|Publication number||USRE38809 E1|
|Application number||US 10/005,745|
|Publication date||Oct 4, 2005|
|Filing date||Nov 2, 2001|
|Priority date||Nov 30, 1995|
|Also published as||US5796510, US5978125, USRE38735|
|Publication number||005745, 10005745, US RE38809 E1, US RE38809E1, US-E1-RE38809, USRE38809 E1, USRE38809E1|
|Inventors||X. Steve Yao|
|Original Assignee||General Photonics Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (11), Non-Patent Citations (10), Referenced by (25), Classifications (21), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention was made with Government support under a Contract awarded by NASA, and is subject to the provisions of Public Law 96-517 (35 U.S.C 202) in which the inventor is granted right to retain title. The government has certain rights in this invention.
This is a broadening reissue application of the U.S. Pat. No. 5,978,125 which has been surrendered to the U.S. Patent and Trademark Office. Another broadening reissue application, Ser. No. 10/002,947, of the same U.S. Pat. No. 5,978,125 was filed on Oct. 31, 2001.
This invention pertains generally to the precision optical path length control, specifically to a photonic variable true time delay device for steering phased array radar, for constructing a transversal filter, and for controlling the optical path in optical interferometry.
Phased array antennas have the important ability of beam steering without mechanical actuators. This feature is highly desirable for applications such as spacecraft, air craft, and mobile platforms where size and mass are restricted. The direction of a microwave (or millimeter wave) beam radiated from a phased array antenna is generally controlled by the relative phase distribution of microwave signals emitted by regularly spaced radiating elements. For a phased array of a wide instantaneous bandwidth, adjusting only the relative phase is not sufficient and so a relative time delay adjustment of the radiating elements must be introduced to avoid the beam pointing error known as squint, which results from the modification of the antenna phase pattern with changing frequency.
Conventional electronic beam forming systems for generating and delivering the requisite time delay and phase information are generally bulky, lossy, inefficient, and of narrow bandwidth. On the other hand, photonic beam forming offers the advantage of high packing density, wide signal bandwidth, light weight, immunity to electromagnetic interference, and remoting capability via optical fiber. Consequently, it has been under intensive investigation in the past few years and many photonic beam forming systems have been proposed and demonstrated. Photonic beam forming network use a lightwave carrier for the electrical signals of the radiating elements of the phased array, and provides the necessary time delay and phase information for beam steering.
For airborne and space-based phased arrays operating at mm-wave frequencies (20 GHz and above), the arrays are usually two-dimensional and a large number of array elements, typically a few thousand, are used. This requires that the beam forming network be two dimensional and have a very high packing density. In addition, the beam forming network must be reversible so that it can be used for both antenna transmitting and receiving. Furthermore, the total delay achievable of the delay network must be sufficiently large so that the maximum scanning angle of the phased array is adequate. Finally, as will be shown later, the delay resolution (the minimum step of delay change) must be fine enough (much less than the wavelength of the signal) to ensure that the angular resolution of the beam scanning is sufficient.
None of the proposed photonic beam forming networks to date meet all of the above requirements. The operation frequencies of the beam forming networks based on acoustooptic modulators (E. Toughlian and H. Zmuda, “A photonic variable RF delay line for phased array antennas,” J. Light-wave Technol., vol. 8, pp. 1824-828, 1990) are limited to below 5 GHz and suffer from poor delay resolution, and therefore not adequate for mm-wave phased arrays. Path-switching time delay devices based on guided wave optics (C. T. Sullivan, S. D. mukherjee, M. K. Hibbs-Brenner, and A. Gopinath, “Switched time-delay elements based on AlGaAs/GaAs optical wave-guide technology at 1.32 mm for optically controlled phased array antennas,” SPIE Proceedings, vol. 1703, pp. 264-21, 1992) are complicated, and are characterized by high loss, high cost, poor delay resolution, and one-dimensional geometry. The free-space path-switching time delay device (N. A. Riza, “Transmit/receive time-delay beam forming optical architecture for phased array antennas,” Appl. Opt., vol. 30, pp. 4594-4595, 1991) shown in
Even for narrow bandwidth phased arrays where true time delay is not necessary, a compact, two dimensional, and programmable phase shifter with high phase-shift resolution is highly desirable. Such a phase shifter can reduce the size and weight, and increase the pointing accuracy of the phased array radar.
Another important application of two dimensional true time delay device is in transversal filters (B. Moslehi, K. Chau, and J. Goodman, “fiber-optic signal processors with optical gain and reconfigurable weights,” Proc. 4th Biennial Department Of Defense Fiber Optics and Photonics Conf., McLean, Va. 1994, pp. 303-309 and D. Nortton, S. Johns, and R. Soref, “Tunable wideband microwave transversal filter using high dispersive fiber delay lines,” Proc. 4th Biennial Department Of Defense Fiber Optics And Photonics Conf., McLean, Va., 1994, pp. 297-301). In such a filter, a microwave or mm-meter wave signal is splitted into many branches and then recombined after the signal in these branches experiences different delays. For a certain set of delays, only the signal with a right frequency will add in phase and exit the beam combining junction with minimum loss. Other frequencies will destructively interfere and suffer severe loss—a bandpass filter is formed. By changing the delay arrangements, the center frequency of the pass band will also change, creating a dynamically tunable filter often referred to as transversal filter. Studies indicate that the bandwidth of the filter is inversely proportional to the number of branches and the frequency tuning resolution is proportional to the delay resolution of the branches. Therefore, a compact, two dimensional, and programmable true time delay with high delay resolution is ideal for constructing such a filter.
Yet another application of a variable delay line with high delay resolution is in optical interferometry, and in auto- and cross-correlation measurements of optical pulses. Presently, variable delay is accomplished by the combination of various forms of mechanical translation and is fine tuned by piezoelectric transducer. Because such a delay line involves mechanical moving parts, it is generally bulky, heavy, difficult to align, and less reliable. In addition, because the piezo-electric transducer suffers from hysteresis and temperature dependent drift, active control using feedback servo loop is required, resulting in a complicated system.
Accordingly, it is an object of this invention to provide a two dimensional and variable true time delay device for phased array radar and for transversal filter applications. The device has the properties of high packing density, low loss, easy fabrication, fast delay variation, and virtually infinite bandwidth. The delay resolution of the device is sufficiently fine for accurate beam steering, and the total delay is adequately large to cover desired scanning angles. This device can be simplified to a phase-shifter beam former for phased arrays of narrow bandwidth, where true time delay is not necessary.
An other object of this invention is to provide a variable delay line which has no moving parts, no hysteresis, and no temperature dependent drift for applications in optical interferometry and optical pulse auto- and cross-correlationl measurements, and in other applications where a precision variable time delay is required.
Yet another object of this invention is to provide a manufacturing method for the mass production of the variable true time delay.
Further objects and advantages of this invention will become apparent from a consideration of the ensuring description and drawings.
Reverse direction optical beam
90° Polarization rotator array
Left Polarization Beamsplitter (PBS)
Left polarization rotator
Right polarization rotator
External polarization rotator
Left external PBS
Right external PBS
Radiating microwave beam
Electrical signal combiner
Left laser array
Left collimating lens array
Left external polarization rotator array
Right external polarization rotator array
Left focusing lens array
Left photodetector array
Right focusing lens array
Right photodetector array
Left collimating lens array
Right diode laser array
Upper corner reflector array
Lower corner reflector array
Upper lens array
Lower lens array
Upper polarization rotator array
Lower polarization rotator array
Birefringent crystal slabs
To lower manufacturing cost, each building block can be injection-molded using glass, acrylic, or other types of materials. As illustrated in
The ladder-structured variable delay device is constructed by stacking multiple basic building blocks on top of each other, as shown in FIG. 3A. Whenever the beam encounters a basic block in reflecting state, it will be directed toward the output. For example, if the ith block is in reflecting state but all the blocks before it are in passing state, the total delay ΔL of the unit is
where n and h are the refractive index and height of the basic building block respectively. The smallest delay increment is thus.
Because the delay unit closely assembles a ladder, the structure of the delay unit is referred to as “ladder” construction. This structure is more compact than that of the conventional path switched delay shown in FIG. 1. The ladder structure is more suited for mass production and therefore less expensive to manufacture.
To minimize the number of basic building blocks used in a ladder-structured variable delay device, the basic building blocks can be arranged in a binary fashion such that the distance between two consecutive blocks increases by a factor of 2, as shown in FIG. 3B. Let M be the total number of the blocks used (or bits), then the maximum value of the delay generated is:
ΔLmax=(20+21+22+. . . 2M−1)Δl=(2M−1)Δl (3)
where Δl=2nh is the smallest delay increment. By properly adjusting the polarization state of the light beam in each block, any time delay in the range from Δl to ΔLmax can be obtained with a resolution (or delay increment) of Δl.
The ladder-structured variable delay device can be made to operate bidirectionally by placing an external polarization beamsplitter 38L at left end of the device, another external polarization beamsplitter 38R at right end of the device, and an external large area polarization rotator 36 covering both the left and right ends, as shown in FIG. 3C. For left to right operation, an optical beam 20 having a horizontal polarization 30A enters the device from left external polarization beamsplitter 38L. External polarization rotator 36 is de-activated to allow optical beam 20 passing through left external polarization beamsplitter 38L and entering the delay device. On the other hand, for right to left operation, an optical beam 20A having a perpendicular polarization 30B enters the device by reflecting off right external polarization beamsplitter 38R. External polarization rotator 36 is activated to rotate perpendicular polarization 30B to horizontal polarization 30A before entering the device. When optical beam 20B reaches the left side of the delay device, external polarization rotator 36 automatically rotates horizontal polarization 30A back to perpendicular polarization 30B so that optical beam 20A exits the delay device by reflecting off left external polarization beamsplitter 38L.
Accordingly, the ladder construction of this invention provides compactness and high packing density. The basic building block is simple and the complete unit consists of many basic building blocks that are stacked together. In addition, two or more units can be cascaded to further increase delay range. Because liquid crystals are used to control the relative delay of each channel, both control voltage and power consumption are low. By injection molding the structure of the device with glass or plastic, the fabrication cost can be greatly reduced. Because the passing states and the reflecting states have orthogonal polarizations, high delay isolation (defined as the optical power of wanted delay divided by the optical power of unwanted delay) is readily achievable with the insertion of polarizers. Finally, the optical loss of the device is low.
The delay resolution of the path switched delay described above is not fine enough for millimeter waves applications, where the delay resolution must be much less than 1 mm. The following describes an index switched variable delay device for achieving high delay resolution for mm wave and other applications where high delay resolution is required.
A delay line can be constructed by putting many such crystal segments together in a linear array, as shown in
To minimize the number of polarization rotators in the device, the lengths of the crystal segments increases increase successively by a factor of 2, as shown in FIG. 5C. The relative optical path delay Δl between the two polarization states in the smallest segment of length l (the least significant bit) is
Let M be the total number of crystal segments (or bits), then the maximum value of the delay generated is:
ΔLmax=(20+21+22+. . . 2M−1)Δl=(2M−1)Δl, ΔLmax=( 2 0 +2 1 +2 2 + . . . 2 M−1)Δl=( 2 M−1)Δl, (5)
By properly adjusting the polarization state of the light beam in each segment 40, any optical path delay in the range from Δl to ΔLmax can be obtained with a resolution (or delay increment) of Δl. Because the length of each crystal segment 40 can be tightly controlled, the accuracy of the device can be very high.
Several delay lines of the design described above can be densely packed in two dimensions to form a compact variable delay device. However, instead of cutting crystal into narrow strips, large area crystal segments 40 and polarzation rotator arrays 22 (spatial light modulators) are used to construct the multiple channel delay device, as shown in FIG. 6. Here polarization rotators in all polarization rotator arrays are aligned element by element and the size of each channel is determined by the size of the rotators. For 2 mm channel spacing, the packing density of the device is 25/cm2. Such a channel spacing is easily attainable in practice, considering that a 1.4 mm diameter Gaussian beam with 1 μm wavelength has a Rayleigh range of 1.54 meters.
The index switched delay device can be used for both transmitting and receiving operations. As shown in
When in the transmitting mode, left external polarization rotator array 68L is inactive. However, right external polarization rotator array 68R is such programmed that it always brings the polarization of light beams in each channel back to be in the plane of the paper after the delay device. This assures that light beams of all channels will pass PBS 38R and be received by right photodetector array 76. A left focusing lens array 74 is placed before left photodetector array 76 to focus light of each individual channel to a corresponding detector on left photodetector array 76. When in the receiving mode, right external polarization rotator array 68R is inactive. However, left polarization rotator array 68L is such programmed that it always brings the polarization of light beam in each channel back to be perpendicular to the plane of paper after the delay device. This assures that all channels will reflect off PBS 38L and be received by a detector array 72, as in
The same concept can also be used to make phase shifters for phased array antennas with narrow bandwidth where true time delay is not necessary. For example, an 8. GHz (X-band) carrier has a wavelength of 3.75 cm. To obtain a total phase shift of 2π for such a carrier, a total length of only 13 cm of Rutile crystal per channel is required. For a Ka band carrier of wavelength of 0.75 cm (40 GHz), only 2.6 cm Rutile crystal per channel is required.
ng − no = Δl/l
Crystal for 1 cm delay
˜6 × 10−4
Table I listed lists the birefringence of potential birefringent materials for fabricating the proposed delay lines. Note that different crystals may be used together to construct a delay line: a crystal with small birefringence can be used to make segments of small delays (less significant bits) and a crystal with large birefringence can be used to make segments of large delays (more significant bits).
The maximum delay ΔLmax required of a beam forming network of a phased array antenna with N×N elements is
where θmax is the maximum beam scanning angle, λ is the wavelength of the carrier (microwave) signal of the phased array, and dmax≡λ/(1+sin |θmax|) is the maximum array spacing allowed before higher order diffraction degrade the antenna gain.
To achieve an angular beam scanning resolution of Δθ, the delay resolution or the minimum path delay between the two adjacent elements Δl is required to be
Table II lists the values of required maximum delay ΔLmax and delay resolution Δl for a phased array with λ=0.75 cm (40 GHz), N=64, and Δθ=1°. The corresponding crystal lengths for the maximum and the minimum delays are also listed. For example, for the case of θmax=30°, LiNbO3 crystal of length 0.87 mm can be used to make the segment of the smallest delay of 76.5 μm and the Rutile crystal of the total length of about 55 cm can be used to make other larger delay segments that have a total delay of 15.75 cm. In the table, the number of bits M is calculated using M=log2(1+ΔLmax/Δl).
θmax = 5°
θmax = 10°
θmax = 30°
θmax = 60°
or 5.04 λ
or 9.33 λ
or 21 λ
or 29.24 λ
No. of bits
It should be noted that Rutile has excellent optical and physical properties: it is transparent to light from 500 nm to 5 μm and its birefringence (ne−no) remains almost unchanged from 430 nm to 4 μm. It has a density of 4.26 g/cm3, a melting point of 2093° K., and a solubility in water less than 0.001.
To reduce the cost and to extend the delay range, the index-changing delay elements may be cascaded with a ladder structured path-switching delay device described previously, as shown in FIG. 8. The birefringent crystal segments are used for the less significant bits of high delay resolution and the path switching concept is used for the more significant bits of large delays. This cascaded construction combines the advantages of both techniques and avoids their short comings. The total length of the crystal segments per channel is now reduced to few centimeters.
In stead of cutting crystals into many segments, the index-switching time delay unit may also be constructed using slabs of crystal, as shown in FIG. 9. Such a unit consists of slabs of birefringent crystal 92, a upper layer of polarization rotators 90A (which may be individually and independently controlled), an optional lower layer polarization rotators 90B, a upper row of corner reflectors 86A, a lower row of corner reflectors 86B, an optional upper lens array 88A, and an optional lower lens array 88B.
Finally, to obtain even finer delay tuning, electrodes 44 can be attached across each crystal slab to apply an electrical field 94 and change the refractive index of the crystal via the electro-optic (or Pockel's) effect of the birefringent crystals, as shown in FIG. 9 and FIG. 10C. The two electrodes on each face of crystal slab 92 can be separated by an insulation layer 98.
In summary, the index switched photonic variable delay device has the properties of high packing density, low loss, easy fabrication, and virtually infinite bandwidth. The device is inherently two dimensional and has a packing density exceeding 25 lines/cm2. The delay resolution of the device can be much less than a femtosecond (one micron in space) and its total delay exceeds 1 nanosecond. The delay accuracy achievable is high, and is only limited by the length accuracy of each crystal segment. The device can also be digitally programmed with low switching power (microwatts per switch or per bit). Such a device is ideal for a beam forming network of a phased array operating at Ka band (˜40 GHz) and above frequencies and for millimeter wave transversal filters. In addition, the delay is reversible so that the same delay device can be used for both antenna transmitting and receiving. Finally, this index-switched variable delay device can be cascaded with a ladder-structured variable device to form a new device which combines the advantages of the two individual devices.
Although the description above contains many specificaties, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.
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|U.S. Classification||359/256, 359/259, 359/245, 359/320|
|International Classification||G02F1/29, G02F1/03, G02F1/01, H01Q3/26, H01Q15/24, G02B6/28, H01Q15/18|
|Cooperative Classification||H01Q15/18, G02F2201/20, G02F1/01, H01Q3/2682, G02B6/2861, H01Q15/246|
|European Classification||H01Q3/26T, H01Q15/24B2, G02F1/01, H01Q15/18|
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