Abstract:
A method of beamforming a radiofrequency array having multiple antenna elements is provided. The method includes transmitting two or more sub-beams of a modulated light beam through a switched fabric, using wavelength switching to designate a respective path through the switched fabric for each sub-beam, and converting each sub-beam to a driving signal for one or more of the antenna elements or to a received signal from one or more of the antenna elements. Each path through the switched fabric has a selected cumulative true time delay.

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to phase control in radiofrequency transmission and reception using arrayed antenna elements. 
       ART BACKGROUND 
       [0002]    It has long been known that arrays of multiple antennas for radar and other radiofrequency transmission and reception offer certain advantages over single-element antennas, such as enhanced spatial selectivity, signal gain, and beam steerability. These and other advantages are greatest when there is precise control over the phases of the antenna elements; i.e., over the relative phase of the wavefront leaving each transmissive element, or of the relative phase, at the detector, of the signal collected by each receptive element. 
         [0003]    Conventional methods of phase control include electronic methods based on the transfer function of a reactive circuit, and delay-based methods that use variable-length delay lines to adjust the phase of each radiofrequency (RF) feed to an antenna element. Neither of these approaches is perfectly adapted for all applications. For example, one drawback of electronic methods is that they are limited in bandwidth. One drawback of delay-based methods is that precise, tuneable phase control is difficult to implement. 
         [0004]    Accordingly, there remains a need for techniques of phase control that combine high precision with high bandwidth. 
       SUMMARY OF THE INVENTION 
       [0005]    We have developed a technique based on optical delay that can provide both high precision and high bandwidth. 
         [0006]    In an embodiment adapted for transmission, a light beam is modulated with an RF signal. The light beam is divided into a plurality of beamlets and distributed through an optical network to an array of transmission elements. At each transmission element, at least one beamlet is converted to an RF signal and transmitted. 
         [0007]    The optical network includes wavelength-selective elements coupled to optical delay lines. The optical network uses wavelength based routing to deliver each beamlet through a designated amount of delay to a designated transmission element. 
         [0008]    In an embodiment adapted for reception, an incoming radiofrequency signal is converted to an electric signal at each of a plurality of reception elements. At each reception element, an optical beamlet is modulated with the electric signal. The respective beamlets are combined into a composite optical signal as a result of propagating them through an optical network of the kind described above. The composite optical signal is detected and further processed, for example by demodulation. While propagating through the optical network, the beamlets are subjected to wavelength based routing to deliver each beamlet through a designated amount of delay before it is combined into the composite optical signal. 
         [0009]    An embodiment of the invention comprises an optical network of the kind described above, as adapted for transmission, reception, or both transmission and reception. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram of a wavelength-selective optical delay device of the prior art. 
           [0011]      FIG. 2  is a schematic diagram of a wavelength-switched optical delay network according to an embodiment of the invention. 
           [0012]      FIG. 3  is a schematic diagram of an optical delay network having three stages, according to an embodiment of the invention. 
           [0013]      FIG. 4  is a schematic drawing of a hypothetical array having eighteen antenna elements. 
           [0014]      FIG. 5  is a partial schematic drawing of a beamforming radiofrequency device including a delay network that includes two stages of frequency-switched optical delay and one stage of electronic phase shifting, operative in transmission. 
           [0015]      FIG. 6  is partial schematic drawing of a beamforming radiofrequency device similar to that of  FIG. 5 , but operative in reception. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A type of optical network useful for the practice of the invention is a network in which passive wavelength-selective optical delay (WSOD) devices are combined with wavelength-shifting devices to provide wavelength-switched optical delay. Such wavelength-switched optical delay networks are known. One example is described in J. D. LeGrange et al., “Demonstration of a time buffer for an all-optical packet router,”  J. Opt. Networking , vol. 6, no. 8 (August 2007) 975-982 (LeGrange 2007). 
         [0017]    With reference to  FIG. 1  one example of a WSOD device  10  as described, e.g., in LeGrange 2007 is a wavelength division multiplexing (WDM) device having a total of N input ports and M output ports. (As will be seen, it will often be advantageous for the port arrangement to be symmetrical, such that N=M.) For purposes of illustration, a total of three input ports and three output ports is shown in the figure. These numbers should not be taken as limiting. Values for N and M of 100 or even more are well within current technical capability. 
         [0018]    WDM device  10  includes an arrayed waveguide grating (AWG)  20  on the input side, and an arrayed waveguide grating  25  on the output side. Each AWG has a number N′ of input ports  30 ,  35  and a number M′ of output ports  40 ,  45 . (In the view of  FIG. 1 , the input ports of AWG  20  are shown as identical to the input ports of device  10 , and the output ports of AWG  25  are shown as identical to the output ports of device  10 . This is by way of illustration and is not meant to exclude other possible arrangements.) 
         [0019]    Although not essential, it will often be advantageous for gratings  20  and  25  to be symmetrically arranged, such that the number of input ports of AWG  20  is matched to the number of output ports of AWG  25 , and likewise that the number of output ports of AWG  20  is matched to the number of input ports of AWG  25 . In the discussion below, we will assume the same number N of ports for the input and output sides of both AWG  20  and AWG  25 . Accordingly,  FIG. 1  shows N=3 input and output ports for each of AWGs  20  and  25 . As explained above, this choice for N is illustrative only, and not intended to be limiting. 
         [0020]    As those skilled in the art will understand, an AWG functions as a two dimensional diffraction grating. As such, it can convert spectral routing to spatial routing. A typical AWG is made from two interconnected star couplers. The connection between the star couplers is made by an array of waveguides having linearly increasing lengths. 
         [0021]    Due to the diffractive behavior of the arrayed waveguides, a suitable optical input will result in light emerging from each waveguide at a particular wavelength. The wavelengths are determined by the lengths of the respective waveguides, in accordance with the laws of optical interference. The length increments between waveguides are typically set to provide a phase shift of 2πA radians from each waveguide to the next, where A is the diffractive order of the grating. 
         [0022]    More particularly, an input signal applied to a given input port will be mapped to different output ports with respective shifts of wavelength. Accordingly, a signal having a given wavelength can enter the AWG on any input port and be routed to a unique output port determined by the given wavelength and by the identity of the input port. 
         [0023]    Known designs for the star couplers and waveguide grating enable the AWG to be used as a spectral multiplexer or demultiplexer with minimal crosstalk between channels. The AWG may be used over multiple grating orders, thereby extending the usable wavelength range and making it possible to form multiple beams simultaneously. One source of further information on the AWG is C. R. Doerr, “Planar Lightwave Devices for WDM” in  Optical Fiber Telecommunications , volume IVA, edited by Ivan Katninow and Tingyc Li, (Academic Press, New York, 2002), pp 405-476. 
         [0024]    Turning back to  FIG. 1 , it will be seen that each of output ports  40  of AWG  20  is coupled to a corresponding one of input ports  35  of AWG  25 . (It should be noted that although the figure shows all of the available ports being used in this manner, it is also possible to select only some of the available ports for such use.) Although not essential, it will often be advantageous for each of output ports  40  to be coupled to the like-numbered one of input ports  35 , as illustrated in  FIG. 1 . The reason is that if the AWGs are coupled in an arrangement with mirror symmetry, then (for a given operating wavelength) light that is injected at a particular input port  30  will exit from the like-numbered output port  45 . 
         [0025]    Each coupling between an output port  40  and an input port  35  is made through a respective optical delay element  50 . Typically, each of the optical delay elements  50  will provide a different amount of delay. 
         [0026]    In view of the foregoing, it will be understood that an AWG arrangement such as that shown in  FIG. 1  provides wavelength-selectable delay. That is, an optical signal injected at a particular one of input ports  30  (of AWG  20 ) will exit at the corresponding output port  45  (of AWG  25 ), irrespective of the input wavelength. However, the input wavelength will determine the output port  40  of AWG  20  to which the signal is mapped. This, in turn, will determine which of the delay elements  50  is used to couple the signal from AWG  20  to AWG  25 . 
         [0027]    It should be noted that if the mapping between input and output ports of each of the AWGs is different for each operating wavelength, then it may be possible to apply input signals simultaneously to all of the input ports  30  without collision. That is, two signals applied to different input ports  30  will be mapped to the same output port  40  only if they are on different operating wavelengths. If they are on different operating wavelengths, they will not affect each other. Similarly, two input signals can be applied to the same input port  30  without colliding if they are on different operating wavelengths. (Although the AWG is described here with linearly incrementing phase and therefore wavelength shifts from channel to channel, it should be noted that in other embodiments, any router design that results in wavelength selection of the output port could be used.) 
         [0028]    Turning now to  FIG. 2 , an example of a wavelength-switched optical delay network includes a master oscillator  110 , which is typically a laser oscillator. The master oscillator produces light beam  120 , which is modulated in modulator  130  with the RF signal from RF source  140 . The modulated light beam is split by splitter  150  into a plurality of beamlets  160 . Each of the beamlets is subjected to a wavelength shifter  170 , controlled by control unit  175 , which places the beamlet on one of the operating wavelengths. The beamlet is then applied as input to a respective one of input ports  180  of WSOD device  190 , which may, e.g., be similar to device  20  of  FIG. 1 . As explained above, the light applied to each of input ports  180  will emerge at a corresponding one of output ports  200 ; having in the meantime been subjected to a discrete amount of delay determined by the applicable input port and operating wavelength. 
         [0029]    The light emerging from each of output ports  200  may be extracted from the optical delay network for further processing and utilization as will be described below, or it may be directed to a next stage of the optical delay network, where it is again split in an optical splitter (not shown), and each output from the splitter is subjected to a further wavelength shifter (not shown) and injected at an input port of a further WSOD device, such as device  210  of the figure. 
         [0030]      FIG. 3 , for example, shows an optical delay network having three stages. If the network is operated in transmission, source  300  injects a radiofrequency modulated optical beam into the first stage. If the network is operated in reception, a composite optical signal (described in more detail below) is extracted from the first stage and directed to receiver  310  for, e.g., detection which converts the signal to the electrical domain, followed by demodulation and further processing. The network as shown in the figure is switchable between transmission and reception modes. In other implementations, the network may be dedicated to one mode or the other. 
         [0031]    Each stage of the network of  FIG. 3  consists of one or more sub-networks. As shown, the first stage has one sub-network  320 , and the second and third stages each have three subnetworks, respectively 331, 332, 333, and 341, 342, 343. These numbers of subnetworks have been chosen solely for purposes of illustration and should not be understood as limiting. 
         [0032]    As shown in inset  350 , each sub-network includes an optical splitter  351 , a set of wavelength-shifters  352  subject to a control unit (not shown), and a WSOD device  353 . 
         [0033]    In the design of antenna arrays, it is often advantageous to organize an array having many elements into a plurality of sub-apertures that are organized hierarchically, so that a sub-aperture at a higher level of organization includes a plurality of sub-apertures at a lower level of organization. Advantageously, each of the sub-networks at each stage of the network is associated with a respective sub-aperture of the array. To illustrate this concept,  FIG. 4  provides a schematic drawing of a hypothetical array having eighteen antenna elements. With reference to insets  360 - 363  of  FIG. 3 , the overall array (inset  360 ) may be subdivided into three sub-apertures, each containing six elements, as shown in inset  361 . Each of these may be further subdivided into two sub-apertures, each containing three elements, as shown in inset  362 . Each of these may be further subdivided into three sub-apertures, each containing a single element, as shown in inset  363 . These subdivisions are purely illustrative and not meant to be limiting. 
         [0034]    Turning again to  FIG. 3 , it will now be understood that each stage illustrated in  FIG. 3  corresponds to one level in the hierarchical division of overall aperture  360  into sub-apertures, and each of the subnetworks shown in the figure corresponds to a respective sub-aperture. Accordingly, stage  1  provides a respective coarse amount of delay to each of the first-level sub-apertures, one of which is shown as shaded in inset  361 . For each of the first-level sub-apertures, stage  2  adds a respective finer amount of delay to each of the second-level sub-apertures, one of which is shown as shaded in inset  362 . For each of the second-level sub-apertures, stage  3  adds a respective still finer amount of delay to each of the third-level sub-apertures. A similar architecture is readily extended to further levels and can be used to provide controllable delay to large arrays of antenna elements, numbering in the hundreds or even in the thousands. 
         [0035]    As noted earlier, two optical signals can enter or exit the same ports of a WSOD device without colliding if they are in different wavelength channels. As a consequence, it may be possible in some implementations to use the optical delay network, or a portion of it, for delay processing of two or more simultaneous signals carrying independent information, if the respective signals are placed on mutually orthogonal sets of operating wavelengths. 
         [0036]    For example, those skilled in the art will appreciate that one of the features of an AWG device is the free spectral range (FSR), having the property that if signals of two wavelengths separated by the FSR are applied to the same input port of an AWG demultiplexer, they will be directed to the same output port. Thus, the FSR defines a (weakly wavelength-dependent) periodic band structure for the responsive behavior of an AWG device. Mutually orthogonal sets of operating wavelengths can be selected on the basis of this band structure. 
         [0037]    Similarly, it may be possible to use the same WSOD device to simultaneously perform the delay processing of an optical signal for two different sub-apertures, if the sets of operating wavelengths corresponding to the respective sub-apertures are chosen appropriately. This may be advantageous if, for example, the various sub-apertures differ only in their corresponding coarse amounts of delay, but add to the coarse delay the same increments of fine delay. Thus, the total amount of hardware could be reduced by reusing one or more of the WSOD devices that provide fine delay. 
         [0038]    It should be noted that if one or more WSOD devices are reused for multiple independent signals or for multiple sub-apertures (at the same level), it will generally be necessary to include one or more wavelength demultiplexers in the network for separating the respective mutually orthogonal sets of operating wavelengths after the last reused device. 
         [0039]    As noted above, the spatial selectivity and beam steerability achievable using arrays of multiple antennas are highly advantageous for radar, communications, and other radiofrequency applications. The signal processing that underlies these capabilities of antenna arrays is beamforming, i.e., the coherent combination of the signals going to or from the respective antenna elements. 
         [0040]    Beamforming is typically achieved using electronic phase shifters, which are well known. However, the performance of electronic phase shifters is frequency-dependent. For that reason, beamforming is disadvantageously limited in bandwidth when it is performed solely by using electronic phase shifters. 
         [0041]    In accordance with the invention, a wavelength-switched optical network such as that described above is used to provide true time delay for at least part of the beamforming. That is, the timing of the phase fronts propagating from individual antenna elements during operation in the transmission mode, or the effective (from the viewpoint of the receiver) timing of the phase fronts propagating toward the individual antenna elements during operation in reception mode, is controlled by optical delay in the signals that the optical delay network directs to or from the antenna elements. Because the optical delays are not affected by the frequencies used for radiofrequency modulation, bandwidths can be achieved that are much greater than those achievable using only electronic phase shifters. 
         [0042]    We believe that because of the precise tolerances achievable in the fabrication of optical delay elements, true time delay can be used to provide controllable delay increments over an extremely wide dynamic range, extending from microseconds or more, down to 0.01 ns or even less. In typical switched fabrics of the kind described here, true time delay provided via optical delay elements will be most useful in the range from 0.1 ns to 100 ns. For the finest phase control at the last stage of the network (i.e., at the stage nearest the antenna elements), we believe it will be most advantageous to use electronic phase shifters. (It should be noted in this regard that the performance of electronic phase shifters is limited by the product of bandwidth times interelement separation. Thus, the electronic phase shifters are most advantageous at the finest level of delay processing, where the corresponding antenna elements are typically clustered within a small spatial volume.) 
         [0043]    For example,  FIG. 5  shows a portion of a beamforming radiofrequency device, including a delay network that includes two stages of frequency-switched optical delay and one stage of electronic phase shifting. Elements common with  FIG. 3  are indicated using like reference numerals. The device is operating in transmission mode. 
         [0044]    As seen in the figure, the coarser two stages of delay processing are done in the optical domain by subnetworks  320  and  330 . However, the finest stage of delay processing, in which the delay increments are mapped to individual antenna elements, is performed in the electrical domain. Accordingly, each output from stage- 2  delay subnetwork  330  is directed to an optical-to-electronic (O/E) converter  500 . Devices for performing O/E conversion using high-speed photodiodes, for example, are well known and need not be described here in detail. (Herein, devices for optical-to-electronic conversion as well as devices for electronic-to-optical conversion will be collectively referred to as “optoelectronic devices”.) 
         [0045]    The electrical output from O/E converter  500  is directed to electronic phase-shifting device  505 . Electronic phase shifters are well known and need not be described here in detail. 
         [0046]    The output from phase shifter  505  is directed to radiative antenna element  515 , from which it is transmitted as electromagnetic radiation. The signal path from O/E converter  500  to radiative element  515  will typically include one or more electronic amplifiers, which have been omitted to simplify the drawing. 
         [0047]      FIG. 6  shows an arrangement similar to that of  FIG. 5 , but operating in reception mode. A plurality of antenna elements having radiofrequency absorbers (which may of course also function as radiators)  605  are grouped into a sub-aperture by stage- 2  delay network  630 . The output of each absorber  605  is directed to a respective electronic phase shifter, where it receives a line increment of phase adjustment (which is equivalent to a fine increment of delay). The output of each phase shifter is directed to a respective electronic-to-optical (E/O) converter  600 . The outputs of the electronic-to-optical (E/O) converters  600  are directed to stage- 2  delay sub-network  630 , where they each receive a coarser increment of delay. The signal path between absorber  615  and sub-network  630  will typically include one or more electronic amplifiers, which have been omitted to simplify the drawing. 
         [0048]    In sub-network  630 , after each input signal (i.e., each signal corresponding to one of the individual absorbers  615 ) has been subjected to optical delay processing, it is shifted onto a common operating wavelength for output from sub-network  630 . Accordingly, the output from sub-network  630  is a composite output signal on one operating wavelength. (As noted above, parallel operation is possible in two or more sets of mutually orthogonal operating wavelengths.) 
         [0049]    In a like manner, the outputs from a plurality of stage- 2  delay networks  630  are collected by stage- 1  delay sub-network  620 , subjected to still coarser increments of delay, shifted onto a common operating wavelength, and combined into a composite optical signal. The composite optical signal output from stage- 1  delay network  620  is directed to receiver  610  for detection and demodulation or other further processing. 
         [0050]    By way of example, the WSOD devices in a network having two stages of optical delay might each include 100 waveguides of various lengths to serve as the delay elements. Thus, for example, the coarse WSOD might have waveguides which span 100 ns of delay in 1 ns increments, and the fine WSOD might have waveguides which span 1 ns of delay in increments of 0.01 ns. As noted, electronic phase shifters may be used to provide still liner increments of delay. 
         [0051]    With further reference to  FIGS. 5 and 6 . O/E conversion. e.g. in converter  500  and receiver  610 , is readily carried out using well-known optoelectronic devices such as high-speed photodiodes. Conversely, E/O conversion, e.g. in converters  600 , may be carried out by well-known techniques such as using a lithium niobate modulator or an electroabsorption modulator to modulate an optical carrier provided by a low-power continuous wave laser. 
         [0052]    The optical signal source, such as source  300 , advantageously uses a modulated high-power laser, or alternatively a modulated low-power laser whose output is subjected to optical amplification. 
         [0053]    The wavelength-shifting devices may use any of various well-known technologies. One example is provided by a silicon optical amplifier (SOA) wavelength converter. A second example is provided by an electroabsorption modulator (LAM) device. 
         [0054]    The EAM device can be used as a wavelength converter by converting the optical data signal to an RF signal via a high speed photodiode. The electrical output of the photodiode is amplified by RF amplifiers and then applied to the EAM. The data modulation is then applied to CW light from a tunable laser transmitted through the LAM, thereby transferring the data modulation to the wavelength of the CW light.