Abstract:
An optical transmitter having a packaging structure by which excellent optical transmitting waveform at high speed transmission is provided. An optical transmitter, comprising, a stem formed almost coaxial shape, a column fixed on said stem, a light emitting element chip provided on said column, terminals insulated with said stem and provided through said stem, and a connecting article connecting between said terminal and said light emitting element chip, wherein, said connecting article is formed by a line which is provided on a insulating substrate and is controlled to have predetermined characteristic impedance, and a resistive element for impedance matching connected to said line serially.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the invention  
           [0002]    The present invention relates to an optical transmitter for use in an optical transmission system, and more specifically to an optical transmitter provided with a packaging structure by which excellent optical transmitting waveform at high speed transmission can be obtained.  
           [0003]    2. Prior Art  
           [0004]    Recently, an optical transmission technique has become to be used commonly in data communicating system, therefore, an optical transceiver which is a key device of an optical transmission has become to be used in large quantities.  
           [0005]    On the other hand, since transmitting speed required to an optical transceiver has become to be high more and more, in future, it seems that an optical transceiver having transmitting speed 10 Gbit/s or more than 10 Gbit/s will become to be used commonly in large quantities. Therefore, techniques to realize high speed transmitting with low cost are required strongly more than former.  
           [0006]    To obtain a high speed optical transmitter, speed up of an optical element itself such as laser diode (LD) or outside modulator is important, but packaging method of an optical element is one of key points, too. Namely, it is necessary to devise a packaging structure which can obtain excellent optical transmitting waveform at high speed transmission, and to make it a realizable structure with low cost.  
           [0007]    In FIG. 1( a ), a front view of a conventional optical transmitter is shown, and in FIG. 1( b ), a plane view of it is shown.  
           [0008]    As shown in FIG. 1( a ) and in FIG. 1( b ), a conventional optical transmitter comprising mainly, the LD chip  30  which outputs light, the semicylindrical column  11  provided on its side with an overhang potion having supporting surface of the LD chip  30 , the disk stem  10  supporting the LD chip  30  vertically to coincide the optical axis of the LD chip  30  with the axis of the optical fiber to which output light is input, and three terminals  12 ,  13  and  14  provided through the stem  10 . Further, either electrode of the LD chip  30  is connected with the drive terminal  14  by a bonding wire  15 , and another electrode of the LD chip  30  is connected electrically with the common terminal  13  through the conductive stem  10  and column  11 .  
           [0009]    Further more, the optical transmitter is provided with the monitoring PD chip  31  which monitors emitting power of output light from the LD chip  30 , on the stem  10  locating under the overhang potion, either electrode of the monitoring PD chip  31  is connected with the PD terminal  12 , and another electrode of the monitoring PD chip  31  is connected with the common terminal  13  by a bonding wire  16 .  
           [0010]    These articles provided on the stem  10  are covered by the cap  20  equipped upper of it with the lens  21  which passes output light from the LD chip  30 .  
           [0011]    The above explained packaging structure is, at present, the most generally used package of light emitting element, and is called TO can package.  
           [0012]    The stem  10  and the cap  20  are formed by metal, and the LD chip  30  is shielded hermetically, therefore, the package is very  3  reliable.  
           [0013]    Further, since the package has already been used enormously, automatic assembling is progressing and exceeding in cost.  
           [0014]    Further more, since this package is coaxial in shape and emits light to axial direction, it can be applied easily to a receptacle type optical transmitter to which an optical connector is inserted directly.  
           [0015]    However, in the conventional technique, because the connection between the LD chip  30  and the drive terminal  14  is made by wire bonding  15 , the bonding wire  15  causes parasitic inductance which lowers transmitting speed. Although length of the bonding wire  15  is designed to be as short as possible, but limitation to shorten the length is existing, problem has been arisen that it is difficult to catch up with demand for high speed transmission more and more recently.  
           [0016]    Further, this parasitic inductance causes disturbance of frequency characteristic, it makes speedup difficult more and more to provide resistance outside of the package so as to dump the disturbance.  
           [0017]    Further more, since parasitic inductance of the bonding wire  15  varies according to a degree of strain or so oh, as shown in FIG. 2, impedance between the LD chip  30  and the drive terminal  14  varies with every product, there is disadvantage that deteriorates reproducibility and causes to lower yield rate.  
           [0018]    Except TO can package, various devises has been made to improve high frequency characteristic, there is a great problem that cost is very expensive.  
           [0019]    Recently, receptacle type optical transmitter with easy handling is required strongly for high speed optical transmitter, there has not been realized TO can package applicable to such optical transmitter that operates at high speed.  
           [0020]    As described above, conventional packaging method of light emitting element is required to improve stability of widebahd characteristic and frequency characteristic,  
         SUMMARY OF THE INVENTION  
         [0021]    An object of the present invention is to provide an optical transmitter provided with reduced parasitic inductance by shortening bonding wire, and high speed transmission and stable frequency characteristic with low cost.  
           [0022]    In accordance with this invention, there is provided an optical transmitter, comprising, a stem formed almost coaxial shape, a column fixed on said stem, a light emitting element chip provided on said column, terminals insulated with said stem and provided through said stem, and a connecting article connecting between said terminal and said light emitting element chip, wherein, said connecting article is formed by a line which is provided on a insulating substrate and is controlled to have predetermined characteristic impedance, and a resistive element for impedance matching connected to said line serially.  
           [0023]    In accordance with this invention, there is provided an optical transmitter, wherein, said resistive element for impedance matching is provided with matching resistor formed directly on said insulating substrate or chip resistor.  
           [0024]    In accordance with this invention, there is provided an optical transmitter, wherein, a branched wiring is provided from middle of said matching resistor or chip resistor and said light emitting element chip, and said branched wiring is connected with a terminal which is connected electrically with outside.  
           [0025]    In accordance with this invention, there is provided an optical transmitter, wherein, said branched wiring is provided with at least resistor and inductor so as to attenuate high frequency signal.  
           [0026]    In accordance with this invention, there is provided an optical transmitter, wherein, diameter of said terminal, diameter of a hole formed through said stem to pass said terminal, and dielectric constant of insulator which is embedded to fix said terminal within said hole are controlled to make almost equalize characteristic impedance of said line with impedance of said terminal.  
           [0027]    In accordance with this invention, there is provided an optical transmitter, wherein, sum of impedance of said light emitting element chip and impedance of said matching resistor or said chip resistor equals characteristic impedance of said line.  
           [0028]    In accordance with this invention, there is provided an optical transmitter, comprising, a stem formed almost coaxial shape, a column fixed on said stem, an outside modulating element provided on said column, terminals insulated with said stem and provided through said stem, and a connecting article connecting between said terminal and said outside modulating element, wherein, said connecting article is formed by a line which is provided on a insulating substrate and is controlled to have predetermined characteristic impedance, and a resistive element for impedance matching connected to said line serially.  
           [0029]    In accordance with this invention, there is provided an optical transmitter, wherein, said resistive element for impedance matching is provided with matching resistor formed directly on said insulating substrate or chip resistor.  
           [0030]    In accordance with this invention, there is provided an optical transmitter, wherein, a branched wiring is provided from middle of said matching resistor or chip resistor and said light emitting element chip, and said branched wiring is connected with a terminal which is connected electrically with outside.  
           [0031]    In accordance with this invention, there is provided an optical transmitter, wherein, said branched wiring is provided with at least resistor and inductor so as to attenuate high frequency signal.  
           [0032]    In accordance with this invention, there is provided an optical transmitter, wherein, diameter of said terminal, diameter of a hole formed through said stem to pass said terminal, and dielectric constant of insulator which is embedded to fix said terminal within said hole are controlled to make almost equalize characteristic impedance of said line with impedance of said terminal.  
           [0033]    In accordance with this invention, there is provided an optical transmitter, wherein, sum of impedance of said outside modulating element and impedance of said matching resistor or said chip resistor equals characteristic impedance of said line, 
       
    
    
     BRIEF DESCRIPTIION OF THE DRAWINGS  
       [0034]    The invention will be explained in more detail in conjunction with the appended drawings, wherein;  
         [0035]    [0035]FIG. 1( a ) is a front view showing a conventional optical transmitter.  
         [0036]    [0036]FIG. 1( b ) is a plane view showing a conventional optical transmitter.  
         [0037]    [0037]FIG. 2 is a circuit diagram showing a conventional optical transmitter.  
         [0038]    [0038]FIG. 3( a ) is a front view showing an embodiment of an optical transmitter of the present invention.  
         [0039]    [0039]FIG. 3( b ) is a plane view showing an embodiment of an optical transmitter of the present invention.  
         [0040]    [0040]FIG. 4 is a circuit diagram showing an embodiment of an optical transmitter of the present invention.  
         [0041]    [0041]FIG. 5( a ) is a front view showing another embodiment of an optical transmitter of the present invention.  
         [0042]    [0042]FIG. 5( b ) is a plane view showing another embodiment of an optical transmitter of the present invention.  
         [0043]    [0043]FIG. 6 is a circuit diagram showing another embodiment of an optical transmitter of the present invention.  
         [0044]    [0044]FIG. 7 is a circuit diagram showing another embodiment of an optical transmitter of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]    Preferred embodiments of the present invention will be explained in conjunction with accompanying drawings.  
         [0046]    In FIG. 3( a ), a front view of inside of optical transmitter in accordance with this embodiment is shown, and in FIG. 3( b ), a plane view of it is shown.  
         [0047]    As shown in FIG. 3( a ) and FIG. 3( b ), optical transmitter comprising mainly, the LD chip  30  which outputs light, the semicylindrical column  11  provided on its side with an overhang potion having supporting surface of the LD chip  30 , the disk stem  10  supporting LD chip  30  vertically to coincide the optical axis of the LD chip  30  with the axis of the optical fiber to which output light is input, and three terminals  12 ,  13  and  14  provided through the stem  10 .  
         [0048]    The ceramic plate  40  is bonded to side of the column  11  almost parallel with the LD chip  30 , and the micro strip line  41  which is connected with the drive terminal  14  is provided on the ceramic plate  40 . This micro strip line  41  is controlled to have characteristic impedance about 25 Ω.  
         [0049]    Further, an impedance matching resistive element is connected to this micro strip line  41  serially. This impedance matching resistive element comprising, the matching resistor  42  having impedance about 20 Ω and the short bonding wire  43  connecting between the matching resistor  42  and either electrode of the LD chip  30 . The matching resister  42  is formed on the ceramic plate  40  directly by printing or so on.  
         [0050]    Another electrode of the LD chip  30  is connected electrically with the common terminal  13  through the conductive stem  10  and column  11 .  
         [0051]    Diameter of the drive terminal  14 , diameter of the hole formed through the stem  10  to pass the drive terminal  14 , and dielectric constant of insulator which is embedded between the drive terminal  14  and the hole and fixing the drive terminal  14  within the hole are formed to equalize characteristic impedance of the micro strip line  41  with impedance of the drive terminal  14 . Impedance of the drive terminal  14  itself is controlled as well as coaxial line.  
         [0052]    Further, this optical transmitter is provided with the monitor PD chip  31  which monitors emitting power of output light from the LD chip  30 , on the stem  10  locating under the overhang potion. And either electrode of the monitor PD chip  31  is connected with the PD terminal  12 , and another electrode of the monitor PD chip  31  is connected with the common terminal  13  by a bonding wire  16 .  
         [0053]    These articles provided on the stem  10  are covered by the cap  20  equipped upper of it with the lens  21  which passes output light from the LD chip  30 , and the LD chip  30  is shielded hermetically.  
         [0054]    Operation of the optical transmitter in accordance with this embodiment will be explained.  
         [0055]    Since the micro strip line  41  is formed that the length is controlled to reach the neighbor of the LD chip  30  in accordance with mounting height of the LD chip  30 , length of the bonding wire  43  is shortened greatly comparing with the conventional optical transmitter.  
         [0056]    Accordingly, it becomes possible to decrease parasitic inductance, and to realize high speed operation.  
         [0057]    As shown in FIG. 4, since impedance of the LD chip  30  is about 5 Ω, matching with the micro strip line  41  having characteristic impedance 25 Ω can be obtained by connecting serially the matching resistor  42  having 20 Ω.  
         [0058]    As explained above, in the conventional optical transmitter, impedance matching for wiring within package had been impossible, but in the present invention, impedance matching near the LD chip  30  became possible.  
         [0059]    Therefore, it became possible to make frequency characteristic flat and variation of frequency characteristic small.  
         [0060]    Characteristic impedance of the micro strip line  41  is not limited to 25 Ω, other value may be applicable so long as matching can be obtained. Further, it is possible to shift matching intentionally to vary frequency characteristic.  
         [0061]    In this embodiment, connecting between the micro strip line  41  and the LD chip  30  is made by short bonding wire  43 , instead of this, if height of the micro strip line  41  and the upper surface of the LID chip  30  is almost same, direct bridging using chip resistor maybe applicable. In this case, matching resistor  42  is not needed.  
         [0062]    According to this constitution, parasitic inductance can be reduced still more.  
         [0063]    In addition, in this embodiment, micro strip line is used as a line formed on insulating plate, other line such as coplanar line or strip line may be applicable so long as characteristic impedance is controlled within predetermined value.  
         [0064]    Other embodiment of the present invention will be explained.  
         [0065]    [0065]FIG. 5( a ) shows a front view of optical transmitter in accordance with another embodiment of the present invention, FIG. 5( b ) shows a plane view of it, and FIG. 6 shows circuit diagram.  
         [0066]    As shown in FIG. 5( a ), this optical transmitter, adding constitution shown in FIG. 3( a ), is provided with the wiring  45  which draws to outside of the package from middle of the matching resistor  42  and LD chip  30  and is connected with outside through the terminal  17 . At the middle of the wiring  45 , the resistor  44  and the inductor  46  are provided.  
         [0067]    Usually, the LD chip  30  is driven by superposing modulating current on bias current. Imposing bias current on the drive terminal  14  possible, but it is disadvantageous for the LD driving circuit because the bias current which flows through the matching resistor  42  heightens applying voltage. Therefore, as shown in FIG. 6, if the wiring  45  which enables to apply the bias current without passing through the matching resistor  42  is provided within the package, high and stable operation becomes possible.  
         [0068]    In addition, the resistor  44  and the inductor  46  are inserted to prevent deterioration of modulating current waveform which flows LD chip  30  through the wiring  45 .  
         [0069]    The structure as explained above can be obtained by packaging ceramic substrate within one package, and other articles can be assembled by same method as conventional TO can package.  
         [0070]    Accordingly, increase of manufacturing costs of this optical transmitter can be held low. Therefore, since advantages of TO can package that is used for conventional low speed and cheap optical transmitter can be applied as it is to very high speed optical transmitter, big cost lowering of high speed optical transmitter can be achieved.  
         [0071]    In addition, in the embodiment, the ceramic substrate  40  is used as a dielectric substrate, material of the dielectric substrate is not limited so long as high frequency characteristic is satisfied.  
         [0072]    [0072]FIG. 7 is showing a circuit diagram in accordance with another embodiment of the present invention. In this embodiment, the termination resistor  47  and the outside modulating element  48  are provided near the LD chip  30 , and output light from the LD chip  30  is emitted through the outside modulating element  48 . The micro strip line  41  is connected to the outside modulating element  48 . In the figure, the terminal resistor  47  is provided paralelly with the outside modulating element  48 , but not limited to this, the terminal resistor  47  may be connected serially with the outside modulating element  48 . An element which integrated a LID chip and an outside modulating element may be used in this embodiment. Not shown in the figure, the outside modulating element  48  is provided on the column  11  within the cap  20 . In accordance with this embodiment which provided the outside modulating element  48  near the LD chip  30 , distortion of waveform during high speed communication may be improved.  
         [0073]    As described above in detail, according to the present invention, a high speed optical transmitter with low costs and stable characteristic can be obtained.  
         [0074]    Although the invention has been described with respect to specific embodiment and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art which fairly fall within the basic teaching herein set forth.