Integrated optical time integrating correlator

An optical time integrating correlator is fabricated in integrated optical form. This integrated optical implementation comprises a number of integrated components including an optical waveguide on a substrate, a diode laser, a guided wave lens, an electro-optic beamsplitter, a pair of surface acoustic wave transducer for generating first and second counterpropagating surface acoustic waves, a spatial filter and a detector array.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention is directed to an optical time integrating correlator 
which is fabricated in integrated optical form. Integrated optics is 
rapidly increasing in importance for large bandwidth optical signal 
processing applications. Some of the advantages of using integrated 
optical techniques over bulk optical techniques include ease of alignment, 
ruggedness, freedom from vibration effects, tolerant to temperature 
changes, small size and low fabrication costs. 
In the prior art certain optical processing systems have been constructed 
using bulk (i.e. discrete) optical components including a time integrating 
correlator. Such an apparatus is described in an article entitled, "A New 
Surface-Wave Acoustooptic Time Integrating Correlator", by N. G. Berg et 
al, App.Physics Lett., 36(4), Feb. 15, 1980, P256-258. It is also 
described in U.S. Pat. No. 4,326,778. FIG. 1 is an example of such prior 
art as shown in the cited article in which a laser beam 10 is split into 
two components via a beamsplitter, a transmitted component 11 and a 
reflected component 12. These two beams enter a SAW (surface acoustic 
wave) delay line with a prescribed angle 4.theta..sub.B between them, in 
which .theta..sub.B denotes the Bragg angle, .theta..sub.B =sin.sup.-1 
[.lambda..sub.1 /(2.lambda..sub.a)], where .lambda..sub.1 is the light 
wavelength and .lambda..sub.a is acoustic wavelength at the design center 
frequency, so that one beam interacts primarily with SAW.sub.1 while the 
other beam interacts primarily with the other counterpropagating 
SAW.sub.2. The resulting diffracted beams 13 and 14 are parallel and 
overlap. They are imaged onto an integrating detector array for some 
period of time; the final array output contains the cross correlation of 
the signals used to generate the two SAWs. 
Our present disclosure describes an integrated optical implementation of 
the time integrating correlator. The integrated device comprises several 
components including a diode laser, an optical waveguide substrate, a 
guided-wave lens, an electro-optic beamsplitter, a pair of surface 
acoustic wave transducers for generating first and second 
counterpropagating surface acoustic waves, a spatial filter and a detector 
array.

DESCRIPTION 
As has been described above, our invention describes the fabrication in 
integrated form of an optical time integrating correlator. Referring to 
the preferred embodiments of FIGS. 2 and 3 there is shown a single crystal 
lithium niobate substrate 20 having diffused into the surface thereof a 
thin layer of titanium to form an optically transmissive surface waveguide 
21. The lithium niobate crystal has piezoelectric, electro-optic and 
waveguiding properties. A light source preferably in the form of a laser 
diode 22 is either integrated into the crystal, FIG. 3, or butt-coupled or 
otherwise mounted to the Ti diffused LiNbO.sub.3 optical waveguide which 
is on the order of one micron thick (FIG. 2). A guided wave lens 23 is 
fabricated by either cutting an aspheric (i.e. curvilinear) depression in 
the surface of the substrate (i.e. a geodesic lens) or depositing a thin 
overlay Luneburg lens on the surface of the waveguide. The lens 23 is 
effective to collimate the light beam from the diode laser. An 
electro-optic (EO) beamsplitter 24 splits the collimated optical beam into 
two components including an undiffracted beam 25 and a diffracted beam 26. 
The EO beamsplitter is fabricated by photolithographically depositing an 
interdigitated electrode pattern on the surface of the waveguide. By 
applying a voltage to the electrode pattern a phase diffraction grating is 
established in the waveguide. Exiting from the EO beamsplitter are two 
beams, the undiffracted beam and the diffracted beam. The amount of 
diffraction is controlled by the strength of the phase grating which is 
proportional to the applied voltage. Anywhere from 0-100% of the energy 
can be diffracted by changing the voltage. The voltage is adjusted for 50% 
efficiency so that the two beams have equal energy. The diffraction angle 
is determined by the electrode spacing. 
The two beams emerging from the EO beamsplitter are identical in both phase 
and power. It is, therefore, unnecessary to include a phase compensation 
element which is required in the bulk device. These two beams are then 
subjected to an acousto-optic interaction with two counterpropagating 
surface acoustic waves produced by two integrated SAW transducers 27 and 
28 placed at their appropriate Bragg diffraction angles. These transducers 
are identical except for their angular positioning. The transducers 27 and 
28 are energized, respectively, by two signals S.sub.1 (t) and S.sub.2 (t) 
which are to be correlated. The two counterpropagating surface acoustic 
waves deflect portions of each of the two optical beams 25 and 26 from the 
EO beamsplitter such that these two deflected beams 30 and 31 overlap each 
other and are propagating parallel to the crystal axis (x axis in the 
figure). These two deflected beams now contain the two signals which are 
to be correlated (i.e. they are coded). 
At this point the undeflected beams 25 and 26 and deflected beams 30 and 31 
are not separated due to the small deflection angles (less than 
1.degree.). If the waveguide was sufficiently long along the x-direction 
it would be possible to separate these beams since they are traveling at 
an angle with respect to each other. Unfortunately this is not a 
satisfactory solution because very long substrates would be required. 
Therefore, a spatial filter 32 as shown in the figures is fabricated 
preferably with high index overlay optics. The spatial filter is 
fabricated with a single lens 33 which images the acoustic pattern onto 
the the detector array 34 where the lens is 2f (f=focal length of the 
lens) from the acoustic field and 2f from the detector array. Or one can 
construct the spatial filter with two lenses 33' and 33" such that the 
lenses are separated by 2f and one of the lenses 33' is a distance f from 
the acoustic field and the other lens 33" is f from the detector array, 
FIG. 2a. In both cases the distance required is 4 times the focal length 
of the lens. Better imaging quality is usually accomplished using the 
latter approach with two lenses. The spatial filtering is accomplished by 
forming high refractive index optics 35 and 36 at the spatial frequency 
plane 37 of the lens 33. These high refractive index optics are preferably 
high refractive index prisms but may be a metallization on the surface 
which is effective to absorb the non-coded optical beams. The optics may 
be deposited overlay optics or may be milled or etched. The index of the 
prisms must be larger than the waveguide index and this causes the 
undeflected non-coded optical beams from the acoustic field to be further 
deflected from the detector array. The two optical beams which contain 
S.sub.1 (t) and S.sub.2 (t) strike the detector array 34 and the output is 
I(t) where 
EQU I(t)=.vertline.S.sub.1 (t)+S.sub.2 (t).vertline..sup.2 
and the cross term is the term of interest for the correlation of S.sub.1 
(t)*S.sub.2 (t). 
In operation the apparatus is used to determine whether and to what extent 
two signals are correlated. The invention may be used at a receiving 
station where a "known" signal is compared to an "unknown" signal 
comprised perhaps of many different signals and noise. If the compared 
signals show significant correlation it can be concluded that the unknown 
signal includes the known signal. The unknown signal S.sub.1 (t) is used 
to drive one SAW transducer while the known signal S.sub.2 (t) is used to 
drive the other SAW transducer. 
In operation the light beam issues from the laser diode and is collimated 
by the first lens 23. The collimated beam is then split into two parts, a 
diffracted beam and an undiffracted beam. The beams are in phase and have 
equal power (due to proper adjustment of the voltage across the 
electro-optic phase grating 24). Each beam is then impinged upon by a 
traveling surface acoustic wave generated by SAW transducers 27 and 28. 
The interaction between light and elastic waves causes a portion of the 
light to diffract (deflect) from each of the split beams. The SAW 
diffracted beams 30 and 31 overlap and propagate colinear with each other 
and with the x axis of the lithium niobate crystal. The spatial filter 32 
is an important element in the operation of the system and the spatial 
filter is used to reject or steer aside portions of the two beams 25 and 
26 that are not diffracted by the SAW transducers 27 and 28. It thus 
provides means to cause the encoded portions of the beams to fall on the 
detector and to effectively eliminate the non-coded portions. The prisms 
35 and 36 are located at or near the spatial frequency plane 37 so that 
they may refract beams 25 and 26 from striking the array and saturating 
it, for only a portion of each beam is diffracted by its corresponding 
SAW. When the SAW-diffracted beams contain or are modulated by signals 
that are somewhat correlated, a peak will appear on one or more of the 
pixels of the detector 34 after a sufficient integration period.