Common path multichannel optical processor

Adverse environmental effects may be canceled from an optical processor which adjusts the phase of spatially distributed frequency components. This is achieved by projecting the optical modulated signal and a local oscillator optical signal along the same optical path and through the same optical components. Multichannel operation is achieved by positioning the optical components for each stage along a common axis.

FIELD OF THE INVENTION 
The present invention relates to any optical processor which is sensitive 
to environmental effects. 
BACKGROUND OF THE INVENTION 
RF signals propagating through a medium generally experience non-linear 
phase characteristics, namely, non-linear phase variation with frequency. 
Without special processing, such a propagated signal will be detected as a 
degraded signal. 
Prior art devices have been satisfactorily employed for years to achieve 
phase and amplitude equalization. However, they are severely restricted in 
the number of frequencies that can be handled by the digital electronics 
circuitry and the speed with which the equalization is activated. 
U.S. Pat. No. 4,771,398, issued Sept. 13, 1988, and assigned to the present 
assignee, utilizes coherent optical processing to perform phase 
equalization corrections of RF signals by providing equalization paths for 
a multitude of discrete frequencies in a parallel operation. By virtue of 
the prior invention, thousands of discrete frequencies may be handled. 
The aforementioned invention utilizes a phase-controlled array in the 
Fourier plane to cancel phase distortion of the propagated signal. The 
array is comprised of individual components that have their birefringence 
electrically altered to correspondingly alter the phase of the particular 
frequency associated with the element. The corrected optical signal then 
undergoes photoelectric transformation at a photomixer and the result is a 
phase-equalized correction signal which corresponds to an input signal 
prior to its propagation-induced phase distortion. 
Although the apparatus of the co-pending application operates 
satisfactorily, at times environmental effects cause problems due to the 
fact that an optically converted RF signal and optical local oscillator 
signal are introduced to the apparatus along parallel paths. These paths 
are subject to different environmental effects due to vibration, 
temperatures, dust, etc. Accordingly, it would be advantageous to 
introduce the RF and local oscillator signals along a common optical path 
to negate the different environmental effects. 
BRIEF DESCRIPTION OF THE PRESENT INVENTION 
The present invention is an improvement of the apparatus in the mentioned 
U.S. patent. The apparatus provides a means for providing a common optical 
train for an optically converted RF signal and associated optical local 
oscillator signal. Therefore, the environmental effects on each are 
identical and environmental disturbances are self-cancelling. With this 
common path approach, sharing of common optical components becomes 
possible thereby affording greater packaging density required in the 
apparatus. 
A further embodiment of the present invention incorporates the common path 
concept in a plurality of parallel optical channels. Such a multi-channel 
array allows the use of a different wavelength laser in each channel, 
should this be desired. Accordingly, this embodiment may be advantageously 
adapted for frequency multiplexing applications.

DETAILED DESCRIPTION OF THE INVENTION 
Prior to a detailed description of the present invention, it is instructive 
to consider the previously conceived optical processor of the mentioned 
U.S. patent shown in FIG. 1, which is sensitive to environmental effects 
as previously discussed. 
A laser beam 10 serves as an optical carrier signal for a modulating RF 
signal 14 which has been previously distorted as a result of propagation. 
The beam 10 and RF signal 14 are introduced to a conventional 
acousto-optical modulator 12, such as the type manufactured by the ISOMET 
Corporation; and a modulated acoustic field (object) 16 is formed by 
modulator 12. 
A Fourier plane 22 is developed between Fourier lens 18 and inverse Fourier 
lens 20. By introducing a phase control array 23 at the Fourier plane 22, 
a phase equalization capability is realized. Specifically, there is a 
spatial frequency distribution of object 16 on the Fourier plane 22; and 
by placing a multi-optical element phase control array 23 in coplanar 
relationship with the spatial distribution, each frequency component of 
object 16, as spatially distributed, may undergo phase modification so 
that a phase-equalized optical signal results. Thus, as will be presently 
explained, the elements of the array produce desired phase control at each 
frequency component of the object 16. 
To better understand the phase control array 23, reference is made to FIG. 
2 wherein a multi-element electro-optic device is illustrated. The 
individual elements are schematically indicated by corresponding spatially 
distributed frequency components F.sub.l -F.sub.n. For purposes of 
simplicity, only a small number of frequency components is illustrated. 
However, it should be understood that the apparatus is intended for a 
large number of frequency components, typically one thousand or more. 
Appropriate electro-optic devices include PLZT, liquid crystal, Kerr 
cells, Pockel cells, Faraday cells, and the like. The purpose of each 
element in the array is to vary the optical path length of the spatially 
distributed frequency components at the Fourier plane 22 so that the 
birefringence of each element is varied as required in order to alter the 
optical path length of each element in a manner that will equalize the 
phase of each frequency component as it passes through the Fourier plane 
22. As a result, the phase of an image located to the right of the inverse 
Fourier lens 20 is phase equalized relative to the distorted object 16. 
The equalized image undergoes processing by combiner 26 which may be a 
conventional semi-silvered mirror. A laser local oscillator beam 28 forms 
a second optical input to the combiner 26 to achieve optical heterodyning 
or down converting thus forming the phase-equalized image 24 which 
impinges upon an intensity-sensitive square law photodetector 30 for 
transforming the corrected phase-equalized image 24 to a corrected RF 
signal at photodetector output 32. As a result, the RF signal at output 32 
is a phase-corrected non-distorted signal resembling the original 
electrical signal which became distorted by propagation prior to 
introduction to the equalization circuitry of FIG. 1. 
It should be pointed out that the phase shift occurring at each of the 
elements in array 23 can be continuously varied, as in the Kerr, Pockel 
cell and liquid crystal devices, or discretely varied as in a Faraday 
cell. The amount of phase shift occurring through each cell is controlled 
by a device which, in its basic form, may resemble a voltage divider 21 to 
which a reference voltage is applied. Individual outputs from the voltage 
divider, as generally indicated by reference numeral 19 (FIG. 2), drive 
each element of the array to a degree corresponding to the desired phase 
shift to be achieved by each element of the array 23. 
The laser local oscillator beam 28, which forms the second optical input to 
the combiner 26 is derived from the laser beam 10. The local oscillator 
beam may be phase-controlled in a manner similar to that disclosed in 
connection with the signal path through the phase-control array 23. This 
is done by including a second phase-control array 33 similar in 
construction to the multi-optical element phase-control array 23. As in 
the case of the first array 23, the second phase-control array 33 modifies 
the phase of the laser beam 10 as it impinges upon each element of the 
array. The lens 36 focuses the phase-modified beam for reflection by 
mirror 35 to form the local oscillator beam 28. In fact, this beam will be 
comprised of phase-modified sections which correspond to the phase 
modifications of the object 16, as a result of phase-control array 23. 
The inclusion of a phase-modified local oscillator beam is not mandatory. 
However, the utilization of both arrays 23 and 33 can be advantageously 
operated in parallel and/or tandem to achieve phase correction of a 
distorted propagated RF signal over a wide range of applications. 
In accordance with the previously conceived invention, phase correction may 
be accomplished in three modes: 
1. utilization of phase-control array 23 and a local oscillator beam 28 
which does not undergo phase control through array 33; 
2. phase control of the local oscillator beam 28 by utilization of array 33 
and no utilization of a phase-control array 23 at the Fourier plane 22; 
and 
3. utilization of phase-control arrays 23 and 33. 
The degree of elemental local oscillator phase control is determined by the 
voltage divider output 19' in the same manner previously described in 
connection with voltage divider output 19, which drives the phase-control 
array 23. 
Although the apparatus illustrates a single pass device, if additional 
phase correction is required, multiple passes through the phase control 
arrays 23 and 33 may be accomplished by a recursive technique which may 
typically utilize mirrors (not shown) for achieving multiple passes. 
The modification of the optical path length through each array element, 
corresponding to phase shift through that element, may be expressed by the 
equation: 
EQU .DELTA..phi.=2.pi.(.DELTA.t n.sub.c C)/.lambda. 
where 
.DELTA.t is the differential delay; 
n.sub.c is equal to the refractive index of the element cell; 
C is equal to the speed of light; and 
.lambda. is the wavelength of the laser beam 10. 
Although the apparatus has been described for radio frequencies, it is 
equally applicable to phase equalizing frequency components of other 
multi-frequency signals, regardless of the medium through which they 
propagate and encounter distortion. 
In situations where amplitude equalization of signal frequency components 
is also necessary, this may be achieved by modifying the frequency 
components of the signal at the Fourier plane; additional amplitude 
equalization being possible by modifying the local oscillator beam. The 
means for so modifying the amplitude of individual frequency components is 
by utilizing arrays of light filtering elements, as disclosed in our 
co-pending patent application entitled METHOD AND APATUS FOR OPTICAL RF 
AMPLITUDE EQUALIZATION, Ser. No. 857,288, now U.S. patent Ser. No. 
4,771,397, issued Sept. 13, 1988. 
The Improvement 
The present invention eliminates the separate path for local oscillator 
beam 28, as employed in the previously conceived apparatus. It has been 
found that, by introducing the local oscillator along a separate optical 
path, different environmental effects are experienced along each of the 
optical paths, which results in a deteriorated RF signal at the output 32. 
Therefore, the common path approach can be used to improve any optical 
heterodyne processor where phase and amplitude stability are important. 
The local oscillator beam and signal beam are directed along a common 
optical path, between shared components, as will be seen from FIG. 3. 
Identical components in FIGS. 1 and 3 have been similarly numbered. The 
embodiment of FIG. 3 illustrates a single channel of signal communication. 
Laser beam 10 is subjected to a beam splitter 9 for generating an upper 
beam serving as a local oscillator (L.O.) beam and a lower carrier signal 
to the input of acousto-optical modulator 12. The RF signal 14 is 
introduced into the modulator as was the case in FIG. 1 and an optically 
modulated signal is generated at the output of modulator 12. Fourier lens 
18' performs the same function as lens 18 in FIG. 1 but handles both the 
local oscillator beam and the optical signal beam. The Fourier lens 18' 
may, for example, be a Casegrainian type. Spatially distributed frequency 
components of an object undergo phase or amplitude modification at array 
34. However, the array includes two separate windows 36 and 38, as shown 
in FIG. 4; the windows respectively passing the signal beam and local 
oscillator beam. Although phase correction may be accomplished in the 
three modes previously mentioned, FIG. 4 shows the construction of a phase 
and amplitude control array in the configuration corresponding to the 
first mode, namely phase or amplitude control of the signal beam but not 
the local oscillator beam. This would be accomplished by including an 
electro-optic device in window 36, of the typical type previously 
mentioned in connection with phase or amplitude control array 23 of FIG. 1 
while passing the local oscillator beam through a clear window 38. The 
local oscillator beam and signal beam individually pass through inverse 
Fourier lens 20' and are combined at combiner 26. As will be appreciated, 
the more significant aspect of the present improvement is the utilization 
of a common optical path wherein the path lengths of both the local 
oscillator beam and signal beam are the same; and the use of common 
optical components ensures that the previously discussed effects from the 
environment are the same upon each beam. Therefore, any adverse 
environmental effects thereof are canceled. The combined beam is converted 
to an electrical signal at output 32 of a photodetecter 30. 
FIG. 5 illustrates a multi-channel apparatus for allowing the use of 
different wavelength lasers for each signal channel, should this be 
desired, such as for frequency multiplexing applications. Only three 
separate channels have been indicated in FIG. 5, but it is to be 
understood that many more may be incorporated in the illustrated 
apparatus. Identical components in FIG. 3 and FIG. 5 are indicated by the 
same reference numerals. 
Considering a first channel of the multi-channel apparatus, a laser beam 10 
is directed to beam splitter 9 which, like a number of similar splitters, 
is coaxially mounted around a fixed ring 40. The laser beam then undergoes 
modulation at 12, while the split laser beam, serving as a local 
oscillator beam, is passed through a central clear portion of a ring 42, 
which coaxially mounts a number of acousto-optical modulators 12. The 
first channel path then continues with Fourier lens 18' which is coaxially 
mounted with other Fourier lenses on a disc 44. 
The channel then continues through phase and amplitude control array 34 
which is mounted to fixed ring 46, coaxially with other similar arrays. 
Inverse Fourier lens 20' is coaxially mounted with other lenses on disc 
48. Optical processing continues with combiner 26, the latter being 
coaxially mounted with similar combiners on fixed ring 50. The combined 
beam is then directed to photodetecter 30 which generates an electrical 
signal output for the first channel along output lead 32. The other 
illustrated axially distributed optical components are respectively 
coaxially aligned so that a plurality of channels is generated in the same 
direction as the first channel. The resultant multi-channel array allows 
the use of different wavelength lasers for each signal channel when 
desired, such as in the application of frequency multiplexing. The various 
optical beams, namely the signal and local oscillator beams from the 
different channels can occupy the same compact volume without interaction. 
This sharing of a common optical direction affords greater packaging 
density. As a result, the present invention produces an optical processor 
with superior operational results while requiring less space than would be 
otherwise required. 
It should be understood that the invention is not limited to the exact 
details of construction shown and described herein, for obvious 
modifications will occur to persons skilled in the art.