Tech Briefs

Narrow-Band WGM Optical Filters With Tunable FSRs

Microwave signals generated by optoelectronic oscillators can be tuned.

NASA’s Jet Propulsion Laboratory, Pasadena, California

Optical resonators of the whispering-gallery-mode (WGM) type featuring DC-tunable free spectral ranges (FSRs) have been demonstrated. Previously, the FSRs of WGM optical resonators were determined solely by the resonator geometries and materials: hence, the FSR of such a resonator could be tailored by design, but once the resonator was constructed, its FSR was fixed. By making the FSR tunable, one makes it possible to adjust, during operation, the frequency of a microwave signal generated by an optoelectronic oscillator in which an WGM optical resonator is utilized as a narrow-band filter.

Posted in: Briefs, ptb catchall, Tech Briefs, Photonics, Mirrors, Calibration, Lasers, Performance upgrades
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Digital Beam Deflectors Based Partly on Liquid Crystals

Laser beams are switched to different directions, without using solid moving parts.

John H. Glenn Research Center, Cleveland, Ohio

A digital beam deflector based partly on liquid crystals has been demonstrated as a prototype of a class of optical beam-steering devices that contain no mechanical actuators or solid moving parts. Such beam-steering devices could be useful in a variety of applications, including free-space optical communications, switching in fiber-optic communications, general optical switching, and optical scanning. Liquid crystals are of special interest as active materials in nonmechanical beam steerers and deflectors because of their structural flexibility, low operating voltages, and the relatively low costs of fabrication of devices that contain them. Recent advances in synthesis of liquid-crystal materials and design of the nematic-liquid-crystal cells have resulted in significant improvements in properties (e.g., short response times and birefringence) that are important for effective beam steering.

Posted in: Briefs, ptb catchall, Tech Briefs, Photonics, Fiber optics, Optics, Switches, Wireless communication systems
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Filter Function for Wavefront Sensing Over a Field of View

Optical performance is more balanced when data from more field points are used.

A filter function has been derived as a means of optimally weighting the wavefront estimates obtained in image-based phase retrieval performed at multiple points distributed over the field of view of a telescope or other optical system. When the data obtained in wavefront sensing and, more specifically, image-based phase retrieval, are used for controlling the shape of a deformable mirror or other optic used to correct the wavefront, the control law obtained by use of the filter function gives a more balanced optical performance over the field of view than does a wavefront- control law obtained by use of a wavefront estimate obtained from a single point in the field of view. (The terms “wavefront sensing,” “image-based,” and “phase retrieval” are defined in the immediately preceding article.)

Posted in: Briefs, Physical Sciences, Mirrors, Calibration, Imaging and visualization, Sensors and actuators
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Wearable Environmental and Physiological Sensing Unit

Safety of operations in hazardous environments could be enhanced.

The wearable environmental and physiological sensing unit (WEPS) is a prototype of systems to be worn by emergency workers (e.g., firefighters and members of hazardous-material response teams) to increase their level of safety. The WEPS includes sensors that measure a few key physiological and environmental parameters, a microcontroller unit that processes the digitized outputs of the sensors, and a radio transmitter that sends the processed sensor signals to a computer in a mobile command center for monitoring by a supervisor. The monitored parameters serve as real-time indications of the wearer’s physical condition and level of activity, and of the degree and type of danger posed by the wearer’s environment. The supervisor could use these indications to determine, for example, whether the wearer should withdraw in the face of an increasing hazard or whether the wearer should be rescued.

Posted in: Briefs, Bio-Medical, Medical, Sensors and actuators, Gases, Hazards and emergency management, Hazards and emergency operations, Protective equipment, Rescue and emergency vehicles and equipment
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Broadband Phase Retrieval for Image-Based Wavefront Sensing

Broadband light can be approximated as monochromatic in phase-retrieval computations.

A focus-diverse phase-retrieval algorithm has been shown to perform adequately for the purpose of image-based wavefront sensing when (1) broadband light (typically spanning the visible spectrum) is used in forming the images by use of an optical system under test and (2) the assumption of monochromaticity is applied to the broadband image data. Heretofore, it had been assumed that in order to obtain adequate performance, it is necessary to use narrow-band or monochromatic light.

Posted in: Briefs, TSP, Physical Sciences, Exterior lighting, Mathematical models, Imaging and visualization
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Iterative-Transform Phase Retrieval Using Adaptive Diversity

High- and low-spatial-frequency contents are recovered with high dynamic range.

A phase-diverse iterative-transform phase-retrieval algorithm enables highspatial- frequency, high-dynamic-range, image-based wavefront sensing. [The terms “phase-diverse,” “phase retrieval,” “image-based,” and “wavefront sensing” are defined in the first of the two immediately preceding articles, “Broadband Phase Retrieval for Image-Based Wavefront Sensing” (GSC-14899-1).] As described below, no prior phase-retrieval algorithm has offered both high dynamic range and the capability to recover high-spatial-frequency components.

Posted in: Briefs, TSP, Physical Sciences, Mathematical models, Remote sensing, Data management
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Wavefront Sensing With Switched Lenses for Defocus

It is no longer necessary to translate a camera to precisely controlled defocus positions.

In an alternative hardware design for an apparatus used in image-based wavefront sensing, defocus diversity is introduced by means of fixed lenses that are mounted in a filter wheel (see figure) so that they can be alternately switched into a position in front of the focal plane of an electronic camera recording the image formed by the optical system under test. [The terms “image-based”, “wavefront sensing”, and “defocus diversity” are defined in the first of the three immediately preceding articles, “Broadband Phase Retrieval for Image-Based Wavefront Sensing” (GSC-14899-1).] Each lens in the filter wheel is designed so that the optical effect of placing it at the assigned position is equivalent to the optical effect of translating the camera a specified defocus distance along the optical axis.

Posted in: Briefs, TSP, Physical Sciences, Calibration, Imaging and visualization, Remote sensing, Switches, Test procedures
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Smooth Phase Interpolated Keying

Excellent performance can be obtained without excessive complexity.

Smooth phase interpolated keying (SPIK) is an improved method of computing smooth phase-modulation waveforms for radio communication systems that convey digital information. SPIK is applicable to a variety of phase-shift-keying (PSK) modulation schemes, including quaternary PSK (QPSK), octonary PSK (8PSK), and 16PSK. In comparison with a related prior method, SPIK offers advantages of better performance and less complexity of implementation.

Posted in: Briefs, Electronics & Computers, Communication protocols, Data exchange, Wireless communication systems
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Maintaining Stability During a Conducted-Ripple EMC Test

Ripple is now injected via amplifier-controlled FETs instead of a transformer.

An improved technique, and electronic circuitry to implement the technique, have been developed for a military-standard electromagnetic- compatibility (EMC) test in which one analyzes susceptibility to low-frequncy ripple conducted into the equipment under test via a DC power line. In the traditional technique for performing the particular test, the ripple is coupled onto the DC power line via a transformer. Depending upon some design details of the equipment under test, the inductance of the transformer can contribute a degree of instability that results in an oscillation of amplitude large enough to destroy the equipment.

Posted in: Briefs, TSP, Electronics & Computers, Electric cables, Electromagnetic compatibility, Electronic equipment, Test procedures, Military vehicles and equipment
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Photodiode Preamplifier for Preamplifier for Weak Signals

This circuit suppresses noise without sacrificing timing accuracy.

An improved preamplifier circuit has been designed for processing the output of an avalanche photodiode (APD) that is used in a high-resolution laser ranging system to detect laser pulses returning from a target. The improved circuit stands in contrast to prior such circuits in which the APD output current pulses are made to pass, variously, through wide-band or narrow-band load networks before preamplification. A major disadvantage of the prior wide-band load networks is that they are highly susceptible to noise, which degrades timing resolution. A major disadvantage of the prior narrow-band load networks is that they make it difficult to sample the amplitudes of the narrow laser pulses ordinarily used in ranging.

Posted in: Briefs, Electronics & Computers, Amplifiers, Lasers, Performance upgrades
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