WO1999027622A1

WO1999027622A1 - Multibeam laser resonator

Info

Publication number: WO1999027622A1
Application number: PCT/FR1998/002493
Authority: WO
Inventors: Bruno Godard; Michel Vampouille; Claude Froehly; Robert Stehle; Pierre Jean Devilder; Vincent Kermene
Original assignee: Universite De Limoges; Societe De Production Et De Recherches Appliquees Sopra
Priority date: 1997-11-21
Filing date: 1998-11-20
Publication date: 1999-06-03
Also published as: FR2771557A1; FR2771557B1

Abstract

The invention concerns an optical resonator (1) designed to transmit a set of spatially quasi uniphase N elementary beams of strong luminance (41, 42, 43), comprising: a large mirror (10), for example concave, and a set of N identical small mirrors (121-123), for example convex, arranged such that their respective focal points are located substantially in the large mirror (10) focal plane (PF). The large mirror (10) and the small mirrors (121-123) define a discharge chamber containing the optical amplifying medium (13). The invention is particularly useful for high precision laser machining systems.

Description

MULTI-BEAM LASER RESONATOR

The present invention relates to a multibeam optical resonator device. It also relates to a laser machining system including this device.

The present invention is part of the improvement of power laser systems.

Such systems are already known including optical resonator devices implementing a so-called unstable optical cavity consisting of a concave mirror and a convex mirror.

Thus, an optical resonator device of the prior art D comprises, with reference to FIG. 1, a discharge chamber 11 containing an amplifying medium 13 within an optical cavity constituted by a concave mirror 10 and a mirror convex 12 arranged so that its focus is located in the focal plane PF of the concave mirror 10. If a converging lens 14 is added thereto, a machining system 2 is then obtained delivering a convergent beam 20 according to an energy distribution 21 of which the diameter in the focal plane FL of the lens 14 approaches more or less the theoretical minimum diameter imposed by the laws of diffraction. The amplifying medium 13 is contained, for example, in a metal enclosure provided with two transparent portholes at the wavelength considered and made of a suitable material.

However, current power laser systems do not always make it possible to best meet various needs such as surface treatments which require a uniform distribution of power over a large area or high precision machining which requires a powerful directive beam. as narrow as possible. Depending on the desired application, two avenues have been explored. One is to increase the total energy delivered by the lasers by increasing the volume of the amplifying medium at the expense of the directivity of the beam.

This then presents a transverse distribution of instantaneous intensity which is very inhomogeneous, but very acceptable when it is integrated over time. It can then be used for certain types of surface treatment.

The other path aims to improve the directivity of the emitted radiation, by seeking to obtain a spatially quasi-uniphase beam by the use of particular resonator configurations. This improvement is particularly desirable in the field of precision machining for which the beam can be focused in a spot of minimum dimension limited by diffraction or adapted. It is for this type of application that the invention was developed. It aims to meet the needs for increased efficiency and higher rates expressed in the industrial sectors of this field.

The aforementioned objectives are achieved with an optical resonator device designed to emit a set of N elementary beams spatially almost uniphases. According to the invention, this resonator device comprises:

- a large mirror, and

- A set of N small mirrors arranged so that their respective focal points are located substantially in the focal plane of said large mirror, this large mirror and these small mirrors each delimiting an optical cavity containing an optical amplifying medium, this optical cavity being unstable in a at least section plans. This produces an optical resonator capable of directly producing a set of spatially quasi-uniphase beams of high luminance, which leads to a significant improvement in energy efficiency compared to the power resonator devices of the prior art.

In a preferred version of the invention, the large mirror is of the concave type while the N small mirrors are of the convex type. However, many other combinations of types of mirrors can be envisaged, provided that these combinations provide instability of the optical cavity in at least one of the section planes.

In a first embodiment, the small mirrors are identical and can be arranged either substantially against each other, or substantially at the vertices of a polygon.

In a second embodiment, the set of N small mirrors comprises mirrors having different focal lengths and spatially arranged so that their respective focal points are located substantially in the focal plane of the large mirror.

The small mirrors are for example held by support means in a predetermined geometric configuration such that their focal points are substantially in the focal plane of the large mirror. They can be formed by any spherical or sphero-cylindrical reflecting surface, in particular by a matrix of micro-mirrors or of micro-lenses associated with mirrors, with reference to document FR 2737786. Convex mirrors can be provided, instead of mirrors with two different radii of curvature in orthogonal planes. These mirrors can also be convex in one axis and planes in the other. One can also provide mirrors whose reflection coefficient is predetermined to obtain the best efficiency in the targeted technical application, for example maximum or semi-reflecting. In addition, you can add treatments to the mirrors.

The output beam (s) are extracted from said resonator, either by the periphery of the convex mirrors, or by a pierced or semi-reflecting reflective plate, placed inside the resonant cavity.

The optical resonator device according to the invention uses for example an excimer laser operating in the wavelength range [0, 1-0, 35] μm.

According to another aspect of the invention, a laser machining system is proposed including at least one optical resonator device according to the invention for generating a beam or a multiplicity of spatially quasi-uniphase beams of high luminance. This laser machining system further comprises a focusing system, for example, a converging lens disposed downstream of the common focal plane containing the set of convex mirrors.

The machining system according to the invention is intended to obtain, at the focal point of the lens placed outside the resonator, N beams of substantially equal intensities, of minimum diameters (limited by the laws of diffraction), arranged in a geometry and controllable deviations.

By using a large concave square or rectangular mirror and a set of small square or rectangular mirrors distributed along a line or a square or rectangular matrix, it is possible to achieve focusing in the form of light lines and to perform linear cuts of low width. Other features and advantages of the invention will appear in the description below. In the appended drawings given by way of nonlimiting examples:

- Figure 1 is a block diagram of an example of an optical resonator device with a convex mirror, representative of the prior art;

- Figure 2 is a block diagram of an example of an optical resonator device 1 according to the invention; - Figure 3 is a block diagram of a machining system according to the invention; FIGS. 4A to 4D represent several possible arrangements of spherical or sphero-cylindrical convex mirrors in the exit plane of a resonator device according to the invention; and

- Figure 5 shows a support equipment for convex mirrors implemented in an optical resonator device according to the invention.

We will now describe several exemplary embodiments of an optical resonator device according to the invention.

In the embodiment shown in FIG. 2, the optical resonator device 1 comprises in the output plane PS several (N) identical convex mirrors (for example, three) 121, 122, 123.

All 121-123 mirrors have a maximum or predetermined reflection coefficient, or are apodized. The N convex mirrors can be placed either at the top of a polygon (triangle, square, hexagon, octagon, ...), or against each other, but always in the same plane, as illustrated in Figure 4.

A cavity, of unstable type, is characterized by a transverse magnification G equal to the ratio of the radius of curvature Rcv of the concave mirror 10 to the radius of curvature Rcx of the convex mirror 121-123: G = Rcv / Rcx

This magnification determines the diameter d of the convex mirrors according to the relation d = Φ / G, where Φ represents the cross section of the amplifying medium 13, this being less than the diameter of the concave mirror 10. The proposed configuration can be compared to the juxtaposition N Cassegrain type telescopes formed by the association of the concave mirror 10 and a convex mirror 12 (cf. FIG. 1). The concave mirror .10 is common to each of the telescopes.

Each telescope selects, in the spontaneous emission noise from the amplifying medium, an average direction corresponding to that of the solid angle under which the concave mirror 10 is seen by one of the convex mirrors 121-123, with reference to the figures 2 and 3. This noise is amplified at each passage through the amplifying medium 13. At the outlet of the discharge chamber, in the plane PS of the convex mirrors 121-123, the laser radiation is composed of N non-collinear collimated beams. The angle between each beam is 2d / (Rcv-Rcx) radians. The near field has a transverse dimension equal to. the section of the amplifying medium 13, or different in the case of apodization.

When a converging lens 14 is placed downstream from the common focal plane, there is then a laser machining system 4 delivering a set of directional beams 41, 42, 43, with reference to FIG. 3. The optical resonator device shown in this figure includes small convex mirrors next to each other 121 ', 122', 123 '. In the focal plane of the converging lens 14, the beams are convergent and not collinear. The number of beams is identical to the number of convex mirrors.

The far field, observed at the focal point of the converging lens 14 with focal length F, consists of N spots of diffraction of minimum dimensions substantially equal to λF / Φ. These N spots are separated by a distance

D = 2dF / (Rcv-Rcx) and have the same arrangement as the convex mirrors. One can also provide, still within the framework of the present invention, convex mirrors having different focal distances but whose respective focal points are all located substantially in the focal plane of the concave mirror 10. One can envisage different possible geometries for the convex mirrors , with reference to FIG. 4. Thus, the number of mirrors and the geometries are not limited. One can thus provide, by way of nonlimiting examples, an arrangement of four spherical square mirrors (FIG. 4A), an arrangement of six spherical mirrors at the tops of a hexagon (FIG. 4B), sphero-cylindrical mirrors arranged on a line

(Figure 4C), or several rows of sphero-cylindrical mirrors (Figure 4D) to allow the production of lines. It should be noted that, even if in the configurations shown the mirrors are separated from each other, it is also possible to envisage configurations in which the mirrors are attached to each other or contiguous. It is also possible to provide for the mirrors to be apodized.

A set 62 of four convex mirrors 620 can for example be fixed, by known techniques of bonding, on a transparent blade 63 held in a support frame 6. But other techniques for fixing lenses or mirrors can be implemented , especially with the use of radial arms common in the field of Cassegrain telescopes.

In all the cases presented, the beam exits through the periphery of each of the mirrors. The N convex mirrors can consist of any spherical reflecting surface such as, for example, a matrix of spherical micro-mirrors. The optical resonator device according to the invention makes it possible to obtain N beams at the ultimate divergence fixed by diffraction in one or more back-and-forth movements inside the discharge chamber. For this, it is necessary that the magnification G is as high as possible. For these reasons, this type of resonator is particularly well suited to lasers with high gains per pass and short duration pulses. The exit beam (s) can be extracted from the resonator, either by the periphery of the N convex mirrors, or by a pierced and inclined reflecting plate, placed inside the discharge chamber, or by a semi-circular plate. transparent.

A machining system according to the invention may have, by way of nonlimiting example, the following dimensions:

- length of the cavity: about 1.25 m;

- diameter Φ of the concave mirror: 2.5.10 ^{~ 2} m;

- diameter d of one of the convex mirrors: 2.5.10 ^{~ 3} nu- focal length of the lens at the exit of the resonator: 3.10 ^"1 m;

- radius of curvature of the concave mirror: 2.5 m; radius of curvature of one of the convex mirrors: 2.5.10 ^_1 m.

Of course, the invention is not limited to the examples which have just been described and numerous modifications can be made to these examples without departing from the scope of the invention. We can in particular vary the number of convex mirrors, their focal distances, their geometric configuration, their reflection coefficient and all the dimensions of the resonator device. according to the invention, depending on the technical objectives sought. In addition, an optical resonator device according to the invention can be advantageously used to perform optical pumping of non-linear materials (BBO, KDP, ...) or light amplifiers. Furthermore, the present invention is not limited to the excimer laser, but can just as easily relate to other types of laser usable for power applications.

Claims

1. Optical resonator device (1, l ') provided for emitting a set of one or more elementary beams (41-43) spatially quasi-uniphases, characterized in that it comprises:

- a large mirror (10), and

- a set of N small mirrors (121-123, 121--123 ^* ) arranged so that their respective focal points are substantially in the focal plane (PF) of said large mirror (10), said large mirror (10) and said small mirrors (121-123) each delimiting an optical cavity containing an optical amplifying medium, this optical cavity being unstable in at least one section plane.

2. Optical resonator device (1, l ') according to claim 1, characterized in that the large mirror is a concave mirror and the small mirrors are convex mirrors in at least one of the section planes.

3. Device (1 ') according to one of claims 1 or 2, characterized in that the small mirrors (121', 122 ', 123') are arranged substantially one against the other.

4. Device according to one of claims 1 or 2, characterized in that the small mirrors are arranged substantially at the vertices of a polygon.

5. Device according to one of claims 1 or 2, characterized in that the small mirrors are arranged in one or more substantially parallel rows.

6. Device according to one of claims 1 or 2, characterized in that the set of N small mirrors comprises convex mirrors having focal distances different and spatially arranged so that their respective foci are located substantially in the focal plane of the concave mirror.

7. Device according to any one of the preceding claims, characterized in that the small mirrors are constituted by any spherical or sphero-cylindrical surface totally or partially reflecting, in particular by a matrix of micro-mirrors or micro-reflectors.

8. Device according to any one of the preceding claims, characterized in that the output beam (s) (x) are extracted from said resonator by the periphery of the small mirrors.

9. Device according to any one of claims 1 to 7, characterized in that the output beam (s) are extracted from said resonator by a reflecting or pierced or semi-transparent and inclined reflective plate placed inside the cavity resonant.

10. laser machining system (4) including an optical resonator device (l ¹ ) according to any one of the preceding claims, for generating a multiplicity of beams of high luminance (41, 43), characterized in that it comprises further a focusing system, for example a converging lens (14) disposed downstream of the small mirrors (121 '-123').