Multi-level architecture for optical time delays in integrated circuits

The present invention provides a method and a circuit for increasing a time delay in an optical signal in an integrated optoelectronic circuit without a proportionate increase in the circuit area. The method uses multiple layers of optical waveguide fabricated in a vertical hierarchy in combination with concentric circular paths. The different sections of the optical waveguide delay line 103, 107 on the multiple layers are connected by electro-optic couplers to pass the optical signal between the sections. In a preferred embodiment, the optical couplers 105, 108 are a poled Electro-Optic (EO) region of cladding 506 allowing the coupler to be electrically addressed. Alternatively, in another embodiment, the couplers 105, 108 may be fabricated with a polymer that is optically nonlinear 406. This type of coupler could be addressed, or activated, by an optical signal, or a component of the optical signal traveling through the coupler.

FIELD OF THE INVENTION 
The invention relates to integrated optic devices, circuits, microwave 
antennas and waveguides, and more particularly, to a multilayer optical 
time delay circuit which can minimize the area used by an optical time 
delay line on an integrated circuit. 
BACKGROUND OF THE INVENTION 
It is often necessary in an electro-optic circuit design to implement a 
delay in a particular signal path. In particular, a delay line network can 
be used to steer the radar beam in a phased array radar system. This type 
of radar system utilizes an antenna which is composed of an array of 
antenna elements that can both receive and send radio frequency (RF) 
energy. The principle of operation is that an antenna beam points in a 
direction normal to its phase front. With phased arrays, the phase front 
is adjusted to steer the beam by individual control of the phase of 
excitation for each radiating element. This requires a controlled delay 
between excitation of successive elements in the array. An embodiment of 
the present invention teaches a method and circuit design to achieve 
delays of this type. 
Typically such delays have been accomplished by sending the signal through 
a loop in an optical waveguide, with the loop a sufficient length to 
provide the desired time delay using the natural propagation delay of the 
signal in the optical waveguide. This method often requires using a very 
large area on the chip to obtain longer delays. Since area on an 
integrated circuit is often limited this increased size makes the circuit 
uneconomical or perhaps unfeasible. An example of this method is disclosed 
by Yap et al. in U.S. Pat. No. 5,222,162 which includes multiple taps for 
the single level delay line. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a method is provided whereby a 
time delay for optical signals in an integrated optoelectronic circuit can 
be implemented without a proportionate increase in the circuit area. The 
method uses a stacked structure whereby multiple layers of optical 
waveguide line are fabricated in a vertical hierarchy. The different 
sections of the optical waveguide delay line on the multiple layers are 
connected by optoelectronic couplers which pass the optical signal between 
the layers and the associated optical waveguides. 
The use of this method can reduce the cost of the circuit by minimizing the 
area needed for the circuit. The method also allows the length of the 
delay to be selectable by controlling the path of the signal though 
alternative sections of the delay line. This is done by independently 
controlling the couplers and using both possible outputs of the coupler 
with different lengths of delay line for each output. This allows the 
circuit to be more versatile and more cost efficient. The present 
invention also allows smaller radar electronic packages to be built, 
saving weight and space as well as providing uses not presently 
achievable. 
This is apparently the first time vertical couplers have been used in 
conjunction with delay lines in integrated optic devices. The method 
specifically provides for implementing an optical time delay in an 
integrated optical circuit, by forming a first delay loop in a first layer 
of optical waveguide, then forming a second delay loop in a second optical 
waveguide stacked on the first optical waveguide, and then switching a 
signal to the second delay loop with a vertical coupler subsequent to 
traversing the first delay loop. 
In a preferred embodiment, the vertical optical couplers are a poled 
Electro-Optic (EO) region of cladding. The coupler may be addressed, which 
means to be activated, by placing the coupler region in the presence of an 
electric field. This causes the region to change in refractive index which 
allows the optical signal to now pass to an adjacent level of waveguide 
core. 
Alternatively, in another embodiment, the couplers are fabricated with a 
polymer that is optically nonlinear. This type of material changes in 
refractive index with the intensity of the optical signal at specific 
wavelengths. Therefore, this type of coupler could be addressed, or 
activated, by the optical signal or a component of the optical signal 
traveling through the coupler.

DETAILED DESCRIPTION OF THE DISCLOSURE 
FIG. 1 illustrates a schematic representation of a preferred embodiment of 
the present invention. An incoming signal on either input 101 is connected 
to a first optical switch 102. Switch 102 directs the incoming signal from 
either of the two inputs 101 to either the waveguide delay line 103 or the 
waveguide bypass line 104. Delay line 103 is connected to the first 
vertical coupler 105 on a first layer. Coupler 105 couples the signal from 
delay line 103 to delay line 107 on a second layer such that delay line 
107 may pass over delay line 103. The second vertical optical coupler 108 
returns the signal from the lower level waveguide path 107 to waveguide 
line 109 on the upper layer which is connected to switch 110. Switch 110 
selects either of the two outputs 111. 
As illustrated, the present invention uses a stacked structure whereby 
multiple layers of optical waveguide delay line 103, 107 are fabricated in 
a vertical hierarchy. These two sections of the optical waveguide delay 
line are connected by electro-optic couplers 105,108 to pass the optical 
signal between the layers and the associated optical waveguides. These 
couplers in this first embodiment can be non-switchable couplers, merely 
passing the signal from one layer to the other. This method reduces the 
size and weight of the circuit by minimizing the area needed for the delay 
circuit. 
In another preferred embodiment, shown in FIG. 2, a similar structure as 
that of FIG. 1 is shown with the additional feature of multiple circular 
shaped paths or spirals on one of the layers and an additional waveguide 
bypass line 206. In this embodiment the incoming signal on either input 
201 is connected to a first optical switch 202. This switch 202 directs 
the incoming signal to either the waveguide delay line 203 or the 
waveguide bypass line 204. Delay line 203 is connected to the first 
vertical coupler 205, a switchable vertical coupler on a first layer. 
Coupler 205 switches the signal from waveguide line 203 to waveguide delay 
line 207 on a second layer or to bypass line 206. Since layers 203 and 207 
are on different layers 207 may pass over delay line 203. The second 
optical coupler 208 directs signals from the delay line 207 to waveguide 
line 209. Signals in waveguide lines 209 and 206 are combined at 
y-junction 210 and then carried by waveguide 212 to switch 211. Switch 211 
selects either the bypass line 204 or line 212 to pass on either of the 
outputs of the circuit 213. 
The second embodiment allows the length of the delay to be selectable by 
controlling the path of the signal though alternative paths. The signal 
could be routed through bypass 204, through delay line 203 and 207 or 
through delay line 203 and bypass 206. This method allows an additional 
delay line bit for a phased array radar delay line circuit. This selection 
of various delay line possibilities is done by independently controlling 
the switches 202, 211 and couplers 205 and using both possible outputs of 
the coupler. This method allows the circuit to be more versatile and more 
cost efficient. This embodiment also illustrates how the method of the 
present invention may be used to make larger delays with concentric 
circular paths or spirals. 
Yet another preferred embodiment is shown in FIG. 3. This embodiment 
illustrates how the method of the present invention may be used to make 
larger delays by using concentric circular paths or spirals on each level 
of delay line. This is done by making one of the delay lines a spiral path 
of decreasing radius 306, then coupling the signal to an adjacent layer 
with a coupler 308 and then making a delay line with a circular spiral of 
increasing radius 307. While FIG. 3 shows the delay lines 306 and 307 
laterally spaced, these lines may be placed to lie directly over one 
another. 
In the present invention and in the preferred embodiments of FIGS. 1, 2 and 
3, the switchable vertical couplers may be either of two preferred types. 
The first preferred type of coupler is implemented with a non-linear 
optical region of cladding as show in FIG. 4. In this embodiment, the 
couplers are fabricated with a polymer that is optically nonlinear. A 
nonlinear photo-optic material changes refractive index with the intensity 
of the signal at specific wavelengths. Therefore, this type of coupler 
could be addressed, or activated, by a separate addressing optical signal 
traveling through the coupler. The structure of this vertical coupler 
comprises two optical waveguides 401, 402 separated by a cladding layer 
403 and bounded by cladding layers 404 and 405. A region of the cladding 
layer separating the two waveguides 406 comprises a nonlinear optical 
polymer material. The coupler 400 may be addressed, which means to be 
activated, by multiplexing with the signal or using a signal which causes 
the region to change in refractive index thereby allowing the optical 
signal to now pass to an adjacent level of waveguide core. The lower 
waveguide core 401 comprises Phosphosilicate glass (PSG) bounded on the 
lower cladding level by SiO.sub.2 cladding 405, and on the upper cladding 
level by polymer cladding 403. The upper waveguide core 402 comprises a 
polymer core bounded below by polymer cladding 403 and above by polymer 
cladding 404. 
The second preferred type of coupler is implemented with an electro-Optic 
(EO) region of cladding as show in FIG. 5. The structure of the vertical 
coupler comprises two optical waveguide cores 501, 502 separated by a 
cladding layer 503 and bounded by cladding layers 504 and 505. A region of 
the cladding layer separating the two waveguides operates as a coupler 506 
and comprises an electro-optic compound such as a poled electro-optic 
polymer. The coupler 500 may be addressed, which means to be activated, by 
placing the coupler region in the presence of an electric field which 
causes the region to change in refractive index thereby allowing the 
optical signal to now pass to an adjacent level of waveguide core. The 
coupler is addressed by applying an electrical field between metal 
electrodes 507 and 508. The electric field acts to change the refractive 
index of the region to switch the optical signal to the adjacent level. 
The lower waveguide core 501 comprises Phosphosilicate glass bounded below 
by SiO.sub.2 cladding 505 and above by polymer cladding 503. The upper 
waveguide core 502 comprises polymer bounded on either side by polymer 
cladding 503, 504. 
In the present invention and in the preferred embodiments of FIGS. 1, 2 and 
3, the non-switchable or non-active vertical couplers may be fabricated 
similar to the switchable optical couplers of FIG. 4. These vertical 
couplers have the same structure but have the refractive index of the 
region shown as element 406 chosen such that the optical signal will 
always couple to the adjacent level. In this case, it is not necessary to 
include an optical or electrical signal to change the refractive index of 
the coupling region 406, the signal will always be coupled. Other vertical 
optical couplers which have various structures could be used to provide 
coupling to adjacent layers within the scope of the present invention. 
The sole Table, below, provides an overview of some embodiments and the 
drawings. 
TABLE 
______________________________________ 
Element 
Specific Name Generic Name Alternatives 
______________________________________ 
101/201 
Input Signal Lines 
Optical Signal Inputs 
301 
102/110 
2 .times. 2 Optical Switch 
Optical Switch 
202/211 
302/310 
103/107 
Delay Waveguide 
Delay Line 
203/207 
306/307 
104/204 
Bypass Waveguide 
Bypass Waveguide 
304 
105/108 
Vertical Optical 
Optical Coupler 
Coupler 
205/208 
305/308 
206 Bypass line Connecting 
Waveguide 
109/209 
Connecting Optical Signal Line 
Waveguide 
212/303 
210 Y-Junction Optical Waveguide 
Junction 
111/213 
Outputs Optical Delay Circuit 
311 Outputs 
401/501 
PSG core Optical Waveguide 
402/502 
Polymer Core Optical Waveguide 
403/503 
EO Polymer Cladding 
Cladding 
405/505 
SiO.sub.2 Cladding 
Cladding 
404/504 
Polymer cladding 
Cladding SiO.sub.2 Cladding 
506 Poled Polymer E-O region Poled Oxide 
E-O region E-O Region 
406 Non-linear 
E-O Region 
507/508 
Electrodes Electrodes 
______________________________________ 
While this invention has been described with reference to illustrative 
embodiments, this description is not intended to be construed in a 
limiting sense. In general the preferred or specific examples are 
preferred over the other alternate examples, however, it is to be 
understood that the scope of the invention also comprehends embodiments 
different from those described, yet within the scope of the claims. It is 
therefore intended that the appended claims encompass any such 
modifications or embodiments. 
In particular, implementation of the method of this invention is 
contemplated in discrete components or fully integrated circuits in 
silicon, germanium, gallium arsenide, or other electronic material 
families. The method also contemplates multiple layers of optical 
waveguide even though the embodiments illustrated are limited to two 
layers. In addition, the shape of the delay loops or paths for the delay 
lines can be varied to be spirals, ovals, concentric circles or variations 
or combinations of these shapes.