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NASA NTRS1991

Adaptive optics in coherent lidar wind measurements: A feasibility study

Laser Doppler radar (lidar) is widely used for remote sensing of wind velocities. Usable wavelengths for the laser are limited by the effects of atmospheric turbulence. An adaptive optical system is proposed to compensate for turbulence effects on signal power. The feasibility of an adaptive system is considered in light of the effects of speckle from the aerosol target. It is concluded that adaptive optics is a promising technique for improving the performance of a 2 micron lidar wind measurement system. The chief technical challenges are a laser that will give the required output and pulse repetition rate, a combined Hartmann sensor and heterodyne detector, and a suitable reconstruction algorithm.

Leland, Robert P.↗

NASA NTRSDate not supplied

Optical Modem Enabling Broadband Datacom Links for Crewed Cis-Lunar Missions

We report on the design, development, and testing of our high-power broadband optical modem supporting NASA’s crewed Artemis-2 mission. The O2O modem will be mounted in the crewed Orion module and provide a broadband 505,000 km bi-directional optical link back to earth while en route to the moon. The full-duplex modem consists of a high-power optical transmitter and receiver optimized for serially-concatenated pulse-position modulation (SCPPM). The transmitter is a master-oscillator power-amplifier optical architecture using efficient cladding-pumped amplification in erbium-ytterbium co-doped fiber. The transmitter outputs up to 1 W at ≈1550 nm (limited for eye safety) and supports 6 different user-rates ranging from 20.39 Mb/s to 260.95 Mb/s using PPM16 and PPM32 modulation formats. The optical receiver supports two user-rates: 10.19 Mb/s and 20.39 Mb/s with both rates employing PPM32. The narrowband receiver filtering is optimized to simultaneously accept four separate wavelength channels to mitigate atmospherics through spatial diversity. A configurable interleaver provides additional protection against atmospherics-based signal fading and a powerful soft-decision error correction scheme enables highly sensitive detection. The measured sensitivities at the two bit-rates are -73.8 and -71.8dBm, respectively. The architecture was designed for reliable operation in space, featuring automatic hardware interlocks, pump sparing for the amplifiers, and autonomous operation of all internal hardware and software control loops. The protoflight unit (PFU) was put through rigorous environmental testing which included pyroshock, vibration, electromagnetic interference/compatibility, and thermal-vacuum testing. The modem successfully passed all the environmental screening and has been declared at Technology Readiness Level (TRL) 6.

space communications↗

NASA NTRS1982

Capacity limit of the noiseless, energy-efficient optical PPM channel

It is shown that even though the capacity per photon can be made to increase without bound, the capacity per channel use (for best power efficiency) is always less than 2 nats per symbol. Furthermore, it approaches 2 nats per symbol as the bandwidth expansion factor goes to infinity.

Lesh, J. R.↗

NASA NTRS1983

Capacity limit of the noiseless, energy-efficient optical PPM channel

Lesh, J. R.↗

NASA NTRS1972

Limits on intergalactic helium from the 3C 273 X-ray spectrum.

An X-ray spectrum of the quasi-stellar object 3C 273 in the interval 0.25-10 keV has been obtained by sounding-rocket observations. The best-fit power-law spectrum has a photon index n = 1.3 with no photoelectric absorption. An upper limit on the X-ray optical depth to 3C 273 has been calculated from the data, permitting upper limits to be set on the absolute abundance of helium in the intergalactic medium.

Margon, B.↗

NASA NTRS2011

Sub-Shot Noise Power Source for Microelectronics

Low-current, high-impedance microelectronic devices can be affected by electric current shot noise more than they are affected by Nyquist noise, even at room temperature. An approach to implementing a sub-shot noise current source for powering such devices is based on direct conversion of amplitude-squeezed light to photocurrent. The phenomenon of optical squeezing allows for the optical measurements below the fundamental shot noise limit, which would be impossible in the domain of classical optics. This becomes possible by affecting the statistical properties of photons in an optical mode, which can be considered as a case of information encoding. Once encoded, the information describing the photon (or any other elementary excitations) statistics can be also transmitted. In fact, it is such information transduction from optics to an electronics circuit, via photoelectric effect, that has allowed the observation of the optical squeezing. It is very difficult, if not technically impossible, to directly measure the statistical distribution of optical photons except at extremely low light level. The photoelectric current, on the other hand, can be easily analyzed using RF spectrum analyzers. Once it was observed that the photocurrent noise generated by a tested light source in question is below the shot noise limit (e.g. produced by a coherent light beam), it was concluded that the light source in question possess the property of amplitude squeezing. The main novelty of this technology is to turn this well-known information transduction approach around. Instead of studying the statistical property of an optical mode by measuring the photoelectron statistics, an amplitude-squeezed light source and a high-efficiency linear photodiode are used to generate photocurrent with sub-Poissonian electron statistics. By powering microelectronic devices with this current source, their performance can be improved, especially their noise parameters. Therefore, a room-temperature sub-shot noise current source can be built that will be beneficial for a very broad range of low-power, low-noise electronic instruments and applications, both cryogenic and room-temperature. Taking advantage of recent demonstrations of the squeezed light sources based on optical micro-disks, this sub-shot noise current source can be made compatible with the size/power requirements specific of the electronic devices it will support.

Strekalov, Dmitry V.↗

NASA NTRS1991

Aberration corrected aspheric grating for far ultraviolet spectrographs - Conventional approach

Two approaches to reducing optical aberrations of concave grating spectrographs have been used, holographically controlling the groove curvature and spacing and reshaping the optical substrate while ruling the grooves conventionally. The latter approach, slightly deforming an ellipsoidal grating blank, can lead to diffraction-limited performance at a single FUV wavelength. When such a grating is used in a slitted Rowland circle spectrograph, the result is an extremely efficient spectrograph with spectral resolving power of about 30,000 and low astigmatism. Optical fabrication technology has advanced to the point where these exotic surface gratings are becoming practical.

Content, David↗

NASA NTRS1982

Tunable diode laser heterodyne spectrometer for remote observations near 8 microns

Infrared heterodyne spectroscopy is a passive technique employing coherent optical detection for the study of spectral features in remote sources. It employs optical components such as mirrors and lenses normally associated with incoherent optics, but because of it's coherent nature, it offers the advantages of ultrahigh spectral resolving power, high frequency precision, and diffraction limited field-of-view. Attention is given to the development of an ultrahigh resolution diode laser heterodyne spectrometer for observational and laboratory use. The instrument is designed for operation in the spectral range from 7.5 to 8.5 microns. A PbSe tuneable diode laser (TDL) is employed as local oscillator. A closed-cycle cooler is employed to keep the oscillator at operating temperatures in the range from 12 to 60 K. Attention is given to factors determining the TDL heterodyne sensitivity, the spectrometer design, and a survey of 8 micron observations, SiO could be detected in the sunspot spectrum.

Glenar, D. A.↗

NASA NTRS2002

Performance Evaluation of 40 cm Ion Optics for the NEXT Ion Engine

The results of performance tests with two 40 cm ion optics sets are presented and compared to those of 30 cm ion optics with similar aperture geometries. The 40 cm ion optics utilized both NSTAR and TAG (Thick-Accelerator-Grid) aperture geometries. All 40 cm ion optics tests were conducted on a NEXT (NASA's Evolutionary Xenon Thruster) laboratory model ion engine. Ion optics performance tests were conducted over a beam current range of 1.20 to 3.52 A and an engine input power range of 1.1 to 6.9 kW. Measured ion optics' performance parameters included near-field radial beam current density profiles, impingement-limited total voltages, electron backstreaming limits, screen grid ion transparencies, beam divergence angles, and start-up transients. Impingement-limited total voltages for 40 cm ion optics with the NSTAR aperture geometry were 60 to 90 V lower than those with the TAG aperture geometry. This difference was speculated to be due to an incomplete burn-in of the TAG ion optics. Electron backstreaming limits for the 40 cm ion optics with the TAG aperture geometry were 8 to 19 V higher than those with the NSTAR aperture geometry due to the thicker accelerator grid of the TAG geometry. Because the NEXT ion engine provided beam flatness parameters that were 40 to 63 percent higher than those of the NSTAR ion engine, the 40 cm ion optics outperformed the 30 cm ion optics.

Soulas, George C.↗

NASA NTRS2011

Optimized Biasing of Pump Laser Diodes in a Highly Reliable Metrology Source for Long-Duration Space Missions

Optical metrology system reliability during a prolonged space mission is often limited by the reliability of pump laser diodes. We developed a metrology laser pump module architecture that meets NASA SIM Lite instrument optical power and reliability requirements by combining the outputs of multiple single-mode pump diodes in a low-loss, high port count fiber coupler. We describe Monte-Carlo simulations used to calculate the reliability of the laser pump module and introduce a combined laser farm aging parameter that serves as a load-sharing optimization metric. Employing these tools, we select pump module architecture, operating conditions, biasing approach and perform parameter sensitivity studies to investigate the robustness of the obtained solution.

808 mm diode pumps↗

NASA NTRS2012

Localization Using Visual Odometry and a Single Downward-Pointing Camera

Stereo imaging is a technique commonly employed for vision-based navigation. For such applications, two images are acquired from different vantage points and then compared using transformations to extract depth information. The technique is commonly used in robotics for obstacle avoidance or for Simultaneous Localization And Mapping, (SLAM). Yet, the process requires a number of image processing steps and therefore tends to be CPU-intensive, which limits the real-time data rate and use in power-limited applications. Evaluated here is a technique where a monocular camera is used for vision-based odometry. In this work, an optical flow technique with feature recognition is performed to generate odometry measurements. The visual odometry sensor measurements are intended to be used as control inputs or measurements in a sensor fusion algorithm using low-cost MEMS based inertial sensors to provide improved localization information. Presented here are visual odometry results which demonstrate the challenges associated with using ground-pointing cameras for visual odometry. The focus is for rover-based robotic applications for localization within GPS-denied environments.

Swank, Aaron J.↗

NASA NTRS2020

Nano-ALSEP: Use of Nano-Sat Technology to Advance Lunar Exploration Science/Engineering

During the Apollo program, unique ALSEP (Apollo Lunar Surface Experiments Package) stations were deployed and left on the surface for long term data collection. Powered by a SNAP-27 electrical power source, the packages continued to transmit useful data until September 30, 1977 (the end of the Fiscal year and termination of the budget). The instruments were varied but included an active seismic experiment (ASE), charged particle experiments, magnetometer experiment, micro-meteoroide/bolide detection experiments. Only only one, the Laser Ranging Retro-Reflector (LRRR) was continued to be used to the present day due to the passive nature of the device. After a dearth of data during an absence of 50 years, is it possible to restart similar packages using the advances in nano-sat technologies? An innovation effort is proposed that would first look at a 2U nano-sat package evolving to an eventual radio-isotope powered system that could be either part of a lunar lander or as an independent package. In the first phase, the packages would only survive during the Lunar day. In a second phase, the later radio-isotope options would place the surface packages in a semi-dormant mode during the Lunar night cycle. Communication capability developed in the TechEdSat-n flight series (UHF, S, X and optical bands) would help to independently close the link and potentially explore IoT devices that were powered only in the sunlight. In addition, limited mobility in the form factor of a nano-rover might also greatly extend the utility of such packages while adding evolutionary capability - and finally continue where the Apollo program was forced to leave off.

M Murbach↗

NASA NTRS1977

A distance limit for a class of model gamma-ray burst sources

Gamma ray burst sources are presumably not larger than 10 to the 9th power cm as inferred from observed flux variations. If they are homogeneous and isotropically radiating, then from photon density considerations, they would have to be optically thick due to gamma-gamma pair production when assumed to be too far away. Deviations of observed photo spectra from an exponential shape around 1 MeV lead to an upper limit of the possible distance of such sources of only 2 kpc from the sun. Thus the sources must be galactic unless the radiation is highly beamed or emerges from a relativistically moving shell. This conclusion depends only on observed parameters. The possible presence of particles and fields in the sources would require them to be even closer.

Schmidt, W. K. H.↗

NASA NTRS1983

The cosmological evolution and luminosity function of X-ray selected active galactic nuclei

The cosmological evolution and the X-ray luminosity function of X-ray selected active galactic nuclei (AGNs) are derived and discussed. The sample used consists of 31 AGNs extracted from a fully identified sample of X-ray sources from the Einstein Observatory Medium Sensitivity Survey and is therefore exclusively defined by its X-ray properties. The distribution in space is found to be strongly nonuniform. The amount of cosmological evolution required by the X-ray data is derived in the framework of pure luminosity evolution and is found to be smaller than the amount determined from optically selected samples. The X-ray luminosity function is derived. It can be satisfactorily represented by a single power law only over a limited range of absolute luminosities. Evidence that the luminosity function flattens at low luminosity or steepens at high luminosity, or both, is presented and discussed.

Maccacaro, T.↗

NASA NTRS1979

High power phase locked laser oscillators

The feasibility of mechanizing an adaptive array of independent laser oscillators for generation of a high power coherent output was experimentally investigated. Tests were structured to evaluate component/system requirements for delivery of energy to a low-earth orbit satellite. Initial experiments addressed the control issues of phase locking unstable resonators at low power levels. A successful phase lock demonstration formed the basis for the design and fabrication of the high power, water-cooled, control mirror subsequently installed in the NASA LeRC high power laser. Tests were performed to characterize the operational limits of the laser system and included quantitative assessment of the frequency stability, noise sources, and optical properties of the beam.

Hayes, C. L.↗

NASA NTRS1991

Displacement response, detection limit, and dynamic range of fiber-optic lever sensors

The authors present the evaluation of the displacement response, detection limit, and dynamic range of fiber-optic lever sensors in a general format to establish their dependence on fiber sizes, optoelectronic detector specifications, input power, and other relevant parameters. The formations for the normalized reflected optical power change are derived for the evaluation of the optimal sensor response, the linearity range, and the minimum detectable displacement. The theoretical models are verified by an experiment which determines sensor response, modulation index, reflected optical power change, and linear response range through dynamic measurement. The application of this theoretical model to the study of a fiber-optic microphone for acoustic pressure detection is considered.

He, Gang↗

NASA NTRS2005

Glucose determination in human aqueous humor with Raman spectroscopy

It has been suggested that spectroscopic analysis of the aqueous humor of the eye could be used to indirectly predict blood glucose levels in diabetics noninvasively. We have been investigating this potential using Raman spectroscopy in combination with partial least squares (PLS) analysis. We have determined that glucose at clinically relevant concentrations can be accurately predicted in human aqueous humor in vitro using a PLS model based on artificial aqueous humor. We have further determined that with proper instrument design, the light energy necessary to achieve clinically acceptable prediction of glucose does not damage the retinas of rabbits and can be delivered at powers below internationally acceptable safety limits. Herein we summarize our current results and address our strategies to improve instrument design. 2005 Society of Photo-Optical Instrumentation Engineers.

Spectrum Analysis, Raman/methods↗

NASA NTRS2006

Silicon-Based Optical Modulator with Ferroelectric Layer

According to a proposal, a silicon dioxide layer in a high-speed, low-power, silicon- based electro-optical modulator would be replaced by a layer of lead zirconate titanate or other ferroelectric oxide material. The purpose of this modification is to enhance the power performance and functionality of the modulator. In its unmodified form, the particular silicon- based electro-optical modulator is of an advanced design that overcomes the speed limitation of prior silicon-based electro- optical modulators. Whereas modulation frequencies of such devices had been limited to about 20 MHz, this modulator can operate at modulation frequencies as high as 1 GHz. This modulator can be characterized as a silicon-waveguide-based metal oxide/semiconductor (MOS) capacitor phase shifter in which modulation of the index of refraction in silicon is obtained by exploiting the free-charge-carrier-plasma dispersion effect. As shown in the figure, the modulator includes an n-doped crystalline silicon slab (the silicon layer of a silicon- on-insulator wafer) and a p-doped polycrystalline silicon rib with a gate oxide layer (the aforementioned silicon dioxide layer) sandwiched between them. Under accumulation conditions, the majority charge carriers in the silicon waveguide modify the index of refraction so that a phase shift is induced in the optical mode propagating in the waveguide. The advantage of using an MOS capacitor phase shifter is that it is possible to achieve high modulation speed because there are no slow carrier-generation or -recombination processes involved in the accumulation operation. The main advantage of the proposed substitution of a ferroelectric oxide layer for the silicon dioxide layer would arise from the spontaneous polarization effect of the ferroelectric layer: This spontaneous polarization would maintain accumulation conditions in the absence of applied voltage. Consequently, once the device had been switched to a given optical state, it would remain in that state, even in the absence of applied voltage (in other words, even with power turned off). A secondary advantage is that because the ferroelectric layer would have an index of refraction larger than that of silicon dioxide, there could be some reduction of optical losses attributable to fabrication of the modulator

Sheldon, Douglas↗