WAVELENGTH DIVISION MULTIPLEXING METHODS AND APPARATUS FOR CONSTRUCTING PHOTONIC BEAMFORMING NETWORKS

Abstract: Methods and apparatus for constructing phased array antenna beamforming networks are provided, that allow to scan multiple beams and select appropriate sets of delaylines simultaneously. The beamforming networks disclosed herein generate less losses than conventional ones and in some caes, do not require active switching, making them completely passive. Three main methods are comprised in the invention: (1) laser wavelength hierarchies, (2) arrangements of Wavelengths Division Multiplexing (WDM) components, (3) re-use of laser wavelengths. Multiple laser wavelengths are arranged in groups and subgroups (wavelength heirarchies) in the wavelength domain. By switching between these wavelength groupies, the arrangements of WDM components disclosed herein enable the beamforming network to direct the beam signals to the proper time delay lines, and to differentiate multiple beams. The method of laser wavelength re-use permits to significantly reduce the number of wavelength utilized, and thus to limit them to the standard wavelengths specified by the ITU.

Description: WAVELENGTH DIVISION MULTIPLEXING METHODS AND APPARATUS FOR CONSTRUCTING PHOTONIC BEAMFORMING NETWORKS Field of the invention This invention relates to the field of phased array antennas that are controlled by networks of optical fibers and other photonic components, such as photonic beamforming networks. More specifically, it relates to methods for constructing low-loss, passive photonic beamforming networks. Background of the Invention Phased array antenna systems are widely used in radar, electronic warfare and high data-rate communications applications. They are sometimes controlled by networks of optical fibers and other photonic components such as lasers, fiber splitters/combiners, and photodetectors. These control networks mainly utilize delay line networks such as the ones shown in Figures la and lb. There are two types of delay line networks which differ in the way time delays are implemented. In the network switched architecture of Figure la, of which the Rotman lens is an example, entire networks of delay lines are switched in/out by a single switch. In the in-line switched arcliitecture of Figure lb, there are several delay lines within each fiber as well as a switch to select them. If F is the number of fibers and P is the number of delay states, the network switched architecture requires one switch with P states, and the in-line switched architecture F switches with P states. Both require IxP splitters to access P delay states, and Fxl combiners to vector sum the outputs. For both types of networks, the signal passes through one IxP splitter, one switch, and one Fxl combiner, so the losses are expected to be comparable. The number of photodetectors required in the network can be a major cost so it is desirable to minimize it. To achieve this, one can place a single photodetector at position A in Figures la and lb, after the Fxl combiner which vector sums the signals on the fibers. However, if all fibers carry the same optical wavelength, as it is the case in most prior art systems, the different signals will interfere and unwanted noise will appear on the detected carrier envelope. In order to avoid this optical coherence problem, photodetectors can be placed at positions B so that photodetection occurs prior to summation, and the optical carriers never interact. However, then a large number of photodetectors is then required and cost is greatly increased. To solve optical coherence problems, while still minimizing the number of photodetectors required, this invention, in one of its aspects, utilizes multiple optical wavelengths. This reduces photodetector count from FxP to 1 in the network switched case (see Figure la), and from F to 1 in the in-line switched case (see Figure lb). Furthermore, in accordance with another aspect of this invention, the lossy splitters/ combiners that form the actively switched prior art networks of Figures la and lb, are replaced by a passive Wavelength Division Multiplexing (WDM) network. The IxP splitters and Fxl combiners are replaced with WDMs, and the functions performed by active switches are realized by separating wavelength groups with passive WDMs. Optical losses in a lxN WDM are less than in a lxN splitter or combiner for N > 6, so in most practical cases losses can be substantially reduced. Prior art photonic networks require active switching, the use of a large number of photodetectors, and inclusion within the network of lossy splitters and combiners. In many cases the prior art also requires specialized or unique optical components. Prior photonic beamforming art such as described in US patent 5,861,845 (Wideband Phased Array Antennas and Methods) alludes to using multiple wavelengths to avoid optical coherence effects, but losses are still high in the combiners which vector sum the optical signals. International Patent application PCT/US00/23200 (Phased Array Antenna Beamformer), published after the priority of the present application, describes a passive receiver network for multiple beams which employs WDMs for beam scanning and delay line selection. However, it does not address optical coherence problems, and uses three- dimensional fiber optics based delay line networks (fiber Rotman lens) which are hard to fabricate. It also utilizes lossy combiners for signal summation. In one aspect, the present invention addresses and solves these problems in a simple, unified manner, and can be implemented using standard ITU (International Telecommunication Union) components developed commercially for fiber optics data networks, and two- dimensional SOS (Silicon on Sapphire) fabrication techniques. Summary of the Invention In accordance with one aspect of the present invention, a number N of incoming RF wavefronts are simultaneously received by an antenna array. Laser light is amplitude modulated to provide B=N synthesized optical beams. The synthesized optical beams are mixed with the incoming electrical wavefronts by optical modulation. The resultant N optical wavefronts, all traveling through common waveguides, are each directed to a predetermined set of delay lines, and subsequently separated and channeled into N separate waveguides. The original incoming wavefronts carried by the synthesized optical beams are now differentiated and can be photodetected and analyzed separately. In accordance with another aspect, the present invention provides novel ways to perform these functions utilizing photonic beamforming networks. It provides methods for constructing low-loss, completely passive, high performance photonic beamforming networks that can simultaneously control beam scanning and delay line selection for multiple beams. The invention comprises, in one aspect thereof, three main methods which include : (1) laser wavelength hierarchies, (2) arrangements of wavelength division multiplexing (WDM) components, and (3) re-use of laser wavelengths. Multiple laser wavelengths are arranged in groups and subgroups (wavelength hierarchies) in the wavelength domain. By switching between these wavelength groupings, the arrangements of WDM components proposed herein enable the beamforming network to direct the beam signals through the proper time delay lines, and to differentiate multiple beams. No switching occurs within the network itself, only at the controlling lasers, and the network is completely passive. Furthermore, signal routing, beam differentiation, and beam vector summation occur with minimal losses due to the arrangements and choice of WDM components and interconnections. The invention also minimizes the number of photodetectors required, and only one photodetector per beam is needed in its most powerful form. The method of laser wavelength re-use permits significant reduction in the number of wavelengths required for the beamformer to function. This allows the wavelengths to be limited to the standard ones specified by the International Telecommunication Union, even with phased array antennas that contain a very large number of elements. Another aspect of the invention, a non-passive, output-switched network, that minimizes the number of wavelengths required is also disclosed. Brief Description of the Drawings Figures la and lb depict the two types of delay line networks conventionally used, Network Switched and In-Line Switched, respectively. Figure 2 is a schematic block diagram overview of an embodiment in accordance with the present invention applied to a Heterodyne True-Time-Delay system. Figure 3 a is a table illustrating the wavelength hierarchy required for the embodiment of Figure 2 Figure 3b is an example of the laser wavelengths activated when beam 1 is directed to port A and beam 2 is directed to port B. Figure 4 shows how a 1x3 WDM conceptually operates. Figure 5 is a block diagram depiction of box 1 from Figure 2. It shows the specific arrangement of WDMs and interconnections required to perform the functions discussed in conjunction with Figure 2 and Figure 3. Figure 6 is a variation of the network illustrated in Figure 5, where WDMs have been replaced with fiber splitters. Figure 7 is a variation of the network illustrated in Figure 5, where WDMs have been replaced with fiber combiners. Figure 8 is another variation of the invention of Figure 5, utilizing port splitters and electrical switches to reduce the number of wavelengths required. Figure 9 is another variation of the invention of Figure 5, where port and beam demultiplexers have been replaced with port-beam demultiplexers. Figure 10 is another variation of the invention of Figure 5, where photodetectors have been placed at the outputs of each fiber multiplexer, and the signals are electrically combined. Figure 11 is a table illustrating the wavelength hierarchy required in conjunction with the embodiment of Figure 8. Figures 12, 12a and 12b depict a three-dimensional structure constituting a Network Switched delay line network for the case of a 4x4 fiber array feeding four delay ports. Figure 13 is a block diagram of the embodiment of Figure 5, where WDMs have been rearranged to place all cross-overs in one section of the network. Figure 14 shows how the 4x4 fiber array of Figures 12a and 12b can be divided into two 2x4 arrays in conjunction with the wavelength re-use technique. Figure 15a shows the WDM architecture to be employed with the wavelength re-use method. Figure 15b is a simplified block diagram of Figure 15a, and represents a network cell. Figure 16 shows how the network cells of Figure 15b would be connected in conjunction with the wavelength re-use method, for the case of a 4x4 fiber array. Figure 17 is a table illustrating the wavelength hierarchy required in conjunction with the embodiment of Figures 18 and 19. Figure 18 is another variation of the invention where WDM filters are used for port and beam operations only BxPxF photodetectors are required. Figure 19 is yet another variation of the invention where WDM filters are used for port and beam operations only as in Figure 18, but requiring only BxF photodetectors because of the use of optical switches. Detailed Description The presently disclosed beam forming networks utilizes simple Wavelength Division Multiplexer (WDM) technology in a number of ways. Figure 4 shows, for N=3, how a lxN WDM operates. A single common fiber 1 carries N wavelengths to a dispersive element 2 which spatially separates these wavelengths, and directs them to N single wavelength fibers 3. This device can be run in reverse as an Nxl combiner. In this mode, it is assumed that if the wrong wavelength is in one of the single wavelength fibers, it does not couple into the common fiber. The wavelength bands passed by the WDM are adjustable during fabrication. Referring to Figure 2, a schematic block diagram overview of an embodiment of the invention applied to a Heterodyne True-Time-Delay system is shown. In this particular embodiment, the system has the capability to receive B=2 incoming wavefronts and direct each one to a separate output utilizing B=2 synthesized laser beams (Beam 1 and Beam 2), F=4 optical fibers (a, β , γ, δ) and P=3 ports or delay lines (A,B,C). For ease of understanding, it is further assumed that the system is set to direct beam 1 to port A and beam 2 to port B. Wavefronts 20 and 21 at respective frequencies f 01 and f 02 are received by antenna array 22. Wavefronts 20 and 21 are detected and then travel down a set of feed lines 23. Analog or digital beam-forming circuit 24 generates local oscillator wavefront 26. Wavefront 26 is in the electrical domain at RF frequency f L01 and is defined by the relative phases of frequency f L01 in four electrical cables or waveguides L01 , β L01 , γ L01 , δ L01 . These electrical signals are used by port/beam selection block 28 to intensity modulate four lasers whose output intensities then bear the same relative phases as the local oscillator wavefront 26. These four optical signals are then output to four optical fibers β γ l3 ό j where the relative phases of the light intensity among the fibers define an optical wavefront 26A. Symmetrically, analog or digital beam-forming circuit 25 generates local oscillator wavefront 27 in the electrical domain at RF frequency f L02 and is defined by the relative phases of 1 - frequency f L01 in four electrical cables or waveguides L02 , β L∞ , γ L02 , δ LQ2 . Port/beam- selection block 29 converts wavefront 27 to optical wavefront 27A and outputs, to each of its four output fibers ( 2 , 2 , γ 2 , δ 2 ), a single wavelength according to the specific delay line (port) desired for beam 2. WDM 30 acting in combine mode, directs laser light from j and 2 into fiber , laser light from β 1 and β 2 into fiber β , laser light from γ j and γ 2 into fiber γ, and laser light from δ 1 and δ 2 into fiber ό . Thus, each optical fiber , β , γ and δ , foπning set of feed lines 33 , carries two wavelengths. This results in two synthesized optical wavefronts 31 and 32, traveling through set of fiber lines 33 at local oscillator frequencies f L01 and f L02 respectively. Incoming wavefront 20 and synthesized wavefront 32 intersect one another in mixers 34 and line by line mixing of the two wavefronts occurs. Such mixing up-converts or down-converts the f Q1 frequency to intermediate frequency f jpj associated with a wavefront 36. Similarly, wavefront 21 and synthesized wavefront 31 intersect one another in mixers 34 and line by line mixing of the two wavefronts produces intermediate frequency f IF2 associated with a wavefront 37. Wavefronts 36 and 37 travel down a set of feed lines 35 and enter WDMs 38. Wavefronts 36 and 37 are each directed to the desired port through the corresponding set of delay lines 39 (port A), 40 (port B) or 41 (port C). In the particular example of Figure 2, WDMs 38 outputs beam 1 to port A and beam 2 to port B. Wavefronts 42 and 43 travel through two sets of delay lines 39 and 40, respectively, and enter WDMs 44. Within WDM 44, wavefront 42 (beam 1), entering through delay lines set 39, is vector summed into a single fiber and directed to photodetecting device 46. An electrical signal S j 48 (corresponding to beam 1) is output by photodetecting device 46 and is sent to a data processing unit. Similarly, wavefront 43, entering through delay lines set 40, is vector summed into a single fiber and directed to photodetecting device 45. Electrical signal S 2 47 (corresponding to beam 2) is output by photodetecting device 45 and is sent to a data processing unit. Thus, incoming wavefronts 20 and 21 which were detected simultaneously have been differentiated by the system forming the present invention, and the information they carry can be processed separately. Although a specific configuration is treated in Figure 2, the same principles hold for, and the invention is applicable to, two-dimensional systems, for any values of B, F, and P, and to other types of be--ιmforming devices such as the Heterodyning Rotman beamformer. Figure 3 a shows the WDM wavelength hierarchy required for the embodiment of the invention shown in Figure 2. For such a system, it is necessary that Nλ = 24 (PxBxF=3x2x4) wavelengths be available to blocks 28 and 29 of Figure 2 (12 for block 28, and 12 for block 29). Block 28 will activate one of wavelength ranges Al, Bl or Cl, depending on which port is selected for beam 1, block 29 will activate one of wavelength ranges A2, B2 or C2 according to which port is chosen for beam 2. Wavelength ranges A 1 , B 1 , C 1 , A2, B2, C2 are each composed of F=4 wavelengths as illustrated in Figure 3 a. Thus, port and beam selection is accomplished by switching groups of F=4 wavelengths for each beam. Figure 3b shows which specific wavelengths need to be activated in order to direct beam 1 to port A and beam 2 to port B. To achieve this, the F=4 wavelengths corresponding to the Al range, namely λAαi , λAβi , λλγi , λAδi , and the F=4 wavelengths corresponding to the Bl range, namely λBα2 , λββ2, λβγ2, λBδ2 are activated. Expansion to include more fibers, beams, or ports is accomplished by adding additional wavelengths to either side of the range shown in Figure 3b. Referring to Figure 5, the specific arrangement of WDM filters and interconnections required to perform the functions discussed in conjunction with Figure 2 and Figure 3a [?], is shown. Figure 5 is a detailed block diagram representation of Box 1 of Figure 2: The light paths for beam 1 going through port A (heavy solid lines) and beam 2 going through port B (heavy dashed lines) are highlighted in the figure. Light in fibers , β, γ, δ, enters a 1x3 port demultiplexer WDMs 60.[?] The input fiber to each of these WDMs carries two wavelengths (one for beam 1 and one for beam 2 corresponding to wavefronts 36 and 37 of Figure 2) and corresponds to common fiber 1 of Figure 4. Each of the three output fibers of each WDM transmits a single wavelength range A, B, or C as defined in Figure 3a. Thus, depending upon which wavelength group Al, Bl, Cl, A2, B2, C2 enters, the WDMs select A, B or C sets of delay lines for beam 1 and beam 2. For example, let us consider fiber and the corresponding WDMα 86 (Figure 5). Two wavelengths (one for beam 1 and one for beam 2) traveling through fiber α enter WDMα . If both wavelengths are in the A wavelength range, one in the A 1 range and one in the A2 range, WDMα will direct both wavelengths to delay line 61 (heavy solid line); If both wavelengths are in the B wavelength range, one in the Bl range and one in the B2 range, WDMα will direct both wavelengths to delay line 62 (heavy dashed line); If both wavelengths are in the C wavelength range, one in the Cl range and one in the C2 range, WDMα will direct both wavelengths to delay line 63 (light solid line). If one wavelength, say λAα, is in the A range (Al or A2) and the other wavelength, say λBα, is in the B range (Bl or B2), then WDMα will direct λAα to delay line 61 and λBαto delay line 62. If one wavelength, say λλ , is in the A range (Al or A2) and the other wavelength, say λcα, is in the C range (Cl or C2), then WDMα will direct λAα to delay line 61 and λcαto delay line 63. If one wavelength, say λBα, is in the B range (Bl or B2) and the other wavelength, say λcα, is in the C range (Cl or C2), then WDMα will direct λBαto delay line 62 and λcαto delay line 63. Note that although WDMα has three output fibers, a maximum of two are active at any given time since only two wavelengths enter the WDM. After passage through delay line set 61, 62 , or 63, light from the four fibers of each delay line set next encounters beam demultiplexer WDM groups 64, 65 or 66. Each of these groups comprises four 1x2 WDMs. Wavelength ranges Al and/or A2 enter WDM group 64 and get separated. Wavelengths in the A 1 range are directed to fiber multiplexer WDM 67, and wavelengths in the A2 range are directed to fiber multiplexer WDM 68. In the same fashion, wavelength ranges Bl and/or B2 enter WDM group 65 to be separated. Wavelengths in the Bl range are directed to fiber multiplexer WDM 69, and wavelengths in the B2 range are directed to fiber multiplexer WDM 70. Lastly, wavelength ranges Cl and/or C2 enter WDM group 66 and get separated. Wavelengths in the Cl range are directed to fiber multiplexer WDM 71, and wavelengths in the C2 range are directed to fiber multiplexer WDM 72. This operation serves to place the beam 1 light on one fiber and the beam 2 light on the other fiber at the output of each 1x2 WDM comprised in WDM groups 64, 65 and 66. Each fiber ' multiplexer WDM 67, 68, 69, 70, 71, and 72 receives light from four input fibers and combines them into a single output fiber. The four input fibers of each fiber multiplexer, each carry the individual wavelengths α, β, γ, δ shown under the Al, Bl, Cl, A2, B2, C2 ranges in Figure 3 a. Combination of the light from the four input fibers by the fiber multiplexers serves to vector sum the envelopes of the of the optical carriers and form the beams. Fiber multiplexers WDMs 67, 69 and 71 direct their single outputs to beam 1 multiplexer 73, through fibers 78, 79 and 80 respectively. Fiber multiplexers WDMs 68, 70 and 72 direct their single outputs to beam 2 multiplexer 74, through fibers 81, 82, 83 respectively. Fibers 78, 79 and 80, are then merged into a single fiber 84 by beam 1 multiplexer 73, and fibers 81, 82, 83 are merged into a single fiber 85 by beam 2 multiplexer 74. The input passbands of beam 1 multiplexer 73 are wavelength ranges Al, Bl, and Cl. The input passbands of beam 2 multiplexer 74 are wavelength ranges A2, B2, and C2. Beam 1, traveling through fiber 84, is photodetected by photodetecting device 75, and beam 2, traveling through fiber 85, is photodetected by photodetecting device 76. Only one photodetecting device per beam is required. Beam 1 always appears at the beam 1 output port and beam 2 at the beam 2 output port, independent of the beam scan angle and which delay line sets were chosen. In an alternative embodiment of the basic invention of Figure 5, the port demultiplexers 60 and beam demultiplexers 64, 65, 66 are replaced with simple fiber splitters as illustrated by Figure 6. This is possible because the filtering performed by the port and beam demultiplexers 60, 64, 65, 66 is redundant to the filtering performed by the fiber multiplexers 67, 68, 69, 70, 71, and 72. Referring to Figure 6, the port and beam demultiplexers of Figure 5, have been replaced by four 1x6 fiber splitters. The rest of the configuration remains the same, and the same functions are performed. Alternatively, the fiber and beam multiplexers can be replaced with combiners without affecting the performance of the network. This configuration is shown in Figure 7., two 12x1 combiners 100 and 101 replace fiber and beam multiplexers 67, 68, 69, 70, 71, 72, 73, and 74 of Figure 5. Referring to Figure 8, yet another embodiment of the invention is illustrated. In this variation, the port demultiplexers 60 of Figure 5 are replaced with the same number of 1x3 fiber splitters 110. Photodetection takes place at the output of fiber multiplexers 113 and is performed by photodetectors 111. Beam multiplexers 73 and 74 of Figure 5 are replaced with electrical switches 112, which permit to select the delay line desired for each beam. The addition of electrical switches eliminates the need for the laser wavelengths used to select a delay line set. Thus the configuration of Figure 8 reduces the total number of wavelengths required fromNλ=PxBxF=24 to Nλ=BxF=8. If the beamformer has more than P=6 ports, the present configuration will have higher losses than the pure WDM configuration of Figure 5, but will require P times fewer wavelengths. Figure 11 shows the wavelength hierarchy required for the configuration of Figure 8. These wavelengths are used only for beam separation and incoherent summation. The wavelength hierarchy of Figure 11 is a truncated version of the one shown in Figure 3b, where the empty wavelength slots of Figure 3 b are eliminated by the use of electrical switches instead of wavelength ranges for port selection. In yet another variation of the invention, the port and beam demultiplexers of Figure 5 can be replaced with a single port beam demultiplexer. In this configuration the delay lines cannot be shared by the beams, and B times as many delay lines are needed. On the other hand, the use of a lxZ WDM, instead of a lxX and a lxY WDM can reduce losses for X<6 and Y<6 but Z=XxY>6. In the particular example where P=3, B=2, and F=4, 1x3 port and 1x2 beam demultiplexers 60, 64, 65, and 66 of Figure 5, are replaced with 1x6 port/beam demultiplexers, as illustrated by Figure 9. These 1x6 port/beam demultiplexers would have six output fibers with passbands Al, A2, Bl, B2, Cl, C2, as shown in Figure 3A. The output fibers are connected to corresponding fiber multiplexers 67, 68, 69, 70, 71 and 72 of Figure 5. Another useful variation of the basic invention presented in Figure 5, is to place a photodetector at the output of each fiber multiplexer 67, 68, 69, 70, 71 and 72, as shown in Figure 10. Then, the outputs of the photodetectors placed after fiber multiplexers 67, 69, and 71 (A1, B1, C1 respectively) can be electrically combined into beam 1, and the outputs of photodetectors placed after fiber multiplexers 68, 70, and 72 (A2, B2, C2 respectively) can be electrically combined into beam 2. This is possible because only one of the three outputs from Al, Bl, Cl is active at any given time, and only one of the three outputs from A2, B2, C2 is active at any given time. This configuration does not require any switching and is completely passive. It has an important application in wavelength re-use networks and is discussed below. Generally, phased array antennas operate in two dimensions and require two-dimensional delay line networks. Figure 12 shows the case of a 4x4 fiber array 120 feeding P=4 delay ports A, B, C, and D. A total of FxP= 16x4=64 delay lines are required. For simplicity, Figure 12a shows only delay lines 121, 122, 123 and 124, connecting fiber γ of the top row of array 120, to the four ports A, B, C, and D. For clarity, the delay lines that feed port C only are illustrated in Figure 12b. Each of the FxP delay lines that constitute the system has a path length well defined that is determined by the system geometry, and the velocity of light in the delay lines. Equations 125 show the path length differences in the x and y directions, ΔLx and ΔLy, from one fiber to the next when given the geometry of the system (i.e., antenna element spacing D, and delay line scan angle components θx and θy) and the velocity of light in the delay line. If implemented using fibers for the delay lines, the delay line network of Figure 12 would be difficult to fabricate, and would require careful cutting to a specific length and splicing FxP fibers. However, this three-dimensional structure can be collapsed into two dimensions and fabricated along with the WDMs in an integrated structure using silica on silicon (SOS) waveguide technology. This collapse to two dimensions is possible because there is a unique mapping of length between input fiber and output port. For example, referring to Figure 12a, fiber 121 (fiber γ of top row) connected to port A, has a length that is unique in the network. This is true for all of the FxP fibers that constitute the network. Consequently, each fiber can be placed on a flat surface. As long as their respective lengths are respected, this new two- dimensional structure is equivalent to the three-dimensional architecture of Figure 12, and can perform the same functions. It is to be noted that when collapsed into two dimensions, the delay line waveguides will cross over each other, and slightly increase network loss. While low-loss cross-overs are easily made using SOS, minimal loss in the network is achieved by minimizing the number of cross-overs. In Figure 5, cross-overs occur in all three areas between the four columns of WDMs. Trying different arrangements of the WDMs within each column, while keeping the interconnections the same, indicates that placing cross-overs in multiple areas minimizes their number. The network of Figure 5 yields the minimum number of cross-overs for the arrangements tried. It is thus a good candidate for a low-loss structure made with SOS. WDMs of the Array Waveguide Grating (AWG) type, can be fabricated using SOS, and easily integrated on the same substrate as the crossing waveguides and/or delay lines. The arrangement of Figure 5 lends itself to standardization and fabrication of the network on one or on multiple substrates. For example, a standard set of interconnections and routing WDMs 132 (Figure 5) could be made on one substrate, and a standard input interface 130 could be made on another. Application specific delay lines 131 could then be made and incorporated into the network. This approach would substantially lower the cost of making large numbers of photonic beamformers that only differ with respect to the delay lines required. Special attention to reducing losses could be paid, since the interconnection and routing section 132 which has the most cross-overs, would be one of the standard pieces. Referring to Figure 13, an example of how the WDMs can be rearranged on the substrate placing all cross-overs in one section, is shown. This arrangement has more cross-overs and thus higher losses than the arrangement of Figure 5. While the network of Figure 5 was developed assuming a network-switched architecture, the rearranged system of Figure 13 looks like an in-line switched network (compare with Figure 1). The WDM system can be thus made to look like either network type simply by moving the WDMs and waveguides around while keeping the interconnections the same. Therefore, this invention may be used equally well in network-switched and in-line switched architectures. If standard ITU wavelengths in the 1550 nm band are used with a spacing of 50 GHz, the system is limited to approximately 100 wavelengths. This number can be doubled using the non-standard 1300 nm band to a maximum of Nλ = PxBxF = 200 wavelengths. Most practical systems have around 100 fibers, leaving little room for beam and port operations. To overcome this limitation, methods of wavelength re-use will now be disclosed. The WDM techniques described herein are well suited to wavelength re-use. The general approach, as illustrated in Figure 14, is to divide the two-dimensional array 134 of input fibers into M sub-arrays or cells 135 and 136, in such way that each cell uses W=PxBx(F/M) wavelengths. Thus, for M > 2 W is a fraction of the number of wavelengths needed in the original network. Each cell uses the same set of W wavelengths, hence the term "wavelength re-use". The outputs of each cell, after going through the delay lines/ports (A, B, or C in Figure 14), are separately photodetected. After photodetection the RF outputs of the photodetectors are electrically summed with equal length or corporate feeds to form the output beam. The cells can be formed of any subset of the fiber array, rows, columns, parts of rows or columns, or even randomly chosen elements throughout the array. All that is required is that the inserted delays be proper. The beamforming network for each cell is independent and self-contained. Figure 14 illustrates the particular example of a 4x4 array of fibers divided into M=2 cells, each cell including 2x4 fibers. Each cell feeds P=3 ports, with only one beam being assumed in this example for simplicity. In general, the cells have different sets of delay lines because they represent different parts of the Rotman lens. However they all use the same wavelengths, WDM structure, and interconnect architecture. Consequently, this configuration lends itself well to the cost-saving standardization earlier described. Indeed, all cell networks preferably use identical input interfaces and interconnect/routing modules, and differ only in the amount of delay inserted by the delay lines utilized. Figure 15a shows the WDM architecture utilized in conjunction with the wavelength re-use method. Note that this structure is identical to the one shown in Figure 5, except for the beam multiplexers which have been removed and replaced by photodetectors disposed at the outputs of each fiber multiplexer. After photodetection the signals can be electrically combined as discussed previously. Figure 15b is a simplified block representation of the Figure 15a. Since wavelength re-use configuration utilizes M times less wavelengths as the embodiment of Figure 5, it will utilize M cells 140 of the type shown in Figure 15b, to accomplish the same functions. The first cell uses a set of W wavelengths, and all subsequent cells "re-use" the same set of W wavelengths. Figure 16 shows an example of how cells 140 can be connected to perform the functions of a 4x4 fiber array, in accordance with the wavelength re-use method. The system of Figure 16 handles B=2 beams and P=3 ports, and is a two dimensional system, contrary to prior art fiber arrays which are three-dimensional. Cells 151, 152, 153, and 154 represent rows 1, 2, 3, and 4 of the 4x4 fiber array, respectively. Each cell is an independent WDM network with PxB electrical outputs Al, Bl, Cl, A2, B2, C2, and each require the same PxBx(F/M) = 3x2x(16/4) = 24 wavelengths. The cells are identical except for the length of the delay lines. The outputs Al, ,A2,B1, B2, Cl, C2 of each cell are directed to the corresponding beam summation junction 155, 156, 157, 158, 159, 160, respectively, via corporate feed 161 (i.e., all Al cell outputs are directed to Al summation junction 155, all A2 cell outputs are directed to A2 summation junction 156, and so on). For clarity, only three feed lines corresponding to Al, A2, and C2 are shown in Figure 16. The four signals entering Al beam summation junction 155 are then vector summed into a single beam, and the same occurs at beam summation junctions 156, 157, 158, 159 and 160. The Al, Bl, Cl junctions, 155, 156, 157, respectively, are then fed to a common beam 1 output 162. The A2, B2, C2 junctions, 158, 159, 160 respectively, are then fed to a common beam 2 output 163. The network requires no switching and is completely passive. A fiber array with F fibers and M cells will require BxPxM photodetectors. The number of photodetectors needed is independent of the number of fibers in the array, and is a significant reduction over more conventional networks that do not use a different wavelength for each fiber involved in the vector summation. For example, if WDMs were used just for port and beam operations in a network-switched architecture using the wavelength hierarchy of Figure 17, a photodetector would have to be placed on every fiber as shown in Figure 18. This would require either BxPxF detectors as shown in Figure 18, or port switching of B detector arrays each containing F detectors (for a total of BxF detectors) as shown in Figure 19. The number of photodetectors needed increases while the number of wavelengths required decreases. For the example of Figure 18, 24 photodetectors are needed while only 6 wavelengths are required for the system to function. Since M can be much less than F, if one re-uses a large number of wavelengths, BxPxM can be much less than both BxPxF and BxF, and thus the number of photodetectors required can be largely reduced. A trade-off, between the cost and complexity of adding laser wavelengths versus reducing the number of photodetectors, must be made for each particular photonic beamforming system. Fiber splitters may be substituted for WDMs as discussed previously. Furthermore, if switching is used at the output of the Al , A2, B 1 , B2, Cl , C2 junctions, a beam-fiber wavelength hierarchy similar to Figure 11 can be used to further reduce the number of wavelengths required to Bx(F/M)=8. In this case, B=2 switches, each having P=3 possible positions, are required. Having described the invention in conjunction with certain embodiments thereof, modifications and variations will now certainly suggest themselves to those skilled in the art. As such, the invention is not limited to the disclosed embodiments except as required by the appended claims.

Claims: CLAIMS : 1. A be--mforming device comprising : (a) a plurality of Wavelength Division Multiplexers (WDMs); (b) a plurality of delay lines connecting at least selected ones of said plurality WDMs; and (c) a plurality of wave guides com ecting at least selected ones of said plurality of WDMs. 2. The beam forming device of claim 1 wherein a first subset of said plurality of WDMs act as port demultiplexers, the elements of said first subset being connected to the elements of a second subset of said plurality of WDMs acting as beam demultiplexers, said first subset and second subset being connected by said plurality of delay lines ; said second subset being connected to a third subset of said plurality of WDMs acting as fiber multiplexers, said second and third subsets being connected by at least selected ones of said plurality of wave guides; said third subset being connected to a forth subset of said plurality of WDMs acting as beam multiplexers, said third and forth subsets being connected by at least selected ones of said plurality of wave guides. 3. The beam forming device of claim 2 further comprising a plurality of photodetectors optically connected to outputs of said forth subset of said plurality of WDMs acting as beam multiplexers. 4. The beam foπning device of claim 1 further comprising a plurality of wave guide splitters, said plurality of wave guide splitters being connected to at least selected ones of said plurality of WDMs, by said plurality of delay lines. 5. The beam forming device of claim 4 wherein said plurality of wave guide splitters are connected to a first subset of said plurality of WDMs acting as fiber multiplexers, by said plurality of delay lines; said first subset is connected to a second subset of said plurality of WDMs acting as beam multiplexers, said first and second subsets being connected through said plurality of wave guides. 6. The beam forming device of claim 15 further comprising a plurality of photodetectors optically connected to outputs of said second subset of said plurality of WDMs acting as beam multiplexers. 7. The beam forming device of claim 1 further comprising a plurality of wave guide combiners, said plurality of wave guide combiners being connected to at least selected ones of said plurality of WDMs, by said plurality of wave guides. 8. The beam forming device of claim 7 wherein a first subset of said plurality of WDMs acting as port demultiplexers is connected to a second subset of WDMs acting as beam demultiplexers, by said plurality of delay lines; said second subset is connected to said plurality of of fiber combiners, said second subse and said fiber combiners being connected by said plurality of wave guides. 9. The beam forming device of claim 8 further comprising a plurality of photodetectors optically connected to outputs of said fiber combiners. 10. The beam forming device of claim 1 further comprising a plurality of wave guide splitters, a plurality of photodetectors and a plurality of electrical switches, said plurality of wave guide splitters being connected to a first subset of WDMs acting as beam demultiplexers by said plurality of delay lines; said first subset being connected to a second subset of WDMs acting as fiber multiplexers by at least selected ones of said wave guides; said second subset of WDMs having their outputs connected to said plurality of photodetectors by at least selected ones of said wave guides; and at least selected ones of said photodetectors having their outputs connected to at least selected ones of said electrical switches by electrical lines. 11. The beam forming device of claim 1 wherein a first subset of said plurality of WDMs acting as port beam demultiplexers is connected to a second subset of WDMs acting as fiber multiplexers, said first subset and said second subset being connected by said plurality of delay lines, said second subset being connected to a third subset of WDMs acting as beam multiplexers, said second subset and said third subset being connected by said plurality of wave guides. 12. The beam forming device of claim 11 further comprising a plurality of photodetectors optically connected to outputs of said third subset of WDMs by at least selected ones of said plurality of wave guides. 13. The beam forming device of claim 2 further comprising a plurality of electrical wave guide combiners and a plurality of photodetectors, a subset (1) of WDMs acting as port demultiplexers being connected to a subset (2) of WDMs acting as beam demultiplexers by said plurality of delay lines; said subset (2) being connected to a subset (3) of WDMs acting as fiber multiplexers by at least selected ones of said wave guides; said subset (3) of WDMs having their outputs connected to said plurality of photodetectors by at least selected ones of said wave guides; and at least selected ones of said photodetectors having their outputs connected to at least selected ones of said electrical wave guide combiners by electrical lines. 14. The beamforming device of claim 2 wherein the elements of said second subset are arranged according to an In-Line-Switched architecture, thereby to concentrate all wave guide cross-overs in an area between said second subset and said third subset. 15. The beamforming device of claim 2 wherein the elements of said second subset are arranged according to a Network-Switched architecture. 16. The beam forming device of claim 1 j-urther comprising a plurality of electrical wave guide combiners and a plurality of photodetectors, wherein a first subset of WDMs acting as port demultiplexers is connected to a second subset of WDMs acting as beam demultiplexers by said plurality of delay lines, said elements of the second subset having their outputs connected to said plurality of photodetectors by at least selected ones of said wave guides, and said photodetectors have their outputs comiected to at least selected ones of said electrical wave guide combiners by electrical lines. 17. The beam forming device of claim 1 ft-rther comprising a plurality of electrical wave guide combiners, a plurality of photodetectors and a plurality of optical switches, wherein a first subset of WDMs acting as port demultiplexers is connected to a second subset of WDMs acting as beam demultiplexers by said plurality of delay lines, elements of the second subset are connected to at least selected ones of said optical switches by at least selected ones of said wave guides, said optical switches having their outputs connected to at least selected ones of said photodetectors by at least selected ones of said wave guides, and said photodetectors have their outputs connected to at least selected ones of said electrical wave guide combiners by electrical lines. 18. The beam forming device of any one of the preceding claims wherein said plurality of wave guide splitters, said plurality of WDMs, said plurality of delay lines, and said plurality of wave guides, are optical components. 19. The beam forming device of any one of the preceding claims wherein said plurality of WDMs comprises Array Waveguide Grating (AWG) filters. 20. The beam forming device of any one of the preceding claims wherein said plurality of waveguide splitters, said plurality of WDMs, said plurality of delay lines, and said plurality of wave guides are integrated a common substrate. 21. A method of beamforming comprising the steps of : (a) synthesizing a plurality of optical wavefronts each comprising of plurality of optical wavelengths; (b) receiving a plurality of incoming electrical wavefronts with an antenna; (c) mixing said plurality of optical wavefronts with said plurality of incoming electrical wavefronts by optical modulation to provide a plurality of resultant optical wavefronts; (d) directing each resultant optical wavefront to a predetermined set of delay lines; and (e) separating each resultant optical wavefronts in order to direct each one to a different output. 22. The method of beamforming as in claim 21 wherein the step of synthesizing a plurality of optical wavefronts, includes the steps of : (a) specifically selecting a subset of said plurality of optical wavelengths in accordance with the set of delay lines desired ; and (b) specifically selecting a subset of said plurality of optical wavelengths in accordance with the output desired. 23. The method of claim 22 wherein the step of directing each resultant optical wavefront to a predetermined set of delay lines, includes providing WDMs capable of channeling each resultant optical wavefront into the predetermined delay line, by dispersing the wavelengths contained in each resultant optical wavefronts. 24. A method of separating a plurality of electromagnetic wavefronts included in an electromagnetic radiation, the method comprising the steps: (a) providing a plurality of optical components; (b) arranging said optical components to form a network having a plurality of inputs and a plurality of outputs, said components being arranged so as to separately direct each one of said electromagnetic wavefronts to one of said outputs. 25. The method of claim 24, wherein said electromagnetic wavefronts each include a plurality of wavelengths, and wherein the directing of said electromagnetic wavefronts to one of said outputs, is achieved by channeling selected ones of said wavelengths into selected ones of said optical components. 26. The method of claim 24 further comprising the step of providing a plurality of electrical components. 27. The method of claim 24, wherein said electromagnetic wavefronts each include a plurality of wavelengths, and wherein the directing of said electromagnetic wavefronts to one of said outputs, is achieved by channeling selected ones of said wavelengths into selected ones of said optical and/or electrical components. 28. The method of claim 24, wherein said optical components include WDMs, delay lines and wave guides. 29. The method of claim 24, wherein said optical components further include splitters, combiners and switches. 30. The method of claim 24, wherein said WDMs comprise Array Waveguide Grating (AWG) filters. 31. The method of claim 24, wherein said WDMs, said delay lines and said wave guides are integrated on a common substrate.

Inventors: STEPHENS RONALD REGIS, STEPHENS, RONALD, REGIS
Applicants: HRL LAB LLC, STEPHENS RONALD REGIS, HRL LABORATORIES, LLC, STEPHENS, RONALD, REGIS
Application year: 2001
Main classification: H01Q3
IPC codes: H01Q3