Abstract:
The inventive optical module has a feeder line for electrically connecting a semiconductor laser with an electric signal input/output unit of a package. The feeder line has a dielectric substrate and a conductor film formed on the dielectric substrate, which consists of a material having thermal conductivity smaller than that of aluminum oxide. Thus obtained is an optical module capable of inhibiting a semiconductor laser from wavelength change caused by heat flowing into the same in a high-density wavelength division multiplex optical transmission system multiplexing a plurality of signals in an optical wavelength region in high density and transmitting the same.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical module employed for a high-density wavelength division multiplex optical transmission system.  
           [0003]    2. Description of the Prior Art  
           [0004]    [0004]FIG. 17 is a schematic sectional view showing the structure of a conventional optical module. Referring to FIG. 17, the conventional optical module mainly includes a semiconductor laser  20  converting an electric signal to an optical signal, a thermistor  70  serving as temperature detection means, a mounting unit  50  mounting the semiconductor laser  20  and the thermistor  70 , a thermoelectric cooling element  80  for heating/cooling the mounting unit  50  for controlling the temperature thereof, a driver IC (integrated circuit)  30 , a feeder line  110 , a package  60  storing these members, and a signal input connector  40  electrically connected to an electric signal input/output unit of the package  60 .  
           [0005]    The feeder line  110  is employed for electrically connecting the semiconductor laser  20  and the electric signal input/output unit of the package  60  with each other. The driver IC  30  is electrically connected between the feeder line  110  and the electric signal input/output unit of the package  60  for amplifying the electric signal input in the semiconductor laser  20 .  
           [0006]    [0006]FIGS. 18 and 19 are a perspective view and a sectional view schematically showing the structure of the feeder line  110  respectively. Referring to FIGS. 18 and 19, the feeder line  110  is a microstrip line prepared by forming conductor films  102  and  103  on an alumina ceramic substrate  101  mainly composed of aluminum oxide. Each of the conductor films  102  and  103  consists of a multilayer structure including at least two layers, i.e., a lower conductor layer  102   b  or  103   b  actually fed with a high-frequency electric signal and a gold plating layer  102   a  or  103   a  necessary for soldering or wire bonding.  
           [0007]    A wavelength division multiplex transmission system is watched with interest as an application of such an optical module. This system, multiplexing a plurality of signals in an optical wavelength region and transmitting the same, readily increases the capacity of an optical communication system. Recently, a high-density wavelength division multiplex transmission system narrowing the multiplexed wavelength interval to 200 GHz or 100 GHz has been defined under international standards for attaining a higher capacity. The optical module must have a sufficiently stable wavelength (preferably not more than about {fraction (1/100)} the wavelength interval) with respect to this wavelength interval.  
           [0008]    In the optical module shown in FIG. 17, heat flows into the mounting unit  50  mounting the semiconductor laser  20  and the thermistor  70  mainly through the aforementioned feeder line  110 . The temperature of the optical module is so controlled that the temperature detected by the thermistor  70  is constant.  
           [0009]    In practice, however, the portion provided with the semiconductor laser  20  and the portion provided with the thermistor  70  are different in thermal resistance from each other as viewed from the heat inflow path, to exhibit different temperatures, as shown in FIG. 20. When the ambient temperature for the optical module changes, therefore, the temperature of the semiconductor laser  20  disadvantageously changes even if the temperature of the optical module is so controlled that the temperature detected by the thermistor  70  is regularly constant.  
           [0010]    Referring to FIG. 20, the descending solid line shows temperature distribution in the case where the package  60  has a higher temperature than the semiconductor laser  20  while the ascending solid line shows temperature distribution in the case where the package  60  has a lower temperature than the semiconductor laser  20 .  
           [0011]    Assuming that the thickness of the substrate  101  of alumina (thermal conductivity: 33 W/m/K) is 254 μm and the thickness of the conductor films  102  and  103  of gold (thermal conductivity: 315 W/m/K) is 3 μm in the microstrip line shown in FIGS. 18 and 19, thermal conduction between the alumina part and the conductor parts is about 9:1 and a larger quantity of heat is transmitted through the alumina part. While thermal conductivity can be lowered by reducing the thickness of the substrate  101  consisting of alumina, the substrate  101  is readily cracked if the thickness thereof is reduced. Therefore, the thermal conductivity cannot be much reduced in practice.  
           [0012]    Wires  90   a  for the driver IC  30  define another heat inflow path, as shown in FIG. 21. The wires  90   a  connected to the driver IC  30  for amplifying the electric signal input in the semiconductor laser  20  are formed on an electric circuit mounting unit  90 A. When the electric signal is input in the connector  40 , the wires  90   a  for the driver IC  30  must be coupled to leads  90 C arranged oppositely to the connector  40 . Therefore, the wires  90   a  are electrically connected to the leads  90 C through wires  90   d  located on the mounting unit  50  and conductor patterns  90   b  located on a lead mounting substrate  90 B.  
           [0013]    In the conventional optical module, the wires  90   a  are coupled to the leads  90 C located oppositely to the driver IC  30  through the mounting unit  50  mounting the semiconductor laser  20 , and hence heat flows into the mounting unit  50  through the wires  90   a  or other wires. When the ambient temperature for the optical module changes, therefore, the temperature of the semiconductor laser  20  disadvantageously changes although the temperature of the optical module is so controlled that the temperature detected by the thermistor  70  is regularly constant, similarly to the aforementioned case where heat flows into the optical module through the feeder line  110 .  
           [0014]    Temperature dependency of the oscillation wavelength of the semiconductor laser  20  is about 10 GHz/° C., and hence the temperature thereof must be controlled with precision of not more than about 0.1° C. within the category temperature range. Therefore, wavelength change of the semiconductor laser  20  caused by heat flowing into the mounting unit  50  through the feeder line  110  or the wires  90   a  comes into question.  
           [0015]    Further, a conventional optical communication system performs no multiplexing in the wavelength region, and hence thermal design is simply based on whether or not the mounting unit  50  can be heated/cooled to a prescribed temperature in the category temperature range. Therefore, the aforementioned problem has been first clarified when the high-density wavelength division multiplex transmission system performing high-density multiplexing in the wavelength region has been watched with interest.  
         SUMMARY OF THE INVENTION  
         [0016]    An object of the present invention is to provide an optical module capable of suppressing wavelength change of a semiconductor laser caused by inflow of heat in a high-density wavelength division multiplex optical transmission system multiplexing a plurality of signals in an optical wavelength region in high density and transmitting the same.  
           [0017]    The optical module according to the present invention comprises a package, an optical device, a mounting unit and a feeder line. The package has an electric signal input/output unit. The optical device is arranged in the package. The mounting unit mounts the optical device. The feeder line is employed for electrical connection between the optical device and the electric signal input/output unit, and includes a dielectric substrate having thermal conductivity smaller than the thermal conductivity of aluminum oxide and a conductor film formed on the dielectric substrate.  
           [0018]    In the optical module according to the present invention, the thermal conductivity of the dielectric substrate employed in the feeder line is smaller than the thermal conductivity of aluminum oxide. Thus, heat can be inhibited from flowing into the optical device through the feeder line as compared with a conventional feeder line employing a dielectric substrate of aluminum oxide. Therefore, a semiconductor laser can be inhibited from wavelength change caused by heat flowing into the same also in a high-density wavelength division multiplex transmission system multiplexing a plurality of signals in an optical wavelength region in high density and transmitting the same.  
           [0019]    In the aforementioned optical module, the thermal conductivity of the dielectric substrate is preferably not more than 3 W/m/K.  
           [0020]    Thus, the thermal conductivity can be remarkably reduced as compared with the thermal conductivity (33 W/m/K) of alumina employed for a conventional dielectric substrate. Therefore, heat can be further inhibited from flowing into the optical device through the feeder line.  
           [0021]    The aforementioned optical module preferably further comprises a thermoelectric cooling element for controlling the temperature of the mounting unit and a temperature detection element mounted on the mounting unit.  
           [0022]    According to the present invention, heat can be inhibited from flowing into the optical device through the feeder line as hereinabove described, whereby the temperature of the optical device can be kept constant by controlling the temperature of the mounting unit with the thermoelectric cooling element in response to the temperature detected by the temperature detection element.  
           [0023]    In the aforementioned optical module, the material of the dielectric substrate is preferably glass ceramic containing silicon dioxide.  
           [0024]    Thus, thermal conductivity lower than that of aluminum oxide, particularly thermal conductivity of not more than 3 W/m/K can be implemented.  
           [0025]    In the aforementioned optical module, the line width of the conductor film in a portion not in contact with the package and the mounting unit is preferably smaller than the width of the dielectric substrate.  
           [0026]    Thus, heat can be inhibited also from flowing into the optical device through the conductor film.  
           [0027]    In the aforementioned optical module, the conductor film for signal transmission is preferably formed only on one of a pair of opposite surfaces of the dielectric substrate.  
           [0028]    Thus, heat can be inhibited from flowing into the optical device through the conductor film.  
           [0029]    In the aforementioned optical module, the feeder line is preferably either a coplanar line or a slot line.  
           [0030]    Thus, the feeder line can be properly prepared from any line in response to the application thereof.  
           [0031]    In the aforementioned optical module, the conductor film preferably has a first terminal portion for electrical connection with the optical device, a second terminal portion for electrical connection with the electric signal input/output unit and a connection portion connecting the first and second terminal portions with each other. The connection portion consists of a material having smaller thermal conductivity than that of gold, and the first and second terminal portions consist of gold.  
           [0032]    When the connection par is prepared from a material having smaller thermal conductivity than gold, heat can be inhibited from flowing into the optical device through the conductor film while soldering or bonding of wires or ribbons is enabled on the first and second terminal portions .  
           [0033]    In the aforementioned optical module, the conductor film preferably has a first terminal portion for electrical connection with the optical device, a second terminal portion for electrical connection with the electric signal input/output unit and a connection portion connecting the first and second terminal portions with each other. Each of the first terminal portion, the second terminal portion and the connection portion have first layer consisting of a material including at least one element selected from a group consisting of nickel, platinum, palladium, tungsten, molybdenum and copper. Each of the first and second terminal portions have second layer consisting of gold formed on the first layers.  
           [0034]    Thus, heat can be inhibited from flowing into the optical device through the conductor film, and soldering or bonding of wires or ribbons is enabled on the first and second terminal portions.  
           [0035]    The aforementioned optical module preferably further comprises an electric circuit for amplifying an electric signal input in the optical device, an electric circuit mounting unit mounting the electric circuit and a wire formed on the electric circuit mounting unit and electrically connected to the electric circuit. The wire is electrically connected to a lead located outside the package without through the mounting unit.  
           [0036]    Thus, heat can be prevented from flowing into the optical device through the wire electrically connected to the electric circuit.  
           [0037]    The aforementioned optical module preferably further comprises a lead mounting substrate mounted on the package and having a conductor pattern for electrical connection to the lead. The lead mounting substrate has an extension part extending toward the electric circuit mounting unit. The conductor pattern extends onto the extension part to be electrically connected to the wire.  
           [0038]    Thus, heat can be prevented from flowing into the optical device through the wire electrically connected to the electric circuit, while the conductor pattern and the wire can be electrically connected with each other without requiring a specific member.  
           [0039]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]    [0040]FIG. 1 is a sectional view schematically showing the structure of an optical module according to a first embodiment of the present invention;  
         [0041]    [0041]FIG. 2 is a perspective view schematically showing the structure of a feeder line employed in the optical module according to the first embodiment of the present invention;  
         [0042]    [0042]FIG. 3 illustrates temperature distribution between a package and a semiconductor laser;  
         [0043]    [0043]FIG. 4 is a sectional view schematically showing the structure of an optical module according to a second embodiment of the present invention;  
         [0044]    [0044]FIG. 5 illustrates temperature distribution between a package and a thermistor;  
         [0045]    [0045]FIG. 6 is a plan view schematically showing the structure of a feeder line employed in an optical module according to a fourth embodiment of the present invention;  
         [0046]    [0046]FIG. 7 is a rear elevational view schematically showing the structure of the feeder line employed in the optical module according to the fourth embodiment of the present invention;  
         [0047]    [0047]FIG. 8 is a plan view schematically showing the structure of a feeder line employed in an optical module according to a fifth embodiment of the present invention;  
         [0048]    [0048]FIG. 9 is a rear elevational view schematically showing the structure of the feeder line employed in the optical module according to the fifth embodiment of the present invention;  
         [0049]    [0049]FIG. 10 illustrates a heat conduction path in the optical module according to the fifth embodiment of the present invention;  
         [0050]    [0050]FIG. 11 is a plan view showing the structure of a coplanar feeder line employed in an optical module according to a sixth embodiment of the present invention;  
         [0051]    [0051]FIG. 12 is a plan view showing the structure of a slot feeder line employed in the optical module according to the sixth embodiment of the present invention;  
         [0052]    [0052]FIGS. 13A and 13B are a plan view and a sectional view showing the structure of a feeder line employed in an optical module according to a seventh embodiment of the present invention respectively;  
         [0053]    [0053]FIGS. 14A and 14B are a plan view and a sectional view schematically showing the structure of a feeder line employed in an optical module according to an eighth embodiment of the present invention respectively;  
         [0054]    [0054]FIG. 15 is a partially fragmented plan view schematically showing the structure of an optical module according to a ninth embodiment of the present invention;  
         [0055]    [0055]FIG. 16 is a partially fragmented plan view schematically showing the structure of an optical module according to a tenth embodiment of the present invention;  
         [0056]    [0056]FIG. 17 is a sectional view schematically showing the structure of a conventional optical module;  
         [0057]    [0057]FIG. 18 is a perspective view schematically showing the structure of a feeder line employed in the conventional optical module;  
         [0058]    [0058]FIG. 19 is a sectional view schematically showing the structure of the feeder line employed in the conventional optical module;  
         [0059]    [0059]FIG. 20 illustrates temperature distribution between a package and a thermistor in the conventional optical module; and  
         [0060]    [0060]FIG. 21 is a partially fragmented plan view schematically showing the structure of the conventional optical module.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]    Embodiments of the present invention are now described with reference to the drawings.  
         [0062]    First Embodiment  
         [0063]    Referring to FIG. 1, an optical module according to a first embodiment of the present invention mainly includes a feeder line  10 , a semiconductor laser  20 , a driver IC  30 , a signal input connector  40 , a mounting unit  50  and a package  60 . The semiconductor laser  20  converts an electric signal to an optical signal, and the mounting unit  50  mounts the semiconductor laser  20 . The package  60  stores the semiconductor laser  20  and the mounting unit  50  therein, and the signal input connector  40  is connected to an electric signal input/output unit of the package  60 .  
         [0064]    The feeder line  10  is employed for electrically connecting the semiconductor laser  20  and the electric signal input/output unit of the package  60  with each other. The driver IC  30  is electrically connected between the feeder line  10  and the signal input connector  40  for amplifying the electric signal input in the semiconductor laser  20 , for example.  
         [0065]    Referring to FIG. 2, the feeder line  10  is a microstrip line prepared by forming conductor films  2  and  3  on a dielectric substrate  1 . The dielectric substrate  1  has smaller thermal conductivity than aluminum oxide (alumina). Each of the conductor films  2  and  3  consists of a multilayer structure including at least two layers, i.e., a lower conductor layer actually fed with a high-frequency electric signal and a gold plating layer necessary for soldering or wire bonding, similarly to the prior art shown in FIG. 19. FIGS. 1 and 2 illustrate the conductor films  2  and  3  as single layers, for convenience of illustration.  
         [0066]    According to this embodiment, the dielectric substrate  1  of the feeder line  10  is prepared from a material having smaller thermal conductivity than alumina employed in the prior art, whereby thermal conduction can be reduced as compared with the prior art employing alumina. Therefore, heat can be inhibited from flowing into the mounting unit  50  for the semiconductor laser  20  through the feeder line  10 .  
         [0067]    The performance of the semiconductor laser  20  is reduced as the temperature thereof is increased. According to this embodiment, however, heat is inhibited from flowing into the mounting unit  50  so that the semiconductor laser  20  is prevented from temperature increase as shown in FIG. 3, whereby the optical module can be used under a higher temperature.  
         [0068]    Second Embodiment  
         [0069]    Referring to FIG. 4, an optical module according to a second embodiment of the present invention mainly includes a feeder line  10 , a semiconductor laser  20 , a driver IC  30 , a signal input connector  40 , a mounting unit  50 , a package  60 , a thermistor  70  and a thermoelectric cooling element  80 .  
         [0070]    The semiconductor laser  20  converts an electric signal to an optical signal, and the thermistor  70  is temperature detection means for detecting the temperature around the semiconductor laser  20 . The mounting unit  50  packages the semiconductor laser  20  and the thermistor  70 , and the thermoelectric cooling element  80  heats/cools the mounting unit  50  for controlling the temperature thereof The package  60  stores these members therein, and the signal input connector  40  is connected to an electric signal input/output unit of the package  60 .  
         [0071]    The feeder line  10 , electrically connecting the semiconductor laser  20  and the electric signal input/output unit of the package  60 , is arranged in the package  60 . The driver IC  30  is electrically connected between the feeder line  10  and the electric signal input/output unit of the package  60  for amplifying the electric signal input in the semiconductor laser  20 .  
         [0072]    The feeder line  10  employed in this embodiment is a microstrip line prepared by forming conductor films  2  and  3  on a dielectric substrate  1  as shown in FIG. 2, for example. The dielectric substrate  1  is prepared from a material having thermal conductivity of not more than 3 W/m/K. Each of the conductor films  2  and  3  consists of a multilayer structure including at least two layers, i.e., a lower conductor layer actually fed with a high-frequency electric signal and a gold plating layer necessary for soldering or wire bonding, similarly to the prior art shown in FIG. 19. FIG. 4 illustrates the conductor films  2  and  3  as single layers, for convenience of illustration.  
         [0073]    According to this embodiment, the dielectric substrate  1  of the feeder line  10  is prepared from the material having thermal conductivity of not more than 3 W/m/K, so that the thermal conductivity of the dielectric substrate  1  can be remarkably reduced as compared with the prior art employing alumina. Thus, heat can be remarkably inhibited from flowing into the mounting unit  50  for the semiconductor laser  20 .  
         [0074]    Thus, heat is remarkably inhibited from flowing into the mounting unit  50  so that temperature difference between the semiconductor laser  20  and the thermistor  70  is reduced regardless of thermal resistance therebetween as shown in FIG. 5, whereby the optical module has a stable wavelength also when the ambient temperature for the semiconductor laser  20  changes.  
         [0075]    Referring to FIG. 5, the descending solid line shows temperature distribution in the case where the package  60  has a higher temperature than the semiconductor laser  20  while the ascending solid line shows temperature distribution in the case where the package  60  has a lower temperature than the semiconductor laser  20 .  
         [0076]    When the thermal conductivity of the dielectric substrate  1  of the feeder line  10  is not more than 3 W/m/K, heat transmitted through the conductor parts of the feeder line  10  is so dominant that wavelength conversion can be further suppressed by devising the constituents and the structure of the conductor parts.  
         [0077]    Third Embodiment  
         [0078]    Referring to FIG. 2, a dielectric substrate  1  of a feeder line  10  employed in an optical module according to a third embodiment of the present invention consists of glass ceramic containing silicon dioxide.  
         [0079]    The remaining structure of the third embodiment is substantially identical to that of the aforementioned first or second embodiment, and hence redundant description is not repeated.  
         [0080]    According to this embodiment, the dielectric substrate  1  consists of glass ceramic containing silicon dioxide. Therefore, the thermal conductivity as well as dielectric loss of the dielectric substrate  1  can be reduced. In particular, the thermal conductivity of the dielectric substrate  1  can be reduced below that of aluminum oxide (alumina) employed in the prior art, and can be further reduced to 3 W/m/K and below.  
         [0081]    When this feeder line  10  is employed, therefore, heat can be remarkably inhibited from flowing into a mounting unit  50  so that the optical module exhibits a stable wavelength and an excellent high-frequency characteristic also when the ambient temperature for a semiconductor laser  20  changes.  
         [0082]    Fourth Embodiment  
         [0083]    Referring to FIGS. 6 and 7, the line widths W 2  and W 3  of portions, not in contact with a package  60  and a mounting unit  50 , of conductor patterns of conductor films  2  and  3  formed on the front and rear surfaces of a dielectric substrate  1  are smaller than the width W 1  of the dielectric substrate  1  in an optical module according to a fourth embodiment of the present invention.  
         [0084]    The remaining structure of the fourth embodiment is substantially identical to that of the aforementioned first or second embodiment. Therefore, members of the fourth embodiment identical to those of the first or second embodiment are denoted by the same reference numerals, and redundant description is not repeated.  
         [0085]    When the dielectric substrate  1  is made of a material having small thermal conductivity, the conductor films  2  and  3  serve as principal heat conduction paths. According to this embodiment, the line widths W 2  and W 3  of the conductor films  2  and  3  are smaller than the width W 1  of the dielectric substrate  1 , whereby the quantities of thermal conduction are reduced in the conductor films  2  and  3 . Therefore, the quantity of heat flowing into the mounting unit  50  packaging a semiconductor laser  20  is reduced so that the optical module exhibits a stable wavelength also when the ambient temperature changes.  
         [0086]    Fifth Embodiment  
         [0087]    Referring to FIGS. 8 and 9, conductor films  2  for signal transmission are formed on only (the front) one of the front and rear surfaces of a dielectric substrate  1  consisting of a material having small thermal conductivity in a feeder line  10  of an optical module according to a fifth embodiment of the present invention. On the other hand, metal patterns  3  are formed on both ends of the rear surface of the dielectric substrate  1  for soldering the feeder line  10  to a package  60  and a mounting unit  50 .  
         [0088]    One of the conductor films  2  formed on the front surface of the dielectric substrate  1  is employed for feeding an electric signal to a semiconductor laser  20 , while a ground potential is applied to the other conductor film  2 .  
         [0089]    The remaining structure of the fifth embodiment is substantially similar to that of the aforementioned first or second embodiment. Therefore, members of the fifth embodiment identical to those of the first or second embodiment are denoted by the same reference numerals, and redundant description is not repeated.  
         [0090]    When the dielectric substrate  1  is made of a material having small thermal conductivity, the conductor films  2  for signal transmission serve as principal heat conduction paths. According to this embodiment, the conductor films  2  are formed only on the front surface of the dielectric substrate  1 , and hence the quantities of thermal conduction in the conductor films  2  are reduced. Therefore, the quantity of heat flowing into the mounting unit  50  packaging the semiconductor laser  20  is reduced so that the optical module exhibits a stable wavelength also when the ambient temperature changes.  
         [0091]    When the conductor films  2  for signal transmission are formed on the upper surface of the dielectric substrate  1  and the lower surface thereof is connected to the package  60  and the mounting unit  50  as shown in FIG. 10, a principal heat conduction path is defined through the package  60 → the dielectric substrate  1 → the conductor films  2 → the dielectric substrate  1 → the mounting unit  50 , as shown by arrow in FIG. 10. Thus, heat passes through the dielectric substrate  1  consisting of a material having small thermal conductivity twice, so that thermal conduction can be more effectively suppressed.  
         [0092]    Sixth Embodiment  
         [0093]    In a feeder line  10  according to a sixth embodiment of the present invention, conductor films  2  for signal transmission are formed only on the front surface of a dielectric substrate  1 , as described with reference to the fifth embodiment. The patterns of the conductor films  2  form a coplanar line as shown in FIG. 11 or a slot line as shown in FIG. 12.  
         [0094]    The remaining structure of the sixth embodiment is substantially similar to that of the aforementioned first or second embodiment. Therefore, members of the sixth embodiment identical to those of the first or second embodiment are denoted by the same reference numerals, and redundant description is not repeated.  
         [0095]    According to this embodiment, the conductor films  2  form a coplanar or slot feeder line, which can transmit an excellent electric signal also with respect to a high-frequency electric signal. Therefore, the optical module can exhibit a stable wavelength and an excellent high-frequency transmission characteristic due to the coplanar or slot feeder line, also when the ambient temperature changes.  
         [0096]    Seventh Embodiment  
         [0097]    [0097]FIG. 13B is a sectional view taken along the line XIII-XIII in FIG. 13A.  
         [0098]    Referring to FIGS. 13A and 13B, conductor films  2  are made of a material having smaller thermal conductivity than gold except portions coming into contact with a package  60  and a mounting unit  50  and portions for bonding wires or ribbons in a feeder line  10  of an optical module according to a seventh embodiment of the present invention. In other words, terminal portions  2   a   1  for bonding wires or ribbons shown in FIG. 13A consist of gold, and connection portions  2   a   2  connecting the terminal portions  2   a   1  located on both ends consist of a material having smaller thermal conductivity than gold. The terminal portions  2   a   1  and the connection portions  2   a   2  of the conductor films  2  are directly formed on a dielectric substrate  1 , with no lower conductor layer formed between the terminal portions  2   a   1  and the connection portions  2   a   2  and the dielectric substrate  1 .  
         [0099]    As shown in FIG. 13B, metal patterns  3  for soldering are formed only on portions coming into contact with the package  60  and the mounting unit  50  on the rear surface of the dielectric substrate  1 .  
         [0100]    The remaining structure of the seventh embodiment is substantially identical to that of the aforementioned first or second embodiment. Therefore, members of the seventh embodiment identical to those of the first or second embodiment are denoted by the same reference numerals, and redundant description is not repeated.  
         [0101]    When the dielectric substrate  1  is made of a material having small thermal conductivity, the conductor films  2  for signal transmission serve as principal heat conduction paths. According to this embodiment, the connection portions  2   a   2  of the conductor films  2  consist of the material having smaller thermal conductivity than gold, whereby the thermal conductivity of the overall conductor films  2  is reduced to reduce the quantities of heat conducted through the transmission paths. Therefore, the quantity of heat flowing into the mounting unit  50  loaded with a semiconductor laser  20  is reduced so that the optical module exhibits a stable wavelength also when the ambient temperature changes.  
         [0102]    Table 1 shows the values of thermal conductivity of nickel (Ni), platinum (Pt), palladium (Pd), tungsten (W) and molybdenum (Mo) employable as metals having smaller thermal conductivity than gold.  
                                                                         TABLE 1                                               Palla-       Molyb-           Gold   Nickel   Platinum   dium   Tungsten   denum                                    Thermal   319   94   72   72   177   139       Conductivity           (W/m/K)           Ratio of   —   29.5   22.6   22.6   55.5   43.6       Thermal           Conductivity           to           Gold (%)                  
 
         [0103]    When the dielectric substrate  1  is made of a material having small thermal conductivity, the conductor films  2  are dominant as heat inflow paths. When neglecting thermal conduction in the dielectric substrate  1 , therefore, the quantity of heat flowing into the mounting unit  50  is halved as compared with the case of employing gold also when employing tungsten having the largest thermal conductivity in Table 1. When employing platinum or palladium having the smallest thermal conductivity, the quantity of heat flowing into the mounting unit  50  is suppressed to ¼ to ⅕ as compared with the case of employing gold.  
         [0104]    Eighth Embodiment  
         [0105]    [0105]FIG. 14B is a sectional view taken along the line XIV-XIV in FIG. 14A.  
         [0106]    Referring to FIGS. 14A and 14B, conductor films  2  of a feeder line  10  of an optical module according to an eighth embodiment of the present invention include conductor films  2   a   1  and  2   b  of different materials. The conductor films  2   b  consist of a metal at least containing any of nickel, platinum, palladium, tungsten, molybdenum and copper individually or in arbitrary combination. The conductor films  2   a   1 , consisting of gold, are formed on the aforementioned conductor films  2   b  only on terminal parts for bonding wires or ribbons.  
         [0107]    On the rear surface of the dielectric substrate  1 , conductor films  3  of gold are formed on portions (both ends) coming into contact with a package  60  and a mounting unit  50 .  
         [0108]    The remaining structure of the eighth embodiment is substantially identical to that of the aforementioned first or second embodiment. Therefore, members of the eighth embodiment identical to those of the first or second embodiment are denoted by the same reference numerals, and redundant description is not repeated,  
         [0109]    As shown in FIG. 19, the conductor films  102  and  103  of the conventional feeder line  110  consist of the multilayer structure including the lower conductor layers  102   b  and  103   b  of nickel, platinum, palladium, tungsten, molybdenum or copper and the gold plating layers  102   a  and  103   a  for soldering or wire bonding, in consideration of adhesion with the dielectric substrate  101  or the like. The lower conductor layers  102   b  and  103   b  of 2 to 3 μm contribute to conduction of the electric signal in practice, and the gold plating conductor layers  102   a  and  103   a  do not much contribute to such conduction.  
         [0110]    When the gold plating conductor films  2   a   1  and  3  are formed only on portions requiring soldering or bonding of wires or ribbons as in this embodiment, therefore, no unnecessary gold plating is performed on portions requiring no gold plating to disadvantageously improve thermal conduction but the quantity of thermal conduction in the feeder line  10  is reduced. Therefore, the quantity of heat flowing into the mounting unit  50  packaging a semiconductor laser  20  is reduced so that the optical module exhibits a stable wavelength also when the ambient temperature changes. When gold plating is performed from above the lower conductor films  2   b , the optical module can be extremely readily manufactured as compared with that according to the seventh embodiment.  
         [0111]    Table 2 shows the values of thermal conductivity of lower conductor films made of nickel, platinum, palladium, tungsten, molybdenum and copper with gold plating performed on overall surfaces and only on necessary portions. Referring to Table 2, the lower conductor films and the gold plating layers are identical in thickness to each other.  
                                                                         TABLE 2                                           Palla-       Molyb-               Nickel   Platinum   dium   Tungsten   denum   Copper                                    Entirely   413   391   391   496   458   722       Plated with           Gold           (W/m/K)           Partially   94   72   72   177   139   403       Plated with           Gold           (W/m/K)           Ratio of   22.8   18.4   18.4   35.7   30.3   55.8       Thermal           Conductivity           (%)                  
 
         [0112]    When the dielectric substrate  1  is made of a material having small thermal conductivity, the conductor films  2  and  3  are dominant as heat inflow paths. When neglecting thermal conduction in the dielectric substrate  1 , therefore, the quantity of heat flowing into the mounting unit  50  is substantially halved in the case of employing copper having the largest thermal conduction in Table 2 and performing gold plating only on necessary portions, as compared with the case of performing gold plating on overall surfaces. When platinum or palladium having the smallest thermal conductivity is employed, the quantity of heat flowing into the mounting unit  50  is suppressed to not more than ⅕ in the case of performing gold plating only on necessary portions, as compared with the case of performing gold plating on overall surfaces.  
         [0113]    Ninth Embodiment  
         [0114]    Referring to FIG. 15, an electric circuit mounting unit  90 A and a lead mounting substrate  90 B are mounted on a package  60  in an optical module according to a ninth embodiment of the present invention.  
         [0115]    A driver IC  30  is mounted on the electric circuit mounting unit  90 A, while wires  90   a  to be electrically connected to the driver IC  30  are formed on the electric circuit mounting unit  90 A. Conductor patterns  90   b  to be electrically connected with leads  90 C are formed on the lead mounting substrate  90 B. The wires  90   a  and the conductor patterns  90   b  are directly electrically connected with each other by wire bonding, for example, without through a mounting unit  50 .  
         [0116]    The remaining structure of the ninth embodiment is substantially identical to that of the aforementioned first or second embodiment. Therefore, members of the ninth embodiment identical to those of the first or second embodiment are denoted by the same reference numerals, and redundant description is not repeated.  
         [0117]    An electric signal input from an electric signal input connector  40  of the package  60  is amplified in the driver IC  30  and input in a semiconductor laser  20 . The wires  90   a  of the driver IC  30  are coupled with the leads  90 C without through the mounting unit  50 , and hence no heat flows into the mounting unit  50  from the wires  90   a  of the driver IC  30 . Therefore, no temperature difference is caused between the semiconductor laser  20  and a thermistor  70  due to heat flowing through the wires  90   a , and the wavelength of the optical module is stable also when the ambient temperature changes.  
         [0118]    Tenth Embodiment  
         [0119]    Referring to FIG. 16, the structure of an optical module according to a tenth embodiment of the present invention is different from that of the ninth embodiment in a point that a lead mounting substrate  90 B has a lead mounting substrate extension part  90 B 1  partially extending toward an electric circuit mounting unit  90 A. Respective conductor patterns  90   b  formed on the lead mounting substrate  90 B extend toward an end of the lead mounting substrate extension part  90 B 1 , to be electrically connected with wires  90   a  on positions closest to the wires  90   a.    
         [0120]    The remaining structure of the tenth embodiment is substantially identical to that of the ninth embodiment. Therefore, members of the tenth embodiment identical to those of the ninth embodiment are denoted by the same reference numerals, and redundant description is not repeated.  
         [0121]    According to this embodiment, the distance between the wires  90   a  and the conductor patterns  90   b  is reduced due to the extension part  90 B 1 . Therefore, the distance for connecting the wires  90   a  and the conductor patterns  90   b  by wire bonding can be reduced so that the wires  90   a  and the conductor patterns  90   b  can be coupled with each other without requiring a specific member. Thus, temperature difference between a semiconductor laser and a thermistor caused by heat flowing through the wires  90   a  can be readily eliminated so that the optical module exhibits a stable wavelength also when the ambient temperature changes.  
         [0122]    While the semiconductor laser  20  is employed as an optical device in each of the first to tenth embodiments, the present invention is not restricted to this but any optical device is applicable to the present invention so far as the same can convert an electric signal to an optical signal.  
         [0123]    While the material for the dielectric substrate  1  is prepared from glass ceramic containing silicon dioxide in the third embodiment, the present invention is not restricted to this but any material is applicable to the present invention so far as the same has smaller thermal conductivity than aluminum oxide.  
         [0124]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.