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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Efficient monolithic MgO:LiNbO3 singly resonant optical parametric oscillator

A monolithic MgO:LiNbO3 singly resonant optical parametric oscillator (OPO) was operated as both a standing-wave and a ring-geometry resonator. The OPO was pumped by the second harmonic of an amplified single-mode diode-laser-pumped Nd:YAG laser. Pump depletions of greater than 60 percent were observed when pumping four times greater than the 35-W threshold. The OPO output at the resonant signal tuned with temperature from 834 to 958 nm, while the corresponding idler tuned from 1.47 to 1.2 microns. The spectral characteristics of the OPO signal output and the relative merits of a standing wave versus a ring geometry are discussed.

Kozlovsky, W. J.↗
NASA NTRS1989

An ultraviolet gas-loaded free-electron laser

By loading the wiggler cavity of an FEL with hydrogen gas, it is possible to tune the FEL to below 130 nm, just above the electronic resonance of molecular hydrogen. The change in wavelength results from the refractive index of the gas, which alters the phase velocity of the wave and so modifies the synchronism condition. Using the parameters of the wiggler on the beamline of the Stanford Superconducting Accelerataor, this FEL can be tuned from 500 to 130 nm with a pressure ranging from 0 to 55 torr. The calculated electronic gain varies between 5.6 and 11 percent over this wavelength interval, comparable to the gain in vacuum.

Reid, Max B.↗
NASA NTRS1988

OPO performance with a long pulse length, single frequency Nd:YAG laser pump

With the advent of new nonlinear materials and single-frequency pump sources, there is renewed interest in optical parametric oscillators (OPOs). A single-mode diode-laser-pumped monolithic Nd:YAG nonplanar ring laser that is both amplified and frequency doubled is used to pump a monolithic MgO:LiNbO3 pulsed singly resonant OPO. The OPO signal output was temperature tuned from 834 to 958 nm, producing an idler tuning from 1.47 to 1.2 microns. Efforts toward a CW all-solid-state doubly resonant OPO are also described.

Kozlovsky, W. J.↗
NASA NTRS1989

Effects of introducing a gas into the free-electron laser

The introduction of a gas into the wiggler section of a free electron laser (FEL) alters the phase velocity of the electromagnetic wave, and so changes the synchronism condition relating wavelength to wiggler parameters and beam energy. This provides a means for tuning the frequency of an oscillator, with the addition of 200 torr of hydrogen gas, the wavelength of a FEL operating in the near infrared without gas was reduced by 0.73 microns. The plasma generated from ionization of the hydrogen molecules by collisions with the electron beam diminished the oscillator gain, but this effect was eliminated by the addition of less than 0.1 percent of an electron attachment gas. Gain is also reduced by multiple scattering of the beam electrons, but this effect is not severe for a 1-m wiggler length. When hydrogen is used, a FEL with fixed wiggler parameters and electron energy can be tuned from the near infrared to about 1200 A, and, with helium, the wavelength can be reduced to 600 A.

Pantell, R. H.↗
NASA NTRS1989

Shuttle relative navigation of a tethered satellite mission with current on board software

A Shuttle mission planned in 1991 will test the feasibility of tethers in space. This mission, a joint effort between Italy and the United States, will connect a satellite (built by the Italians) to the Shuttle with a 20 km long tether. This mission poses unique navigation problems. The flight software on the Shuttle was never designed to account for the low level acceleration that is generated by the gravity gradient. IMUs on the Shuttle was never designed to account for the low level acceleration that is generated by the gravity gradient. Inertial Maneuvering Units on the shuttle will sense the acceleration of the tether but it turns out that incorporating the continuous accelerometer noise also generates large error growth. Relative navigation is another important issue since the majority of the mission will be conducted while the satellite is out of the visual range of the crew. Some kind of feedback on the motion of the satellite will be desirable. Feedback of the satellite motion can be generated by using the rendezvous radar. To process the radar measurements, the flight software uses a 13 state Kalman Filter, but unfortunately with the filter currently tuned as it is, valid measurements tend to be ignored. This is due to the constraint of the tether on the satellite, which is an unmodeled force. Analysis shows that with proper tuning, relative navigation is possible.

Lee, Kevin A.↗
NASA NTRS1988

Advances in SIS receiver technology

Significant advances in SIS receiver technology since the last Asilomar meeting include: superconductor materials, integrated inductive tuning elements, and planar mounting structures. The effect of these advances is to push the upper frequency operating limit from about 600 to 1500 GHz, and to enhance the feasibility of focal plane arrays of heterodyne receivers. A fundamental high frequency operating limit of SIS mixers is set by the superconducting energy gap. A practical limitation for high frequency operation of SIS junctions is their parasitic capacitance and resistance. The performance of the mixer will be degraded by the Resistor-Capacitor rolloff. Several designs were reported for inductive elements integrated on the same substrate as the SIS junctions to tune out the bulk junction capacitance. Most millimeter SIS-based heterodyne receivers have used waveguide coupling structures. Technology has advanced to the state where programs that have a high probability of success can be defined to produce arrays of SIS receivers for frequencies as high as 1500 GHz.

Frerking, M. A.↗
NASA NTRS1990

An expert system to perform on-line controller restructuring for abrupt model changes

Work in progress on an expert system used to reconfigure and tune airframe/engine control systems on-line in real time in response to battle damage or structural failures is presented. The closed loop system is monitored constantly for changes in structure and performance, the detection of which prompts the expert system to choose and apply a particular control restructuring algorithm based on the type and severity of the damage. Each algorithm is designed to handle specific types of failures and each is applicable only in certain situations. The expert system uses information about the system model to identify the failure and to select the technique best suited to compensate for it. A depth-first search is used to find a solution. Once a new controller is designed and implemented it must be tuned to recover the original closed-loop handling qualities and responsiveness from the degraded system. Ideally, the pilot should not be able to tell the difference between the original and redesigned systems. The key is that the system must have inherent redundancy so that degraded or missing capabilities can be restored by creative use of alternate functionalities. With enough redundancy in the control system, minor battle damage affecting individual control surfaces or actuators, compressor efficiency, etc., can be compensated for such that the closed-loop performance in not noticeably altered. The work is applied to a Black Hawk/T700 system.

Litt, Jonathan S.↗
NASA NTRS1989

Atmospheric methane measurement instrument using a Zeeman-split He-Ne laser

The construction of an atmospheric methane measurement instrument based on a Zeeman-split IR He-Ne laser is reported. The laser has a tranverse magnetic field over about 2/3 of its gain length and can oscillate at an (unsplit) frequency (2947.91/cm) centered on a methane absorption line, or on either of two frequencies split by + or - 0.055/cm from the center, with low CH4 absorption. The laser is tuned to dwell sequentially at each frequency, giving two differential absorption measurements in each 46-ms tuning cycle. Atmospheric measurements are made using two multiple pass absorption cells, one with fast (0.75-s) and one with slow (5-s) flow response times. Fluctuations in ambient CH4 of about 20-ppb (rms, 1-s averaging) are detected, with interference fringe effects the dominant noise source. The instrument has operated in a field experiment (NASA GTE/ABLE-3A) in Alaska.

Mcmanus, J. Barry↗
NASA NTRS1990

Adaptive noncolocated velocity feedback for vibration damping

A method is proposed for adaptive noncolocated velocity feedback control of flexible structure vibrations. The approach, denoted as auto-tuning, is to drive the system into a sequence of controlled oscillations to provide accurate knowledge of the plant characteristics in the vicinity of the phase cross-over frequencies. An allpass phase notch filter cascade is used as the control architecture to phase stabilize each destabilizing mode in the plant transfer function. The allpass phase notch filter cascade is tuned precisely by the information extracted from the controlled oscillations.

Bayard, David S.↗
NASA NTRS1989

Single mode wavelength control of modulated AlGaAs lasers with external and internal etalon feedback

Single mode lasing without mode hops has been obtained for VSIS and CSP laser diodes with an external etalon attached to the laser's front facet for up to an 8 C range CW and a 4 C range pulsed, with .07 nm/C tuning. Tests of thin tapered-thickness (TTT) laser diodes show CW and pulsed single mode lasing over 10 C and 2 C ranges, respectively, with .08 nm/C tuning. An analysis of the TTT structure reveals the equivalent of an internal etalon. The time-resolved pulsed behavior for both types of lasers show single mode lasing within the proper temperature ranges with minor modes present only early in the optical pulse, if at all. The external etalon produces noticeable interference fringes in the farfield pattern, while those of the TTT lasers are smooth. Ongoing CW lifetest results indicate stability to within one longitudinal mode after a few hundred hours of operation, along with at least several thousand hours lifetime.

Maynard, William L.↗
NASA NTRS1991

Airborne water vapor DIAL system and measurements of water and aerosol profiles

The Lidar Applications Group at NASA Langley Research Center has developed a differential absorption lidar (DIAL) system for the remote measurement of atmospheric water vapor (H2O) and aerosols from an aircraft. The airborne H2O DIAL system is designed for extended flights to perform mesoscale investigations of H2O and aerosol distributions. This DIAL system utilizes a Nd:YAG-laser-pumped dye laser as the off-line transmitter and a narrowband, tunable Alexandrite laser as the on-line transmitter. The dye laser has an oscillator/amplifier configuration which incorporates a grating and prism in the oscillator cavity to narrow the output linewidth to approximately 15 pm. This linewidth can be maintained over the wavelength range of 725 to 730 nm, and it is sufficiently narrow to satisfy the off-line spectral requirements. In the Alexandrite laser, three intracavity tuning elements combine to produce an output linewidth of 1.1 pm. These spectral devices include a five-plate birefringent tuner, a 1-mm thick solid etalon and a 1-cm air-spaced etalon. A wavelength stability of +/- 0.35 pm is achieved by active feedback control of the two Fabry-Perot etalons using a frequency stabilized He-Ne laser as a wavelength reference. The three tuning elements can be synchronously scanned over a 150 pm range with microprocessor-based scanning electronics. Other aspects of the DIAL system are discussed.

Higdon, Noah S.↗
NASA NTRS1991

CW laser strategies for multi-parameter measurements of high-speed flows containing either NO or O2

Measurements of gasdynamic quantities were performed using a rapid-tuning CW dye laser to resolve Doppler-shifted spectral features in either the O2 Schumann-Runge bands or the NO gamma band near 225 nm. With the rapid-tuning capability, spectral features were acquired at a repetition rate of 4 kHz. Monitoring O2 transitions provided estimates of velocities while monitoring collision-broadened NO line pairs provided simultaneous measurements of velocity, temperature and pressure. Experiments were first performed in absorption within the transient one-dimensional flows generated in a shock tube. Agreement between measured and theoretical values, as calculated from one-dimensional shock relations, was typically better than 5 percent. The method was extended to fluorescence detection of NO in a static cell. Temperature and pressure were extracted from recorded profiles, and the results agreed well with expected values.

Dirosa, M. D.↗
NASA NTRS1990

An iterative algorithm combining model reduction and control design

A design strategy which integrates model reduction by modal cost analysis and a multiobjective controller design is proposed. The necessary modeling and control algorithms are easily programmed in Matlab standard software. Hence, this method is very practical for controller design for large space structures. The design algorithm also solves the very important problem of tuning multiple loop controllers (multi-input, multi-output, or MIMO). Instead of the single gain change that is used in standard root locus and gain and phase margin theories, this method tunes multiple loop controllers from low to high gain in a systematic way in the design procedure. This design strategy is applied to NASA's Mini-Mast system.

Hsieh, C.↗
NASA NTRS1991

Performance of NbN superconductive tunnel junctions as SIS mixers at 205 GHz

Small area (less than 1 sq micron), high-current-density NbN/MgO/NbN tunnel junctions with I-V characteristics suitable for high-frequency mixers have been fabricated. Mesa-geometry junctions with an area of about 1 sq micron and critical current density of 5-10 kA/sq cm are integrated with superconducting microstrip lines designed to resonate out the junction capacitance. A study was made of the mixer gain and noise performance near 205 GHz as a function of the inductance provided by the microstrip line. This has confirmed, at a high millimeter-wave frequency, values of junction capacitance of 85 fF/sq micron and recently measured values of a magnetic penetration depth of 380 nm. Mixer noise temperatures as low as 134 K at 1.5 K have been obtained for properly tuned junctions. A significant improvement in mixer performance on cooling from 4.2 K to 1.5 K was observed. Edge-geometry junctions with an area of 0.3 sq microns and critical current density of 18-25 kA/sq cm have also been fabricated. These junctions give a mixer noise temperature of 145 K at 4.2 K without the use of integrated tuning elements. These are the best results ever achieved for NbN-based SIS mixers.

Mcgrath, W. R.↗
NASA NTRS1991

Characterization of NbN films and tunnel junctions

Properties of NbN films and NbN/MgO/NbN tunnel junctions are discussed. NbN junctions are being developed for use in high-frequency, SIS quasiparticle mixers. To properly design mixer circuits, junction and film properties need to be characterized. The specific capacitance of NbN/MgO/NbN junctions has been measured as a function of the product of the normal-state resistance and the junction area (RnA), and it is found to vary by more than a factor of two (35-85 fF/sq microns) over the range of RnA measured (1000-50 ohm sq microns). This variation is important because the specific capacitance determines the RC speed of the tunnel junction at a given RnA value. The magnetic penetration depth of NbN films deposited under different conditions is also measured. The magnetic penetration depth affects the design of microstrip line used in RF tuning circuits. Control of the magnetic penetration depth is necessary to fabricate reproducible tuning circuits. Additionally, the critical current uniformity for arrays of 100 junctions has been measured. Junction uniformity will affect the design of focal-plane arrays of SIS mixers. Finally, the relevance of these measurements to the design of Josephson electronics is discussed.

Stern, J. A.↗
NASA NTRS1991

Continuous wave laser absorption techniques for gasdynamic measurements in supersonic flows

Line-of-sight measurements of velocity, temperature, pressure, density, and mass flux were performed in a transient shock tube flow using three laser absorption schemes. All methods employed an intracavity-doubled ring dye laser tuned to an OH transition at 306 nm. In the first scheme, the gas was labeled by 193.3-nm excimer photolysis of H2O, and the passage of the generated OH was detected downstream. In the second method, the laser was tuned at a rate of 3 kHz over the R1(7) and R1(11) line pair, and absorption was simultaneously monitored at 90 and 60 deg with respect to the flow. Velocity was determined from the Doppler shift of the profiles and the temperature from the intensity ratio of the lines. Pressure was determined from both the magnitude of absorption and the collisional broadening. In the third method, the laser wavelength was fixed at a single frequency, and a continuous measurement of velocity and pressure was obtained using the signals from the two beam paths. All methods gave results which compare favorably to calculated values.

Davidson, David F.↗
NASA NTRS1991

Birefringent filter design

A birefringent filter is provided for tuning the wavelength of a broad band emission laser. The filter comprises thin plates of a birefringent material having thicknesses which are non-unity, integral multiples of the difference between the thicknesses of the two thinnest plates. The resulting wavelength selectivity is substantially equivalent to the wavelength selectivity of a conventional filter which has a thinnest plate having a thickness equal to this thickness difference. The present invention obtains an acceptable tuning of the wavelength while avoiding a decrease in optical quality associated with conventional filters wherein the respective plate thicknesses are integral multiples of the thinnest plate.

Bair, Clayton H.↗
NASA NTRS1992

Fully integrated aerodynamic/dynamic optimization of helicopter rotor blades

A fully integrated aerodynamic/dynamic optimization procedure is described for helicopter rotor blades. The procedure combines performance and dynamic analyses with a general purpose optimizer. The procedure minimizes a linear combination of power required (in hover, forward flight, and maneuver) and vibratory hub shear. The design variables include pretwist, taper initiation, taper ratio, root chord, blade stiffnesses, tuning masses, and tuning mass locations. Aerodynamic constraints consist of limits on power required in hover, forward flight and maneuvers; airfoil section stall; drag divergence Mach number; minimum tip chord; and trim. Dynamic constraints are on frequencies, minimum autorotational inertia, and maximum blade weight. The procedure is demonstrated for two cases. In the first case, the objective function involves power required (in hover, forward flight and maneuver) and dynamics. The second case involves only hover power and dynamics. The designs from the integrated procedure are compared with designs from a sequential optimization approach in which the blade is first optimized for performance and then for dynamics. In both cases, the integrated approach is superior.

Walsh, Joanne L.↗