LASER DIODE MODULE WITH INTEGRAL COOLING

Abstract: A laser module with integral cooling is disclosed. The module is intended for coupling to a fiber optic cable for various applications. The module is generally cylindrical and has a laser diode array (32) mounted centrally therein. A cooling channel (62) is in thermal relation with the array (32) either directly or via a diamond substrate. A fiber optic coupler (46) mates with the portion of the housing carrying the array and carries a lens subassembly (16, 18). During assembly, the coupler (46) is positioned to accurately focus the output of the array into a beam suitable for transmission to the fiber optic cable.

Description: LASER DIODE MODULE WITH INTEGRAL COOLING This invention relates to semiconductor laser diodes. More specifically, it relates to laser diode modules which may include a plurality of semiconductor laser diodes. Such laser diodes are capable of generating a relatively high beam output for their size and are used in such applications as fiber optic communications and laser surgery. Typically, such semiconductor diode lasers emit energy in the wavelength range of 0.805 to 1.55 micrometers for efficient coupling to fiber optics. For a background discussion of such semiconductor diode lasers see Laser Focus World, August, 1992, pgs. 79 et seq. One type of a laser diode suitable for use with the present invention is disclosed in United States Patents Nos. 4,860,298 and 4,985,897 both to Dan Botez et al. hereby incorporated by reference. The laser diode there disclosed typically comprise between twenty to forty laser elements and having a size of 100 microns thick by 0.5 millimeters wide by 1.0 millimeters long. Such modules are capable of generating in excess of 15 watts. These devices are capable of many applications, but a principal problem with their use is the dissipation of heat, which is considerable at this relatively high output to size ratio. Typically, the laser outputs are concentrated into a beam focused by a lens system into a single beam which may, if desired, thereafter be coupled to a fiber optic cable or similar device. Details of the lasers and the means for optically focusing the beams are disclosed in the aforementioned Botez et al. patents, although they are discussed herein as necessary to an understanding of the invention. Such devices typically have a current input of between 0.5 and ten amps or higher and can create an output of between 0.2 and ten watts or more of optical power. One limiting parameter on Ught output is heat build-up. Without efficient cooling, the optical power output decreases rather dramatically as temperature increases. As a rule of thumb, if ten watts of optical power are created, there are approximately ten watts of heat which must be dissipated. Previous designs have been able to obtain only approximately 200 milliwatts of optical power without some form of cooling. Thus, the laser module according to the present invention is capable of optical power orders of magnitude greater than the prior art. Such a high optical output device has many applications if it can be appropriately packaged, focused and cooled at a reasonable cost. Some of these applications include marking substrates with alpha-numeric information, medical applications, laser type-setting, desktop manufacturing, manufacture of semi-conductor devices and many others. In short, a high output, low-cost laser module is desirable and would be well received in the marketplace. As indicated, the prior art has not heretofore been capable of such high output without significant additional expense to add on cooling and lens systems external to the module. It is an object of the present invention to overcome these disadvantages by incorporating both into the module in a simple and inexpensive manner. Existing lower power (less than 1 watt) laser diodes are currently available in three general packaging formats. The most basic is an open heat sink package, usually in the form of a C-mount. This format is intended for custom integration with optical lens coupling and cooling systems which must be added on to the laser module. A second form of packaging, cylindrical packaging such as the TO series, is also a common format used where coupling to fiber optics is required but cooling is not required or can be satisfactorily accomplished by mounting the device to a cold (usually metallic) surface. Where higher power output is required a high heat load (HHL) package is commonly used. The HHL is a rectangular package often provided with fiber coupling. The package may incorporate thermoelectric cooling, but usually requires mounting to an external cooling device or providing a submount that provides for micro-channel cooling, impingement cooling or back cooling. All of these high efficiency cooling and mounting techniques require further integration into a package that does not by itself provide for fiber coupling. In short, the lack of a unified design results in excessive cost, undue size and thereby limits the usefulness of the device. In order to provide for practical application of high powered laser diodes, it is necessary to provide efficient cooling and fiber optic coupling in a low-cost integrated package. It is necessary to overcome the inherent difficulties of the HHL rectangular package which does not allow for low-cost fiber optic coupling and which often requires electrical connections to be made by soldering to a glass insulated feedthrough. Furthermore, such packaging is inherently expensive and difficult to interface. According to the invention there is provided a laser module with integral cooling comprising: (a) a first body section carrying a laser diode array and including means for cooling said array; and (b) a second body section joined to the first body section carrying a lens subassembly for receiving laser energy from said array and projecting it as a beam along an optical path. Further according to the invention there is provided a laser module with integral cooling comprising: (a) a body member carrying a laser diode array and including means for cooling said array; (b) a lens subassembly secured to the body member for receiving laser energy from the laser diode array and projecting it as a beam along an optical path. A laser module in a cylindrical form embodying the invention has an advantage that it incorporates direct water cooling of the laser chip, integrated fiber optic coupling and is easy to connect and disconnect to electrical leads. The invention will now be described further by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view of the concept of the invention employing a cylindrical laser module which is suitable for connection to a fiber optic cable; Figure 2A is a sectional view of a first embodiment of the invention; Figure 2B is a top view of the disk on which the diode module is mounted; Figure 2C is a side sectional view of the disk of Figure 2B showing the coolant and mounting details; Figure 3A is a side sectional view of a second embodiment of the invention; Figures 3B and 3C are sectional schematics illustrating the manner in which the module of Figure 3 A is coupled to a cooling manifold; Figure 4A illustrates a third embodiment of the invention utilizing a tubular passage for the liquid coolant and in which the laser module is directly mounted to the tubing; Figure 4B illustrates the electrical connections for the embodiment of Figure 4A; Figure 4C is an enlarged perspective view of the Figure 4A embodiment; Figure 5 is a fourth embodiment of the invention illustrating a simplified and, in some applications, preferred embodiment in which the liquid coolant is introduced radially into the cylindrical module; Figure 6 is a fifth and presently preferred embodiment of the invention; Figure 6A is an exploded perspective view of the Figure 6 embodiment; and Figure 6B is an exploded perspective view on an enlarged scale of the laser mounting and coolant tube. Referring to Figure 1, a cylindrical laser module is conceptually illustrated. The laser diodes are mounted on a substrate positioned at 10 within a cylindrical body 12. Electrical contacts to the lasers are provided on the cylindrical body as at 14. Collimating lens 16 and objective lens 18 serve to collimate and focus the beams from the lasers into a single beam suitable for coupling to fiber optics. A modified ST-coupler 20 is integrally provided as part of the laser module. Thus, a fiber optic cable or the like is simply connected to the ST- coupler and automatically couples the fiber optics to the laser output as it passes through the objective lens 18. Not shown in Figure 1 is the manner in which this module is intended to be mounted and cooled. Referring to Figures 2A-C, a first embodiment of the invention is illustrated. A ceramic disk 30 has mounted thereon at the central portion thereof, a laser module chip 32 (Figures 2B and 2C). Electrical connections to the module are plated on the outer periphery of the cylindrical mounting for eventual connection to the laser chip 32. Thus, as shown in Figure 2C electrode 34 is connected to the chip. The chip itself is preferably not mounted directly to the ceramic but to an industrial diamond submount 36. The output from the laser module is coupled through a lens system to a fiber optic output. For that purpose, ceramic disk 30 is secured to a plastic body 40. This is accompUshed, as shown in Figure 2A, by inserting epoxy into openings 42 so as to create epoxy bonds 44 around the circumference of the disk 30 joining the plastic body 40 thereto. Thus, the plastic body 40 has ceramic disk 30 bonded thereto with epoxy. The ceramic disk incorporates two electrical contact bands deposited around its circumference providing the electrical interface to a laser power supply (not shown). The laser and diamond are inserted into the ceramic substrate secured thereto and electrical contact made using a standard baU bond or ribbon connections as shown in Figure 2C at 34. A structural adhesive may be provided around the perimeter of the interface to ensure a hermetic seal against the entry of moisture. The ST-type coupler is shown in Figure 2A at 46. The coupler is joined to the plastic body 40 by screw threads as at 48. Mounted within the coupler, in a recess 50, is a spherical objective lens 18. The collimating lens 16 is mounted therebehind. As the coupler 46 is threadably inserted into the plastic body 40, aUgnment is achieved ensuring that the lens accurately focuses the output of the laser device. When the coupler is correctly positioned, epoxy can be placed in the screw threads permanently to secure the coupler in relation to the plastic body 40 which in turn is bonded to the ceramic disk 30. This eliminates the need for a fiber optic pigtail. Thus, the collimating lens and focusing optics are mounted inside the ST-coupler 46 that is threaded into the plastic housing. The pitch of the threads incorporated into the coupler and the plastic body aUow accurate focusing of the laser beam into a fiber optic 52. Lateral positioning adjustments are performed by moving the plastic body over the surface of the ceramic to obtain maximum Ught coupling into the fiber. Angular adjustment is performed by the rotation of the plastic housing with respect to the ceramic base. Optical aUgnment is fixed by injecting adhesive between the plastic body 40 and the ceramic base 30 to form a sheer bond. Preferably a metal or plastic retaining ring is then instaUed to improve the structural integrity of the outer body. The module as thus far described interfaces with a cooling fixture. The position of the cooUng fixture is indicated generaUy at 60 in Figure 2A. As illustrated in Figure 2C, the cooUng fixture permits a Uquid coolant to circulate in a channel 62 in the ceramic disk 30. Channel 62 is in contact with the diamond 36 on which the laser module 32 is mounted for heat dissipation purposes. Preferably cooUng fixture mates with a coolant manifold having a check valve to prevent coolant from reaching the ceramic until the manifold has been secured thereto. Another valve may be provided to prevent leakage when the coolant manifold is disconnected from the module. Thus, cooUng for the laser diode is provided along with a mechanism for quickly disconnecting the laser module from the coolant source. Suitable disconnect features and manifolds are detailed in the remaining embodiments described hereafter. Referring to Figures 3A-C, a second embodiment of the invention is Ulustrated. In this embodiment, the ST-coupler 70 and plastic body 72 are the same as in the first embodiment and the focusing and collimating lens are similarly mounted. Thus, an ST- coupler 70 is secured to a plastic body 72. The lenses 74 are secured to the coupler and move relative to the plastic body until proper focus and aUgnment are achieved. Thereafter epoxy is employed to permanently secure the assembly together. A structural support ring 78 is deployed around the assembly. In this embodiment, the laser diode 77 is secured in a cavity 79 to a diamond substrate 80 mounted to the plastic block 82. Note the absence of the ceramic disk utilized in the first embodiment. Incorporated into the block 82 is a cooling fixture which contacts the substrate 80 and is adapted to engage a cooUng manifold as shown in Figures 3B and 3C. The aUgnment procedure is relatively straightforward. The coupler 70, containing the lenses 74, are held stationary while the plastic block or base 82 containing the diamond substrate 80 on which the laser diode 77 is mounted, is manipulated whUe the diode generates a laser beam. The beam is aUgned with the center line of the lenses 74. When that occurs, the epoxy is injected as previously described to lock the base and coupler together. The aUgnment process may be accomplished in an aUgnment fixture typicaUy consisting of an aUgnment base and an observation stand. The base permits 360° rotation as weU as XY translational movement. A fiber optic sleeve coupled, for example, to a CCD Camera is connected to a monitor. Once the laser beam is observed to be in its optimized position, the epoxy is injected. Thereafter the structural support ring 78 is tightened to provide additional mechanical holding force. Referring to Figures 3B and 3C, the details of the cooUng fixture and its interconnection to a manifold coupler 96 are iUustrated. Referring to Figure 3C, it wiU be seen that the block 82 includes a horseshoe shaped cavity 84 through which a Uquid coolant can flow and contact the diamond 80 on which the laser diode is disposed. A water coupler 86 is secured to the back block 82, as by epoxy or other means. The water coupler 86 includes a central bore 88 communicating with the horseshoe shaped channel 84 and an exit port 90. As indicated by the arrows in Figure 3C, the channel communication is intended to permit Uquid coolant to enter through the central bore 88, circulate through the horseshoe shaped channel 84 thereby to contact the diamond 80 and carry heat therefrom and finaUy exit through port 90. The water coupling 86 terminates in a nipple fitting 92 having an O- ring 94 to prevent leakage. Fitting 92 is adapted to be received in a manifold coupler 96 (Figure 3B) which in turn is secured to a source of Uquid coolant. Referring to Figure 3B the details of the manifold coupler 96 are iUustrated. The nipple fitting 92 is received in the receptacle 98 where the O-ring 94 prevents leakage. Liquid flows from a supply conduit 100 into the central bore 88 when the two units are secured together. Coolant which has been circulated is returned from the module through the outlet port 90 into conduit 102 in the coupler 96 to a heat exchange device which removes the heat from the coolant so that it may be recirculated for further use. Preferably, the manifold coupler 96 not only couples the Uquid coolant to the laser module but also carries the electrical contacts which power the laser module. As iUustrated in Figure 3B, spring contacts 104 and 106 are provided on the outer circumference of the coupler. Note that the contacts terminate in spring arms 108 and 110 which are intended to engage electrical contact surfaces 112 and 114 on the outer circumference of the block 82 in Figure 3C. In summary, the second embodiment includes a premolded plastic block 82 including a fluid coupler that also serves as the mounting base for the cylindrical laser module. This reduces the size of the laser submount, principaUy by eliminating the ceramic structure of the first embodiment. In this embodiment, the laser submount is partiaUy submersed in the cooUng fluid. That is, a portion of the diamond on which the laser module is mounted is disposed in the horseshoe shaped channel 84 (see Figure 3C). FinaUy, in this embodiment the electrical contacts are placed on the manifold coupler to provide a quick disconnect feature for both the electrical and the cooUng required by the module. Referring to Figures 4A and B, a third embodiment of the invention is iUustrated. This embodiment is suited for appUcations requiring higher coolant flow to dissipate increased heat load. Such an appUcation would occur, for example, during quasi-continuous wave (CW) operation. In the embodiment of Figures 4A and B, the horseshoe shaped conduit 84 of Figure 3 and the channel 88 are replaced with a copper tube 110 formed in the shape of the letter C. A diamond submount 114, to which the laser diode module is secured, is inserted through an opening in the tube 110 for direct cooling by the Uquid. It is secured in place using solder or structural adhesive. As shown in Figure 4B, the electrical contacts 116 and 118 may be terminal lugs molded into the plastic housing representing the positive and negative terminals. Contacts attached to the laser diode and laser submount are soldered to the terminal lugs. This design is preferable to the other designs for high heat dissipation because the copper tube 110 has a relatively simple straight in and out path which is more efficient in conducting heat away. In addition, the tube can be leakproof tested and checked prior to instaUation into the housing assembly. Also, aUgnment is easier due to the relaxed tolerances of the tube design versus the fixed tolerances in the molded embodiment of Figures 3B and 3C. The design iUustrated in Figure 4A is adapted to be secured to a source of Uquid coolant by means of an appropriate manifold coupler of the type generaUy iUustrated in Figure 3B. The O-rings 120 and 122 iUustrated in Figures 4 A and B, are provided for sealing the unit to such a manifold coupler. As iUustrated in the drawings, the copper tube 110 is round at the input and output points 124 and 126. Its cross-section, however, is preferably rectangular at its center section as iUustrated in the perspective view in Figure 4C. This section of the copper tube 128 is notched out to receive the laser submount 129 which is then epoxy bonded or soldered into place. The laser diode is disposed on the upper surface of the submount. An alternative, which is possible with this design, is the complete elimination of the diamond submount. The laser diode can be mounted directly to the tubing 110, on the outside thereof, thereby further simplifying the design and fabrication process. Referring to Figure 5, a fourth embodiment of the laser module according to the invention is iUustrated. In this embodiment, the ST-coupler again serves as the lens mount. The coupler 130 has the lens assembly 132 mounted therein. A retaining ring 134 secures the ST-coupUng and housing 136 securely after the aUgnment and assembly procedure. The laser diode submount 138 is soldered directly to a copper tube 140 which extends radiaUy through the cylindrical package. The rear portion of the laser submount is exposed directly to the fluid coolant through an opening in the tube 140. The tube 140 is disposed between a base 142 which is mated to the ST-coupUng block 130 and the forward section of the housing 136. Liquid coolant is suppUed to the tube 140 by means of insertable plugs 144 and 146 and elastomer tube 154 and 156. These tubes are adapted to engage the tube 140 at either end thereof thereby to form a conduit through which Uquid can be transported into and out of the module in a straight path thereby reducing the fluid pressure drop. Disposed in the base 142 are threaded bores 150 and 152. Screws are disposed in the bores for purposes of permitting aUgnment of the laser module during assembly. As iUustrated, the screws bear against the tube 140 which in turn carries the laser diode 138. By adjusting the angular position of the copper tubing using these screws it is possible to precisely aUgn the laser module output with the lens 132 after the housing and ST-coupler block have been securely bonded. Thereafter the screws may be permanently fixed in the bores 150 and 152 to prevent any change in aUgnment. Because of its simple and straight through design, the embodiment of Figure 5 is highly efficient, simple to manufacture and operate and relatively low in cost as compared with the prior art. The electrical connections to the laser module can be made in any of the various ways described in connection with the previous embodiments or in other ways known to those skiUed in this art. For many appUcations this design is preferred. The aUgnment process is performed in four steps. First, the device is mounted into an aUgnment fixture that interfaces to water and the drive electronics. WhUe operating at a low duty cycle an imaging fiber optic camera system is attached to the ST-coupler. The plastic body is then rotated to set the focus adjustment for minimum spot size (best focus) while the coupler is held fixed. In the second step, lateral angular adjustment is performed until the laser diode output is normal with respect to the plane defined by the interface of the body and optics housing. In the next step, the angular position of the collimating lens is aUgned with the laser diode output. For this purpose, the optics housing is rotated unt a symmetrical coUimated beam profile is obtained. FinaUy, lateral adjustment of the optics housing is accomplished to maximize power output. It is at this point, that the entire assembly may be epoxy injected as previously described to permanently secure the device in optical aUgnment and to hermeticaUy seal it against unwanted intrusion of moisture. The embodiment of Figure 6 is similar to the Figure 5 embodiment with a few exceptions. In the Figure 6 embodiment, the length of the coolant tube is shorter than in the Figure 5 embodiment. Also, the coolant channel is provided with a tubular fitting adapted to engage a quick disconnect coupler 202. More significantly, the Figure 6 embodiment differs from the embodiment of Figure 5 by utiUzing a two-part fitting to permit better alignment of the lens assembly. The optical housing 136 screw-threadably receives a lens fitting 204 which contains the lens assembly 132. This permits precise focusing of the output from the laser 138 after which the fitting 204 is secured in place with epoxy as previously described. A separate fiber optic fitting 206 is provided which carries the ST-coupler assembly 208 intended to engage a fiber optic cable element. The fitting 206 shdably bears against the lens fitting 204 to permit aUgnment of the fiber optic element with the output of the lens assembly 132. When properly aUgned, it is bonded in place and secured by a separate retaining ring element 210, si ular to retaining ring 134 which now secures only the lens fitting 206. In summary, the Figure 6 embodiment is a sUght improvement upon the Figure 5 embodiment in that it permits more precise and easier aUgnment of the lens system with the laser and the fiber optic with the lens system. The aUgnment process is siπύlar to that described for the Figure 5 embodiment modified to account for the separate fittings 204 and 206. Figure 6A is an exploded perspective view of the Figure 6 embodiment. Note in particular, the laser element 138 disposed inside the coolant tube 140 and the electrical contacts 220 and 221 which provide electrical power to the diode. Figure 6B is an exploded perspective view of a portion of the Figure 6 embodiment illustrating the manner in which the mounting block and laser diode are received in rectangular portion of the coolant tube 140. The laser diode 230 is mounted on the diamond substrate 232 which, in turn, is secured to the mounting block 234. The mounting block is then received as an integral part of the coolant tube 140. Coolant passing through circular openings 250 and 252 removes the heat generated during laser operation. As each of the foregoing embodiments has particular advantages and disadvantages, the selection of which module is preferred wiU depend upon the appUcation including the output required, cost considerations and other factors. Nevertheless, the present invention, in its different embodiments, discloses a cylindrical laser module which is low in cost, can be easUy assembled and aUgned, permanently secured in the aUgned condition and quickly connected and disconnected to a Uquid coolant and a laser power supply. This capabiUty and the features of the device have not heretofore been avaUable.

Claims: CLAIMS: 1. A laser module with integral cooUng comprising: (a) a first body section (30, 82, 150, 152, 153) carrying a laser diode array (32, 77, 138, 230) and including means (60, 62, 86, 140) for cooUng said array; and (b) a second body section (40, 72, 136) joined to the first body section (30, 82) carrying a lens subassembly (16,18, 74,132) for receiving laser energy from said array and projecting it as a beam along an optical path. 2. A module as claimed in Claim 1, wherein said second body section (40) mcludes means (46, 70, 130) for coupling the laser beam for transmission externaUy of the module. 3. A module as claimed in Claim 1 or 2, wherein the means (60, 62, 84, 86, 88, 90, 96) for cooUng said array includes means (62) for circulating a Uquid coolant in thermal relation with said diode array (32). 4. A module as claimed in Claim 3, wherein said means (62) for circulating includes a channel (62) in thermal relation with said diode array (32) through which said coolant is circulated. 5. A module as claimed in Claim 4, wherein said diode array (138) is mounted directly to said channel (140). 6. A module as claimed in any one of the preceding claims, wherein said diode array (32) is mounted on a substrate (36, 80, 232) which is in thermal relation with said channel (62). 7. A module as claimed in Claim 6, wherein said substrate (36, 80) is formed of diamond. 8. A module as claimed in Claim 3, wherein the means for circulating includes means (62, 86, 96, 140) for connecting a coolant supply to said first body section. 9. A module as claimed in Claim 8, wherein said means for connecting includes a coolant coupler (86) permitting coolant to enter and exit said first body section and further including a manifold (96) for releasably engaging said coolant coupler (86), said manifold (96) being in communication with said coolant supply. 10. A module as claimed in Claim 8, wherein said means for connecting includes plugs (144) for releasably engaging a tube (140) disposed in said channel through which coolant flows, said plugs communicating with said coolant supply, said tube (140) being in thermal relation with said diode array (138). 1 1. A module as claimed in Claim 1, wherein said first body section includes means (150, 152, 153) for permitting aUgnment of said laser diode array with said lens subassembly during assembly of said module. 12. A module as claimed in Claim 1, wherein said second body section (40) is bonded to said first body section (30). 13. A module as claimed in Claim 1, wherein the optical subassembly is disposed in an optical coupler (46, 70, 130) secured to said second body section. 14. A module as claimed in Claim 13, wherein said optical coupler (46) is secured to said second body section (40) at a position selected to accurately focus the laser beam along said optical path. 15. A module as claimed in Claim 13, wherein said optical coupler (46, 70, 130) includes means for coupling an external optical cable to said optical path. 16. A module as claimed in Claim 1, wherein a ceramic disk (30) is disposed between and secured to said first and second body sections, said laser diode array (32) being mounted to said disk. 17. A module as claimed in Claim 16, wherein said laser diode array (32) is mounted to said disk (30) via a diamond substrate (36). 18. A laser module with integral cooUng comprising: (a) a body member (60, 82) carrying a laser diode array (32) and including means (62) for cooUng said array; (b) a lens subassembly (16, 18, 72) secured to the body member for receiving laser energy from the laser diode array (32) and projecting it as a beam along an optical path. 19. A module as claimed in Claim 18, wherein said lens subassembly includes means (46, 70) for coupling the laser beam for transmission externaUy of the module. 20. A module as claimed in Claim 18, wherein the means for cooUng said array includes means (62) for circulating a Uquid coolant in thermal relation with said diode array (32). 21. A module as claimed in Claim 20, wherein said means for circulating includes a channel (62) in thermal relation with said diode array through which said coolant is circulated. 22. A module as claimed in Claim 21, wherein said diode array is mounted directly to said channel. 23. A module as claimed in Claim 21, wherein said diode array (20) is mounted on a substrate (36) which is in thermal relation with said channel. 24. A module as claimed in Claim 20, wherein the means for circulating further includes means (86, 96) for connecting a coolant supply to said body member. 25. A module as claimed in Claim 18 further including means for permitting aUgnment of said laser diode array (32) with said lens subassembly (16, 18).

Inventors: JACKSON JOHN EDWIN, PETERSON BRIAN LEE, RYAN DANIEL JOSEPH, SARRO CARL JOHN, JACKSON, JOHN, EDWIN, PETERSON, BRIAN, LEE, RYAN, DANIEL, JOSEPH, SARRO, CARL, JOHN
Applicants: GEN ELECTRIC CO PLC, THE GENERAL ELECTRIC COMPANY, PLC
Application year: 1998
Main classification: H01S3
IPC codes: G02B6, H01S5