Abstract:
To improve a diode laser device comprising at least one laser diode, a power supply for the laser diode, a cooling device including a heat sink with the laser diode disposed thereon, and a coolant supply allowing a coolant to flow through the heat sink, so that simple monitoring of its operation is possible without interfering with use of the diode laser device, it is proposed that an operation monitoring device be provided, and that the operation monitoring device detect a quantity representing the electric current flowing through the laser diode and a quantity representing the temperature of the coolant exiting from the heat sink and determine from these an operational quantity representing operation of the diode laser device.

Description:
[0001]    The present disclosure relates to the subject matter disclosed in international application PCT/EP01/03165 of Mar. 20, 2001, which is incorporated herein by reference in its entirety and for all purposes.  
         BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to a diode laser device comprising at least one laser diode, a power supply for the laser diode, a cooling device including a heat sink with the laser diode disposed thereon, and a coolant supply allowing a coolant to flow through the heat sink.  
           [0003]    Such diode laser devices are known from the prior art. The problem with these is that it is only possible to check whether they are operating properly by measuring the emitted laser radiation with calorimetric measuring instruments or devices for measuring the optical power, and, depending on the configuration, measurement of the optical power may prove problematic.  
           [0004]    This requires arrangement of a corresponding measuring head in the radiation path of the laser radiation, with the result that the diode laser device cannot be used for its intended purpose, for example, for pumping a solid-state laser during the measuring.  
           [0005]    The object underlying the invention is, therefore, to so improve a diode laser device of the generic kind that simple monitoring of the operation is possible without interfering with use of the diode laser device.  
         SUMMARY OF THE INVENTION  
         [0006]    This object is accomplished with a diode laser device of the kind described at the outset, in accordance with the invention, in that an operation monitoring device is provided, and in that the operation monitoring device detects a quantity representing the electric current flowing through the laser diode and a quantity representing the temperature of the coolant exiting from the heat sink and determines from these an operational quantity representing operation of the diode laser device.  
           [0007]    The advantage of the solution according to the invention is that the operational quantity can be determined without interrupting the radiation path of the laser radiation, and, consequently, also while the diode laser device is being used for its intended purpose, for example, for pumping a solid-state laser.  
           [0008]    The solution according to the invention has the further great advantage that determination of the operational quantity can be carried out with simple means, as merely determination of the quantity representing the current flowing through the laser diode and the quantity representing the temperature of the coolant exiting from the heat sink is required, and these can be detected with simple means.  
           [0009]    A multiplicity of direct and indirect methods are conceivable for detecting the quantity representing the temperature of the coolant exiting from the heat sink. If, for example, the coolant exiting from the heat sink is cooled to a certain temperature, it is conceivable to detect the cooling power required for the cooling.  
           [0010]    For reasons of simplicity, however, it is particularly advantageous for the operation monitoring device to detect the temperature of the coolant exiting from the heat sink by means of a sensor.  
           [0011]    For even more precise determination of the operational quantity, it has proven advantageous for the operation monitoring device to determine a quantity representing the temperature difference between the coolant entering the heat sink and the coolant exiting from the heat sink.  
           [0012]    Such determination of the quantity representing the temperature difference can be carried out, for example, indirectly by determining the cooling power required for reaching a specified temperature for the coolant entering the heat sink.  
           [0013]    It is, however, particularly simple for the operation monitoring device to detect the temperature of the coolant entering the heat sink by means of a sensor.  
           [0014]    In principle, the operational quantity can be determined with sufficient precision on the basis of the temperature measurements, but the precision can be further increased by the operation monitoring device detecting the flow rate of the coolant through the heat sink.  
           [0015]    This can be accomplished directly with particular advantage by the operation monitoring device detecting the flow rate of the coolant through the heat sink with a flowmeter.  
           [0016]    A particularly favorable solution allowing the operational quantity to be determined with a high degree of precision provides for the operation monitoring device to determine a thermal quantity representing the thermal output conducted away from the laser diode by the cooling device and enabling the thermal output that is not emitted by the laser diode in the form of radiation to be determined at least approximately.  
           [0017]    In principle, when determining the quantity representing the electric current flowing through the laser diode, it is conceivable to detect setting parameters of the power supply from which one can at least indirectly draw conclusions about the electric current flowing through the laser diode.  
           [0018]    With a view to determining the operational quantity as precisely as possible, it is, however, particularly expedient, in particular, in order to determine the changes in the operational quantity as exactly as possible, for the operation monitoring device to comprise a current measuring device for determining the electric current flowing through the laser diode.  
           [0019]    In the case of a laser diode it can be assumed that the voltage dropping at it will be approximately constant. However, in order to determine the operational quantity with as high a degree of accuracy as possible, provision is preferably made for the operation monitoring device to comprise a voltage measuring device for determining the voltage dropping at the laser diode during operation.  
           [0020]    When determining the operational quantity, a particularly high precision is achievable by the operation monitoring device determining an electrical quantity representing the electric power supplied to the laser diode, which constitutes the total electric power supplied to the laser diode.  
           [0021]    A particularly high accuracy is achievable in determining the operational quantity when the operation monitoring device determines the operational quantity from the electrical quantity and the thermal quantity.  
           [0022]    In addition to the electrical quantity and the thermal quantity, other parameters may be incorporated into the determining of the operational quantity.  
           [0023]    It is preferable for the parameters incorporated into the determining of the operational quantity to be selected such that the operational quantity represents the optical output power of the laser diode, so that the operational quantity is a direct measure of the optical output power of the laser diode and therefore directly supplies the most important information for operation of the laser diode.  
           [0024]    Regarding the configuration of the diode laser device it has merely been assumed that this comprises one laser diode. However, the solution according to the invention can be utilized with particular advantage when the diode laser device comprises several laser diodes.  
           [0025]    In this case, the several laser diodes are preferably fed by a common power supply.  
           [0026]    The operational quantity can also be advantageously determined with sufficient precision when the quantity representing the electric current flowing through the entirety of the laser diodes is determined.  
           [0027]    A quantity representing the electric current flowing through the totality of the laser diodes and a quantity representing the voltage dropping at the entirety of the laser diodes are therefore preferably incorporated into the electrical quantity, for which purpose the laser diodes are electrically connected in series.  
           [0028]    When several laser diodes are provided in the diode laser device, the determining of the quantity representing the temperature of the coolant exiting from the heat sink also focuses on the temperature of the coolant exiting from the totality of the heat sinks.  
           [0029]    Coolant preferably flows in parallel through the heat sinks of the several laser diodes so that each of the laser diodes is subjected to substantially the same cooling power.  
           [0030]    Details of the design of the cooling device will now be given. It is, for example, conceivable to conduct the coolant exiting from the heat sink or heat sinks away freely. It is, however, particularly advantageous for the cooling device to comprise a cooling circuit from which the heat is conducted away via a heat exchanger.  
           [0031]    Details of the way in which the operation monitoring device according to the invention operates will now be given. It is, for example, conceivable to operate the operation monitoring device continuously and to thus determine the operational quantity continuously.  
           [0032]    It is, however, sufficient to determine the operational quantity after specified time intervals or after specified operating cycles, as it can usually be assumed that the operational quantity will change only slowly and not necessarily abruptly.  
           [0033]    Further features and advantages of the solution according to the invention will be apparent from the following description and the appended drawings of an embodiment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIG. 1 is a schematic representation of a diode laser device according to the invention;  
         [0035]    [0035]FIG. 2 is an enlarged representation of two superimposed laser units;  
         [0036]    [0036]FIG. 3 is a section taken along line  3 - 3  in FIG. 2; and  
         [0037]    [0037]FIG. 4 is a block diagram schematically representing a mode of operation of an operation monitoring device according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    An embodiment of a diode laser device shown in FIG. 1 comprises a laser diode array generally designated  10  which is formed by a stack of individual laser units  12 .  
         [0039]    Each of the individual laser units  12  comprises, as shown in FIG. 2, a laser bar  14  representing a laser diode, from which laser radiation  18  exits at a front side  16  thereof.  
         [0040]    The laser bar  14  rests on a heat sink generally designated  20  comprising a cover plate  22  on which the laser bar  14  rests in a front area  24  thereof in thermal contact therewith. The heat sink  20  further comprises a base plate  26  and lying between the cover plate  22  and the base plate  26  intermediate plates  28 ,  30  and  32 . These serve to form in the heat sink  20  a cooling channel system  34  extending between an inflow channel  36  and a return flow channel  38 , which penetrate the cover plate  22 , the intermediate plates  28  to  32  and the base plate  26  substantially perpendicularly to the area of their extent.  
         [0041]    For example, starting from the inflow channel  36 , the intermediate plate  28  forms a first cooling channel section  42  of the cooling channel system  34 , which cooling channel section  42  extends from the inflow channel  36  beneath the cover plate  22  in the direction of the front area  24  and close to the front area  24  of the cover plate  22  passes via an opening  44  into a second cooling channel section  46  of the cooling channel system  34 , which cooling channel section  46  leads from the opening  44  on a side of the cooling channel section  42  that is remote from the cover plate  22  to the return flow channel  38 .  
         [0042]    Hence, the cover plate  22  can be directly cooled by the coolant entering the cooling channel system  34  via the inflow channel  36 , more particularly, as far as into the front area  24  thereof, on which the laser bar  14  rests and into which heat is discharged from the laser bar  14 .  
         [0043]    The entire heat sink  20  also serves as first connection electrode for the laser bar  14 . As second connection electrode  50 , a contact plate  50  is disposed on a side of the laser bar  14  that is located opposite the heat sink  20 . The contact plate  50  extends over the laser bar  14 , more particularly, over the entire width B thereof, and, starting from the laser bar  14 , also over part of the area of the cover plate  22  and is thereby supported by the cover plate  22 , and an insulator  52  is provided between the contact plate  50  and the heat sink  20 .  
         [0044]    Hence, the laser bar  14  is electrically contacted, on the one hand, by the heat sink  20  and, on the other hand, by the contact plate  50 .  
         [0045]    With its heat sink  20  the next laser unit  12  rests on the contact plate  50  in direct electrical contact therewith, so that, seen electrically, all the laser bars  14  of the laser diode array  10  are connected in series, and, for example, a first power supply line  56  leads from a power supply  54  to the heat sink  20  of the lowermost laser unit  12  of the laser diode array  10 , while a second power supply line  58  leads from the power supply  54  to the contact plate  50  of the uppermost laser unit  12  of the laser diode array  10 .  
         [0046]    The inflow channel  36  and the return flow channel  38 , which pass through each of the heat sinks  20 , are arranged in alignment with each other in all laser units  12 , so that the inflow channels  36  and the return flow channels  38  of the entirety of the heat sinks  20  of the laser diode array  10  are interconnectable by annular seals  62  and  64  provided between the individual heat sinks  20 . The annular seals  62  and  64  each enclose the outlet openings of the inflow channel  36  and the return flow channel  38  in one heat sink and the inlet openings of the inflow channel  36  and the return flow channel  38  in the other heat sink and thereby establish a connection between the inflow channels  36  of all heat sinks  20  of the laser diode array  10  and the return flow channels  38  of all heat sinks  20  of the laser diode array  10 .  
         [0047]    Consequently, the cooling channel systems  34  of all heat sinks  20  are supplied in parallel with coolant, and all laser bars  14  of the laser diode array  10  are therefore cooled in the same way.  
         [0048]    An inflow line  66  and a return flow line  68  are provided for supplying the laser diode array  10  with coolant. The inflow line  66  and the return flow line  68  lead to the laser diode array  10  and are, for example, connected to the inflow channel  36  and the return flow channel  38  of the heat sink  20  of the lowermost laser unit  12 .  
         [0049]    Coolant is fed by a pump  70  from a reservoir  72  into the inflow line  66 , whereas the heated coolant withdrawn via the return flow line  68  is fed to a cooler  74  which then discharges the cooled coolant into the reservoir  72 , from which the pump  70  draws in coolant again.  
         [0050]    Therefore, the reservoir  72 , the pump  70 , the inflow line  66 , the entirety of the inflow channels  36  of the laser diode array  10 , the entirety of the cooling channel systems  34  in the heat sinks  20  of the laser diode array  10 , the entirety of the return flow channels  38  of the laser diode array  10 , the return flow line  68  and the cooler  74 , which, in turn, leads to the reservoir  72 , together form a cooling device generally designated  76  with a cooling circuit  78 .  
         [0051]    The diode laser device according to the invention is also provided with an operation monitoring unit generally designated  80 . The operation monitoring unit  80  comprises an evaluation circuit  82  which is connected to a temperature sensor  86  in the inflow line  66 , to a temperature sensor  88  in the return flow line  68  and to a flowmeter  90  in either the inflow line  66  or the return flow line  68  for detecting the amount of coolant flowing through the cooling circuit  78  per time unit.  
         [0052]    Hence the evaluation circuit  82  can detect the inflow temperature T z  by means of the temperature sensor  86 , the return flow temperature T R  by means of the temperature sensor  88  and the flow rate D of coolant per time unit through the cooling circuit  78  by means of the flowmeter  90 .  
         [0053]    The evaluation circuit  82  is also connected to a voltage detection unit  92  for detecting the electrical voltage U applied to the entirety of the laser bars  14  connected in series, and to a current detection unit  94  for detecting the current I flowing from the power supply  54  through the entirety of the laser bars  14  of the laser diode array  10 .  
         [0054]    The evaluation circuit  82  operates, as shown in FIG. 4, for example, by means of a processor, in the manner that the temperature difference ΔT is first determined from the inflow temperature T z  and the return flow temperature T R  by subtraction. The temperature difference ΔT is then multiplied by the flow rate D and a further parameter K to determine a thermal quantity G T , which represents the thermal output dissipated by the cooling device  76 . In the simplest case, the parameter K is a constant, but it may be a parameter area which is dependent upon the temperature difference ΔT and/or the flow rate D.  
         [0055]    As shown in FIG. 4, the evaluation circuit  82  also determines from the voltage U at the entirety of the laser bars  14  and the electric current I passing through the entirety of the laser bars  14 , by multiplication, an electrical quantity G E  which represents the electric power supplied to the laser diode array  10 .  
         [0056]    Finally, an operational quantity F representing the optical power emitted by all laser bars  14  is determined by forming the difference between the electrical quantity G E  and the thermal quantity G T .  
         [0057]    The operational quantity F makes it possible to monitor operation of the entirety of the laser bars  14  in the laser diode array  10  during use thereof for its intended purpose, for example, for pumping a solid-state laser, and, therefore, to ascertain whether, for example, due to aging of the laser bars  14 , the optical power output is decreasing, which is shown by a decrease in the operational quantity F.