Patent Publication Number: US-2023156932-A1

Title: Optical Communication Element

Description:
TECHNICAL FIELD 
     The present invention relates to a highly functional optical communication component having a structure in which an optical communication element and an electronic circuit element are integrated. 
     BACKGROUND ART 
     In recent years, in optical communication technology for an optical communication system, an optical information processing system, and the like, due to explosive prevalence of a mobile terminal represented by a smartphone, enriched video delivery service, and the like, a demand for an increased transmission capacity of an optical network has grown day by day. To respond to the demand, a further technical development is in demand. To meet the demand, an optical communication system using signal processing in an electric stage such as, e.g., digital coherent communication, an ultra-high-speed communication system featuring a transmission capacity over 100 Gbit/s, and the like have been practically implemented. 
     In addition, it is technically important to achieve a size reduction and a cost reduction for each of optical communication components used in these systems. In such an optical communication component, an optical communication element and an electronic circuit element are used in most cases as a set. An optical modulator serving as an example of the optical communication element outputs either of an intensity-modulated signal obtained by modulating an intensity of an optical signal incident thereon and a phase-modulated signal obtained by modulating a phase of light. An optical modulator driver integrated circuit (hereinafter referred to as an optical modulator driver IC) serving as an example of the electronic circuit element changes either of the intensity-modulated signal and the phase-modulated signal each from the optical modulator to an optical signal that can be processed in the electric stage and then outputs the optical signal. A light receiving element serving as another example of the optical communication element receives an optical signal transmitted thereto, converts the optical signal to an electric signal, and then outputs the electric signal. A transimpedance amplifier (hereinafter referred to as the TIA) serving as another example of the electronic circuit element amplifies and processes the electric signal from the light receiving element such that the electric signal can be processed in the electric stage. 
     By the way, to implement a compact and low-cost optical communication component that handles a high-speed signal, the optical communication element and the electronic circuit element are mostly mounted in one package to be configured as an optical communication component. This is because, by integrating the individual devices with each other, it is possible to prevent attenuation and reflection of an electric signal between the devices and also reduce connected portions of wires. This serves as a contribution to prevention of a degraded electrical characteristic and to achievement of a size reduction and a cost reduction. 
     As an example of such an optical communication component, an integrated coherent receiver (hereinafter abbreviated as the ICR) that receives an optical signal by digital coherent communication can be listed. The ICR is an optical communication component in which an optical circuit that separates optical signals resulting from multi-value phase modulation by respective phases thereof, a light receiving element that converts the optical signals to electricity, a TIA that amplifies electric signals, and the like are mounted in one package. 
     In the ICR, each of the electric signals output from the TIA is retrieved from the package to the outside thereof via RF wiring. However, when a direct-current (hereinafter abbreviated as DC) voltage is applied from an external electronic circuit element to the ICR, an electronic circuit element such as the TIA may break down. Accordingly, a capacitor component referred to as a DC block that allows a high-frequency signal to pass therethrough, but does not allow the DC signal to pass therethrough is needed on the RF wiring. Note that, in the ICR, it is typical to embed even the DC block in the package. The high-frequency signal, which is in a short wavelength band including a versatile radio frequency (RF), is hereinafter referred to as the RF signal. 
     The DC block is made of a chip capacitor disposed on the RF wiring and has a property of allowing RF signals having frequencies ranging from several megahertz in a low frequency band to several tens of gigahertz in a high-frequency band to pass therethrough and not allowing the DC signal of up to kHz order to pass therethrough. For the DC block embedded in an optical communication component such as the ICR, a structure in which a chip capacitor of a size of about 0.6×0.3 mm or 0.4×0.2 mm is disposed on RF wiring having a width of about 100 μm is used. Accordingly, to a portion corresponding to the DC block, layout consideration is preferably given by increasing the width of the RF wiring or locating a ground (hereinafter abbreviated as GND) electrode at a long distance therefrom such that a characteristic impedance is controlled not to greatly vary compared to that in a portion corresponding to the RF wiring. However, when such control is performed, it is required to reduce reflection, radiation, and the like of the RF signals. 
       FIG.  1    is an outer appearance perspective view when viewed from obliquely above in which a structural shape of a DC block in a package  11  in an ICR  10  as a known optical communication component is partially broken and exposed. Note that the ICR  10  typically has a function of receiving polarization-multiplexed signals having two phase components perpendicular to each other and outputting four pairs of differential electric signals. 
     Referring to  FIG.  1   , the package  11  of the ICR  10  is preferably made of a ceramic and has, as a base, a pedestal structure in which a recessed portion for mounting the component is formed in a frame-shaped erecting wall portion. On a flat surface of the recessed portion of the package  11 , the DC block including a pair of two RF wires  12  that transmit differential electric signals and DC block capacitors  14  is provided. The RF wires  12  include grounded coplanar lines circumferentially surrounded by GND electrodes  13  to receive an output signal from a TIA input thereto and output the pair of differential electric signals. The DC block capacitors  14  connected to the RF wires  12  allow the output signal from the TIA transmitted through the RF wires  12  to pass therethrough and cut off a DC signal when the DC signal is included in the output signal. 
     In the ICR  10 , the output signal from the TIA is transmitted through the RF wires  12  and passes through the DC block capacitors  14 . A portion of the package  11  located below the flat surface of the recessed portion thereof serves as a laminated ceramic package  11   a  formed by alternately laminating ceramic layers and metal layers. The output signal after the DC signal is cut off is transmitted to the lower layers via metal via wires placed to extend through the ceramic layers in the laminated ceramic package  11   a . The output signal is further led out from signal output lead wires  15  formed to be exposed on a rear side of the laminated ceramic package  11   a.    
     In the case of the ICR  10 , a potential of a signal line of the external electronic circuit may not be the same as the GND potential and, at this time, a DC voltage is applied to each of the lead wires  15  serving as signal lines. When the voltage of the lead wire  15  at this time is applied directly to the TIA via the RF wires  12 , a withstand voltage of a semiconductor forming the TIA may be exceeded, and consequently the DC block is required to protect the electronic circuit of the TIA. 
       FIG.  2    is a partially enlarged view illustrating a result of simulating an electric field intensity of the RF signal in the package  11  of the ICR  10  illustrated in  FIG.  1   . Specifically, in  FIG.  2   , the electric field intensity of the RF signal in a portion of the package  11  where the RF wires  12  for outputting the two pairs of differential electric signals and the total of four DC block capacitors  14   a  and  14   b  included in two pairs are present is illustrated. 
     Referring to  FIG.  2   , the output signal from the TIA is transmitted through the RF wires  12  and passes through the DC block capacitors  14   a  and  14   b . It can be seen that, at the time of the passage, the electric field intensity is higher in regions E 1  and E 2  before the DC block capacitors  14   a  and  14   b . Note that the electric field intensity indicates reflection of the RF signal, radiation thereof to the outside, and the like. Such reflection, radiation, and the like are caused by mismatched characteristic impedances, mismatched propagation modes of the RF signal, and the like. From a result of more detailed analysis, it was found that the higher electric field intensities represented returning of energy of the RF signal radiated in the package  11  as noise at an unintended place. In such a case, the returning of the RF signal as the noise serves as a factor causing degradation of performance of the optical communication component. 
     To prevent performance degradation due to the reflection of the RF signal in such a DC block portion, radiation thereof to the outside, and the like, it is necessary to adjust the respective characteristic impedances of a portion corresponding to the RF wires  12  and the DC block portion. It may also be possible to achieve a match between RF propagation mode shapes or the like, but either the characteristic impedance adjustment or the match achievement needs to be performed carefully. 
     For example, to achieve connection to each of the RF wires  12  having a width of about 100 μm, a compact chip capacitor having not a 0.6×0.3 mm size, but a 0.4×0.2 mm size is used. In addition, by using a technique of reducing a gap between the signal line of each of the RF wires  12  and the GND electrode  13  or reducing a size difference when the propagation mode is converted, it is possible to prevent the performance degradation to a given degree. 
     However, the RF signal transmitted by each of the RF wires  12  formed on flat surfaces of the grounded coplanar lines and the RF signal passing through the chip capacitor having a height differ in propagation mode shape. Specifically, compared to the RF wire  12  on a flat surface having a width of about 100 μm, the 0.6×0.3 mm chip capacitor has a height as large as 0.3 mm, and accordingly the RF signal passing through the chip capacitor is transformed into a propagation mode having an extent which is about three times as wide as and about three to five times as high as that of a propagation mode of the RF signal transmitted by the RF wire  12 . As a result, even when the characteristic impedances adjustment is carefully performed, the chip capacitor results in a place where a certain degree of reflection of the RF signal, a certain degree of radiation thereof to the outside, and the like occur. 
     When the reflection of the RF signal thus occurs in the DC block portion, signal quality degradation such as attenuation of the output signal and a reduction in an output amplitude at a specified frequency occurs. Meanwhile, when the radiation of the RF signal to the outside occurs in the DC block portion, the RF noise may expand in an inner space of the optical communication component and begin to turn back to the RF signal at an unexpected place. In addition, the radiation of the RF signal to the outside may result in amplification of the RF signal by the optical modulator driver IC and the TIA, oscillation thereof, and the like and lead to significant performance degradation of the optical communication component. 
     Due to such circumstances, it is difficult to completely eliminate reflection and radiation between the flat RF wires  12  and the chip capacitors each having a given height. Accordingly, a coping method such as placement of a radio wave absorber for absorbing the high-frequency energy radiated in the space in the optical communication component may occasionally be used. The radio wave absorber comes in a type that absorbs an electric current generated by a radio wave by using an inner resistance of a material, a type that uses a dielectric loss, a type that uses a magnetic loss in a magnetic material such as ferrite, or the like. The radio wave absorber of any type absorbs the high-frequency energy radiated in the inner space of the package  11  and prevents returning of the energy to be able to achieve an effect of noise reduction, oscillation prevention, or the like. 
     However, when the radio wave absorber is used, setting of a layout presents a problem. For example, when the radio wave absorber is present immediately above the RF wires  12 , the radio wave absorber undesirably serves as a factor which attenuates the RF signal. In addition, when the radio wave absorber is fixed in the package  11  also, an actual method encounters difficulty. Moreover, an application of the radio wave absorber is difficult unless the radio wave absorber satisfies conditions that a material thereof can efficiently absorb a radio wave and there is no large difference between a thermal expansion coefficient thereof and that of each of constituent elements in the package  11 . 
     Specifically, unless it is guaranteed that the radio wave absorber drops off within a temperature range of −5° C. to 85° C. corresponding to an operating temperature of the optical communication component, it becomes difficult to use the radio wave absorber over a long period of time. Besides, no generation of a gas that may affect the optical communication component, no long-term quality deterioration, and the like is also required. This results in a situation where use of an extra radio wave absorber as a countermeasure against performance degradation increases the number of component parts, leads to a detriment to a cost reduction, and is therefore hard to practically implement. 
     There are another configuration of an optical communication component in which an optical communication element and an electronic circuit element are mounted in one package. Specifically, as another example, a coherent optical sub-assembly (hereinafter abbreviated as COSA) using silicon photonics technology can be listed. The COSA has a silicon photonics chip (hereinafter referred to as the SiP chip) in which an optical circuit, an optical modulator, a germanium optical receiver, and the like are integrated in one chip by using the silicon photonics technology of forming an optical element on a silicon substrate. Then, the SiP chip serving as an optical communication element and an optical modulator driver IC and a TIA each serving as an electronic circuit element are contained together in one package to allow the COSA to be configured as the optical communication component. In the COSA also, for the protection of the optical modulator driver IC and the TIA, a DC block is embedded in the package. 
       FIG.  3    is a diagram illustrating a cross section of an example of a basic structure of a COSA  20  as a known optical communication component in a side surface direction. 
     Referring to  FIG.  3   , a package (PKG)  21  of the COSA  20  is preferably made of a ceramic or of an organic substrate material, and has a base having a flat plate shape. An upper portion of the package  21  is covered with a lid (LID)  27  for protecting various devices such as an optical modulator driver IC  26 , a SiP chip  25 , and DC block capacitors  24   a  and  24   b  each mounted on an upper surface of the package  21 . On a lower surface of the package  21 , a solder BGA (Ball Grid Array)  31  for effecting connection and fixation to a printed circuit board (PCB) as a connection partner is juxtaposed. The optical modulator driver IC  26  and the SiP chip  25  are connected and fixed by individual Au bumps  32  provided in juxtaposition to a conductive pattern on the upper surface of the package  21 . 
     The conductive pattern provided on the package  21  includes RF wires, GND electrodes, metal via wires, and the like. To the package  21  of the COSA  20  also, a laminated ceramic structure can be applied. For example, it is possible to provide connection between the various devices with the RF wires and place the metal via wires for routing and connection of the inner-layer GND electrode. However, a detailed configuration of the conductive pattern is not specified herein except that the DC block capacitor  24   a  is interposed between the RF wires to be able to protect the optical modulator driver IC  26  serving as the electric circuit element. 
     Preferably, the lid  27  is formed of a metal material having a high thermal conductivity such as aluminum or a copper alloy and the like. In such a case, devices such as the optical modulator driver IC  26  and the TIA generate heat during operation thereof, and accordingly a heat dissipation structure is used as countermeasures against heat generation. In the example illustrated in  FIG.  3   , a heat dissipation paste  28  is interposed between a projecting portion  27   a  corresponding to an inner projecting portion of the lid  27  and an upper surface of the optical modulator driver IC  26 . Since the lid  27  is bonded to the package  21 , the heat dissipation paste  28  may be applied appropriately to, e.g., the inner projecting portion  27   a  of the lid  27 . This can provide a structure in which heat generated from the optical modulator driver IC  26  is transferred to the lid  27  via the heat dissipation paste  28  present on the upper surface of the optical modulator driver IC  26  to be dissipated. 
     An exemplary case can be illustrated in which, as a material of the package  21 , a low-temperature co-fired ceramic (hereinafter abbreviated as LTCC) as a type of ceramic is used. The LTCC in use has a thermal expansion coefficient of about 11 ppm/K close to that of the printed circuit board to be connected to the package  21  via the solder BGA  31 , and is excellent in terms of mountability of the optical communication component. Note that, when aluminum is used as a metal material to be used for the lid  27 , the lid  27  has a thermal expansion coefficient of about 23 ppm/K and, when a copper alloy is used as the metal material to be used for the lid  27 , the lid  27  has a thermal expansion coefficient of about 17 ppm/K. 
     It is said that, unlike an optical element formed of an indium phosphide InP material or the like, the COSA  20  does not require hermetic sealing, and a non-airtight package structure that can easily be produced at low cost is used. Of the non-airtight package structure, a portion corresponding to the package  21  based on ceramic or the like and a portion corresponding to the lid  27  can be bonded together by an easy method using an adhesive or the like, not by a bonding method such as silver brazing or welding. When a copper alloy having a relatively small thermal expansion coefficient difference with the LTCC is used as the metal material to be used for the lid  27 , cost is slightly higher than when aluminum is used as the metal material of the lid  27 . When lower-cost aluminum is used as the metal material of the lid  27 , a problem to be solved is how to overcome a thermal expansion coefficient difference with the LTCC. 
     In the COSA  20  having the non-airtight package structure illustrated in  FIG.  3   , as the countermeasures against heat generation, the heat dissipation paste  28  is interposed between the local inner projecting portion  27   a  of the lid  27  and the upper surface of the optical modulator driver IC  26  to improve heat dissipation efficiency. However, with such a mere inventive modification, even though a heat dissipating effect is obtained during heat generation from the devices in the package  21 , a problem that energy radiated at an unintended place in the package  21  returns as noise to result in performance degradation cannot be solved. 
     Note that, as a known technique related to an optical communication component having an integrated structure, a form of the COSA is shown in NPL 1. The COSA is configured by flip-chip mounting a SIP chip, a driver IC, and a TIA on a package made of an LTCC material and disposing chip capacitors serving as DC blocks on the package. In addition, in the same manner as in the case described with reference to  FIG.  3   , the COSA is configured by covering an upper portion of the entire package with a lid. Meanwhile, in NPL 2, as an example of an optical communication element, an InP-based 90 0  hybrid integrated light receiving element for 100 Gbit/s compact coherent receivers is disclosed. 
     CITATION LIST 
     Non Patent Literature 
     
         
         [NPL 1] C. Doerr, J. Heanue, L. Chen, R. Aroca, S. Azemati, G. Ali, G. McBrien, Li Chen, B. Guan, H. Zhang, X. Zhang, T. Nielsen, H. Mezghani, M. Mihnev, C. Yung, and M. Xu “Silicon Photonics Coherent Transceiver in a Ball-Grid Array Package”, 2017 Optical Fiber Communications Conference and Exhibition (OFC), March 2017, Post-Deadline paper, Th5D. 5. 
         [NPL 2] N. Inoue, H. Yagi, R. Masuyama, T. Katsuyama, Y. Yoneda, and H. Shoji, “InP-Based Photodetector Monolithically Integrated with 90° Hybrid for 100 Gbit/s Compact Coherent Receivers”, SEI Technical Review No. 185, July 2014, pp. 61-66. 
       
    
     SUMMARY OF THE INVENTION 
     Embodiments according to the present invention are achieved in order to solve the problems described above. An object of the embodiments according to the present invention is to provide an optical communication component capable of preventing energy radiated at an intended place in a package from returning as noise and causing performance degradation. 
     To attain the object described above, an aspect of the present invention is an optical communication component including: a package having a flat plate shape; an optical communication element mounted on an upper surface of the package; an electronic circuit element mounted on the upper surface of the package and located at a position different from that of the optical communication element; a DC block device mounted on the upper surface of the package and located at a position different from those of the optical communication element and the electronic circuit element to cut off a DC signal included in a RF signal transmitted to the electronic circuit element via a conductive pattern provided on the package; and a lid provided over an upper portion of the package to cover the optical communication element, the electronic circuit element, and the DC block device, the lid having a separation projecting portion that projects toward the upper portion of the package to separately define a region where the DC block device is present and a region where the optical communication element and the electronic circuit element are present. 
     The optical communication component having the configuration described above can prevent energy radiated at an intended place in a package from returning as noise and causing performance degradation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is an outer appearance perspective view when viewed from obliquely above in which a structural shape of a DC block in a package in an ICR as a known optical communication component is partially broken and exposed. 
         FIG.  2    is a partially enlarged view illustrating a result of simulating an electric field intensity of an RF signal in the package of the ICR illustrated in  FIG.  1   . 
         FIG.  3    is a diagram illustrating a cross section of an example of a basic structure of a COSA as a known optical communication component in a side surface direction. 
         FIG.  4    is a diagram illustrating a cross section of a basic structure of a COSA as an optical communication component according to a first embodiment of the present invention in the side surface direction. 
         FIG.  5    is a diagram illustrating a cross section of another example of a basic structure of a COSA as an optical communication component according to a comparative example in the side surface direction. 
         FIG.  6    is a diagram illustrating a cross section of a basic structure of a COSA as an optical communication component according to a second embodiment of the present invention in the side surface direction. 
         FIG.  7    is a cross-sectional view illustrating a partially broken conductive pattern of a grounded coplanar line including RF wires placed in a surface layer and GND electrodes disposed in an inner layer and in the surface layer, which is applicable to a package related to a principal portion of the COSA illustrated in  FIG.  5   . 
         FIG.  8    is a perspective view of a conductive pattern on the package illustrated in  FIG.  7   , which is partially illustrated in a cross section. 
         FIG.  9    is a cross-sectional view illustrating a partially broken conductive pattern including RF wires placed in an inner layer and GND electrodes disposed in the inner layer and in a surface layer, which is applicable to a package related to a principal portion of the COSA illustrated in  FIG.  6   . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Referring to the drawings, a detailed description will be given of an optical communication component according to each of embodiments of the present invention. 
     First Embodiment 
       FIG.  4    is a diagram illustrating a cross section of a basic structure of a COSA  20 A as an optical communication component according to the first embodiment of the present invention in a side surface direction. 
     Referring to  FIG.  4   , the COSA  20 A is similar to the COSA  20  in  FIG.  3    in that a lid  27 A has the projecting portion  27   a  and different from the COSA  20  in  FIG.  3    in having a separation projecting portion  27   b  projecting toward the upper surface of the package  21  having the flat plate shape. The separation projecting portion  27   b  has a function of separately defining a region where the DC block capacitor  24   a  is present and a region where the SiP chip  25  serving as the optical communication element and the optical modulator driver IC  26  serving as the electronic circuit element are present. 
     The DC block capacitors  24   a  and  24   b  also cut off herein the DC signal included in the RF signal transmitted to the optical modulator driver IC  26 , the TIA, and the like via the conductive pattern provided on the package  21 . The conductive pattern may also be regarded as including the RF wires and the GND electrodes. This allows the DC block capacitors  24   a  and  24   b  to function as DC block devices protecting the various devices. 
     A one portion of the GND electrodes of the conductive pattern is electrically connected to a tip surface of the separation projecting portion  27   b  of the lid  27 A. When a state of the connection is mechanically stable, sufficient contact is provided between the GND electrode and the tip surface of the separation projecting portion  27   b . When the state of the connection is not mechanically stable, the GND electrode of the conductive pattern mentioned above and the tip surface of the separation projecting portion  27   b  mentioned above may also be bonded together using an adhesive or the like. However, when, e.g., a non-conductive adhesive is used as the adhesive, side walls of the separation projecting portion  27   b  in the vicinity of the tip surface thereof may be adhesively fixed appropriately to a top surface of the package  21  by using the adhesive so as to prevent the adhesive from being applied to the tip surface of the separation projecting portion  27   b . Meanwhile, when a conductive adhesive is used as the adhesive, the tip surface of the separation projecting portion  27   b  may be adhesively fixed appropriately to the surface of the package  21  by using the adhesive. In either case, mechanical stability as well as an electrically connected state is maintained. When conduction is provided between the GND electrode and the tip surface of the separation projecting portion  27   b  of the lid  27 A, the lid  27 A is at a GND potential. Alternatively, the electrical connection between the lid  27 A and the package  21  may also be provided not at the separation projecting portion  27   b , but at another region such as an outer peripheral edge portion. In such a case, since the RF wires are placed on the surface of the package  21  to be brought into contact with the separation projecting portion  27   b , when the lid  27 A at the GND potential is brought closer thereto, it is required to give sufficient consideration so as not to affect a RF characteristic. 
     Note that, to the package  21  of the COSA  20 A also, the laminated ceramic structure can be applied. For example, it is possible to connect the various devices with the RF wires included in the conductive pattern and place the metal via wires for routing and connection of the inner-layer GND electrode. However, it is also assumed that a detailed configuration of the conductive pattern is not specified herein except that the DC block capacitor  24   a  is interposed between the RF wires to be able to protect the optical modulator driver IC  26  serving as the electric circuit element. 
     The configuration is otherwise the same as in the case of the COSA  20 . Specifically, the SiP chip  25  is configured by integrating an optical circuit, an optical modulator, a germanium optical receiver, and the like in one chip by using the silicon photonics technology of forming an optical element on a silicon substrate. An upper portion of the package  21  is covered with the lid  27 A for protecting devices such as the optical modulator driver IC  26 , the SiP chip  25 , and the DC block capacitors  24   a  and  24   b  each mounted on the upper surface of the package  21 . On the lower surface of the package  21 , the solder BGA  31  for effecting connection and fixation to the printed circuit board as the connection partner is juxtaposed. The optical modulator driver IC  26  and the SiP chip  25  are connected and fixed by the individual Au bumps  32  provided in juxtaposition to the conductive pattern on the upper surface of the package  21 . 
     Preferably, the lid  27 A is formed of a metal material having a high thermal conductivity such as aluminum or a copper alloy and the like. Devices such as the optical modulator driver IC  26  and the TIA also generate heat during operation thereof, and accordingly a heat dissipation structure is used herein as countermeasures against heat generation. Specifically, the structure is such that the heat dissipation paste  28  is interposed between the projecting portion  27   a  corresponding to the inner projecting portion of the lid  27 A and the upper surface of the optical modulator driver IC  26 . Since the lid  27 A used herein is also bonded to the package  21 , the heat dissipation paste  28  may be applied appropriately to the inner projecting portion  27   a  of the lid  27 A. This can provide a structure in which the heat generated from the optical modulator driver IC  26  is transferred to the lid  27 A via the heat dissipation paste  28  present on the upper surface of the optical modulator driver IC  26  to be dissipated. 
     In the case of the COSA  20 A according to the first embodiment, on the inner side of the lid  27 A, the separation projecting portion  27   b  as well as the projecting portion  27   a  for using the heat dissipation paste  28  is provided. The separation projecting portion  27   b  separately defines the region where the DC block capacitor  24   a  is present and the region where the electronic circuit element is present. Consequently, the energy of the RF signal reflected by the portion corresponding to the DC block and radiated in the inner space of the package  21  is confined to a space formed by inner walls of the lid  27 A and the top surface of the package  21 . As a result, it is possible to sufficiently prevent the energy radiated at an unintended place in the package  21  from returning as noise and causing performance degradation. 
     Also, in the case of the COSA  20 A according to the first embodiment, as the metal material of the lid  27 A, a copper alloy or the like having a relatively small thermal expansion coefficient difference with the LTCC as the material of the package  21  is used preferably. In this case, it is possible to provide a compact and high-performance optical communication component that is hardly affected by thermal expansion. However, for a cost reduction, aluminum having a relatively large thermal expansion coefficient difference with the LTCC as the material of the package  21  can also be used as a metal material of the lid  27 A. In such a case, due to the presence of the separation projecting portion  27   b  other than the projecting portion  27   a , the lid  27 A has an improved mechanical strength. As a result, even when aluminum is used as the metal material of the lid  27 A, influence of thermal expansion is reduced, and it is possible to provide a compact and low-cost optical communication component. 
     Note that, in the exemplified structure of the COSA  20 A according to the first embodiment, the separation projecting portion  27   b  separately defines the region where the DC block capacitor  24   a  is present. However, in the COSA  20 A, the conductive pattern provided on the package  21  also differs depending on a mode of each of the various devices mounted on the upper surface of the package  21 . Accordingly, it is possible to provide the COSA  20 A with a structure in which an additional separation projecting portion is provided to separately define even the region where the DC block capacitor  24   b  is present. In other words, the number of the separation projecting portions to be disposed and places where the separation projecting portions are to be disposed can freely be changed depending on various devices mounted on the package  21  and the conductive pattern for connection thereof. 
     Second Embodiment 
       FIG.  6    is a diagram illustrating a cross section of a basic structure of a COSA  20 C as an optical communication component according to the second embodiment of the present invention in the side surface direction. Note that, in the second embodiment, referring to  FIG.  5    illustrating a cross section of another example of a basic structure of a COSA  20 B as an optical communication component according to a comparative example, a description will be given of a difference between the respective basic structures of the COSA  20 C and the COSA  20 B, while consideration is given thereto. 
     Referring to  FIG.  5   , the COSA  20 B according to the comparative example is different from the COSA  20  in  FIG.  3    in a routing and connection structure of a conductive pattern on a package  21 B having a flat plate shape. To the package  21 B, a laminated ceramic package  21 Ba is applied, and routing of the RF wires  22 , GND electrodes  23 , and metal via wires described later is performed. The RF wires  22  are placed so as to connect, as a whole, the DC block capacitor  24   a , the optical modulator driver IC  26 , and the SiP chip  25  such that the DC block capacitor  24   a  is interposed between the RF wires  22 . Note that the lid  27  is merely configured to have a heat dissipation projecting portion  27   a.    
     In the laminated ceramic package  21 Ba, the metal via wire connecting the GND electrode  23  on an upper surface of the package  21 B and the solder BGA  31  in a direction of lamination is applied. Note that, around the metal via wire, via wires of the GND electrode  23  are similarly illustrated. Additionally, the metal via wire connecting the RF wire  22  connected to the DC block capacitor  24   a  on the upper surface of the package  21 B and the solder BGA  31  in the direction of lamination is also applied. Still additionally, the metal via wire connecting the inner-layer GND electrode  23  of the package  21 B and the solder BGA  31  in the direction of lamination is also applied. 
     The via wires of the GND electrodes  23  are placed so as to surround the metal via wire of the RF wire  22  and, by using a structure similar to that of a coaxial line, it is possible to implement a characteristic with reduced reflection and attenuation of the PF signal. Note that the GND electrodes  23  formed in layers underlying the RF wires  22  formed on the top surface of the package  21 B include the GND electrode  23  and the metal via wire in the surface layer not shown, and a layout thereof is illustrated in  FIG.  7    described later. 
     Referring to  FIG.  6   , the COSA  20 C is different from the COSA  20 B in  FIG.  5    in that a laminated ceramic package  21 Aa is applied as a routing and connection structure in a conductive pattern on a package  21 A having a flat plate shape. Various devices provided on an upper surface of the package  21 A are the same as those used in the case of the first embodiment. The lid  27 A is configured to have the projecting portion  27   a  and also have the separation projecting portion  27   b . Note that, as illustrated in the first embodiment, as the material of the lid  27 A, the metal material having the high thermal conductivity is used and, as the material of the package  21 A, the LTCC material or the like having the thermal expansion coefficient closer to that of the printed circuit board as the connection partner is used. 
     The laminated ceramic package  21 Aa has a wiring structure in which a portion of the RF wire  22  temporarily extends into the inner layer of the package  21 A via one of metal via wires  29  and then returns again to the surface layer via another of the metal via wires  29  at another position. The RF wires  22  are placed so as to connect, as a whole, the DC block capacitor  24   a , the optical modulator driver IC  26 , and the SiP chip  25  such that the DC block capacitor  24   a  is interposed between the RF wires  22 . Note that the GND electrodes  23  formed in the layers overlaying and underlying the RF wires  22  formed in the inner layer of the package  21 A have metal via wires not shown, and a layout thereof is illustrated in  FIG.  9    described later. 
     In the case of the COSA  20 C also, the separation projecting portion  27   b  separately defines the region where the DC block capacitor  24   a  is present and the region where the electronic circuit element is present. Consequently, the energy of the RF signal reflected by the portion corresponding to the DC block and radiated in an inner space of the package  21 A is confined to a space formed by the inner walls of the lid  27 A and a top surface of the package  21 A. As a result, it is possible to prevent the energy radiated at an unintended place in the package  21 A from returning as noise and causing performance degradation. 
     Additionally, in the case of the COSA  20 C, the portions of the RF wires  22  connecting the DC block capacitor  24   a  and the optical modulator driver IC  26  are placed in the inner layer of the package  21 A and, in the surface layer of the inner-layer portion in which the RF wires  22  are placed, the GND electrode  23  is disposed. Such a configuration allows the tip surface of the separation projecting portion  27   b  of the lid  27 A to come into contact with the GND electrode  23  on the upper surface of the package  21 A and be electrically connected thereto without bringing the tip surface of the separation projecting portion  27   b  into contact with the RF wire  22 . 
     In other words, such a form allows the space in which the DC block is provided to be provided as a closed space. Elements forming the closed space include the GND electrode  23  on the top surface of the package  21 A to be connected to the tip surface of the separation projecting portion  27   b  of the lid  27 A and inner walls of the lid  27 A at the GND potential in such a state of connection. Such elements also include the GND electrode  23  formed on an end side of the top surface of the package  21 A to be bonded to an end surface of an edge portion of the lid  27 A. 
     In the COSA  20 C having such a configuration, the energy of the RF signal reflected by the DC block portion and radiated in the inner space of the package  21 A is confined to a space formed by the lid  27 A at the GND potential and the GND electrode  23  on the upper surface of the package  21 A. A confining effect achieved by the COSA  20 C is more remarkable than that achieved by the COSA  20 A in the first embodiment. 
     In other words, the COSA  20 C in the second embodiment achieves the same actions and effects as those achieved by the COSA  20 A in the first embodiment and can more reliably prevent occurrence of performance degradation. As a result, it is possible to provide a compact and low-cost optical communication component having an excellent RF characteristic. In particular, in the case of the COSA  20 C, the portions of the RF wires  22  are placed in the inner layer of the package  21 A to allow the tip surface of the separation projecting portion  27   b  of the lid  27 A to be connected mechanically solidly to the GND electrode  23  on the top surface of the package  21 A. As a result, to maintain a connected state, bonding the side walls in the vicinity of the tip surface of the separation projecting portion  27   b  of the lid  27 A and the top surface of the package  21 A together using, e.g., the non-conductive adhesive  30  or the like is effective, as illustrated in  FIG.  6   . When the non-conductive adhesive  30  is used, it is preferable to prevent the adhesive  30  from being applied to the tip surface of the separation projecting portion  27   b  and provide conduction between the GND electrode  23  and the lid  27 A through the tip surface of the separation projecting portion  27   b . Meanwhile, when the conductive adhesive is used, the tip surface of the separation projecting portion  27   b  and the GND electrode  23  on the upper surface of the package  21 A are adhesively fixed to each other by using the adhesive. In either case, it is possible to provide a form in which mechanical stability and an electrically connected state are simultaneously maintained. Alternatively, as described in the first embodiment, it is also possible to provide electrical connection between the lid  27 A and the package  21  not at the separation projecting portion  27   b , but at another place, and points to be considered in that case are also as described above. In this state, the lid  27 A is at the GND potential. 
     The electronic circuit element to be used in the optical communication component comes in various types such as a type that requires heat dissipation and a type that requires a potential at a back surface thereof to be reduced to the GND potential. For example, the optical modulator driver IC  26  illustrated in each of  FIGS.  3 ,  4 ,  5 , and  6    is of a type that requires heat dissipation and also requires a potential at a back surface thereof to be reduced to the GND potential. This results from flip-chip mounting of the optical modulator driver IC  26  with the back surface thereof facing upward. The SiP chip  25  illustrated in each of the drawings mentioned above is also flip-chip mounted. 
     When consideration is given to such circumstances, there is a case where a paste for which a material having an optimal property is to be selected and which need not necessarily have a strong bonding force, such as a thermally conductive paste having an excellent heat dissipation property or a low-resistance conductive paste, may be applied. In such a point also, it can be said that the form in the second embodiment in which a bonding area can be increased to ensure a bonding strength between the lid  27 A and the package  21 A is advantageous. In other words, by ensuring the bonding strength between the lid  27 A and the package  21 A as appropriate, it is possible to use the optical communication component for a long period without separating the lid  27 A from the package  21 A in a range of −5° C. to 85° C. corresponding to an operating temperature of the optical communication component. In such a case, it is possible to provide the optical communication component excellent in mechanical stability and long-term reliability. 
     A structure in which the portions of the RF wires  22  connecting the DC block capacitor  24   a  and the optical modulator driver IC  26  are placed in the inner layer of the package  21 A and the GND electrode  23  is disposed in the surface layer, such as that of the COSA  20 C, has various advantages. This structure allows a structure in which the RF wires  22  are circumferentially surrounded by the GND electrodes  23  to be provided, which is effective in improving a characteristic compared to the grounded coplanar line applicable to a case where the RF wires  22  are on the top surface of the package  21 A. In other words, by also covering the upper portions of the RF wires  22  with the GND electrodes  23 , it is possible to reduce likelihood of entrance of noise from the outside and reduce radiation of the RF signal to the outside. 
     The following will add technical supplementary notes with respect to the laminated ceramic package  21 Ba of the COSA  20 B according to the comparative example described above and the laminated ceramic package  21 Aa of the COSA  20 C according to the second embodiment. 
       FIG.  7    is a cross-sectional view illustrating a partially broken conductive pattern applicable to the package  21 B related to a principal portion of the COSA  20 B according to the comparative example described above. The conductive pattern on the laminated ceramic package  21 Ba is a grounded coplanar line including the RF wires  22  placed in the surface layer and the GND electrodes  23  disposed in the inner layer and in the surface layer.  FIG.  8    is a perspective view of the conductive pattern on the package  21 B, which is partially illustrated in a cross section. Note that, in  FIG.  7   , the layout of the GND electrode  23  in the surface layer and the metal via wires  29 , which is not illustrated in  FIG.  5   , is also illustrated. 
       FIG.  9    is a cross-sectional view illustrating a partially broken conductive pattern applicable to the package  21 A related to a principal portion of the COSA  20 C according to the second embodiment described above. The conductive pattern on the laminated ceramic package  21 Aa includes the RF wires  22  placed in the inner layer and the GND electrodes  23  disposed in the inner layer and in the surface layer. In other words,  FIG.  9    illustrates a form in which the inner-layer RF wires  22  are surrounded by the GND electrodes  23  in the surface layer and in the inner layer and by the metal via wires  29  in the inner layers. Note that, in  FIG.  9   , a layout of the metal via wires  29  not illustrated in  FIG.  6    is also illustrated. 
     In each of the modes in  FIGS.  7  to  9   , a line form of a structure (GSSG structure) in which the GND electrode  23  is absent between the two RF wires  22  serving as the differential lines is illustrated. However, as in the case of the ICR  10  illustrated in  FIG.  1   , a line form of a structure (GSGSG) in which the GND electrodes  23  are interposed between the two RF wires  22  can also be used instead. Therefore, the optical communication component of the present invention is not limited to the configuration disclosed in each of the embodiments. 
     Note that, in the exemplified structure of the COSA  20 C according to the second embodiment also, the separation projecting portion  27   b  separately defines the region where the DC block capacitor  24   a  is present. However, in the COSA  20 C, the conductive pattern provided on the package  21 A also differs depending on a mode of each of the various devices mounted on the upper surface of the package  21 A. Accordingly, it is possible to provide the COSA  20 C with a structure in which, in the same manner as described in the first embodiment, an additional separation projecting portion is provided to separately define even the region where the DC block capacitor  24   b  is present. In other words, the number of the separation projecting portions are to be disposed and places where the separation projecting portions are to be disposed can freely be changed depending on various devices mounted on the package  21 A and the conductive pattern for connection thereof.