Engineering topics

Marshall, William K.

Publications and source records attributed to Marshall, William K..

11 records

NASA NTRS1992

Optical Phase-Locked Loop For Optical Communication

Experimental apparatus includes two diode-pumped solid-state lasers of non-planar-ring-oscillator type, one represents the transmitting oscillator, other local oscillator in receiver. Output of transmitting oscillator is amplitude-modulated with pulse-position-modulated data stream. Phase-error variance of less than 0.05 mrad(Sup 2) achieved with relatively-narrow-band loop. Applications include free-space and fiber-optic communications, navigation, and scientific instrumentation in which receivers are required to track phases of transmitted signals.

Win, Moe Z.↗

NASA NTRS1991

Low data rate coherent optical link demonstration using frequency stabilized solid state lasers

A low data rate laboratory heterodyne link demonstration operating at 1.06 micron using frequency stabilized Nd:YAG lasers was implemented. The data was modulated onto the optical carrier at 100 kbps using binary pulse position modulation. Phase coherent reception was achieved by closing the receiver phase tracking loop around the piezo-electric and temperature-tuned local oscillator laser. Initial frequency acquisition was accomplished by linearly scanning the LO frequency over the uncertainty range while a combined frequency and phase tracking loop searches for a lock indicator signal. The link performance was experimentally evaluated and compared to the theoretical predictions.

Chen, CHIEN-C.↗

NASA NTRS1990

Analyzing Optical Communications Links

Optical Communication Link Analysis Program, OPTI, analyzes optical and near-infrared communication links using pulse-position modulation (PPM) and direct detention. Link margins and design-control tables generated from input parameters supplied by user. Enables user to save sets of input parameters that define given link and read them back into program later. Alters automatically any of input parameters to achieve desired link margin. Written in FORTRAN 77.

Marshall, William K.↗

NASA NTRS1988

Calculating Optical-Transmitter Radiation Pattern s

New formula gives more-accurate gains and pointing losses. Set of approximate formulas predicts angular dependence of far radiation field coherent optical transmitter, telescope having central obscuring disk. Formulas derived without recourse to simplifying assumption of uniform plane-wave illumination used to derive less-accurate traditional formulas.

Marshall, William K.↗

NASA NTRS1988

Synchronization Scheme For PPM Communication

Synchronization achieved under difficult, low-signal conditions. Synchronization scheme for pulse-position modulation (PPM) matches pulse, word, and frame timeslots of receiver to received signals to ensure proper identification of received pulses. Involves brief sequence of synchronizing pulses within each transmitted frame and relies on periodic repetition of synchronizing pulses for verification. Use of verification reduces number of synchronizing pulses required but increases time required to achieve synchronization.

Marshall, William K.↗

NASA NTRS1988

Monolithic electro-optic modulator array

A PIN GaAlAs diode structure is provided with parameters for index guiding of light in a single mode. The index of refraction of the central layer I (which in practice may be lightly doped .pi. or .nu.) is greater than the p- and n-layers to create a slab waveguide in the transverse direction. Stripe contacts define separate waveguide channels that are separated electrically and optically by implanting protons or etching grooves between the stripe contacts in the upper layer. Separate reverse biasing voltages may be applied to the stripe contacts for modulation of the light in proportions to the voltage, either with absorption modulation, if the light wavelength is within about 500.ANG. of the bandgap of the .pi.-material, or phase-delay modulation, if the wavelength is separated from the bandgap of the .pi.-material by at least 900.ANG..

Robinson, Deborah L.↗

NASA NTRS1987

Diffractively Coupled, Refractively Guided Lasers

Semiconductor-laser arrays more reliable, more powerful, and easier to make. Improved design intended to eliminate undesired electromagnetic modes and mode shifts sometimes occuring in gain-guided variety. Reflected from mirror/window at end of common resonator section of laser, energy refracted from each laser enters adjacent laser. Mutual coupling establishes phase relationships among lasers. Monolithic laser array made by standard epitaxial techniques. Made in part with polymeric materials to mitigate some deleterious effects of all-expitaxial processing. Potential applications include optical communications, ranging, printing, and recording.

Katz, Joseph↗

NASA NTRS1987

Approximate Analysis of Semiconductor Laser Arrays

Simplified equation yields useful information on gains and output patterns. Theoretical method based on approximate waveguide equation enables prediction of lateral modes of gain-guided planar array of parallel semiconductor lasers. Equation for entire array solved directly using piecewise approximation of index of refraction by simple functions without customary approximation based on coupled waveguid modes of individual lasers. Improved results yield better understanding of laser-array modes and help in development of well-behaved high-power semiconductor laser arrays.

Marshall, William K.↗

NASA NTRS1987

Transmitter pointing loss calculation for free-space optical communications link analyses

In calculating the performance of free-space optical communications links, the transmitter pointing loss is one of the two most important factors. It is shown in this paper that the traditional formula for the instantaneous pointing loss (i.e., for the transmitter telescope far-field beam pattern) is quite inaccurate. A more accurate and practical approximation is developed in which the pointing loss is calculated using a Taylor series approximation. The four-term series is shown to be accurate to 0.1 dB for the theta angles not greater than 0.9 lambda/D (wavelength/telescope diameter).

Marshall, William K.↗

NASA NTRS1986

Simple method for designing or analyzing an optical communication link

A simple method is described for determining the performance of a free space optical communication link. The method can be used either in the system design (synthesis) mode, or in the performance evaluation (analysis) mode. Although restricted to photo counting based detection (e.g., photomultiplier tube or equivalent) of pulse position modulated signals, the method is still sufficiently general to accommodate space-based, as well as ground-based, reception.

Lesh, James R.↗

NASA NTRS1986

Word and frame synchronization with verification for PPM optical communications

A method for obtaining word and frame synchronization in pulse position modulated optical communication systems is described. The method uses a short sync sequence inserted at the beginning of each data frame and a verification procedure to distinguish between inserted and randomly occurring sequences at the receiver. This results in an easy to implement sync system which provides reliable synchronization even at high symbol error rates. Results are given for the application of this approach to a highly energy efficient 256-ary PPM test system.

Marshall, William K.↗