Abstract:
A low voltage modulation signal can be realized while reducing reflection and radiation of a high frequency modulation signal in an optical modulator. The optical modulator includes an electrooptic effect element having a signal electrode and a ground electrode thereupon each provided with a pad, and varying optical phase by the electrooptic effect and; a relay substrate constituted of a dielectric wafer on which a coplanar waveguide connected to the signal electrode pad and the ground electrode pad on the electrooptic effect element is formed; and a connector having a center conductor and an external conductor respectively connected to the coplanar waveguide of the relay substrate, and supplying a modulation signal of microwave band to the signal electrode of the electrooptic effect element, wherein, when the modulation signal includes a component of 30 GHz and a pad space between the signal electrode pad and the ground electrode pad of the electrooptic effect element is defined as S [μm] and a pad height is defined as H [μm], the pad height is no greater than 300 and the relation is set as 
     −0.002H 2 +1.3H−160&lt; S &lt;0.0025H 2 −1.6H+550

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an optical modulator in which an element for varying optical phase by the electrooptic effect is mounted.  
         BACKGROUND OF THE INVENTION  
         [0002]    An optical communication system is used as a large capacity broadband communication system. In such an optical communication system, higher bit rate is required in transmission as demand for larger communication capacity increases.  
           [0003]    Meanwhile, in the optical communication system, there is employed an optical modulator having an element, which varies optical phase by the electrooptic effect changing a refractive index when electric field is applied on a ferroelectric crystal, etc.  
           [0004]    Such an element varying the optical phase by the electrooptic effect (hereinafter the element is simply referred to as electrooptic effect element) to be mounted on the optical modulator is provided with an optical waveguide formed on a wafer cut out of an electrooptic crystal of LiNbO 3 , LiTaO 2 , etc. with a metallic film of Ti, etc. produced thereon through patterning and thermal diffusion or proton exchange in benzoic acid by the IC production technique. Further a required electrode is formed in the vicinity of the optical waveguide.  
           [0005]    The optical modulator has such a configuration that an optical signal is supplied from outside the electrooptic effect element to the optical waveguide so as to supply a high frequency modulation signal of a microwave band to an electrode formed in the vicinity of the optical waveguide.  
           [0006]    [0006]FIG. 1 is a top plan view of one configuration example of the optical modulator with a cover removed. An electrooptic effect element  2  is housed in a shielding case  1 . FIGS. 2A through 2C are schematic configuration diagrams of electrooptic effect element  2 .  
           [0007]    To function as an optical modulator, an exemplary optical waveguide  10  formed on electrooptic effect element  2  is made to branch into two parallel waveguides to configure a Mach-Zehnder waveguide. FIG. 2B is a cross-sectional view along line ‘a’ in the plan view shown in FIG. 2A. Also FIG. 2C is a cross-sectional view along line ‘b’.  
           [0008]    As an example, when using a Z-cut wafer for electrooptic effect element  2  cut out from an LiNbO 3 crystal in the Z-axis direction, constituting an electrode of a single electrode, and applying a modulation scheme of the intensity modulation, a signal electrode  20  is disposed right on top of either one of the parallel branch waveguides, while a ground electrode  22  is disposed right on top of the other branch waveguide. Further, a buffer layer constituted of SiO 2 , etc. is provided between the substrate and signal electrode  20  and between the substrate and ground electrode  22 , so as to prevent the optical signal traveling in the two parallel waveguides from being absorbed by signal electrode  20  and ground electrode  22 .  
           [0009]    In FIG. 2A, an optical signal is input to an incident side (Opt In) of waveguide  10 . To function as an optical modulator, a rectangular microwave signal output from a signal source  25  is supplied to signal electrode  20  as a modulation signal in the same direction as the traveling direction of the optical signal. Accordingly, the refractive indexes of each parallel optical waveguide branching into two are varied in the mutually opposite directions, producing variation of optical phase difference in the parallel optical waveguides. An intensity modulated optical signal is then output from an output side (Opt Out) of optical waveguide  10  in FIG. 2A.  
           [0010]    Here, in the configuration of the optical modulator shown in FIG. 1, the high frequency microwave signal supplied from signal source  25  as a modulation signal is supplied to between signal electrode  20  and ground electrodes  21 ,  22  through an RF connector  3  having a center conductor  30  and an external conductor  31 .  
           [0011]    Center conductor  30  of RF connector  3  is inserted into a sliding contact member  32  and is connected between signal electrode  20  of electrooptic effect element  2  and sliding contact member  32  with bonding. Also external conductor  31  of RF connector  3  is connected to ground electrodes  21 ,  22  of electrooptic effect element  2  with wire bonding  23 .  
           [0012]    Further, in the case the wavelength of high frequency signal is long as compared to the size of the electrodes in electrooptic effect element  2 , the characteristics of electrooptic effect element  2  is not substantially affected. However, when the wavelength becomes shorter, this affects the high frequency characteristic of electrooptic effect element  2 , resulting in producing radiation and reflection on the high frequency signal. As a result, it becomes difficult to obtain wideband transmission characteristic in electrooptic effect element  2 . Further, the sizes of sliding contact member  32  and center conductor  30  of RF connector  3  are minute on the order of several tens of μm and therefore it is very hard to assemble.  
           [0013]    To solve the aforementioned problem, the inventors of the present invention have been studying a method of connecting RF connector  3  and electrooptic effect element  2  via a relay substrate. In this case, it is to be considered that each of the own characteristic impedance of RF connector  3 , the relay substrate and electrooptic effect element  2  is designed as 50Ω.  
           [0014]    In particular, in the case wire bonding is used to connect between electrooptic effect element  2  and the relay substrate, similar to the case of connecting RF connector  3  to electrooptic effect element  2  shown in FIG. 1, it is to be considered to maintain the characteristic impedance as 50Ω so as to prevent microwave reflection, thus to broaden the gap between the wire bonding area (hereafter referred to as pad) of signal electrode  20  and ground electrodes  21 ,  22 .  
           [0015]    However, if the aforementioned gap between the pad of signal electrode  20  and the pads of ground electrodes  21 , 22  is broadened, there arises a problem that the electric field becomes broaden and the radiating microwave component becomes increased. This produces deterioration of transmission property. Moreover, when the frequency in use becomes higher, the degree of radiation becomes greater.  
           [0016]    Meanwhile, if the pad width W of signal electrode  20  is set narrower, there arises another problem of difficulty in connecting to the relay substrate with bonding.  
         SUMMARY OF THE INVENTION  
         [0017]    Accordingly, it is an object of the present invention to provide an optical modulator mounting an element which varies optical phase by the electrooptic effect, having a feature of refraining the increase of radiation ratio even when microwave frequency in use becomes higher, as well as a feature of easily connecting to the relay substrate with bonding.  
           [0018]    As a first embodiment of the present invention to attain the aforementioned object, an optical modulator includes; an electrooptic effect element having a signal electrode and a ground electrode thereupon each provided with a pad and varying optical phase by the electrooptic effect and; a relay substrate constituted of a dielectric wafer on which a coplanar waveguide connected to the signal electrode pad and the ground electrode pad on the electrooptic effect element is formed; and a connector having a center conductor and an external conductor respectively connected to the coplanar waveguide of the relay substrate, and supplying a modulation signal of microwave band to the signal electrode of the electrooptic effect element, wherein when the modulation signal includes a component of 30 GHz and a pad space between the signal electrode pad and the ground electrode pad of the electrooptic effect element is defined as S [μm] and a pad height is defined as H [μm], the pad height is no greater than 300 and the relation is set as 
           −0.002H 2 +1.3H−160&lt; S&lt; 0.0025H 2 −1.6H+550 
           [0019]    As a second embodiment of the present invention, an optical modulator includes; an electrooptic effect element having a signal electrode and a ground electrode thereupon each provided with a pad, and varying optical phase by the electrooptic effect and; a relay substrate constituted of a dielectric wafer on which a coplanar waveguide connected to the signal electrode pad and the ground electrode pad on the electrooptic effect element is formed; and a connector having a center conductor and an external conductor respectively connected to the coplanar waveguide of the relay substrate, and supplying a modulation signal of microwave band to the signal electrode of the electrooptic effect element, wherein when the modulation signal includes a component of 40 GHz and a pad space between the signal electrode pad and the ground electrode pad of the electrooptic effect element is defined as S [μm] and a pad height is defined as H [μm], the pad height is no greater than 300 and the relation is set as 
           −0.002H 2 +1.3H−160&lt; S&lt; 0.001H 2 −0.8H+370 
           [0020]    As a third embodiment of the present invention, an optical modulator includes; an electrooptic effect element having a signal electrode and a ground electrode thereupon each provided with a pad, and varying optical phase by the electrooptic effect and; a relay substrate constituted of a dielectric wafer on which a coplanar waveguide connected to the signal electrode pad and the ground electrode pad on the electrooptic effect element is formed; and a connector having a center conductor and an external conductor respectively connected to the coplanar waveguide of the relay substrate, and supplying a modulation signal of microwave band to the signal electrode of the electrooptic effect element, wherein when the modulation signal includes a component of 30 GHz and a characteristic impedance by the signal electrode pad and the ground electrode pad of the electrooptic effect element is defined as Z 0  [Ω] and a pad height is defined as H [μm], the pad height is no greater than 300 and the relation is set as 
           −0.0005H 2 +0.32H−19&lt; Z   0 &lt;0.00061H 2 −0.34H+98 
           [0021]    As a fourth embodiment of the present invention, an optical modulator includes; an electrooptic effect element having a signal electrode and a ground electrode thereupon each provided with a pad, and varying optical phase by the electrooptic effect and; a relay substrate constituted of a dielectric wafer on which a coplanar waveguide connected to the signal electrode pad and the ground electrode pad on the electrooptic effect element is formed; and a connector having a center conductor and an external conductor respectively connected to the coplanar waveguide of the relay substrate, and supplying a modulation signal of microwave band to the signal electrode of the electrooptic effect element, wherein when the modulation signal includes a component of 40 GHz and a characteristic impedance by the signal electrode pad and the ground electrode pad of the electrooptic effect element is defined as Z 0  [Ω] and a pad height is defined as H [μm], the pad height is no greater than 300 and the relation is set as 
           −0.0005H 2 +0.32H−19&lt; Z   0 &lt;0.000093H 2 −0.061H+57 
           [0022]    As a fifth embodiment of the present invention, in the first embodiment or the second embodiment, the pad space between the signal electrode pad and the ground electrode pad is set as 130 [μm].  
           [0023]    As a sixth embodiment of the present invention, in the third embodiment or the fourth embodiment, the characteristic impedance by the signal electrode pad and the ground electrode pad of the electrooptic effect element is set as 42 [Ω].  
           [0024]    As a seventh embodiment of the present invention, in either of the first embodiment to the sixth embodiment, a width of the signal electrode pad of the electrooptic effect element is between 30 μm and 70 μm.  
           [0025]    As an eighth embodiment of the present invention, in either of the first embodiment to the sixth embodiment, widths of the signal electrode pad and the ground electrode pad of the electrooptic effect element are 50 ìm.  
           [0026]    As a ninth embodiment of the present invention, in either of the first embodiment to the sixth embodiment, each characteristic impedance of the relay substrate and the RF connector is set as 50Ω.  
           [0027]    Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 shows a top plan view of an exemplary configuration of an optical modulator with the cover thereof removed.  
         [0029]    [0029]FIGS. 2A through 2C show schematic diagrams of electrooptic effect element  2 .  
         [0030]    [0030]FIG. 3 shows an exemplary configuration of the optical modulator studied under the concept by the inventors of the present invention.  
         [0031]    [0031]FIGS. 4A and 4B show diagrams each illustrating electrode pattern formed on the electrooptic effect element.  
         [0032]    [0032]FIG. 5 shows an embodiment of electrooptic effect element  2 , an element varying optical phase by electrooptic effect, provided in an optical modulator as an object of the present invention.  
         [0033]    [0033]FIGS. 6A and 6B show charts illustrating the modulation signal loss caused by reflection at the modulation frequency of 30 GHz.  
         [0034]    [0034]FIGS. 7A and 7B show charts illustrating the modulation signal loss caused by radiation at the modulation frequency of 30 GHz.  
         [0035]    [0035]FIGS. 8A and 8B show charts illustrating the modulation signal loss caused by reflection and radiation at the modulation frequency of 30 GHz.  
         [0036]    [0036]FIGS. 9A and 9B show charts illustrating the modulation signal loss caused by reflection at the modulation frequency of 40 GHz.  
         [0037]    [0037]FIGS. 10A and 10B show charts illustrating the modulation signal loss caused by radiation at the modulation frequency of 40 GHz.  
         [0038]    [0038]FIGS. 11A and 11B show charts illustrating the modulation signal loss caused by reflection and radiation at the modulation frequency of 40 GHz.  
         [0039]    [0039]FIG. 12 shows a chart illustrating the tolerable upper limit of a pad space S as well as the tolerable lower limit thereof when varying a pad height H at the modulation frequency of 30 GHz.  
         [0040]    [0040]FIG. 13 shows a chart illustrating the tolerable upper limit of the characteristic impedance Z 0  as well as the tolerable lower limit thereof when varying pad height H at the modulation frequency of 30 GHz.  
         [0041]    [0041]FIG. 14 shows a chart illustrating the tolerable upper limit of a pad space S as well as the tolerable lower limit thereof when varying a pad height H at the modulation frequency of 40 GHz.  
         [0042]    [0042]FIG. 15 shows a chart illustrating the tolerable upper limit of the characteristic impedance Z 0  as well as the tolerable lower limit thereof when varying pad height H at the modulation frequency of 40 GHz. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    The preferred embodiments of the present invention are described hereinafter referring to the charts and drawings. Prior to the description of the embodiments, for the sake of easy understanding of the present invention, there is explained a detailed configuration for connecting RF connector  3  and electrooptic effect element  2  using a relay substrate which has been studied by the inventors of the present invention.  
         [0044]    [0044]FIG. 3 is a diagram illustrating a configuration example to connect RF connector  3  to electrooptic effect element  2  using a relay substrate which has been studied by the inventors of the present invention.  
         [0045]    In FIG. 3, relay substrate  40  has signal electrode  400  connected to signal electrode  20  and ground electrodes  21 ,  22  of electrooptic effect element  2 , and a coplanar waveguide having ground electrodes  401 ,  402  each disposed on either side of signal electrode  400  on a dielectric wafer such as AL 2 O 3 .  
         [0046]    Electrooptic effect element  2  is connected to relay substrate  40  by the connection between each pad  200 ,  210 ,  220  of electrooptic effect element  2  and each corresponding electrode  400 ,  401 ,  402  of the waveguide with wire bonding  23 .  
         [0047]    Here, in FIG. 3, though the connection between relay substrate  40  and RF connector  3  is shown for the input side only, the output side is also connected to a non-illustrated RF connector via relay substrate  40 .  
         [0048]    Also, pad  200  of signal electrode  20  has electrode width W so as to connect to signal electrode  400  on the coplanar waveguide of relay substrate  40 , as shown in FIGS. 4A, 4B in which only electrode patterns are illustrated.  
         [0049]    Meanwhile, a non-illustrated RF cable for connecting signal source  25  and RF connector  3  generally has a characteristic impedance of 50Ω. Accordingly, as for RF connector  3  and relay substrate  40 , each of the own characteristic impedance is designed to be 50Ω.  
         [0050]    Therefore, as for pads  200 ,  210  and  220  in electrooptic effect element  2 , it is considered necessary to maintain each characteristic impedance as 50Ω so as to prevent microwave reflection. One method to satisfy the above is to broaden a gap S between pad  200  of signal electrode  20  and pads  210 ,  220  of ground electrodes  21 ,  22 .  
         [0051]    However, as mentioned earlier, if gap S between pad  200  of signal electrode  20  and pads  210 ,  220  of ground electrodes  21 ,  22  is broadened, such a problem arises that the electric field is broadened and accordingly the radiated microwave component increases and the transmission property is deteriorated. If the microwave frequency of use becomes higher, the degree of the radiation becomes greater.  
         [0052]    Meanwhile, if the width W of pad  200  is made narrower, there arises a problem that the connection between relay substrate  40  and pad  200  with bonding becomes difficult.  
         [0053]    Taking this into consideration, the inventors of the present invention have studied to find that an optimal condition for transmission property which enables to reduce the microwave reflection and radiation in the input/output portions of electrooptic effect element  2 , i.e. pads  200 ,  210  and  220  even when the characteristic impedance deviates as a result of broadening the size of gap S between the aforementioned pad  200  of signal electrode  20  and pads  210 ,  220  of ground electrodes  21 ,  22 .  
         [0054]    According to the present invention, there is provided an optical modulator which mounts an element varying optical phase by the electrooptic effect with this newly found optimal condition for transmission property.  
         [0055]    [0055]FIG. 5 is an embodiment of electrooptic effect element  2  which varies optical phase by the electrooptic effect in the optical modulator of the present invention.  
         [0056]    Signal electrode  20  and ground electrodes  21 ,  22  having mutually functioning electrode length L with a similar electrode pattern shown in FIGS. 4A, 4B are formed by patterning on a LiNbO 3  substrate  40  having 2 mm in width, 1 mm in height and 50 mm in length.  
         [0057]    Here, desirably the voltage of the high frequency signal is set lower because it is difficult to produce high power of a drive circuit located at the last stage of signal source  25  for producing high frequency signals. However, in the case the loss of high frequency signals is large, a limit exists to make low voltage of the high frequency signals. From this viewpoint, it is important to reduce the loss of high frequency signals.  
         [0058]    Now, considering loss of high frequency signal as a modulation signal, the loss is produced in input/output RF connector  3  and relay substrate  40 , in input portion of electrooptic effect element  2 , i.e. pads  200 ,  210  and  220 , and by mutually functioning length L of electrodes  20 ,  21  and  22 .  
         [0059]    Here, the loss produced in RF connector  3  and relay substrate  40  is on the order of 0.5 dB at the input side and the output side of RF connector  3 , respectively, when the characteristic impedance is set to 50Ω. Also, the loss caused by mutually functioning electrode length L is fixedly determined by the lengths of electrodes  20 ,  21  and  22 .  
         [0060]    Meanwhile, the loss produced in the input portion of electrooptic effect element  2 , i.e. pads  200 ,  210  and  220 , is caused by reflection and radiation. It has been considered essential that the impedance of these pads are to be fit to the characteristic impedance of RF connector  3  and relay substrate  40 , that is 50Ω.  
         [0061]    However, in regard to the loss produced in pads  200 ,  210  and  220 , the inventors of the present invention have found after studying various cases that there exists an optimal condition rather than fitting to the characteristic impedance, 50Ω, of RF connector  3  and relay substrate  40 .  
         [0062]    This condition is studied in the following embodiments.  
         [0063]    [First Embodiment] 
         [0064]    In the electrode pattern shown in FIGS. 4A, 4B, width W of pad  200  of signal electrode  20  is assumed to be 50 μm. Also the modulation signal frequency is assumed to be 30 GHz.  
         [0065]    Loss S 21  was calculated, which was caused by reflection by measuring S 21  when space S was varied between pad  200  and pads  210 ,  220  with height H of pad  200  varied to 300 μm, 200 μm and 100 μm, respectively. Loss S 21  was also calculated caused by reflection when varying the characteristic impedance.  
         [0066]    Here, loss S 21  is a value obtained from the ratio of an input signal level and an output signal level.  
         [0067]    [0067]FIGS. 6A and 6B show the results of the aforementioned measurement. FIG. 6A is a chart representing loss by reflection when pad space S is varied, and FIG. 6B represents loss by reflection when varying the characteristic impedance.  
         [0068]    Meanwhile, using the same condition as the above, loss caused by radiation is measured, the results of which are shown in FIGS. 7A and 7B. The chart shown in FIG. 7A represents loss by radiation when varying pad space S, and the chart shown in FIG. 7B represents loss by radiation when varying the characteristic impedance.  
         [0069]    Further, FIGS. 8A and 8B are charts in which the loss by reflection shown in FIGS. 6A, 6B is added to the loss by radiation shown in FIGS. 7A, 7B, respectively. FIG. 8A represents loss when varying pad space S and FIG. 8B represents loss when varying the characteristic impedance.  
         [0070]    It is understood from FIG. 8A that the loss caused by reflection and radiation becomes minimum when pad space S is set as 130 μm, irrespective of the height H of pads  200 ,  210  and  220  set as 300 μm, 200 μm or 100 μm. Also, from FIG. 8B, it is shown that the loss becomes minimum when the characteristic impedance is set as 40Ω.  
         [0071]    As mentioned earlier, the loss produced in relay substrate  40  and the input side of RF connector  3  is approximately 0.5 dB, as well as the loss produced in relay substrate  40  and the output side of RF connector  3 .  
         [0072]    Therefore, considering the loss allowed in the input and output portion of electrooptic effect element  2  is up to the equivalent degree, the upper limit and the lower limit in the tolerable range of pad space S when loss S 21  is no greater than 0.5 dB varies with pad height H in FIG. 8A.  
         [0073]    [0073]FIG. 12 is a chart illustrating upper limit A and lower limit B of pad space S when varying pad height H obtained from the charts shown in FIGS. 8A, 8B.  
         [0074]    Accordingly, when the modulation signal has a component of 30 GHz, the tolerable range of pad space S has the relation shown below: height H is less than 300 μm, and 
         −0.002H 2 +1.3H−160&lt; S&lt; 0.0025H 2 −1.6H+550 
         [0075]    where the space between the signal electrode pad and the ground electrode pad of the electrooptic effect element is defined as S [μm], and the pad height is defined as H [μm].  
         [0076]    From this relation, for example when H=300 μm, the tolerable size of S becomes between 50 μm and 280 μm.  
         [0077]    As for the characteristic impedance, FIG. 8B indicates that the value which minimizes the loss is 41Ω. In a similar manner to the above, when the loss of no greater than approximately 0.5 dB is allowed, which is equivalent to the loss produced in the input side and the output side of RF connector  3  and relay substrate  40 , the tolerable characteristic impedance Z 0  [Ω] is within the range of upper limit A and lower limit B shown in FIG. 13, which satisfies the relation shown below: 
         −0.0005H 2 +0.32H−19&lt; Z   0 &lt;0.00061H 2 −0.34H+98 
         [0078]    where Z 0  is the characteristic impedance.  
         [0079]    [Second Embodiment] 
         [0080]    In a similar manner to the first embodiment, width W of pad  200  of signal electrode  20  is assumed to be 50 μm. However, in this second embodiment, the modulation signal frequency is assumed to be 40 GHz.  
         [0081]    Loss S 21  was calculated, which was caused by reflection by measuring S 21  when space S was varied between pad  200  and pads  210 ,  220  with height H of pad  200  varied to 300 μm, 200 μm and 100 μm, respectively. Loss S 21  was also calculated caused by reflection when varying the characteristic impedance.  
         [0082]    [0082]FIGS. 9A and 9B are the results of the aforementioned measurement, where FIG. 9A is a chart representing loss by reflection when pad space S is varied, and FIG. 9B represents loss by reflection when varying the characteristic impedance.  
         [0083]    Meanwhile, using the same condition as the above, loss caused by radiation is measured, the results of which are shown in FIGS. 10A and 10B. The chart shown in FIG. 10A represents loss by radiation when varying pad space S, and the chart shown in FIG. 10B represents loss by radiation when varying the characteristic impedance.  
         [0084]    Further, FIGS. 11A and 11B are charts in which the loss by reflection shown in FIGS. 9A, 9B is added to the loss by radiation shown in FIGS. 10A, 10B, respectively. FIG. 11A represents loss when varying pad space S and FIG. 11B represents loss when varying the characteristic impedance.  
         [0085]    It is understood from FIG. 11A that the loss caused by reflection and radiation becomes minimum when pad space S is set as 120 μm. Also, from FIG. 11B, it is shown that the loss becomes minimum when the characteristic impedance is set as 40Ω.  
         [0086]    As mentioned earlier, the loss produced in relay substrate  40  and the input side of RF connector  3  is approximately 0.5 dB, as well as the loss produced in relay substrate  40  and the output side of RF connector  3 .  
         [0087]    Therefore, considering the loss allowed in the input and output portion of electrooptic effect element  2  is up to the equivalent degree, the upper limit and the lower limit in the tolerable range of pad space S when loss S 21  is no greater than 0.5 dB varies with pad height H in FIG. 11A.  
         [0088]    [0088]FIG. 14 is a chart illustrating upper limit A and lower limit B of pad space S when varying pad height H obtained from the charts shown in FIGS. 11A, 11B.  
         [0089]    Accordingly, when the modulation signal has a component of 40 GHz, height H is less than 300 μm and the tolerable range of pad space S has the relation shown below: 
         −0.002H 2 +1.3H−160&lt; S&lt; 0.001H 2 −0.8H+370 
         [0090]    where the space between the signal electrode pad and the ground electrode pad of the electrooptic effect element is defined as S [μm], and the pad height is defined as H [μm].  
         [0091]    From this relation, for example when H=300 μm, the tolerable size of S becomes between 50 μm and 220 μm.  
         [0092]    As for the characteristic impedance, FIG. 11B indicates that the value which minimizes the loss is 42Ω. In a similar manner to the above, when the loss of no greater than approximately 0.5 dB is allowed, which is equivalent to the loss produced in the input side and the output side of RF connector  3  and relay substrate  40 , a tolerable characteristic impedance Z 0  [Ω] is within the range of upper limit A and lower limit B shown in FIG. 15, which satisfies the relation shown below: 
         −0.0005H 2 +0.32H−19&lt; Z   0 &lt;0.000093H 2 −0.061H+57 
         [0093]    where Z 0  is the characteristic impedance.  
         [0094]    From the measured results of the aforementioned embodiments, the loss caused by radiation and reflection in the input portion of electrooptic effect element  2  neither depends on the modulation frequency nor pad height H. The loss becomes minimum either when pad space S is 130 μm or when the characteristic impedance is 42Ω.  
         [0095]    In the aforementioned embodiments, the width of pad  200  of signal electrode  20  is assumed to be 50 μm. The tolerable range which hardly affects the characteristic impedance is the range between 30 μm and 70 μm.  
         [0096]    In the above explanation of the embodiments, electrooptic effect element  2  in the optical modulator is assumed to configure with a Z-cut dielectric wafer and a single electrode, and intensity modulation is employed as a modulation scheme. The application of the present invention is not limited to the above structure.  
         [0097]    Namely, other optical modulator maybe configured with electrode of dual electrode type in the case of a Z-cut dielectric wafer and the intensity modulation scheme. It is also possible to introduce the phase modulation or the polarization modulation as the modulation scheme. Further, in the case the dielectric wafer is formed of X-cut, similar combinations to the above in respect of the electrode and the modulation scheme may also be applied.  
         [0098]    The embodiments of the present invention have been described in accordance with the accompanied drawings. By applying the optimal condition on the sizes of pads  200 ,  210  and  220  provided in the input portion of electrooptic effect element  2 , a low voltage modulation signal can be realized while reducing reflection and radiation of a high frequency modulation signal.  
         [0099]    The foregoing description of the embodiments is not intended to limit the invention to the particular details of the examples illustrated. Any suitable modification and equivalents may be resorted to the scope of the invention. All features and advantages of the invention which fall within the scope of the invention are covered by the appended claims.