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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Laser Frequency Stabilization

Laser ranging and interferometry are essential technologies allowing for many astounding new spacebased missions such as the Laser Interferometer Space Antenna (LISA) to measure gravitational radiation emitted from distant super massive black hole mergers or distributed aperture telescopes with unprecedented angular resolution in the NIR or visible regime. The requirements on laser frequency noise depend on the residual motion and the distances between the spacecraft forming the interferometer. The intrinsic frequency stability of commercial lasers is several orders of magnitude above these requirements. Therefore, it is necessary for lasers to be stabilized to an ultrastable frequency reference so that they can be used to sense and control distances between spacecraft. Various optical frequency references and frequency stabilization schemes are considered and investigated for the applicability and usefulness for space-based interferometry missions.

Donelan, Darsa↗
DOE OSTI2020

Terahertz Spectroscopy of Gas Mixtures with Dual Quantum Cascade Laser Frequency Combs

Terahertz laser frequency combs based on quantum cascade lasers provide coherent, broadband, electrically pumped, THz radiation sources for use in future spectroscopic applications. In this work, we explore the feasibility of such lasers in a dual-comb spectroscopy configuration for the detection of multiple molecular samples in the gas phase. The lasers span approximately 180 GHz of optical bandwidth, centered at 3.4 THz, with submilliwatt total optical power. One of the main advantages of dual-comb spectroscopy is its high speed, which opens up the possibility for direct observations of chemical reaction dynamics in the terahertz spectral region. As a proof-of-concept, we recorded continuously evolving spectra from gas mixtures with 1 ms temporal resolution.

42 ENGINEERING↗
NASA NTRS2006

Pattern-Recognition Algorithm for Locking Laser Frequency

A computer program serves as part of a feedback control system that locks the frequency of a laser to one of the spectral peaks of cesium atoms in an optical absorption cell. The system analyzes a saturation absorption spectrum to find a target peak and commands a laser-frequency-control circuit to minimize an error signal representing the difference between the laser frequency and the target peak. The program implements an algorithm consisting of the following steps: Acquire a saturation absorption signal while scanning the laser through the frequency range of interest. Condition the signal by use of convolution filtering. Detect peaks. Match the peaks in the signal to a pattern of known spectral peaks by use of a pattern-recognition algorithm. Add missing peaks. Tune the laser to the desired peak and thereafter lock onto this peak. Finding and locking onto the desired peak is a challenging problem, given that the saturation absorption signal includes noise and other spurious signal components; the problem is further complicated by nonlinearity and shifting of the voltage-to-frequency correspondence. The pattern-recognition algorithm, which is based on Hausdorff distance, is what enables the program to meet these challenges.

Karayan, Vahag↗
NASA NTRS2010

Swept Frequency Laser Metrology System

A swept frequency laser ranging system having sub-micron accuracy that employs multiple common-path heterodyne interferometers, one coupled to a calibrated delay-line for use as an absolute reference for the ranging system. An exemplary embodiment uses two laser heterodyne interferometers to create two laser beams at two different frequencies to measure distance and motions of target(s). Heterodyne fringes generated from reflections off a reference fiducial X(sub R) and measurement (or target) fiducial X(sub M) are reflected back and are then detected by photodiodes. The measured phase changes Delta phi(sub R) and Delta phi (sub m) resulting from the laser frequency swept gives target position. The reference delay-line is the only absolute reference needed in the metrology system and this provides an ultra-stable reference and simple/economical system.

Zhao, Feng↗
NASA NTRS2005

Laser frequency stabilization for LISA

The requirement on laser frequency noise in the Laser Interferometer Space Antenna (LISA) depends on the velocity and our knowledge of the position of each spacecraft of the interferometer. Currently it is assumed that the lasers must have a pre-stabilized frequency stability of 30Hz/square root of Hz over LISA'S most sensitive frequency band (3 mHz - 30 mHz). The intrinsic frequency stability of even the most stable com- mercial lasers is several orders of magnitude above this level. Therefore it is necessary to stabilize the laser frequency to an ultra-stable frequency reference which meets the LISA requirements. The baseline frequency reference for the LISA lasers are high finesse optical cavities based on ULE spacers. We measured the stability of two ULE spacer cavities with respect to each other. Our current best results show a noise floor at, or below, 30 Hz/square root of Hz above 3 mHz. In this report we describe the experimental layout of the entire experiment and discuss the limiting noise sources.

Mueller, Guido↗
NASA NTRS2006

Automatic Locking of Laser Frequency to an Absorption Peak

An electronic system adjusts the frequency of a tunable laser, eventually locking the frequency to a peak in the optical absorption spectrum of a gas (or of a Fabry-Perot cavity that has an absorption peak like that of a gas). This system was developed to enable precise locking of the frequency of a laser used in differential absorption LIDAR measurements of trace atmospheric gases. This system also has great commercial potential as a prototype of means for precise control of frequencies of lasers in future dense wavelength-division-multiplexing optical communications systems. The operation of this system is completely automatic: Unlike in the operation of some prior laser-frequency-locking systems, there is ordinarily no need for a human operator to adjust the frequency manually to an initial value close enough to the peak to enable automatic locking to take over. Instead, this system also automatically performs the initial adjustment. The system (see Figure 1) is based on a concept of (1) initially modulating the laser frequency to sweep it through a spectral range that includes the desired absorption peak, (2) determining the derivative of the absorption peak with respect to the laser frequency for use as an error signal, (3) identifying the desired frequency [at the very top (which is also the middle) of the peak] as the frequency where the derivative goes to zero, and (4) thereafter keeping the frequency within a locking range and adjusting the frequency as needed to keep the derivative (the error signal) as close as possible to zero. More specifically, the system utilizes the fact that in addition to a zero crossing at the top of the absorption peak, the error signal also closely approximates a straight line in the vicinity of the zero crossing (see Figure 2). This vicinity is the locking range because the linearity of the error signal in this range makes it useful as a source of feedback for a proportional + integral + derivative control scheme that constantly adjusts the frequency in an effort to drive the error to zero. When the laser frequency deviates from the midpeak value but remains within the locking range, the magnitude and sign of the error signal indicate the amount of detuning and the control circuitry adjusts the frequency by what it estimates to be the negative of this amount in an effort to bring the error to zero.

Koch, Grady J.↗
NASA NTRS1992

Diode-laser frequency stabilization based on the resonant Faraday effect

The authors present the results of a method for frequency stabilizing laser diodes based on the resonant Faraday effects. A Faraday cell in conjunction with a polarizer crossed with respect to the polarization of the laser diode comprises the intracavity frequency selective element. In this arrangement, a laser pull-in range of 9 A was measured, and the laser operated at a single frequency with a linewidth less than 6 MHz.

Wanninger, P.↗
NASA NTRS1999

Construction and Calibration of a Difference Frequency Laser Spectrometer and New THZ Frequency Measurements of water and Ammonia

A three laser system based on 852nm DBR lasers has been constructed and used to generate radiation in the 750 GHz to 1600 GHz frequency region. The system works by locking two of the three lasers to modes of an ultra low expansion Fabry-Perot cavity. The third laser is offset locked to one of the cavity locked lasers with conventional microwave techniques. The signal from the offset laser and the other cavity locked laser are injected into a Master Oscillator Power Amplifier (MOPA), amplified and focused on a low temperature grown GaAs photomixer, which radiates the difference frequency. The system has been calibrated with molecular lines to better than one part in 10(exp 7). In this paper we present the application of this system to the v(sub 2) in inversion band of Ammonia and the ground and v(sub 2) states of water. A discussion of the system design, the calibration and the new spectral measurements will be presented.

Pearson, J. C.↗
NASA NTRS1983

Modification of the photoionization of hydrogen by a low-frequency laser

The modification of the photoionization cross section due to the presence of an intense low-frequency laser is obtained for the hydrogen-atom target. Previous calculations of this effect have neglected the effects of the Coulomb potential in the final state. A sum rule was obtained which indicates that the photoelectric current is unaffected by the presence of the low-frequency laser to lowest order in the laser frequency. It is found that the Coulomb potential changes this result and that the photoelectric current is changed by a factor which depends upon the laser parameters and the electron momentum. A simple experiment is suggested to confirm this prediction.

Fiordilino, E.↗
NASA NTRS1991

On the control aspect of laser frequency stabilization

Realization of frequency stable lasers is viewed as key to progress in many areas of research; therefore, the search for more effective techniques of frequency stabilization has intensified significantly in recent years. Investigating and validating the fundamental linewidth and frequency stability limits of a Nd:YAG laser oscillator, locked to a high finesse reference cavity in the microgravity and vibration-free environment of space, is the objective of a NASA project called SUNLITE at LaRC. The objective of this paper is to further investigate the application of feedback control theory in active frequency control as a frequency stabilization technique and determine the most appropriate control strategy to be used in general and particularly in the SUNLITE Project.

Zia, Omar↗
NASA NTRS1973

The CO2 laser frequency stability measurements

Carbon dioxide laser frequency stability data are considered for a receiver design that relates to maximum Doppler frequency and its rate of change. Results show that an adequate margin exists in terms of data acquisition, Doppler tracking, and bit error rate as they relate to laser stability and transmitter power.

Johnson, E. H., Jr.↗
NASA NTRS2015

Apparatus and Method to Enable Precision and Fast Laser Frequency Tuning

An apparatus and method is provided to enable precision and fast laser frequency tuning. For instance, a fast tunable slave laser may be dynamically offset-locked to a reference laser line using an optical phase-locked loop. The slave laser is heterodyned against a reference laser line to generate a beatnote that is subsequently frequency divided. The phase difference between the divided beatnote and a reference signal may be detected to generate an error signal proportional to the phase difference. The error signal is converted into appropriate feedback signals to phase lock the divided beatnote to the reference signal. The slave laser frequency target may be rapidly changed based on a combination of a dynamically changing frequency of the reference signal, the frequency dividing factor, and an effective polarity of the error signal. Feed-forward signals may be generated to accelerate the slave laser frequency switching through laser tuning ports.

Chen, Jeffrey R.↗
DOE OSTI2025

Experimental and numerical evaluation of laser shinethrough in polystyrene at 3 ω and 4 ω laser frequencies

In laser-direct-drive inertial confinement fusion (ICF), laser shinethrough is the process associated with laser propagation through the target plastic ablator, polystyrene is often used in ICF experiments, and leads to energy deposition in the target until the critical density is established at the ablator front surface. This process causes material modifications that may change the shock propagation and lead to density and pressure perturbations at the interface between the polystyrene and cryogenic deuterium-tritium (DT) layer. This work reports on experimental observations and numerical investigations of laser shinethrough at the third and the fourth harmonic of ICF class laser systems (1 ω corresponds to a wavelength of ∼1.06 μm ). The experiments evidence that, at 4 ω , the laser is absorbed over a distance of about a few microns and laser shinethrough is completely eliminated in contrast with the behavior at 3 ω . These experimental observations are reproduced by numerical simulations. The laser fluence required to reach the critical density at the ablator front surface is also found to be more than one order of magnitude higher at the third harmonic. Finally, modeling suggests that the back surface of the ablator exhibits ion temperatures greater than the melting temperature of cryogenic DT and electron pressure larger than the bulk and shear moduli of cryogenic DT for the case of using a 3 ω laser.

Pineau, A. [Univ. of Rochester, NY (United States)↗
NASA NTRS1991

Far infrared laser frequencies of CH3OD and N2H4

The frequencies of 26 laser lines with wavelengths between 57 and 534 microns are measured in the optically pumped laser gases CH3OD and N2H4. A pair of 12CO2 lasers are employed as a frequency standard for the heterodyne frequency measurements. As the measurements are oriented toward spectroscopic applications of FIR lasers where dense, broad, frequency coverage is essential, every strong pump absorption of the two molecules for new lines that might be made to lase by varying the laser gas pressure and the tuning of the pump laser are investigated.

Radford, H. E.↗
NASA NTRS2002

Simple approach to laser frequency stabilization

We describe a simple approach to laser frequency stabilization. It was originally developed for the metrology subsystem for NASA's StarLight mission, a space-based separated-spacecraft stellar interferometer.

starlight↗