MEASUREMENTS ON THE RAMAN COMPONENT OF LASER ATMOSPHERICBACKSCATTERJ. A. Cooney Citation: Applied Physics Letters 12, 40 (1968); doi: 10.1063/1.1651884 View online: http://dx.doi.org/10.1063/1.1651884 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/12/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Vertical Profiling of Atmospheric Backscatter with a RamanAerosol Lidar AIP Conf. Proc. 1203, 388 (2010); 10.1063/1.3322473 Temporally resolved Raman backscattering diagnostic of high intensity laser channeling Rev. Sci. Instrum. 73, 2259 (2002); 10.1063/1.1475348 Stimulated Raman backscattering instability in short pulse laser interaction with helium gas Phys. Plasmas 3, 1682 (1996); 10.1063/1.871688 Raman backscattering in laser wakefield and beatwave accelerators Phys. Fluids B 3, 468 (1991); 10.1063/1.859889 LASER BACKSCATTER CORRELATION WITH TURBULENT REGIONS OF THE ATMOSPHERE Appl. Phys. Lett. 12, 72 (1968); 10.1063/1.1651904

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Volume 12, Number 2 APPLIED PHYSICS LETTERS 15 January 1968

MEASUREMENTS ON THE RAMAN COMPONENT OF LASER ATMOSPHERIC BACKSCATTER

J. A. Cooney Radio Corporation of America. Astro-Elcctronics Division

Princeton. New Jersq (Received 8 September 1967; in final form '27 November 1!1(7)

Density profiles of the gaseous atmosphere up to 3 km have been measured using laser backscatter. The ambi­guity in the return at the laser frequency (6943 A) due to the aerosol scatter component was avoided by monitor­ing the frequency-shifted Raman vibrational-rotational band of the nitrogen component centered at 8285 A.

The atmospheric backscatter from giant pulse lasers has been used for various purposes by a number of investigators.I-~ In this Letter, prelimi­nary observations are reported on a frequency­shifted component of atmospheric backscatter. De­tailed calculations:! had demonstrated the feasi­bility of this approach. Using a ruby laser system with an output at 6943 A, the backscatter at wave­lengths in the vicinity of 8285 A due to the Raman vibrational-rotational band of nitrogen has been observed.

The purpose of this experiment is to provide a means of obtaining density profiles of the gaseous atmosphere. It might be expected that observation of the Rayleigh backscatter would suffice. However, because of the quantity of aerosols present in the atmosphere, the backscattered return, unshifted in frequency from that of the transmitter, consists of both gaseous and aerosol backscatter in unknown proportions and hence is ambiguous. The fre­quency-shifted backscatter at 8285 A due to tht­fundamental Raman vibrational-rotational band of nitrogen whose intensity is proportional only to the nitrogen molecular density in the atmos­phere, is used to avoid this ambiguity. To within the measurement precision of the present system, the cross sections for the pertinent Raman process, as calculated from the observed data, agrees with the measurement of Stanbury4 et al.

The apparatus consists of a monostatic in-line laser transmitter and optical receiver. The pulsed ruby laser is capable of a maximum output of 250 MW in a 20-nsec pulse. It operates nominally at 6943 A with a beam width of half power of 10- 12 mrad. The receiver consists of a 30" diam, 21" focal length mirror with a *,' circle of confusion. At the mirror focus, a selected RCA-7265 multiplier photo­tube (dark current 2 X IO-H A at 2400 V) is posi­tioned. The tube is encased in a light-tight mount­ing on the front end of which a lens and optical filter system is installed. The measured transmis­sion at 8285 A of the combined ISO A pass-band

40

filter and blocking filter was 35% and the power transmitted by the combination at 8285 A was greater than I (F of that transmitted at 6943 A. Thus, although the return power at 6943 A is greater by approximately a factor of 500 than that at 8285 A, its contribution to the measured intensity is negligible.

The measured response of the entire receiver in the vicinity of 8285 A, which included line envelope shape, filter response, light collecting efficiency of the mirror and lenses, and the absolute spectral response characteristics of the photomultiplier, was 5.4 X 10-4 A per /LW of power over the Raman band. The output of the photomultiplier was fed across a 104 anode resistor, then through a cathode follower network of 0.48 amplification, and dis­played on an oscilloscope.

The measurements were made at a field test site well after dark on clear nights and with the axis of the system pointing in the vertical.

In a monostatic coaxial arrangement of trans­mitter and receiver, the return power P, accurate enough for present purposes is given by

P =PTB(Cr)~A I 47Trt

PI' is the transmitted power, B is a system constant which involved the light collecting efficiency, (Cr) is the pulse length in units of distance, A is the mir­ror area, r is the altitude, and ~ is the Rayleigh at­tenuation or scattering coefficient multiplied by the ratio of Raman to Rayleigh cross section. As ~ de­pends directly on the local value of the number density of N~ molecules, the return power from a given altitude is directly proportional to the nitrogen density at that altitude. The Rayleigh attenuation coefficient for the incident light for the ambient temperature and pressure5 at a wavelength of 7000 A for 78% N~ composition at 1.5 km is 3.08 X lO-fi m- I. Stansbury et al. 4 give the ratio of Raman vibra­tional to Rayleigh cross section for N~ including only the Q branch as 4.42 X 10--1. Based on their

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Volume 12, Number 2 APPLIED PHYSICS LETTERS 15 January 1968

measurements and including the Sand 0 branches, the effective Raman/Rayleigh cross section ratio is 1.88 X 10-:1• Thus g = 5.79 X 10-9 m- I

The laser was nominally operated at 20-30 MW in 7 5-} 00 nsec pulses. For the case shown in Fig. 1, measurements in the laboratory showed that the output power was about 25.0 MW in a 100 nsec pulse. Using g = 5.79 X 1O-9 /m, the return power expected from 1.5 km, assuming 1 m2 mirror area and 100% collecting efficiency, is 1.6 X 10-1 f.,tW. The actual mirror area and light collection efJiciency are factored into the system response. The return power of 1.6 X 10-7 W would lead to an output of 426 m V at 1.5 km. For the curve6 shown in Fig. 1, the measured value is approximately 400 m V.

The measured curve departs from 1/r2 depend­ence at the lower altitudes, due primarily to circuit limitations. This circuit roundoff was caused by a combination of cathode follower tube cutoff and residual stray capacitance giving rise to 2-3 f.,tsec response time of the receiver network. Small align­ment error of transmitter and receiver contributed a few percent to the error. Although some depar­ture from l/r~ behavior can be seen on the experi­mental curve, anticipated variations of density

1000~--T---~--~----r---~---T--~

900

en ~800 o >

.... 600 => a.. .... => 500 o 0: w ~400 o ...J ...J 0300 I..L.

\ --- EXPECTED RETURN

\ MEASURED

RETURN

\ \ \ \ \ \ \ \ \ \

\ '\ ~

" '" 100 __ . __ ~Q!§~!-~'LEl-________ _

Fig. 1. Typical trace of Raman return at 8285 A.

with altitude are less than present measurement accuraCies.

At the time of the measurement1 shown in Fig. 1, the local weather conditions were as follows: tem­perature 25°F; pressure, 1028.0 millibars; wind con­ditions, calm. This added 8% beyond normal tem­perature and pressure to the atmospheric density. A relatively high ambient light level existed, due to the proximity of artificial light sources. The local maxima at 2.0 and 2.5 km are typical of noise pulses that have been found to arise from the ambient light.

The relatively close correspondence of expected and measured curves over much of the range shows that the measured return follows a l/r2 altitude dependence quite closely. The so called expected return does not include the variation of density with altitude which for 1.5 km would be about 13% below ground density. Although the error appears to be small for the present measurements over certain portions of the return trace, no meaningful assessment of overall error associated with the meas­urement can be made until sufficient data is obtained with a more accurate means of monitoring laser en­ergy output. Nonetheless, the preliminary estimates indicate that the present absolute measurement should be correct to within a factor or two. The ulti­mate minimum error on the absolute measurement will probably never get below several percent. How­ever, if such measurements are rectified by inde­pendent ground level measurements of temperature and pressure, an overall error of the order of 1 % appears feasible.

Preliminary checks were made on possible spuri­ous contributions to the signal arising from the elec­tronics by firing the laser with multiplier phototube capped. In addition, the effects on the signal from the laser flash-lamp light and ruby fluorescence were found to be negligible. Additional measure­ments, which will be reported in a later communi­cation, are presently in progress. These include measurements using optical filters, whose center frequencies are placed off the maximum of the N2 vibrational-rotational Raman band, as well as meas­urements of the pure rotation band for which an increase in cross section of about 50 is available. In addition, measurements are in progress to de­termine the amount of fluorescence generated in the optical filters. Also, a less noisy ambient light background is being used. Finally, frequency dou­bling of the laser output is anticipated. Assuming }-5 % power conversion efficiency, this increases the return signal by 20-} 00 due primarily to in­creased photocathode sensitivity.

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Volume 12, Number 2 APPLIED PHYSICS LETTERS 15 January 1968

The author acknowledges the help of J. Orr of this laboratory on the design and fabrication of the receiver equipment. Helpful discussions were held with Prof. R. Schotland of New York University. In addition, O. Ziemelis, T. Faith, and C. Tomasetti of this laboratory contributed to the measurements.

I R. Collis and M. Ligda. Nature 203, 508 (1964). 'B. R. Clemesha. G. S. Kent. and R. W. H. Wright. Nature 209,

184 (1966).

"J. Cooney. Proceedings oj the Symposium on Electromagnetic Sens­ing oj the Earth from Satellites. (Polytechnic Press. Brooklyn. New York. 1965). page P-P-IO.

4E. S. Stansbury, M. F. Crawford. and H. L. Welsh. Can . .f. Phys. 31,954 (1953).

5 Obtained by use of ground measurement and assumed lapse rates.

"This curve was drawn from a photo of an oscilloscope trace. 'This measurement program was conducted in Princeton. N.

.J.. during the fall and winter of 1966.

LENGTH CHANGE OF COPPER AFTER ELECTRON IRRADIATION*

R. H anada and J. W. Kauffman Department of Materials Science

The Technological Institute Northwestern University

Evanston. Illinois (Received 16 November 1967; in final form 6 December (967)

The length change of pure copper after 2.2 MeV electron irradiation was measured with a sensitivity of 3 x 10-" at liquid-helium temperature. Subsequent isochronal annealing showed the existence of substructures in stage 1.

These substructures in length change are found to be similar with resistivity recovery results.

Length change as well as electrical resistivity is a fundamental quantity in relation to lattice imper­fection. No previous measurements have been re­ported on length change due to electron irradiation in metals. The present Letter reports such measure­ment with a sensitivity of 3 X 10-H in 2.2 MeVelec­tron-irradiated pure copper.

The purposes of the present experiments are (I) determination of volume change by a Frenkel pair in copper and (2) study of the five substages in stage I by means of length change.

The measurement of length change in metals after particles irradiation so far reported are the lattice parameter change of copper after deuteron irradiation by Simmons and Balluffi,l the length change of copper after deuteron irradiation by Vook and Wert,2 the length measurement of copper and aluminum after internal fission damage by Blewitt,a and the lattice parameter change of copper after neutron irradiation by Wenzl et al.4

The experimental method is the same type suc­cessfully used by North and Buschert5 for their electron-irradiated germanium. The principle of the method is an accurate measurement of parallel plate capacitances, which -are formed between the top surface of the specimen and vacuum-deposited thin metal films on a flat sapphire plate. The con-

*Work supported by U. S. Atomic Energy Commission.

figuration and the purposes of the four capacitances are shown in Fig. I. The copper specimen was cut from one piece of rod by means of machining and spark cutting and has no junctions to avoid possible hysteresis upon isochronal annealing. The ma­terial is 5-9' polycrystalline copper obtained from ASARCO. The specimen was annealed in vacuum for 1 hr at 450°C after completion of machining and spark-cutting work. The irradiated area is 6.5 mm long, 7 mm wide, and 0.25 mm thick. Capaci­tances were measured by a General Radio Capaci­tance Bridge (ModelI615A) with a PAR model lock-

Fig. 1

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