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Testing of A Photon-Counting Optical Ground Receiver With Emulated Space-to-Ground Link Effects

The National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) developed and previously characterized a photon-counting optical ground receiver system. The receiver is compliant with the Consultative Committee for Space Data Systems (CCSDS) Optical Communications Coding and Synchronization High Photon Efficiency (HPE) Standard. The standard will be used on the Optical Artemis-2 Orion (O2O) communications demonstration and the Deep Space Optical Communication (DSOC) project aboard the Psyche spacecraft. The receiver system consists of a fiber interconnect, up to sixteen superconducting nanowire single-photon detectors (SNSPDs), and a field programmable gate array (FPGA) based receive modem. Previously, the receiver system architecture was described and test results without emulated link effects were presented. SNSPD device properties, which impact detection jitter and time delay, can limit the receiver dynamic range, especially when operating with varying flux rates (>10 dB) between detectors. Codeword error-rate curves with and without simulated clock drifts attributed to Doppler shift and space transmitter clock differences are presented. Results with ±66 ppm clock differences show minimal performance impact (<0.2 dB). Test results show that the receiver dynamic range (>28 dB) is limited by changing SNSPD detection delays at high photon flux rates.

optical communications↗

NASA NTRS1999

The Earth Occultation Technique with the Burst and Transient Source Experiment

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) is an uncollimated all-sky instrument sensitive to photons in the 20 keV to 1 MeV range. Since CGRO orbits the Earth at an altitude of 450 km, about 33% of the sky, as viewed with BATSE, is covered by the Earth at any given time. The entire sky is subject to Earth Occultation for some portion of CGRO's 52 day precession period. When a source sets below (or rises above) the Earth's limb, atmospheric attenuation produces step-like features in the BATSE data. The observed change in count rate in several energy bands provides a measurement of the source intensity and spectrum without sophisticated background models. These occultation features are used to locate and monitor astrophysical sources with BATSE when the source signal can be separated from the detector background. Examples of step searches, spectra light curves, and transform imaging are presented. Systematic uncertainties due to source confusion, detector response, and rapid background variations are discussed.

Wilson, C. A.↗

NASA NTRS1982

A statistical model of the photomultiplier gain process with applications to optical pulse detection

A Markov diffusion model was used to determine an approximate probability density for the random gain. This approximate density preserves the correct second-order statistics and appears to be in reasonably good agreement with experimental data. The receiver operating curve for a pulse counter detector of PMT cathode emission events was analyzed using this density. The error performance of a simple binary direct detection optical communication system was also derived.

Tan, H. H.↗

NASA NTRS1982

A statistical model of the photomultiplier gain process with applications to optical pulse detection

Tan, H. H.↗

NASA NTRS1991

The Berkeley EUV spectrometer for ORFEUS

A novel EUV spectrometer is presented for the ORFEUS-SPAS mission. It uses a set of four varied line-space spherical diffraction gratings to obtain high-resolution spectra of point sources at wavelengths between 390 and 1200 A. The spectra are recorded with two detector units, each containing curved-surface microchannel plates and a delay-line anode-readout system. An independent optical system detects the image of the source in the entrance aperture and tracks the source as it drifts during an observation, enabling a reconstruction of the spectra postflight. The overall system performance is discussed and illustrated by synthetic spectra.

Hurwitz, M.↗

NASA NTRS1981

Curved-channel microchannel array plates

The microchannel array plate (MCP) is a photoelectric detector with an imaging capability comparable to that of a photographic plate. Recently MCPs in which the channels are curved to inhibit ion feedback have become available. These devices represent a major advance in MCP technology, since a single curved-channel MCP can be operated stably at high gain in the pulse-counting mode without any of the problems of stability of response or short lifetime reported for 'chevron' MCP detectors. Attention is given to the mode of operation of channel electron multipliers (CEM) and MCP, curved-channel MCP, test procedures, and performance characteristics. The accumulated test data show that the fundamental operating characteristics of the curved-channel MCP are directly related to those for the CEM.

Timothy, J. G.↗

NASA NTRS1999

Time-Frequency Analysis of the Dispersion of Lamb Modes

Accurate knowledge of the velocity dispersion of Lamb modes is important for ultrasonic nondestructive evaluation methods used in detecting and locating flaws in thin plates and in determining their elastic stiffness coefficients. Lamb mode dispersion is also important in the acoustic emission technique for accurately triangulating the location of emissions in thin plates. In this research, the ability to characterize Lamb mode dispersion through a time-frequency analysis (the pseudo-Wigner-Ville distribution) was demonstrated. A major advantage of time-frequency methods is the ability to analyze acoustic signals containing multiple propagation modes, which overlap and superimpose in the time domain signal. By combining time-frequency analysis with a broadband acoustic excitation source, the dispersion of multiple Lamb modes over a wide frequency range can be determined from as little as a single measurement. In addition, the technique provides a direct measurement of the group velocity dispersion. The technique was first demonstrated in the analysis of a simulated waveform in an aluminum plate in which the Lamb mode dispersion was well known. Portions of the dispersion curves of the AO, A I , So, and S2 Lamb modes were obtained from this one waveform. The technique was also applied for the analysis of experimental waveforms from a unidirectional graphite/epoxy composite plate. Measurements were made both along and perpendicular to the fiber direction. In this case, the signals contained only the lowest order symmetric and antisymmetric modes. A least squares fit of the results from several source to detector distances was used. Theoretical dispersion curves were calculated and are shown to be in good agreement with experimental results.

Prosser, W. H.↗

NASA NTRS1999

Time-Frequency Analysis of the Dispersion of Lamb Modes

Accurate knowledge of the velocity dispersion of Lamb modes is important for ultrasonic nondestructive evaluation methods used in detecting and locating flaws in thin plates and in determining their elastic stiffness coefficients. Lamb mode dispersion is also important in the acoustic emission technique for accurately triangulating the location of emissions in thin plates. In this research, the ability to characterize Lamb mode dispersion through a time-frequency analysis (the pseudo Wigner-Ville distribution) was demonstrated. A major advantage of time-frequency methods is the ability to analyze acoustic signals containing multiple propagation modes, which overlap and superimpose in the time domain signal. By combining time-frequency analysis with a broadband acoustic excitation source, the dispersion of multiple Lamb modes over a wide frequency range can be determined from as little as a single measurement. In addition, the technique provides a direct measurement of the group velocity dispersion. The technique was first demonstrated in the analysis of a simulated waveform in an aluminum plate in which the Lamb mode dispersion was well known. Portions of the dispersion curves of the A(sub 0), A(sub 1), S(sub 0), and S(sub 2)Lamb modes were obtained from this one waveform. The technique was also applied for the analysis of experimental waveforms from a unidirectional graphite/epoxy composite plate. Measurements were made both along, and perpendicular to the fiber direction. In this case, the signals contained only the lowest order symmetric and antisymmetric modes. A least squares fit of the results from several source to detector distances was used. Theoretical dispersion curves were calculated and are shown to be in good agreement with experimental results.

Prosser, W. H.↗

NASA NTRS1998

Effective Area of the AXAF High Resolution Camera (HRC)

The AXAF High-Resolution Camera (HRC) was calibrated at NASA MSFC's X-Ray Calibration Facility (XRCF) during 1997 March and April. We have undertaken an analysis of the HRC effective area using all data presently available from the XRCF. We discuss our spectral fitting of the beam-normalization detectors (BNDs), our method of removing higher order contamination lines present in the spectra, and corrections for beam non-uniformities. We apply a model of photon absorption depth in order to fit a smooth curve to the quantum efficiency of the detector. This is then combined with the most recent model of the AXAF High-Resolution Mirror Assembly (HRMA) to determine the ensemble effective area versus energy for the HRC. We also address future goals and concerns.

Patnaude, Daniel↗

NASA NTRS1974

Edge and line detection in ERTS imagery: A comparative study

Several local edge detection operators were applied to a set of ERTS pictures of the Monterey, Calif. area. Gradient operators performed consistently better than laplacian operators in detecting edges. It was also found that if a grayscale normalization operation, histogram flattening, was applied to the pictures first, the edge detector outputs were greatly enhanced. The use of interpolation for more accurate location of edges on a digital picture was also briefly investigated. Curve detection operators were applied to the edge detector outputs; this had the effect of enhancing the edges while suppressing noise.

Eberlein, R.↗

NASA NTRS1977

A focussing iron line crystal spectrometer for Spacelab

A crystal spectrometer system is described which employs conical focusing of 12 curved LiF crystal panels to minimize the detector size and reduce the background counting rate. The wavelength range from 1.70 to 1.98 A is covered, including the resonance lines of Fe XXV and Fe XXVI as well as the Fe I K-alpha line and absorption edge. Operation of the spectrometer is discussed, noting that diffracted X-rays are registered in one-dimensional position-sensitive detectors and that the arrival position of a photon in a detector is related to its wavelength due to the fixed curvature of the crystal panels in the dispersion plane. Some characteristics of the multianode position-sensitive detectors are reviewed along with the crystal arrangement and mounting. The instrument sensitivity is evaluated in relation to the strengths of 6.7-keV emission features detected by the Ariel 5 and OSO 8 proportional-counter spectrometers.

Catura, R. C.↗

NASA NTRS2001

Synchronized Electronic Shutter System (SESS) for Thermal Nondestructive Evaluation

The purpose of this paper is to describe a new method for thermal nondestructive evaluation. This method uses a synchronized electronic shutter system (SESS) to remove the heat lamp's influence on the thermal data during and after flash heating. There are two main concerns when using flash heating. The first concern is during the flash when the photons are reflected back into the camera. This tends to saturate the detectors and potentially introduces unknown and uncorrectable errors when curve fitting the data to a model. To address this, an electronically controlled shutter was placed over the infrared camera lens. Before firing the flash lamps, the shutter is opened to acquire the necessary background data for offset calibration. During flash heating, the shutter is closed to prevent the photons from the high intensity flash from saturating the camera's detectors. The second concern is after the flash heating where the lamps radiate heat after firing. This residual cooling introduces an unwanted transient thermal response into the data. To remove this residual effect, a shutter was placed over the flash lamps to block the infrared heat radiating from the flash head after heating. This helped to remove the transient contribution of the flash. The flash lamp shutters were synchronized electronically with the camera shutter. Results are given comparing the use of the thermal inspection with and without the shutter system.

Zalameda, Joseph N.↗

NASA NTRS2000

The Earth Occultation Technique with the Burst and Transient Source Experiment

The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) is successfully being used as an all-sky hard X-ray monitor. The experiment consists of a set of eight uncollimated detectors sensitive to photons in the 20 keV to 2 MeV range. Since CGRO orbits the Earth at an altitude of about 450 km, about 33% of the sky, as viewed with BATSE, is covered by the Earth at any given time. The entire sky is subject to Earth occultation for some portion of CGRO's 52 day precession period. When a source sets below or rises above the Earth's limb, atmospheric attenuation produces step-like features in the BATSE data. The observed change in count rate in several energy bands provides a measurement of the source intensity and spectrum without sophisticated background models. These occultation features are used to locate and monitor astrophysical sources with BATSE when the source signal can be separated from the detector background. Examples of step searches, spectra, light curves, and transform imaging are presented.

Wilson, C. A.↗

NASA NTRS2002

The Burst and Transient Source Experiment Earth Occultation Technique

An Earth orbiting detector sensitive to gamma-ray photons will see step-like occultation features in its count rate when a gamma-ray point source crosses the Earth's limb. This is due to the change in atmospheric attenuation of the gamma rays along the line of sight. In an uncollimated detector, these occultation features can be used to locate and monitor astrophysical sources provided their signals can be individually separated from the detector background. We show that the Earth occultation technique applied to the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (CGRO) is a viable and flexible all-sky monitor in the low-energy gamma-ray and hard X-ray energy range (20 keV-1 MeV). The method is an alternative to more sophisticated photon imaging devices for astronomy and can serve well as a cost-effective science capability for monitoring the high energy sky. Here we describe the Earth occultation technique for locating new sources and for measuring source intensity and spectra without the use of complex background models. Examples of transform imaging, step searches, spectra, and light curves are presented. Systematic uncertainties due to source confusion, detector response, and contamination from rapid background fluctuations are discussed and analyzed for their effect on intensity measurements. A sky location-dependent average systematic error is derived as a function of Galactic coordinates. The sensitivity of the technique is derived as a function of incident photon energy and also as a function of angle between the source and the normal to the detector entrance window. Occultations of the Crab Nebula by the Moon are used to calibrate Earth occultation flux measurements independent of possible atmospheric scattering effects.

Harmon, B. A.↗

NASA NTRS1992

Development of high resolution imaging detectors for x ray astronomy

This final report summarizes our past activities and discusses the work performed over the period of 1 April 1990 through 1 April 1991 on x-ray optics, soft x-ray (0.1 - 10 KeV) imaging detectors, and hard x-ray (10 - 300 KeV) imaging detectors. If microchannel plates (MCPs) can be used to focus x-rays with a high efficiency and good angular resolution, they will revolutionize the field of x-ray optics. An x-ray image of a point source through an array of square MCP pores compared favorably with our ray tracing model for the MCP. Initial analysis of this image demonstrates the feasibility of MCPs for soft x-rays. Our work continues with optimizing the performance of our soft x-ray MCP imaging detectors. This work involves readout technology that should provide improved MCP readout devices (thin film crossed grid, curved, and resistive sheets), defect removal in MCPs, and photocathode optimization. In the area of hard x-ray detector development we have developed two different techniques for producing a CsI photocathode thickness of 10 to 100 microns, such that it is thick enough to absorb the high energy x-rays and still allow the photoelectrons to escape to the top MCP of a modified soft x-ray imaging detector. The methods involve vacuum depositing a thick film of CsI on a strong back, and producing a converter device that takes the place of the photocathode.

Murray, S. S.↗

NASA NTRS1977

Wideband 1.064 micrometer detector evaluation

Several types of communications detectors for use in a 400 Mbps 1.064 micrometer laser communication system were evaluated and characterized. The communication system Bit Error Rate (BER) performance was measured, and test results for the best detector of each type are summarized. The complete BER curves are presented. The 400 Mbps 1.064 micrometer communication system receiver test bed is described. The best performance levels which can be achieved by focusing the signal to diffraction limited spots on the photosensitive area are cited.

Green, S. I.↗

NASA NTRS1980

The direct deconvolution of X-ray spectra

The method of deconvolution of proportional counter X-ray spectra as outlined by Blissett and Cruise is reviewed with particular emphasis on low-energy spectra. This method involves the expansion of the incident spectrum in terms of a set of orthonormal singular functions which propagate independently through the detector matrix. When applied to low-energy detectors which typically exhibit absorption features in their efficiency curves, the initial Blissett and Cruise prescription is shown to be unstable. Alternative methods for handling the efficiency are described and evaluated. In the deconvolution of steep spectra, additional distortions are shown to arise as a result of sidelobe oscillations in the effective response functions. A selective weighting procedure is thus introduced to suppress spurious features of this type. Finally, the role of filtering in the deconvolution procedure is discussed and several suitable forms for the filter are suggested.

Kahn, S. M.↗

NASA NTRS1990

Dispersive spectroscopy on AXAF

The designs of two transmission grating spectrometers and a Bragg crystal spectrometer that are being developed for the Advanced X-ray Astrophysics Facility (AXAF) are described. The gratings, which are composed of arrays of small facets mounted on plates which can be inserted immediately behind the AXAF telescope, divide the AXAF energy band (80 eV-10 keV) into three regions and attain very high resolving powers for point sources. The Bragg Crystal Spectrometer (BCS) is a focal plane instrument, in which X-rays that satisfy the Bragg law are reflected from a curved crystal which refocuses the beam onto an imaging detector. The BCS achieves the highest spectral resolutions of the AXAF spectrometers: for E values between 500 and 1600 eV, the FWHM of a narrow line is less than 1 eV.

Markert, T. H.↗