Immersion Liquids for Ruby Lasers M. E. Graham, B. 1. Davis, and D. V. Keller A search was made for high index of refraction liquids suitable for immersion of ruby crystals in high- energy lasers. One liquid, SnCl, 2H20 in glycerin, is reported which can exactly match the index of ruby (1.76) and which, at the same time, is stable to the flash-lamp environment and transparent at the ruby pumping and lasing wavelengths. Another liquid, saturated aqueous SnCl,22H20, has a slightly lower index (1.60), but is more generally useful because of its low viscosity. Eight additional liquids with indices in the range from 1.43 to 1.54 are reported and tables are included which list the relevant properties of all of the liquids considered. Introduction Sapphire cladding of small ruby laser rods has been used successfully to reduce threshold pumping power and increase the available output.' Subsequently water, glycerin, and benzyl benzoate have been used as immersion liquids for the same purpose with the addi- tional feature that they can be circulated for more rapid cooling. 2 ' 3 More recently, uniform amplitude regularly spaced laser pulses have been produced by complete immersion of a ruby rod in glycerin or potassium iodide. 4 The number of reported liquids suitable for these applications is small because of the simultaneous requirements of (1) high index of refrac- tion (preferably approaching nfruby = 1.76), (2) stability to the severe flash-lamp environment, and (3) high transmission at the pump bands. In addition, for ex- periments in which the ends of the ruby are also im- mersed 4 the liquid must be transparent to the laser wavelength and have low viscosity so that schlieren effects are minimized in the beam path. This paper gives the results of a search for liquids satisfying the above requirements. Meyerowitz5 has made a compilation of high index of refraction liquids (n > 1.74) for use in the identifica- tion of crystalline materials; however, color and photo- chemical stability were not considered important cri- teria in his investigation. Johannsen 6 gives methods for preparation of some of the liquids. Our general approach was to consider high-index organic liquids, high-index salts with high solubilities, and high-index, low-melting point salts for eutectic mixtures. Because of requirement (3) above only colorless liquids were considered. The transmission at the pump bands was The authors are with the Northrop Corporation, Ventura Division, Newbury Park, California. Received 7 January 1965. not measured quantitatively. Stability to the flash- lamp environment was determined for those liquids with an index of refraction >1.45. Experimental Methods Refractive index measurements of the liquids were made by simultaneously observing a scale by two paths: one direct and one through a small-angle wedge filled with the liquid to be tested. With the scale at a large distance R from the wedge and a small wedge angle a, the refractive index n is related to the scale reading x by the equation a(n - 1) = tan-' (x/R). Using a calibrated direct-reading scale and sodium lamp illumination the measurements were made to three significant figures. The measured values varied from sample to sample of a given liquid because of differences in temperature and aging. To test flash stability the liquid was placed in a test tube between two EG & G Type FX 47A linear flash lamps 4.45 cm apart in a highly reflecting MgO-coated U-shaped cavity 2.54 cm deep, 6.35 cm wide, and 17.15 cm long. The flash lamps were connected in series with a 15,000-J, 5-kV capacitor bank through an 850-/.H inductor providing a 3-msec bell-shaped current pulse. The system was fired at 0.5 kV intervals from 2.0 kV to 4.5 kV or to the voltage at which decomposition occurred. The liquids were cooled between firings. The 4.5-kV shots were repeated with the liquid still hot. The relative stabilities of the liquids were determined by the largest amount of energy which could be delivered to the flash lamps without causing visible decomposi- tion. In the tables this threshold energy for a particular liquid is termed its "flash stability". It is emphasized that these results are relative stabilities for a par- ticular pumping pulse shape. They may be altered somewhat for other shapes. May 1965/ Vol. 4, No. 5 / APPLIED OPTICS 613
Transcript
Immersion Liquids for Ruby Lasers
M. E. Graham, B. 1. Davis, and D. V. Keller
A search was made for high index of refraction liquids suitable for immersion of ruby crystals in high-energy lasers. One liquid, SnCl, 2H20 in glycerin, is reported which can exactly match the index ofruby (1.76) and which, at the same time, is stable to the flash-lamp environment and transparent at theruby pumping and lasing wavelengths. Another liquid, saturated aqueous SnCl,22H20, has a slightlylower index (1.60), but is more generally useful because of its low viscosity. Eight additional liquidswith indices in the range from 1.43 to 1.54 are reported and tables are included which list the relevantproperties of all of the liquids considered.
Introduction
Sapphire cladding of small ruby laser rods has beenused successfully to reduce threshold pumping powerand increase the available output.' Subsequentlywater, glycerin, and benzyl benzoate have been used asimmersion liquids for the same purpose with the addi-tional feature that they can be circulated for morerapid cooling.2' 3 More recently, uniform amplituderegularly spaced laser pulses have been produced bycomplete immersion of a ruby rod in glycerin orpotassium iodide.4 The number of reported liquidssuitable for these applications is small because of thesimultaneous requirements of (1) high index of refrac-tion (preferably approaching nfruby = 1.76), (2) stabilityto the severe flash-lamp environment, and (3) hightransmission at the pump bands. In addition, for ex-periments in which the ends of the ruby are also im-mersed4 the liquid must be transparent to the laserwavelength and have low viscosity so that schliereneffects are minimized in the beam path. This papergives the results of a search for liquids satisfying theabove requirements.
Meyerowitz5 has made a compilation of high indexof refraction liquids (n > 1.74) for use in the identifica-tion of crystalline materials; however, color and photo-chemical stability were not considered important cri-teria in his investigation. Johannsen 6 gives methodsfor preparation of some of the liquids. Our generalapproach was to consider high-index organic liquids,high-index salts with high solubilities, and high-index,low-melting point salts for eutectic mixtures. Becauseof requirement (3) above only colorless liquids wereconsidered. The transmission at the pump bands was
The authors are with the Northrop Corporation, VenturaDivision, Newbury Park, California.
Received 7 January 1965.
not measured quantitatively. Stability to the flash-lamp environment was determined for those liquids withan index of refraction >1.45.
Experimental Methods
Refractive index measurements of the liquids weremade by simultaneously observing a scale by two paths:one direct and one through a small-angle wedge filledwith the liquid to be tested. With the scale at a largedistance R from the wedge and a small wedge angle a,the refractive index n is related to the scale reading xby the equation a(n - 1) = tan-' (x/R). Using acalibrated direct-reading scale and sodium lampillumination the measurements were made to threesignificant figures. The measured values varied fromsample to sample of a given liquid because of differencesin temperature and aging.
To test flash stability the liquid was placed in a testtube between two EG & G Type FX 47A linear flashlamps 4.45 cm apart in a highly reflecting MgO-coatedU-shaped cavity 2.54 cm deep, 6.35 cm wide, and 17.15cm long. The flash lamps were connected in series witha 15,000-J, 5-kV capacitor bank through an 850-/.Hinductor providing a 3-msec bell-shaped current pulse.The system was fired at 0.5 kV intervals from 2.0 kVto 4.5 kV or to the voltage at which decompositionoccurred. The liquids were cooled between firings.The 4.5-kV shots were repeated with the liquid still hot.The relative stabilities of the liquids were determined bythe largest amount of energy which could be deliveredto the flash lamps without causing visible decomposi-tion. In the tables this threshold energy for a particularliquid is termed its "flash stability". It is emphasizedthat these results are relative stabilities for a par-ticular pumping pulse shape. They may be alteredsomewhat for other shapes.
May 1965 / Vol. 4, No. 5 / APPLIED OPTICS 613
Table I. Organic Liquids and Organic Solutions
FlashLiquid Indexa Color stability (kJ) Viscosityb
Glycerin + SnCl,2211201,1,2,2-tetrabromoethane1,1,2,2-tetrachloroethaneTetrachloroethyleneDow Chemical Co. CMDPO-17Dow-Corning 200 silicone fluidDow-Corning 200 silicone fluidGeneral Electric SF 1017 silicone fluidV-C Chemical Company Vircol-82
Water cleareWater clearWater clearWater clearWater clearWater clearWater clearWater clearWater clear
4.5>14.5
2.9
4.514.5
>14.5
>14.5
>14.54.5
<2.92.9
<2.9
8.8>14.5
LowLow
Low
Very highLow
MediumLow
MediumLowLow
Medium
MediumLowLowLowLow
Low (20 cS)Very high (500,000 cS)
MediumLow
a The numbers (1), (2), and (3) in the second column indicate the source of the data: (1) experimental; (2) International CriticalTables; (3) manufacturers' data.
b The qualitative terms low, medium, and high may be taken to imply viscosities comparable to those of water, glycerin, and syrup,respectively.
c No measurable absorption at 6328 A for a 42-cm path.
Results
The relevant properties of all the liquids consideredare reported in Tables I-III in three categories: (1) pureorganic liquids and organic solutions; (2) saturatedaqueous solutions; and (3) eutectics and molten salts.
SnCl2 2H20 in glycerin can be used for an exact indexmatch with ruby (n = 1.76). It is colorless and hashigh flash stability; however, it is quite viscous, andschlieren effects can be troublesome. 4 SnCl1 2H,0 inwater was found to be most generally useful because itcan be prepared with a relatively high index (n = 1.60),is water clear with low viscosity, and has high flashstability. However, this solution is quite corrosive.The ideal liquid, with low viscosity and index 1.76, hasnot yet been found.
Most of the inorganic aqueous solutions tested hadhigh flash stability. Notable examples are AgNO3(n = 1.48), CdCI2 (n = 1.48), SnCl4 (n = 1.54),Hg(NO3)2 + HgBr2 (n > 1.54), and Hg(NO3)2 + HgI2(n > 1.52).
The only organic solutions, other than SnCl 2H20in glycerin, that exhibit high flash stability aredimethyl sulfoxide (n = 1.48), epibromohydrin (n =1.52), glycerin (n = 1.47), and Vircol-82 (n = 1.43).These all have high heats of vaporization, high specificheats, and are water clear. All except glycerin havelow viscosity.
It is a pleasure to thank R. J. Yackley for numeroushelpful discussions and for providing us with many ofthe chemicals.
References
1. G. E. Devlin, J. McKenna, A. D. May, and A. L. Schawlow,Appl. Opt. 1, 11 (1962).
2. 0. Svelto and M. DiDomenico, Jr., Appl. Opt. 2, 431 (1963).3. W. Sooy, R. Congleton, B. Dobratz and W. Ng, in Quantum
Electronics III, P. Grivet and N. Bloemberger, eds. (Colum-bia University Press, New York, 1964), p. 1103.
4. B. I. Davis and D. V. Keller, Appl. Phys. Letters 5, 80 (1964).5. R. Meyerowitz, Am. Mineralogist 40, 398 (1955).6. A. Johannsen, Manual of Petrographic Methods (McGraw-Hill,
New York, 1918).
614 APPLIED OPTICS / Vol. 4, No. 5 / May 1965
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Table II. Aqueous Solutions
Liquid Indexa Color Flash stability Viscosityb
H20 1.33 (1) Clear >14.5 LowAgNO3 1.48 (1) Water clear >14.5 LowBaBr2 1.44 (1) Yellow brown - LowBaCl2 1.40 (1) Water clear - LowBaBr2 + HgBr 1.63 (1) Brown - LowBiCl3 (+HCl) 1.69 (1) Water clear 11.5 LowCdCl2 1.48 (1) Water clear >14.5 LowHg(NO3)2 H20 + HgBr2c 1.54 (1) Water clear >14.5 LowHg(NO3)2 H20 + HgI2c 1.52 (1) Water clear >14.5 LowKBr 1.39 (1) Water clear - LowKBr + HgBr 1.60 (1) Yellow brown - LowKI + HgI 1.73 (1) Yellow - MediumKId 1.46 (1) Clear to yellow >14.5 LowLiCl 1.43 (1) Water clear - Low
1.4373 (2)Na2O SiO2 -XH20 (water glass) 1.40 (1) Water clear - MediumPbCl2 1.36 (I) Water clear - LowPb(NO3)2 1.40 (1) Water clear - LowPerchloric acid (71%) 1.40 (1) Water clear - LowSbCl3 (+HCl) 1.70 (1) Brown violet - LowSnCl2-2H20 1.60 (1) Water cleare >14.5 LowSnCl4 1.54 (1) Water clear >14.5 LowSrCl2 (+HCl) 1.41 (1) Water clear - Low
a The numbers (1) and (2) in this column indicate the source of the data: (1) experimental; (2) International Critical Tables.b The qualitative terms low and medium may be taken to imply viscosities comparable to those of water and glycerin respectively.rNot saturated solutions. Hg(NO3)2- HO forms eutectic mixtures with HgI2 and with HgBr2. Meyerowitz (ref. 5) states that a
water solution of mercuric nitrate and mercuric iodide has index 1.80. We found, however, that the high-index mixtures have a yellowcolor and high viscosity. The eutectic mixture of Hg(NO3)2 * H20 with HgBr2 is also very viscous.
d The liquid turned yellow when flashed, but became clear again when in contact with Sn metal.e No measurable absorption at 6328 A for a 42-cm path.
a The data in this column were obtained from experimental results.b The qualitative term may be taken to imply a viscosity comparable to that of water.
___''W~ii'¢i'4 Pentaron. Wah ........... D.C. FSanmP Kol.9 Ad.hn-.oC d
m eeting Sai e n aMay5 OSA Pittsburgh Sect. Mtg., Interference Filters and
Thin Optical Films by R. J. Pegis, St. John FisherCol., Rochester, N.Y.
5 OSA San Diego Sect. Mtg., Transformation of irbackgrounds to electrical noise, by George Car-michael of Astronautics
5-7 1965 Microwave Theory & Techniques Symp., JackTar Harrison Hotel, Clearwater J. E. Pippin,Sperry Microwave Electronics Co., Box 1828, Clear-water, Fla.
10-21 Lasers: Theory, Technology, and Applications,C. Kikuchi and G. W. Stroke, Co-chairmen Engi-neering Summer Conferences, W. Engr. Bldg., Univ.of Michigan, Ann Arbor, Michigan 48104
13 ARPA Lecture on numerical simulation of the earth'satmosphere by C. E. Leith, Rm. 5A 1070, The
Research Projects Agency, Wash. 25, D.C.13-14 Symp. on Signal Transmission & Processing, Colum-
bia Univ., New York City Omar Wing, Dept. Elec.Engr., Columbia Univ., New York City 10027
17-21 SPSE Ann. Conf., Sheraton Hotel, Cleveland, OhioE. R. Craig, SPSE, P. 0. Box 1609, Wash., D.C.
19 OSA Chicago Sect. Mtg., Discussion of some papersgiven at ICO, Japan, by Philip N. Slater of IITAnton Weigandt, 1434 W. Catalpa Ave., Chicago 40
24-29 3rd Congress IFIPS, Hilton Hotel, N.Y.C. Evan Her-bert, IFIPS Congress 65 Office, 345 E. 47th St.,New York, N.Y. 10017
24-June 1 IAMA Internatl. Conf. on Cloud Physics, Tokyo andSapporo, Japan H. Weickmann, Meteorological Br.,Signal Corps Engr. Labs., Ft. Monmouth, N.J.07701 or H. Hatakeyama, Japan MeteorologicalAgency, Otemachi Chiyoda-ku, Tokyo
24-June 4 Introduction to Optical Data Processing, E. Leithand C. J. Palermo, Co-chairmen, EngineeringSummer Conferences, W. Engr. Bldg., Univ. of Mich-igan, Ann Arbor, Michigan 48104
May 1965 / Vol. 4, No. 5 / APPLIED OPTICS 615
Report on the Conference on Undergraduate Research Programsin Optical Physics Held 8-12 June 1964 at Southwestern College,Memphis, Tennessee
Van Zandt Williams and J. H. Taylor
The initial activity of Task IV of Optics-An Action Pro-gram was the mailing of a letter to the chairman of each physicsdepartment that offers a Bachelor of Science degree in physics(these number about 750). This letter described the purpose ofTask IV and also inquired as to whether or not that departmentwas interested in starting research in optical physics. 284 replieswere received. Of this number, 188 expressed an interest in in-itiating a program of research in the field of optical physics orclosely related subjects. A later mailing to this group of 188 in-dicated that 106 of them were interested in attending a conference.Grants were obtained for the conference sufficient to invite 80 toattend. In addition to the 80, several others attended at theirown expense.
The majority of physics departments that expressed interestin Task IV are small, i.e., with staffs numbering 2, 3, 4, 5, and 6men. Data obtained during the conference yielded profile in-formation of the participants that will permit a base for progressmeasurement in the future. Most of the participants were inthe 30-45 year age bracket. The majority of the participantsoffer optics in either the junior or senior years, use Jenkins andWhite as a text, and devote about 30% of their course to geo-metrical optics, 60% to physical optics, and 10% to specialtopics.
This conference provided an opportunity to assemble and getto know some of those college and university professors who areinterested in optical physics research at the undergraduate level.The invited lecturers brought to the attention of conferenceparticipants some of the topics included in modern optical phys-ics and also many ideas for research projects.
The week's schedule, the invited lecturers, and their subjectsare shown in Table I on page 619.
The participants had an opportunity to make known to TaskIV leaders some of the ways in which they can be helped in thefield of optical physics. Although some of the suggestions wereprobably unrealistic, one desire very obviously runs throughout:that the Optical Society of America take a more active leadership.Participants also repeatedly requested opportunities to updatetheir education in modern optical physics.
V. Z. Williams is with the American Institute of Physics, 335 E.45th Street, New York, N.Y. 10017. J. H. Taylor is with South-western College, Memphis, Tennessee 38112, and is the leader ofTask IV (Undergraduate Research Programs).
This conference was conducted under grants from the NationalScience Foundation and the Office of Naval Research.
Prior to the conference, each participant was assigned to oneof eight groups. Each group was assigned a chairman and a vice-chairman. In addition to attending the invited lectures, each ofthe eight groups met several times during the conference to discussthe various topics it had been assigned. On the last day of the con-ference each group presented a report of its work. Groups I-Vwere concerned with research in optical physics; the topics as-signed them were:
Group I-Coherence effects, lasers, nonlinear optics, magneto-optics.
Group II-Spectroscopy-atomic, molecular, solid state; inter-ferometry; fluorescence and luminescence.
Group III-Radiometry and photometry; sources and de-tectors; far ultraviolet and infrared.
Group IV-Atmospheric and space optics; astronomy; mete-orology and space surveillance; aurora and airglow.
