresnel half-period zones. Experimental radial intensity profiles are presented for some of these positions.

Nonlinear Loss in Ge in the 2.5–4-μm Range

Because of its high reflectivity and low absorption Ge has often been used as an output coupler or beam splitter in pulsed CO2 lasers operating at 10.6 μm. We report here some measurements which indicate that Ge may not be suitable for these uses in the 2.5-4 μm range, since severe attenuation can exist for moderate-power pulses. Measurements were made on a 5.1-mm thick plane-parallel disk of optical quality Ge. When the Ge was placed in the output beam of a pulsed HF laser fitted with an unstable resonator, transmission values were lower than the constant value of 0.46 ± 0.01 obtained in the 2-11.5 μm range with a spectrophotometer. The anomalous transmission effect was investigated over a wider range of intensity by placing the Ge in the converging beam from a 79-cm focusing mirror. After passing through the Ge the beam entered a calorimeter. A sapphire splitter before the Ge directed a fraction of the incident beam into a second calorimeter. The two calorimeter readings, together with the ratio obtained without the Ge, gave the Ge transmission as a function of incident laser energy. Figure 1 shows the reciprocal transmission as a function of incident peak intensity for the HF spectrum (2.6-3.1 μm) with the laser operating on SF6:C3H8. The data fits approximately a linear relation to be expected for a material in which the absorption coefficient is proportional to the intensity, as will be shown below. At the Ge position, 40 cm from focus, the beam cross section had an area of ~2 cm; the beam power was calculated using the known pulse length of 100 nsec FWHM. A second set of transmission measurements was made with the Ge 28 cm from focus. It should be noted that the transmission observed

The Los Alamos FEL photoinjector drive laser

The drive laser system of the photoelectric injector for the Los Alamos free-electron laser (FEL) is described. This system consists of a 1.053 mu m mode-locked Nd:YLF oscillator, a grating-rhomb pulse compressor, a fast electrooptic shutter to reduce pulse repetition frequency, a pulsed Nd:YLF amplifier chain, and a KTP frequency doubler. The bright electron beams required for high performance FELs demand stringent specifications on drive laser output levels, beam quality, and stability. The design specifications, laser architecture, and operating performance following the most recent round of upgrades are presented. >

Studies of Stokes generation in XeF-pumped CH 4

Using a diffraction-limited XeF pump laser, the beam parameters of the first-Stokes output from a single-pass CH4 Raman cell were studied. The apparent Stokes origin depended on the pump energy, and the onset of second Stokes production depended on the pump f/number.

Oscillations of gaussian beam parameters in periodic lens waveguides

Abstract Gaussian beams propagating through periodic thin-lens waveguides are found to oscillate in Rayleigh range and waist longitudinal position. A differential expansion of expressions derived from gaussian beam theory leads to approximate analytic expressions for the amplitude, period, and phase of the oscillations. The amplitude and phase are found to depend on the input parameters, while the period depends on the waveguide parameters.