FLASHLAMP PUMPED COLOUR CENTRE LASERS IN LiF AND NaF

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FLASHLAMP PUMPED COLOUR CENTRE LASERS IN LiF AND NaF

J. Doualan

To cite this version:

J. Doualan. FLASHLAMP PUMPED COLOUR CENTRE LASERS IN LiF AND NaF. Journal de

Physique Colloques, 1987, 48 (C7), pp.C7-355-C7-357. �10.1051/jphyscol:1987785�. �jpa-00227089�

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JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au n012, T o m e 48, dkcembre 1987

FLASHLAMP PUMPED COLOUR CENTRE LASERS IN LiF AND NaF

J.L. DOUALAN

Laboratoire de Spectroscopic Atomique, CNRS LA-19,

ISMRa-Universite de Caen, Bd Marechal Juin, F-14032 Caen Cedex, France

+We describe pulsed flashlamp+pumped colour centres laser.+The laser emission of F, centres in LiFOH at 7 7 K , F, centres in NaF at RT and (F,IA centres in NaFLi at R.T.

INTRODUCTION

The only colour centre which have been the active elements of flashlamp pumped are F (111, FB(II) on one hand at 7 7 K ^[l]['] and F,

C3]

of F,

[*I

^centres

in LiF at

~ . 4

on the other hand. Flashlamp pumped lasers are interesting because of their simple realization and their low cost. Moreover the flashlamp has a very broad emission spectrum, which gives an ample choice or wavelength for the colour centre pumping. It is also possible to use a part of the flashlamp spectrum to rege- nerate the use ul centres. We describe here flaqhlamp pumped, tunable,+colour centre

f

lasers using F, centres in LiFOH at 7 7 K

[ = I ,

F, centres in NaF and (F,)A centres in NaFLi at R.T.

DESCRIPTION OF THE LASER

LiF and NaF rods are prepared by a Bridgman technique, they are 2 to 5cm in length and 0.3 or 0.4cm in diameter. The NaF rods are more difficult to realize than the LiF ones. The samples are irradiated at 200K by a beam of electrons from a van de Graaff accelerator (1.5MeV, 4.5pA/cmZ). The rod is along one of the focal axes of a cylindro elliptical mirror (fig.l). A xenon flashlamp (VOX R5 P2.5) from Verre et Quartz is along the second focal axis, it is surrounded by a pyrex jacket

Fig. 1 : Diagram of colour centre laser : CF cold finger, LR laser rod, F. flashlamp, CE cylindro-elliptical mirror, VC vacuum chamber M, gold coated mirror R=lrn,M, multidielectric plane output mirror, SW silicawindowat Brewster's angle, P.prism, LF Lyot filter.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987785

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C7-356 JOURNAL DE PHYSIQUE

to allow the circulation of an aqueous solution ofCr,.O,K, to cut the flashlamp light below 510 or 550nm. The pulse repetition frequency can be chosen between 0.5 and 15Hz, its length is between 10 and 9 0 ~ s FWHM. The laser optical cavity istermi- natedby-agoldcoated concavemirror with lm curvature radius at one end and by a multidielectric plane output mirror at the other end. It contains tuning devices

(prism, Lyot filter) and possibly losses devices (glass plates, neutral filters).

We have two types of laser. In the first one the active rod is cut at Brewster's angle, in the second one the.termina1 faces are perpendicular to the optical axis, the losses are more important but the gain is fully sufficient to compensate them, in return the initial adjustement of the laser is much easier.

LASER EMISSION FROM F; CENTRES IN L ~ F O H - AT 77K

['I , [ = I

The tuning range is 840 to 1050nm. Output energy measurements have been performed without any tuning element in the cavity and with a 15% transmitting output mirror. It is 2.4m.J for an electrical input energy of 12.55 (fig.2) and the threshold is about 25 electrical power. We investigate sample heating effects. The repetition rate is increased gradually from 0.5Hz to 15Hz, the length of pulses being 1 0 ~ s FWHM and the electrical energy 125. The fading appears for repetition rates higher than 12Hz because the sample cooling becomes inefficient.At llHz the fading is less than 5% after 160.000 pulses. We increase the length of the pump pulses from 1 0 ~ s to 9 0 ~ s FWHM AT 0.5Hz (fig.3). The threshold is the same for swi- tching~ on and off. The repetition rate and the length of pulses cannot be increased because the threshold is too high and the flashlamp power supply cannot deliver a higher average intensity.

The unexplained losses in the cavity vary from 10% to 20%. We will try to lower the threshold by decreasing the losses and to progress toward C.W. lamp pumped laser operation.

LASER EMISSION

3

;'l CENTRES IN NaF and (F;)A CENTRES IN NaF:Li AT ROOM TEMPERATURE The rods of NaF a rather poor optic quality, they contain bubbles and inclusions. The laser emission of F; centres in NaF is obtained at R.T and 77K.

At R.T the tuning range is between 890nm to 1070nm, the electrical energy threshold is 3.55 for pulses of 1 0 ~ s . The half life of F; centre is -6h

['I.

However we obser- ve a laser emission during 5 days because the flashlamp regenerates the F; centres

L81.

2hu

Fig. 2

I . . . . I - . -

. . .

- . . .

0 5 0 100 150 t Psk

Fig. 3

Laser pulse energy (E ) versus Time dependence of laser (a) and flashlamp input electrical energy (E.) II (b) pulses the input electrical energy is

805.

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But we 3DSerVe an irreversible bleaching of the sample because the electron centres (F;,F,,

...

⁾finally recombine with hole centres created during electron irradiation For (F') in NaF:Li (with 260ppm by weight of Li), the laser emission is also obtained at R.T. A

I ['

and 77K. At R.T, the tuning range is 920nm to 1150nm, the electrical energy threshold is 2J for 1 0 ~ s FWHM pump with a 2.5cm long rod. Contrarily to F, centres it was possible to increase the concentration of (F,) by a U.V irradiation at R.T (fig.6). The same mecanism convert F to (F ) in Khll Li[lo] and

KCll Na[ll]

.

^A

The half life in darkness at R.T of (F;)A is several weeks but the am- bient light destroys then in a few minutes. The flashlamp regenerates the (F') centre. However after 20000 shots at 0.5Hz with 85 electrical energy by puls:,Athe laser intensity decreases by a factor of 3, due to an irreversible destruction because the hole centres recombine with electron centres.

We will try to stabilize thecentresby additive colouration

\& .

^{We also}

intend flashlamp pumping for lasers using the (F;)* [13][14] and (F;)' [ ] centre in NaF as well as F;-like centres in NaCL OH- [16].

REFERENCES

rll

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f 2 j

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['I

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