Max Garbuny

Pollutant Detection by Absorption Using Mie Scattering and Topographic Targets as Retroreflectors

Remote pollutant measurement by absorption using topographical reflectors or atmospheric Mie scattering as a distributed reflector offers increased range and sensitivity compared to that achieved by Raman or resonance backscattering methods. The use of opographical reflectors offers the advantage of a single-ended absorption measurement for ranges up to 10 km and sensitivities to ess than 0.01 ppm for a 10-mJ, 100-nsec transmitted pulse. The distributed Mie reflector permits absorption measurements over a depth ctau/2, determined by the pulse length tau, and allows ranging by time-of-flight measurement. For a 100-mJ, 100-nsec pulse sensitivities to 0.3 ppm at a 15-m depth resolution to ranges of 1-4 km are possible. This sensitivity is 10(4)to 10(5) times better than that achieved by the Raman method.

Remote detection of CO by parametric tunable laser

Resonance absorption of the radiation from a parametrically tuned laser permits the remote detection and density measurement of pollutant gases at large ranges with single‐ended systems if topographical features or atmospheric aerosol are used as backscattering means. We have built and demonstrated such a transmitter‐receiver system operating at 2.3 μm in the infrared. The presence and amount of a CO sample placed at a range of 107 m against a topographical backscattering target was established by the absorption of rotational lines in the first overtone transition. The results indicate a range limit of the present equipment at 1.5 km and a sensitivity in good agreement with predictions based on measured linewidths and cross sections.

Laser engines operating by resonance absorption

The coherence properties and power levels of lasers available at present lend themselves to the remote operation of mechanical engines by resonance absorption in a working gas. Laser radiation is capable of producing extremely high temperatures in a gas. Limits to the achievable temperatures in the working gas of an engine are imposed by the solid walls and by loss of resonance absorption due to thermal saturation, bleaching, and dissociation. However, it is shown that by proper control of the laser beam in space, time, and frequency, as well as by choice of the absorbing gas, these limits are to a great extent removed so that very high temperatures are indeed attainable. The working gas is largely monatomic, preferably helium with the addition of a few volume percent of an absorber. Such a gas mixture, internally heated, permits an optimization of the expansion ratio, with resulting thermal efficiencies and work ratios, not achievable in conventional engines. A relationship between thermal efficiency and work ratio is derived that is quite general for the optimization condition. The performance of laser piston engines, turbines, and the Stirling cycle based on these principles is discussed and compared with conventional engine operation. Finally, a brief discussion is devoted to the possibility and concepts for the direct conversion of selective vibrational or electronic excitation into mechanical work, bypassing the translational degrees of freedom.

Method for the Generation of Very Fast Light Pulses

A method is described in which a visible or infrared light beam is repeatedly reflected between a rotating and stationary mirror system. The resulting sweep speed is proportional to the number of such reflections. Light pulses of 4×10−8 sec duration were measured. A considerable increase in the sweep speed appears feasible. Various applications of the system are mentioned.

Image Converter for Thermal Radiation

The development of the phothermionic image converter is described, a device which reproduces scenes by virtue of differences in their thermal radiation. The method uses the principle of projecting a temperature image on a free multilayer film about 0.05 μ thick which contains a photosurface with thermally sensitive yield. Image conversion is achieved by probing the temperature distribution with a flying light spot and displaying synchronously the thermally modulated photocurrent on a monitor. Section 1 discusses the limits of temperature imaging. Under idealized shielding conditions, the relative temperature distributions in the image assume a very simple and general relationship to that of the object, e.g., are equal in radiation equilibrium. Practical heat exchange processes will present other signal limitations which, in combination with the random noise inherent in any conversion process, determine the system performance. Section 2 deals with the experimental work of searching for, and improving on, the phenomena on which the operation of the image converter is based. This includes theory and experiments on temperature sensitive photoelectric yields and on the electrical and optical properties of thin films. Certain components such as sealed crystal windows, cooled radiation shields, and free thin films had to be developed for the image tube. Several models were built and accommodated by suitable optical and electronic systems. When completed, the device had television bandwidth and motion portrayal capabilities and the ability to detect, or image, temperature differences of 10° to 50°C.

Photoconductive Time Constants and Related Characteristics of p-Type Gold-Doped Germanium

To measure the very short photoconductive time constants of p-type gold-doped germanium, two alternative methods were applied and compared. The first is indirect, using the relationship between the magnitude of generation-recombination noise and carrier lifetimes. The second method is direct, employing a high-speed light-pulsing technique. If no other noise sources are important, the results of the indirect method approach those of the direct method as a lower limit. A combination of such time-constant measurements was performed on a series of crystals in which impurity densities and carrier concentrations had been evaluated by Hall coefficient and conductivity measurements. From these data quantum yields of carrier generation, and cross sections for photon capture and carrier recombination were evaluated. The photon capture cross section of the 0.15 ev gold acceptor level at 5 μ is 1.3×10−16 cm2, averaging 0.9×10−16 cm2 for 2–9 μ. The hole capture cross section by the Au− ion in germanium was found to be 2.3×10−14 cm2.

Analysis of the spectrum of a laser beam modulated by a periodic electrooptic Doppler shift.

An analysis is made of the spectrum of a laser beam modulated by the electrooptic doppler shift for two important cases: where the applied electric field is a sawtooth function and where it is a triangular function. The spectra reduce to simple forms when the maximum frequency shift is an integral multiple of the modulation frequency.

Vacuum Tight Windows with Wide Band Transmission Characteristics

A vacuum tight barium fluoride window with transmission range from 1800 A to 14 μ is described. The window consists of a barium fluoride disk sealed into a suitably formed fine silver spinning with silver chloride. Evacuated envelopes incorporating this window have been made and kept for months without indication of leakage, based on the performance of photoelectric cathodes in the vacuum space. This technique has been equally successful in sealing lithium fluoride windows. The sealing procedure is fully discussed.

Pressure Dependence of Resonance Fluorescence Lifetimes in SF 6 and SF 6 –Air Mixtures

A pilot study is described in which the resonance of the 10.6-microm CO(2) laser radiation with the nu(3) vibrational level of the SF(6) molecule is used to explore the effect of gas density on the time response of fluorescence. A slow increase of the fluorescent decay time with pressure of pure SF(6) is found that is interpreted in terms of a degree of equilibrium between vibrational and translational temperatures. In SF(6)-air mixtures, however, the increase of air pressure from 0 atm to 1 atm produces a proportional increase of the time constant from milliseconds to seconds in a process in which the vibrational-translational relaxation time is very small compared with the bulk cooling time determined by the thermal diffusivity. The results are compared with previous studies using Q-switched pulse excitation.

Graphical Representation of Particle Trajectories in a Moving Reference System

A graphical method is derived for the analysis of microwave electron tubes, ion accelerators etc., which refers particle positions and velocities to a moving reference system. If the forces are dependent on time only, the trajectories are transformed into straight lines. For inhomogeneous fields an approximation procedure applies. To demonstrate the capabilities of this method a brief treatment of the transit time phenomena in cavity triodes is outlined.