Waveguide type optical device

In a waveguide type optical device which has an optical waveguide and modulation electrodes for varying its refractive index, both formed in the top of a substrate and a ferroelectric crystal having a pyroelectric effect, the top of the crystal substrate being parallel to the direction of its spontaneous polarization, conductive films are formed in two surfaces of the crystal substrate which cross the direction of the spontaneous polarization. The conductive films are electrically interconnected to thereby prevent a change in the operating temperature characteristic of the optical device which is caused by the pyroelectric effect of the crystal substrate.

BACKGROUND OF THE INVENTION 
The present invention relates to a waveguide type optical device which has 
an optical waveguide and electrodes formed in a substrate of a 
ferroelectric crystal which has a pyroelectric effect. 
Waveguide type optical devices utilizing an electrooptic effect are a phase 
modulator, an intensity modulator, an optical switch and so forth. In the 
case of using, for an optical device, a crystal as of lithium niobate 
(LiNbO.sub.3) which has a pyroelectric effect, i.e. an effect that 
spontaneous polarization varies with temperature change, however, the 
operation of the optical device becomes unstable due to unnecessary 
electric fields resulting from the generation of electric charges in the 
crystal surfaces which cross the direction of spontaneous polarization. 
FIG. 1 is a diagrammatic showing of a conventional waveguide type optical 
device 10 having its substrate 11 formed of lithium niobate crystal, for 
explaining its unstable operation due to a temperature change. The crystal 
substrate 11 has X-surfaces X1, X2, Y-surfaces Y1, Y2 and Z-surfaces Z1, 
Z2 perpendicular to the X, Y and Z axes, respectively, and the direction 
of spontaneous polarization P of the crystal is assumed to be the Z-axis 
direction. The X-surfaces X1 and X2 in the drawings will be referred to 
also as the top and bottom surfaces, respectively. An optical waveguide 12 
and modulation electrodes 13 and 14 are formed in a surface parallel to 
the direction P of spontaneous polarization, i.e. in the X-surface X1 in 
this example. Light which propagates in the optical waveguide 12 is phase 
modulated by an electrooptic effect (i.e. by the Pockels effect) in 
accordance with a voltage which is applied to the electrodes 13 and 14. 
In a steady state in which the optical device 10 is held at a fixed 
temperature, polarization charges in the Z-surfaces Z1 and Z2 are 
neutralized by stray charges in the air which stick to the polarization 
charges. A temperature change of the crystal substrate 11 causes a change 
in the amount of polarization, and as a result, positive and negative 
surface charges develop in the Z-surfaces Z1 and Z2 (which generally, are 
surfaces crossing the direction of polarization P at an arbitrary angle 
and which are called polarization planes), respectively, generating 
electric fields as indicated by their electric lines of force Ef. The 
resultant electric fields are applied to the optical waveguide 12 directly 
or indirectly through the electrodes 13 and 14. This changes the phase of 
the light propagating through the waveguide 12, as is the case with the 
modulation drive voltage which is applied to the modulation electrodes 13 
and 14, and hence is a cause of the unstable operation of the optical 
device accompanying the temperature change. 
With such a structure as shown in FIG. 2, in particular, in which the 
ferroelectric crystal substrate 11 in FIG. 1 is mounted on a mount 30 
having a substantially equal coefficient of thermal expansion, and 
terminal electrodes 33 and 34 are provided on the marginal portions of the 
mount 30 along the planes of polarization (i.e. the Y-surfaces Y1 and Y2) 
and connected by bonding wires 31 and 32 to the modulation electrodes 13 
and 14, respectively, so that the terminal electrodes 33 and 34 are each 
supplied with the modulation drive voltage, the electric lines of force Ef 
of electric charges resulting from polarization are readily caught by the 
bonding wires 31 and 32, and consequently, the modulation electrodes 13 
and 14 are supplied with a voltage produced by the pyroelectric effect. 
In Japanese Application Laid Open No. 73207/87 entitled "Waveguide Type 
Optical Device" an arrangement is proposed for preventing deterioration of 
the temperature characteristic of the optical device caused by its 
pyroelectric effect. According to this prior art literature, a slightly 
conductive film is formed between the electrodes to that prevent that 
electric charges generated by the pyroelectric effect from remaining in 
the electrode portions. With such a structure, however, there is a 
possibility that if the resistance value of the film is too low, the 
device will be destroyed by a large current which flows between the 
electrodes when the electric fields are applied. On the other hand, when 
the resistance value of the film is too high, the charges caused by the 
pyroelectric effect cannot completely be driven out of the electrode 
portions, and hence the intended object cannot be attained. Moreover, if 
the insulation between the electrodes is lowered by the film, then no 
effective electric fields are applied to the optical waveguide, resulting 
in the reduction of the modulation efficiency. Besides, variations in the 
resistance value of the film lead to variations in the modulation 
characteristic. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a waveguide 
type optical device whose temperature characteristic is less affected by 
the pyroelectric effect of the optical device. 
The waveguide type optical device according to a first aspect of the 
present invention includes: optical waveguide means and modulation 
electrode means for changing the refractive index thereof, both formed in 
a first surface of the ferroelectric crystal substrate parallel to the 
direction of polarization; first and second conductive films respectively 
formed in second and third surfaces of the crystal substrate which cross 
the direction of polarization; and connection means for electrically 
interconnecting the first and second conductive films. 
In the above, the connection means may be means for connecting the first 
and second conductive films to a common ground, or a short-circuit 
conductive film formed in a fourth surface of the crystal substrate 
opposite the first surface thereof and having its two ends connected to 
the first and second conductive films. 
By electrically interconnecting the first and second conductive films, it 
is possible to neutralize the surface charges caused by spontaneous 
polarization due to a temperature change; hence, the temperature stability 
of the optical device is improved far more than in the past. 
According to a second aspect of the present invention, a ferroelectric 
crystal substrate having an optical waveguide and modulation electrodes 
formed on both sides thereof is mounted on a mount which has a planar 
surface larger than that of the substrate. Terminal electrodes are 
provided on the marginal portion of the mount along one or both of the 
side surfaces of the ferroelectric crystal substrate crossing the planes 
of polarization and the planar surface of the mount. The modulation 
electrodes and the terminal electrodes are interconnected by bonding wires 
extended over the above-mentioned side surface or surfaces of the 
ferroelectric crystal substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Since the waveguide type optical device 10 according to a present invention 
can be made to have an outward form of a flat parallelepiped as is the 
case with the prior art example depicted in FIG. 1, embodiments of the 
invention described hereinbelow are shown only in section in parallel to 
the Y-surface of the crystal substrate. FIG. 3 is a schematic 
representation of an embodiment according to the first aspect of the 
invention, in which the parts corresponding to those in FIG. 1 are 
identified by the same reference numerals. Also in this embodiment, there 
are formed, in the top surface X1 of the crystal substrate 11 of a 
ferroelectric substance (lithium niobate, for example) parallel to the 
direction of spontaneous polarization P, the straight optical waveguide 12 
and the modulation electrodes 13 and 14 disposed adjacent both sides for 
varying its refractive index. One of the features of the present invention 
is that the entire areas of the Z-surfaces Z1 and Z2 of the crystal 
substrate 11 (in general, a plurality of surfaces crossing the direction 
of spontaneous polarization P) are covered with conductive films 21 and 
22, respectively. Furthermore, the FIG. 3 embodiment includes, as means 
for shorting the conductive films 21 and 22, a shorting film or conductor 
23 formed in the bottom surface X2 of the crystal substrate 11 and 
electrically connected at both ends to the conductive films 21 and 22, 
respectively. With such a structure, even if positive and negative charges 
are generated in the Z-surfaces Z1 and Z2 of the substrate 11 by a 
temperature change, they neutralize each other via the shorting conductor 
23 and no electric field is formed, so that no influence is exerted on the 
refractive index of the optical waveguide 12. 
FIG. 4 illustrates a modification of the embodiment of FIG. 3, which is the 
same as the FIG. 3 embodiment in that the Z-surfaces Z1 and Z2 of the 
substrate 11 are covered with the conductive films 21 and 22, but 
different therefrom in that the conductive films 21 and 22 are 
electrically interconnected by connecting them to a common ground G, i.e. 
a common potential point, by lead wires 24 and 25, instead of forming a 
shorting conductive film 23 in the bottom surface X2 of the substrate 11 
therefor. 
Incidentally, it is evident that the conductive films 21 and 22 and the 
shorting conductor 23 in FIG. 3 may be grounded at arbitrary points as 
required. This is exemplified in FIG. 5, in which the conductive film 21 
is grounded at one point. 
The material for the conductive films 21 and 22 needs only to permit 
migration of electric charges and may also be semiconductive, and its 
conductivity is not critical. The conductive films 21 and 22 can easily be 
formed, for example, by coating a conductive point or evaporating metallic 
films. The shorting conductor 23 can similarly be formed but may also be 
replaced with a conductive wire. 
FIG. 6 illustrates an embodiment of the waveguide type optical device 
according to the second aspect of the present invention. The ferroelectric 
crystal substrate 11, which has the optical waveguide 12 and the 
modulation electrodes 13 and 14 formed on both sides thereof, as is the 
case with the prior art example of FIG. 2, is mounted on a mount 30 which 
has a planar surface 30S larger than that of the substrate 11. The mount 
30 is a square plate or block of a material which is the same as that of 
the substrate 11 or different therefrom but substantially equal thereto in 
the coefficient of thermal expansion. Terminal electrodes 33 and 34 are 
formed in the marginal portion or portions of the planar surface 30S of 
the mount 30 along one or both of Y-surfaces Y1 and Y2 of the 
ferroelectric crystal substrate 11 perpendicular to the polarization 
planes Z1 and Z2 thereof, in this embodiment along the Y-surface Y2. 
Bonding wires 31 and 32 are extended from the terminal electrodes 33 and 
34 over the Y-surface Y2 and are connected to the modulation electrodes 13 
and 14, respectively. 
Thus, the bonding wires 31 and 32 do not extend over the polarization 
planes Z1 and Z2, and hence hardly catch the electric lines of force 
caused by the pyroelectric effect. This structure permits realization of a 
waveguide type optical device which is far less affected by temperature 
change than the optical device depicted in FIG. 2. It is also possible to 
form one of the terminal electrodes 33 and 34 in the marginal portion of 
the planar surface 30S of the mount 30 along the Y-surface Y1 of the 
crystal substrate 11 and connect it to the corresponding one of the 
modulation electrodes 13 and 14 by a bonding wire extended over the 
Y-surface Y1. Moreover, any one of the embodiments shown in FIGS. 3 
through 5 may also be combined with the FIG. 6 embodiment. 
As described above, according to the first aspect of the present invention, 
in the optical device which has the optical waveguide and the electrodes 
for the modulation thereof formed in one surface of the ferroelectric 
crystal substrate parallel to the direction of spontaneous polarization, 
conductive films are formed on a plurality of surfaces of the substrate 
which are charged by spontaneous polarization and the conductive films are 
electrically interconnected. With such a structure, since charges 
generated in the plurality of surfaces of the substrate can be neutralized 
by each other, no electric field develops--this precludes the possibility 
of the refractive index of the optical waveguide being affected by such 
changes and hence prevents the deterioration of the temperature 
characteristic of the optical device which is caused by the pyroelectric 
effect. Moreover, according to the present invention, there is no of 
reducing the insulation resistance between the modulation electrodes which 
leads to the degradation of the modulation characteristic. 
According to the second aspect of the present invention, the modulation 
electrodes are connected to the terminal electrodes formed on the marginal 
portion of the mount, by bonding wires extended from the former over the 
side surface or surfaces of the ferroelectric crystal substrate which 
cross the planes of polarization. This structure affords substantial 
reduction of electric lines of force due to electric charges caused by the 
pyroelectric effect which are caught by the bonding wires. 
It will be apparent that many modifications and variations may be effected 
without departing from the scope of the novel concepts of the present 
invention.