FR2909490A1 - WINDOW FORMING FILTER, AND IN PARTICULAR SEPARATOR OF HARMONIC FOR LASER - Google Patents

Abstract

The invention relates to a spectral filtering device for an incident beam (20) comprising a window (2), each face of which comprises a zone (4, 8) of a coating for reflecting, in the window, a radiation at a desired wavelength (lambda1), and transparent to at least one other undesired wavelength (lambda2, lambda3), the coating of one of the faces being located partially opposite the coating of the other face.

Description

1 WINDOW FORMING FILTER, AND IN PARTICULAR SEPARATOR OF HARMONIC FOR LASER

  TECHNICAL FIELD AND PRIOR ART The invention relates to the field of high-power lasers, and in particular to the problem of filtering and separating harmonic wavelengths at the output of a laser. This problem arises especially for high power lasers operating in the field of ultraviolet. Frequency conversion techniques, and harmonic generation in particular, make it possible to generate laser beams at desired wavelengths.

  For certain applications, however, it is necessary to separate the harmonics generated from the fundamental wavelengths or other wavelengths. For example, when using the third harmonic of an Nd: YAG laser, in order to carry out fluorescence excitation in the ultraviolet range, it is necessary to perform a filtering of the second harmonic and the length of fundamental wave that otherwise would interfere with the fluorescence signal. Similarly, in the field of material processing, the residual power of the second harmonic and the fundamental wavelength can cause thermal effects affecting the quality of the treated parts.

The most common approach to achieving harmonic separation is to use dispersive elements such as prisms, or to use dichroic mirrors with dielectric coatings. However, high-power lasers require harmonic separators that meet certain constraints; dielectric coatings have limited strengths at high powers (either peak power or average power). Another limitation is the formation, on the optical surfaces, assisted by the laser beam itself, of particles or pollutants which increase the losses (by diffusion and / or absorption) and can lead to damage to these optical surfaces in a serious manner, due to thermal effects. This is particularly problematic for ultraviolet lasers: short wavelengths are indeed able to photodissociate residual contaminants, for example organic molecules, located in the laser head. These contaminants will then be deposited on all the optical surfaces located in the path of the beam, downstream of the head, and this using the electric field 25 of ultraviolet radiation itself. To solve these problems, several techniques are known. According to a first technique, used today in the laser industry, an assembly is carried out in a clean environment, using only materials whose degassing is low, and finally the laser head is sealed hermetic to prevent external contamination during its lifetime. However, there is still a residual amount of contaminants which leads to long-term degradation of the laser, particularly with regard to high power lasers. US 6,798,813 discloses means for allowing a closed loop purge system to remove any contaminant from the laser head. However, this approach is complex, increases the size of the device, as well as the power consumption and operating cost of the laser system. US 6,782,033 discloses a laser head with a rotatably mounted exit window so that a new optical surface can be selected after another surface has been damaged by the UV beam, thereby recovering the initial performance. of this bundle. However, all the optical elements disposed within the laser head, in particular those used for harmonic separation, are still degraded during use. Another approach is to use uncoated surfaces only, with Brewster angle incidence being used to limit reflection losses. US 6,980,358 utilizes a prism with this Brewster angle incidence and total internal reflection to effect 90-rotation of an ultraviolet beam. However, the device described does not allow the separation of harmonic wavelengths.

US 5,052,780 uses a Fresnel prism under Brewster incidence: the desired harmonic beam is deflected in total internal reflection, which is not the case for unwanted harmonics. This approach has several disadvantages: the angular positioning of the prism is critical because of the very small difference in total internal reflection angle at different wavelengths. As a result, efficient selection in wavelengths requires several round trips in the prism, which is then cumbersome. Additional wavelength separation can be achieved with additional optical elements, such as absorbent filters which, in fact, further increases the problem of damage to optical surfaces. US 5,850,407 discloses an optical device in which the harmonic separation is performed by the output surface of a nonlinear crystal itself, which is cut at the Brewster angle. Due to the small angular separation between the different wavelengths at the crystal output, complete beam separation requires a long optical path, which increases the size and cost of the laser head.

There is therefore the problem of providing an optical component for separating harmonic beams with reduced exposure areas. Preferably, such a component is compatible with hermetic sealing of a laser head to provide a better quality of operation. PRESENTATION OF THE INVENTION The invention firstly relates to a spectral filtering device of an incident beam comprising a window each face of which comprises a zone without a coating and an area provided with a coating making it possible to reflect, in the window, radiation at a predetermined wavelength, or wavelength range, (21), and at least partially transparent to at least one other wavelength, or range of wavelengths undesired and predetermined (22,23), the coating of one of the faces being located partially opposite the coating of the other face. An incoming beam undergoes at least two reflections in the window, on the treated or coated areas, before forming an outgoing beam. The beam enters and exits the window through regions or untreated areas. Reflections taking place inside the window, the interaction of the beam with any contaminant that would be deposited on the faces or the treated areas of the window, in contact with the environment outside the window, is reduced.

In addition, no additional element or additional component is required between the output of the harmonic generating means and the output of the laser beam after passing through the window.

The reflection characteristics of the two areas with coatings may be the same or different from each other. The window may be mounted in means for hermetically sealing the window in the path of a laser beam. For example, the window is held in the holding means by at least one O-ring, or glue, or solder on glass, or a metal weld. According to a preferred embodiment, the angle of incidence of the beam with the window is equal to the Brewster angle for the desired wavelength. According to yet another embodiment, the untreated portion of at least one of the input and output faces 20 define a pan forming an angle (9o) that is not zero with the treated part of this same face. The window may be mounted in means making it possible to vary the overlap of the treated regions with respect to the path of the incident beam. For example, these means may be means for mounting the window in rotation or in translation with respect to an optical axis. The areas on which the coatings are made may have rectilinear, or non-rectilinear, boundaries.

The untreated portion of at least one of the input and output faces defining a pan may form a non-zero angle (9o) with the treated portion of the same face.

The invention also relates to a laser device, comprising a laser active medium that can generate a laser beam, and filtering means as above. The invention relates in particular to a laser device, comprising means for generating an ultraviolet laser beam, and filtering means as described above, mounted so as to form a sealed exit window of a beam. laser.

Such a device may further comprise means for generating harmonics of the beam. Preferably, no optical component being disposed in the path of a beam generated by the laser cavity, between the exit of this cavity and the exit sealed window, or between the harmonic generation means and the sealed window. Release. The invention also relates to a method for separating harmonics from an incident beam by means of a device according to the invention, in which process this incident beam enters the window through the uncoated zone of the face of the incident beam. the window entrance, undergoes at least two reflections on the coated areas, and exits the window through the uncoated area of the exit face.

BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A, 1B and 1C show, in front, side and rear view, an embodiment of a laser window for harmonic separation, according to the invention. FIG. 2 represents beam paths in a laser window according to the invention. Figure 3 shows the mounting of a harmonic separation window according to the invention in a laser head. FIG. 4 represents a window for harmonic separation, according to the invention, with angles of incidence and controlled reflection. FIG. 5 shows the path of a laser beam in a laser window, according to the invention, with multiple reflection. FIGS. 6 and 7 each represent a laser device implementing filtering means according to the invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS The invention will firstly be explained with reference to FIGS. 1A-1C and 2. In these figures, the separation of a harmonic beam at the wavelength 23r of FIG. Other unwanted beams in the output beam, at wavelengths 22, 23, ..., are obtained by means of an exit window of the laser head, denoted by the reference 2. The lengths of 22, 23, ... are for example the fundamental wavelength, from which harmonics are obtained, as well as the wavelengths of one or more other harmonics generated from the length fundamental wave. On a portion of the input surfaces 6 and 5 of output 10 of this window are applied dielectric coatings 4, 8, which overlap partially between the input and output faces. The other areas 14, 18 of the input and output faces are not treated with such coatings.

The treated areas are shown in these figures with a truncated circular shape, with a straight line 5, 7 between treated and untreated areas. In FIGS. 1A and 1B there can be seen a coating 4 which partially covers the entrance face 6 of the window 2. A coating 8 partially covers the exit face 10 of the window 2. According to the wavelengths considered, the beam intensities and the desired transmission coefficients, these coatings may consist of dielectric multilayers (based on SiO 2 and / or TiO 2 and / or HfO 2 and / or Ta 2 O 5 and / or ZrO 2 for example) or thick monolayer controlled (MgF2 for example) or any other usual treatment.

As can be seen clearly in the side view of FIG. 1B, coatings 4 and 8 cover a common area 11 of window 2 (hatched area in this figure). The degree of overlap (measured by the height h of the overlap zone in FIG. 1B) of the two coated parts depends on the desired reflections, as explained below.

The thickness e (see FIG. 1B) of the window is, for example, between 1 mm and 10 mm, whereas its diameter or its widest dimension (the window is not necessarily of circular shape) is example between 5 mm and 50 mm. The beam 20 of the laser, comprising all the wavelengths generated by the active laser medium, and possibly by the various harmonic generation means used, encounters the window 2 in an untreated portion 14 of the input face. 6 of the window. In the window, the beam 20 undergoes a path defined by the laws of refraction, and therefore a function of the index of the material of the window 2, this path being represented in FIG. 2 by the references 20 ', 20 ", 20 ' ". The reference 22 designates the output beam that emerges from the exit face 10 of the window 2. This beam is parallel to the input beam 20. During the path in the window, the beam will meet the coatings 4 and 8 input and output faces. In the example shown in FIG. 2, these coatings are at least partially transparent for the wavelengths 22, 23 (in fact for all the wavelengths that it is desired to eliminate), and highly reflective. for the wavelength 21r which is the desired wavelength of the output beam 22.

In the diagram of FIG. 2, the input beam, which arrives at incidence 9e with respect to the normal Ne at the input face, is deflected to the treated surface 8 of the output face. The deflected beam 20 'will encounter this treated surface at the point RI, from where it is reflected, in the form of a reflected beam 20 ", towards the input surface 6, and more specifically towards its treated zone 4, to be reflected at point R2, in R2 there is still a reflection towards the exit point S of beam 22.

In fact, it is especially the component at the wavelength 21 of this beam that will be reflected at the points R1 and R2. The wavelengths 22, 23 are at least partially transmitted to the outside of the window by transmission through the output faces at the point R1 and input at the point R2. According to a preferred embodiment, an angle of incidence 9e (see FIG. 2) is used equal to the Brewster angle for the wavelength 21 in order to minimize the reflection losses, whether at the entrance or exit surface. However, other angles can be used. FIG. 4 shows an embodiment in which the angle 90 between the treated surfaces 4, 8 and the untreated surfaces 14, 18 of the same face is chosen so that the angle of incidence 9R of the beam at the points RI , R2, ... allows to simplify the design of the coatings 4, 8 (for example, 9R = 45). This embodiment may be particularly advantageous in the case where the beams at different wavelengths 21, 22, 23, .... are associated with different polarizations. Indeed, since the coefficient of reflection of the coatings depends on both the wavelength and the angle of incidence, the control of the angle θ R thanks to the choice of 00 offers an additional degree of freedom in the design. coatings 4.8. Here again, this output beam 22 is parallel to the input beam 20. As illustrated in FIG. 5, the number of reflections inside the window 2 can be greater than 2. This number can be controlled by adjusting the surfaces of the treated and untreated areas of the window faces, as well as the ratio or coverage ratio of the treated sections on both sides (height h of Figure 1B). The number of rounds of the beam in the window is chosen so as to achieve a desired level of elimination of certain harmonics, which also depends on the reflection characteristics of the coatings 4, 8 and the spectrum of the incident beam.

In the diagram of FIG. 5, the input beam, which arrives at incidence 9e with respect to the normal Ne at the input face, is deflected towards the treated surface 8 of the exit face. The deflected beam 20 'will meet this treated surface at the point R1r from which it is reflected, in the form of a reflected beam 20 ", to the input surface 6, and more specifically its treated area 4, for At R2, reflection is again reflected at the exit face, where the beam 20 '' is reflected at the point R3. Another path in the window leads to the point R4 where the beam 20 (4) is reflected. A last path leads the reflected beam 20 (5) to the exit point S of the beam 22. In fact it is especially the component at the wavelength 2 of this beam which will be reflected at the points R1 R2 R3. and R4. The wavelengths 22, 23 are, at least partially, transmitted to the outside of the window, by transmission through the exit faces, at the points R1, R3, and input, at the points R2, R4.

Preferably, the characteristics of the coatings 4, 8 are chosen so as to minimize the desired number of reflections within the window 2, in order to limit the possible losses due to propagation in the window, in particular in the window. case of an ultraviolet radiation for which the absorption by the material of the window can be important. Other embodiments make it possible to control the number of reflections in the window, such as, for example, the existence of the non-parallel faces on which the treatments are made (as in FIG. 4). In addition, on one or both faces of the window, the treated areas 4, 8 of a face, or on which the coatings 4, 8 are made, may have, with respect to the zones 14, 18 untreated (and not constituting the outer periphery of the window), limits 5, 7 (Figure 1A, 1C) non-rectilinear.

According to yet another embodiment, a rotational or translational mounting of the window 1494 with respect to the optical axis of the laser head may be provided. Such means are for example indicated in document US Pat. No. 6,782,033. These various means make it possible to vary the recovery of the treated regions with respect to the path of the incident beam. In the examples given above, the treated areas 4, 8 of the input and output faces 10 may have the same characteristics in terms of reflection and transmission. But, alternatively, the coatings 4, 8 deposited on both sides of the window may have different reflection and transmission coefficients from one another.

The shape of the window is not limited to a circular shape, as in FIGS. 1A-1C, but may also be an elliptical or rectangular shape. The treated area may also be surrounded by a non-circular edge. treated for the manipulation and fixing of the window, for example during the formation of coatings 4, 8 on the faces of inputs and outputs. FIG. 3 represents an example of mounting of a harmonic separation window, according to any of the embodiments described above, in a laser head: the window 2 is fixed between two holding pieces 30, 32, assembled by means of two screws 34. The window is held in a compartment 36 by means of O-rings 38, 40. Such an assembly makes it possible to hermetically seal the laser head, so that to eliminate any contamination. The two parts 30, 32 of the support are such that the beams transmitted at the wavelengths 22, 23, which would pass through the treated surfaces of the window 2 are absorbed or scattered and do not leave the laser head. Only the beam at wavelength 2 can exit from this head via port 44. Other wavelengths are absorbed or scattered by the walls of cavity 36. Other means can be used to mount the window fixedly in its compartment: this window can be glued, for example with adhesive means such as an epoxy glue, or a glass solder can be used. According to yet another variant, the window may have a peripheral metallization for its welding. The window can also be maintained (as in FIG. 3) by O-rings, the hermetic nature of the compartment being ensured by the tightening of the screws 34. For all these holding means, it will be preferable to use materials with very low outgassing, to minimize the risk of contamination of the laser head.

The invention offers, with respect to the devices described in the prior art, several advantages. In particular, it makes it possible to produce a hermetic seal capable of ensuring both reduced contamination and separation of the harmonics, and thus maximum compactness, minimum cost and a very simple assembly process; in addition, the number of surfaces to be treated that the laser beam must encounter on its path is reduced to a minimum since, in its most basic version, only two faces of a single window 5 make it possible to filter or selecting the desired harmonics. The invention requires only reduced alignment: the output beam 22 is collinear or parallel to the incident beam, regardless of the angle of incidence. The only effect of the latter is a slight increase in losses at wavelength 21 (or, more generally, at the desired wavelength) when one deviates from the Brewster angle of incidence. As a result, the device according to the invention is much more robust with respect to the misalignment, which could be induced by temperature variations and / or mechanical vibrations, and / or aging of the adhesives, which the devices of the prior art. A device according to the invention thus makes it possible to achieve a minimum sensitivity to possible misalignments, which increases the robustness and simplifies the assembly process. A device according to the invention also makes it possible to produce a minimum number of interfaces, and thus to minimize the potentially contaminating surfaces. The same device can be used for the separation of harmonics, as well as for the hermetic sealing of the laser head, thus reducing the costs, the size and the number of interfaces processed. Finally, the number of reflections in the window can also be easily controlled, which makes it possible to control the rejection rate of unwanted harmonics. FIG. 6 represents a laser device implementing filtering means according to the invention. It comprises a laser active medium 50, for example a YAG bar, harmonic generation means 51, for example a frequency doubling crystal, means 52, 54 for pumping this active medium, for example flash lamps, and input and output mirrors 56 58 forming a laser cavity.

A window 2 according to the invention makes it possible to separate the harmonics according to the invention. An output beam 22 therefore has the desired properties in terms of wavelengths and harmonics. The window 2 is for example maintained by holding means of the type 20 described above in connection with FIG. 3. FIG. 7 represents a microlaser device 60 with which a nonlinear crystal 62 is associated with a view to the generation of harmonics. , and filtering means according to the invention, which makes it possible to provide harmonic filtering, in accordance with the invention. Again, an output beam 22 has the desired properties in terms of wavelengths and harmonics and the window 2 is for example maintained by holding means of the type described above in connection with FIG.

In a laser device according to the invention, the beam coming from either the laser cavity or harmonic generation means does not encounter any optical element other than the output window according to the invention, which also ensures sealing in the entire device or laser head. For example, there is a pressure in the laser head, for example of dry air, of approximately 1.5 bar, and the window makes it possible to ensure a minimum of exchanges between the inside of the laser head and the outside atmosphere.

Claims (18)

  1. Apparatus for spectrally filtering an incident beam (20) comprising a window (2), each face of which comprises a zone (14, 18) without a coating and a zone (4, 8) provided with a coating for reflecting, in the window, radiation at a desired wavelength (21), and at least partially transparent to at least one other undesired wavelength (22,23), the coating of one of the faces being located partially in look at the coating on the other side.   2. Device according to claim 1, an incoming beam undergoing at least two reflections in the window before forming an outgoing beam (22).   3. Device according to claim 1 or 2, the window being maintained inclined in the path of the laser beam at an angle close to the Brewster angle for the desired wavelength (21).   4. Device according to one of claims 1 to 3, the reflection characteristics of the two zones (4, 8) provided with a coating being identical to one another.   5. Device according to one of claims 1 to 3, the reflection characteristics 2909490 20 of the two zones (4, 8) provided with a coating being different from one another.   6. Device according to one of claims 1 to 5, further comprising means (30, 32) for holding the window in the path (20, 22) of a laser beam.   7. Device according to claim 6, the window being held tightly in the means (30, 32) of maintenance.   8. Device according to claim 7, the window being held tightly in the holding means (30, 32) by at least one O-ring, or glue, or solder on glass, or a metal weld.   9. Device according to one of claims 1 to 8, further comprising means for performing a variation of the recovery of the treated regions relative to the path of the incident beam. 25   10. Device according to one of claims 1 to 9, the areas on which the coatings (4, 8) are made having non-rectilinear boundaries. 30   11. Device according to one of claims 1 to 10, further comprising means 2909490 21 for mounting the window in rotation or in translation with respect to an optical axis.   12. Device according to one of claims 1 to 11, the untreated portion (14, 18) of at least one of the input and output faces defining a pan forming an angle (9o) non-zero with the part treated (4, 8) of this same face. 10   13. Device according to one of claims 1 to 12, the input and output faces being parallel to each other.   14. Laser device, comprising a head 15 for generating an ultraviolet laser beam, this head itself having a laser cavity for generating this laser beam, and filtering means, according to one of claims 1 to 13. , mounted to form a sealed exit window of said head. 20   15. Device according to claim 14, further comprising means (51, 62) for generating harmonics of the beam. 25   16. Device according to claim 14 or 15, no optical component being disposed in the path of a beam generated by the laser cavity, between the output of this cavity and the exit sealed window, or between the generation means harmonics 30 and the exit sealed window. 2909490 22   17. A method of separating harmonics from an incident beam (20) using a device according to one of claims 1 to 16, wherein said incident beam enters the window (2) by the zone 5. (14) without coating the inlet face (6) of the window, undergoes at least two reflections on the coated areas (4, 8), and leaves the window through the uncoated area (18) of the exit face (8). 10   18. The method of claim 17, the angle of incidence of the incident beam (20) on the input face being equal to the Brewster angle for the desired wavelength (21).