EP1889341A2

EP1889341A2 - Stack of laser diodes forming a laser device

Info

Publication number: EP1889341A2
Application number: EP06742865A
Authority: EP
Inventors: Fabrice Monti Di Sopra; Bruno Frei
Original assignee: Lasag AG
Current assignee: Lasag AG
Priority date: 2005-05-13
Filing date: 2006-05-10
Publication date: 2008-02-20
Also published as: WO2006122691A3; WO2006122692A3; US20080310469A1; WO2006122692A2; US7848371B2; EP1889342A2; WO2006122691A2

Abstract

The laser device (22) is formed by a stack of laser diodes (4) placed on wafers (6) made of an electrically conductive material and having a good heat conductivity. In order to obtain a good efficiency in the evacuation of heat toward the cooling body (10) and to avoid the problems associated with an electrical short circuit, each wafer has, on its lower end (24), an electrically non-conductive layer placed on its surface before fastening to the cooling body by a fastening material (26) that preferably conducts heat well, formed, in particular, by a solder layer. According to the invention, the non-conductive layer has a width that is sufficient for enabling the fastening material to partially cover the lateral sides of the non-conductive layers, and the fastening material extends between the wafers, i.e. so that it forms a continuous layer on the cooling body.

Description

D ISPOSITI F LASER SHAPED BY A STACK OF LASER DIODES

The invention relates to a laser device formed by a stack of laser diodes each arranged on a wafer formed of an electrically conductive material and a good conductor of heat. These plates provide on the one hand an electrical connection between the laser diodes to allow the passage of an electric supply current and on the other hand to conduct the heat produced by these laser diodes in the direction of a cooling body which evacuates or dissipate the heat of the laser diodes.

In particular, the invention relates to a quasi-continuous or continuous light wave laser diode stack (QCW or CW laser diode). In Figure 1 is shown partially a laser device of the type mentioned above. This device 2 comprises laser diodes 4 in bar form, shown schematically, and metal plates 6 between which the laser diodes are arranged. To supply the laser diodes, the plates 6 are provided electrically conductive. The electrical connection between the wafers 6 and the laser diodes 4 is performed in a manner known to those skilled in the art. To protect the laser diode itself, the latter can be associated with a lower electrically insulating portion and of greater thickness than the material constituting the laser diode. In the figures appended to the description of the present invention, the various elements or layers associated with a laser diode are schematically represented therewith in the form of a bar 4.

The arrangement of the wafers 6 and the laser diodes thus allows the passage of an electric current in the direction X. Some heat transmission is also obtained in this direction X via the metal plates 6. In particular, the wafers 6 are copper. However, to allow efficient cooling, it is necessary that the heat produced by the diodes can be discharged by the wafers 6 towards the cooling body 10. This cooling body conventionally presents a conduit 12 for a circulation of water. Since the electric current must pass through the diodes in the X direction, it is necessary that the wafers 6 are electrically insulated from one another. To do this, in the prior art of FIG. 1, an electrically insulating ceramic layer 14 is welded to the body 10 by means of a solder forming a film 16. Next, each wafer 6 is welded to the layer 14 also at the same time. 18. The welding of the pads 6 must be carried out in a structured manner, to avoid short circuits between these plates 6. It is therefore necessary to prevent the solder between a wafer 6 and the ceramic layer 14 is in contact with the solder for welding another wafer.

The device of the prior art described herein before has several disadvantages. First, performing a structured weld to attach the wafers to the insulating layer is a complex operation that requires special precautions. In addition, it is difficult to guarantee a good industrial yield for such an operation since the solder 18 tends to extend during welding. It will also be noted that a weld defines an interface which forms a brake on the transfer of heat towards the cooling body 10. In the case of FIG. 1, two solder layers are present on either side of the layer 14 , which decreases the efficiency of the cooling of the laser diodes.

Also known from US 2004/0082112 a laser diode stack which differs from that shown in Figure 1 essentially in that the insulating layer is structured. According to the teaching of this document, each plate is assembled in its lower part to an electrically insulating layer by welding. This insulating layer is flat and has the same dimensions as those of the end of the wafer. Each plate is therefore first welded to a clean insulating layer. Then, each assembly thus formed is welded to the cooling body by means of a solder provided on the cooling body and structured to correspond to the separate zones provided for the plurality of "wafer-insulating layer" assemblies.

This last embodiment of the prior art, like that shown in Figure 1, has manufacturing problems. The structured weld defines a precise location for each wafer on the cooling body. This poses a machining problem for the various elements, in particular tolerances for the thickness of the wafers and for the thickness of the diodes. Indeed, variations in these thicknesses cause a problem of alignment of the stack of diodes and platelets with solder areas. But decreasing the tolerances in the machining of wafers and diodes increases the manufacturing price. In addition, the thickness of the diodes can vary substantially in the standard assortment of a laser diode manufacturer. It should also be noted that this thickness is not standardized so that it also varies from one supplier to another. A method as described in document US 2004/0082112 thus poses a real problem of assembly. Another problem is that the layer of structured solder must be of low height to remain substantially in the separate areas provided. Thus the tolerance in the machining of the height of the wafers is also critical. If during the prior assembly of the stack of wafers and laser diodes the lower ends of the wafers are arranged in the same geometrical plane to ensure that each plate will rest well on the solder deposited in the corresponding zone on the cooling body during the welding of the wafers to this body, the laser diodes attached to the wafers on the side of their upper ends will not emit in the same emission plane, this which then poses a problem of collimation or focusing of the laser beam generated by the plurality of diodes.

An object of the present invention is to solve the aforementioned problems by providing a laser device with a stack of laser diodes that can be manufactured by a reliable industrial process whose implementation is relatively easy. For this purpose, the present invention relates to a laser device formed by a stack of laser diodes each arranged on a wafer formed of an electrically conductive material and having a good thermal conductivity, these wafers being arranged next to each other, two wafers adjacent ones being located on either side of one of the laser diodes. These plates provide on the one hand an electrical connection between the laser diodes which allows the passage of an electric supply current and, on the other hand, to conduct the heat produced by these laser diodes in the direction of a cooling body to which the plates are fixed by means of a fixing material, each plate being provided at its end on the side of said cooling body with an electrically insulating layer forming an interface between, on the one hand, the cooling body and said material fastening and, on the other hand, the wafer. This laser device is characterized in that said electrically insulating layer has a thickness of about 100 microns or greater and said fixing material forms a through layer between said platelets.

According to one embodiment of the invention, the electrically insulating layer is a thick layer arranged on the lower face of the end of each wafer located opposite the cooling body. The thickness of the insulating layer is selected so that the fixing material, in particular a solder, can go up a little along the lateral faces of this insulating layer while remaining below the height of the insulating layer. The invention will be described hereinafter with the aid of the following description, made with reference to the appended drawing, given by way of non-limiting example, and in which:

FIG. 1, already described, schematically represents a laser device with a stack of laser diodes according to the prior art; FIG. 2 diagrammatically represents an embodiment of the present invention; and

- Figure 3 shows in perspective an alternative embodiment of the invention. - AT -

The laser device 34 comprises, as in the prior art, pads 6 formed of an electrically conductive material and good heat conductor. Laser diodes 4 are arranged between the plates 6 so as to allow the passage of an electric supply current of these diodes. The number of diodes and platelets may vary, in particular depending on the intended application for the laser devices. The wafers 6 may be entirely of metal or of another metallized material on the surface.

The plates 6 are arranged above a cooling body 10 having a conduit 12 for the circulation of a cooling fluid. The plates 6 and the cooling body 10 are made for example of copper. Other materials that are good conductors of heat are obviously conceivable. Preferably, the wafers 6 are formed of an electrically conductive material with good thermal conductivity to allow evacuation of the heat produced by the laser diodes towards the cooling body 10. The wafers are attached at their lower end to the body of the cooling by means of a fixing material 26 selected so as to conduct the heat sufficiently. In this embodiment, the fastening material 26 is also an electrical conductor.

Each plate 6 has at its lower end, that is to say at the end fixed to the cooling body 10, an electrically insulating layer 38 which is arranged to form an interface between, on the one hand, the body 10 and the fixing material 26 and, on the other hand, the wafer 6 considered. In the embodiment described here, the fastening material is a solder. More particularly, each plate 6 has at its lower end 24 located opposite the cooling body 10 a relatively thick layer of electrically insulating material. For example, the thickness H1 of the insulating layer 38 is about 100 microns. Given the thickness of this layer 38, a material having a good thermal conductivity will be selected. In the context of the present invention, a thickness H1 greater than 100 microns is preferred.

The brazing layer covers the underside of the layer 38 and also rises a little along the lateral faces of this layer 38, so as not to rise above the height H1 of the thick layer 38. For example, the solder layer 26 has about a height H2 less than the height H1 from the lower surface of the insulating layer 38.

The insulating layer 38 on the lower end 24 of each of the wafers defines an insulating block which is fixed to the cooling body by a fixing material preferably having a good thermal conductivity. According to the invention, the solder 26 forms a relatively thick through layer. It is thus possible to deposit solder necessary for welding the wafers on the cooling body so as to form a continuous layer, that is to say unstructured, and the wafers are then made to be welded. It is also conceivable to provide the solder 26 once the plates 6 are arranged facing the body 10. All welding methods known to those skilled in the art are available to optimize the method of fixing the wafers 6 to 10. The fact that the pads 6 have their lower parts immersed in the solder layer 26 to a certain height increases the heat transfer area between each plate and the solder layer, which increases the efficiency of the solder layer. heat transfer platelets 6 to the solder 26, then to the cooling body 10 on which the solder layer is disposed.

Figure 3 partially shows a variant of the laser device of Figure 2 which highlights the advantages of the present invention. The wafers 6 have varying lengths. However, thanks to the thick insulating layer, for example between 150 and 250 microns, and a continuous solder layer 26 which is also thick, for example between 200 and 300 microns, it is ensured that each insulating layer is correctly coated with solder on its lower face. The stack of wafers and diodes is made so that the diodes emit substantially in one and the same transmission plane. The machining tolerance on the height of the pads is therefore reported from the side of the weld to the cooling body. The average height H of penetration of the insulating layers 38 in the solder 26 is in this example approximately equal to half the thickness of this insulating layer. Then, thanks to the continuous solder layer, the thickness W of the plates 6 and the thickness D of the diodes 4 can vary without causing any problem for welding to the cooling body.

Claims

Laser device (22; 34) formed by a stack of laser diodes (4) each arranged on a wafer (6) formed of an electrically conductive material and having a good thermal conductivity, these wafers being arranged adjacent to each other. others, two adjacent plates being located on either side of one of the laser diodes, these plates ensuring on the one hand an electrical connection between the laser diodes which allows the passage of an electric supply current and, of on the other hand, to conduct the heat produced by these laser diodes towards a cooling body (10) to which the plates are fixed by means of a fixing material (26), each plate being provided at its end on the side of said a cooling body of an electrically insulating layer (28; 38) forming an interface between, on the one hand, the cooling body and the said fixing material and, on the other hand, the wafer, this laser device being characterized in that said electrically insulating layer has a thickness of about 100 microns or greater and said fixing material forms a through layer between said platelets.

2. Laser device according to claim 1, characterized in that at least the majority of the electrically insulating layers have their side walls partially covered by said fixing material.

Laser device according to claim 1 or 2, characterized in that said fixing material is a solder forming a continuous layer on the surface of said cooling body.