Method and device for conditioning the light emission of a laser diode array

Abstract: A method for conditioning the light emission of a laser generator (1), comprising an array (2) of laser diodes positioned side by side along a spatial axis (X), to equalize brightness in the direction (X) of the array (2) and in the direction (Y) perpendicular thereto, said conditioning being such as to enable optimal use of the laser beam (L) resulting from the compounding of the individual beams (L1, L2, L3) emitted by said laser diodes, the method comprising the following steps: a) separating by deflection the laser beams (L1, L2, L3) emitted by the laser diodes; b) conveying at least the adjacent laser beams (L1, L2, L3) emitted by said laser diode array (2) into different spatial planes; c) recombining said beams (L1, L2, L3) in different planes along an axis (Y) perpendicular to the axis (X) along which the laser diodes lie; and d) collimating said laser beams into a resultant single laser beam (L). The method is implemented by a device comprising means for separating the individual laser beams emitted side by side by the laser diode array, and means for recombining said beams such as to superpose these latter in order to enable their subsequent collimation.

Description: This invention relates to a method of conditioning, or suitably modifying, the spatial distribution of the laser light emission of a array of power laser diodes (defining a laser diode bar) in order to optimize the use of the resultant laser beam emitted by said array, in accordance with the main claim. The invention also relates to a device for implementing the aforesaid method. A laser generator of laser diode bar type is known to require optimization for proper use; for example if using optical fibres, the coupling efficiency in coupling such a device to a multimodal optical fibre has to be optimized. This requirement arises from the fact that light emission by a laser diode bar results from the compounding of a plurality of laser beams each emitted by a corresponding power diode of the laser diode array. By virtue of the geometrical characteristics of this array, each beam tends to open as a "fan" and has hence to be suitably conditioned in order for the laser beam resulting from the compounding of all the individual laser beams to attain the correct dimensions (substantially point-like on a target) and angular aperture for its insertion into an optical fibre. Bars of high power laser diodes also frequently require devices for their coupling to the multimodal optical fibre to facilitate their use in various applications in which it is preferable to physically separate the bar from the object onto which the laser is to be applied. This ensures, inter alia, a better protection of the bar itself, this often being an element of merit in the architecture of the laser source, while at the same time achieving a modular system with a simple connection via optical fibre which for example facilitates work required for replacing the bar itself, or for coupling several laser beam generating bars to this fibre. The difficulty of implementing a method for efficiently transferring light energy from the bar to the optical fibre (and of providing a corresponding device) derives from the emission characteristic (or modality) of the bar which, as stated, is strongly asymmetrical. Its individual constituent emitters are aligned along a first spatial axis X, they Y are rectangular in shape, with that side along the second axis Y (perpendicular to X) much shorter than that along the X axis, and they emit light along a third axis Z (perpendicular to X and Y). To quantify the concept of spatial quality of a laser beam of wavelength λ, normally the product Q = πW/λ is evaluated for the angle of divergence (associated with diffraction)  x and  y in each of the two directions X, Y, and the corresponding characteristic dimension W X and W Y of the light distribution in the plane perpendicular to the axis of propagation in which the area of this distribution is a minimum. The significance of the merit parameter Q (spatial quality related in known manner to brightness) therefore represents the ratio of the real beam divergence, , to the theoretical minimum divergence λ/(πW) which it can possess by virtue of diffraction, from which it follows that Q ≥ 1. The directions X and Y can be associated with a characteristic brightness, inversely proportional to Q X and Q Y respectively. For example, in the emission from a single bar element, Q Y is close to 1 and Q X = 20-30. In any event, the overall emission along the X direction, achieved by incoherently superposing the emissions of the various constituent diodes (normally > 10) of a bar, is substantially worse in terms of spatial quality and brightness (Q X > 1000) than the entire distribution in the X direction (Q Y equal to about 1), where the emitter beams do not mix. This means that the distribution of the light emitted by a laser diode bar can be easily focused in the Y direction but is difficult to focus on a small dimension in the X direction with a focusing angle adequate for example for efficiently propelling (ie inserting) the laser beam into the optical fibre. Even in direct power bar applications (ie those not requiring the use of optical fibres for light transportation) there remains the problem of spatial quality of the laser beam. In this respect, it is often desirable for the laser light not only to be focusable within sufficiently small dimensions, but also to reasonably remain within these dimensions for a certain length along its axis of propagation. WO 98 19 202 and US 5 805 748 disclose a device and a method according to the pre-characterising portion of claim 1 and 4. An object of this invention is to provide a method for modifying the original emission distribution of a laser bar in order to equalize its spatial quality (and hence brightness) in the X and Y directions in the case of direct applications, and its coupling efficiency in an optical fibre. A further object is to provide a device for implementing the aforesaid method which is of low constructional and operating costs. These and further objects which will be apparent to an expert of the art are attained by a method and device in accordance with the accompanying claims. The invention is defined in claims 1 and 4. Embodiments of the invention are set out in the dependent claims. The invention will be more apparent from the accompanying drawing, which is provided by way of non-limiting example and on which: Figure 1 is a schematic view of a device of the invention taken from above; Figure 2 is a side view of the device of Figure 1; Figure 3 is a section on the line 3-3 of Figure 1; Figure 4 is a section on the line 4-4 of Figure 1; Figure 5 is a section on the line 5-5 of Figure 1; Figure 6 is a schematic view from above showing a first device for focusing the laser beam leaving the device of the invention; Figure 7 is a side view of the device of Figure 6; Figure 8 is a schematic view from above showing a second device for focusing the laser beam leaving the device of the invention; Figure 9 is a side view of the device of Figure 8; Figure 10 is a view from above showing a variant of the device of the invention associated with the focusing device of Figures 6 and 7; Figure 11 is a side view of the device of Figure 10. In Figures 1, 2, 6, 7, 8, 9, 10, 11 the cartesian axes are shown in order to clarify the description. Furthermore the focusing devices of Figures 6 to 11 coupled to the device of the invention are known per se. With reference to Figures 1 to 5, the device of the invention is shown associated with a laser generator 1 comprising a bar or array 2 of power diodes providing continuous or pulsating laser light emission. The bar 2 is powered by a usual unit 3 for electrically powering said bar and for controlling its operating temperature. The bar 2 is of known form. It is of rectangular parallelepiped shape and, with reference to the cartesian axes shown in Figures 1 and 2, extends along the X and Y axes. The laser beams from the power diodes are emitted in the direction of the Z axis. As can be seen from the figures, and as is the norm, the bar 2 extends along the X axis further than along the Y axis. For greater clarity of explanation, this description will consider a bar consisting of three individual emitters or three emitter groups, to which there correspond three laser beams L1, L2, L3. This consideration serves only to facilitate the description of the invention, and in no way represents a limitation on the method and device of the invention. In front of the bar 2 there is positioned a separator member 5 comprising for example a cylindrical microlens 6 or a lens having a dioptric power other than zero along only one of the directions perpendicular to the axis of propagation of the laser beam or of the light in general. It has a substantially rectangular cross-section and extends along a major axis W. This microlens is suitably inclined to the bar 2 such that its axis A forms an angle α to the plane of the X axis, as shown in Figure 3. The purpose of the member 5 is to collimate the beams L1, L2 and L3 (of substantially blade-shaped cross-section) in the direction of the Y axis, their axis of propagation being shifted angularly in the direction of the Y axis (visible in Figures 2 and 4) to an extent depending on the position occupied by each individual emitter in the bar 2, on the basis of known geometrical optics relationships. As stated, the axis W of the member 5 is inclined to the plane of the X axis by an angle α for example of between 0.1° and 5°. In order to avoid movements of the beams L1, L2, L3 along the Y axis which are too small, the angle α should not fall below too small a value (for example 0.05°); this angle in any event depends on the planarity of the bar 2 along the X axis. The device of the invention also comprises a recombining member 8 spaced from the member 5. Between the two there can be inserted (as in the embodiment of Figures 1 and 2) a spherical lens 10. The purpose of the recombining 8 member is to make the propagation directions of the centroids of the individual beams L1, L2 and L3 perfectly parallel, by aligning them in the direction of the Y axis (or along it), as shown in Figure 5. Downstream of the member 8, the image of the bar emission conditioned in this manner has its spatial quality (and hence brightness) equalized in the X and Y directions, and can hence be focused in optimal manner with minimum divergence, for direct applications or for coupling to an optical fibre 12. In this respect, if N is the number of beams, the now conditioned spatial distribution has Q' Y = NQ Y and Q' X = Q X /N, with Q' X = Q' Y , the optimal choice for the number of beams hence being N = √(Q X /Q Y ). By way of example, four possible arrangements for the recombining member 8 are described hereinafter with reference to Figures 6 to 11. In the embodiment of Figures 6 and 7, the member 8 comprises a plurality of flat-convex cylindrical lenses of positive focal length 13, adjacent on each other, their centres being positioned along an axis lying in a plane perpendicular to the Z axis and inclined to the Y axis to make the propagation directions of the centroids of the beams L1, L2 and L3 leaving the member 8 parallel to the Z axis, in accordance with geometrical optics relationships. In the figures under examination, two cylindrical lenses 14 and 15 are positioned at a suitable distance from the member 8 such that the image of the beams L1, L2 and L3, newly conditioned by the member 8, is focused in the two directions X and Y in one and the same plane F-F with dimensions defined on the basis of the application of the resultant laser beam leaving these lenses. For example, in order for the beam L to be able to be inserted into an optical fibre 50, this beam must be focused within the fibre core, with an entry cone contained within the acceptance cone of the optical fibre. In the figures under examination, the lens 10 has a focal length F and is positioned at a distance F from the member 5 so as to collimate the beams emitted by the laser diodes and form on the recombining member 8, also positioned at a distance F from the lens, a figure consisting of an image of the individual beams aligned in the direction of the Y axis (see Figure 5). In the embodiment shown in Figures 8 and 9, the member 8 comprises a plurality of flat-concave cylindrical lenses of negative focal length 20, their centres being positioned on a straight line lying in a plane perpendicular to the Z axis and inclined to the Y axis to make the propagation directions of the centroids of the beams L1, L2 and L3 leaving the member 8 parallel to the Z axis. As in the case of Figure 6 and 7, in the figures under consideration the lens 10 and the focusing lenses 21 and 22 are also provided, their purpose being identical to that of the aforedescribed lenses 14 and 15. In this case, the lens 21 is a cylindrical lens the focal length and position of which are chosen to collimate the newly conditioned laser beam in the X direction with a suitable direction, in such a manner as to obtain in the focal plane of the spherical lens 22 a light distribution with defined dimensions in the X and Y direction. In Figures 10 and 11 the member 8 comprises a plurality of flat mirrors 25, the normals of which are progressively rotated (in advancing along an axis parallel to the Y axis) parallel to the X-Z plane. These mirrors deflect the laser beams L1, L2 and L3 in a direction perpendicular to the Z axis, and hence along the X axis, where lenses 26 and 27 similar to the lenses 14 and 15 of Figures 6 and 7 are positioned. Figure 11 shows the cross-section through the beams L1, L2 and L3 on the lens 26. The lenses 26 and 27 operate as the lenses 14 and 15, and consequently reference should be made to the description of these latter. Again in the figures under examination, the beams L1, L2 and L3 strike the lens 10 (not shown) positioned between the member 8 and the member 5. Using the member 8 as represented in one of the aforedescribed Figures 6 to 11, the entry plane of a multimodal optical fibre 50 can be presented with a virtually square light distribution, with similar and opposite X and Y dimensions, and divergence angles in the two perpendicular directions which are compatible with the characteristic acceptance angle. This enables light to be efficiently transferred from the laser diode bar to the optical fibre.

Claims: A method for conditioning the light emission of a laser generator (1) comprising an array (2) of laser diodes to equalize brightness along a first axis in the direction (X) of the array (2) and along a second axis in the direction (Y) perpendicular thereto, said conditioning being such as to enable optimal use of the laser beam (L) resulting from the compounding of the individual beams (L1, L2, L3) emitted by said laser diodes, for example for its optimal coupling to an optical fibre (50), said laser diodes being positioned side by side along the first spatial axis (X) perpendicular to which the individual laser beams are emitted along a third axis (Z), the method comprising the following steps: a) separating by deflection the laser beams (L1, L2, L3) emitted by the laser diodes, said separation taking place in proximity to the laser diode array; b) shifting angularly relatively. To each other within the plane defined by the second and third axis (Y, Z) to convey at least the adjacent laser beams (L1, L2, L3) emitted by said laser diode array (2) into different spatial planes; c) recombining said beams (L1, L2, L3) in different planes along the second axis (Y), perpendicular to the axis (X) along which the laser diodes lie by deflecting the beams so that the centroids of the beams are aligned along the second axis (Y) direction and propagate in parallel directions, and d) collimating said laser beams into a resultant single laser beam (L), characterised by comprising a step e) of collimating the beams along the second axis direction (Y) between said step b) of conveying and said step c) of recombining. A method as claimed in claim 1, characterised in that following the recombining of the laser beams (L1, L2, L3) emitted by the laser diode array (2), said beams (L1, L2, L3) are propagated along the said emission direction (Z). A method as claimed in claim 1, characterised in that following the recombining of the laser beams (L1, L2, L3) emitted by the laser diode array (2), said beams (L1, L2, L3) are propagated along a direction perpendicular to the emission direction (Z). A device for conditioning the light emission of a laser generator (1) comprising a laser diode array (2) having a rectangular parallelepiped body extending mainly along a principal spatial axis (X) and to a lesser extent along a second axis (Y) perpendicular to this latter, the laser beams (L1, L2, L3) being emitted by said diodes along a third axis (Z) perpendicular to said axes (X, Y), said device comprising, for said laser beams (L1, L2, L3), separator and deflector means (5) positioned in proximity to the laser diode array (2) and arranged to shift angularly relatively to each other said beams and to convey said beams into different spatial planes parallel to the plane defined by the aforesaid first and third axis (X, Z), and recombining means (8) arranged to deflect said beams so that their centroids propagate side-by-side along an axis parallel to the second axis, said laser beams (L1, L2, L3) being then focused by focusing means (14, 15; 21, 22; 26, 27) in the focal plane, characterised in that a lens (10) is positioned between the separator means (5) and the recombining means (8), said lens (10) having focal length F in the second direction (Y) and being positioned at a distance F from said separator and deflector means (5) so as to collimate along the second direction (Y) the beams emitted by the laser diodes. A device as claimed in claim 4, characterised in that the separator and deflector means are a lens having a dioptric power other than zero along only one of the directions perpendicular to the axis of propagation of the laser beam. A device as claimed in claim 5, characterised in that the lens (6) has its axis of symmetry (W) inclined to the plane defined by the first and third axis (X, Z). A device as claimed in claim 4, characterised in that the recombining means (8) comprise a plurality of adjacent flat-convex cylindrical lenses (13) of positive focal length with their centres positioned along an axis lying in a plane perpendicular to the axis (Z) and inclined to the second axis (Y), said lenses (13) being placed along the third axis (Z) in a position where the beams are aligned along the second axis (Y). A device as claimed in claim 4, characterised in that the recombining means (8) comprise a plurality of superposed flat-concave cylindrical lenses (20) of negative focal length with their centres positioned along an axis lying in a plane perpendicular to the axis (Z) and inclined to the second axis (Y), said lenses (20) being placed along the third axis (Z) in a position where the beams are aligned along the second axis (Y). A device as claimed in claim 4, characterised in that the recombining means (8) comprise a plurality of mirrors (25), the normals of which, in advancing along an axis parallel to the second axis (Y), are progressively rotated parallel to the plane defined by the first and third axis (X, Z). A device as claimed in claim 4, characterised in that the focusing means are cylindrical lenses (14, 15) positioned in succession along the path of the laser beams (L1, L2, L3). A device as claimed in claim 4, characterised in that the focusing means are a cylindrical lens (21) and a spherical lens (22) positioned along the path of the laser beams (L1, L2, L3).

Inventors: AGNESI ANTONIANGELO, PICCINNO GIULIANO, AGNESI, ANTONIANGELO, PICCINNO, GIULIANO
Applicants: BRIGHT SOLUTIONS SOLUZIONI LAS, BRIGHT SOLUTIONS SOLUZIONI LASER INNOVATIVE S.R.L
Application year: 2000
Main classification: G02B27
IPC codes: H01S5