Lasing Characteristics of Seventeen Visible-WavelengthDyes Using a Coaxial-Flashlamp-Pumped Laser

J. B. Marling, J. G. Hawley, E. M. Liston, and W. B. Grant

The performance and tuning characteristic of organic dye lasers were measured using commercially avail-able coaxial flashlamps as the pumping source. Tuning by a diffraction grating permitted continuous cov-erage of the 4200-7500-A region at up to 0.6% tuned output energy efficiency. Preliminary results with a600-J Marx-Bank coaxial laser at 4500 A show 1% efficiency and single-pass unsaturated gain greater than20 at 4500 A.

1. Introduction

Coaxial flashlamp-pumped dye lasers have been inuse for several years as a relatively simple and effi-cient tunable laser source. Although tuning rangesfor dyes pumped by linear flashlamps cover the visi-ble spectrum,' improved performance and tuningrange are anticipated from the electrically faster andoptically more efficient coaxial flashlamp. In addi-tion, the availability of several improved types of las-ing dyes2 now allows energetic dye laser output in allregions of the visible spectrum. The lasing proper-ties of seventeen dyes were compared to permit theoptimum selection of a dye for any wavelength in the4200-7500-A region.

11. Experimental Apparatus and Technique

The laser used commercially available coaxialflashlamps with an arc length of 15 cm and inner boreof 10 mm. 3 4 A 0.3-ptF coaxial capacitor permittedenergy storage up to 100 J although all performancemeasurements were taken with the capacitor chargedto 22 kV (73 J) or 24 kV (86 J). The stored electricalenergy was switched by a triggered spark gap and ap-plied to the lamp by means of a close-coupled, low-inductance coaxial geometry similar to commerciallyavailable units. The front window was fused silicawith 30-min wedge angle and an antireflection coat-ing on the air-exposed surface. The back windowformed the diverging element of a beam-expandingtelescope. This window was a 1-in.-diameter plano-concave lens with a 5-cm radius of curvature.5 The

When this work was done all authors were with Stanford Re-search Institute, Menlo Park, California 94025; J. B. Marling isnow with Lawrence Livermore Laboratory, Livermore, California94550.

Received 7 March 1974.

plane surface was in contact with the dye and theconcave surface was antireflection-coated. This lens,combined with a condensing element that had a 20-cm focal length and antireflection coating on bothsides, resulted in a simple and low-loss 2X beam-ex-panding telescope.

Tuning was achieved by using an 1800-1/mmBausch & Lomb diffraction grating in the first orderwith 5000 A blaze wavelength. The grating wasmounted in a polar rotator 6 adjustable to an anglethat was readable to within 5 sec of arc. The angularposition could be rapidly converted to wavelength.Peak grating efficiency was 80% and the output mir-ror was 70 cm from the grating, with 50% reflectivitybeing typical for most dyes.

The dye was circulated in polyethylene tubingusing a magnetically coupled stainless steel gear-pump.7 It was filtered through an on-line 0.6- or2.0-gm disk-type PVC filter in a PVC or stainlesssteel holder.8 Both pore sizes seemed equally effec-tive in this application.

The lamp and electrical driver yielded laser pulsesthat were typically 240 nsec full width at half-maxi-mum. Linewidths were measured using a visualspectrometer and found to be between 2 A and 20 Adepending on telescope adjustment. Without a tele-scope, linewidths were typically 1 A to 3 A, but grat-ing damage was avoided at the higher energies (above100 mJ) only by use of the 2X telescope. Repetitionrate was about once every 8 sec, limited by the timefor thermal gradients to subside. A triaxially config-ured lamp, with cooling fluid between the dye celland inside bore of the coaxial flashlamp, was oper-ated at up to 4 pps provided the dye and cooling fluidwere kept at the same temperature to within 0.2 C.Output of the triaxial configuration, with a 6-mmdye-cell bore, was about half that of the 10-mm-borestandard lamp. For all energy measurements a disk

October 1974 / Vol. 13, No. 10 / APPLIED OPTICS 2317

Table 1. Dyes Used in this Research, along with the Parameters Used for the Curves Depicted in Fig. 1

Flash-Output lampmirror Grating Flash- thresh-

Concentration reflec- effi- lamp oldDye Solvent used for Fig. 1 tivity ciencyb energy energy EarliestNo. Name (additive) (mole/liter) (%) (%) (J) (J) Supplier Ref.

1 Coumarin 120 Methanol 2 X 10-4 50 78 73 40 c k2 4,6-dimethyl-7-ethylamino Methanol 2 X 10-4 50 77 86 32 d I

coumarin3 Carbostyril 165 Methanol 2 X 10-4 77 86 76 c4 4-methyl-umbelliferone H 20 2 X 10-4 77 86 36 e m

(+ NaOH) (6 X 10-4)

5 Esculin monohydrate Methanol 4 X 10-4 38 73 86 30 d n(+ NaOH) (3 X 10-3)

6 Coumarin 102" Methanol 2 X 10-4 50 71 86 25 c k7 Coumarin 30 Methanol 2 X 10-4 50 65 73 34 c k8 Coumarin 6 Methanol 2 X 10-4 50 66 73 c k9 Disodium fluorescein \ Methanol 3 X 10-4 80 67 86 43 C 0

+2 J 10-410 4-amino-1,8-naphthalimide Methanol 1.5 X 10-4 80 70 73 43 f

+2f J 1.3 X 10-411 Rhodamine 6G, process 2" Methanol 5 X 10-6 30 72 73 15 9 m12 Kiton Red S Methanol 10-4 30 74 73 25 h, i p13 Cresyl violet acetate l Methanol 2 X 10-5 30 74 73 22 h, j q

+ R6G I 5 X 10-514 Cresyl violet nitrate ) Methanol 8 X 10-6 30 76 73 20 h, j q

+ R6G | 5 X 10-515 Carbazine with triethylamine Methanol 2 X 10-5 42 77 73 27 h, j

+ R6G | 2 X 10-416 Nile blue nitrate Methanol 7.5 X 10-4 65 78 86 38 h, j r

+ R6G f Methanol 5 X 10-517 Oxazine 1 perchlorate \ Methanol 8 X 10-6 78 78 86 60 c s

+ R6G J 10-4

* a Ammonyx-LO, Onyx Chemical Co., New York, N.Y.;40% of the lasting energy of methanol solutions.

b Bausch & Lomb, private communication.

5% commercial solution added to water solutions of these dyes yielded 20 to

Eastman Organic Chemicals, Eastman Kodak Co., Rochester, N.Y. 14650.d Aldrich Chemical Co., Alfred Bader Library of Rare Chemicals, 940 West St. Paul Avenue, Milwaukee, Wisc. 53233.Matheson, Coleman and Bell, East Rutherford, New Jersey.

f Aldrich Chemical Co., 940 West St. Paul Ave., Milwaukee, Wisc. 53233.a Pilot Chemicals Division, New England Nuclear Corp., 36 Pleasant Street, Watertown, Mass. 02172.

h Research samples provided by R. N. Steppel.i J. M. Drake, R. N. Steppel, D. Young and associates, to be published.i R. N. Steppel and associates, to be published.kRef. 2.R. Srinivasan, IEEE J. Quantum Electron. QE-5, 552 (1969).B. B. Snavely, 0. G. Peterson, and R. F. Reithel, Appl. Phys. Lett. 11, 275 (1967).P. 0. Sorokin, J. R. Lankard, V. L. Moruzzi, and E. C. Hammond, J. Chem. Phys. 48, 4726 (1968).

o P. 0. Sorokin and J. R. Lankard, IBM, J. 11, 148 (1967).P D. W. Gregg, M. R. Querry, J. B. Marling, S. J. Thomas, C. V. Dobler, N. J. Davies, and J. F. Belew, IEEE J. Quantum Electron

QE-6, 270 (1970).J. B. Marling, D. W. Gregg, and L. L. Wood, Appl. Phys. Lett. 17, 527 (1970).P. K. Runge, Opt. Commun. 4, 195 (1971).

* K. H. Drexhage, "Design of Laser Dyes," presented at the 7th International Quantum Electronics Conference, Montreal, Canada,8-11 May 1972.

calorimeters was used. It was accurate to 3%, by cal-ibrations traceable to NBS standards, according tothe manufacturer.

Ill. Experimental Results

The dyes tested are listed in Table I, along withthe operating conditions, output characteristics, sup-

plier, and references to earlier work. The solventused was reagent-grade methanol. Dyes 4, 5, 6, and11 were also tried in water. Using water + 1.5% Am-monyx-LO (see Table I, footnote a) as solvent, Cou-marin 102 and Rhodamine 6G yielded Y4 and '/ theoutput achievable with methanol, respectively. Forbest results, dye 2 was recrystallized before use.

2318 APPLIED OPTICS / Vol. 13, No. 10 / October 1974

LU

-J

zj

400

300

200

100

4000 4500 5000 5500 6000

WAVELENGTH -A

Others were used as supplied. The output energywas dependent on concentration and output-mirrortransmission. These two parameters were varied forsome, but not all, of the dyes, so that further im-provement could be obtained in some cases (seeTable I). Tuning curves for a given concentration ofeach dye are presented in Fig. 1.

The dyes tended to degrade in output energy fairlyrapidly; usually, a 1-liter solution was used, and theenergy decreased by 10% after the first five shots.The curves shown in Fig. 1 are smooth fits to the datataken during the first few shots when degradationwas limited to 10%. Further degradation proceedsmore slowly.10 The values shown for the curves areestimated to be accurate to at least 20% with re-spect to the parameters tabulated in Table I. Itmight also be pointed out that when a smaller borediameter is used, a higher dye concentration shouldbe used, since the flashlamp light should be absorbedin only one pass through the dye cell.

Optimum performance was obtained for some dyesby adding a second dye to the mixture which shiftedflashlamp energy into the usable absorption bands ofthe lasing dye by an absorption/fluorescence process.This second dye is referred to as a spectral shift-er." " 2 The addition of Rhodamine 6G to dye solu-tions intended for laser action beyond 6400 A (theoxazine dye family, for example) always resulted inimproved performance. For example, the addition of5 X 10-5 mole/liter of R6G to 10-6 mole/liter cresylviolet acetate permitted laser action from a single dyemixture tunable from 5650 A, although with laser en-ergy somewhat reduced from that in Fig. 1.

Two of the dyes, disodium fluorescein and 4-amino-1,8-naphthalimide, lased only upon the addi-tion of about 10-3 mole/liter 1,3,5,7-cyclooctate-traene (COT)13 and the bubbling of argon throughthe solution to remove the oxygen. Dye 2 was addedas a spectral shifter. The naphthalimide, dye 10, isdissolved first in dimethylformamide and then dilut-ed in methanol.

As has been noted by several authors,14"5 the tun-ing characteristics can be altered by changing theconcentration of the dye or the reflectivity of the out-put mirror. This effect arises from changes in the

II \ / A 1 775!I Fig. 1. Experimental tuning6500 7000 7500 curves for seventeen dyes, with pa-

rameters listed in Table I.

wavelength dependence of relative gain and loss inthe lasing medium-greater reflectivity increases theoutput at longer wavelength, as does higher concen-tration. For cresyl violet acetate or nitrate with 5 X10-5 mole/liter of R6G, the wavelength for peak out-put energy shifted from 6210 A to 6670 A as the con-centration of cresyl violet was varied between 10-6and 8 X 10-5 mole/liter (see also Ref. 12). The de-pendence of the peak output wavelength of cresyl vi-olet acetate (CVA) and cresyl violet nitrate (CVN)was very similar, but the cresyl violet nitrate tendedto be more energetic by as much as 50%.

The laser output energy Eo was found to vary inaccordance with the usual formula:

E = K (E - Et) (1)

where K is the slope efficiency of the dye, Ei is theelectrical input energy, and Et is the electrical inputat the threshold for lasing. K can be obtained fromthe values given in Table I and Fig. 1. Saturation ef-fects that would cause deviations from Eq. (1) werefound to be insignificant at energy levels measuredhere.

Preliminary tests have been performed on a 600-Jcoaxial flashlamp pulsed by a Marx-Bank electricaldriver system. The electrical configuration is similarto that reported by Ewanizky and Wright,16 with two1.0-,uF, 25-kV capacitors being used for energy stor-age. The unit was commercially supplied3 and yield-ed extremely high efficiency as a pump for organicdye lasers. Figure 2 shows an energy performancecurve for broadband output from 4,6-dimethyl-7-eth-ylaminocoumarin at about 4500 A. It can be seenthat 5 J were obtained at 22 kV (484 J) input for 1%electrical energy conversion. When used as an am-plifier, small-signal single-pass gain was 16 with thesame coumarin at 4510 A, and 22 with coumarin 102at 4900 A at 20-kV charging voltage. Such a systemwill have great potential as an energetic amplifier ofnarrow-line dye lasers (see also Refs. 17 and 18).

In summary, this paper presents the lasing charac-teristics of seventeen organic dyes spanning the visi-ble spectrum, pumped by commercially availablecoaxial flashlamps. Figure 1 and Table I permit se-lection of the optimum dye for a given desired laser

October 1974 / Vol. 13, No. 10 / APPLIED OPTICS 2319

U,LU

-J

0

zLULU

U,

-S

In

3

2

00 100 200 300 400

ELECTRICAL INPUT ENERGY (JOULES)

References

500

Fig. 2. Laser output vs electrical input energy for the 600-JMarx-Bank coaxial laser, using 4,6-dimethyl, 7-ethylamino-cou-

marin at 4500 A, and mirrors with 99% and 30% reflectivity.

output wavelength, and also provide a standard forevaluation of other flashlamp-pumped dye laser sys-tems. Coaxial flashlamps are also indicated as effec-tive dye laser amplifiers.

The authors thank John Edighoffer for help intesting the laser performance of some of the dyes andR. N. Steppel for providing some of the dyes used inthe tests.

1. J. B. Marling, L. L. Wood, and D. W. Gregg, IEEE J. QuantumElectron. QE-7,498 (1971).

2. S. A. Tuccio, K. H. Drexhage, and G. A. Reynolds, Opt. Com-mun. 7, 248 (1973).

3. Manufacturer, Phase-R Corp., P.O. Box G-2, New Durham,N.H. 03855.

4. Manufacturer, Candela Corp., 14 Charles St., NeedhamHeights, Mass. 02194.

5. Manufacturers, Coherent Radiation, 3210 Porter Drive, PaloAlto, Calif. 94304, and Laser Energy, 320 N. Washington St.,Rochester, N.Y. 14625.

6. Manufacturer, Ardel Kinematic Corp., 84-49 164th Street, Ja-maica, N.Y. 11432.

7. Manufacturer, Micropump Corp., 1021 Shary Court, Concord,Calif. 94520.

8. Manufacturer, Millipore Filter Corp., Ashby Road, Bedford,Mass. 01730.

9. Manufacturer, Scientech, Inc., 1727-14th Street, Boulder,Colo. 80302.

10. R. N. Steppel and associates, to be published.11. C. E. Moeller, C. M. Verber, and A. H. Adelman, Appl. Phys.

Lett. 18, 278 (1971).12. W. Schmidt, W. Appt, and N. Wittekindt, Z. Naturforsch. 27,

37 (1972).13. R. Pappalardo, H. Samelson, and A. Lempicki, Appl. Phys.

Lett. 16, 267 (1970).14. B. I. Stepanov and A. N. Rubinov, Sov. Phys.- Usp. 11, 304

(1968); original in Usp. Fiz. Nauk, 95,44 (1968).15. S. L. Chin and G. Bedard, Opt. Commun. 3,299 (1971).16. T. F. Ewanizky and R. H. Wright, Jr., Appl. Opt. 12, 120

(1973).17. Y. H. Meyer and P. Flamant, Opt. Commun. 9, 227 (1973).18. G. Magyar and H.-J Schneider-Muntau, Appl. Phys. Lett. 20,

406 (1972).

Vance Hoffman of Varian's Applied Physics Laboratory

2320 APPLIED OPTICS / Vol. 13, No. 10 / October 1974