Optical phase array radar

In an optically controlled phased array radar control information in the form of microwave frequency modulation of an optical carrier is transmitted by optical waveguide from a central processor to each of the antenna elements. Phase shifts introduced by each of the individual waveguides are then monitored to provide a compensation signal used either to regulate the phase shift or to offset the phase of the modulation applied to the waveguide at the central processor end.

BACKGROUND OF THE INVENTION 
This invention relates to phased array radars and is particularly concerned 
with how to drive the individual antenna elements of such an array from a 
central processor. 
PRIOR ART STATEMENT 
Electro-optical apparatus for phased arrays are known in the prior art. For 
example, see Wright et al. U.S. Pat. No. 3,878,520 issued Apr. 15, 1975, 
and Levine U.S. Pat. No. 4,028,702 issued June 7, 1977. Both of these 
prior art patents require switching between optical fibers. 
Though the concept of phased array radar would provide significant 
operation advantages, particularly in the military scenario, the 
development of such systems has been held back by the per element phase 
control problem. In detail the system considerations can be different for 
transmitting or receiving arrays and are dependent upon the technical 
approach taken. Basically, in transmission it is necessary to pass to the 
antenna elements from a central processor: 
(I) instantaneous microwave frequency 
(II) microwave phase 
(III) pulse edge timing. 
Differential phase between the elements establishes the beam direction and 
the pulse edge timing is tailored to maintain a constant pulse sidelobe 
level in the time domain for different beam direction. 
On receive, control to the phased array radar elements again involves 
microwave frequency and phase for the local oscillator, but in addition 
the target information is required to be collected from each element. 
Hitherto, in the standard approach to both receiver and transmitter phased 
array radar systems, a signal at the transmit frequency or LO, or at a 
subharmonic of these frequencies, is fed to the elements via a radio 
frequency (RF) manifold, usually a coaxial line, microstrip or waveguide. 
In the transmit mode a phase shifter is incorporated in each element to 
set the output phase of the element. A greater variety of techniques can 
be used in a receive array by providing the phase shifting at RF, 
intermediate frequency (IF), or baseband. 
SUMMARY OF THE INVENTION 
The present invention is however concerned with phased array radar systems 
in which the control information is transmitted to each of the antenna 
elements by microwave frequency modulation of an optical carrier. The RF 
manifold may then be replaced by a bundle of single mode optical fibers 
thereby affording the possibility of much greater compactness. 
It will be appreciated that, unless the individual optical fibers by which 
the individual antenna elements are connected to the central processor all 
have the same length, the relative phases of any microwave modulation 
impressed on the optical signals at the central processor end will not in 
general be the same as the relative phases appearing at the individual 
antenna elements. It is in principle possible to arrange for the fibers to 
be accurately cut to the same effective optical length and to be 
fabricated with low temperature material so that temperature variations 
have no significant effect upon that length. However, such an approach is 
critical and uncertain. The present invention is particularly concerned 
with an alternative approach in which changes in path length are monitored 
and appropriate measures taken. 
According to the present invention there is provided an optically 
controlled phased array radar, wherein the operation of the antenna 
elements of the radar is controlled from a central processor by microwave 
frequency signals impressed on optical carriers relayed from the central 
processor to the individual antenna elements on individual optical 
waveguide transmission links. Each link includes means for monitoring the 
microwave frequency phase shift introduced by that link to provide a 
compensation signal which is used either to regulate the optical path 
length of that link or to offset the phase of the microwave frequency 
modulation of an optical signal applied to that link by the central 
processor. 
A preferred way of monitoring the phase shift is to insert a four-port 
directional coupler into the link at the central processor end and to use 
photodetectors connected to two of the ports to detect respectively a 
portion of the launched light and a portion of the light returning after 
reflection at the antenna end. The detector outputs are mixed to give a 
signal representative of the phase shift, at the microwave modulation 
frequency, introduced by the round trip path.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram of an optically controlled phased array radar. It 
shows a central processor 10 connected with the individual members of an 
array of antenna elements 11 by way of individual optical transmission 
links 12. In the central processor microwave modulation has to be applied 
to an optical carrier, which is conveniently the output of an injection 
laser diode. 
There are three contending techniques for applying a high frequency 
modulation to a laser diode source. A separate modulator at the laser 
output, or square wave current modulation of the laser may be used in 
conjunction with one modulation bit delay in a series Mach-Zehnder 
interferometer. Alternatively two lasers may be operated in a phase locked 
loop configuration at the offset frequency of the required modulation. The 
last method has the advantage that it is applicable to millimeter wave 
frequencies just as easily as it is to low microwave frequencies. This 
contrasts with separate modulators which currently are only realizable at 
the lower microwave band. However, the phase locked loop approach involves 
the penalty that phase control is rather less conveniently implemented 
because a phase reference at the microwave frequency has to be provided, 
rather than the phase being set directly by adjusting the relative optical 
phase via an analog input to a phase control. 
FIG. 2 illustrates an implementation of the direct modulation approach. At 
the input end light, propagating in a lithium niobate integrated optics 
waveguide 20 is launched into a two-way splitter 21, one of whose branches 
feeds a second two-way splitter 22. One branch of this second splitter 
passes through phase retarding elements 23 and 24, while the other branch 
passes through a phase retarding element 25, and then the two branches are 
combined in a recombiner 26. The output of this recombiner feeds one input 
branch of a second recombiner 27 whose other branch is fed by the other 
output of the first two-way splitter 21 having first passed through a 
phase retarding element 28. Phase retarding elements 24 and 25 are driven 
from a microwave source (not shown in FIG. 2) with a 90.degree. microwave 
phase shifter 29 in one of the drive paths so that the drives are in phase 
quadrature. This makes that part of the circuitry from the two-way 
splitter 22 to the recombiner 26 a straight optical analog of a 
conventional single-side-band modulator, with the phase retardation of 
element 23 being set to a fixed value that compensates for any optical 
phase mismatch at recombiner 26 attributed to differences in optical path 
length in the two branches in the absence of any modulation applied to 
elements 24 and 25. The phase of the microwave modulation on the optical 
carrier emerging from recombiner 27 is controlled by the optical phase 
retardation introduced by phase retarding element 28, with an optical 
frequency phase change of x.degree. at element 28 producing an equivalent 
phase change of x.degree. in the microwave frequency modulation of the 
output from recombiner 27. 
FIG. 3 illustrates an implementation of the alternative phase locked loop 
controlled microwave modulation system. An injection laser 30 is driven by 
frequency stabilization control circuitry 31 to provide a frequency 
stabilized optical output at a frequency f.sub.1, which is launched into 
an optical waveguide 32. This light is heterodyned in a directional 
coupler 33 with light from a second injection laser 34 operating at a 
frequency f.sub.2. The difference frequency (f.sub.1 -f.sub.2) is the 
frequency of the required microwave modulation to be impressed on the 
optical carrier. For this purpose the second laser 34 is driven by 
frequency control circuitry 35 which derives its control signal from a 
phase sensitive detector 36 fed with a first signal from a microwave 
frequency local oscillator 37 operating at the desired modulation 
frequency and a second microwave frequency signal derived by detecting a 
portion of the modulated optical carrier. For this purpose a second 
directional coupler 38 is used to tap off a small proportion of the 
optical power. This is fed to a detector 39 whose output is fed to the 
phase sensitive detector 36. Normally the same local oscillator 37 will be 
used for driving each modulator of the array, and hence a microwave 
frequency phase shifter 300 may be included between the local oscillator 
37 and the phase sensitive detector 36 to control the phase of the 
microwave modulation of the optical carrier. FIG. 3 also shows an optical 
switch 301 controlled by a pulse timer 302 for switching the modulation on 
and off. 
FIG. 4 is a schematic representation of the optical link 12 between the 
central processor 10 and one of the antenna elements. the first part of 
this link is constructed in an integrated optics subsystem 40 which may be 
formed integrally with part of the central processor, while the remainder 
of the link is provided by a length 41 of a single mode optical fiber. The 
integrated optics subsystem has a directional coupler 42 with one port 
connected to receive light from the central processor, another port 
connected to the optical fiber 41 and the remaining ports connected to 
photodiodes 43 and 44. Diode 44 receives a portion of the light 
transmitted from the central processor to the antenna element 11. The 
coupling of the fiber 41 to the antenna element is deliberately designed 
to produce a reflection, and may incorporate a partial reflector (not 
shown) at this point. The reflected signal returns to the integrated 
optics subsystem 40 where a portion of it is coupled to diode 44. The 
relative phases of the microwave frequency signal outputs from the two 
diodes 43 and 44 therefore depends upon the optical path length of the 
link. These signals are therefore fed to a phase sensitive detector 45 to 
provide a control signal on line 46 which is used either to regulate the 
optical path length of the optical link so as to maintain a substantially 
constant predetermined value for the microwave modulation frequency phase 
shift introduced by the link, or to compensate for this phase shift in the 
control of the phase of the modulation applied to the link by the central 
processor. 
One way of maintaining a substantially constant optical path length for the 
link is to include in the optical path a switching network such as that 
illustrated at 47 by which additional lengths of optical waveguide can be 
electrically switched into the optical path. In the network shown 
schematically at 46 there are five four-port optical waveguide switches 
48a to 48e, and four waveguide `loops` introducing respectively additional 
optical path lengths of 8s, 4s, 2s and s. Switch 48a is controllable to 
direct light from the central processor either into the 8s loop or into 
its shunt. Switch 48b is controllable to direct light from the 8s loop or 
its shunt into the 4s loop or its shunt, and so on. Switches 48a and 48e 
may be realized by single electrically switched directional couplers. 
Greater flexibility is however required for switches 48b, 48c and 48d, 
which can be realized by tandem pairs of electrically switched directional 
couplers. 
The alternative approach of using the signal on control line 46 to 
compensate elsewhere for the microwave modulation frequency phase shift 
introduced by the link between the central processor and the antenna 
element is simpler to implement when using the type of modulator described 
previously with reference to FIG. 2. In this instance the signal can be 
applied directly to the phase retarding element 28, or to a further phase 
retarding element (not shown) in tandem with element 28, because at this 
point a change in optical phase will produce an equivalent change in phase 
of the microwave frequency modulation. When using the type of modulator 
described previously with reference to FIG. 3 the control signal is 
applied to the microwave frequency phase shifter 300. 
It will be noticed that the particular phase delay monitoring technique 
exemplified with reference to FIG. 4, being dependent upon a comparison 
between the phase of a launched signal and that of a reflected signal, 
actually measures the phase delay involved in a double transit of the link 
and thus leaves an unresolved ambiguity in the measure of the phase delay 
introduced by a single transit. (A single transit phase delay of x.degree. 
will provide a double transit phase delay of 2x.degree., and the same 
double transit phase delay will also be provided by a single transit phase 
delay of (x+180).degree..) 
In practice however this need not be a problem if the system is chosen so 
that the links are all dimensioned to provide substantially the same phase 
delay in the first instance, and the design ensures that the maximum range 
of the environmentally induced excursions lies safely beneath the 
180.degree. ambiguity limit. 
Turning attention now to the antenna element 11 whose structure is depicted 
in FIG. 5, the main consideration, assuming the correct microwaved phase 
has been delivered by the link to a photodetector 50, is to eliminate 
variations in phase through a power amplifier stage 51. This is achieved 
by inserting an electrically controlled microwave frequency phase shifter 
52 between the output of the photodetector 50 and the input of the 
amplifier 51. Some of the signal output from the photodetector is tapped 
off and fed to a phase sensitive detector 53 where its phase is compared 
with that of power tapped from the output of the amplifier to produce the 
requisite control signal for controlling the phase shifter 52. 
Although the foregoing specific description with reference to the drawings 
has referred exclusively to phased array radars of the transmission mode 
type, it will be clear that a receive mode type phased array radar 
requires substantially the same information to be fed to the antenna 
elements from the central processor as is required by the elements of the 
transmit type radar. Hence it will also be clear that the present 
invention is also applicable to receive type optically controlled phased 
array radars, though many of the components of the antenna elements will 
be entirely different in order to suit the quite different function of 
those elements. Typically in the antenna element 11' (FIG. 6) of a receive 
type radar the incoming radar signal will be fed to a mixer/frequency 
changer 60 where it is compared with a signal from an amplifier/oscillator 
61 whose frequency and phase is controlled by the output of a photodiode 
62. The amplifier/oscillator 61 may incorporate the same type of phase 
locked loop phase control as described previously with reference to the 
amplifier 51 of the antenna element of FIG. 5. The signal output of the 
mixer/frequency changer 60 may be at base band or at an intermediate 
frequency considerably lower in frequency than the microwave operational 
frequency of the radar and hence its transmission back to the central 
processor does not present the problems of the transmission of the 
microwave frequency control signals from the central processor to the 
antenna elements. Thus there is no need to impress it upon an optical 
carrier. 
It will also be apparent that the invention is also applicable to an 
optically controlled phased array radar operable alternately in transmit 
and receive modes.