Engineering topics
Publications and source records attributed to Yeh, C..
This paper presents a new way to design a low-loss couipler for high- or low-dielectric constant, dielectric waveguide for optical or millimeter/submillimeter waves.
This document is a progress report of work done in 1985 on the Communications and Control for Electric Power Systems Project at the Jet Propulsion Laboratory. These topics are covered: Electric Field Measurement, Fiber Optic Temperature Sensing, and Optical Power transfer. Work was done on the measurement of ac and dc electric fields. A prototype sensor for measuring alternating fields was made using a very simple electroscope approach. An electronic field mill sensor for dc fields was made using a fiber optic readout, so that the entire probe could be operated isolated from ground. There are several instances in which more precise knowledge of the temperature of electrical power apparatus would be useful. This report describes a number of methods whereby the distributed temperature profile can be obtained using a fiber optic sensor. The ability to energize electronics by means of an optical fiber has the advantage that electrical isolation is maintained at low cost. In order to accomplish this, it is necessary to convert the light energy into electrical form by means of photovoltaic cells. JPL has developed an array of PV cells in gallium arsenide specifically for this purpose. This work is described.
Fermat number transformation convolutes two digital data sequences. Very-large-scale integration (VLSI) applications, such as image and radar signal processing, X-ray reconstruction, and spectrum shaping, linear convolution of two digital data sequences of arbitrary lenghts accomplished using Fermat number transform (ENT).
The emergence of new applications requiring high data traffic necessitates the development of high speed local area networks. Optical fiber is selected as the transmission medium due to its inherent advantages over other possible media and the dual optical bus architecture is shown to be the most suitable topology. Asynchronous access protocols, including token, random, hybrid random/token, and virtual token schemes, are developed and analyzed. Exact expressions for insertion delay and utilization at light and heavy load are derived, and intermediate load behavior is investigated by simulation. A new tokenless adaptive scheme whose control depends only on the detection of activity on the channel is shown to outperform round-robin schemes under uneven loads and multipacket traffic and to perform optimally at light load. An approximate solution to the queueing delay for an oscillating polling scheme under chaining is obtained and results are compared with simulation. Solutions to the problem of building systems with a large number of stations are presented, including maximization of the number of optical couplers, and the use of passive star/bus topologies, bridges and gateways.
Because of the small loss tangent of ice at 30-50 GHz, significant depolarization of satellite-to-ground communication signals could occur due to the presence of cirrus clouds in their paths. It is known that cirrus clouds basically consist of ice needles and plates. Extensive calculations have been performed for the problem of scattering by a family of ice needles and plates for various length-to-width ratios ranging from 1 to 10. Scattering results with special attention to induced cross-polarized fields will be presented.
(Previously announced in STAR as N81-30327)
Optimum design of modern ground-satellite communication systems requires the knowledge of rain-induced differential attenuation, differential phase shift, and cross polarization factors. Different available analytical techniques for raindrop scattering problems were assessed. These include: (1) geometrical theory of diffraction; (2) method of moment; (3) perturbation method; (4) point matching methods; (5) extended boundary condition method; and (6) global-local finite element method. The advantages and disadvantages of each are listed. The extended boundary condition method, which was determined to yield the most scattering results, is summarized. The scattered fields for Pruppacher-Pitter raindrops with sizes ranging from 0.5 mm to 3.5 mm at 20 C and at 30 GHz for several incidence angles are tabulated.
Utilizing round optical fibers as communication channels in optical communication networks presents the problem of obtaining a high efficiency coupling between the optical fiber and the laser. A laser is made an integral part of the optical fiber channel by either diffusing active material into the optical fiber or surrounding the optical fiber with the active material. Oscillation within the active medium to produce lasing action is established by grating the optical fiber so that distributed feedback occurs.
Optimum threshold conditions are derived for oscillations of transversely bounded distributed-feedback lasers in the cases of fiber and diffused waveguides, and it is shown that these structures are feasible with available active materials. The results of calculations show that a fiber laser has an optimum operating region which requires the least gain for self-sustained oscillation, while optimum operation occurs at the highest frequency possible in a diffused waveguide. It is noted that these gains can be achieved with dye in the case of a fiber waveguide and with dye or semiconductors in the case of a diffusion guide.
The reflection and transmission of rectangular and Gaussian pulses impinging on a periodically stratified slab are studied and a number of examples are illustrated and analyzed. The coupled waves approach is used to derive the reflection and transmission coefficient, then the fast Fourier transform is used to illustrate the transient responses.
Bounded distributed-feedback (DFB) lasers are studied in detail. Threshold gain and field distribution for a number of configurations are derived and analyzed. More specifically, the thin-film guide, fiber, diffusion guide, and hollow channel with inhomogeneous-cladding DFB lasers are considered. Optimum points exist and must be used in DFB laser design. Different-modes feedback and the effects of the transverse boundaries are included. A number of applications are also discussed.
Mode conversion in a periodically perturbed thin-film optical waveguide is studied in detail. Three different types of perturbations are considered: periodic index of refraction of the film, periodic index of refraction of the substrate, and periodic boundary. The applications in filters, mode converters, and distributed feedback lasers are discussed.
A frequency selective coupler which consists of two thin film waveguides imbedded in a periodic medium is studied using the Brillouin diagram. Detailed results for the relative bandwidth and the coupling factor are plotted as a function of normalized frequency for a representative case.
Temperature dependence of ionization rates in aluminum gallium arsenides for samples with varying Al contents
Geometric optics analysis of arbitrarily shaped dual reflector antennas