Edward S. Fry

ULTRASLOW GROUP VELOCITY AND ENHANCED NONLINEAR OPTICAL EFFECTS IN A COHERENTLY DRIVEN HOT ATOMIC GAS

We report the observation of small group velocities of order 90 meters per second, and large group delays of greater than 0.26 ms, in an optically dense hot rubidium gas (≈ 360 K). Media of this kind yield strong nonlinear interactions between very weak optical fields, and very sharp spectral features. The result is in agreement with previous studies on nonlinear spectroscopy of dense coherent media.

Measurement of the Mueller matrix for ocean water

The normalized light scattering polarization matrix has been measured for ocean water using an electrooptic light scattering polarimeter. Measurements were done on samples from the Atlantic and Pacific Oceans and the Gulf of Mexico. The polarization effects in the matrices were found to have, in general, a form which is similar to polarization effects in the Rayleigh scattering approximation; for example, all off-diagonal matrix elements except S12 and S21 were zero. Mueller matrix elements were calculated using a Mie computer code and compared to the measured matrices for ocean water. A simple one-component distribution was found to produce a reasonably good fit.

Measurement of polarized light interactions via the Mueller matrix.

A new instrument for rapid and accurate measurement of the Mueller matrix is described. Distinct measurements of all sixteen elements are made simultaneously and with an absolute accuracy of 1-5%. The instrument employs electrooptic modulators. Results are presented for several simple optical devices and systems.

Photoacoustic tomography of biological tissues with high cross-section resolution: Reconstruction and experiment

A modified back-projection approach deduced from an exact reconstruction solution was applied to our photoacoustic tomography of the optical absorption in biological tissues. Pulses from a Ti:sapphire laser (4.7 ns FWHM at 789.2 nm) were employed to generate a distribution of photoacoustic sources in a sample. The sources were detected by a wide-band nonfocused ultrasonic transducer at different positions around the imaging cross section perpendicular to the axis of the laser irradiation. Reconstructed images of phantoms made from chicken breast tissue agreed well with the structures of the samples. The resolution in the imaging cross section was experimentally demonstrated to be better than 60 microm when a 10 MHz transducer (140% bandwidth at -60 dB) was employed, which was nearly diffraction limited by the detectable photoacoustic waves of the highest frequency.

Relationships between elements of the Stokes matrix

Although there are sixteen elements of the Stokes matrix, they are constructed from basically four amplitudes and three phase differences. This of course implies that there exist nine independent relationships connecting the elements. These relationships are equalities for scattering by a single particle in a fixed orientation and in a fixed direction. When Stokes matrices from an ensemble of particles differing in size, orientation, morphology, or optical properties are added incoherently, only six equalities become one-way inequalities. These relations will prove to be very useful for providing consistency checks on experimental measurements of all sixteen elements.

Integrating cavity absorption meter

Scattering effects have always been an important systematic problem in absorption measurements. A new integrating cavity absorption meter has been developed that, in principle, is rigorously independent of scattering effects. The theoretical basis for this integrating cavity device is developed and applied to a generic experimental device: a one-dimensional model is described that demonstrates qualitatively the observed deviations from ideal; details of an actual device are provided; and experimental results for the absorption coefficient of aqueous solutions with various absorptions and with various concentrations of scatterers are presented.

Fast resonance-detection technique for single-frequency operation of injection-seeded Nd:YAG lasers.

A new technique for maintaining single-frequency output from injection-seeded Nd:YAG lasers is described. It involves quickly sweeping the slave-cavity longitudinal-mode spectrum when the flash lamps have created a maximum population inversion. An interference signal is detected by fast electronics, and the Q switch is opened when the slave cavity is resonant with the injected field.

ACCURACY LIMITATIONS ON BRILLOUIN LIDAR MEASUREMENTS OF TEMPERATURE AND SOUND SPEED IN THE OCEAN

There are five mutually dependent variables relevant to Brillouin lidar measurements of temperature and sound speed in the ocean; they are (1) the Brillouin shift, (2) the sound speed, (3) the index of refraction, (4) the temperature, and (5) the salinity. We use three well-known relations to analyze rigorously the interdependence of these five variables. Clearly, a Brillouin shift measurement does not provide a stand-alone determination of temperature or sound speed; one more variable or one more relation must be known. The use of mean values of salinity that have been obtained by an analysis of a large set of historical in situ data is considered for this additional relation.

Aircraft laser sensing of sound velocity in water : Brillouin scattering

Abstract A new real-time data source for sound speed in the upper 100 m has recently been proposed for exploratory development at the Naval Oceanographic and Atmospheric Research Laboratory. This data source would be generated via a ship- or aircraft-mounted, optical pulsed laser using the technique of spontaneous Brillouin scattering. This system should be capable (from a single 10 ns 500 mJ pulse) of yielding range resolved (≈ 1 m resolution) sound speed profiles in water to depths of 75–100 m to an accuracy of 1 m/s. At aircraft speeds of 300 miles/h (134 m/s) and a laser pulse repetition rate of 10 pps, vertical profiles of sound speed would be obtained spatially every 13 m. These 100 m profiles would provide the capability of rapidly monitoring the upper-ocean vertical structure for much of the worlds oceans and for most seasons. They would provide new perspectives on upper-ocean mixing and the oceanic internal wave field. These profiles would also provide an extensive, new, subsurface-data source to existing real-time, operational ocean nowcast/forecast systems. The present systems are dependent on sparse bathythermograph data and information inferred from sea-surface data. Extension of the 100 m profiles to depth using existing and proposed techniques would further increase the value of the laser-derived information.

Temperature dependence of the Brillouin linewidth in water

In the frequency spectrum of light that is scattered in liquid water there is a central elastically scattered peak that is due mainly to scattering by suspended particles; and, there is a peak on each side of the central peak that is displaced by the Brillouin frequency shift. The Brillouin shift is a direct measure of sound speed. The linewidth of the Brillouin shifted lines is dependent on the bulk and shear viscosity of water as well as its density, thermal conductivity, and specific heat. The linewidth of the Brillouin peaks has been investigated in laboratory experiments over a range 1°C to 35°C. The frequency spectrum of back-scattered laser light was analysed using a scanning Fabry-Perot etalon. A strong dependence of the linewidth of the Brillouin shifted lines on temperature was found. In particular, for low temperatures in the range 10°C down to 1°C the linewidth shows an increase from 750 MHz to 1.4 GHz.