Abstract:
A laser device includes: an optical modulator that is optically coupled to a semiconductor laser mounted on a first mounting portion; a second mounting portion that is separately away from the first mounting portion; a bridge that couples the first mounting portion and the second mounting portion; a driver IC that is mounted on the second mounting portion and drives the optical modulator through a transmission pathway provided on the bridge; and a capacitor that is provided on the bridge and is coupled to the transmission pathway.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-074981, filed on Mar. 25, 2009 and Japanese Patent Application No. 2010-047607, filed on Mar. 4, 2010, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    (i) Technical Field 
         [0003]    The present invention relates to a laser device having a semiconductor laser. 
         [0004]    (ii) Related Art 
         [0005]    A laser device having a semiconductor laser and an optical modulator is known. A driver IC drives the optical modulator through a transmission pathway. Here, when an optical modulator requiring direct-current bias for driving is used, a capacitor is connected to the optical modulator in order to prevent inflow of the direct-current bias to the driver IC. 
         [0006]    A direct-current cutting capacitor is susceptive to heat of a driver IC, and the property of the direct-current cutting capacitor gets variable, when the direct-current cutting capacitor is located near the driver IC. This may influence a signal transmitted to the optical modulator of the semiconductor laser. And, the modulation property may be degraded. Further, reliability may be degraded because of breaking of the capacitor. 
       SUMMARY 
       [0007]    It is an object of the present invention to provide a laser device having favorable modulation property. 
         [0008]    According to an aspect of the present invention, there is provided a laser device including: an optical modulator that is optically coupled to a semiconductor laser mounted on a first mounting portion; a second mounting portion that is separately away from the first mounting portion; a bridge that couples the first mounting portion to the second mounting portion; a driver IC that is mounted on the second mounting portion and drives the optical modulator through a transmission pathway provided on the bridge; and a capacitor that is provided on the bridge and is coupled to the transmission pathway. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates a schematic top view of a laser device in accordance with a comparative example; 
           [0010]      FIG. 2  illustrates a schematic top view of a laser device in accordance with a first embodiment; 
           [0011]      FIG. 3  illustrates a schematic cross sectional view of the laser device in accordance with the first embodiment; and 
           [0012]      FIG. 4  illustrates a schematic top view of a laser device in accordance with a second embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    A description will be given of a laser device in accordance with a comparative example, in order to state a problem to be solved by the following embodiment. 
       Comparative Example 
       [0014]      FIG. 1  illustrates a schematic top view of a laser device  100  in accordance with the comparative example. A package  10  houses a sub carrier  22  and a heat sink  30 . A bridge  40  couples the sub carrier  22  to the heat sink  30 . As illustrated in  FIG. 3 , there is a space under the bridge  40 . A temperature control device  20  is separated away from the heat sink  30 . 
         [0015]    A carrier  21  is mounted on the temperature control device  20 . The sub carrier  22  is mounted on the carrier  21 . A semiconductor laser  23  and an optical modulator  24  are mounted on the sub carrier  22 . The optical modulator  24  is optically coupled to the semiconductor laser  23 , and modulates a light emitted from the semiconductor laser  23 . A lens  26  is mounted on the carrier  21 . The light emitted from the semiconductor laser  23  is modulated by the optical modulator  24 , and is emitted from an outputting portion  12  of the package  10  through the lens  26 . An optical axis of the light emitted from the semiconductor laser  23  to outside overlaps with a centerline of the package  10  in a width direction. That is, the semiconductor laser  23  is located on the centerline of the package  10  in the width direction. 
         [0016]    A driver IC  32  for driving the optical modulator  24  is mounted on the heat sink  30 . A plurality of noise cutting capacitors  34  are located on the heat sink  30  around the driver IC  32 . The noise cutting capacitor  34  is coupled to a direct-current terminal of the driver IC  32  and removes a noise included in a signal fed into the driver IC  32 . The signal fed into the driver IC  32  is provided from an external terminal  50 . The external terminal  50  is coupled to the driver IC  32  through a transmission pathway  56  on a substrate  54 . Two kinds of inputting signals are needed, because the driver IC  32  is driven with complementary signals. Therefore, the laser device  100  has two external terminals  50 . The driver IC  32  is located on a center (a centerline of the package  10  in the width direction) of the two external terminals  50 . 
         [0017]    A transmission pathway  60  made of a MSL (Micro Strip Line) and so on couples the driver IC  32  to the optical modulator  24 . The transmission pathway  60  is provided on a transmission pathway substrate  33  on the heat sink  30 , on the bridge  40 , and on a transmission pathway substrate  27  on the sub carrier  22 . The transmission pathway  60  transmits a driving signal from the driver IC  32  to the optical modulator  24 . A direct-current cutting capacitor  62  and a conical coil  64  are coupled to the transmission pathway  60  on the transmission pathway substrate  33  in order from the driver IC  32  side. The conical coil  64  provides a direct current carrying the signal from the driver IC  32  to the transmission pathway  60 . The direct-current cutting capacitor  62  prevents inflow of the direct current provided from the conical coil  64  into the driver IC  32 . A capacity of the direct-current cutting capacitor  62  is set so that the direct-current cutting capacitor  62  transmits the outputting signal of the driver IC  32 . 
         [0018]    The heat of the driver IC  32  causes relatively high temperature of the heat sink  30 , in the laser device  100 . The direct-current cutting capacitor  62  is susceptible to the heat from the heat sink  30 , because the direct-current cutting capacitor  62  is mounted on the heat sink  30 . Therefore, property of the direct-current cutting capacitor  62  gets variable. The direct-current cutting capacitor  62  has direct influence on a signal provided to the optical modulator  24 , because the direct-current cutting capacitor  62  is coupled to the transmission pathway  60  from the driver IC  32  to the optical modulator  24 . This may result in degradation of modulation property of the laser device  100 . 
         [0019]    As mentioned above, it is difficult to obtain favorable modulation property in the laser device  100  in accordance with the comparative example. 
       First Embodiment 
       [0020]      FIG. 2  illustrates a schematic top view of a laser device  110  in accordance with a first embodiment.  FIG. 3  illustrates a cross sectional view taken along A-A′ dashed line of  FIG. 2 . The same components as those illustrated in  FIG. 1  (the first comparative example) have the same reference numerals in order to avoid a duplicated explanation. 
         [0021]    As illustrated in  FIG. 2  and  FIG. 3 , the heat sink  30  is more downsized in the laser device  110  than in the comparative example. And, a distance between the temperature control device  20  and the heat sink  30  is enlarged. The bridge  40  couples the heat sink  30  to the sub carrier  22  on the temperature control device  20 , as is the case with the comparative example. In the embodiment, the direct-current cutting capacitor  62  is mounted on the bridge  40  and is coupled to the transmission pathway  60  on the bridge  40  in series. The conical coil  64  is mounted on another carrier other than the heat sink  30  and is coupled to the transmission pathway  60  between the direct-current cutting capacitor  62  and the optical modulator  24 . The other structure of the laser device  110  is the same as the comparative example. 
         [0022]    Thermal conductance from the heat sink  30  to the direct-current cutting capacitor  62  is restrained, because in the laser device  110 , the direct-current cutting capacitor  62  is mounted on the bridge  40 . Therefore, property variability of the direct-current cutting capacitor  62  caused by heat generation of the driver IC  32  is restrained. Accordingly, the laser device  110  has favorable modulation property, because signal transmission from the driver IC  32  to the optical modulator  24  is improved. Breaking of the direct-current cutting capacitor  62  caused by the heat generation of the driver IC  32  is restrained. And, reliability degradation is restrained. 
         [0023]    On the other hand, the other capacitors such as the noise cutting capacitor  34  are not coupled to the transmission pathway  60  between the driver IC  32  and the optical modulator  24 , and have not so much influence on the modulation property of the laser device  110  even if a quantity of heat is generated in the driver IC  32 . The capacitors may be mounted on the heat sink  30 , as is the case with the comparative example. In other words, the direct-current cutting capacitor  62  connected to an alternating-current signal such as high frequency signal from the driver IC  32  is provided on the bridge  40 , and the noise cutting capacitor  34  connected to a direct-current power supply is mounted on the heat sink  30 . This allows an efficient arrangement of the capacitors having different purposes. 
         [0024]    In the laser device  110 , it is not necessary to generate a space for locating the direct-current cutting capacitor  62  and the conical coil  64  on the heat sink  30 . Therefore, the heat sink  30  may be downsized. And a distance between the heat sink  30  and the temperature control device  20  may be enlarged. Therefore, thermal conduction from the heat sink  30  to the temperature control device  20  may be restrained. This allows reduction of power consumption of the temperature control device  20 . 
         [0025]    In the laser device  110 , it is easy to exchange the direct-current cutting capacitor  62  and the conical coil  64 . For example, in general, whole of a block of the transmission pathway  60 , to which the direct-current cutting capacitor  62  and the conical coil  64  are coupled, is exchanged, when the direct-current cutting capacitor  62  and the conical coil  64  are to be exchanged by mistaking in mount processing. In the comparative example, it is necessary to exchange the transmission pathway substrate  33  on the heat sink  30 . However, it is difficult to exchange the transmission pathway substrate  33  because all of a lower face of the transmission pathway substrate  33  is adhered to the heat sink  30  with brazing or the like. In contrast, a user only has to exchange the bridge  40 . In the embodiment, only the end of the bridge  40  is connected to the temperature control device  20  and the heat sink  30  with brazing or the like. Therefore, it is easy to exchange the bridge  40 , compared to exchanging of the transmission pathway substrate  33 . 
       Second Embodiment 
       [0026]    A second embodiment is an embodiment where the location of the driver IC  32  and the transmission pathway  60  is changed.  FIG. 4  illustrates a schematic top view of a laser device  120  in accordance with the second embodiment. The same components as those illustrated in  FIG. 2  (the first embodiment) have the same reference numerals in order to avoid a duplicated explanation. 
         [0027]    In the laser device  110  in accordance with the first embodiment, the transmission pathway  60  is curved at the transmission pathway substrate  33  on the heat sink  30  side. In contrast, in the laser device  120  in accordance with the second embodiment, the transmission pathway  60  is not curved at the transmission pathway substrate  33 . And, the driver IC  32  is located on a straight line in an extension direction of the transmission pathway  60  on the bridge  40 . The semiconductor laser  23  and the optical modulator  24  are located on a centerline of the package  10  in the width direction, as is the case with the first embodiment. The bridge  40  and the transmission pathway  60  are located off the centerline in the width direction. The transmission pathway  60  is located on a straight line in parallel with the centerline of the package  10 , on the bridge  40 . The driver IC  32  is located on the straight line on the heat sink  30 . Therefore, the driver IC  32  is located off the centerline in the width direction of the package  10 . The two transmission pathways  56  coupling the driver IC  32  to the external terminal  50  have the same length. 
         [0028]    In the laser device  120 , the driver IC  32  is located on the straight line of the transmission pathway  60  on the bridge  40 . It is therefore not necessary that the transmission pathway  60  is curved on the transmission pathway substrate  33 . This allows downsizing of the heat sink  30 . And, it is possible to enlarge the distance between the temperature control device  20  and the heat sink  30 . Therefore, thermal isolation may be improved between the temperature control device  20  and the heat sink  30 . 
         [0029]    In the first and the second embodiments, the conical coil  64  is used as an inductor for providing direct-current to the transmission pathway  60 . However, another inductor (for example, a coil having constant coil diameter) may be used. 
         [0030]    The direct-current cutting capacitor  62  coupled to the transmission pathway  60  is mounted on the bridge  40 . However, another capacitor may be mounted on the bridge  40 . For example, if a capacitor for filtering may be coupled to a transmission pathway, thermal influence of the driver IC  32  is restrained when the capacitor is coupled to the transmission pathway  60  on the bridge  40 . That is, thermal influence of a driver IC on a capacitor to be coupled to a transmission pathway is restrained when the capacitor is mounted on a bridge. This restrains degradation of high frequency wave caused by property changing of the capacitor. 
         [0031]    The present invention is not limited to the specifically disclosed embodiments and variations but may include other embodiments and variations without departing from the scope of the present invention.