Technical field

The present invention relates to a laser arrangement for production of a sum-frequency- generated (SFG) laser beam. More specifically the present invention relates to such a laser arrangement for generation of short wavelength light. The invention also relates to a method for production of a sum-frequency-generated (SFG) laser beam.

Description of the prior art

The first-generation lasers, mostly gas and liquid lasers, with the wavelength in the visible region, are today generally replaced by solid-state lasers as they are more compact, efficient, with lower power consumption, higher output power and often lower noise. Today, most lasers in the visible region are either diode lasers, with limited output power and often poor spectral characteristics and beam quality, or frequency doubled solid-state lasers. The latter group of lasers can be made with high power while maintaining an excellent beam quality. They can also have lower noise than gas lasers and most diode lasers.

Even though most gas lasers have been replaced by solid-state lasers, gas lasers are still used in some instances. As an example, Krypton-ion gas lasers are still used. These lasers have many applications like, holography, printing, digital imaging, nondestructive testing, spectroscopy, optical pumping, confocal microscopy, flow cytometry, compact disc and DVD mastering, photomask direct imaging, and precision optics inspection, hence a solid-state replacement would be highly desirable. The Kr-ion lasers are large, unwieldly, have a high power consumption and require water cooling. Indeed, some lasers consume as much as 50 kW of electrical power and need an additional 100 kW of power for water cooling, which for full time use in production would mean an annual bill for electricity of approximately€ 100000 (cost 2018). These lasers have also a substantial noise and a limited life time before the gas tube needs to be serviced or exchanged. Thus, a solid-state alternative to the Krypton-ion gas laser would be highly desirable.

Solid-state lasers are tailor made by exploiting efficient emission lines in rare-earth elements. Most of these are in the IR wavelength range, but with frequency conversion techniques, for example, frequency doubling (second harmonic generation, SHG) or sum-frequency generation (SFG), laser lines in the visible range, as short as 457 nm, can be obtained. In this case by frequency doubling the 914 nm Nd-line in YV0 ₄ or GdV0 ₄ , see for example www.Cobolt.se or www.Coherent.com. This is also described in US 7,362,783.

To obtain a desired wavelength one needs to find suitable laser lines or combination of laser lines. A successful example of this is the "Calypso laser" from Cobolt AB which has a non-linear sum frequency mixing crystal, which is placed inside two collinear laser cavities. Lasing is obtained in two Neodymium laser crystals place together emitting two different laser lines simultaneously, the 914 nm line and the 1064 nm line. When these lines are sum frequency mixed 491 nm is generated, which is sufficiently close to 488 nm to become a compact replacement for the Ar-laser at 488 nm. However, a disadvantage with Cobolt' s sum-frequency mixing concept is that it requires the two lasers to be single-frequency to get stable operation. This is obtained by utilizing special mode filters, which can be a cumbersome complication in the construction. If the lasers are not forced to operate single-frequency there will be mode hops during the frequency conversion process due to longitudinal mode coupling between the cavities, which leads to large amplitude fluctuations for the frequency doubled signal. An additional limitation of these lasers has been the limited number of wavelengths that can be reached, ultimately set by the emission band of the rare-earth ions. To reach shorter wavelength one can utilize frequency tripling or frequency quadrupling. However, these processes are considerably more inefficient than the nonlinear processes described above. In most cases they also require very efficient stabilization, which is an expensive complication in the laser design. Furthermore, to achieve UV wavelengths by frequency conversion other materials are required than for visible radiation due to transmission limitations. Such materials have much lower non- linearities which converts into low efficiency as such. To overcome the efficiency limitations frequency tripling and quadrupling is mostly used with pulsed laser systems, which provide high peak power even at moderate average power, or with resonant intra- cavity frequency doubling which require sophisticated laser control.

Description of the invention

An object of the present invention is to provide a laser arrangement which at least alleviates one of the problems with the prior art. Another object of the present invention is to provide a laser arrangement with which it is possible to reach shorter wavelengths and scalable power than what is possible with the presently available frequency doubled and sum-frequency mixed solid-state lasers. A further object of the present invention is to provide a laser arrangement with which it is possible to reach short wavelengths, and which is less complex and more stable than presently available laser arrangements.

At least one of these objects is achieved with a laser arrangement according to the independent claim 1.

A method for provision of short wavelength laser radiation is provided in the independent method claim.

Further advantages are achieved with the features of the dependent claims.

According to a first aspect of the present invention a laser arrangement for production of a sum-frequency-generated (SFG) laser beam is provided. The laser arrangement according to the first aspect comprises an incoupling mirror and an outcoupling mirror constituting a laser cavity. A laser medium is arranged in the laser cavity. The laser arrangement also comprises a pump beam generator for generation of a pump beam directed to the laser medium, for generation of a resonating laser beam at a first wavelength (λ^ within the laser cavity, and a non-linear crystal arranged in the laser cavity between the laser medium and the outcoupling mirror. The laser arrangement is characterized in that it comprises an external laser for generation of a second laser beam at a second wavelength (A ₂ ). The external laser is arranged so that the second laser beam enters the laser cavity between the laser medium and the non-linear crystal and is directed through the non-linear crystal and the outcoupling mirror. The SFG laser beam at a third wavelength (λ ₃ ) is generated by sum- frequency-generation in the non-linear crystal from the resonating laser beam and the second laser beam. The laser cavity comprises a mirror in the form of a volume Bragg grating (VBG) to stabilize the first wavelength (λ^.

A laser arrangement according to the present invention provides the possibility to generate short wavelength laser radiation with a laser arrangement which is less complex than the laser arrangements according to the prior art for generation of short wavelength laser radiation. A laser arrangement according to the present invention is more stable than laser arrangements in which two laser cavities share the same laser medium. The present laser arrangement also provides for the use of a frequency doubled second laser beam. This enables the generation of shorter wavelength radiation than what is possible with the laser arrangement of the prior art. Thus, in other words the external laser is not affected by the laser cavity and may be arranged independently of the laser cavity. Thus, frequency doubling in the external laser does not interfere with the laser cavity. This facilitates the construction of the laser arrangement.

In the case of efficient conversion in the SFG process a substantial amount of the resonating laser beam at the first wavelength A _{l 5} typically more than 5-10%, will be converted to the SFG wavelength. This constitutes a significant loss at the first wavelength, and the resonating laser beam will thereby tend to shift its wavelength, within the gain bandwidth, to another point where it has less loss, but still strong gain. When the resonating laser beam shifts wavelength, A _{l 5} outside the phasematching bandwidth for the SFG process the SFG signal disappears. The desired SFG signal is then gone even though both the laser at Α ₁₍ and the second laser beam, from the external laser, are of high power. The problem with unstable intra-cavity oscillation and frequency conversion was first analyzed by T Baer ( J. Opt. Soc. Am. B, 3, 1175-1180 (1986) for intra-cavity frequency doubled laser, and it was then named "the green problem". It was seen for the first generation of frequency doubled (green) Nd:YAG lasers and constituted a major practical problem. For Nd:lasers at 1 μηι, which still have a comparatively narrow bandwidth this problem has to a large extent been solved by some more or less advanced design methods. However, for broad emission bandwidth laser like Yb:lasers this problem still remains and few frequency converted lasers of this type exists. This problem is also more accentuated at the high power levels aimed at with the present invention. The invention proposes a solution to the problem of the changing fundamental wavelength, A _{l 5} at high conversion efficiency for the SFG process by using a VBG as one of the mirrors in the laser cavity to lock the wavelength of the laser at λ _χ A stable laser is thereby achieved despite a possibly high loss obtained by the SFG process. This method works well both for narrower bandwidth laser ions like Nd ³⁺ and broad band emission ions, like Yb ³⁺ . It should be emphasized that the VBG bandwidth should be narrow enough that the laser emission fully falls within the phasematching bandwidth of the nonlinear crystal for SFG generation.

Preferably the pump beam is directed to the laser medium through the incoupling mirror. This arrangement ensures a good beam quality of the resonating laser beam as well as the SFG laser beam. However, it is possible to pump the laser medium in other ways known per se by persons skilled in the art. The laser arrangement according to the invention may be arranged to output the SFG laser beam as a continuous laser beam or as a pulsed laser beam. The only main difference on the laser arrangement when it is operated as a pulsed laser is that one of the resonating laser beam and the external laser has to be pulsed. This will result in that the SFG laser beam becomes pulsed. Additionally, adjustments to the doping levels of the laser crystal and non-linear crystal may have to be made for pulsed operation.

Preferably, the laser cavity is arranged so that no more than 10 % of at least one of, and preferably both of, the second laser beam and the SFG laser beam is reflected back into the non-linear crystal. By arranging the laser cavity any sum-frequency-generation in the backwards direction is essentially avoided. Thus, no measures have to be taken to get rid of laser radiation of the second and third wavelength in the backwards direction. There are different possibilities to achieve that no more than 10 % of at least one of, and preferably both of, the second laser beam and the SFG laser beam is reflected back into the non-linear crystal. The outcoupling mirror may have a reflectivity of no more than 10 % for at least one of the second wavelength and the third wavelength, and preferably both wavelengths. The outcoupling mirror still should be highly reflective for the first wavelength to provide for the resonating wave in the laser cavity.

The external laser may be a frequency doubled laser or a sum- frequency-laser. By having the external laser as a frequency doubled laser the SFG laser beam can have a shorter wavelength than has been possible to achieve with the lasers according to the prior art.

The laser arrangement may comprise a dichroic output mirror arranged between the non-linear crystal and the output mirror, for reflecting a majority of the second laser beam out from the laser cavity, preferably more than 90 % of the second laser beam. The dichroic mirror must have a high transmittance for the resonating laser beam. This is an alternative to making the outcoupling mirror low reflecting for the second laser beam.

The laser cavity may be a folded laser cavity and comprise a folding mirror arranged between the laser medium and the non-linear crystal. One of the reasons for arranging the laser cavity as a folded laser is that the laser cavity becomes more compact.

Furthermore, a folded cavity also provides the possibility to arrange the external laser so that the second laser beam enters the laser cavity through the folding mirror. This simplifies the construction of the laser arrangement.

The laser arrangement may comprise a dichroic input mirror arranged between the laser medium and the non-linear crystal and arranged to reflect the second laser beam into the laser cavity. The dichroic mirror preferably has a high reflectivity for the second wavelength and, more importantly, has a high transmittance for the first wavelength. The transmittance for the first wavelength should be as high as possible to minimize the effect on the resonating laser beam.

At least one of the incoupling mirror and the outcoupling mirror may be a volume Bragg grating (VBG). A VBG provides the advantage that frequency locking at a specific wavelength, in a particular emission band, but off the main wavelength peak can be achieved. A VBG reflects a very narrow spectrum and transmits all other wavelengths.

The laser cavity may be a folded laser cavity with a second folding mirror arranged between the laser medium and the non-linear crystal, wherein the incoupling mirror, the outcoupling mirror, the first folding mirror and the second folding mirror, are arranged to form a ring laser cavity. This is an alternative to a resonant cavity. In a ring laser cavity, a component for unidirectional oscillation is arranged so that the resonating laser beam travels in a closed loop instead of back and forth. This component is normally called an optical isolator, or an optical diode, as it allows the light to go in one direction, only. A benefit with this design is that by including the optical isolator in the cavity, a unidirectional traveling beam can be generated, and all power at the first wavelength λ _χ can be utilized in the SFG process.

In the case of a folded cavity comprising a folding mirror the folding mirror may be a volume Bragg grating (VBG). In case the folded cavity has more than one folding mirror, preferably no more than one of them is a VBG. Also, preferably, no more than one of the incoupling mirror, the outcoupling mirror and the folding mirror is a VBG. It is especially advantageous to have a folding mirror as a VBG.

In the cases where the second laser beam and the SFG beam are transmitted through the dichroic cavity mirrors, each mirror acts as a negative lens on the transmitted beam if standard plano-concave substrates are used for manufacturing the mirrors. This can have a deteriorating effect on the quality of the laser beams. To solve this problem, at least one of the mirrors can be manufactured using a convex-concave substrate, having a convex surface and a concave surface, wherein the ratio of the radius of curvature of the convex surface to the radius of curvature of the concave surface is in the interval 0.8-1.25, preferably 0.95-1.05 and most preferred 0.99-1.01. If the convex surface and the concave surface both have the same radius of curvature, the lensing effect is minimized.

The resonating laser beam may have a wavelength in the interval 1010-1110 nm, wherein the second laser beam has a wavelength in the interval 457-675 nm, so that the SFG laser beam has a wavelength in the interval 315-420 nm. The wavelength interval 457-675 nm is easily achievable with a frequency doubled laser thus making it suitable for the external laser. Also, the wavelength interval 1010-1110 is easily achievable using a Yb or Nd doped solid-state element. The wavelength interval 315-420 nm has not been readily achievable with solid-state lasers according to the prior art, unless utilizing the more cumbersome frequency tripling technique.

As mentioned above, the laser medium may be an Yb or Nd doped solid-state element. This is preferable to reach the wavelength interval of 315-420 nm. The external laser may be a frequency doubled Nd doped solid-state laser, preferably a frequency doubled Nd:YV0 ₄ or Nd:GdV0 ₄ laser at 671 nm. It consists of an already frequency doubled laser (the 1342 nm Nd:YV0 ₄ or Nd:GdV0 ₄ laser line) at 671 nm. The second laser beam is sum-frequency mixed with the resonating laser beam at 1074 nm to achieve an SFG laser beam close to 413 nm to replace the Kr-ion laser.

The solid-state element is a laser crystal chosen from the group consisting of yttrium aluminium garnet (YAG), yttrium orthovanadate (YVO), yttrium gadolinium vanadate (GdVO), yttrium lithium fluoride (YLF), or a tungstate crystal chosen from the group consisting of potassium yttrium tungstate (KYW) and potassium gadolinium tungstate (KGW). These laser crystals are favourable for the wavelength interval 1010-1110 nm.

The non-linear crystal is chosen from the group consisting of potassium titanyl phosphate (KTP), rubidium doped potassium titanyl phosphate RKTP, potassium titanyl arsenate (KTA), magnesium doped lithium tantalate (MgOLT), magnesium doped lithium niobate (MgOLN), lithium triborate (LBO), beta barium borate (BBO) cesium lithium borate (CLBO), bismuth triborate (BiBO), or LaBGeOs (LGBO), wherein the non-linear crystal (3) preferably is periodically poled for quasi-phase-matching where so is possible. These crystals are preferable for SFG into the wavelength interval 315- 420 nm.

Partial absorption in the non-linear crystal of the generated SFG laser beam, λ ₃ causes heating of the non-linear crystal. To some extent also the second laser beam, λ ₂ , causes heating of the non-linear crystal. However, as the absorption is stronger for short wavelengths the absorption of the SFG laser beam causes more heating than the second laser beam. Both the second laser beam and the SFG laser beam pass only one way through the non-linear crystal and their optical power change along the propagation direction, the second laser beam decrease in power and the generated beam increase in power. However, due to the stronger absorption at shorter wavelengths the absorption will result in an uneven heating of the non-linear crystal along the direction of propagation of the resonating laser beam with more heating taking place at the back end of the non-linear crystal, where the second laser beam exits the non-linear-crystal. To compensate for the uneven heating of the non-linear crystal the laser arrangement may comprise a heating, or cooling, device arranged to control the temperature of the nonlinear crystal along the direction of propagation of the resonating laser beam, to be within 2 K and preferably within 1 K over the whole crystal. Maintenance of a uniform temperature of the non-linear crystal is important as the phasematching condition for the non-linear crystal, particularly in the case of quasi-phase matching, is narrow in temperature, often 1 K or narrower. It is particularly the short wavelength radiation which is partly absorbed, which then would result in a temperature gradient in the nonlinear crystal as the SFG laser beam is growing in strength along the non-linear crystal. The temperature control could be maintained with an oven, a heater or at least one Peltier element.

One way of achieving a constant temperature in the non-linear crystal at high conversion would, thus, be to rise the temperature of the front end of the non-linear crystal, where the second laser beam, X enters the non-linear crystal, and to lower the temperature at the back end of the non-linear crystal, where the second laser beam exits the non-linear-crystal, by external heater/coolers to compensate for the heating caused by absorption of the SFG laser beam. For good thermal contact the nonlinear crystal should be mounted in a metal block to which the heater/cooler are contacted. In one realization the laser arrangement may comprise a heat sink and at least two Peltier elements, wherein the non-linear crystal is mounted in contact with the heat sink, and the Peltier elements are arranged in contact with the heat sink at opposite ends of the heat sink, along the direction of propagation of the second laser beam. The heat sink may be a metal mount or any other suitable material with high thermal conductivity. It might be possible to omit the heat sink and to arrange the Peltier elements in contact with the non-linear crystal. The first of the Peltier elements, arranged at the end of entrance of the second laser beam, is arranged to be set to a first desired temperature. A second of the Peltier elements, arranged at the end of exit of the second laser beam, is arranged to be set to a second desired temperature. The first desired temperature is higher than the second desired temperature. By controlling the Peltier elements in this way, the desired temperatures of the Peltier elements compensate for the gradient of the absorption of the second laser beam. Thus, an almost constant temperature in the non-linear crystal may be achieved. Preferably, the first and second Peltier elements are in thermal contact with the nonlinear crystal through the heat sink. According to a second aspect of the present invention a method for production of a sum- frequency-generated (SFG) laser beam is provided. The method comprises providing a laser cavity comprising an incoupling mirror, an outcoupling mirror, a laser medium, and a non-linear crystal. The method also comprises generating a resonating laser beam at a first wavelength (A- _L ) in the laser cavity. The method is characterized in that it comprises providing a volume Bragg grating (VBG) as a mirror in the laser cavity to stabilize the first wavelength (A- _L ), generating a second laser beam at a second wavelength (A ₂ ) outside the laser cavity, directing the second laser beam to enter the laser cavity between the laser medium and the non-linear crystal, and through the nonlinear crystal and the outcoupling mirror, and generating by sum-frequency-generation in the non-linear crystal from the first laser beam and the second laser beam the SFG laser beam at a third wavelength (A ₃ ).

In the following description of embodiments similar features will be denoted by the same reference numeral in the different figures.

Short description of the figures

Fig. 1 shows a laser arrangement according to a first embodiment of the invention. Fig. 2 shows a laser arrangement according to a second embodiment of the invention.

Fig. 3 shows a laser arrangement according to a third embodiment of the invention. Fig. 4 shows a laser arrangement according to a fourth embodiment of the invention.

Fig. 5 shows a laser arrangement according to a fifth embodiment of the invention. Description of embodiments

The most basic embodiment of a laser arrangement 100 according to the present invention is shown schematically in Fig. 1. The laser arrangement comprises an incoupling mirror 4 and an outcoupling mirror 5, which together form a laser cavity. A laser medium 1, comprising a lanthanide element, and a non-linear crystal 3 are arranged in the laser cavity. The laser arrangement 100 in Fig. 1 also comprises a pump beam generator 12 which emits a pump beam 6 which transfer energy to the laser medium with a wavelength matching the absorption band for the lanthanide ion in the laser medium 1. The pump beam 6 is launched into the laser cavity through the incoupling mirror 4, which is transparent to the pump light, and absorbed in the lanthanide ion in the laser medium 1. A first laser beam, below called resonating laser beam, with a first wavelength λ _χ is built up through stimulated emission between the incoupling mirror 4 and the outcoupling mirror 5. The laser arrangement 100 also comprises an external laser 2, which emits a second laser beam 13 at a second wavelength λ ₂ , and a dichroic input mirror 7 arranged between the laser medium 1 and the non-linear crystal 3. The second laser beam 13 from the external laser 2, with the second wavelength λ ₂ , is launched into the laser cavity via the dichroic input mirror 7 and into the non-linear crystal 3. The resonating laser beam and the second laser beam 13 are SFG mixed in the non-linear crystal 3. The generated SFG laser beam 14 is then coupled out through the outcoupling mirror 5 to provide the desired SFG laser beam 14 at the wavelength, λ ₃ . Both the incoupling mirror 4, and the outcoupling mirror 5 are highly reflecting at the first wavelength input mirror 4 has a high transmittance at the pump wavelength to let the pump power into the cavity. The outcoupling mirror 5 has a low reflectivity at the third wavelength λ ₃ to couple out the desired SFG light. An additional important feature is that the outcoupling mirror 5 also has a low reflectivity for the second wavelength, λ ₂ , so that the second wavelength λ ₂ passes only one time through the non-linear crystal. The dichroic input mirror 7 transmits the laser beam at λ _χ oscillating in the laser cavity, while it reflects the second laser beam 13 with wavelength λ ₂ from the external laser 2. The dichroic input mirror is arranged so that the reflected second laser beam 13 becomes collinear and overlapping with the resonating laser beam in the non-linear crystal 3. The SFG laser beam 14 is generated in the non-linear crystal 3 from the second laser beam 13 and the resonating laser beam, which is oscillating between the incoupling mirror 4 and the outcoupling mirror 5. A primary advantage with this arrangement is that the intra-cavity power will be very high at the first wavelength, A _l5 as the incoupling mirror 4 and the outcoupling mirror 5 are highly reflecting at this wavelength. A high intra-cavity power is a prerequisite for high conversion in the non-linear crystal 3. Besides loss through undesired absorption or scattering there is essentially no other way out from the laser cavity for the resonating laser beam than through the sum-frequency generation. Into the bargain, all sum- frequency generation will be generated in the "forward direction", i.e., only from left to right in Fig. 1. This is of course because the incoupled second laser beam 13 at the second wavelength λ ₂ is traveling in this direction only. This is an important difference from conventional intra-cavity frequency doubled and intra-cavity sum-frequency generating lasers, in which the SFG laser beam 14 is obtained in both directions, as the oscillating laser fields are propagating in both directions. The generation of the SFG laser beam 14 in only one direction is very favourable as no measures have to be taken to take care of a laser beam in the undesired direction. The generation of an SFG laser beam 14 in two opposite directions is a severe problem for the lasers according to the prior art, where two short wavelength beams (SHG or SFG) are propagating in opposite directions, and one of them either has to be "thrown away", or they have to be beam combined one way or the other. The latter is delicate and can lead both to alignment and interference problems as it is a phase sensitive process which requires half wavelength alignment accuracy and stability over time.

The shortest wavelength reachable with conventional solid-state lasers commercially available today is 457 nm which, as mentioned above, is the frequency doubled Nd:YV0 ₄ or Nd:GdV0 ₄ laser oscillating at the 914 nm line. To obtain shorter wavelengths than 457 nm one needs to look into the frequency mixing equation:

λ ₃ λ _χ λ ₂ ' ^ ^

where λ _χ and λ ₂ are the two laser wavelengths and λ ₃ is the wavelength for the sum frequency wave. At least one of the waves λ _χ or λ ₂ must then be shorter than the shortest available laser wavelength obtainable with conventional solid-state lasers, i.e. 914 nm. In our invention this is obtained by using an already frequency converted laser as λ ₂ .

To obtain an efficient sum-frequency generation it is preferable that a tailor made quasi- phase matching crystal is used. This crystal could be of any of the preferred materials for visible generation, periodically poled potassium titanyl phosphate (PPKTP), periodically poled rubidium doped potassium titanyl phosphate PPRKTP, periodically poled potassium titanyl arsenate (PPKTA), periodically poled magnesium doped lithium tantalate (PPMgOLT) or periodically poled magnesium doped lithium niobate (PPMgOLN). For high power versions of the short wavelength laser, and pulsed version of the laser, the non-linear element can be one of the commonly used crystals for UV generation, benefiting from the much lower absorption, and thereby avoiding heating and potential risk for detuning of the phasematching condition for the non-linear crystal. This can be lithium triborate (LBO), beta barium borate (BBO), bismuth triborate (BiBO), cesium lithium borate (CLBO) or periodically poled LaBGeOs (PPLGBO). However, these crystals have much lower non-linearity than the ones mentioned above and require much higher laser fields to be efficient and are therefore the primarily used crystals in pulsed operation.

A particularly interesting realization of our invention is a construction that can replace the Kr-ion laser at 413 nm. It consists of an already frequency doubled laser (the 1342 nm Nd:YV0 ₄ or Nd:GdV0 ₄ laser line) at 671 nm, (A ₂ ), which is sum-frequency mixed with a laser at 1074 nm (Aj). 1074 nm is not a normal laser wavelength and to obtain it frequency locking of a Yb:laser or a Nd:laser is required. The frequency locking is necessary as the laser otherwise would oscillate at the maximum of the emission peak of the laser medium 3. Frequency locking at specific wavelengths, in a particular emission band, but off the main peak can best and simplest be done by using a volume Bragg grating (VBG) as one of the cavity mirrors. This is shown in Fig 1, where the incoupling mirror 4 consist of a VBG which reflects a very narrow spectrum and transmits all other wavelengths. The VBG can be constructed with a wavelength specific reflectivity, in most cases very close to 100 %, at the desired wavelength (1074 nm) for the laser design. As mentioned above, a high reflectivity on the output coupling mirror 5 will result in that a high intensity at the first wavelength is built up in the cavity, which is desirable for our design, as the power P ₃ of the SFG laser beam 14 depends on the product of the power Pi of the first laser beam and the power P ₂ of the second laser beam 13, in the undepleted case. In contrast to intra-cavity SFG of two lasers, like the Cobolt laser at 491nm, the sum frequency generation in the laser arrangement 100 according to the invention can be maximized simply by adjusting the power of the external laser at the second wavelength A ₂ ■

There are several alternative variations of the invented laser which can be used to reach the desired Kr-ion wavelength at 413 nm, or close to it. One can for example mix a frequency doubled Nd:laser in the 660 - 675 nm band with a Yb-laser at around 1100 nm. However, the 1100 nm line is a normal laser wavelength neither for Yb nor Nd so it has to be obtained by frequency locking using a grating, as discussed above. In Fig. 2, 3, 4, and 5 additional embodiments are shown. In Fig. 2 a laser arrangement 100 according to an embodiment of the invention with a folded cavity is shown. The laser cavity in the embodiment shown in Fig. 2 is folded by the introduction of a first folding mirror 8. In the embodiment in Fig. 2 the resonating laser beam is focused into the non-linear crystal 3 to obtain, at the same time a stable laser oscillation, and an optimized sum- frequency generation. The second laser beam 13, at the second wavelength λ ₂ , is launched through the first folding mirror 8 to give a focused spot with a size suitable for efficient and stable sum-frequency generation in the non-linear crystal 3. The first folding mirror 8 is a dichroic mirror, i.e., it is highly reflecting for the first wavelength A _l5 and highly transmitting for the second wavelength λ ₂ . The first folding mirror 8 is preferably a focusing mirror to allow the resonating laser beam, at the first wavelength λ _χ , to be focused in the non-linear crystal 3, as is indicated in Fig. 2. Also, in this embodiment, the use of a VBG as the incoupling mirror 4, to lock the wavelength of the resonating laser beam at the first wavelength A _l5 is fruitful as in the embodiment shown in Fig. 1. Furthermore, an alternative design is to utilize a double folded cavity as is shown in the embodiment of Fig. 3, where the first folding mirror 8 is a VBG which is highly reflective for the first wavelength λι at a small incident angle but transparent for the second wavelength λ ₂ , allowing it to put through the second laser beam 13 at the second wavelength λ ₂ . The second cavity fold is achieved by employing a second folding mirror 11 which is highly reflective for both the first wavelength λι and the second wavelength λ ₂ . In Fig. 3 the outcoupling mirror has been manufactured using a convex-concave substrate, where both of the curved surfaces have the same radius of curvature, which minimizes the lensing effect and thus improves the quality of the SFG laser beam 14. Yet, another realization of the invention, as a variation of the one in Fig. 2, is described in Fig. 4. In this case the beam with wavelength, λ ₂ , is coupled out of the cavity using a dichroic output mirror 9 with the same, or similar reflective properties as the dichroic input mirror 7. The dichroic output mirror 9 in Fig. 4 has a high transmittance for the third wavelength so that the SFG laser beam 14 passes through the dichroic output mirror 9 and through the outcoupling mirror 5. This put less stringent requirement on the outcoupling mirror 5, which otherwise has to be specified at three wavelengths, i.e., that the outcoupling mirror 5 has low reflectivity at λ ₂ and λ ₃ and high reflectivity at . The only difference between the laser arrangement 100 shown in Fig. 4 and the laser arrangement 100 in Fig. 1 is that the dichroic output mirror 9 has been added. Alternatively, the dichroic output mirror 9 could have a high reflectivity for the third wavelength so that both the second laser beam 13 and the SFG laser 14 beam exits upwards in Fig. 4.

Fig. 5 shows a laser arrangement according to a fifth embodiment of the invention. The main differences between the embodiment of Fig. 3 and the embodiment in Fig. 5 is that the incoupling mirror 4 is a plano-concave element instead of a planar element. And that the outcoupling mirror 5 is a planar element instead of a plano-concave element. The outcoupling element is arranged to reflect the resonating laser beam back to the incoupling mirror 4, which in turn is arranged to reflect the resonating laser beam along the direction of the pump beam through the laser medium 1. Thus, a ring cavity is formed in which the resonating laser beam at λι travels in only one direction through the non-linear crystal. This improves the efficiency of the sum-frequency-generation. Also shown in Fig. 5 is the optical isolator 19 which allows the resonating laser beam to propagate in only one direction.

Also shown in Figs. 4 and 5 is an embodiment for controlling the temperature of the non-linear crystal 3. The laser arrangement 100 comprises a heat sink 10, preferably in the form of a metal mount, and at least two Peltier elements 17, 18. The non-linear crystal 3 is mounted in contact with the heat sink 10 and the Peltier elements 17, 18, are arranged in contact with the heat sink 10 at opposite ends of the heat sink 10, along the direction of propagation of the second laser beam 13. To obtain high conversion efficiency to λ ₃ and stability in the cavity it might be required to temperature control the non-linear crystal to handle heating of it due to partial absorption of the propagating beams. This is important as the phasematching condition for the non-linear crystal in the case of quasi-phase matching is narrow in temperature, often 1 K or narrower. It is particularly the short wavelength radiation which is absorbed. Thus, the main problem is the absorption of the SFG laser beam, which increases in strength along the nonlinear crystal. Thus, the absorbed energy from the SFG laser beam increases along the non-linear crystal, which results in a temperature gradient in the non-linear crystal as the SFG signal is growing along the non-linear crystal 3. The temperature control could be maintained with an oven, a heater or with Peltier elements as is shown in Figs. 4 and 5. This is done with a temperature gradient to minimize any distortion in the sum- frequency conversion, i.e., the part of the non-linear crystal that absorbs most of the SFG laser beam 14 is heated the least and vice versa. This might be achieved by setting the first of the Peltier elements 17, arranged at the end of entrance of the second laser beam 13, to a first desired temperature, and by setting the second of the Peltier elements 18, arranged at the end of exit of the second laser beam 13, to a second desired temperature, wherein the first desired temperature is higher than the second desired temperature. This will result in the desired temperature control in the non-linear crystal. The control of the Peltier elements 17, 18, may be performed in a manner known per se from the prior art.

A second realization of our invention is a construction that can replace the blue HeCd laser at 442 nm. It consists of a Nd:YAG laser at A _{l 5} with wavelength 1320 nm, into which a second laser beam 13 at 660 nm (A ₂ ) is launched from an external laser 2 in the form of a frequency doubled 1320 nm laser. The sum- frequency generated laser beam 14, A ₃ , at 440 nm is extracted, which is sufficiently close to the blue HeCd line to make the laser arrangement 100 a functioning replacement laser in most applications. The wavelength can be slightly shifted by locking one, or both, of the lasers with a VBG as described previously. Additionally, other Nd hosts, like YV0 ₄ or GdV0 ₄ with slightly different laser wavelengths can also be utilized for one or both of the laser lines.

A third realization of the invention is the construction of a laser arrangement 100 for substitution of the HeCd laser UV line at 325 nm based on a Nd:YV0 ₄ or Nd:GdV0 ₄ laser operating at the 914 nm line (A- _L ) as the resonating laser beam. The resonating laser beam at 914 nm is sum-frequency mixed with a second laser beam 13 at 504 nm (A ₂ ) from an external laser 2 in the form of a frequency doubled VBG locked Yb-laser operating at 1008 nm. This results in a SFG laser beam 14 at 325 nm.

A fourth realization of the invention is the construction of a laser arrangement 100 for substitution of the HeCd laser UV line at 325 nm based on a Yb-laser operating at 1040 nm (A- _L ) as the resonating laser beam. The resonating laser beam at 1040 nm is sum frequency mixed with a second laser beam 13 at 473 nm (A ₂ ) from an external laser 2 in the form of a frequency doubled Nd:YAG laser operating at 946 nm. The sum- frequency generated laser beam 14 is then at 325 nm.

The described embodiments may be amended in many ways without departing from the scope of the invention which is limited only by the appended claims. It is for example possible to include the thermal control from Figs. 4 and 5 also in the other embodiments.