GENERATION OF CW COHERENT RADIATION IN THE NEAR U.V.

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GENERATION OF CW COHERENT RADIATION IN THE NEAR U.V.

B. Couillaud

To cite this version:

B. Couillaud. GENERATION OF CW COHERENT RADIATION IN THE NEAR U.V.. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-115-C8-125. �10.1051/jphyscol:1981814�. �jpa-00221709�

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JOURNAL DE PHYSIQUE

CoZ Zoque C8, suppZe'ment au nO1 2, Tome 42, de'cembre 1981 page C8-115

GENERATION OF CN COHERENT RADIATION I N THE NEAR U a V ,

B. Couillaud

Centre de Physique MoMcuZaire Gptique e t Hertzienne, Universitg de Bordeaux I, 33405 TaZence, France and Department of Physics, Stanford University, Stanford CA. 94305, U.S.A.

Abstract.- Following the demonstration of the second harmonic generation in m w ~ r a n k e n , numerous non linear methods have been used associated with pulsed lasers to generate U.V. radiation in the range 200-300 nm. The corres- ponding sources are now widely used in the spectroscopic studies, but their limited monochromaticity makes them of poor interest in high resolution spectroscopy. There is for long time now a pronounced interest to have at one's disposal a radiation source which provides extremely narrow bandwith, convenient intensity and tunability, but it is only recently, with the advent of powerful CW dye lasers, and the development of enhancement technics that the first reliable sources have been demonstrated, and the first high resolution spectroscopy studies performed. We give in this paper a brief and then certainly incomplete review of the different progress that have been done in the generation of higly monochromatic U.V. radiation, as well as a review of the different experiments that have been performed in this frequency domain.

We mainly deal with the use of non linear crystals in second harmonic generation and sum frequency mixing schemes, leaving for the conclusion the listing of the other possibilities of coherent CW U.V. generation that could be used in a near future.

Introduction.- The advent in the early seventies of the new methods of non linear spectroscopy providing Doppler free measurements in gases has greatly increased the need of highly monochromatic, powerful and tunable sources. Due to their outstandin?

features, the single mode CW dye lasers have become the favourite devices. However the spectral range covered by these devices is limited between 400 and 960 nm by the presently available dyes. The advent of new lasing color centers has recently filled the gap which was existing in the deep red, in such a way that the range between400nm and 3.6u is now completely covered by direct lasing effect. The high frequency domain

(namely wavelengths shorter than 400 nm), at the excepeion of a very limited number of discrete lasing transitions cannot be presently directly covered,

The generation of coherent U.V. radiation by interaction of a laser radiation with a material presenting a non linear response is known for nearly 20 years now and has been extensively used with pulsed lasers where high fundamental power densities are available for an efficient wavelength conversion. The low power densities available from CW sources permits at the present time only to use those non linear efcects presenting the highest efficiency in order to get a convenient 1T.V. power. CW U.V.

radiation can be generated utilizing an apropriate non linear medium either by frequency doubling the output of a CW laser operating in the visible or by mixing the output of two CId laser sources. These technics are referred to as second harmonic generation (SHG) and sum frequency mixing (SFM).

Efficient mixing of radiation at wl and w2 to produce radiation at the sum frequency w3 given by w3=w2+wl is only possible when the phase matching condition

~heren(w~),n(~~)~l(w~)arerespectively the refractive index of the non linear medium for the frequencies wl, w2, w3. This condition reduces n(w)=n(2w) in the second

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981814

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JOURNAL DE PHYSIQUE

harmonic generation case.

All optical materials have sufficient spectral dispersion, to require that special techics be used to satisfy the conditions of the preceeding equation. Two technics using birefringent materials are commonly used, The first technic, critical or angle phase matching, utilizes uniaxial crystals and propagation of the fundamental waves at some angle to the optical axis ; the polarization of the different beams is choosen in such a way that the birefringence overcomes the spectral dispersion. The second, non critical or temperature phase matching, allows propagation of the conveniently polarized beams, perpendicularly to the optical axis of a uniaxial crystal and utilizes the temperature dispersion of the birefringence to overcome the spectral dispersion. Phase matching schemes are utilized in which the two interacting fundamental waves propagating in the bifringent medium have either parallel or othogonal polarizations. These two schemes are respectively referred as type 1 and 2 phase matching.

1 . Second harmonic generation (SHG).- The production of U.V. radiation for high

resolution spectroscopy imposes a high monochromaticity ot the fundamental radiations used in the non linear generation process. The single mode CW laser are the only sources fulfilling this requirement. Their output power is generally low (few watts at most) in such a way that great care has to be taken in the design of the doubling or summing system, in order to get the maximum efficiency. Among the two possible technics of phase matching described above, the angle phase matching has to be avoided when possible, since the double refraction effect, which occurs for directions of propagation other than parallel or perpendicular to the optical axis, greatly reduces the overlap of the interacting beams and then the efficiency of the system. Only some of the non linear crystals presently available, present a large 90" phasematched temperature tuning range, over a significaht range of fundamental wavelengths. In terms of highly monochromatic U.V. generation, the situation can be summarized as follows. Among the existing non linear crystals, ADP (Ammonium dihydrogen phosphate) and ADA (Ammonium dihydrogen arsenate) are of particular interest because of their large 90" phase matched temperature tuning range. These two crystals will normally cover in SHG the whole rangebetween 245 nm to 310 nm at the exception of a small domain around 280 nm. The fundamental radiation can khen be provided either by gaz lasers (krypton or Argon) for some particular frequencies where high U.V. power is required or by single mode CW dye lasers operating between 490 nm and 620 nm. Other crystals like KDP, RDP, RDA can also he used but their narrow temperature tuning range restricts greatly their interest.

This brief enumeration would not be complete if we didlnot include UREA") and the potassium pentaborate KB~(~). Although they cannot be used in 90" temperature phase matching, because in particular of the very low sensitivity of their refractive index with temperature, they nevertheless present some interesting characterics which make them attractive. Although not commercially available at present, recent studies have shown that UREA has properties which point to its future widespread application as a non linear medium. UREA has a non linear coefficient that is larger than those of the KDP isomorphs and provides efficient SHG for fundamental wavelength down to 480 nm. The KB5 crystal in angle matched SHG is the only crystal available to generate U.V. under 240 nm, and has then to be considered, in despite of its small non linear coefficient, which, associated to the double refraction effect, provides low conversion efficiencies.

The output power to be expected in SHG, when using Gaussian input beams can be calculated by use of the theory developed by G.D. BOYD and D.A. KLEIMFIAN(~~.

Under small signal conditions, the output power P2, is proportional to the square of the input power Po. The conversion efficiency P2,/P$ depends on many parameters, including the direction of propagation, the focusing and linewidth of the input beam, the non linear coefficient of the medium and the length of the non linear crystal.

For a 90° phase matching, under the optimal focusing conditions established by BOY0 and KLEIMMAN, the conversion efficiency is about I O - ~ W - ~ for a 25 mm long ADP or ADA crystal. This value falls rapidly when the incidence angle is decreased to reach for exemple 5. ~ o - ~ w I at =60°.

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It is obvious from the preceeding considerations on the conversion efficiency, and from the value of the avalaible output power of the CV lasers, that the U.V.

power generated by SHG is expected to be low, typically in the mW range. Tunable CW U.V. radiations have been produced by frequency doubling the output of a gas laser

dye laser, using non linear crystals located external to the laser cavity

, in configuration of angle or temperature tuning. This simple approach provides powers under the mW and a tunable U.V. source of sufficient intensity for a certain number of spectroscopic studies. However in order to obtain higher powers, an elegant solution consists to locate the non linear crystal within a laser cavity where the losses have been reduced as much as possible, in order to raise the effec- tive intracavity power of the fundamental wave This a roach first tested with argon ion lasers and linear CW dye lasers(6-7-6-9-10-1y~ has found its full power with the advent of ring lasers.

The first intracavity doubling experiment, using a CW ring dye laser, was performed in 1976(12). From this date till 1980 several studies of the generation of radiation at wavelengths in the vicinity of 300 nm, by intracavity doubling a Rhoda- mine 6G ring dye laser, have been reportedC13-la). However the problems encountered in the different set up were such that practically no spectroscopic studies were completed. The difficulties in intracavity doubling are mainly caused by the inser- tion losses afforded by the non linear crystals. These crystals are soft, and an optical polish is difficult to obtain ; morever they are hygroscopic in such a way that they have to be used in a water vapor free environment. The solution comonly used to get rid of these problems, is to protect the crystal against environmental conditions by sealing it in a cell filled with an index matching fluid, which also serves as a good heat conductor and minimizes the reflection losses at the crystal surfaces. Typical output powers of a few tens of milliwatts have been achieved with this method, but only during few minutes, the U.V. degrading with time. The nominal U.V. power could be restored in these experiments by slightly translating the non linear crystal in order to change the beam path in the active medium. The matching index fluid used during the last few years in the different intracavity doubling attempts, has been recently prooved to be responsible of power instability in the U.V. generation. Whether these instabilities are related to the photochemistry of the matching index fluid under U.V. excitation, or to more complex reactions, is not presently known, but the recent experiments performed with no liquid coatings on the optical surfaces of the crystal, have clearly demonstrated that the U.V. degradation was directly attributable to the presence of this fluid.

An example of an intracavity frequency-doubled Rhodamine 6G ring dye laser system, is the system recently developed in ST AN FORD(^^) and shown schematically in fig.1.

FUNDAMENTAL

BLOCKING FILTER CORNING

VERTEX MOUNTED BREWSTER PLATE

P U M P M I R R O R U V y

Q

"

C O L L

' .." "" . ^r^.^,^.

OUTPUT

COMPENSATION

P U M P M I R R O R

COATED B E A M SPLITTER

Fig.]. Arrangement of the ring laser cavity for SHG

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C8-118 JOURNAL DE PHYSIQUE

In this experi-ment a Coherent Radiation model 699-21 ring cavity CW dye laser has been employed to produce single-frequency ultraviolet powers of more than 10 mW near 296 nm, stable over many hours, and continuously scannable over 60 GHz.

A water based Rhodamine 6G solution dye permitted pumping by all visible lines (19W) of an argon ion laser. The solvent was a mixture of 75% Ammonyx LO detergent and 25% ethylene glycol, cooled near 10 centigrades to obtain the proper viscosity for the dye jet. The pressure at the jet was increased from 40 to 70 ~ s i . Up to 2.5W of visible single mode output power could be generated in this way with the standard output coupler of 15% transmission.

For ultraviolet operation, the light direction inside the dye laser was reversed by changing the polarity of the magnet for the Faraday rotator of the optical diode. The standard output coupler was replaced by a highly reflective mirror transmitting only about 50 mW of visible radiation, and a coated beamsplitter was used to send most of this light into the external cavity and into the reference diode used for electronic frequency stabilization. In order to extract the second harmonic output, the standard upper folding mirror was replaced by a quartz sub- strate, coated to give 70% transmission in the U.V. while maintaining high reflec- tivity at the fundamental wavelength.

A 18 mm long crystal of ADA with optical surfaces at Brewster angle served as the frequency doubler. It was inserted into the dye laser cavity at the auxiliary beam waist between the upper folding mirror and the "high reflector", replacing the astigmatic compensation rhomb. The crystal, together with a small electric heater, was enclosed in a protective housing with openings for the beam, and a continuous flow of dry nitrogen gas helped to protect the delicate crystal surfaces.

Using this source, the technique of saturation spectroscopy lias been extended into the ultraviolet. The CW dye laser with internal £re uency doubler has been used to record Doppler free spectra of the Hg I transitions 6'PO-b3Di at 296.728 nm and 6 3 ~ ~ - 6 1 ~ ~ at 296.759 nm. The isotope shifts for naturally abundant mercury could be mesured to within a few MHz.

A spectrum of the 6 3 ~ o - 6 3 ~ 1 line is shown in fig.2. The saturation intensity is low, since the metastable level can be accumulatively depleted by optical pumping.

Very strong signals with obvious power broadening were recorded with less than 0.5 mw/mm2. At lower intensities, the resolution approached the natural line width of 27 MHz (FWHM in the U.V.). The closely spaced lines of the even isotopes and the hyperfine components of the odd isotopes appear completely resolved. The intensities of the even components appear distorted in the upper trace because the absorption in the cell exceeded 90%. An expanded spectrum recorded at reduced discharge intensity is shown below.

Other intracavity doubling experiments, presenting interesting features, have been recently published. Among those, we would like to point out the generation of continuous wave, tunable U.V. radiation (250-260 nm) by intracavity doubling a coumarin 515 ring dye laser(19). A cooled (200-280 K) ADP crystal with end faces cut at Brewster angle, was placed inside the laser ring cavity. U.V. powers at 254 nm of 120 VW and 60 UW were achieved with the laser operating multimode (bandwidthz20GHz) and single mode (bandwidthdOM~z) respectively. The capabilities of the laser system are illustrated in fig.3 where the sub-Doppler fluorescence excitation spectrum of natural mercury has been investigated in an atomic beam for the 253.7 nm 3 ~ 1 - 1 ~ 0 transition.

While high output power on the second harmonic can be obtained by using the high density of energy existing inside a laser cavity, the introduction of a new birefringent and lossy element in the resonator makes the system difficult to handle and a certain skill is required to use it. Moreover, the design of the commercial systems make difficult the introduction in the laser cavity of a second laser beam for SFM purposes. Finally, the requirements on the focussing in the crystal and in the amplifier medium remove some flexibility to the system.

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I I I I I

- 2 - 1 0 l G H z

ULTRAVIOLET FREQUENCY DETUNING

Fig.2. Ultraviolet Hg line 6 3 ~ 0 - 6 3 ~ 1 at 296.72 nm, recorded by CGJ saturated absorption spectroscopy. The central portion with the closely spaced even isoto ic components, has been recorded below at reduced intensity.

yfrom B. COUILLAUD et a1 (18) 1

Fig.3. Single mode continuous scans over selected hyperfine components of

3 ~ y - 1 ~ 0 tqansition around 253 nm.

from C.R. WBSTER et al. (1911

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C8-120 JOURNAL DE PHYSIQUE

In order to overcome these inconvenients a method using an external ring enhancement cavity has recently been proposed(21). The use of enhancement cavi ies to increase the U.V. power has been proposed as early as 66 by ASHKII et al.(~'), but has very scarcely been used since. The system that has been developped at STANFORD by B. COUILLAUD, L. BLOOMFIELD, Ph. DABKIEWICZ and T.W. HANSCH (to be published) is shown in fig.4.

BLOCKING FILTER

Fig.4. Experimental set up for SHG in an external ring cavity.

The output of a coherent ring laser CR 699-21 working single mode and frequency stabilized on an external reference cavity was fed into an external ring cavity, image of the 699 cavity except for the foldingmirror, which borrowed from a 599 model, had a radius of curvature of 75 m. In order to enhance the incoming energy, the externl ring cavity was locked on the laser frequency using the electronic control, brewster plate and PZT translator of a CR 599 dye laser. In order to lock at the top of the fringe, the scheme recently proposed by two of us was used(2O). In this method, the laser beam is reflected off the passive cavity. Because of the presence of internal polarizing elements, any detuning from resonance makes the reflected beam ellipti- cally polarized. A simple polarization analyzer produces the error-signal for feedback locking. Using a 18 m long ADA crystal and an input power of 1W, 8mW of U.V. were available after the blocking filter and a collimating lens. The linewidth was of the order of IMHz and the scan range I5GHz.

Using this source, the technique of intermodulated fluorescence has been extended in the ultraviolet. Doppler free spectra of the ~ ~ s transition of - 5 ~ ~ 3 ~ e and 4 ~ e have been recorded around 294.5 nm. In order to get rid of the velocity changing

collisions which smear out the signal, an electron gun was used to excite the helium atoms at a pressure of 10 to 20 mT. A typical spectrum, with relevant cross overs is shown in fig.5.

In conclusion, a CW ring laser with intracavity frequency doubler or asso ciated to an enhancement cavity can produce a stable, highly monochromatic tunable radiation at the second harmonic frequency, Such sources extend the range of CW Doppler free spectroscopy into the important ultraviolet spectral region as has been demonstrated in the different studies of ultraviolet transitions in atomic I!e andFg The crystals now available do not permit however to cover contin~lously the whole domain between 200 and 300 nm. The complement to the range cpvered by SHG can nevertheless be obtained by sum frequency mixing of the dye laser and a fixed frequency ion laser.

2. L u ~ E q ~ n c e x i n g (S.F.M.).- ultraviolet radiation can be generated by sum frequency mixing in non linear materials, but, although straight forward, this technique has not found as wide application as harmonic generation. The requirement of two laser sources explains certainly partly this situation, as well as the troublesome problems of focusing and aligning of the two fundamental beams. It turns out however that the sum frequency mixins has certain advantages over second harmonic generation. Preminent among these advantages are a broader o~~tput tuning range

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and, in many cases, higher oi~tptit power.

UV FREQUENCY DETUNING (GHz)

Fig.5. Doppler free spectrum of the 2 S-5 P transition in He.

We have already given the condition which has to be fulfilled in SFK, in order to assure the maximum U.V. power. This condition known as the "phase matching condition" is given by :

It can be achieved in non linear crystals with a proper choice of input beam polarizations, direction of propagation and crystal temperature. The two different arrangements of the respective polarizations of the fundamental beams already mentioned and referred as type 1 and type 2 are commonly used. The non linear crystals available for SFM are the same than those already mentioned for SHG ; the low CW fundamental powers available, impose for the reasons already developed, the 90' temperature phase matching every time it is possible. ADA and ADP are therefore, the more appropriate non linear crystals for generation of highly monochromatic U.V. by SFM. To illustrate some of the advantages of SFM over SHG, the set of sum wavelengths h g that can be generated for different temperatures of an ADP crystal used in a 90' type I phase matching scheme, has been plotted versus the two corres- ponding fundamental wavelengths X1 and A 2 in fig.6. The apex of the different curves

(where h l and A2 are degenerate) corresponds to the SHG. Since the Curie point in ADP limits at -126°C the lower temperature at which the crystal can be used, the generation of U.V. in SHG is limited at roughly 245 nm. Because of their concavity, turned toward the highest frequencies, it is readily seen from the curves that the use of SFM schemes extends toward the high frequencies, the range of U.V. that can be produced in SHG. Another interesting feature of SFM is that the temperature tuning affords some flexibility in the input wavelength combinations A 1 and X p that may be used to generate a particular wavelength. Finally, in certain instances, radiations at wavelengths that can be generated by frequency doubling, can be more efficiently generated by mixing, resulting in higher output powers. This is possible because the use of SFM may permit utilization of more eqficient lasers, or the use of more efficient non linear mediums, or a more favorable phase matching condition.

In terms of sub Doppler spectroscopy, and then of single mode lasers,

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C8-122 JOURNAL DE PHYSIQUE

r a d i a t i o n s t u n a b l e a c c r o s s a broad range of U.V. w a v e l e n g t h s , can be simply gene- r a t e d by mixing t h e o u t p u t o f a commercial CTJ dye l a s e r w i t h v a r i o u s o u t p u t l i n e s from a k r y p t o n o r a r g o n i o n l a s e r .

F i g . 6 . 90' phase matched sum f r e q u e n c y g e n e r a t i o n i n ADP.

w

- -

3 3 4 . 5 ^351.1^363.8_-413.1 457.9 r----*

-

476.5 4 8 8 . 0

"3

w

-

^514.5

0 z 5 3 0 . 9

u a c---* 568.2

0

-

^647.1

Z 676.4

Z

3 752.5

C

R h 6 G

-

^799;.3 ¹⁰⁹⁰

SECOND HARMONlC CONVERSION

I I I I I I , I I I I , I I I I I I

2 5 0 3 0 0 350

S U M FREQUENCY WAVELENGTH (nm)

~ i g . 7 . Tuning r a n g e s o b t a i n e d by mixing t h e o u t p u t of a Rhodamine 6G l a s e r w i t h s e l e c t e d i o n l a s e r l i n e s

/ f r o m F.B. DUNNING 6 2 3 ) 1

The f i g . 7 . shows t h e sum f r e q u e n c y wavelength t u n i n g r a n g e t h a t can be

o b t a i n e d by mixing t h e o u t p u t of a Rhodamine 6G dye l a s e r w i t h s e l e c t e d o u t p u t l i n e s from a k r y p t o n o r argon i o n l a s e r . The r e s u l t a n t o u t p u t t u n i n g r a n g e , e x t e n d i n g

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from 211 to 406 nm, can be compared in this figure with the range that can be obtained by directly doubling the output of the dye laser.

Although frequency mixing seems quite promising to generate highly monochromatic U.V. in the range 200-300 nm, this technique has not been yet really used in high resolution spectroscopy. Till now people has been more concerned with the

technology of the systems than with applications. Among the different results that have been published in the past, we must mention the generation of continuously tunable U .V. between 257 and 357 nm by S. BLIT and coworkers (24) (25) . This radiation was obtained by mixing the output of a Rhodamine 6G laser with selected output lines from an ion laser. Several non linear crystals have been used in an angle matching configuration to cover the whole range : ADP, ADA, KDP and RDP. The output power of the U.V. generated radiation was typically of the order of few tens of microwatts.

We have already mentioned that even for the wavelength in the SHG range, sum frequency mixing, because of the high output power available.in several of the lines of ions lasers is expected to yield higher U.V. power. This has been recently demonstrated(2b) in studies of the production of CIvT radiation in the vicinity of 247.5 nm, which can be generated, in a 90" phase matched temperature-tuned ADP crystal, both by mixing the 413.1 nm output of a krypton ion laser with radiation from a Rhodamine 6G laser and by frequency doubling the output of a coumarin 480 laser(26). The use of SFM yielded output powers of =ImlJ an order of magnitude greater than those obtained by SHG. Moreover the phase matching temperature required for SPM was close to -60°C, about 4 0 " ~ above the temperature for SHG, making the system more easy to handle.

A certain number of attempts have been made to generate efficiently U.V. radiation by 90" temperature matching SFM in ADP, at wavelengths below those attai- nable directly by SHG in this material. For example, radiation at 243.5 nm has been

generated by mixing the 406.7 nm line of a krypton ion laser with 606.6 nm radiation from a Rhodamine 6G laser(27). Another interesting combination has been used to generate highly monochromatic U.V. radiation at the same wavelength (twice Lyman a) in order to perform sub Doppler two photon spectroscopy on the 1'3-2s transition in hydrogen(28). This radiation was produced by mixing the 413.1 nm line of a single mode frequency locked krypton ion laser, with 590.6 nm radiation from a Rhodamine 6r;

single mode, frequency stabilized ring laser. With 600 mW output power for the ion laser and IW from the dye laser, as much as 0.7mW was generated with a linewidth under 2MHz and a scannable range of 30GHz. In order to obtain a reasonable intensity for the two photon experiment, an enhancement cavity was used to provide intracavity power higher than 5mW in each direction. This experiment however has not yet been completed, because of a degradation with time of the U.V. power, whose mecha- nism is not completely understood now.

In order to generate a coherent U.V. radiation with enough power to perform non linear spectroscopy experiments in helium, a scheme using an external ring cavity to enhance one of the two fundamental frequencies has very recently been proposed and demonstrated by B. COUILLAUD, L. BLOOMFIELD, Ph. DABKIEWICZ and

T.W. HANSCH (to be published). The goal was to generate a highly monochromatic radiation at 272.4 nm and 269.7 nm in order to study respectively the 2 3 ~ - 8 3 ~ and Z3s-g3p transition in atomic helium. A Schematic representation of the source is given in fig.8. It is basically the set up already depicted in fig.4. for SHG, where the ADA is changed for an ADP crystal, while the output of a single mode frequency stabilized ion argon laser at 488 nm is focussed into the non linear crystal through a dichroic mirror. The respective polarizations of the fundamental beams as well as the cut of the ADP crystal were such that a type I 90' phase matching condition was fullfilled. With a circulating dye laser power of 12 watts in the enhancement cavity, and 3.5 watts from the argon laser, more than 4 mwatts of U.V. were disponible in a bandwith of the order of lMHz, after passage through the blocking filter and a collimating lens. Work is now in progress to complete the helium studies.

To close this, certainly, very uncomplete list of results, let us mention that radiation tunable from 211 nm to 215 nm, a wavelength region not directly accessible by SHG, has been generated by mixing, in an angle tuned KB5 cr*stal, the output of a Rhodamine 6G laser with the 334.5 nm line of an argon laser(29).

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BLOCKING FILTER

UV

SINGLE MODE

. .

F i g . 8 . E x p e r i m e n t a l s e t up f o r SFM u s i n g an e x t e r n a l r i n g c a v i t y t o enhance one of t h e f u n d a m e n t a l f r e q u e n c i e s .

Conclusion.- I n summary, a s i t i s e v i d e n t from t h e f o r e g o i n g d i s c u s s i o n , c o h e r e n t CW r a d i a t i o n c a n now b e g e n e r a t e d a c c r o s s a n e x t e n d e d r a n g e of U.V. w a v e l e n g t h s , by u s e o f SHG and SFM t e c h n i c s . I t r e m a i n s n e v e r t h e l e s s t h a t t h e o u t p u t powers g e n e r a t e d i n t h e U.V., even a f t e r t h e a d v e n t of t h e p o w e r f u l r i n g dye l a s e r s , a r e low ( t y p i c a l l y o f t h e o r d e r o f t e n m i l l i w a t t s ) . Moreover even t h e more t r a n s p a r e n t c r y s t a l s p r e s e n t a s i g n i f i c a n t a b s o r p t i o n a r o u n d 200 nm which l i m i t s t h e t e c h n i c s o f SEG and SFM around t h i s v a l u e . As a m a t t e r of f a c t , c o h e r e n t CW g e n e r a t i o n i s o n l y b e g i n i n g t o b e s e r i o u s l y s t u d i e d and s y s t e m s which would r e p l a c e t h e non l i n e a r c r y s t a l s , o r e x t e n d t h e r a n g e of f r e q u e n c y toward t h e f a r U.V. a r e o n l y a t t h e v e r y b e g i n i n g of t h e i r development. Among t h e d i f f e r e n t s y s t e m s which seem promi- s i n g i n a m o r e o r l l e s s d i s t a n t f u t u r e , a r e t h e e x c i m e r l a s e r s , two p h o t o n s l a s e r s and t h e f r e e e l e c t r o n l a s e r . While many p e o p l e t h i n k t h a t excimer l a s e r s c a n b e r u n CIJ, t h i s b e h a v i o ~ h a s n o t b e e n y e t d e m o n s t r a t e d i n o u r knowledge. Moreover even i f t h i s r e g i m c a n b e o b t a i n e d t h e t u n a b l e r a n g e w i l l b e l i m i t e d a s w i l l b e t h e t u n a b l e r a n g e of t h e two p h o t o n s l a s e r s . The f r e e e l e c t r o n l a s e r , t i l l now, h a s o n l y been r u n i n t h e i n f r a r e d and i n a p u l s e d regim . The e x t e n s i o n towards a CW r e g i o n and a s h o r t e r w a v e l e n g t h r a n g e w i l l c e r t a i n l y t a k e an a p p r e c i a b l e amount of t i m e , which a s s o c i a t e d t o t h e c o m p l e x i t y o f t h e s y s t e m w i l l n o t make t h i s s o u r c e e a s i l y a v a i l a b l e f o r s p e c t r o s c o p i c p n r p o s e s a t s h o r t term.

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