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NASA NTRS1975

The evolution of the SEASAT imaging radar

The paper describes the design parameters and development of a synthetic aperture radar for use on the SEASAT spacecraft. This imaging radar is designed to operate at altitudes of 800 km with an orbital inclination of 108 deg, a nominal resolution of 25 m, and a swath width of 100 km. The design evolved from planetary imaging radar studies conducted over many years where an L-band imaging radar was developed and tested on aircraft flights as a prototype system to map the surface of Venus. A solid-state transmitter is used where the pulse repetition frequency is a function of altitude and will be about 2kHz for a 12-m long antenna. The receiver consists of the receiver protector, input filters, the gain control, and the RF amplifier. The ground station uses the standard NASA receiver with a 10-m antenna. The correlator, either optical or digital, must be able to compensate for the pitch and yaw variations of the spacecraft as well as the inherent effective yaw caused by the rotation of the earth, and extract the range curvature and range walk effects.

Brown, W. E., Jr.↗

NASA NTRS1989

Eye-Safe Lidar

Laser infrared radar (lidar) undergoing development harmless to human eyes, consists almost entirely of solid-state components, and offers high range resolution. Operates at wavelength of about 2 micrometers. If radiation from such device strikes eye, almost completely absorbed by cornea without causing damage, even if aimed directly at eye. Continuous-wave light from laser oscillator amplified and modulated for transmission from telescope. Small portion of output of oscillator fed to single-mode fiber coupler, where mixed with return pulses. Intended for remote Doppler measurements of winds and differential-absorption measurements of concentrations of gases in atmosphere.

Byer, Robert L.↗

NASA NTRS2008

NASA Tech Briefs, November 2008

Topics covered include: Digital Phase Meter for a Laser Heterodyne Interferometer; Vision System Measures Motions of Robot and External Objects; Advanced Precipitation Radar Antenna to Measure Rainfall From Space; Wide-Band Radar for Measuring Thickness of Sea Ice; Vertical Isolation for Photodiodes in CMOS Imagers; Wide-Band Microwave Receivers Using Photonic Processing; L-Band Transmit/Receive Module for Phase-Stable Array Antennas; Microwave Power Combiner/Switch Utilizing a Faraday Rotator; Compact Low-Loss Planar Magic-T; Using Pipelined XNOR Logic to Reduce SEU Risks in State Machines; Quasi-Optical Transmission Line for 94-GHz Radar; Next Generation Flight Controller Trainer System; Converting from DDOR SASF to APF; Converting from CVF to AAF; Documenting AUTOGEN and APGEN Model Files; Sequence History Update Tool; Extraction and Analysis of Display Data; MRO DKF Post-Processing Tool; Rig Diagnostic Tools; MRO Sequence Checking Tool; Science Activity Planner for the MER Mission; UAVSAR Flight-Planning System; Templates for Deposition of Microscopic Pointed Structures; Adjustable Membrane Mirrors Incorporating G-Elastomers; Hall-Effect Thruster Utilizing Bismuth as Propellant; High-Temperature Crystal-Growth Cartridge Tubes Made by VPS; Quench Crucibles Reinforced with Metal; Deep-Sea Hydrothermal-Vent Sampler; Mars Rocket Propulsion System; Two-Stage Passive Vibration Isolator; Improved Thermal Design of a Compression Mold; Enhanced Pseudo-Waypoint Guidance for Spacecraft Maneuvers; Altimetry Using GPS-Reflection/Occultation Interferometry; Thermally Driven Josephson Effect; Perturbation Effects on a Supercritical C7H16/N2 Mixing Layer; Gold Nanoparticle Labels Amplify Ellipsometric Signals; Phase Matching of Diverse Modes in a WGM Resonator; WGM Resonators for Terahertz-to-Optical Frequency Conversion; Determining Concentration of Nanoparticles from Ellipsometry; Microwave-to-Optical Conversion in WGM Resonators; Four-Pass Coupler for Laser-Diode-Pumped Solid-State Laser; Low-Resolution Raman-Spectroscopy Combustion Thermometry; Temperature Sensors Based on WGM Optical Resonators; Varying the Divergence of Multiple Parallel Laser Beams; Efficient Algorithm for Rectangular Spiral Search; Algorithm-Based Fault Tolerance Integrated with Replication; Targeting and Localization for Mars Rover Operations; Terrain-Adaptive Navigation Architecture; Self-Adjusting Hash Tables for Embedded Flight Applications; Schema for Spacecraft-Command Dictionary; Combined GMSK Communications and PN Ranging; System-Level Integration of Mass Memory; Network-Attached Solid-State Recorder Architecture; Method of Cross-Linking Aerogels Using a One-Pot Reaction Scheme; An Efficient Reachability Analysis Algorithm.

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NASA NTRS1993

All solid-state high power visible laser

The overall objective of this Phase 2 effort was to develop and deliver to NASA a high repetition rate laser-diode-pumped solid-state pulsed laser system with output in the green portion of the spectrum. The laser is for use in data communications, and high efficiency, short pulses, and low timing jitter are important features. A short-pulse 1 micron laser oscillator, a new multi-pass amplifier to boost the infrared power, and a frequency doubler to take the amplified infrared pulsed laser light into the green. This produced 1.5 W of light in the visible at a pulse repetition rate of 20 kHz in the laboratory. The pulses have a full-width at half maximum of near 1 ns. The results of this program are being commercialized.

Grossman, William M.↗

NASA NTRS1998

Ultrafast Modulation and Switching of Quantum-Well Lasers using Terahertz Fields

Modulation and switching of semiconductor lasers are important for laser-based information technology. Typically the speed of modulation and switching is limited by interband processes such as stimulated and spontaneous recombinations which occur on a nanosecond time scale. This is why the diode laser modulation has been restricted to tens of GHz. Modulation at higher speed is highly desirable as the information technology enters into the so-called tera-era. In this paper, we study the possibility of utilizing THz-field-induced plasma heating to modulate quantum-well lasers. This is a timely study since, with the advancement of THz solid-state sources and free-electron lasers, THz physics and related technology is currently coming out of its infancy. The investigation of interplaying THz and optical fields is also of intruiging fundamental interest. First, we introduce theoretical plasma heating results for the quantum-well optical amplifier in the presense of an intense half-cycle THz pulse. The heated carrier distributions are then utilized to calculate the THz-pulse-induced change in refractive index and gain profile. Since the electron-hole-plasma is heated using intraband transitions, we circumvent the usual complications due to an overall change in density, and the nonlinear recovery is governed solely by the carrier-LO-phonon interactions, typically 5 ps for a complete recovery. This procedure implies THz and sub-THz switching and recovery rates, respectively; using either gain modulation or index modulation. Plasma heating via steady-state THz fields is also studied. Finally, numerical simulation of a coupled set of equations to investigate the THz modulation based on a simplified model for quantum-well lasers is presented. Our results show that a semiconductor laser can be modulated at up to 1 THz with little distortion with a THz field amplitude at the order of a few kV/cm. Laser responses to a change in THz frequency will be shown. Constraints, practicalities, and applications will be discussed.

Ning, Cun-Zheng↗

NASA NTRS2000

A High Energy 2-microns Laser for Multiple Lidar Applications

Solid-state 2-microns laser has been receiving considerable interest because of its eye-safe property and efficient diode pump operation, It has potential for multiple lidar applications to detect water vapor. carbon dioxide and winds. In this paper, we describe a 2-microns double pulsed Ho:Tm:YLF laser and end-pumped amplifier system. A comprehensive theoretical model has been developed to aid the design and optimization of the laser performance. In a single Q-switched pulse operation the residual energy stored in the Tm atoms will be wasted. However, in a double pulses operation mode, the residual energy stored in the Tm atoms will repopulate the Ho atoms that were depleted by the extraction of the first Q-switched pulse. Thus. the Tin sensitized Ho:YLF laser provides a unique advantage in applications that require double pulse operation, such as Differential Absorption Lidar (DIAL). A total output energy of 146 mJ per pulse pair under Q-switch operation is achieved with as high as 4.8% optical to optical efficiency. Compared to a single pulse laser, 70% higher laser efficiency is realized. To obtain high energy while maintaining the high beam quality, a master-oscillator-power-amplifier 2-microns system is designed. We developed an end-pumped Ho:Tm:YLF disk amplifier. This amplifier uses two diode arrays as pump source. A non-imaging lens duct is used to couple the radiation from the laser diode arrays to the laser disk. Preliminary result shows that the efficiency of this laser can be as high as 3%, a factor of three increases over side-pump configuration. This high energy, highly efficient and high beam quality laser is a promising candidate for use in an efficient, multiple lidar applications.

Yu, Jirong↗

NASA NTRS2013

Compact Radar Transceiver with Included Calibration

The Digital Beamforming Synthetic Aperture Radar (DBSAR) is an eight-channel phased array radar system that employs solid-state radar transceivers, a microstrip patch antenna, and a reconfigurable waveform generator and processor unit. The original DBSAR transceiver design utilizes connectorized electronic components that tend to be physically large and heavy. To achieve increased functionality in a smaller volume, PCB (printed circuit board) transceivers were designed to replace the large connectorized transceivers. One of the most challenging problems designing the transceivers in a PCB format was achieving proper performance in the calibration path. For a radar loop-back calibration path, a portion of the transmit signal is coupled out of the antenna feed and fed back into the receiver. This is achieved using passive components for stability and repeatability. Some signal also leaks through the receive path. As these two signal paths are correlated via an unpredictable phase, the leakage through the receive path during transmit must be 30 dB below the calibration path. For DBSAR s design, this requirement called for a 100-dB isolation in the receiver path during transmit. A total of 16 solid-state L-band transceivers on a PCB format were designed. The transceivers include frequency conversion stages, T/R switching, and a calibration path capable of measuring the transmit power-receiver gain product during transmit for pulse-by-pulse calibration or matched filtering. In particular, this calibration path achieves 100-dB isolation between the transmitted signal and the low-noise amplifier through the use of a switching network and a section of physical walls achieving attenuation of radiated leakage. The transceivers were designed in microstrip PCBs with lumped elements and individually packaged components for compactness. Each transceiver was designed on a single PCB with a custom enclosure providing interior walls and compartments to isolate transceiver subsystems from radiated interference. The enclosure also acts as a heat sink for the voltage regulators and power amplifiers inside the system. The PCB transceiver design produces transmit pulses of 2 W with an arbitrary duty cycle. Each transceiver is fed by an external 120-MHz signal transmit and two 1,140-MHz local oscillator signals. The received signal is amplified and down-converted to 120 MHz and is fed to the data processor. The transceiver dimensions are approximately 3.5 11.5 0.6 in. (9 29 1.5 cm). The PCB transceiver design reduces the volume and weight of the DBSAR instrument while maintaining the functionality found in the original design. Both volume and weight are critical for airborne and flight remote sensing instrumentation.

McLinden, Matthew↗

NASA NTRS2006

Advanced 2-micron Solid-state Laser for Wind and CO2 Lidar Applications

Significant advancements in the 2-micron laser development have been made recently. Solid-state 2-micron laser is a key subsystem for a coherent Doppler lidar that measures the horizontal and vertical wind velocities with high precision and resolution. The same laser, after a few modifications, can also be used in a Differential Absorption Lidar (DIAL) system for measuring atmospheric CO2 concentration profiles. The world record 2-micron laser energy is demonstrated with an oscillator and two amplifiers system. It generates more than one joule per pulse energy with excellent beam quality. Based on the successful demonstration of a fully conductive cooled oscillator by using heat pipe technology, an improved fully conductively cooled 2-micron amplifier was designed, manufactured and integrated. It virtually eliminates the running coolant to increase the overall system efficiency and reliability. In addition to technology development and demonstration, a compact and engineering hardened 2-micron laser is under development. It is capable of producing 250 mJ at 10 Hz by an oscillator and one amplifier. This compact laser is expected to be integrated to a lidar system and take field measurements. The recent achievements push forward the readiness of such a laser system for space lidar applications. This paper will review the developments of the state-of-the-art solid-state 2-micron laser.

Yu, Jirong↗

NASA NTRS1995

Real-Time, Holographic, Dynamic Image-Storage Device

Solid-state device developed for high-speed acquisition, dynamic storage, and amplification of three-dimensional holographic images. Holograms generated via four-wave mixing in two or more photorefractive crystals (or subelements of single crystal) to create single-crystal or multicrystal oscillator. Apparatus provides dynamic storage of holographic image of object after electronic shutter closed to turn off object beam. Provides capability to store, amplify, process, and transmit time-varying, two-dimensional, spatial information. Developments include sensors, actuators, and optical computers operating at speeds on order of speed of light. Potential in applications in which need for high-speed acquisition and storage of three-dimensional holographic images.

Montgomery, Raymond C.↗

NASA NTRS1997

Solid-state Image Sensor with Focal-plane Digital Photon-counting Pixel Array

A solid-state focal-plane imaging system comprises an NxN array of high gain. low-noise unit cells. each unit cell being connected to a different one of photovoltaic detector diodes, one for each unit cell, interspersed in the array for ultra low level image detection and a plurality of digital counters coupled to the outputs of the unit cell by a multiplexer(either a separate counter for each unit cell or a row of N of counters time shared with N rows of digital counters). Each unit cell includes two self-biasing cascode amplifiers in cascade for a high charge-to-voltage conversion gain (greater than 1mV/e(-)) and an electronic switch to reset input capacitance to a reference potential in order to be able to discriminate detection of an incident photon by the photoelectron (e(-))generated in the detector diode at the input of the first cascode amplifier in order to count incident photons individually in a digital counter connected to the output of the second cascade amplifier. Reseting the input capacitance and initiating self-biasing of the amplifiers occurs every clock cycle of an integratng period to enable ultralow light level image detection by the may of photovoltaic detector diodes under such ultralow light level conditions that the photon flux will statistically provide only a single photon at a time incident on anyone detector diode during any clock cycle.

Fossum, Eric R.↗

NASA NTRS2012

Simple Cell Balance Circuit

A method has been developed for continuous cell voltage balancing for rechargeable batteries (e.g. lithium ion batteries). A resistor divider chain is provided that generates a set of voltages representing the ideal cell voltage (the voltage of each cell should be as if the cells were perfectly balanced). An operational amplifier circuit with an added current buffer stage generates the ideal voltage with a very high degree of accuracy, using the concept of negative feedback. The ideal voltages are each connected to the corresponding cell through a current- limiting resistance. Over time, having the cell connected to the ideal voltage provides a balancing current that moves the cell voltage very close to that ideal level. In effect, it adjusts the current of each cell during charging, discharging, and standby periods to force the cell voltages to be equal to the ideal voltages generated by the resistor divider. The device also includes solid-state switches that disconnect the circuit from the battery so that it will not discharge the battery during storage. This solution requires relatively few parts and is, therefore, of lower cost and of increased reliability due to the fewer failure modes. Additionally, this design uses very little power. A preliminary model predicts a power usage of 0.18 W for an 8-cell battery. This approach is applicable to a wide range of battery capacities and voltages.

Johnson, Steven D.↗

NASA NTRS2010

Power Amplifier Module with 734-mW Continuous Wave Output Power

Research findings were reported from an investigation of new gallium nitride (GaN) monolithic millimeter-wave integrated circuit (MMIC) power amplifiers (PAs) targeting the highest output power and the highest efficiency for class-A operation in W-band (75-110 GHz). W-band PAs are a major component of many frequency multiplied submillimeter-wave LO signal sources. For spectrometer arrays, substantial W-band power is required due to the passive lossy frequency multipliers-to generate higher frequency signals in nonlinear Schottky diode-based LO sources. By advancing PA technology, the LO system performance can be increased with possible cost reductions compared to current GaAs PAs. High-power, high-efficiency GaN PAs are cross-cutting and can enable more efficient local oscillator distribution systems for new astrophysics and planetary receivers and heterodyne array instruments. It can also allow for a new, electronically scannable solid-state array technology for future Earth science radar instruments and communications platforms.

Fung, King Man↗

NASA NTRS1992

Development of Mesospheric Sodium Laser Beacon for Atmospheric Adaptive Optics

A solid-state source of long pulse length, sodium-resonance radiation was developed for the purpose of generating an artificial star in the earth's mesospheric sodium layer. This radiation is generated by sum-frequency mixing the output of a 1.064 micron Nd:YAG laser with the output of a 1.319-micron Nd:YAG laser. By operating these lasers at wavelengths very close to the peak of their tuning curves, it is possible to match the wavelength of the sum-frequency radiation to that of the sodium D2 adsorption wavelength. Two pulsed laser systems were constructed, one producing as much as 0.6 J of sodium resonance radiation at a 10-Hz repetition rate and another producing as much as 24 mJ at a 840-Hz repetition rate. In both laser systems, the 1.06-micron and 1.32-micron Nd:YAG lasers are configured as mode-locked master oscillators followed by power amplifiers. Other aspects of this project are presented.

Jeys, T. H.↗

NASA NTRS1993

Every Good Virtue You Ever Wanted in a Q-switched Solid-state Laser and More: Monolithic, Diode-pumped, Self-q-switched, Highly Reproducible, Diffraction-limited Nd:yag Laser

The applications of Q-switched lasers are well known, for example, laser radar, laser remote sensing, satellite orbit determination, Moon orbit and 'moon quake' determination, satellite laser communication, and many nonlinear optics applications. Most of the applications require additional properties of the Q-switched lasers, such as single-axial and/or single-transverse mode, high repetition rate, stable pulse shape and pulse width, or ultra compact and rugged oscillators. Furthermore, space based and airborne lasers for lidar and laser communication applications require efficient, compact, lightweight, long-lived, and stable-pulsed laser sources. Diode-pumped solid-state lasers (DPSSL) have recently shown the potential for satisfying all of these requirements. We will report on the operating characteristics of a diode-pumped, monolithic, self-Q-switched Cr,Nd:YAG laser where the chromium ions act as a saturable absorber for the laser emission at 1064 nm. The pulse duration is 3.5 ns and the output is highly polarized with an extinction ratio of 700:1. It is further shown that the output is single-longitudinal-mode with transform-limited spectral line width without pulse-to-pulse mode competition. Consequently, the pulse-to-pulse intensity fluctuation is less than the instrument resolution of 0.25 percent. This self-stabilization mechanism is because the lasing mode bleaches the distributed absorber and establishes a gain-loss grating similar to that used in the distributed feedback semiconductor lasers. A repetition rate above 5 KHz has also been demonstrated. For higher power, this laser can be used for injection seeding an amplifier (or amplifier chain) or injection locking of a power oscillator pumped by diode lasers. We will discuss some research directions on the master oscillator for higher output energy per pulse as well as how to scale the output power of the diode-pumped amplifier(s) to multi-kilowatt average power.

Chen, Y. C.↗

NASA NTRS1997

Multi-pass light amplifier

A multiple-pass laser amplifier that uses optical focusing between subsequent passes through a single gain medium so that a reproducibly stable beam size is achieved within the gain region. A confocal resonator or White Cell resonator is provided, including two or three curvilinearly shaped mirrors facing each other along a resonator axis and an optical gain medium positioned on the resonator axis between the mirrors (confocal resonator) or adjacent to one of the mirrors (White Cell). In a first embodiment, two mirrors, which may include adjacent lenses, are configured so that a light beam passing through the gain medium and incident on the first mirror is reflected by that mirror toward the second mirror in a direction approximately parallel to the resonator axis. A light beam translator, such as an optical flat of transparent material, is positioned to translate this light beam by a controllable amount toward or away from the resonator axis for each pass of the light beam through the translator. The optical gain medium may be solid-state, liquid or gaseous medium and may be pumped longitudinally or transversely. In a second embodiment, first and second mirrors face a third mirror in a White Cell configuration, and the optical gain medium is positioned at or adjacent to one of the mirrors. Defocusing means and optical gain medium cooling means are optionally provided with either embodiment, to controllably defocus the light beam, to cool the optical gain medium and to suppress thermal lensing in the gain medium.

Plaessmann, Henry↗

NASA NTRS1999

Diode Laser Velocity Measurements by Modulated Filtered Rayleigh Scattering

The ability of solid-state lasers to be tuned in operating frequency at MHz rates by input current modulation, while maintaining a relatively narrow line-width, has made them useful for spectroscopic measurements. Their other advantages include low cost, reliability, durability, compact size, and modest power requirements, making them a good choice for a laser source in micro-gravity experiments in drop-towers and in flight. For their size, they are also very bright. In a filtered Rayleigh scattering (FRS) experiment, a diode laser can be used to scan across an atomic or molecular absorption line, generating large changes in transmission at the resonances for very small changes in frequency. The hyperfine structure components of atomic lines of alkali metal vapors are closely spaced and very strong, which makes such atomic filters excellent candidates for sensitive Doppler shift detection and therefore for high-resolution velocimetry. In the work we describe here we use a Rubidium vapor filter, and work with the strong D(sub 2) transitions at 780 nm that are conveniently accessed by near infrared diode lasers. The low power output of infrared laser diodes is their primary drawback relative to other laser systems commonly used for velocimetry. However, the capability to modulate the laser frequency rapidly and continuously helps mitigate this. Using modulation spectroscopy and a heterodyne detection scheme with a lock-in amplifier, one can extract sub-microvolt signals occurring at a specific frequency from a background that is orders of magnitude stronger. The diode laser modulation is simply achieved by adding a small current modulation to the laser bias current. It may also be swept repetitively in wavelength using an additional lower frequency current ramp.

Mach, J. J.↗

NASA NTRS1979

Analysis of S-band solid-state transmitters for the solar power satellite

The possibility of replacing the Reference System antenna in which thermionic devices are used for the dc-to-microwave conversion, with solid-state elements was explored. System, device, and antenna module tradeoff investigations strongly point toward the desirability of changing the transmitter concept to a distributed array of relatively low power elements, deriving their dc power directly from the solar cell array and whose microwave power outputs are combined in space. The approach eliminates the thermal, weight, and dc-voltage distribution problems of a system in which high power tubes are simply replaced with clusters of solid state amplifiers. The proposed approach retains the important advantages of a solid state system: greatly enhanced reliability and graceful degradation of the system.

Belohoubek, E. F.↗

NASA NTRS1998

Electro-Optic Lightning Detector

Electric field measurements are fundamental to the study of thunderstorm electrification, thundercloud charge structure, and the determination of the locations and magnitudes of charges deposited by lightning. Continuous field observations can also be used to warn of impending electrical hazards. For example, the USAF Eastern Range (ER) and NASA Kennedy Space Center (KSC) in Florida currently operate a ground-based network of electric field mill sensors to warn against lightning hazards to space vehicle operations/launches. The sensors provide continuous recordings of the ambient field. Others investigators have employed flat-plate electric field antennas to detect changes In the ambient field due to lightning. In each approach, electronic circuitry is used to directly detect and amplify the effects of the ambient field on an exposed metal conductor (antenna plate); in the case of continuous field recordings, the antenna plate is alternately shielded and unshielded by a grounded conductor. In this work effort, an alternate optical method for detecting lightning-caused electric field changes is Introduced. The primary component in the detector is an anisotropic electro-optic crystal of potassium di-hydrogen phosphate (chemically written as KH2PO4 (KDP)). When a voltage Is placed across the electro-optic crystal, the refractive Indices of the crystal change. This change alters the polarization state of a laser light beam that is passed down the crystal optic axis. With suitable application of vertical and horizontal polarizers, a light transmission measurement is related to the applied crystal voltage (which in turn Is related to the lightning caused electric field change). During the past two years, all critical optical components were procured, assembled, and aligned. An optical housing, calibration set-up, and data acquisition system was integrated for breadboard testing. The sensor was deployed at NASA Marshall Space Flight Center (MSFC) in the summer of 1998 to collect storm data. Because solid-state technology is used, future designs of the sensor will be significantly scaled down In physical dimension and weight compared to the present optical breadboard prototype. The use of fiber optics would also provide significant practical improvements.

Koshak, William J.↗