Abstract:
A semiconductor laser device includes: a semiconductor laser including a plurality of emission regions into which currents are injected to emit laser beams and first and second major surfaces opposite to each other; and a plurality of first wires bonded to the first major surface of the semiconductor laser, wherein the first major surface of the semiconductor laser has a first stripe region corresponding to one of the plurality of emission regions, and a second stripe region corresponding to another of the plurality of emission regions, and the number of the first wires bonded to the first stripe region is larger than the number of the first wires bonded to the second stripe region.

Description:
BACKGROUND OF THE INVENTION 
     Field 
       [0001]    The present invention relates to a semiconductor laser device for use as a visible light source for a projector or the like or as an excitation light source for a processing machine or the like. 
       Background 
       [0002]    A semiconductor laser has merits such as reduction in size, good color reproducibility, low power consumption and high luminance, and is expected as a light source for projectors or projection-type displays for cinemas or the like (see, for example, JP 2011-49338 A). However, when laser beam is applied to a surface to be illuminated, a speckled pattern called speckle noise appears, showing a flickering image. The cause of this phenomenon is interference due to single-wavelength high-coherence characteristics of the laser beam. Speckle noise causes a video viewer to have a feeling of annoyance and fatigues the viewer&#39;s eyes. It is desirable to reduce the speckle noise. 
         [0003]    As a method of reducing the above-described speckle noise, a method of vibrating the illuminated surface and a method of placing a diffusing plate in the optical path between the semiconductor laser and the illuminated surface may be mentioned. Each of these methods, however, requires a markedly high cost of implementation. There is a relationship Cs∝1/√Δλ between the magnitude Cs of speckle noise and the half-width Δλ of the oscillation spectrum of laser beam. Increasing the half-width of the oscillation wavelength of the light source is effective as a method for reducing speckle noise. 
         [0004]    A plurality of semiconductor lasers differing in wavelength may be used to increase the half-width of the oscillation wavelength of the light source. However, there is a need to manufacture a plurality of semiconductor lasers having active layers different from each other. Also, a plurality of laser beams differing in wavelength cannot be emitted from one semiconductor laser. 
       SUMMARY 
       [0005]    In view of the above-described problem, an object of the present invention is to provide a semiconductor laser device in which the half-width of the oscillation wavelength of one semiconductor laser can be increased to reduce speckle noise. 
         [0006]    According to the present invention, a semiconductor laser device includes: a semiconductor laser including a plurality of emission regions into which currents are injected to emit laser beams and first and second major surfaces opposite to each other; and a plurality of first wires bonded to the first major surface of the semiconductor laser, wherein the first major surface of the semiconductor laser has a first stripe region corresponding to one of the plurality of emission regions, and a second stripe region corresponding to another of the plurality of emission regions, and the number of the first wires bonded to the first stripe region is larger than the number of the first wires bonded to the second stripe region. 
         [0007]    In the semiconductor laser device according to the present invention, wire bonding positions are set one-sidedly on one emission region to make uneven currents flowing into the emission regions, and heat generated in the wires is caused to flow one-sidedly into the one emission region, thereby obtaining different active layer temperatures. The oscillation wavelengths of laser beams emitted from the emission regions are thereby made different from each other, thus increasing the half-width of one semiconductor laser and reducing speckle noise. 
         [0008]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a side view of a semiconductor laser device according to a first embodiment of the present invention. 
           [0010]      FIG. 2  is a front view of the semiconductor laser device according to the first embodiment of the present invention. 
           [0011]      FIG. 3  is a sectional view of the semiconductor according to the first embodiment of the present invention. 
           [0012]      FIG. 4  is a plan view of essential portions of the semiconductor laser device according to the first embodiment of the present invention. 
           [0013]      FIG. 5  is a diagram showing the results of obtaining temperature distributions in the active layer in the resonator of the semiconductor laser according to the first embodiment of the present invention. 
           [0014]      FIG. 6  is a plan view showing essential portions of a semiconductor laser device according to a comparative example. 
           [0015]      FIG. 7  is a diagram showing the results of obtaining temperature distributions in the active layer in the resonator of the semiconductor laser according to the comparative example. 
           [0016]      FIG. 8  is a diagram showing the results of obtaining temperature distributions in the active layer in the resonator in a case where the n-side electrode and the gold plating layer are formed on the entire surface. 
           [0017]      FIG. 9  is a plan view of essential portions of a semiconductor laser device according to a second embodiment of the present invention. 
           [0018]      FIG. 10  is a plan view of essential portions of a semiconductor laser device according to a third embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    A semiconductor laser device according to the embodiments of the present invention will be described with reference to the drawings. The same components will he denoted by the same symbols, and the repeated description thereof may be omitted. 
       First Embodiment 
       [0020]      FIG. 1  is a side view of a semiconductor laser device according to a first embodiment of the present invention.  FIG. 2  is a front view of the semiconductor laser device according to the first embodiment of the present invention. A semiconductor laser  1  is incorporated in a φ9.0 mm-stem package frequently used in light sources for communication, disk systems or projectors. 
         [0021]    The semiconductor laser  1  is joined to a submount  2 . The submount  2  is mounted on a block  3  on a stem. Copper is ordinarily used as a material for the block  3  in order to reduce the thermal resistance of the package. The block  3  is bonded to an eyelet  4  on the stem. The diameter of the eyelet  4  is 9.0 mm. 
         [0022]    Leads  5  and  6  are passed through holes provided in the eyelet  4 . Sealing glass  8  for electrically separating the eyelet  4  and the leads  5  and  6  from each other is provided between the eyelet  4  and the leads  5  and  6 . An upper surface of the semiconductor laser  1  and the lead  5  are electrically connected to each other by gold wires  8 . An upper surface of the submount  2  and the lead  6  are electrically connected to each other by gold wires  9 . 
         [0023]    A bottom surface  10  of the eyelet  4  is in contact with a frame (not shown) for the system. As a result, heat generated from the semiconductor laser  1  is released to the outside through the submount  2 , the block  3 , the eyelet  4  and the bottom surface of the eyelet  4 . The stem package may be capped in order to encapsulate the semiconductor laser  1 . 
         [0024]      FIG. 3  is a sectional view of the semiconductor laser  1  according to the first embodiment of the present invention. An n-type AlInP lower cladding layer  12 , an undoped AlInP lower optical guide layer  13 , a GaInP active layer  14 , an undoped AlGaInP upper optical guide layer  15 , a p-type AlInP upper cladding layer  16  and p-type GaAs contact layers  17  and  18  are successively formed on an n-type GaAs semiconductor substrate  11 . The contact layers  17  and  18  are etched to form ridge stripes. 
         [0025]    The semiconductor substrate  11  has a thickness of 50 to 150 μm. The lower cladding layer  12  has a thickness of 0.5 to 4.0 μm and a carrier concentration of 0.5 to 1.5×10 18  cm −3 . Each of the lower optical guide layer  13  and the upper optical guide layer  15  has a thickness of 0.02 to 0.4 μm. The active layer  14  has a thickness of 3.0 to 20 nm. The upper cladding layer  16  has a thickness of 0.5 to 4.0 μm and a carrier concentration of 0.5 to 2.0×10 18  cm −3 . Each of the contact layers  17  and  18  has a thickness of 0.05 to 0.5 μm and a carrier concentration of 1.0 to 4.0×10 19  cm −3 . 
         [0026]    Insulating film  19  such as silicon nitride film is formed on the upper cladding layer  16  and at opposite sides of the contact layers  17  and  18 . Openings are formed in the insulating film  19  on the contact layers  17  and  18 . Each opening has a width of 60 μm. 
         [0027]    A p-side electrode  20  is formed on the contact layers  17  and  18  and the insulating film  19  and is low-resistance-joined to the contact layers  17  and  18  through the openings of the insulating film  19 . An gold plating layer  21  is formed on the p-side electrode  20 . The p-side electrode  20  is a multilayer structure formed of thin films of, for example, Ti, Pr or Au and has a total thickness of 0.05 to 1.0 μm. The gold plating layer  21  has a thickness of 1.0 to 6.0 μm. An n-side electrode  22  is joined to a lower surface of the semiconductor substrate  1  i. An gold plating layer  23  is formed on a lower surface of the n-side electrode  22 . The n-side electrode  22  is a multilayered structure formed of thin films of, for example, Ti, Pr or Au and has a total thickness of 0.05 to 1.0 μm. The gold plating layer  23  has a thickness of 1.0 to 6.0 μm. 
         [0028]    Regions in the active layer  12  located right below the two openings of the insulating film  19  are formed as first and second emission regions  24  and  25  into which currents are injected to emit laser beam. Thus, laser beams are emitted from two places in the semiconductor laser  1 . The n-side electrode  22  and the gold plating layer  23  are formed on only the first emission region  24  side of the semiconductor substrate  11 . 
         [0029]      FIG. 4  is a plan view of essential portions of the semiconductor laser device according to the first embodiment of the present invention. The semiconductor laser  1  shown in  FIG. 3  is shown upside down. The gold plating layer  21  n the front surface of the semiconductor laser  1  is joined to the submount  2 . The active layer  14  of the semiconductor laser  1  is therefore markedly close to the submount  2 . A plurality of gold wires  8  are bonded to the gold plating  23  on the back surface of the semiconductor laser  1 . A plurality of gold wires  9  are bonded to the submount  2 . Each of the number of gold wires  8  and the number of gold wires  9  is 6, and each gold wire has a diameter of 25 μm and a length of 2.0 mm. 
         [0030]    The back surface of the semiconductor laser  1  has a first stripe region  26  corresponding to the first emission region  24 , and a second stripe region  27  corresponding to the second emission region  25 . The n-side electrode  22  and the gold plating layer  23  are formed on the first stripe region  26 , not on the second stripe region  27 . The plurality of gold wires  8  are bonded only to the first stripe region  26 . 
         [0031]    A method of manufacturing the semiconductor laser  1  according to the present embodiment will subsequently be described. First, the lower cladding layer  10 , the lower optical guide layer  11 , the active layer  12 , the upper optical guide layer  13 , the upper cladding layer  14  and a contact layer are successively formed on the semiconductor substrate by a crystal growth method such as metal organic chemical vapor deposition (MOCVD). The contact layer is then selectively removed by etching to leave the contact layers  17  and  18  only on the first and second emission regions  24  and  25 . The insulating film  19  is thereafter formed on the entire surface, and portions of the insulating film  19  on the first and second emission regions  24  and  25  are removed by etching. Subsequently, the p-side electrode  20  and the gold plating layer  21  are formed. Subsequently, the back surface of the semiconductor substrate  11  is polished so that the semiconductor substrate  11  has a desired thickness, and the n-side electrode  22  and the gold plating layer  23  are formed. The semiconductor substrate  11  is cleaved so that the resonator length is 1.5 mm, a coating having a reflectance of 10% is formed on the front end surface, and a coating having a reflectance of 90% is formed on the rear end surface. The semiconductor laser  1  according to the present embodiment is manufactured according to the above-described method. 
         [0032]    Currents flow through the first and second emission regions  24  and  25 , and laser beam is guided through the first and second emission regions  24  and  25 . In the first and second emission regions  24  and  25 , therefore, heat is generated due to threshold currents, non-emission recoupling, light absorption or Joule beat.  FIG. 5  is a diagram showing the results of obtaining temperature distributions in the active layer in the resonator of the semiconductor laser according to the first embodiment of the present invention. Heat simulation was made by assuming that the total value of currents injected into the semiconductor laser  1  is 5.0 A and the outside temperature is 0° C. and by considering Joule heat with respect to the first and second emission regions  24  and  25 . In  FIG. 5, 0  on the abscissa corresponds to the position of the front end surface and 1.5 mm on the abscissa corresponds to the position of the rear end surface. Here, heat generation due to light absorption and non-emission recoupling is excepted. However, temperature distributions in the two emission regions due to light absorption and non-emission recoupling are equal to each other because the front and rear end surface reflectances, the optical gains and the threshold current values of the two emission regions are substantially equal to each other. 
         [0033]    There is a difference of about 1.5° C. between the active layer temperature in the first emission region  24  and the active layer temperature in the second emission region  25 . The cause of this is as described below. As shown in  FIG. 4 , the gold wires  8  are connected one-sidedly on the first emission region  24 , and the electrode exists only on the first emission region  24  side. The current flowing from the gold wires  8  into the first emission region  24  is therefore larger than the current flowing from the gold wires  8  into the second emission region  25 . As a result, Joule heat in the first emission region  24  is larger than that in the second emission region  25 . Further, since the gold wires  8  are connected on the first emission region  24  side, Joule heat generated in the gold wires S is transferred to the vicinity of the first emission region  24  to further increase the temperature difference between the first emission region  24  and the second emission region  25 . 
         [0034]    The temperature dependence of the oscillation wavelength, which varies depending on the materials of the semiconductor laser and the oscillation wavelength, is about 0.20 nm/° C. in the semiconductor laser  1  according to the present embodiment using an A[GaInP-based material. In the semiconductor laser  1 , therefore, the wavelength difference between the first emission region  24  and the second emission region  25  is about 0.3 nm. Thus, laser beams differing in wavelength are emitted from one semiconductor laser  1 . 
         [0035]      FIG. 6  is a plan view showing essential portions of a semiconductor laser device according to a comparative example. For uniform current injection into a plurality of emission regions, the first and second stripe regions  26  and  27  respectively corresponding to the first and second emission regions  24  and  25  are evenly bonded.  FIG. 7  is a diagram showing the results of obtaining temperature distributions in the active layer in the resonator of the semiconductor laser according to the comparative example. Heat simulation was made under the same conditions as in  FIG. 5 . The temperature difference between the first emission region  24  and the second emission region  25  is small. Accordingly, the wavelengths of beams emitted from the two emission regions are substantially equal to each other. The half-width of the emission wavelength of one semiconductor laser  1  is therefore narrow, so that speckle noise occurs. 
         [0036]    On the other hand, in the semiconductor laser device according to the present embodiment, wire bonding positions are set one-sidedly on one emission region to make uneven currents flowing into the emission regions, and heat generated in the wires is caused to flow one-sidedly into the one emission region, thereby obtaining different active layer temperatures. The oscillation wavelengths of laser beams emitted from the emission regions are thereby made different from each other, thus increasing the half-width of one semiconductor laser  1  and reducing speckle noise. 
         [0037]      FIG. 8  is a diagram showing the results of obtaining temperature distributions in the active layer in the resonator in a case where the n-side electrode and the gold plating layer are formed on the entire surface. It can be understood that the temperature difference between the first emission region  24  and the second emission region  25  is reduced in comparison with the case of the present embodiment shown in  FIG. 5 . Moderation of current one-sidedness by current spread on the electrode can thus be prevented by forming the n-side electrode  22  and the gold plating layer  23  only on one of the emission regions as shown in  FIG. 4 , 
         [0038]    The wire-bonded first stripe region  26  of the semiconductor laser  1  is closer to the wire-bonded region of the submount  2  than the second stripe region  27  not wire-bonded. The distance between the two wire-bonded regions is reduced as described above, thereby increasing the current flowing through the wire-bonded first emission region  24  while reducing the current flowing through the second emission region  25  not wire-bonded. As a result, the temperature difference between the first emission region  24  and the second emission region  25  is increased and the advantage of the present invention is heightened. 
       Second Embodiment 
       [0039]      FIG. 9  is a plan view of essential portions of a semiconductor laser device according to a second embodiment of the present invention. The number of the plurality of gold wires  8  bonded to the first stripe region  26  is larger than the number of gold wires  8  bonded to the second stripe region  27 . Thus, it is not necessarily required that all the gold wires  8  be bonded to only one of the emission regions. As long as the numbers of wires are uneven, a temperature difference occurs between the first emission region  24  and the second emission region  25 , enabling reduction of speckle noise. 
       Third Embodiment 
       [0040]      FIG. 10  is a plan view of essential portions of a semiconductor laser device according to a third embodiment of the present invention. A plurality of gold wires  8  are bonded only to the first stripe region  26 . The temperature difference between the first emission region  24  and the second emission region  25  is thereby made larger than that in the second embodiment, thus achieving a further reduction of speckle noise. However, the larger temperature difference can be achieved in the case where the n-side electrode  22  and the gold plating layer  23  are formed only on the first stripe region  26  as in the first embodiment. 
         [0041]    The first to third embodiments have been described with respect to a case where the number of emission regions of the semiconductor laser  1  is two. The present invention, however, is not limited to this. It is apparent that the same advantage can also be achieved in a case where the semiconductor laser  1  has three or more emission regions. In the case where the number of emission regions is two, however, the spacing between the emission regions is so large that the current flowing mainly into one of the emission regions cannot easily flow into the other adjacent emission region. As a result, the difference between the currents flowing into the two emission regions is increased and the advantage of the present invention is thereby heightened. 
         [0042]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
         [0043]    The entire disclosure of Japanese Patent Application No. 2016-045757, filed on Mar. 9, 2016 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.