Chi C. Sung

Second-order Rytov approximation

Abstract : An explicit and useful formulation of the solution for the second- order Rytov approximation is given. From this solution a condition of validity for the Rytov solution is obtained. It is concluded that, in general, both the Born and Rytov approximations have the same domain of validity.

Investigation of the effects of temporal thermal gradients in fiber optic gyroscope sensing coils, part 2

We investigate the effects of the nonreciprocity caused by time-varying, radial thermal gradients in interferometric fiber gyroscope sensing coils. Four identical sensing coils have been wound with elliptical core and Panda polarization-maintaining fibers in straight and quadrupolar wind configurations. The induced gyro bias drift, measured as a function of the elevated temperature and time, is presented and compared with a simple heat-conduction model. Qualitative agreement is reached with suitable choices of material parameters.

Doppler and collision-broadening effects in the profile of spectral lines

The effect of velocity-dependent collision broadening on the shape of spectral lines is investigated. Computations show that the modified line shape has a narrower half-width than the conventional Voigt profile.

Study of ultraminiature sensing coils and the performance of a depolarized interferometric fiber optic gyroscope

A study of the performance of a depolarized fiber optic gyro- scope utilizing miniature single-mode fiber sensing coils is conducted. The study includes the analysis of incorporating long and short Lyot depolarizers coupled with sensing coils with diameters of 0.6 and 2.4 in. The roles of the length of the depolarizers and size of the sensing coils are quantitatively correlated, based on the known material parameters in the theoretical model and the experimental data. Minimum depolarizer requirements necessary to minimize the standard mean deviation of the random drift in miniature interferometric fiber optic gyroscopes are pre- sented.

Reduction of nonreciprocity noise in wound fiber optic interferometers

Nonreciprocity caused by temperature-rate-induced phase bias in fiber optic interferometers is experimentally investigated. A series of temperature ramp rates is applied to several fiber optic test coils to produce time-varying thermal gradients along the axial and radial directions. Data on the induced peak rate are presented for coils wound using the conventional (end-to-end) wind and the quadrupolar (center-to-end) wind. The quadrupolar-wound coil exhibits substantially lower sensitivity to temperature transients. It is found that the conventional wind can be considered for low-perlormance, low-cost systems operating in moderately adverse environments.

Wavelet transform applied to synthetic aperture radar - optical implementation and adaptive techniques

The wavelet transform is applied to signal processing of synthetic aperture radar and techniques for determining the range, cross-range, and rotation of the target as well as adaptive process for better resolution are studied. We also optically implement the wavelet transform in the laboratory for real-time processing of radar data, and describe methods for adaptive processing.

Scattering of an electromagnetic wave from dielectric bodies of irregular shape

Cross sections for an electromagnetic wave scattered from dielectric bodies of irregular shape have been obtained. The boundary conditions are reduced to a set of linear algebraic equations whose variables are the expansion coefficients of the electric and magnetic fields. Numerical results are compared with the exact solution for oblate spheroids and good agreement is obtained despite the Rayleigh hypothesis implicitly used in the formulation. The procedures used by Chylek, Grams, and Pinnick to fit the experimental data are also examined and discussed.

Explanation of the experimental results of light backscattered from a very rough surface

We have calculated reflected and refracted electromagnetic waves from a rough surface with the boundary conditions exactly satisfied. The surface model consists of three parameters: the dimension of the independent cells, h; the parameter in the slope distribution function of the tangent plane t0; and the average radius of curvature of the surface, R. Within the reasonable range of these parameters, we have calculated the polarized component P and the depolarized component D measured by Renau et al. and have found the following features of the scattering of the reflected wave from a rough surface that have not been previously explained theoretically. (i) P and D have very different angular dependences. (ii) Whereas P is a sensitive function of t0, D is almost independent of t0. (iii) D does not vanish at all angles. A qualitative comparison between the data and the calculation is given.

Progress in miniaturizing fiber optic gyroscope components for tactical weapon systems

The US Army Missile Command, Research, Development and Engineering Center has been conducting research and development activities on solid-state inertial components for next generation weapon systems for the past decade. The fiber optic gyroscope (FOG), which is on an upward trend as evidenced by the increase in patents and users, warrants a closer examination to address the technology limitations associated with ultra-miniature FOG components that are designed to operate in adverse environments. Several FOG testbeds are constructed for the characterization and evaluation of the optical components operating under a series of test conditions. Analytical models are developed to aid in explaining the experimental results. The data are used in assessing the size limitations for the optical components and design approaches that tend to degrade gyroscope performance. Some remarks are mad on the effects of bending-induced birefringence in single mode fiber on the performance of depolarized FOGs. It is found that reducing the inner diameter of the sensor coil below one inch has a detrimental effect on the gyroscope performance. Furthermore, the inherent fiber crossovers in the conventional precision wound coils tend to degrade the performance of the performance of the depolarized gyroscope. The findings of our investigation will aid FOG designers in optimizing FOG optical components for applications requiring small or restricted volume for various performance regimes.

Temperature effect on the performance of polarization-maintaining fibers

The cross coupling of the polarization modes of polarization-maintaining fibers is measured in a temperature control chamber. The ntemperature dependence of the extinction ratio is analyzed in terms of a steady temperature gradient and a fluctuating component that are experimentally controlled. The consistency of the experimental results with the theoretical analysis in this study can be used to limit the applications of fibers to specific desirable designs. Temperature range for the required accuracy is also addressed.