R. H. Pantell

Thermal effects in doped fibers

A theoretical analysis of the pump-induced temperature change and associated thermal phase shift occurring in a pumped doped fiber is presented. Although the primary devices targeted are all-optical switches based on doped fibers, where such effects can be detrimental, this analysis is also applicable to lasers, amplifiers, and other doped fiber devices. The effects of a single pump pulse, multiple pulses and continuous wave (CW) pumping are investigated, both in the dynamic and steady-state regimes. Simple expressions are derived for the thermal relaxation time constant of a fiber, and for its steady-state temperature rise and thermal phase shift under CW pumping. This study predicts that in all-optical fiber switches utilizing a reasonably good dopant the thermal effect due to a single short pulse is negligible in all interferometers, while the steady-state effect can be sizable in a standard fiber Mach-Zehnder but is negligible in a twin-core fiber, a two-mode fiber, and a specially designed Mach-Zehnder interferometer.

EFFICIENT, TUNABLE OPTICAL EMISSION FROM LiNbO3 WITHOUT A RESONATOR

We report the first observation of tunable stimulated optical emission from a nonlinear crystal without the use of an external resonator, and with greater than 50% conversion of the pump beam to the tunable frequency. Opposite faces of a LiNbO3 crystal were polished flat and parallel to provide for multiple reflections of the signal radiation (Stokes) inside the crystal. The A1 symmetry 248‐cm−1 polariton mode was excited with a Q‐switched ruby laser, and the signal frequency was tuned by varying the angle of incidence of the laser beam relative to the normal to the crystal surface. Operation was at room temperature; no crystal damage was observed over most of the tuning range; and laser depletion occurred within the first few nanoseconds of the Q‐switched pulse. The difference between the Stokes frequency and laser frequency was varied from 42 to 200 cm−1, and the corresponding idler wavelength was varied from 238 to 50 μ.

Continuously tunable submillimeter wave source

A method for spanning the 100−1000−μ portion of the spectrum with continuously tunable coherent radiation is described. The approach is based upon laser light scattering from the long−wavelength side of the A1−symmetry soft mode in LiNbO3. In contrast with other techniques, this method uses a single fixed−frequency pump source, requires no magnetic field, provides continuous rather than discrete tuning, can cover most of the 100−1000−μ range, operates at room temperature, and is simple to tune. The experimental data show that tuning was obtained from approximately 150 to 700 μ.

A high-energy, laser accelerator for electrons using the inverse Cherenkov effect

A laser method for accelerating electrons is described, based on the inverse Cherenkov effect in a gas. The laser fields are in the form of a cylindrical cone of plane waves on whose axis travel the electrons, with the cone angle and the gas refraction index such that each electron sees constant fields in time. Expressions are obtained relating the overall energy transfer to total laser power and wavelength, and to gas index and interaction length. With laser powers now available, energy increments of tens of GeV are possible. For comparative purposes, a related alternative scheme involving electrons in vacuum and evanescent laser fields is also analyzed. It is found that the method applies particularly well to adding energy to the electron bunches produced by large microwave accelerators, as collision effects are less troublesome at high injection energies.

Coherent emission and gain from a bunched electron beam

The radiation from a modulated electron beam, such as that produced in a radio-frequency accelerator, passing through a magnetic undulator is analyzed. The authors show that in a waveguide free electron laser (FEL), this may lead to an emission of a significant amount of coherent radiation in the far infrared to millimeter wave range. A simple and powerful method of calculating the spectral distribution of the radiated power is presented along with an analysis of the gain and the saturated power. The experimental results of spontaneous emission measurements on an RF driven FEL, are presented and compared to theoretical predictions. >

Analysis of nonlinear optical switching in an erbium-doped fiber

A mathematical model of the strong, resonantly enhanced nonlinear phase shift recently reported in Er-doped fibers which relates the phase shift and signal loss to the fiber parameters and the pump and signal wavelengths, is presented. Predictions are in fair agreement with the phase shift and loss measured in an experimental Er-doped fiber switch based on this effect. A strong, nearly wavelength independent contribution to the nonlinear phase shift is observed in the switch. The model suggests that this effect is due to the same nonlinear effect arising from one or more vacuum ultraviolet (VUV) transitions in Er/sup 3+/. >

Surface plasmon immunoassay.

The most sensitive existing assays used to determine antibody levels in blood serum samples require a tracer material, e.g., radioisotope, fluorofore, or enzyme, to identify the specific analyte. Surface plasmon spectroscopy has been applied recently as a no-label technique for the assay of specific antibody solutions with the antigen proteins immobilized on a metal surface. It is found that the metal surface configuration originally proposed for the surface plasmon immunoassay (SPI) is unstable and unsuitable for the assay of specific antibodies in a large mixture of proteins such as in a blood serum. Nevertheless, by properly designing the metal surface structure, the SPI can be made an extremely practical device. Preliminary results for the assay of dinitrophenyl (DNP) and keyhole limpet hemocyanin (KLH) antibodies in blood serum samples, indicate that the SPI, in addition to providing a simple and fast measurement, is comparable with existing approaches, such as radioimmunoassy or enzyme-linked immunosorbent assay both in sensitivity and specificity.

Ferroelectric Harmonic Generator and the Large‐Signal Microwave Characteristics of a Ferroelectric Ceramic

At microwave frequencies, ferroelectric ceramics behave as nonlinear dielectrics and exhibit large signal scalar nonlinear characteristics in both the dielectric constant and the rf conductivity. The nonlinear behavior of a ceramic of 73% BaTiO3‐27% SrTiO3 has been used to construct a harmonic generator operating from 3 kMc to 9 kMc. An efficiency of 8.5% for a peak input power of 2200 w was attained. In addition, a new measurement technique has been developed for measuring the large signal properties of this ceramic as well as for other nonlinear materials. This technique makes explicit use of the anharmonic response of a resonant system that contains a nonlinear element. The analysis of both the harmonic generator and the nonlinear resonant system takes into account nonlinear reactance as well as nonlinear resistance. For temperatures about 30°C above the Curie temperature (the Curie temperature for the ceramic composed of 73% BaTiO3‐27% SrTiO3 is approximately 20°C) the large signal dielectric constant...

Radiation characteristics of planar channeled positrons

An analysis is presented to determine the characteristics of the radiation from planar channeled positrons. This radiation has interesting properties as a source for the keV photon energy range, and may also be used to investigate the behavior of channeled particles and to characterize the channeling crystal. Several important aspects of this problem are considered, such as line‐broadening mechanisms, dependence of the radiation on the properties of the incident particle beam, and effects of anharmonicity in the crystal field potential. The emission has a full width at half‐maximum (FWHM) ?13%, is an order of magnitude brighter than bremsstrahlung, is tunable, is linearly polarized, and is highly directional.

OPTICAL PATTERNS OF THERMALLY SELF‐DEFOCUSED LIGHT

In this Letter we discuss the observation of a pattern of concentric annuli formed as a result of thermal effects upon a light beam. To our knowledge, this is the first time such patterns have been reported.When a beam of light passes through an absorbing medium whose refractive index decreases as a function of temperature, the medium behaves as a negative lens. Gordon et al.1 observed this phenomenon by placing a cell containing an absorbing liquid between the mirrors of a helium‐neon laser. The absorption constants for several liquids with very low absorbencies have been determined using this effect.2 Rieckhoff3 measured the increase in the diameter of a light beam that passed through a thermal self‐defocusing liquid placed external to the laser inter‐ferometer. Recently Leite et al.4 demonstrated that this effect may be used as a power limiting device.