o.k. deutschbein et c. c. pautrat- cw laser at room temperature using vitreous substances, - ieee...

8/13/2019 O.K. DEUTSCHBEIN et C. C. PAUTRAT - CW Laser at Room Temperature Using Vitreous Substances, - IEEE Journal

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CW LASER AT ROOM TEMPERATUREUSING VITREOUS SUBSTANCES

BY

0. K. DEUTSCHBEINAND

C C. PAUTRAT

Reprinted from IEEE JOURNAL OF QU NTUM ELECTRONICSVol. QE-4 Number 2 February 1968

Pp. 48-51Copyright 1968 and reprinted by permission of the copyright owner

PRINTED IN THE U S A


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X. A .10-3 cm l 2 05 22 P1/24 G 22G 9 2

2 03 22 1(15/24G 3 2_

4 G 7/2

2 K13/24G 5

laser transition X.1. 13 611 _

20

5 200

5800

6 300

6800

7

8000

8800

-10

17 000

24 000

49 000

CW Laser at Room Temperature UsingVitreous Substances

0. K. DEUTSCHBEIN AND C. C. PAUTRAT

bstractWith the aim of finding laser glasses operating in CWoom temperature, a comparative study was made of variousdymium-doped vitreous substances : silicate, borate, germanate,

sphate, and fluoride glasses.About 500 samples of phosphate glasses, doped with neodymium,e prepared and their optical properties studied.The phosphate glasses have three main advantages for laseron.

1) The fluorescence band is narrower than in silicate glasses andcan be as sharp as 153 A

2) The optical pumping is more efficient in phosphate glasses.3) The lifetime of the 4F312 level is about 300 tiS

The phosphate glasses have very interesting laser characteristics,in pulsed operation the threshold is 1 joule. Such a rod exhibitsicontinuous operation at room temperature pumped by a kryptonlamp; when excited in ac, the threshold is about 1 kW. It was

nd that the fluoride glasses have even more promising lasererties.

I N T R O D U T I O N

A MONG the different types of lasers, the solidfluorescent lasers have the advantage of h avinga relatively high concentration of active atoms,., 1.6 X 10 2 atoms/cm for Nd-doped YAG, and

5X 10 2 atoms/cm for LG55 glass of the Schott

Manuscript received June 9, 1967; revised October 21, 1967.s research was partially supported, at its first stage, by theS. Department of Army, through its European Research Office.spaper was presented at the Conference on Laser EngineeringApplications, Washington, D. C., June 6-9, 1967.The authors are with Centre National d Etudes des Telecom-

nications, 92 Issy-lea-Moulineaux, France.

Fig. 1. Energy levels of free Nda+ ion.

Company. For this reason powerful and compact lasersare feasible. From the various rare earths neodymiumis the most attractive dopant because it has severaladvantages as seen in Fig. 1).

1) It has a four-level energy scheme at room tempera-ture, because the final level of laser transition is

essentially unoccupied.2) It has several absorption bands, stronger than the

other trivalent rare earths, in the visible and near


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DEUTCHBEIN AND PAUTRAT: CW LASER USING VITREOUS SUBSTANCES

infrared region where powerful radiation sourcesare available.

3) The wavelength of em ission is in the 1-m icron region,wh ere fast detectors exist.

Neodym ium-doped silicate glasses are valuable laserm aterials for pow erful emission in pulse operation, ' butCW operation at room temperature seems practicallyimpossible' ' I because the thresh old pow er is relativelyhigh. This is due essentially to two reasons:

1) the emission bands are broad, and2) the different absorption bands have unequal in-

tensities.

G E N E R A L I T I E S

In laser action, the optical gain coefficient is inverselyproportional to the half width of the emission ban d, fora given pumping power, and so materials with sharperem ission bands w ill have low er threshold values. In solidfluorescent lasers, the emission spectra are not formedby several isolated lines as in gases but by a more orless com plex band w hose origin is the splitting up of theenergy levels of the atom s by the interionic Stark effectand the dynam ic interaction of phono ns. In glasses, theinherent fluctuations of com position produce inhom ogen-eously broadened ban ds which are m arkedly wider thanin crystals.

In order to find glasses having low er thresho lds thansilicate glasses, com parative study wa s m ade of v ariousneodym ium-doped vitreous substances: silicate, borate,gerraanate, phosphate, and fluoride. As far back as 1 938,Tomaschek and Deutschbein had observed that thefluorescence spectra of Eu3 +are sharper in phosphate andfluoride glasses than in silicate and bo rate glasses, andfor this reason a systematic study on neo dymium -dopedphosphate glasses has b een carried out.

P H O S P H A T E G L A S S E S

About 500 samples of phosphate glasses of variouscompositions have been prepared and their optical prop-erties studied.' The volume of each sample was about25 cre, sufficient for optical and physical me asurementsbut not enough for casting laser ingots of good opticalhom ogeneity. With the aim of determining the influenceof the different com ponents on the w idth of the em issionband, sim ple series of mo novalent, divalent, and trivalentcations in glasses of metaphosphate type have beenstudied, as well as mixed glasses with several cations.Furthermo re, the rates P20 5/metal oxide have been variedw ithin the lim its of the vitreous phase.

Fluorescence Spectra

The m easurem ents of the fluorescence spectrum in the1.05-micron region g ave the following results.

a) The fluorescent bandwidth is relatively small for

monovalent cations (about 190 A.), broader fordivalent cations, and even broader (up to 400 A.)for trivalent cations (A l).

b) For a given valency, heavier ions have smallerbandwidths than do lighter ones.

c) Mixed glasses of a m onovalent and a divalent catio[e.g., of the form ula CdK (P0 3 6] have a sm allerbandw idth (166 A) than glasses with only one cati[185 A for KPO5 and 232 A for Cd (P03)2].

d) Phosphate glasses of rather com plex compositionas described in several patents ' 8 , have band-

widths of more than 300 A .Consequently, it is possible to prepare Nd-doped glass

having m uch smaller fluorescent bandw idths (153 A) thsilicate glasses (240 A ) as com pared in Fig. 2.

bsorption Spectra

As seen from Fig. 1, the Nd' ion has several absorptibands in the visible and nea r infrared region, w hich agenerally of different intensities. In the case of silicglasses, the band at 5800 A is very strong, about threetimes more intense than the other bands in the nearinfrared region, Fig. 3. Th ese features are advantageo

for the excitation of thick rods because the pum ping ligis absorbed in different depths of the section of the robut on the other side the threshold values are higher.In phosphate glasses, the band s at 5800 A, 7500 A and8000 A have approxim ately the sam e intensities, and ththe pumping light is absorbed by the three bands inthe same w ay, and so low er thresholds can be obtaineespecially for thin rods (of 3 m m diameter).

ifetime

The lifetime of the 4F 3 2level is about 300 /is forneodymium concentrat ion of 0.25 gram-atom per l it

This value corresponds to a natural linewidth of 2 X 10' Laser ffect

In order to test the laser potentiality of phosphateglasses, one m elt of a volume of on e liter was preparof the composition Zn Li, (P0 3 6 which does not havea very small fluorescence bandw idth (190 A.), but whicis relatively easy to prepare. Cylindrical rods of 50 mlength and 3 nun diam eter have been cut from this ingow ith confocal ends coa ted with m ultidielectric m irroThese rods hav e been tested in pulsed operation at rootemperature by wrapping a silver foil around the rod

and a straight xenon flash lamp PEK X E 1-2. Underthese con ditions typical thresholds h ad been of 1 joulUnder the same condition very good Ca W0 4 rods ofthe same dimensions have thresholds of 0.5 joule, andcom m ercial silicate glass rods of the same dim ension4 joules.

It should be m entioned that our phosphate glass rohave poorer optical quality than the c om m ercial silicaglass rods, as can be seen from the mode patterns showin Fig. 4. This can be explained by the small quantityof our melt, as compared to the much higher volumesof com m ercial laser glasses. Semicontinuous laser opertion at room temperature has been obtained with anapparatus already used for crystals. ' The pum p reflectois an elliptical silver cylinder of 73.2 and 66.7 mm for


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1.02 1.04 1.06 1.08 1.10 +.12

1.0

0

eeeee cascorelative intensity)

Fig. 2. Fluorescence spectra of Nd-doped glasses.

Alk Optical Density

Ii

Nd doped

phosph at egass

Nd d oped

sili cate gass

Emssi on of a

ungsten iodi ne lamp

/

0.0

........ .......

0.5 6 7 .8 9

Fig. 3. Absorption spectra of Nd-doped glasses.

a) (b)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Fig. 4. Emission modes, (a) for a phosphate glass, (b) for a commercial silicate glass.


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D E U T C H B E I N A N D P A U T R AT : O W L A S E R U S IN G V I T R E O U S S U B S T A N C E S 1

A t , :7 : t , ` .* . :..tr 0

0

20 ms

Fig. 5. Variations of emission of glass rod versus time.

Optical ensity

1 12

Fig. 6. Fluoresence spectra of Nd-doped glasses.

0 9

0 7

0 6

05

0 4

0 3

0 2

0 1

00

Nd dop ed

amp

fl uoride ass Nd dope d

silicate assEmssi on

f a

tungsten iodine

0 5 6 7 0 8 9

Fig. 7. Absorption spectra of Nd-doped glasses.

major and minor axes. The rod is pumped by a kryptonarc lamp supplied with alternative current (50 Hz).The lamp and the rod are cooled by tap water.

W ith only one alternation- of electric current 5 0 Hz)supplied to the arc lamp, the threshold was 720 watts

and with the two alternations 1100 watts, as comparedwith 500 w atts for a Ca W04Nd rod of the same dimen-sions. (Fig. 5) shows the variation of the emission ofthe rod versus time. Rods can operate with 1 -kW pumpingpower w ithout deterioration.

F L U O R I D E G L A S S E S

It was found that the fluoride glasses have even morepromising properties. Their width of fluorescence bandis smaller Fig. 6), and their optical pumping efficiencyis still better, as seen on Fig. 7. The near infrared absorp-tion bands are the most intense, and correspond to the

region of maximum emission of tungsten-iodine lamps.On the contrary, their preparation is much more difficultand a special technology for the elaboration of laser ingots

and for manufacturing of rods is in progress. One litermelts have not yet been prepared, and the rods madeat this time of melts of a cubic inch volume only, areof very poor optical quality. In spite of this their laserthresholds 6 to 7 joules) are equivalent to those of th

best commercial silicate glasses for pulsed operation.When homogeneous rods will be available, real CW operation at room temperature should be possible.

R E F E R E N C E S1 For a review article on glass lasers see E. Snitzer, Appl. Optics

vol. 5, no. 10, pp. 1487-1499, 1966.21 C. G. Young, Appl. Phys. Lett. vol. 2, pp. 151-152, April 15,

1963.61 C. G. Young, Appl. Optics vol. 5 , pp. 993-997, June 1966.4/ R. Tomaschek and 0. Deutschbem, Glastech. Ber. vol. 16,

p. 155, 1938.6/ 0. Deutschbein, C. C. Pautrat, and I. MISvirchevsky, Rev

Phys. Appl. vol. 2, pp. 29-37, 1967.6/ J. R. van Wazer

Phosphorus and Its Compounds vol. 1

Chemistry. New York: Interscience, 1958.71 Grimm and Hoopert, U. S. Patent 1964-239, 1934.

Schott, French Patent 1302-063.61 0. K. Deutschbein, G. Grimouille, C. Pautrat, and G. Petit

Le Du, Rev. Phys. Appl. vol. 1, pp. 128-132, June 1966.

o.k. deutschbein et c. c. pautrat- cw laser at room temperature using vitreous substances, - ieee...

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