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A surface acoustic wave (SAW) sensor constructed from single-mode fiber-optic components is described. An analysis of reciprocal and nonreciprocal modes of operation of the sensor is presented. Results from measurements on a variety of SAW devices illustrate the use of the sensor. The amplitude sensitivity is 0.0003 A for an integration time of 0.1 s.
Fiber/jacket interactions affect performance. Attenuation and phase measurements made to assess effect of temperature on signal delay and attenuation in two fiber-optic cable samples.
A circular-scan streak tube designed for use in a laser ranging system is described. Electrostatic focusing is used between the photocathode and a microchannel plate, and electrons from the microchannel plate are proximity focused onto an output phosphor screen. Electron beam deflection is achieved by driving two orthogonal sets of deflection plate assemblies in phase quadrature at a frequency of 200 MHz. The light intensity in the output beam trace is measured by using a circular Reticon array of 720 photodiodes, which is fiber-optically coupled to the output phosphor screen of the tube. Sample measurements of frequency-doubled Nd:YAG laser-pulse waveforms are given. Also the output beamwidth has been measured at the 200-MHz deflection frequency. The results suggest a limiting resolution of 33 psec for the circular-scan streak tube used for these tests.
The design of a clamped-diaphragm pressure switch is described in which diaphragm motion is detected by a simple fiber-optic displacement sensor. The switch was evaluated in a pressure measurement system where it detected the zero crossing of the differential pressure between a static test pressure and a tank pressure that was periodically ramped from near zero to fullscale gage pressure. With a ramping frequency of 1 hertz and a full-scale tank pressure of 69 N/sq cm gage (100 psig), the switch delay was as long as 2 milliseconds. Pressure measurement accuracies were 0.25 to 0.75 percent of full scale. Factors affecting switch performance are also discussed.
Consideration is given to the development of a low-noise fiber-optic ring interferometer gyroscope. A technique for measuring the Fresnel drag coefficient of optical fibers is described, and the accuracy of the technique is considered. An experiment is performed which allows verification of the Einstein velocity addition theorem to the first nonlinear term. An experimental setup for measuring Fresnel drag is described: it consists of a Sagnac interferometer and a Fresnel drag measurement configuration.
An assessment and determination of technology requirements for developing a demonstration model to evaluate feasibility of practical cryogenic liquid level, pressure, and temperature sensors is presented. The construction of a demonstration model to measure characteristics of the selected sensor and to develop test procedures are discussed as well as the development of an appropriate electronic subsystem to operate the sensors.
A research data system developed for in-flight measurement of direct and nearby lightning-strike characteristics is described. The measurement system consists of a wide-band analog recorder which records the continuous lightning scenario and fast sample-rate digital transient recorders with augmented memory capacity for increased time resolution of specific times of interest. Electromagnetic sensors with bandwidths exceeding 100 MHz are used which respond to rates of change of the quantities being measured. Data system immunity from electromagnetic interference is accomplished by the use of a dynamotor for power isolation, shielded system enclosure and fiber-optic data links.
Current work on optical sensors and optically controlled actuators for use in air-breathing engine control systems is reviewed with particular reference to the design and operation of several new fiber-optic devices. These include a tachometer, a rotary position encoder, a Fabry-Perot interferometer and a rare-earth sensor for measuring engine gas temperatures, a high-temperature photoswitch designed for the range -55 to 260 C, and optical cables and connectors. The advantages of optics over conventional wire systems used for sensing and actuator control are briefly discussed.
Large surface areas (approximately 1 sq m) of the Skynet I qualification model satellite were exposed to the environments of a flux of monoenergetic electrons and pulsed high-intensity X-ray photons from an exploding-wire-radiator source, separately and simultaneously. Experiments were performed with both the cylindrical solar cell panels and the end thermal blanket exposed to these environments. The satellite was instrumented with fiber-optics isolated fast-response sensors capable of sensing and recording the time-varying electric fields and surface currents on the satellite. Spontaneous discharges of the two surfaces were characterized prior to measurements of the interaction of the system-generated electromagnetic pulse from the X-ray photon burst with the electron precharged surfaces. A two-dimensional self-consistent finite difference computer code was used to predict the currents on the Skynet both with and without precharging of the thermal-blanket. These calculations, which used an thermal-blanket voltage profile, predicted a factor of 2 increase in satellite exterior axial currents.
Phase 2 (development) which was concluded for the ultrasonic Doppler device and the light-pipe-reflectance device is reported. An ultrasonic Doppler breadboard system was assembled which accurately measured runout in the J-2 LOX pump impeller during operation. The transducer was mounted on the outside of the pump volute using a C-clamp. Vibration was measured by conducting the ultrasonic wave through the volute housing and through the fluid in the volute to the impeller surface. The impeller vibration was also measured accurately using the light-pipe probe mounted in an elastomeric-gland fitting in the pump case. A special epoxy resin developed for cryogenic applications was forced into the end of the fiber-optic probe to retain the fibers. Subsequently, the probe suffered no damage after simultaneous exposure to 2150 psi and 77 F. Preliminary flash X-radiographs were taken of the turbine wheel and the shaft-bearing-seal assembly, using a 2-megavolt X-ray unit. Reasonable resolution and contrast was obtained. A fast-neutron detector was fabricated and sensitivity was measured. The results demonstrated that the technique is feasible for integrated-time measurements requiring, perhaps, 240 revolutions to obtain sufficient exposure at 35,000 rpm. The experimental verification plans are included.
Digital fiber-optic data-link terminal modules are being developed in a number of laboratories, and often the modules take the form of black boxes without convenient access to internal signals. This paper discusses types of measurements which can be made using only the digital terminals of the link. Bit-error-rate (BER) vs signal-power data can, of course, provide a means for proving overall link performance, but it can also be used to determine certain internal parameters of the receiver. The theoretical dependence of the BER-vs-light-signal-power curve shape on receiver input-noise equivalent power and comparator slicing level is discussed. BER-vs-light-signal-power measurements obtained with an experimental link are presented and interpreted with the aid of the theoretical curves. The effects of other problems, such as hum or oscillation, on the BER data are mentioned. Sensitivity of the system BER to bit rate is also discussed, with application to determining system operating limits. Since BER measurements are simple to make and can easily be automated, they may prove to be a useful means for either laboratory evaluation or production-line acceptance testing of sealed terminal modules.
A planar Rowland spectrometer was fabricated and characterized as a wavelength demultiplexer for multimode fiber-optic applications. The spectrometer consisted of a planar multimode glass waveguide with two curved end faces and a cylindrical concave attached to one of the end faces. Semiconductor lasers with wavelengths between 0.825 and 0.845 micron were used for the measurements. Cross-talk isolation between two adjacent fibers with center-to-center separation of 175 microns (100 A in wavelength difference) was measured to be 18 dB. The device's performance was limited by grating diffraction efficiency, optical aberration, waveguide dispersion, and waveguide losses.
A newly developed electro-optic voltage transducer is described. It is capable of measuring 30 ns rise time, floating or single-ended, 0 to 20 kV transients in a high noise environment such as a fast discharge laser or energetic switch system. The voltage transient is detected by a specially developed resistor in series with a light emitting diode. The light output (proportional to voltage) is transmitted remotely via a fiber-optic cable to a detector and recorder. Typical signals and comparison with commercial probe system signals are described.
A research data-gathering system being developed for inflight measurement of direct and nearby lightning strike characteristics is described. Wideband analog recorders used to record the lightning scenario are supplemented with high-sample-rate digital transient recorders with augmented memory capacity for increased time resolution of specific times of interest. The endless-loop data storage technique employed by the transient recorders circumvents problems associated with oscilloscopic techniques and allows unattended operation. System integrity and immunity from induced effects is accomplished by fiber-optics signal-transmission links, shielded system enclosures, and the use of a dynamotor for power system isolation.
The design and fabrication of an annular ring detector consisting of optical fibers terminated with photodetectors is described. The maximum width of each concentric ring has to be small enough to permit the resolution of a Ronchi ruling transform with a dot spacing of 150 microns. A minimum of 100 concentric rings covering a circular area of 2.54 cm diameter also is necessary. A fiber-optic array consisting of approximately 89,000 fibers of 76 microns diameter was fabricated to meet the above requirements. The fibers within a circular area of 2.5 cm diameter were sorted into 168 adjacent rings concentric with the center fiber. The response characteristics of several photodetectors were measured, and the data used to compare their linearity of response and dynamic range. Also, coupling loss measurements were made for three different methods of terminating the optical fibers with a photodetector.
The seminar focused on infrared systems, the space telescope, new design for space astronomy, future earth resources systems, and planetary systems. Papers were presented on infrared astronomy satellite, infrared telescope on Spacelab 2, design alternatives for the Shuttle Infrared Telescope Facility, Spacelab 2 infrared telescope cryogenic system, geometrical theory of diffraction and telescope stray-light analysis, Space Telescope scientific instruments, faint-object spectrograph for the Space Telescope, light scattering from multilayer optics, bidirectional reflectance distribution function measurements of stray light suppression coatings for the Space Telescope, optical fabrication of a 60-in. mirror, interferogram analysis for space optics, nuclear-pumped lasers for space application, geophysical fluid flow experiment, coherent rays for optical astronomy in space, optical system with fiber-optical elements, and Pioneer-Venus solar flux radiometer.
Passive optical sensors using fiber-optic signal transmission to a remote monitoring station are explored as an alternative to electrical sensors used to monitor the status of explosive propellants. The designs of passive optical sensors measuring liquid level, pressure, and temperature in cryogenic propellant tanks are discussed. Test results for an experimental system incorporating these sensors and operating in liquid nitrogen demonstrate the feasibility of passive sensor techniques and indicate that they can serve as non-hazardous replacements for more conventional measuring equipment in explosive environments.
This paper reviews the recent progress in producing picosecond optical pulses from semiconductor laser diodes. The discussion concentrates on the mode-locking of a semiconductor laser diode in an external resonator. Transform-limited optical pulses ranging from several picoseconds to subpicosecond durations have been observed with active and passive mode-locking. Even though continuing research on the influence of impurities and defects on the mode-locking process is still needed, this technique has good promise for being utilized in fiber-optic communication systems. Alternative methods of direct electrical and optical excitation to produce ultrashort laser pulses are also described. They can generate pulses of similar widths to those obtained by mode-locking. The pulses generated will find applications in laser ranging and detector response measurement.