Abstract:
A multiple-wavelength laser device includes a first semiconductor laser chip having two modulable unit laser portions, outputs of the unit laser portions being optically coupled to a single output optical axis; a second semiconductor laser chip having two or less than two modulable unit laser portions, outputs of the unit laser portions being optically coupled to a single output optical axis; an optical coupler that combines the output optical axes of the first and the second semiconductor laser chips; and a plurality of drive current pathways or a plurality of signal transmission pathways that are coupled to each of the unit laser portions of the first and the second semiconductor laser chips with a connection conductor.

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. 2009-073579, filed on Mar. 25, 2009, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    (i) Technical Field 
         [0003]    The present invention relates to a multiple-wavelength laser device having a plurality of semiconductor laser chips. 
         [0004]    (ii) Related Art 
         [0005]    An optical communication system using an optical fiber is being built according to speeding up of information communication. The optical communication system may use multiple-wavelength transmission method. Japanese Patent Application Publication No. 11-54842 (hereinafter referred to as Document 1) discloses a laser device having a plurality of semiconductor laser chips. 
         [0006]    With the art of Document 1, there are many wires for providing electrical power or signal. This results in greater density of the wires. It is possible to produce many semiconductor laser chips from a wafer when the semiconductor laser chips are located on a small area. Therefore, an interval between unit laser portions is reduced. This results in greater density of the wires. In this case, freedom degree of wire track design is reduced. Therefore, there is a problem that modulation property may be degraded because of interference of high frequency wave signal. 
       SUMMARY 
       [0007]    It is an object of the present invention to provide a multiple-wavelength laser device having favorable modulation property. 
         [0008]    According to an aspect of the present invention, there is provided a multiple-wavelength laser device including: a first semiconductor laser chip having two modulable unit laser portions, outputs of the unit laser portions being optically coupled to a single output optical axis; a second semiconductor laser chip having two or less than two modulable unit laser portions, outputs of the unit laser portions being optically coupled to a single output optical axis; an optical coupler that combines the output optical axes of the first and the second semiconductor laser chips; and a plurality of drive current pathways or a plurality of signal transmission pathways that are coupled to each of the unit laser portions of the first and the second semiconductor laser chips with a connection conductor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  illustrates a schematic plane view of a semiconductor laser chip in accordance with a comparative embodiment; 
           [0010]      FIG. 1B  illustrates an arrangement of bonding wires connecting a semiconductor laser chip and a printed circuit substrate; 
           [0011]      FIG. 2  illustrates a schematic view of a main part of a multiple-wavelength laser device in accordance with a first embodiment; 
           [0012]      FIG. 3  illustrates a plane view of a multiple-wavelength laser device; 
           [0013]      FIG. 4  illustrates a schematic plane view of a multiple-wavelength laser device in accordance with a second embodiment; and 
           [0014]      FIG. 5  illustrates a schematic plane view of a multiple-wavelength laser device in accordance with a third embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    A description will be given of a multiple-wavelength laser device in accordance with a comparative embodiment in order to state a problem solved in the following embodiments. 
       Comparative Embodiment 
       [0016]      FIG. 1A  illustrates a schematic plane view of a semiconductor laser chip  300  in accordance with a comparative embodiment. As illustrated in  FIG. 1A , the semiconductor laser chip  300  has four unit laser portions  20   a  to  20   d  arranged in order. The unit laser portions  20   a  to  20   d  are arranged in array so that longitudinal directions thereof are substantially in parallel with each other. The unit laser portions  20   a  to  20   d  have a structure in which optical modulators  22   a  to  22   d  and SOAs (Semiconductor Optical Amplifier)  23   a  to  23   d  are coupled to outputting ends of laser portions  21   a  to  21   d  in order. 
         [0017]    Optical signals from the unit laser portions  20   a  to  20   d  transmit in an optical waveguide having an optical axis different from each other. The optical waveguides are coupled to an optical waveguide having a single output optical axis in an optical multiplexer  24 . Thus, the optical signals from the unit laser portions  20   a  to  20   d  are multiplexed at the optical multiplexer  24  and are output outward. 
         [0018]    The unit laser portions  20   a  to  20   d , the optical waveguides, and the optical multiplexer  24  make the semiconductor laser chip  300 . The semiconductor laser chip  300  can transmit data at 100 Gb/s at a maximum if the unit laser portions  20   a  to  20   d  can operate at 25 Gb/s. 
         [0019]      FIG. 1B  illustrates an arrangement of bonding wires connecting the semiconductor laser chip  300  and a printed circuit substrate. As illustrated in  FIG. 1B , the bonding wires are dense even if the bonding wires are connected to the semiconductor laser chip  300  from both sides of the array, because the bonding wires are connected to the two unit laser portions from a single side. This results in reduction of freedom degree of track design of a connection conductor to be connected to an optical modulator. Therefore, favorable modulation property may not be obtained. And, wire density causes reduction of yield ratio in a manufacturing process. 
         [0020]    A length of a bonding wire connected to the optical modulator  22   a  and  22   d  may be at a minimum, because the optical modulators  22   a  and  22   d  are arranged outside of the array. However, a bonding wire connected to the optical modulators  22   b  and  22   c  is longer than the bonding wire connected to the optical modulators  22   a  and  22   d , because it is necessary to connect the bonding wire and the optical modulators  22   b  and  22   c  across the optical modulators  22   a  and  22   d . In this case, modulation property of the optical modulators  22   b  and  22   c  may be degraded. And, circuit designation may be complicated if each optical modulator having different length operates at the same property. 
         [0021]    The four unit laser portions generate heat when operating in the semiconductor laser chip  300 . In this case, temperature relation between the unit laser portions  20   b  and  20   c  and the unit laser portions  20   a  and  20   d  may be asymmetric. Therefore, operation property of each of the unit laser portions is different from each other. 
       First Embodiment 
       [0022]      FIG. 2  illustrates a schematic view of a main part of a multiple-wavelength laser device  100  in accordance with a first embodiment. As illustrated in  FIG. 2 , the multiple-wavelength laser device  100  has a semiconductor laser chip  10   a  and a semiconductor laser chip  10   b . The semiconductor laser chip  10   a  has unit laser portions  20   a  and  20   b . Longitudinal directions of the unit laser portions  20   a  and  20   b  are substantially in parallel with each other. The semiconductor laser chip  10   b  has unit laser portions  20   c  and  20   d . Longitudinal directions of the unit laser portions  20   c  and  20   d  are substantially in parallel with each other. 
         [0023]    The unit laser portions  20   a  to  20   d  respectively have a structure in which the optical modulators  22   a  to  22   d  and the SOAs  23   a  to  23   d  are respectively connected to outputting ends of the laser portions  21   a  to  21   d  in order. In the unit laser portion  20   a , an optical signal from the laser portion  21   a  is fed into the optical modulator  22   a . The optical modulator  22   a  modulates the optical signal and provides a modulation signal into the SOA  23   a . The SOA  23   a  amplifies the modulation signal and outputs the amplified modulation signal. In the unit laser portions  20   b  to  20   d , modulation signals are output from the SOAs  23   b  to  23   d  with the same processes. 
         [0024]    The modulation signals from the SOAs  23   a  and  23   b  are multiplexed at a wavelength multiplexer in an optical waveguide and are output as a modulation signal S 1 . The modulation signals from the SOAs  23   c  and  23   d  are multiplexed at a wavelength multiplexer in an optical waveguide and are output as a modulation signal S 2 . In the embodiment, an optical axis of the modulation signal S 1  and an optical axis of the modulation signal S 2  are at right angle with each other. The modulation signal S 1  is fed into an optical coupler  30  through a lens  25 . The modulation signal S 2  is fed into the optical coupler  30  through a lens  26 . 
         [0025]    In the embodiment, a PBS (Polarization Beam Splitter) is used as the optical coupler  30 . The modulation signals S 1  and S 2  are multiplexed at the optical coupler  30  and are output outside through a lens  27 . 
         [0026]    With the structure, the semiconductor laser chip  10   a  is separated away from the semiconductor laser chip  10   b . In this case, bonding wire density is restrained. Therefore, flexibility of track design of the bonding wires connected to the optical modulators  22   a  to  22   d  is improved. Accordingly, favorable modulation property is obtained. And yield ratio in a manufacturing process may be improved if the wire density is restrained. 
         [0027]    Multiplexing loss at the optical coupler  30  may be restrained because the optical coupler  30  is a polarization beam splitter. 
         [0028]      FIG. 3  illustrates a plane view of the multiple-wavelength laser device  100 . As illustrated in  FIG. 3 , the multiple-wavelength laser device  100  has a structure in which a main part thereof illustrated in  FIG. 2  is housed in a package  40 . There are provided temperature control devices  50   a  and  50   b , printed circuit substrates  60   a  to  60   d , driver ICs  70   a  to  70   d  and external connection terminals  80   a  and  80   b  in the package  40 . There is provided an optical connector  28  at a sidewall of the package  40 . 
         [0029]    The semiconductor laser chip  10   a  and the lens  25  are arranged on the temperature control device  50   a . The semiconductor laser chip  10   b  and the lens  26  are arranged on the temperature control device  50   b.    
         [0030]    In the embodiment, an output optical axis of the unit laser portions  20   a  and  20   b  is different from that of the unit laser portions  20   c  and  20   d . In this case, the unit laser portions  20   a  and  20   b  may be arranged away from the unit laser portions  20   c  and  20   d . Therefore, the printed circuit substrates  60   a  to  60   d  can be respectively arranged adjacent to the unit laser portions  20   a  to  20   d . In  FIG. 3 , reference numerals of each part of the unit laser portions  20   a  to  20   d  are omitted. 
         [0031]    The printed circuit substrate  60   a  is arranged on the unit laser portion  20   a  side, compared to the temperature control device  50   a . Metal wires  61   a  to  63   a  acting as drive current pathway or a signal transmission pathway are provided on the printed circuit substrate  60   a . One end of the metal wire  61   a  is connected to the laser portion  21   a  with a bonding wire  91   a . One end of the metal wire  62   a  is connected to the optical modulator  22   a  with a bonding wire  92   a . The metal wire  63   a  is connected to the SOA  23   a  with a bonding wire  93   a . The bonding wires  91   a  to  93   a  act as a connection conductor. 
         [0032]    Another end of the metal wires  61   a  to  63   a  is connected to the driver IC  70   a . Therefore, the laser portion  21   a  receives a laser driving current through the metal wire  61   a . The optical modulator  22   a  receives a modulation signal through the metal wire  62   a . The SOA  23   a  receives a SOA driving current through the metal wire  63   a.    
         [0033]    The printed circuit substrate  60   b  is arranged on the unit laser portion  20   b  side, compared to the temperature control device  50   a . Therefore, the printed circuit substrate  60   b  is arranged in an opposite side of the unit laser portion  20   a . The printed circuit substrate  60   b  has metal wires  61   b  to  63   b . One end of the metal wire  61   b  is connected to the laser portion  21   b  with a bonding wire  91   b . One end of the metal wire  62   b  is connected to the optical modulator  22   b  with a bonding wire  92   b . The metal wire  63   b  is connected to the SOA  23   b  with a bonding wire  93   b.    
         [0034]    Another end of the metal wires  61   b  to  63   b  is connected to the driver IC  70   b . Therefore, the laser portion  21   b  receives a laser driving current through the metal wire  61   b . The optical modulator  22   b  receives a modulation signal through the metal wire  62   b . The SOA  23   b  receives a SOA driving current through the metal wire  63   b.    
         [0035]    With the structure, a distance may be reduced to the minimum between each part of the unit laser portion  20   a  and the metal wires  61   a  to  63   a  and between each part of the unit laser portion  20   b  and the metal wires  61   b  to  63   b . Therefore, degradation of modulation property may be restrained. And it is possible to design a structure in which a length of the bonding wire  92   a  connecting the optical modulator  22   a  and the metal wire  62   a  is the same as that of the bonding wire  92   b  connecting the optical modulator  22   b  and the metal wire  62   b . In this case, the optical modulators  22   a  and  22   b  may operate at the same modulation property. 
         [0036]    Similarly, the printed circuit substrate  60   c  is arranged on the unit laser portion  20   c  side, compared to the temperature control device  50   b , and the printed circuit substrate  60   d  is arranged on the unit laser portion  20   d  side, compared to the temperature control device  50   b . In this case, a distance may be reduced to the minimum between each part of the unit laser portion  20   c  and the metal wires of the printed circuit substrate  60   c . And a distance may be reduced to the minimum between each part of the unit laser portion  20   d  and the metal wires of the printed circuit substrate  60   d . Therefore, degradation of modulation property of the semiconductor laser chip  10   b  may be restrained. And it is possible to design a structure in which a length of a bonding wire connecting the optical modulator  22   c  and a metal wire is the same as that of a bonding wire connecting the optical modulator  22   d  and a metal wire. In this case, the optical modulators  22   c  and  22   d  may operate at the same modulation property. The SOA and the optical modulator may be arranged in order with respect to the unit laser portion. 
         [0037]    The unit laser portions  20   a  and  20   b  are arranged symmetrically on the temperature control device  50   a . Therefore, a temperature difference may be restrained between the unit laser portion  20   a  and the unit laser portion  20   b . And, a temperature difference may be restrained between the unit laser portion  20   c  and unit laser portion  20   d . Therefore, operation property difference between each unit laser portion may be restrained. 
         [0038]    The optical coupler  30  multiplexes an optical signal from the semiconductor laser chip  10   a  and an optical signal from the semiconductor laser chip  10   b . The optical coupler  30  outputs the multiplexed signal outward through the lens  26 . From a view of restrain of polarized wave, the semiconductor laser chips  10   a  and  10   b  may be arranged by rotating with respect to the output optical axis thereof. 
       Second Embodiment 
       [0039]      FIG. 4  illustrates a schematic plane view of a multiple-wavelength laser device  100   a  in accordance with a second embodiment. As illustrated in  FIG. 4 , the multiple-wavelength laser device  100   a  is different from the multiple-wavelength laser device  100  of  FIG. 3  in a point that an optical axis of the semiconductor laser chip  10   a  is substantially in parallel with that of the semiconductor laser chip  10   b . In the embodiment, a PLC (Planar Lightwave Circuit), a WDM (Wavelength Division Duplexing), and so on may be used as the optical coupler  30 . 
         [0040]    In the embodiment, the semiconductor laser chip  10   a  may be separated away from the semiconductor laser chip  10   b , because the semiconductor laser chips  10   a  and  10   b  have two or less than two unit laser portions. Therefore, bonding wire density may be restrained. The printed circuit substrates may be arranged on both sides of the semiconductor laser chips  10   a  and  10   b  with respect to each of the unit laser portions. The length of the bonding wires connected to the optical modulators  22   a  to  22   d  may be reduced to the minimum, and may be the same. 
       Third Embodiment 
       [0041]    The optical coupler  30  may be provided outside of the package  40 .  FIG. 5  illustrates a schematic plane view of a multiple-wavelength laser device  100   b  in accordance with a third embodiment. As illustrated in  FIG. 5 , the multiple-wavelength laser device  100   b  is different from the multiple-wavelength laser device  100   a  of  FIG. 4  in a point that the optical coupler  30  is provided outside of the package  40 . In this case, the optical coupler  30  receives an output light of the semiconductor laser chip  10   a  through an optical connector  28   a  provided on a sidewall of the package  40 . The optical coupler  30  receives an output light of the semiconductor laser chip  10   b  through an optical connector  28   b  on a sidewall of the package  40 . 
         [0042]    In the above-mentioned embodiments, two semiconductor laser chips having two unit laser portions are provided. 
         [0043]    However, the structures are not limited. One of the semiconductor laser chips has only one unit laser portion. 
         [0044]    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.