Abstract:
A laser module capable of improving the heat exchange properties between a thermoelectric module and a semiconductor laser than previously possible. The laser module has a thermoelectric module including a plurality of thermoelectric elements and first and second substrates for sandwiching the plurality of thermoelectric elements therebetween, at least the first substrate being made of silicon; and a semiconductor laser formed on the first substrate of the thermoelectric module.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a laser module which includes a thermoelectric module adapted to cool a semiconductor laser, to have a cooling function. Further, the present invention relates to a method of manufacturing such a laser module.  
           [0003]    2. Description of a Related Art  
           [0004]    Research and development is recently progressing for applying a thermoelectric module utilizing thermoelectric effect (e.g., Seebeck effect, Peltier effect, and Thomson effect) to the temperature control of the semiconductor laser. FIG. 8 shows an exemplary configuration of a laser module provided with such a thermoelectric module.  
           [0005]    As shown in FIG. 8, the laser module designated generally by  100  comprises a thermoelectric module  101  mounted within the interior of a housing  113 . To provide connections with external circuits, a plurality of lead wires  114  extend parallel to the top surface of the housing  113 . The thermoelectric module  101  comprises a plurality of thermoelectric elements  102  and two substrates  103  and  104  which sandwich the thermoelectric elements  102  therebetween. The substrates  103  and  104  are both made of alumina.  
           [0006]    A semiconductor laser  105  is formed on the top surface of the substrate  103 . The semiconductor laser  105  includes a silicon substrate  106  having a top surface to which are attached a laser chip  108 , a photodiode  109 , a thermistor  110 , etc. The substrate  106  is soldered to the top surface of the substrate  103  of the thermoelectric module  101 , with a solder layer  107  intervening between the substrates  103  and  106 .  
           [0007]    In the semiconductor laser  105 , the photodiode  109  receives laser beams emitted leftward in the diagram from the laser chip  108  and issues electric signals which depend on the intensity of the beams. The thermistor  110  is used for, e.g., detection of an extraordinary rise in temperature of the substrate  106 . Laser beams output rightward in the diagram from the laser chip  108  pass through the optical isolator  111  and the lens  112  in the mentioned order and transmit through the interior of an optical fiber  115  which extends to the exterior of the housing  113 .  
           [0008]    In the conventional laser module  100  as shown in FIG. 8, however, heat generated by the semiconductor laser  105  is transmitted through the three layers, i.e., the substrate  106 , the solder layer  107  and the substrate  103  in the mentioned order. A poor thermal conductive efficiency was therefore present between the thermoelectric module  101  and the semiconductor laser  105 , which was desired to be improved.  
           [0009]    By the way, Japanese Patent Laid-open Publication JP-8-46248 discloses a thermoelectric module having two silicon substrates for sandwiching a plurality of thermoelectric elements therebetween. However, the thermoelectric module disclosed in the above publication is not intended for use of cooling the semiconductor laser included in the laser module.  
         SUMMARY OF THE INVENTION  
         [0010]    In view of the above circumstances, the object of the present invention is to provide a laser module capable of achieving an improved heat exchange efficiency between a thermoelectric module and a semiconductor laser, and to provide a method of manufacturing the laser module.  
           [0011]    In order to solve the above problems, according to a first aspect of the present invention, there is provided a laser module comprising: a thermoelectric module including a plurality of thermoelectric elements, and a first substrate and a second substrate for sandwiching the plurality of thermoelectric elements therebetween, at least the first substrate being made of silicon; and a semiconductor laser formed on the first substrate of the thermoelectric module.  
           [0012]    In order to solve the above problems, according to a second aspect of the present invention, there is provided a method of manufacturing a laser module comprising the steps of: (a) assembling a thermoelectric module by sandwiching a plurality of thermoelectric elements between a first substrate and a second substrate, at least the first substrate being made of silicon; and (b) forming a semiconductor laser on the first substrate of the thermoelectric module.  
           [0013]    In the laser module of the present invention, heat generated by the semiconductor laser is conveyed to the thermoelectric element by way of the first substrate made of silicon so that the semiconductor laser is cooled. Silicon has a higher thermal conductivity than alumina, and hence it is possible to keep a uniform temperature within the surface of the first substrate and enhance the heat exchange efficiency between the thermoelectric module and the semiconductor laser as compared with the prior art, to thereby achieve a rapid effective cooling of the semiconductor laser.  
           [0014]    Since silicon is superior in micro-processibility including etching, markings indicative of the positions of elements (e.g., a laser chip and a thermistor) constituting the semiconductor laser or fixing grooves for fixing the elements can be formed in the surface of the first substrate by, e.g., etching, whereby the positioning accuracy upon mounting of the elements on the first substrate can be improved.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:  
         [0016]    [0016]FIG. 1 is a diagram showing the configuration of a laser module according to one embodiment of the present invention;  
         [0017]    [0017]FIG. 2 is a perspective view showing the configuration of a thermoelectric module as shown in FIG. 1;  
         [0018]    [0018]FIG. 3 illustrates manufacture steps of a substrate on one hand as shown in FIG. 1;  
         [0019]    [0019]FIG. 4 is a partially enlarged view of a silicon wafer after etching step as shown in FIG. 3;  
         [0020]    [0020]FIG. 5 illustrates manufacture steps of a substrate on the other hand as shown in FIG. 1;  
         [0021]    [0021]FIG. 6 illustrates manufacture steps of a thermoelectric element as shown in FIG. 1;  
         [0022]    [0022]FIG. 7 illustrates assembly steps of the laser module as shown in FIG. 1; and  
         [0023]    [0023]FIG. 8 is a diagram showing the configuration of a conventional laser module. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.  
         [0025]    [0025]FIG. 1 is a diagram showing a configuration of a laser module according to one embodiment of the present invention.  
         [0026]    As seen in FIG. 1, the laser module is generally designated by  10  and comprises a thermoelectric module  11  mounted within the interior of a housing  25 . The housing  25  has a bottom  25   a  which is provided with a Cu—W for heat radiation design. In order to provide a connection with an external circuit, a plurality of lead wires  26  extend in parallel with the top surface of the housing  25 . These lead wires are designed to transmit high-frequency signals. The thermoelectric module  11  comprises a plurality of P-type thermoelectric elements  12   a , a plurality of N-type thermoelectric elements  12   b , and a couple of substrates  13  and  14  which sandwich the P-type and N-type thermoelectric elements therebetween. A most preferred material of the substrate  13  is silicon single crystal.  
         [0027]    [0027]FIG. 2 is a perspective view showing a configuration of the thermoelectric module.  
         [0028]    In the thermoelectric module  11 , the adjacent P-type thermoelectric element  12   a  and N-type thermoelectric element  12   b  are connected with each other by way of electrodes  15   b  disposed on the top surface of the substrate  14 , to form a element pair. A plurality of element pairs are connected in series by way of electrodes  15   a  disposed on the undersurface of the substrate  13 . Terminals  16  and  17  for introduction of current are connected respectively to opposite ends of a series circuit formed by the element pairs.  
         [0029]    The top surface of the substrate  13  is formed with a plurality of grooves  18  and  28  which are V-shaped in section. It is preferred that the edge of the substrate  13  be appropriately machined (e.g., chamfered in R). In this case, it is possible to restrain the edge of the substrate  13  from suffering any breakage due to silicon having a lower toughness than that of alumina or the like.  
         [0030]    Referring to FIGS. 1 and 2, a semiconductor laser  19  is formed on the top surface of the substrate  13 . The semiconductor laser  19  includes a laser chip  20  emitting laser beams, a photodiode  21  for receiving some of the laser beams emitted from the laser chip  20 , a thermistor  22  for detecting the temperature, an optical isolator  23  for allowing the laser beams from the laser chip  20  to pass therethrough in a predetermined direction, and a lens  24  for collecting the laser beams which have passed through the optical isolator  23 .  
         [0031]    In this embodiment, the laser chip  20  and the photodiode  21  are fixedly secured to the grooves  18  of the substrate  13  while the optical isolator  23  and the lens  24  are firmly fastened to the grooves  28  of the substrate  13 . The laser chip  20 , photodiode  21  and thermistor  22  are electrically connected to respective predetermined lead wires  26 . The lead wires  26  in turn are electrically connected to a power supply circuit for supplying the power to the semiconductor laser  19  and electrically connected to a control circuit for controlling the oscillating operations. On the basis of outputs from the photodiode  21  and the thermistor  22 , for example, the control circuit changes the power to be supplied to the laser chip  20  by the power supply circuit, to thereby control the oscillating operations of the semiconductor laser  19 .  
         [0032]    In the semiconductor laser  19 , the photodiode  21  receives a laser beam output leftward in FIG. 1 from the laser chip  20  and provides as its output an electric signal which depends on the intensity of the laser beam. The thermistor  22  is utilized for, e.g., the detection of an extraordinary rise in temperature of the substrate  13 . A laser beam output rightward in FIG. 1 from the laser chip  20  passes through the optical isolator  23  and the lens  24  in the mentioned order and thereafter transmits through the interior of an optical fiber  27  which extends outward from the housing  25 .  
         [0033]    This embodiment allows heat generated by the semiconductor laser  19  to be conveyed to the thermoelectric elements  12   a  and  12   b  by way of only the substrate  13  made of silicon and the electrode  15   a  made of an electrically conductive material, to thereby cool the semiconductor laser  19 . Silicon has a higher thermal conductivity than alumina (thermal conductivity of alumina: 36.0 W/mK, thermal conductivity of silicon: 148.0 W/mk). It is thus possible to keep a uniform temperature within the surface of the substrate  13  and enhance the heat exchange efficiency between the thermoelectric module  11  and the semiconductor laser  19  as compared with the prior art, to thereby achieve a rapid effective cooling of the semiconductor laser  19 .  
         [0034]    Since silicon is superior in micro-processibility including etching, markings for positioning the laser chip  20 , etc., or fixing grooves for fixing the elements can be formed on the substrate  13  by, e.g., etching, whereby the positioning accuracy upon mounting of the elements on the substrate  13  can be improved.  
         [0035]    Reference is then made to FIGS.  3  to  7  to describe a method of manufacturing the laser module.  
         [0036]    A method of manufacturing the substrate  13  will first be described. FIG. 3 shows manufacture steps of the substrate  13 . At first, the surface of a silicon wafer  30  is oxidized and then a resist is formed thereon. A lithography process is then effected so as to form the grooves  18  to which the laser chip  20  and the photodiode  21  (see FIG. 1) are fitted and the grooves  28  to which the optical isolator  23  (see FIG. 1) is fitted.  
         [0037]    Next, at the etching step, the exposed oxide film is etched by using the remaining resist as a mask, and then the surface of the exposed silicon wafer  30  is anisotropically etched by using the remaining oxide film as a mask. As a result of the two etchings, the grooves  18  and  28  are formed in the surface of the silicon wafer  30 . Afterward, the remaining oxide film and resist are removed from the surface of the silicon wafer  30 .  
         [0038]    It is preferred that the first etching be a dry etching. The reason is that if the first etching is a wet etching, “under-etching” tends to occur as indicated by a broken line in FIG. 4, as a result of which the second etching proceeds from the exposed surface of the silicon wafer, which may possibly impair the pit accuracy between the grooves. Thus, by effecting the first etching in dry, the silicon wafer can be processed with an extremely high accuracy, thereby facilitating the positioning of the optical elements included in the semiconductor laser.  
         [0039]    Referring again to FIG. 3, at the insulation film forming step, an insulation film is formed by means of, e.g., oxide film method or polyimide method over the entire surface of the silicon wafer  30  formed with the grooves  18  and  28 . And then, at the plating step, plating is effected thereon by means of, e.g., thin-film adhesion method. Next, at the cutting step, the plated silicon wafer  30  is cut into an appropriate size to obtain the substrate  13 .  
         [0040]    A method of manufacturing the substrate  14  will hereinafter be described. FIG. 5 shows manufacture steps of the substrate  14 . These steps are carried out in parallel with the manufacture steps of the substrate  13 .  
         [0041]    At the insulation film forming step, an insulation film is formed over the entire surface of a silicon wafer  31  by means of, e.g., oxide film method or polyimide method. And then, at the plating step, plating is effected thereon by means of, e.g., thin-film adhesion method. Next, at the cutting step, the plated silicon wafer  31  is then cut into blocks each having an appropriate size to obtain the substrate  14 .  
         [0042]    A method of manufacturing the thermoelectric element will then be described. FIG. 6 shows manufacture steps of the thermoelectric element. These steps are carried out in parallel with the manufacture steps of the substrates  13  and  14  for example.  
         [0043]    At the cutting step, a bulk material  40  is cut into blocks each having an appropriate size. And then, at the plating step, plating is effected on the entire surface of an element material  41 . Next, at the dicing step, the plated element material is then diced into blocks each having an appropriate size to obtain the thermoelectric element  12  (P-type thermoelectric element or N-type thermoelectric element).  
         [0044]    A method of assembling the laser module will then be described. FIG. 7 shows assembly steps of the laser module.  
         [0045]    At the assembly step, the thermoelectric module  11  is assembled by sandwiching the plurality of thermoelectric elements  12  formed in accordance with the manufacture steps of FIG. 6 between the substrate  13  formed with the electrodes in accordance with the manufacture steps of FIG. 3 and the substrate  14  formed with the electrodes in accordance with the manufacture steps of FIG. 5. Then, at the attachment step after inspection of the assembled thermoelectric module  11 , the laser chip  20  and the photodiode  21  are fixedly secured to the grooves  18  of the substrate  13 , while the optical isolator  23  and the lens  24  are firmly fastened to the grooves  28  of the substrate  13 . The thermistor  22  is then attached onto the substrate  13 . The semiconductor laser  19  is thus formed on the thermoelectric module  11 . Then, as shown in FIG. 1, the thermoelectric module  11  provided with the semiconductor laser  19  is mounted within the interior of the housing  25  and the optical fiber  27  is fitted via the lens  24  to the optical isolator  23 , to consequently complete the manufacture steps of the laser module  10 .  
         [0046]    According to the present invention, as set forth hereinabove, at least one of the two substrates included in the thermoelectric module is made of silicon and the semiconductor laser is formed on the silicon substrate, whereby it is possible to enhance the heat exchange efficiency between the thermoelectric module and the semiconductor laser than ever before.  
         [0047]    While the illustrative and presently preferred embodiment of the present invention has been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.