High resolution spatially extended 1D laser scattering diagnostics using volume Bragg grating notch filters.

Laser light scattering systems with volume Bragg grating (VBG) filters, which act as spectral/angular filters, have often been used as a point measurement technique, with spatial resolution as low as a few hundred µm, defined by the beam waist. In this work, we demonstrate how VBG filters can be leveraged for spatially resolved measurements with several µm resolution over a few millimeters along the beam propagation axis. The rejection ring, as determined by the angular acceptance criteria of the filter, is derived analytically, and the use of the ring for 1D laser line rejection is explained. For the example cases presented,i.e., for a focused probe beam waist with a diameter of ∼150 µm, the rejection ring can provide resolution up to several millimeter length along the beam propagation axis for a 1D measurement, which is also tunable. Additionally, methods to further extend the measurable region are proposed and demonstrated, using a collimation lens with a different focal length or using multiple VBG filters. The latter case can minimize the scattering signal loss, without the tradeoff of the solid angle. Such use of multiple VBGs is to extend the measurable region along the beam axis, which differs from the commonly known application of multiple filters, to improve the suppression of elastic interferences. 1D rotational Raman and Thomson scattering measurements are carried out on pulsed and DC discharges to verify this method. The system features compactness, simple implementation, high throughput, and flexibility, to accommodate various experimental conditions.

Phase-matched waveguide four-wave mixing scaled to higher peak powers with large-core-area hollow photonic-crystal fibers.

Hollow photonic-crystal fibers with large core diameters are shown to allow waveguide nonlinear-optical interactions to be scaled to higher pulse peak powers. Phase-matched four-wave mixing is predicted theoretically and demonstrated experimentally for millijoule nanosecond pulses propagating in a hollow photonic-crystal fiber with a core diameter of about 50 microm , suggesting the way to substantially enhance the efficiency of nonlinear-optical spectral transformations and wave mixing of high-power laser pulses in the gas phase.

Switching intense laser pulses guided by Kerr-effect-modified modes of a hollow-core photonic-crystal fiber.

A Kerr-nonlinearity-induced profile of the refractive index in the hollow core of a photonic-crystal fiber (PCF) changes the spectrum of propagation constants of air-guided modes, effectively shifting the passbands in fiber transmission, controlled by the photonic band gaps (PBGs) of the cladding. This effect is shown to allow the creation of fiber switches for high-intensity laser pulses. The Kerr-nonlinearity control of air-guided modes in PCFs and the performance of a PCF switch are quantified by solving the propagation equation for the slowly varying envelope of a laser pulse guided in Kerr-effect-modified PCF modes. The spatial dynamics of the light field in a PBG waveguide switch is analyzed with the use of the slowly varying envelope approximation, demonstrating high contrasts of optical switching with PBG waveguides and hollow PCFs.

Carbamazepine in Fabry's disease: effective analgesia with dose-dependent exacerbation of autonomic dysfunction.

Seven patients with Fabry's disease and severe pain received carbamazepine (CMZ). Five of 7 patients had moderate to complete relief based upon self-assessment of pain levels. Preexisting autonomic dysfunction was exacerbated by CMZ in 2. Complications encountered were ileus, urinary retention, and gastrointestinal disturbance. Although CMZ was useful in treatment of pain, caution should be employed in this disease.

Laser beacon system for aircraft collision hazard determination.

A laser beacon collision hazard determination system is capable of simultaneously determining range, bearing, and heading of threat aircraft. Calculations demonstrate that threat aircraft may be observed at > 10 km under good visibility conditions. When the visibility is limited to 5.6 km (3 nautical miles), the shortest possible warning time for aircraft below 3000 m (10,000 ft) can be > 15 sec. A wide variety of detection systems may be chosen based on cost, detection range, and sophistication. Traffic saturation is not a problem since closer aircraft produce easily distinguishable signals so traffic may be prioritized. Preliminary tests demonstrate that accurate range measurements are possible under daylight conditions.

Display and perception of 3-D space-filling data.

We discuss the fundamental challenges associated with visual interpretation of space-filling data and describe a display method which has proved to be very useful and flexible for this task. This display method, which is based on computational corollaries to physical optics and the dynamic use of conventional CRT displays, results in excellent 3-D perception. The discussion outlines the algorithm used for implementation on a PC/AT and its extension to achieve a real-time interactive display environment.

High-energy pulse-burst laser system for megahertz-rate flow visualization.

A high-pulse-energy megahertz-repetition-rate Nd:YAG-based pulse-burst laser system has been developed. The laser can produce a burst of more than 30 pulses, with an average energy per pulse of 70 mJ, at up to 1-MHz repetition rate. The burst repetition rate is 9 Hz. Coupled with a megahertz-framing-rate CCD camera, the frequency-doubled pulse-burst laser system has been successfully used in the visualization of shock evolution in a supersonic flow.

Ultraviolet filtered Rayleigh scattering temperature measurements with a mercury filter.

We report the development of ultraviolet filtered Rayleigh scattering as a diagnostic tool for measurements of gas properties. A frequency-tripled narrow-linewidth Ti:sapphire laser illuminates a sample, and Rayleigh scattered light is imaged through a mercury-vapor absorption filter. Working in the ultraviolet improves the signal-to-noise ratio compared with that previously obtained in the visible as the result of an enhanced scattering cross section as well as the nearly ideal properties of the mercury filter. Tuning the laser through the absorption notch of the filter is a means of probing the scattering line shape, which contains temperature information. Temperature measurements of air are shown to have uncertainties of less than 3%.

Observation of a 100-MHz frequency variation across the output of a frequency-doubled injection-seeded unstable-resonator Q-switched Nd:YAG laser.

We report high-resolution measurements of the spatial variation of the optical frequency of an injection-seeded unstable-resonator Q-switched Nd:YAG laser. Images of the second harmonic taken through a molecular-iodine notch filter show frequency variations of as much as 100 MHz (second harmonic) between the center and the edge of the beam.

Narrow-linewidth passband filter for ultraviolet rotational Raman imaging.

We present a narrow-passband spectral filter capable of frequency-resolved imaging of rotational Raman light scattering with strong spectral rejection of out-of-band Raman, Rayleigh, and Mie scattering. The filter is based on mercury-vapor absorption, and subsequent resonant fluorescence and has a passband of less than 1 cm(-1). It is paired with an injection-seeded, cavity-locked, frequency-tripled Ti:sapphire laser that produces >30 mJ/pulse of single-mode, tunable light in the vicinity of 253.7 nm. The laser and filter are combined to spectrally resolve scattering from individual rotational Raman lines of nitrogen and oxygen.

Corrected and calibrated I2 absorption model at frequency-doubled Nd:YAG laser wavelengths.

We present a computer model for accurately predicting absorption profiles for molecular iodine cells over the tuning range of frequency-doubled Nd:YAG lasers. The model is compared with experimental data for a number of different cell conditions. This model is intended for use in the design and optimization of absorption filters and for data analysis in applications in which the accuracy of the measurement is related closely to the accuracy with which the filter profile is known.

Hemispherical-field-of-view, nonimaging narrow-band spectral filter.

Two compound parabolic concentrators are used to create a 180 degrees -field-of-view spectral filter. The collection optics are reflective and are designed to collimate the light through a multilayer interference filter and then to refocus it onto an optical detector. Assuming unit reflectance and no loss through the optical filter, this device operates at the thermodynamic collection limit.

Characterization of laser seeding by use of group-velocity dispersion in an atomic-vapor filter.

A method for measuring laser seeding efficiencies by use of group-velocity dispersion has been developed. By tuning the laser near a resonance in an atomic-vapor filter it is possible to temporally decouple the seeded (narrow-band) light from the unseeded (broadband) light. We measured a seeding efficiency of 99.8% of the third harmonic of an injection-seeded Ti:sapphire laser. A model for the observed dispersion has been developed and tested. The group-velocity dispersion in the filter may also be used to chirp pulses for spectral analysis in the time domain.

Motor program memory storage in Parkinson's disease patients tested with a delayed response task.

We used a delayed response paradigm to test the hypothesis that the prolonged reaction time in patients with Parkinson's disease (PD) is related to a deficiency in their ability to store a motor program in memory while waiting to move. PD patients, both on and off medication, were compared with age-matched normal subjects during arm movements directed toward a target light. The target light was displayed either during a 3- to 9-s delay or for only 1 s followed by a 2- to 8-s delay before the go signal. At the end of the delay, subjects were required to begin movement rapidly. The reaction time of PD patients was longer than normal and increased slightly when the patients were off medication. The patients had no excessive increase in reaction time with delay in either task compared with the control subjects. We conclude that patients with PD can hold a motor program in memory storage for at least 8 s.

Dispersion filter for spectral and spatial resolution of pure rotational Raman scattering.

A new filtering technique for Raman spectroscopy utilizes atomic vapor to suppress strong elastic and Rayleigh scattering while simultaneously resolving individual rotational Raman lines. Filtered images capture high-resolution spectral information in one dimension and spatial resolution in the other dimension. The filter is based on resonance enhanced dispersion, where the index of refraction varies dramatically. In a simple prism geometry the vapor disperses incident radiation according to frequency. A mercury-vapor-based dispersion filter has been fabricated, modeled, and demonstrated to capture pure rotational Raman scattering from CO(2).

Proposed single-pulse two-dimensional temperature and density measurements of oxygen and air.

A combination of laser-induced fluorescence and Rayleigh scattering from an ArF laser tuned onto an oxygen transition is proposed for instantaneous two-dimensional measurements of temperature and density. A model giving the temperature and frequency dependence of these mechanisms is developed, and supporting experimental results are presented.

One-dimensional coherent four-wave mixing as a way to image the spatial distribution of atoms in a laser-produced plasma.

An experimental technique based on coherent one-dimensional hyper-Raman-resonant four-wave mixing in broad cylindrically focused light beams has been developed for line-by-line imaging of spatial distribution of excited atoms in a low-temperature plasma of optical breakdown. The technique was applied to map excited lead atoms in a low-temperature laser-produced plasma.