Abstract:
A laser diode array, having one or a plurality of electrically mounted laser diode bars, in which current paths through the array are sufficiently parallel and close together to result in a substantial reduction of inductance. As a result, higher duty cycles at lower pulse widths are possible. In some embodiments, a heatsink is provided as part of an overall assembly. The heatsink may provide part or all of a return portion of the current path for the array. Alternatively, the heatsink may be insulated from the rest of the array. The array itself may be fabricated in any of a number of known manners.

Description:
The present application claims benefit of domestic priority from Provisional Application No. 60/345,940, filed Nov. 7, 2001. The present application incorporates by reference the disclosure of that provisional application. 
    
    
     FIELD OF THE INVENTION 
     High speed on-off switching of laser diode arrays requires the fabrication of laser diode array assemblies with the lowest inductance possible. To accomplish lower inductance, the current path into the array (from the power source), through the array, and out of the array (back to the power source) needs to be as short as possible. The input and output lines through the array need to be as close to each other as possible and, if possible, parallel or substantially parallel to each other. 
     BACKGROUND OF THE INVENTION 
     Laser diode arrays can be run continuously, in a manner known as continuous wave (CW) operation. CW operation usually involves providing drive current to the arrays on the order of 0 to 40 amps continuously. 
     Laser arrays can also be operated in a pulsed fashion, which is known as quasi continuous wave (QCW) operation. QCW operation usually involves providing drive current to the arrays on the order of 0 to 200 amps. Even higher drive currents are expected in the near future. Pulsed operation usually involves pulse widths in a range of less than 1 us to more than 100 ms, with most applications being in the 100 to 500 us pulse width range. These pulses are then usually repeated at intervals from 1Hz to several kHz. 
     FIG. 1 shows a configuration of a typical laser diode array  100  with laser diode bars  105  disposed in a substrate  115 . When current is applied to the array  100 , the laser diode bars  105  emit laser light generally in a direction  110 . 
     To apply current to the array  100 , a cathode connection  120  and an anode connection  130  are connected to opposite sides of the array  100 . As also shown in FIG. 1, a heatsink  140  is provided below array  100 , and insulation  150  is interposed between heatsink  140  and the respective cathode and anode connections  120 ,  130 . 
     Metallization  155  around the substrate  115  and, as necessary or appropriate, around the laser diode bars  105  (except over the emission points of those bars, of course), enables the wrapping of current around the end of the laser diode array. The current flows in the direction of arrows  160 . 
     In the just-described arrangement, distances between cathode and anode connections  120 ,  130  will be a function of the width of the heatsink  140 , which in turn will be a function of the width of the laser diode array  100 . The distance between those connections  120 ,  130  tends to be relatively substantial, on the order of 10 mm, depending on considerations such as the number of laser diode bars in the array  100  (dictating, to some extent, the width of the array  100 ); the dimensions of the heatsink  140  on which the array is mounted; the amount of insulation  150  that is needed between the cathode and anode connections  110 ,  120  and the heatsink  140 ; and the like. 
     With the configuration of FIG. 1, pulsed operation like that described earlier is attainable. 
     When operating at somewhat longer pulse widths, on the order of  100  us and above, the “Rise and Fall Time” of the current pulse is of less of a concern. The “Rise and Fall Time” of a current pulse from 0 current to full drive, say, 100 amps, and then from full drive current back to 0 amps is usually on the order of 1 us to 20 us, which is not a large concern when the actual pulse width itself is on the order of 100 us or more. 
     The “Rise and Fall” time of a current pulse becomes more of a concern as the actual pulse width becomes shorter and shorter, and the intervals between pulses become shorter and shorter. For example: If the required pulse width is 1 us, then the rise and fall time of the current pulse has to be considerably shorter. Rise and fall times are affected greatly by the inductance of the current path, especially when trying to switch on and off high current pulses. So, in order to have faster rise times, it is important to keep the inductance of the entire system as low as possible. 
     Inductance in high current pulsed systems can be minimized by keeping the power leads as close to each other as possible and, if possible, as parallel or substantially parallel to each other as possible, in order to cancel out magnetic fields which high current flows create. However, while making power leads that are close and parallel to each other is relatively easy, creating the same lower inductance conditions in laser diode arrays is not. 
     Because of prior operating parameters for laser diode arrays, it has not been necessary to investigate the need for low inductance arrays. However, as needs have changed, with shorter pulse widths and faster repetition rates, it has become necessary to investigate array configurations, and in particular anode/cathode configurations, which will provide sufficiently low inductance to enable fast rise and fall times. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, there has arisen a need for laser diode arrays, still capable of operating at very high drive currents, on the order of 50 to 200 amps per pulse, but operating with pulse widths of less than 1 us and at very short intervals between pulses, leading to laser diode arrays with pulsed operations on the order of 100 kHz and above, more than two orders of magnitude higher than QCW rates at which laser diode arrays have typically operated. It is the need for these shorter and shorter pulse widths, combined with higher and higher drive currents, that has led to a need to fabricate of laser diode arrays with the lowest inductance possible. 
     To achieve these and other objects, according to the present invention, a low inductance laser diode array is provided in which, in an exemplary embodiment, the cathode and anode connections to the array are positioned substantially parallel to each other, and more closely together than has been the case previously. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a typical laser diode array mounted on a heatsink, with its anode and cathode connections. 
     FIG. 2 shows one configuration of the inventive low inductance laser diode array, with its anode and cathode connections. 
     FIG. 3 shows the array of FIG. 2, mounted on a heatsink. 
     FIG. 4 shows another mounting configuration of the inventive low inductance laser diode array, with an insulated heatsink. 
     FIG. 5 shows yet another mounting configuration of the inventive low inductance laser diode array. 
     FIG. 6 shows a configuration similar to that of FIG. 5, but with an insulated heatsink. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     By way of introduction, it is noted that, in a typical laser diode array, there will be a current path by which current passes from a current source and enters the array, passes through one or more diode bars which are mounted in the array, and then returns to the current source. Typically at least part of this current path will be along the upper surface of the array, to some extent, because that will be the easiest way to provide a current path through all of the laser diode bars. Diode bar mounting techniques such as various known-flavors of “Rack&#39;n Stack,” or the inventor&#39;s own patented technique, shown for example in U.S. Pat. No. 5,040,187, which is incorporated by reference here, will provide a current path as just described. Alternatively, diode bars may be mounted in a conductive substrate, as in another of the inventor&#39;s patents, U.S. Pat. No. 5,128,951, which also is incorporated by reference here. 
     In any of the above-mentioned laser diode array structures, there is an upper conductive layer, of one form or another, through which current enters the array from one “end,” travels through the array, exits from the other “end,” and goes back to the current source. FIG. 1 shows an example of this path. 
     However, the FIG. 1 path is relatively long. As a result, as discussed earlier, the inbound and outbound current paths will not be in close proximity to each other. The distance between the inbound and outbound current paths can cause relatively substantial inductance as compared to an array with the current paths in closer proximity to each other. 
     In accordance with the invention, then, an array is fabricated with two current paths, one of which is the current path through the array, and the other of which is the current path back to the source. These two current paths are provided in close proximity to each other and, if possible, parallel or substantially parallel to each other. As a result, it is possible to lower the overall array inductance, thus enabling a decrease in the rise and fall times of the array, and thus in turn allowing an array to be driven effectively by faster pulses. 
     FIG. 2 shows one configuration of the inventive array  200  with the inventive current path configuration. In FIG. 2, laser diode bars  205  are mounted in substrate  215 . Cathode connection  220  is provided on top of the array  200 , and anode connection  230  is provided on the bottom of the array  200 . A conductive layer  255  is provided around the array so that there is an electrical path between the cathode and anode connections  220 ,  230 . With this electrical path, current flows in the direction of arrows  260 . Effectively, the distance between the cathode and anode connections  220 ,  230  is substantially the thickness of substrate  215 , rather than the width of the substrate, as is the case in the configuration of FIG.  1 . 
     Comparing FIGS. 1 and 2, it is apparent that current paths in FIG. 2 are much closer to each other than in the configuration of FIG.  1 . The FIG. 2 current paths also are substantially parallel to each other. As a result, the FIG. 2 configuration will provide lower inductance as compared with the FIG. 1 configuration. 
     FIG. 3 shows a laser diode array  300  with laser diode bars  305  mounted in substrate  315 , yielding laser emission direction  310  as shown. Array  300  is attached to a heatsink  340 . The heatsink  340  typically is made of copper, though other metals or materials can be used. In this embodiment, in the absence of insulation between the array and the heatsink, the heatsink  340  will not be electrically isolated from the array  300 . The array  300  may be mounted to the heatsink  340  in any of a number of known manners, well within the abilities of the ordinarily skilled artisan, and so the mounting details need not be described here. As with the embodiment of FIG. 2, cathode connection  320  and anode connection  330 , and the provision of a conductive layer  355  around the substrate  315  provide a current path  360  in which the “inbound” and “outbound” paths are substantially parallel to each other, and are separated from each other essentially by the thickness of substrate  315 . 
     FIG. 4 shows a variant of FIG.  3 . In FIG. 4, a laser diode array  400  comprises a plurality of laser diode bars  405  mounted in a substrate  415 , yielding laser emission direction  410 , with cathode connection  420  and anode connection  430  as shown. A conductive layer  455  yields a current path  460  as shown. The array  400  is mounted on heatsink  440 , this time with an electrical isolation layer  480  is interposed between the heatsink  440  and the array  400 , and a solder or adhesive joint  490  provided between the array  400  and the isolation layer  480 . The isolation layer  480  may be beryllium oxide (BeO), another ceramic, or any such suitable electrical isolator. 
     FIG. 5 shows a further variant in which a laser diode array  500  has laser diode bars  505  mounted in a conductive layer  525  which constitutes the array substrate. In the preceding embodiments shown in the earlier Figures, the laser diode array  100 ,  200 ,  300 ,  400  could be fabricated using any number of techniques which are well known to ordinarily skilled artisans, such as the various known flavors of a technique known as “Rack&#39;n Stack”. Another technique, different from “Rack&#39;n Stack,” is described in U.S. Pat. No. 5,040,187, which is incorporated by reference herein. 
     In contrast to the arrays of FIGS. 1-4, the array  500  of FIG. 5 may be fabricated as described, for example, in U.S. Pat. No. 5,128,951, which is incorporated by reference herein. One difference between what is shown expressly in U.S. Pat. No. 5,128,951 and the array shown in FIG. 5 is a further conductive portion  565  at the right hand side of FIG. 5, to facilitate the completion of a current path  560  from the cathode connection  520  to the anode connection  530 . The portion  565  may be an integral part of conductive layer  525 . 
     In the FIG. 5 embodiment, there also is a insulative portion, or isolation layer  585 . The FIG. 5 isolation layer  585  does not extend all the way between the array  500  and the heatsink  540 . Instead, the further conductive portion  565  connects the conductive layer  525  to the heatsink  540 , completing the current path from cathode portion  520  to anode portion  530 . 
     Finally, the embodiment shown FIG. 6 has aspects similar to those of FIGS. 4 and 5, as follows. In FIG. 6, a laser diode array  600  comprises laser diode bars  605  mounted in a conductive layer  625 , yielding laser emission direction  610  as shown. An additional conductive portion  665 , which may be an integral part of the conductive layer  625 , extends downwardly, a little more deeply than the rest of conductive layer  525 , similarly to the FIG. 5 embodiment. An isolation layer  680  is interposed between the array  600  and heatsink  640 , similarly to the FIG. 4 embodiment. In addition, a solder or adhesive joint  690  is provided between isolation layer  680  and array  600 . Cathode connection  620  and anode connection  630  are at the left-hand side of FIG. 6, similarly to the other embodiments. In one version of the FIG. 6 embodiment, conductive layer  695  may be provided underneath insulative layer  685 , to complete the current path  660  between cathode connection  620  and anode connection  630 . The conductive  695  may be the same material as (and even integral with) conductive layer  625 , or may be made of another material, as appropriate. Conductive layer  695  also may be dispensed with, where solder joint  690  is sufficient to complete the current path. 
     In the described embodiments, the laser diode arrays are shown as having upper and lower surfaces that are parallel to each other. However, practically speaking, the upper and lower surfaces may not be perfectly parallel, but instead may be substantially parallel. Moreover, the second current path may be laid down along a surface that is not perfectly parallel to the first current path, with the result that the first and second current paths may not be perfectly parallel to each other, but instead may be substantially parallel. For purposes of achieving the goals of the invention, it is sufficient that, over the distance that the first and second current paths travel; the distance between them not change significantly. The degree of acceptable “parallelism” will be a function of the performance goals to be accomplished, and the corresponding degree of necessary decrease in inductance. 
     In summary, the present invention relates not to laser diode arrays themselves, but rather to current paths into, through, and out of the array. The configurations detailed herein yield lower inductance than previously has been achieved in laser diode arrays, and hence enables much faster rise and fall times, and much faster pulse repetitions. 
     While the invention has been described with reference to several embodiments, various modifications within the scope and spirit of the invention will be apparent to those of working skill in this technological field. Accordingly, the scope of the invention is to be measured by the appended claims.