Method of and apparatus for providing a high powered ultraviolet laser beam with high repetition frequency
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ultraviolet laser beam and, more in particular, a method of and an apparatus for providing a high powered ultraviolet laser beam. The invention can be used in a number of industrial applications from the book industry to the production of printed cards and in the medical treatments as well. 
     2. Description of the related art including information disclosed under 37 CFR §§12.97-1.99. 
     It is known that lasers commercially available in the wavelength range lower than 300 nm are essentially of four types: 
     (a) excimer lasers with powers up to a hundred of watts but at very low repetition frequency (up to some hundreds of Hz); 
     (b) dye lasers which can be operated both in continuous wave mode and in pulsed mode (some Hz), powers of some hundreds of milliwatts being provided in the first case, and some in the second case; 
     (c) gas lasers operating only in continuous wave mode and with very low powers of the order of one hundred of milliwatts; 
     (d) solid state lasers with doubling of frequency operating at high repetition frequency with powers lower than one watt. 
     SUMMARY OF THE INVENTION 
     The present invention is aiming at overcoming the limits of the commercially available lasers by providing a laser beam of 266 nm with a surprisingly high average power and at a frequency of 100 Mhz from a commercial infrared laser. 
     The doubling of the frequency of an infrared laser Nd-Yag laser as a result of two passages through a LiNbO 3  crystal has already been reported in the literature. 
     The inventive step at the base of the present invention is that the generally disclosed multi-passage geometry can be associated with a storage ring for photons in full analogy with the storage rings generally used for electrons and protons. 
     Thus the invention provides the conversion of the infrared light from a commercial infrared laser, for example a mode-locked Nd-Yag laser, into green light by doubling the frequency in a non-linear crystal placed within a ring cavity acting as storage ring for the green light. 
     The green light which has been generated propagates around the ring and is partially converted into UV-light in a second doubling crystal located in the cavity. 
     The residual green continues around the ring to the first doubler where, if the ring transit time matches exactly with the repetition frequency of the mode-locked infrared laser, it is coherently added to the green generated by the next IR (infrared) pulse. 
     It should be noted that the ring cavity allows advantageously: 
     (1) the efficiency of the infrared-green conversion to be enhanced as a result of the phase addition of the green generated in the first doubling crystal to the green previously generated and propagating in the cavity; 
     (2) green power levels not available with the conventional laser to be provided at the input of the second doubling crystal due to the storage process in the cavity; 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will be apparent from the following detailed description with reference to the accompanying drawing showing by way of a non-limitative example a preferred embodiment. In the drawing: 
     FIG. 1 is a block diagram of the laser system with the ring cavity; 
     FIGS. 2a and 2b show the light compensation of KTP-crystal and the equivalent circuit with the auxiliary crystal of length Z x . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     With the reference to the Figures, the diagram of FIG. 1 shows an embodiment of the invention in which IR is the infrared light beam from a laser source of the commercial type, for example a mode-locked Nd-Yag laser with output power of 30 W. 
     C 1  is a non-linear crystal for the infrared-green doubling (for example KTP); C 2  is a non-linear crystal for the green-UV doubling (for example KD*P or β-BaB 2  O 4 ); M 1 , M 2 , M 3 , M 4 , are mirrors reflecting totally the green; L 1 , L 2 , L 3 , L 4  are lenses for focusing the green in both crystals C 1  and C 2  and stabilizing the cavity; P 1  is a prism used to superimpose along the same path the input infrared on the green propagating into the cavity; P 2  is a prism used for extracting from the cavity the infrared not converted by the first crystal C 1  ; P 4  is a prism used for extracting from the cavity the ultraviolet generated by the second crystal. 
     Let&#39;s now assume that the first infrared light pulse emitted by the laser to crystal C 1  has a density power I 1 . At the output of crystal C 1 , there will be a residual infrared pulse and a green pulse, the power of which will be determined by the conversion efficiency of crystal C 1  according to the expression: 
     
         Σ.sub.1 =tanh.sup.2 (Z.sub.1 √K.sub.1 ·I.sub.1)(1a) 
    
     where Z 1  is the length of the crystal and K 1  is a specific coefficient for the crystal. 
     The residual infrared pulse will leave the cavity after prism P 2 , while the green pulse will reach crystal C 2  with power I c . 
     Under the same conditions as for C 1  an UV-pulse will be supplied at the output of C 2  with power I uv  : 
     
         I.sub.uv =Σ.sub.2 *I.sub.c ; Σ.sub.2 =tanh.sup.2 (Z.sub.2 √K.sub.2 ·I.sub.c)                        (1b) 
    
     where Z 2  and K 2  are the same as for C 1  above. 
     This UV-pulse leaves the cavity after prism P 4 , while the residual green pulse continues around the ring until crystal C 1  is reached with power I(A), wherein if the transit time (revolution) of the green in the ring is exactly the same as the repetition period of infrared pulses, the green pulse is introduced into crystal C 1  together with the second infrared pulse. 
     The conversion efficiency from infrared to green is now affected by the presence of the previously generated green. 
     Thus, as shown in FIGS. 2a and 2b, the condition that the infrared and the green are introduced at the same time into the crystal can be made similar to the condition of having an infrared pulse of power I in  which is converted in a first crystal of length Z x  such as to have at the output of the latter a residual infrared of power I 1  and a green of power I(A) which are then introduced into the second crystal of length Z 1 . 
     As one can see, it is as if a laser of density power I in  =I+I(A), i.e. greater than that of the real laser, and a crystal of length Z x  +Z 1  were present. That is why a very high infrared-green conversion efficiency is obtained. 
     Therefore, when the equilibrium condition is reached after the pulse is passed through the cavity a sufficient number of times, the infrared is almost converted into green so that the power of the latter is considerably enhanced. 
     This will also cause the green-UV conversion efficiency to increase. In fact, the UV power leaving the cavity will be equal to the IR power entering the cavity less the residual IR diminished by the green power losses due to reflexion and absorption in the various optical elements of the cavity. As a whole, an IR-UV conversion efficiency of the order of 70% is obtained. Thus, if a commercial infrared mode-locked Nd-Yag laser having an output power of 30 W is used, the laser in accordance with the invention supplies a light at 266 nm with average power up to 20 W. 
     It should be appreciated that the combination of a multi-passage geometry with the optical ring cavity of the described apparatus is advantageous as any event causing a drop of the conversion efficiency in the single passage has a little impact on the power levels reached at the equilibrium. The less the instantaneous conversion efficiency, the more passages around the ring are required to reach about the same conversion efficiency at the equilibrium. For example, by reducing both K 1  and K 2  values in relationship (1) by a factor of 2, the UV-power at the equilibrium diminishes by approximately 13%. Accordingly, the number of passages necessary to reach the equilibrium varies from 6 to 11, which is always well within the limits imposed by the expected coherence time of the mode-locked Nd-Hag laser. Under these circumstances, the ring cavity shifts the burden of difficulties from maintaining a high single-passage conversion efficiency to controlling the beam quality and the optical path length. Although the latter is not trivial, it is far lens problematic. 
     Since unlike the known laser systems the laser of the invention provides an almost continuous wave UV beam having high average power, it has several applications both in the industrial and medical field. Among the most significant applications the following can be mentioned: 
     laser pantography wherein the UV laser is used to directly engrave the printed circuits on the semiconductor wafers, and wherein Nd-Yag lasers whose frequency is conventionally quadrupled are at present used; 
     laser lithography which is intensively used for printing in the book industry where Argon-lasers having longer wavelengths are used at present; 
     angioplastics permitting blood-clots in the arteries to be removed. 
     The high powered UV-laser of the present invention is an ideal instrument to the above purposes as the artery walls absorb UV-beams much less than the blood-clots. 
     The present invention has been described and illustrated according to a preferred embodiment. It should be understood, however, that many changes and modifications can be made by the skilled in the art without parting from the scope of the present industrial invention as claimed in the following claims.