WO2007134645A1

WO2007134645A1 - Optical system for shaping and focusing the radiation from a set of laser bars

Info

Publication number: WO2007134645A1
Application number: PCT/EP2006/069788
Authority: WO
Inventors: Alessandro Dalla Lana; Paolo Villoresi
Original assignee: Think Laser Srl
Priority date: 2006-05-19
Filing date: 2006-12-15
Publication date: 2007-11-29
Also published as: ITPD20060201A1

Abstract

The present invention is an optical system for geometrical shaping an focusing of radiation originated from N high power semiconductor laser sources (for example a stack of N linear bars) on M spots of reduced size and spatially separated. This goal is achieved by means of an efficient and compact system composed of a small number of optical elements. The present invention is particularly suitable for the injection of the beams produced into one or more optical fibres used for the optical pumping of high power fibre laser or for direct use in material processing applications.

Description

Description OPTICAL SYSTEM FOR SHAPING AND FOCUSING THE

RADIATION FROM A SET OF LASER BARS

Technical Field

[1] The present invention is an optical system for geometrical shaping and subsequent focusing of electromagnetic radiation characterized by a variable spatial quality when this characteristic is assessed abng axes laying on a plane orthogonal to the propagating direction.

[2] The main purpose of the beam-shaping device is to obtain one or more beams having a spatial quality and brightness more homogeneous than the initial one. This technique allows the radiation to be handed with extreme flexibility by potential optical systems which take the beams thus produced as input.

[3] In particular the present invention overcomes the obstacle to obtaining one or more spots of reduced size from an array of laser sources. This goal is achieved by means of a compact system composed of a smal number of optical elements.

[4] This kind of optical system can be successfully applied to the beam shaping of the radiation produced by one or more high power semiconductor laser bars, in order to colmate and to focus it on a set of symmetrical spots with a reduced size. It is particularly suitable for the injection of the beams produced into one or more optical fibres used for the optical pumping of high power fibre laser.

Background Art

[5] Techniques for focusing the radiation produced by a set of sources on a set of guides are currently under intense devebpment, because they could be used in several industrial applications. For example applications can vary from the delivery of hundreds of Watts through a bundle of optical fibres for material processing to the recent need for injecting the radiation in a bundle of optical fibres for the pumping of high power fibre lasers.

[6] At present these techniques are realized in different ways, depending on the particular application pursued, but in most cases they represent a compromise between the spatial quality of the beam produced and its optical power. The spatial quality of the radiation is wel represented by the Beam Parameter Product (BPP) which is calculated as the product of the radius of the beam waist times the far field half- divergence of the beam times the refractive index of the propagation medium.

[7] A technique commonly used for the injection of the radiation produced by N sources in N fibres consists in replicating N times the optical system used for the injection of a single source in a single fibre. Each system for example can be composed of a set of optics for the homogenization, colmation and focusing of a single beam. An example of this approach applied to the pumping of high power fibre lasers is directly inherited and derived from telecommunications: it consists in the use of N solid-state single laser emitters, each of them independently colmated and fibre injected. Some drawbacks in this approach are the limited output power available from every laser source (a few Watts, due to its derivation from telecommunications systems) and the replication N times of the optics and of the cooling system, which lead to a significant increase in overall dimension and maintenance of the system.

[8] Another interesting approach consists in the injection of a great amount of power in each optical fibre, in order to obtain fibre laser resonators characterized by a superior modal quality, high power and consequently high brightness. High power sources can be used in this technique: for example linear diode laser bars, composed of a group of single emitters with typical total dimension of 10mm x lμm and arranged along the sbw axis direction, or diode laser stacks, composed of a set of bars arranged abng the fast axis direction.

[9] There are several solutions proposed for the homogenization and the subsequent fibre focusing of the radiation coming from a high power diode laser bar: for example those described in the documents DE- 19920293, US-5887096, US-6377410 and US- 5592333. However none of them makes any reference to any methods for the injection in N fibres of the radiation produced by N bars, and it is impossible to devise solutions for this purpose from information directly related to these documents, except for the obvious solution consisting of the use of N identical systems. Of course the replication of identical systems results in significant drawbacks in respect of overall dimension, cost and the need for precise alignment of N complex optical systems.

[10] Solutions for fibre laser pumping based on a high power diode laser stack have been seldom proposed in literature. For example, the document US-2004076197 claims a technique for pumping a fibre laser with the radiation emitted by a high power diode laser stack, but the specific fibre composition used and described in the document differs radically from present industry standards (double cladding doped fibres). In particular that invention describes an optical system which concentrates radiation produced by the stack in a rectangular spot with one side considerably bnger than the other. The specific fibre device is placed at the end of the focusing system and consists of a cladding with one or more cores inside, doped with active material and pumped by the diodes, thus forming a guide and laser oscilator. It is clear that such a device can be applied neither to the pumping of fibre lasers, which usually use a bundle of optical fibres, nor to the high efficiency injection of beams in a bundle of fibres for direct material microprocessing.

[11] It is useful to cite as examples some other documents, in order to highlight the novelty of the present invention. These documents deal with fibre injection of the radiation produced by a diode laser array and with methods for the pumping of fibre lasers by double cladding technology: WO-9515510, WO-2005002005, WO- 2004112207, CA-2296279. However, an in-depth analysis of these documents reveals no reference to methods or direct extensions of the inventions that may achieve the focusing on M fibres of the radiation coming from N diode laser bars through a system or device which is compact, straightforward and free from complicate adjustments, such as is detailed by the present invention. Disclosure of Invention

Technical Problem

[12] The present invention is an optical system for the focusing of the radiation originated from N high power semiconductor laser sources (for example a stack of N linear bars) on M spots of reduced size and spatialy separated. In this way a method is disclosed for obtaining hundreds of Watts of laser power concentrated in a discrete set of separated spots, and is particularly suitable for the injection in M different fibres, for example for direct use in material microprocessing or for high power fibre laser pumping.

Technical Solution

[13] In the most general case the invention is composed of a set of high power laser sources, of a set of shaping optics, aimed at achieving the homogenization of the beam parameter products, and of a set of focusing optics, aimed at the formation of spatialy resolved spots. The optical system gathers the radiation, emitted for example by a stack of N high power diode laser bars, and concentrates it in M spots which have smal diameter and are spatialy separated. The core of the system is a novel Beam Shaping Optical Device (BSOD) which allows to perform a parallel homogenization of the N beam parameter products, using a single optical component for the entire stack of N laser bars.

Advantageous Effects

[14] Hereinafter some important advantages of this invention wl be discussed when it is compared to currently available solutions or solutions which can be derived from previous documents.

[15] The overall size of the system is considerably reduced, especially if considered in relation to the power available for injection to the fibres. This is obtained by grouping the sources together in a smal space (for example, using a stack of linear arrays), by using a single beam shaping optics (for the parallel shaping of the radiation produced by the N sources) and by using a single optical system specifically designed to focus the shaped beams on M spots in a reduced space.

[16] The fiber coupling efficiency is high, due to a beam shaping optical system that permits to homogenize the spatial quality of the beams and to focus them with a numerical aperture and a spot size which are optimal for fibre injection.

[17] In a high power system the use of a reflective optical system (for example metalc) for beam shaping has significant advantages over a system based on refractive elements because it avoids the need for high power antireflection coatings, the absorption of radiation by the optical material and mechanical deformations induced by heating that could affect the optical performance of the device.

Description of Drawings

[18] Fig. 1 and 2 show schematic drawings of an example of one implementation of this invention, aimed at illustrating the principal elements of the system and representative paths followed by the optical radiation (ray tracing).

[19] Fig. 3 refers to the schematic of beam shaping optics used in this invention.

[20] Fig. 4, 5 and 6 report three orthogonal views of the part of the system incbding the laser sources, the optics for fast axis colmation and the beam shaping optics. These figures are intended to show a possible configuration for the positioning of the listed elements in relation to the entire system.

[21] In the following the elements used in Fig. 1-6 are listed:

1. Array of N toroidal lenses for the colmation of the fast axis divergence for each one of the N diode linear bars

2. Toroidal colmation lens for the diode linear bar 12

3. Toroidal colmation lens for the diode linear bar 13

4. Beam Shaping Optical Device (BSOD)

5. Output beam of the diode linear bar 12 after the beam shaping carried out by the BSOD 4

6. Output beam of the diode linear bar 13 after the beam shaping carried out by the BSOD 4

7. Toroidal lens for the colmation of residual slow axis divergence 8. Aspheric focusing lens

9. Optical axis of the aspheric focusing lens 8

10. Spot in the focal plane of the aspheric focusing lens 8 corresponding to the diode linear bar 12

11. Spot in the focal plane of the aspheric focusing lens 8 corresponding to the diode linear bar 13

12. Dode linear bar no. 1

13. node linear bar no. N

14. Reflecting surface of the BSOD 4

15. Reflecting surface of the BSOD 4

16. Angle (90° in the preferred embodiment) between the reflecting surfaces 14 and 15

17. Width (400μm in the preferred embodiment) of each reflecting surface 14 and 15

18. Length of the BSOD 4

19. Width of the BSOD 4

20. New virtual source after the beam shaping for the beam emitted from bar 12

21. New virtual source after the beam shaping for the beam emitted from bar 13.

Best Mode For Carrying Out The Invention

[22] The optical system described in this invention has the capability to focus the optical radiation generated by a set of N high power semiconductor laser bars on M spots of reduced size and spatialy resolved. Hereinafter an illustrative implementation of the system wil be described which does not imply any limit to the generality of the overall invention or of the single parts which compose the entire system. [23] Reference is made to Fig. 4, 5 and 6 and the Cartesian coordinates system x-y-z used therein.

[24] In this application example the source is composed of a stack of N diode linear laser bars. In particular each bar is composed of a set of single emitters placed abng the x- axis direction, for a total length of about 10mm. Consequently each bar has in the y- axis the dimension of each single emitter, normally lμm. The radiation emitted from each of the N linear bars has a divergence of about 12° (measured including 95% of the optical power) in the x-z plane (sbw axis) and of about 70° (measured incbding 95% of the optical power) in the y-z plane (fast axis). Moreover each bar has a toroidal fast axis colmating lens (FAC) laying in the x-axis direction; in the figures, lens 2 is relative to bar 12, while lens 3 is relative to bar 13. [25] The radiation from each bar, after the colmation in the y-z plane, reaches the

BSOD 4, which carries out the homogenization of the spatial quality of the N beams. The BSOD is composed of a group of reflecting surface pairs 14 and 15, which form an angle 16 of about 90°. Each of these surfaces has a width 17 of about 400μm that has to be chosen in accordance with the beam dimension abng the y-axis after the fast axis colmation. The BSOD length 18 and the BSOD width 19 are related to the number of linear bars and to their spacing abng the y-axis. In general the BSOD can be considered as an echele diffraction grating with a blazing angle of 45°.

[26] In order to locate in a simple way the correct position of the BSOD 4 and of the toroidal lenses array 1 (and consequently of the diode stack) it is possible to proceed in two steps. For rotations, the convention is made that, considering an axis coming out from the sheet towards the reader, a clockwise rotation is positive. Starting from lens array 1, placed as shown in Fig. 5, the BSOD 4 is bcated so that the plane identified by the lines of intersection formed by each pair of reflecting surfaces 14 and 15 is parallel to the x-y plane, so that its reflecting surfaces face lens array 1 and the direction determined by its side 19 is parallel to the x-axis. The BSOD is rotated by 45° around the x-axis and then by -45° around the z-axis. The configuration obtained after these steps is shown in Fig. 4, 5 and 6. Each ray emitted by the source hits both surfaces of one of the couples of reflecting surfaces 14 and 15 and is directed towards the toroidal lens 7.

[27] For the description of the fdbwing elements reference is made to Fig. 1 and 2, where another orthogonal axis system, x'-y'-z' is used. The coordinate system x'-y'-z' is derived from system x-y-z through the foDowing steps. Rotate x-y-z by 45° around the z-axis and then by 45° around the new x-axis. Then x becomes x', y becomes y' and z becomes z', defining the new x'-y'-z' system.

[28] Fig. 1 and 2 explain the beam shaping function carried out by the BSOD 4 and highlight the role of the colmating optics 7 and of the focusing optics 8.

[29] Each of the N beams coming from the respective linear bar is shaped by the BSOD

4 so that the optics 7 and 8 which foflow in the radiation path see N new virtual sources (for example 20 and 21) each with the foDowing characteristics. Consider for example the beam emitted by linear bar 12. The spatial extension of the new source 20 abng the y'-axis is comparable to the width of the colmated beam of the lens 2 abng the y-axis (for example 700μm), but it is characterized by a divergence in the x'-y' plane which is equal to that of source 12 in the x-z plane before it reaches the BSOD (for example 12°, measured including 95% of the optical power). The spatial extension of the new source 20 abng the z'-axis is similar to the width of the beam that hits the BSOD abng the x-axis (for example 10mm), but characterized by a divergence in the x'-z' plane equal to the divergence of the source 12 in the y-z plane after being colmated by lens 2 and before hitting the BSOD 4 (for example about 0.2°, measured including 95% of the optical power). In this manner it is possible to homogenize in parallel the spatial quality in the planes x'-y' and x'-z' for each of the N sources, where the spatial quality of the laser is expressed in terms of the Beam Parameter Product (BPP) which is calculated as the product of the radius of the beam waist times the far field half-divergence of the beam times the refractive index of the propagation medium.

[30] Measurements of the BPP before the BSOD 4 show, for each of the N beams, a value of about lOOOmm-mrad measured in the plane x-z and a value of about lmm-mrad measured in the plane y-z.

[31] Measurements of the BPP after the BSOD 4 show, for each of the N beams, a value of about 130mm-mrad measured in the plane x'-y' and a value of about 25mm-mrad measured in the plane x'-z': it is evident the more homogeneous spatial quality of the shaped beams.

[32] Each of the N virtual sources (for example 20 and 21) is characterized by homogeneous spatial quality, by substantial overlapping along the z' direction and by spatial separation abng the y' direction. This fact implies that with a suitable anamorphic imaging system it is possible to obtain N spots of reduced size aligned abng the z' direction and spatially resolved abng the y' direction. In this example the anamorphic imaging optics is composed of a colmation toroidal lens 7 and of a focusing aspheric lens 8. The toroidal lens 7 has optical power in the x'-y' plane. With reference to the radiation emitted from source 12, lens 7 has the capability to colmate the radiation 5 coming from the new homogenized virtual source 20, which inherits the divergence of the original sbw axis. Moreover , lens 7 is rotated around the y'-axis so that its surfaces are substantially parallel to the direction abng which the new sources (for example 20 and 21) expand in the x'-z' plane: this fact alows to compensate for the optical path difference due to the tilt of the new sources with respect to the z'-axis, in the x'-z' plane.

[33] After the toroidal lens 7 the radiation is colmated both in the fast and in the slow axis and hits on the focusing lens 8. Lens 8 focuses the incoming radiation on a reduced spot and with a numerical aperture suitable for optical fibre injection. To obtain these results it has to be wel corrected for spot aberrations, both in axis and off axis, for example spherical aberration and coma. In this way it is possible to obtain a set of N spots on its focal plane, aligned along the z' direction and spatialy resolved abng the y' direction. For example, in Fig. 1 and 2, the parameters 10 and 11 represent the image spots relative to the linear bars 12 and 13. The distance between two consecutive spots is determined by the magnification of the anamorphic imaging system abng the y' direction, and in this case it can be changed by varying the focal length of the colmating lens 7 and of the focusing lens 8 or by varying their distance with respect to the BSOD 4 and the focal plane or by varying the bar to bar pitch of the stack abng the fast axis direction.

[34] In this way it is possible to obtain a set of N spots whose dimensions and distances can be adjusted for the particular application. In particular it is possible to inject a high amount of power in the fibre by aligning the entrance of the N fibres with the N image spots of the N linear diode array, with a variety of industrial applications as described earlier.

[35] In order to concentrate on M spots the radiation emitted by a set of N diode laser bars, where M and N are different, several techniques can be operated, for example the polarization coupling or decoupling or wavelength coupling. In the first case the beams are combined or separated according to their polarizations states which can be manipulated by using suitable birefringent components or similar. In the second case the use of bars in the stack with different wavelengths permits to operate the beam combining using dichroic mirrors.

[36] The above description is only an example of a possible application and it wl be apparent to those skiled in the art that further modifications, variations and other uses and applications of the present invention may be made therein without departing from the aims and purposes of the present invention. AI such modifications, variations and other uses and applications that do not depart from the aims and purposes of the present invention are deemed to be covered by the present invention, which has been described with reference to an example application.

Claims

[1] 1. An apparatus for geometrical shaping and concentrating of radiation beams emitted by a stack of N diode laser bars. These emit radiation along a propagation direction z and have the fast axis and the slow axis laying respectively on y-z plane and on x-z plane, where x,y,z is an orthogonal coordinates system. This apparatus has the following characteristic components: a) a reflective optic composed of a plurality of pairs of planar reflecting surfaces that form an opening angle of approximately 90°, wherein each pair of reflecting surfaces forms a line of intersection oriented approximately at an angle of 60° with respect to the x direction and approximately at an angle of 45° with respect to the z direction; b) some optical components for transforming the optical parameters of the beams to be shaped by the reflective optic described above and for focusing the radiation emerging frcm there as M spots.

[2] 2. An apparatus according to claim 1, wherein the nurrber N of diode laser bars and the nurrber M of the spots are equal.

[3] 3. An apparatus according to claim 1, wherein the mutual distance between the

M spots can be controlled by varying the bar to bar pitch along the fast axis direction and/or by a suitable design of the optical components of claim 1 section b).

[4] 4. An apparatus according to claim 1, wherein the set of optical elements of claim 1 comprises 4 different optics: one for collimating the radiation with respect to the fast axis (if not already included in the set of N diode laser bars), one for the hcmogenization of the beam parameter products and two for concentrate the radiation on M spots.

[5] 5. An apparatus according to claim 1, wherein the set of optical elements of claim 1 section b) includes at least one element with a toroidal surface.

[6] 6. An apparatus according to claim 1, wherein the the set of optical elements of claim 1 section b) includes at least one element with a cylindrical surface.

[7] 7. An apparatus according to claim 1, wherein the the set of optical elements of claim 1 section b) includes at least one element with an a-cylindrical surface.

[8] 8. An apparatus according to claim 1, wherein the imaging optical system includes at least one element with an axial-symmetric surface.

[9] 9. An apparatus according to claim 1, wherein the set of optical elements of claim 1 section b) includes at least one element with a graded refraction index profile.

[10] 10. An apparatus according to claim 1, wherein the design realizes the compensation of the optical path length mismatch frcm the N sources to the M spots.

[11] 11. An apparatus according to claim 10, wherein the method for the compensation of the optical path length mismatch is carried out by a lens tilted with respect to the propagating direction of the radiation in order to reduce the dimensions of each spot.

[12] 12. An apparatus according to claim 1 used for the injection of the radiation caning frcm the N bars in M optical fibres.

[13] 13. An apparatus according to claim 1, to be used for the pumping of fibre lasers.