High-Speed Sequence Photography of a Ruby Laser

E. S. Dayhoff and B. Kessler

The sequence of phenomena occurring when a ruby laser crystal is flashed is studied on a microsecondtime scale by means of a high-speed framing camera making about 500,000 frames per second. Tworuns of photographs are presented. The individual bursts of light constituting the laser flash showthe following characteristics: Each burst involves the whole active volume of the crystal; each burstshows a grainy or flocculated distribution of light across the face of the crystal, and this distributionchanges in fine detail from frame to frame; the grains of brightness are often arranged in stripes andbands in patterns which change from frame to frame; there are some permanently dark regions; thereare some pinholes in the silver coating which scatter an appreciable amount of light out of the main beam.

Introduction

The light emitted by oscillating ruby lasers is widelyknown to occur in a series of short bursts of a few micro-seconds duration. The whole train of such burstsmay typically occupy an interval of time of 50 to 200usec each time the ruby is pumped above its threshold.Each burst of light presumably represents oscillationin one mode or one class of modes of whatever typemay be available for the electromagnetic oscillations ofthe laser crystal.' In order to study the modes ofoscillation and the mechanism of oscillation initiation,it therefore appears necessary to observe the individualbursts of light in their natural sequence.

Camera

The camera used by us to make sequential observa-tions of the light bursts is a Beckman-Whitley Model192 Framing Camera. 2 It records a strip of 80 in-dependent exposures in each operation. Since itruns continuously it will make multiple exposures ifthe duration of the illumination is greater than the timerequired to make 80 exposures. Shuttering of in-dividual frames is accomplished by means of a turbinedriven rotating mirror in conjunction with severalother optical elements. The framing rate may bevaried over wide ranges by adjusting the air pressuresupplied to the turbine. Speeds of 500,000 frames persecond are easily obtained using compressed air to

The authors are at the U. S. Naval Ordnance Laboratory,White Oak, Silver Spring, Maryland.

Received 10 September, 1961.

drive the turbine, while higher speeds are availablewhen helium gas is used. Focusing of the camera islimited to the range from about 2 meters to infinitybut auxiliary lenses will extend this range. Theentrance pupil is split, each half being used for 40frames of the 80 frame strip, and each pupil is in theshape of a narrow horizontal diamond, so it is necessaryto use care in directing the laser beam into the camerato prevent vignetting by the edges of the entrancepupil.

The pictures shown in Figs. 1 and 2 were all made byphotographing one oscillating ruby crystal in thefollowing way: The camera was focused on the silveredend face of the crystal, though the accuracy of focusingwas better for some runs than for others. The rubycrystal was aligned so that the spot of red light wasthrown into one of the entrance pupils. When lightwas allowed to enter only one entrance pupil, onlyone-half of the resulting frames carry the impression oflaser light, but there is no vignetting. The otherset of 40 frames shows only a pattern of pinholes in thesilvering, since light from pinhole sources is moredivergent than the main laser beam and can enter bothentrance pupils. Without an auxiliary lens the laserspot at the plane of the entrance pupil of the camerawas about as large as the entrance pupil itself so itis difficult to guarantee a complete absence of vignet-ting, but when an auxiliary lens of 30.5 cm focal lengthwas used the spot at the entrance pupil could be madeabout 1 cm in diameter, and it was a simple matter toinsure that it was not clipped by the edge of theentrance pupil. The auxiliary lens was a 30.5 cmf.l. Aero Ektar f/2.5. Use of the auxiliary lens in-

May 1962 / Vol. 1, No. 3 / APPLIED OPTICS 339

chromium doped ruby of laser grade supplied by theLinde Company. The pumping source was a GeneralElectric FT 524 flashtube fed from a capacitor bank of200 f (for Fig. 1) and 300 Af (for Fig. 2) photoflashcapacitors rated at 4000 volts. The crystal wasshaped like a cylinder 5 cm long and 5 mm in diameterand was cut with the c axis of the crystal approximatelyparallel to the axis of the cylinder; an actual measure-ment with the back scattered x-ray technique indicatedan error of about 1 in the parallelism of these axes.The sides were rough ground and the ends were polishedflat and parallel to a limit of 1 minute of arc. Theends were vacuum coated with silver and overcoatedwith SiO. One end was opaque and the other had atransmission of roughly 0.3%.

Fig. 1. End face of "Lasing" ruby. The ruby had shiftedin its holder allowing a thin lune of pumping light to show at thetop. The frames are labeled in microseconds but it is not knownat which frame the flash begins. The camera was focused onthe face of the ruby but focusing was somewhat imperfect.This flash was made with a pumping energy about 9% abovethreshold, crystal at room temperature, and with a silver coat-ing of rather indifferent quality. No auxiliary lens was usedand the original image was about 0.16 cm in diameter. Plus-Xfilm.

creases the size of the image on the film (Kodak Plus-X) from about 0.16 cm diameter to about 1.3 cm.Since the laser beam is quite monochromatic and ofsmall cross section, the geometric and chromaticaberrations of the lens should be negligible. Thedistance between the auxiliary lens and the crystalwas such that off-axis rays would not cross the axis ofthe lenses.

Flash HeadBoth photographs were taken with light from the

same crystal. The crystal was an 0.05%-0.06%

Fig. 2. End face of "Lasing" ruby. Frames labeled inmicroseconds with arbitrary zero. Camera better focused onruby face than in Fig. 1. Pumping energy about 110% abovethreshold, crystal at room temperature, silver coating of goodquality protected by layer of SiO which had probably oxidizedto Sio2. Aero Ektar auxiliary lens used providing an imageon the film of about 1.3 cm diameter. Plus-X film.

340 APPLIED OPTICS / Vol. 1, No. 3 / May 1962

I

I

I

Results

Some general characteristics may be recognized inthe results. Individual bursts of light of duration ofabout a microsecond are seen and also others whoseduration is of the order of 4 or 5 usec. These are oftenseparated by periods of almost complete extinction.A granular pattern of light is seen in each flash, and thepattern is seen to change in detail from flash to flashthough it may be said in general that each flash oc-cupies all of the active part of the crystal. (Onecrescent-shaped side of our crystal seems never toflash and hence is ignored as being inactive.) Thepatterns seen do not have any simple geometricconfiguration but do seem to have dark and light bandswhich move from frame to frame. In some cases thereseems to be a sweeping motion, especially for thedark bands in Fig. 2. Figure 2, which is more ac-curately focused and less enlarged from the negativethan Fig. 1, shows a number of bright pinholes whichalso appear in the mating 40 exposures made with theother entrance pupil. These pinholes remain in thesame places from frame to frame and their brightnessvaries more or less with the laser flashing, suggestingthat they are simply diffracting laser light into aspreading beam.

Discussion

A preliminary interpretation of these results may beattempted although much work remains to be done toverify these ideas. The inactive crescent shapedregion of our crystal seems likely due to a departurefrom parallelism of the end faces along this side of thecrystal due to a roundness of the ends. Certain otherdark inactive areas may be due to polishing defects orother defects on the crystal. The generally grainyappearance of the light may result from a graininessof the deposited silver coat and, if so, should be dif-ferent for crystals using multiple layer coatings of othermaterials. In certain frames (such as 60 usec in Fig.2) there seems to be an interference pattern in thegrains. Other runs, not reproduced here, seem toconfirm that patterns rather strongly suggestiveof interference patterns vill form under certainconditions.

Some interesting conclusions can be drawn from thegeneral appearance of the pictures. The fact that eachburst of light involves the whole active width of thecrystal seems to indicate that, despite inhomogeneitiesin the crystal and irregularities in the polishing of theends, the electromagnetic interaction responsible for

the regenerative action links the whole crystal togetherfrom one side to the other, even though various prop-erties such as angular spread of the beam suggest thatcoherence of the wave front is limited to a small frac-tion of the crystal area. (The angular spread ob-served for this crystal is about 0.50.) One of ourrecent film strips not shown here provides some evi-dence that the wave front is rather uneven and irregular.

It is interesting to look for evidence of recognizablemode patterns. The unknown geometry variations ofthe end face of the crystal would probably so distortthe mode patterns that they would become unrec-ognizable in detail. It is nevertheless possible that thepatterns of bars in some frames are the result of in-dividual modes or mixtures of modes of the betterknown types as they appear in the presence of dis-tortions due to various imperfections. The bar pat-terns in Fig. 2 are particularly intriguing as they seemto have a growing and then a sweeping motion over atime interval of 5 usec or so in a number of places.

Conclusion

It is apparent that the accumulation of additionaldata of this same general kind should do much toclarify the details of the laser process. Especiallyuseful will be comparison studies on various crystalsground in different ways. The processes occurring insuch crystals are obviously quite complex and mustinvolve such things as switching and mixing of ordinarymodes of a right circular cylinder at least. *

The authors wish to thank B. J. Crapo and T.Marshall for valuable assistance with the camera.

References1. Good reviews of laser phenomena have been given by A. L.

Schawlow, Solid State J. 2, 21 (1961), and by T. H. Maimanetal. Phys. Rev. 123, 1151 (1961).

2. A description of this type camera has been given by M. C.Kurtz, J. Soc. Motion Picture Television Engrs. 68, 16(1959).

* The authors feel it might well be useful for the word "LASER"to apply to devices in which these complex processes are impor-tant, as is generally the case with optical devices, and to use theword "MASER" in the apparently simpler case where themodes of oscillation are rather rigidly enforced by the apparatus,as is generally the case with microwave masers. Such usagewould generally conform to current practice and assist in dis-tinguishing certain physical processes which may otherwiserequire a new name.

May 1962 / Vol. 1, No. 3 / APPLIED OPTICS 341

BOOKScontinued from page 334

1.1 and 1000l at temperatures between 20 and 13,000'K. Thesedata are given to four figures.

Table III includes: (1) the relative spectral radiance referredto the maximum radiance at the indicated temperature, (2) arestatement of the wavelength ratios of Table I-2, and (3) anauxiliary function for computing the derivatives of the Planckfunction. These quantities are plotted for values of the function\T in the range of 0.01 to 0.99 cm 'K. Procedures for evaluatingthe functions at higher values of XT are described in the text.

Table IV comprises the relations: (1) the total radiance of ablackbody, (2) the maximum radiance of a blackbody, and (3)the wavelength at which the maximum radiance occurs. Allthese quantities are given for temperatures from 1000 to 2500'Kat 20 intervals, from 2500 to 5500'K at 50 intervals and from 5500to 10,0000K at 100 intervals.

Table V is similar to Table III but with the quantities tabu-lated against wave numbers rather than wavelengths, and TableVI is similarly related to Table IV with the argument in recipro-cal temperature units.

Table VII consists of two parts-a compilation of the lumi-nance (photometric brightness) of a blackbody between 800 and17960K at 4° intervals and a table of the luminance and thethree chromaticity coordinates in the temperature intervals ofTable I.

Finally, Table VIII is an auxiliary temperature correctiontable to protect the tables against obsolescence with subsequentrevision of the values of the physical constants.

Examples of the use of these tables are included in the booktogether with a collection of excellent bibliographies.

The magnitude of the effort contained in this magnificentcompilation makes one hesitate before offering an adverse com-ment. This reviewer has found it necessary to interpolateto intermediate values of the temperatures in Tables I-3and II-3. Using linear interpolation, as suggested in the pre-face, it was found that values midway between the tabulatedtemperatures were rarely accurate to more than three figures.The reviewer has no suggestion to offer other than the illogicalsuggestion that the tables be expanded still further. In general,the book will be an invaluable reference volume for people en-gaged in radiation, high-temperature, and spectroscopy work.Where considerable interpolation is needed, the radiation sliderule will be a useful adjunct.

S. KATZ

Tables of the Fractional Function for the Planck Radiation Law.By M. CZERNY AND A. WALTHER. Springer-Verlag, Berlin,1961. 60 pp. 28DM

This book is about as short as the book of Pivavonsky and

Nagel, reviewed above, is long. With it and a little study, how-

ever, one can perform certain calculations involving blackbodyradiation functions that are not possible with the large volume.Every library should have both of these volumes, as together theypresent a very thorough treatment of the Planck radiation func-tion.

Ten or fifteen years ago there existed only very limited tablesof blackbody functions, and many of these tables were madeusing different values of the radiation law constants cl and c2from those generally accepted today. Parry Moon publishedtables in J. Opt. Soc. Am. 38, 291 (1948) and A. N. Lowen

and G. Blanch published tables in J. Opt. Soc. Am 30, 70 (1940)

(the so-called W.P.A. Tables) which this reviewer has found very

useful in his work. Another handy short set of tables is containedin the book Tables of Functions by Jahnke and Emde, 4th ed.,Dover Publications, New York (1945). The Second World Warproduced many engineering applications of infrared radiationand a renewed interest in blackbody functions. Prof. Czernyproduced a "radiation slide rule" that was useful for quick calcu-lations. M. W. Makowksi, while at the Polish Refugee Collegein London, designed a much more elaborate radiation slide rule[Rev. Sci. Instr. 20, 876 (1945)] which is available commerciallyfrom A. G. Thornton, Ltd. (and various distributors). A. H.Canada, while at the General Electric Schenectady Laboratory,designed the G.E. slide rule which is usually found in everyinfrared man's briefcase. This is described in the GeneralElectric Review, December 1948. While we are mentionin r suchhandy aides for blackbody calculations we should mention an-other G.E. author, J. P. Chernoch, who published some nomo-grams in the July 1957 issue of Aviation Age, entitled "InfraredCalculations Made Simple." Here, by laying a rule across thetemperature scale and wavelength scale, one can determine thepercentage of the radiation of a blackbody at that temperaturethat falls below that wavelength. Another combination nomo-gram and slide rule has been recently described by Block Associ-ates in their advertisement in J. Opt. Soc. Am. of January 1962.

For many engineering purposes small slide rules of this sort arecompletely adequate. Other uses began to arise in the post-warperiod which demanded higher accuracy. The simultaneousdevelopment of high-speed electronic computers made more pre-cise calculation of the Planck function practical. The set oftables which Dr. Nagel had begun on a desk-type calculator atDarmstadt during the war was completed on a computer at theHarvard Computation Laboratory by Mark Pivavonsky and isdescribed in the preceding review. The set of tables describedhere was also begun using a desk calculator but was completed in1956 on an IBM 650 at the Institut fur Praktische Mathematik(IPM) at the Technische Hochschule at Darmstadt. In 1959A. G. DeBell of Rocketdyne Division, North AmericanAviation, needed an improved set of tables for some calculationshe was making on flame temperatures, so he programmed an

IBM 704 to make these calculations which are available asRocketdyne Research Report 59-32. (Unfortunately he useda value 1.4388 for C2, which is the most recent but not the gen-erally accepted value.) In 1959 C. Ferriso of Convair, alsomaking flame calculations, prepared a similar set of tables usingan IBM 704. His tables however are in terms of intensity versuswavenumber rather than wavelength, so that they do fill a gapin the literature. In 1960 S. Twomey of the N.A.S.A. made aset of calculations for the blackbody function between 180'Kand 315'K and between 3400 cm-' and 20 cm-', which is moreuseful for meteorological calculations.

One is tempted to remark it is good that at long lastdetailed tables now exist for the Planck function, so that futureusers may be spared the trouble and expense of computing thisfunction. This is not quite true, however; in this new modernworld of 7090 super-high-speed computers, present-day workerssimply program the function into whatever they are calculating.This is quicker for them than looking up the function in a table.So, we are lucky that these books have appeared; future scientistswould have been too busy to tabulate these intermediate steps.

To discuss this book specifically, 20 of the 60 pages are devotedto a discussion of the Planck function. This book deals chieflywith the fractional functions of a blackbody radiator, that is, theratio of the radiation between X1 = 0 and X2 = X to the totalradiation of the blackbody radiator at that temperature (as

continued on page 358

342 APPLIED OPTICS' / Vol. 1, No. 3 / May 1962