Tapered velocity electro-optical waveguide switch

A four-part electro-optical switch for switching optical radiation from one aveguide to another. Parallel waveguides are embedded in the surface of a substrate and provided with separating ends for transmitting radiation through the waveguides. Curved electrodes on the outer side of each waveguide in cooperation with a central electrode and separate voltage sources change the electro-optic properties of the waveguide such that radiation may be switched from one waveguide to the other when the voltage source is in the "on" state.

BACKGROUND OF THE INVENTION 
This invention relates to optical waveguides and more particularly to a 
switch arrangement for switching radiation from one waveguide to another 
and for modulating optical signals. 
Optical radiation has been switched in and out of optical waveguides by use 
of gratings, prisms, and acousto-optic devices. Optical radiation has been 
switched from one waveguide to another by use of parallel waveguides in 
which one or both waveguides included a section of electro-optical 
material whose index of refraction is changed by an electrical voltage. 
Electrodes placed parallel with spaced waveguides have also been used, as 
shown in U.S. Pat. Nos. 3,909,108 and 3,964,819. The device of U.S. Pat. 
No. 3,909,108 requires that electro-optic interaction occurs over a 
critical length which is voltage-independent but depends upon the coupling 
constant between the two waveguides. U.S. Pat. No. 3,964,819 requires a 
3dB coupler at each end of each waveguide which adds to the length of the 
device. In prior art devices, the lengths of the waveguides in which 
radiation is transfered are critical in order to present the proper 
coupling length. 
SUMMARY OF THE INVENTION 
A four-part electro-optical switch between channel optical waveguides 
including electrodes which are placed relative to the waveguides. A 
voltage applied to the electrodes can be adjusted such that the coupling 
constant between the two waveguides and the rate of change of the 
difference in propagation constant, .DELTA..beta., is held low enough to 
ensure adiabatic propagation within the center of the device to transfer 
power from one waveguide to the next. The device is bidirectional since 
radiation may be coupled in either direction and admitted in either 
waveguide at either end of the switching device. One of the main 
advantages of the device is that the device may be relatively short and 
the length is not critical since errors in the proportionality between the 
length, L, and the difference in propagation constant, .DELTA..beta., can 
be compensated for by changing the voltage. The voltages may be applied to 
the electrodes with either polarity depending on the direction of the 
radiation and the waveguide to be switched.

DETAILED DESCRIPTION 
This device includes a substrate 10 of electro-optic material such as 
LiNbO.sub.3 within which a pair of spaced waveguides 11 and 12 of an 
electro-optic material having an index of refraction higher than the 
substrate is formed. The waveguides have a rectangular cross section and 
are embedded in the substrate 10 such that their upper surfaces are 
coplanar with the substrate surface and each other. The major lengths of 
the waveguides are parallel with each other with their ends diverging from 
each other. The separation between the ends of the waveguides causes them 
to act as power dividers. An electrode 13 is laid down on the surface of 
the substrate centrally between the pair of waveguides paralleling a major 
portion of the waveguides. One end 14 of the electrode 13 goes off at an 
angle crossing over the waveguide 11 and the opposite end 15 is at an 
angle crossing over the waveguide 12. An insulator may be placed between 
the electrodes and the waveguide at their crossover. 
A pair of curved electrodes 16 and 17 are disposed on the surface of the 
substrate with one electrode on the outside of each waveguide. Electrodes 
16 and 17 curve uniformly away from the waveguides with the end of each 
electrode closest to the adjacent waveguide being opposite the end of the 
central electrode which is directed off at an angle in a direction away 
from the end of the curved electrode closest to the waveguide. The curved 
electrodes are related to each other such that the spacing separating them 
is equal, at equal distances on opposite sides of a center line 
perpendicular to the length of the waveguides. Curved electrode 16 and the 
angular end 14 of central electrode 13 are connected electrically to a 
double-pole, double-throw, polarity reversing switch 20 connected 
electrically to a voltage source 18. Curved electrode 17 and the angular 
end 15 of the central electrode 13 are connected electrically to a 
double-pole, double-throw, polarity reversing switch 30 which is connected 
electrically to a voltage source 19. The double-pole, double-throw 
switches and the voltage sources are connected such that in the "on" state 
opposite polarity is applied to the curved electrodes 16, 17, while in the 
"off" state the same polarity is applied to the curved electrodes. In the 
"on" state, switching takes place, while in the "off" state the radiation 
passes straight through without any switching. 
FIG. 2 is a cross-sectional view of the device taken across its center 
which illustrates the relationship of the various parts. 
FIG. 3 is a modification of the device shown in FIGS. 1 and 2. The device 
includes a substrate 10 of an electro-optic material, such as LiNbO.sub.3 
with waveguides 11 and 12 such as described above for FIG. 1. An elongated 
electrode 21 is deposited on the surface of the substrate directly between 
waveguides 11 and 12. The electrode 21 extends beyond the parallel 
sections of the waveguides and is connected at one end electrically to the 
negative side of voltage source 18 and at the opposite end to the positive 
side of voltage source 19. A pair of curved electrodes 22 and 23 are 
deposited opposite the outer side of waveguide 11 and a pair of curved 
electrodes 24 and 25 are deposited opposite the outer side of waveguide 
12. Each pair of electrodes are deposited such that the end toward the 
outer ends of the waveguides are closest to the surface of the adjacent 
waveguide with the electrodes directed toward each other and curved away 
from the waveguide, with the ends of the electrodes approaching a center 
line perpendicular to the waveguides. Thus, the corresponding parts of the 
curved electrodes of each pair are equidistant from the center line. 
Electrodes 22 and 24 are opposite the same end of the waveguides and are 
shown connected electrically to the positive side of voltage source 18. 
Likewise electrodes 23 and 25 are opposite the other end of the waveguides 
and are connected electrically to the negative side of voltage source 19. 
With the switching arrangement shown in the drawings and described above, 
with the switch in the "on" state the waveguide device behaves as a 
tapered velocity coupler. In the "on" state the voltage is applied so that 
the difference in propagation constant, .DELTA..beta. between the 
interacting channel varies slowly over the length of the device changing 
polarity sign in the middle of the structure. In the "on" state V.sub.1 
=+V.sub.2. If the voltage is adjusted such that .sub.0.sup.L 
.sqroot..DELTA..beta..sup.2 +4.vertline.K.vertline..sup.2 dz=m.pi. where m 
is an odd integer, K is the coupling constant between the two guides, and 
the rate of change of .DELTA..beta. with propagation direction is held low 
enough to ensure adiabatic propagation within the center of the device, 
then power will be transfered from one guide to the next. In the "off" 
state, V.sub.1 =-V.sub.2 where the voltage is applied such than 
.DELTA..beta. has the same sign over the length (L) of the device. If the 
voltage is such that .sub.0.sup.L .sqroot..DELTA..beta..sup.2 
+4.vertline.K.vertline..sup.2 dz=n.pi. where n is an even integer then no 
power will transfer between the two channels. L is the length of the main 
body of the optical waveguides between the angular spreading ends. The 
most efficient length is L=2.pi./.sqroot..vertline.K.vertline..sup.2. 
In the arrangement shown in FIG. 3, .DELTA..beta.=0 in the center of the 
device in both the "off" and "on " state. The device may be constructed 
without the center electrode 21 if the proper orientation of the 
electro-optic crystal is chosen. 
In operation of the device of FIG. 1, the double pole-double throw switches 
are closed so that electrode 16 is more positive then electrode 13, 
electrode 13 is more positive than electrode 1 which occurs when V.sub.1 
=+V.sub.2. In this position, with the proper magnitude of V.sub.1 
radiation entering waveguide 11 at A will be switched out on waveguide 12 
at D. Radiation entering waveguide 12 at B will be switched to waveguide 
11 and out at C. With the switches in the same position, radiation 
entering at C will be switched out on B and radiation entering D will be 
swtiched out on A. Changing the polarity of the electrodes by changing the 
position of one of the switches such that V.sub.1 =-V.sub.2 then with the 
proper magnitude of v.sub.1 the radiation entering A will leave at C, and 
if it enters B it will leave at D. Also if radiation enters C, it will 
leave on A, and if it enters D, it will leave on B. Thus, it can be seen 
that if V.sub.1 =+V.sub.2 radiation will be switched from one waveguide to 
the other and if V.sub.1 =-V.sub.2, the radiation will pass straight 
through in the waveguide in which it enters. 
The modification shown in FIG. 3 will switch radiation the same as 
described above if V.sub.1 =+V.sub.2, with the proper magnitude of 
V.sub.1. 
With a waveguide-electrode structure as described above, the waveguides may 
be made without a critical length. Errors in length may be compensated for 
by changing the voltage. With a waveguide structure with a length of 
L=2.pi./.sqroot..vertline.K.vertline..sup.2, the voltages V.sub.1 and 
V.sub.2 may be 10 volts. 
Obviously many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims the invention may be 
practiced otherwise than as specifically described.