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NASA NTRS2010

Measuring a Fiber-Optic Delay Line Using a Mode-Locked Laser

The figure schematically depicts a laboratory setup for determining the optical length of a fiber-optic delay line at a precision greater than that obtainable by use of optical time-domain reflectometry or of mechanical measurement of length during the delay-line-winding process. In this setup, the delay line becomes part of the resonant optical cavity that governs the frequency of oscillation of a mode-locked laser. The length can then be determined from frequency-domain measurements, as described below. The laboratory setup is basically an all-fiber ring laser in which the delay line constitutes part of the ring. Another part of the ring - the laser gain medium - is an erbium-doped fiber amplifier pumped by a diode laser at a wavelength of 980 nm. The loop also includes an optical isolator, two polarization controllers, and a polarizing beam splitter. The optical isolator enforces unidirectional lasing. The polarization beam splitter allows light in only one polarization mode to pass through the ring; light in the orthogonal polarization mode is rejected from the ring and utilized as a diagnostic output, which is fed to an optical spectrum analyzer and a photodetector. The photodetector output is fed to a radio-frequency spectrum analyzer and an oscilloscope. The fiber ring laser can generate continuous-wave radiation in non-mode-locked operation or ultrashort optical pulses in mode-locked operation. The mode-locked operation exhibited by this ring is said to be passive in the sense that no electro-optical modulator or other active optical component is used to achieve it. Passive mode locking is achieved by exploiting optical nonlinearity of passive components in such a manner as to obtain ultra-short optical pulses. In this setup, the particular nonlinear optical property exploited to achieve passive mode locking is nonlinear polarization rotation. This or any ring laser can support oscillation in multiple modes as long as sufficient gain is present to overcome losses in the ring. When mode locking is achieved, oscillation occurs in all the modes having the same phase and same polarization. The frequency interval between modes, often denoted the free spectral range (FSR), is given by c/nL, where c is the speed of light in vacuum, n is the effective index of refraction of the fiber, and L is the total length of optical path around the ring. Therefore, the length of the fiber-optic delay line, as part of the length around the ring, can be calculated from the FSRs measured with and without the delay line incorporated into the ring. For this purpose, the FSR measurements are made by use of the optical and radio-frequency spectrum analyzers. In experimentation on a 10-km-long fiber-optic delay line, it was found that this setup made it possible to measure the length to within a fractional error of about 3 10(exp -6), corresponding to a length error of 3 cm. In contrast, measurements by optical time-domain reflectometry and mechanical measurement were found to be much less precise: For optical time-domain reflectometry, the fractional error was found no less than 10(exp -4) (corresponding to a length error of 1 m) and for mechanical measurement, the fractional error was found to be about 10(exp -2) (corresponding to a length error of 100 m).

Tu, Meirong↗

NASA NTRS1991

A wide-band monolithic quasi-optical power meter for millimeter- and submillimeter-wave applications

A novel monolithic power meter has been developed for submillimeter-wave applications (100 GHz to 10 THz). The detector is a large-area bismuth bolometer integrated on a 1.2-micron-thick dielectric membrane. This approach results in a wide-band, high-responsivity detector. The power meter is simple to fabricate, is inexpensive, and can be easily calibrated using a low-frequency network. Quasi-optical measurements at 90, 140, and 240 GHz show that the bolometer is polarization-independent and could be modeled by a simple transmission line model. Absolute power measurements at 90, 140, and 240 GHz show a + or - 5 percent accuracy and agree well with a calibrated Anritsu power meter at 90 GHz. Potential application areas are power calibration, antenna coupling efficiency measurements, and absolute power measurements from solid-state devices and far-infrared lasers at submillimeter wavelengths. Absolute output power measurements on a 220-280 GHz tripler using the quasi-optical power meter are presented as an application example.

Ling, Curtis C.↗

NASA NTRS1972

Acousto-optic filter for electronic laser tuning

Electronically tunable lithium niobate filter utilizes acoustic-optic diffraction for tuning laser to desired frequencies. Filter placed inside laser cavity diffracts incident optical signal of one polarization into orthogonal polarization by collinearly propagating acoustic beam to desired wavelength.

Harris, S. E.↗

NASA NTRS1986

Evidence for beamed electrons in a limb X-ray flare observed by Hard X-Ray Imaging Spectrometer (HXIS)

The limb flare of November 18, 1980, 14:51 UT, was investigated on the basis of X-ray images taken by the Hard X-ray Imaging Spectrometer (HXIS) and of X-ray spectra from the Hard X-Ray Burst Spectrometer (HXRBS) aboard the Solar Maximum Mission (SMM). The impulsive burst was also recorded at microwave frequencies between 2 and 20 GHz whereas no optical flare and no radio event at frequencies below 1 GHz were reported. The flare occurred directly at the SW limb of the solar disk. Taking advantage of the spatial resolution of HXIS images, the time evolution of the X-radiation originating from relatively small source regions can be studied. Using Monte Carlo computations of the energy distribution of energetic electrons traversing the solar plasma, the bremsstrahlung spectra produced by the electrons were derived.

Haug, Eberhard↗

NASA NTRSDate not supplied

Detecting Short Term Drought Impact in the Southwest US Using GOES-16 ABI Data

Satellite optical remote sensing has been often used for monitoring broad-region vegetation change, for example, phenology observations and the year-to-year leaf area index (LAI) responses to climate oscillations. However, rapid responses of vegetation to day-to-day weather perturbations are difficult to detect using available optical remote sensing satellites because of the low frequency of the observations. Sun-synchronous optical sensors, such as Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Very High Resolution Radiometer (AVHRR), can observe a target area once a day. Daily observations cannot distinguish whether short-term changes in Normalized Differential Vegetation Index (NDVI) are actual LAI change or cloud contamination. NOAA’s GOES satellites make observations every 10 to 15 minutes using the Advanced Baseline Imager (ABI). In this study, we used the Geostationary-NASA Earth Exchange (GeoNEX) L1G Top-of-Atmosphere (TOA) ABI data to detect drought impact on NDVI time series in the Southwest US. We used an empirical method to cancel the BRDF effect of varying solar zenith angle. The ABI was able to detect short term drought impacts as well as an NDVI decrease in the dry season. Increased NDVI right after a rainfall followed by an immediate decrease was observed. These ABI NDVI changes were correlated with the RGB time series from PhenoCam Network data. Results indicated that the ABI can be used for short-term analysis of LAI and can detect small LAI changes caused by drought in an arid area, suggesting the potential for its use in near-real time drought monitoring applications.

ABI↗

NASA NTRS1985

Two-frequency laser-induced fluorescence technique for rapid velocity-field measurements in gas flows

A technique is presented for measurements of two-dimensional velocity fields in gas flows. The single-mode frequency of an argon-ion laser is fixed in the wing of an absorption line of iodine molecules that are seeded at low level in the flow of interest. The emitted fluorescence is detected with an image-intensified 100 x 100 photodiode-array camera. two pairs of counterpropagating laser sheets sequentially probe the flow to determine two velocity components. The frequency in one pair is shifted with respect to the other by an acousto-optic modulator. This two-frequency scheme eliminates the need to determine the slope of the line externally and offers the potential for combined pressure and velocity-field measurements.

Hiller, B.↗

NASA NTRS1988

Optical Isolator For Use With Single-Mode Fiber

Assembly of commercially available components acts as single-mode fiber-optic isolator with lower forward-transmission loss and higher attenuation of reverse transmission than previously achieved in single unit. New design reduces cost and improves performance of optical gyroscopes, precise time- and frequency-signal-distribution systems, and other systems that include fiber optics and isolators.

Lutes, George F.↗

NASA NTRS2006

Simplified Lasercom System Architecture Using a Disturbance-Free Platform

A simplified laser communications (lasercom) system architecture, primarily for a deep-space flight transceivers, can be realized by decoupling the lasercom optical components from the host spacecraft using a disturbance-free platform (DFP) developed by Lockheed Martin Space System Company. Unlike conventional lasercom system architectures where a high bandwidth control loop is used to stabilize the optical line-of-sight in the presence of platform disturbance, the DFP package isolates the optical train from the high frequency platform jitter produced by the host. By preventing the vibration from coupling into the optics train, the need for a high bandwidth beam stabilization control loop (including fast steering mirror, detectors, controls and the associated relay optics) is eliminated with possible mass savings.

deep space↗

NASA NTRSDate not supplied

3D Geolocation of Simulated Lightning Sources from Low-Earth Orbit

The recent removal of the Lightning Imaging Sensor from the International Space Station has left an observational gap in lightning detection from low-Earth orbit (LEO). However, new studies have demonstrated the potential for 3D geolocation of lightning sources using orbiting sensors. The Cubespark mission concept aims to take advantage of these developments by deploying a constellation of satellites with radio frequency (RF) sensors and optical imagers to not only map lightning locations, but also to collect bi-spectral flash images. These new capabilities include mapping storm charge structure, flash channel structure, and distinguishing microphysical processes throughout flash development, helping link microphysics and convective processes with overall flash and storm structure around the globe from LEO. In this study, we simulate lightning RF sources in the very high frequency (VHF) band, extrapolate their signals to space-based detection using an improved ionospheric model, and reconstruct their 3D locations using a time-of-arrival (TOA) minimization algorithm. Various constellation configurations, locations, and atmospheric conditions are considered in order to identify and quantify the three main sources of geolocation error: geometric, ionospheric, and instrumental effects. The promising results of this study emphasize the potential of space-based 3D lightning mapping under diverse conditions. 3D resolution is shown to be better than 1-2 km in many cases, enabling new global applications in meteorology and climate sciences. Here we present a selection of these geolocation results as seen from space alongside recent advancements, paving the way for a future generation of LEO lightning mappers.

CubeSpark↗

NASA NTRS1981

Rapid oscillations in cataclysmic variables. V - H2252-035, a single-sideband X-ray and optical pulsar

Photometric and spectroscopic observations of the optical counterpart of the X-ray source H2252-035 reveal the existence of stable periodicities of 3.59 hr and 14.31 min. Both radial velocity and photometric variations occur at the longer period, suggesting that it is the true orbital period of the underlying cataclysmic binary. The shorter period appears as a 10% modulation in the light curve, superposed on the 3.59 hr variation. The frequency is the lower orbital sideband of the X-ray pulsation frequency, suggesting that the optical pulses arise from reprocessing in the atmosphere of the secondary. Unlike the majority of X-ray pulsars, H2252-035 appears to contain a white dwarf rather than a neutron star.

Patterson, J.↗

NASA NTRS2018

Deep Space Relay Terminals for Mars Superior Conjunction

The Sun periodically blocks direct communication between the Earth and Mars, creating a need for a relay when missions have a critical need for communication during these times. We examined several approaches based on optical or radio-frequency relays placed in deep space between the Earth and Mars, exploring multiple possible placements of relays, including periodic orbits in the Sun-Earth and Sun-Mars rotating frames, and eccentric, sun-centered orbits. L4 and L5 long-period orbits in the sun-Mars system provide suitable communications geometry continuously for very long durations. In such an orbit, a deep space relay terminal with two 50 cm optical telescopes and two 75 cm Ka-band dish antennas, along with associated receivers and transmitters, would be capable of supporting Mars superior conjunctions with an optical data rate of 28 to 44 Mbps for return links, and 30-36 Mbps in the forward direction. The relay should use efficient, low-noise optical detectors, such as appropriately cooled Avalanche Photo Diode or Superconducting Nanowire Single Photon Detectors, to achieve these data rates. The single relay discussed in this study might have additional value beyond communications, providing a synergistic platform for solar observation, solar wind observation, gravitational studies, the search for near-earth asteroids, or a navigational beacon.

Lau, Chi-Wung↗

NASA NTRS2016

Point-Wise Phase Matching for Nonlinear Frequency Generation in Dielectric Resonators

An optical resonator fabricated from a uniaxial birefringent crystal, such as beta barium borate. The crystal is cut with the optical axis not perpendicular to a face of the cut crystal. In some cases the optical axis lies in the plane of the cut crystal face. An incident (input) electromagnetic signal (which can range from the infrared through the visible to the ultraviolet) is applied to the resonator. An output signal is recovered which has a frequency that is an integer multiple of the frequency of the input signal. In some cases a prism is used to evanescently couple the input and the output signals to the resonator.

Yu, Nan↗

NASA NTRS1993

VUV thin films, chapter 7

The application of thin film technology to the vacuum ultraviolet (VUV) wavelength region from 120 nm to 230 nm has not been fully exploited in the past because of absorption effects which complicate the accurate determination of the optical functions of dielectric materials. The problem therefore reduces to that of determining the real and imaginary parts of a complex optical function, namely the frequency dependent refractive index n and extinction coefficient k. We discuss techniques for the inverse retrieval of n and k for dielectric materials at VUV wavelengths from measurements of their reflectance and transmittance. Suitable substrate and film materials are identified for application in the VUV. Such applications include coatings for the fabrication of narrow and broadband filters and beamsplitters. The availability of such devices open the VUV regime to high resolution photometry, interferometry and polarimetry both for space based and laboratory applications. This chapter deals with the optics of absorbing multilayers, the determination of the optical functions for several useful materials, and the design of VUV multilayer stacks as applied to the design of narrow and broadband reflection and transmission filters and beamsplitters. Experimental techniques are discussed briefly, and several examples of the optical functions derived for selected materials are presented.

Zukic, Muamer↗

NASA NTRS2019

Rising Above the Cloud - Toward High-Rate Delay-Tolerant Networking in Low-Earth Orbit

The High Data Rate Architecture (HiDRA) project is implementing a High-rate Delay Tolerant Networking (HDTN) capability that can support Low Earth Orbit (LEO) applications and environments. The present state of the effort, future work, and other elements of the work to date are described in this paper. This implementation is intended to support applications that run at 1+ Gbps, per the requirements of modern optical and high-frequency RF links. Uniquely, this implementation is also tuned to support relay and data trunking applications, which might require support for large numbers of small bundles per second. The design for this platform is based entirely on commercial-off-the-shelf (COTS) components, and possesses buffering capabilities in the 5 TB range. This document takes results from previous individual tests and integrates them to demonstrate results in the presence of a coherent use-case: consider a network aboard the ISS which intends to utilize an upcoming optical communications capability. For this use-case, orbital analysis software is used to analyze orbital dynamics, from which a list of access times are generated that might take in to account weather, schedule competition, etc. A variant of Contact Graph Routing (CGR) is applied to these windows to determine an optimal schedule. This schedule is then loaded into the HDTN prototype and, in conjunction with various measurement tools, a complete end-to-end analysis of HDTN's performance is conducted. Various bottlenecks (including storage) are identified: these bottlenecks are expected to help us focus our future work on the elements of the system that are most likely to present issues moving forward. Finally, we discuss possible paths for evolution beyond the present rates supported by the system, including (but not limited to) hardware acceleration.

Hylton, Alan↗

NASA NTRS1978

The central, compact source in the Cygnus A galaxy

Single, optically thin, component models as well as double, optically thick at low frequencies, component models have been examined in trying to explain the central source in the nucleus of Cygnus A. In the course of exploring those models, it is found that this compact radio source may be quite similar to other compact and variable sources in Seyfert galaxies and quasars. Double component models generally do better because they fit the X-rays, and there is a good chance that the X-rays arise in the central source; these models can naturally explain variability which may already have been detected in X-rays. The IR is most likely due to emission by cold dust in the nucleus of Cygnus A. Optical emission is, very likely, primarily thermal, except perhaps at UV wavelengths. A number of observations are suggested to check the predictions of this work.

Kafatos, M.↗

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↗

NASA NTRS1994

Raman-shifting an ArF excimer laser to generate new lines for obtaining optical diagnostic based information in flow fields

The application of tunable excimer lasers in combustion and flow diagnostics is almost routine nowadays. The properties of this laser system that enable density and temperature measurements in supersonic and hypersonic flow fields to be conducted are its high power, high repetition rate, and high spectral brightness. The limitation imposed by this system on these measurements is the paucity of lines in the wavelength region, the vacuum-ultraviolet, where species of interest, such as OH, N2, O2, H2, H2O, CO, NO, etc., are susceptible to electronic excitation to high-lying states. To circumvent this problem one normally resorts to nonlinear optical techniques such as frequency conversion via stimulated Raman scattering (SRS), more commonly known as Raman shifting or Raman mixing, to extend these nonintrusive and nonperturbing techniques to the shorter wavelengths in the VUV region and, for that matter, to longer wavelengths in the infrared region, if the need arises. The theoretical basis of SRS and its application are well documented in the literature. In essence, the Raman shift is a consequence of the inelastic scattering of the incident radiation by the sample. Most of the scattered radiation from the molecules of the sample is unchanged in frequency. However, a small fraction of the incident radiation is changed in frequency. This shift is a result of the fact that some of the incident photons on colliding with the molecules of the sample give up some of their energy and emerge with a lower energy resulting in the lower-frequency Stokes radiation. Other incident photons may increase their energy by colliding with the vibrationally excited molecules of the medium and emerge as higher-frequency antistokes radiation. The generation of the latter is the main objective of this project. The process, however, depends on several factors, including the beam quality of the pump laser, the cross-section of the gaseous medium, the gas pressure, and the ambient temperature of the gas near the focal region. Furthermore, since the Raman shifting process is polarization sensitive, it is necessary to have all of the laser energy in a single polarization. These factors were taken into consideration in the execution of the project. The implementation of the Raman shift was accomplished by focusing the 193 nm output of an ArF excimer laser (Lamda-Physik LPX 150) into a 1-meter long high pressure recirculating Raman cell filled with H2 gas. The laser system was modified in order to improve the mode quality of the pump beam to enhance the Raman shifting. To accomplish this feat, a prism beam expander and grating on the oscillator discharge provided wavelength tuning over the excimer gain profile. Furthermore, a triple-pass configuration, as opposed to unstable resonator optics, was employed in the operation of the amplifier cavity so that when the oscillator output radiation, focused by a 51-cm focal length fused silica lens through a 50 micron pinhole (serving as a spatial filter) and recollimated with a 25-cm focal length lens, was fed into the amplifier, it was injection locked, thereby providing tunable radiation with relatively low divergence. The forward scattered radiation emanating from the impingement of the modified pump beam on the Raman cell was detected using an energy meter after the latter had been separated from it using a dispersing prism.

Koker, Edmond B.↗

NASA NTRS1988

Millimeter-wave diode-grid frequency doubler

Monolithic diode grids were fabricated on 2-cm square gallium-arsenide wafers in a proof-of-principle test of a quasi-optical varactor millimeter-wave frequency multiplier array concept. An equivalent circuit model based on a transmission-line analysis of plane wave illumination was applied to predict the array performance. The doubler experiments were performed under far-field illumination conditions. A second-harmonic conversion efficiency of 9.5 percent and output powers of 0.5 W were achieved at 66 GHz when the diode grid was pumped with a pulsed source at 33 GHz. This grid had 760 Schottky-barrier varactor diodes. The average series resistance was 27 ohms, the minimum capacitance was 18 fF at a reverse breakdown voltage of -3 V. The measurements indicate that the diode grid is a feasible device for generating watt-level powers at millimeter frequencies and that substantial improvement is possible by improving the diode breakdown voltage.

Jou, Christina F.↗