The present invention relates to an optical modulator in which an element for varying optical phase by the electrooptic effect is mounted. 
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. 
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. 
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 LiNbO3, LiTaO2, 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. 
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. 
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. 
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 xe2x80x98axe2x80x99 in the plan view shown in FIG.  2A. Also FIG.  2C is a cross-sectional view along line xe2x80x98bxe2x80x99. 
As an example, when using a Z-cut wafer for electrooptic effect element 2 cut out from an LiNbO3 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 SiO2, 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. 
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.
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. 
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, 32 of electrooptic effect element 2 with wire bonding 23. 
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 xcexcm and therefore it is very hard to assemble. 
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 50 xcexa9. 
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 50 xcexa9 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. 
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. 
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. 
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. 
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 xcexcm and a pad height is defined as H xcexcm, the pad height is no greater than 300 and the relation is set as
xe2x88x920.002H2+1.3Hxe2x88x92160 less than S less than 0.0025H2xe2x88x921.6H+550
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 xcexcm and a pad height is defined as H xcexcm, the pad height is no greater than 300 and the relation is set as
xe2x88x920.002H2+1.3Hxe2x88x92160 less than S less than 0.001H2xe2x88x920.8H+370
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 Z0 xcexa9 and a pad height is defined as H xcexcm, the pad height is no greater than 300 and the relation is set as
xe2x88x920.0005H2+0.32Hxe2x88x9219 less than Z0 less than 0.00061H2xe2x88x920.34H+98
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 Z0 [xcexa9] and a pad height is defined as H [xcexcm], the pad height is no greater than 300 and the relation is set as
xe2x88x920.0005H2+0.32Hxe2x88x9219 less than Z0 less than 0.000093H2xe2x88x920.061H+57
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 xcexcm. 
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 xcexa9. 
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 xcexcm and 70 xcexcm. 
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 lm. 
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 50 xcexa9. 
Further scopes and features of the present invention will become more apparent by the following description of the embodiments with the accompanied drawings.