We demonstrate a 4-bit optical true time-delay module for synthetic aperture radars based on the integration of polymer channel waveguide and electro-optic Bragg gratings. The demonstrated device is a truly integrated module that eliminates the most difficult packaging problem associated with the delicate interfaces between optical fibers and optical switches. The total insertion loss of the 4-bit optical true-time-delay line is less than 3 dB with switching time <50 □s and driving voltage of 25 V.
It has been realized that the lack of enabling technology of beam forming and steering devices significantly slows down the process of implementing wideband phased array antenna systems. In this paper, we present our research in developing an integrated electro-optic switched true-time-delay module as a boradband beam forming device for wideband phased array antennas. The unique feature of our approach is that both the true-time-delay waveguide circuit and electro-optic switching elements are monolithically integrated in a single substrate. As a result, this integration significantly reduces the device size while eliminating the most difficult packaging problem associated with the delicate interfaces between optical fibers and optical switches. Such a monolithic approach offers greater precision for the RF phase control than the fiber-delay-lines thanks to the sub-micrometer accuracy of lithography-defined polymeric waveguides. More important, the proposed optical switched true-time-delay network requires very low electrical power consumption due to the low power soncumption of electrically-switchable waveguide gratings. Furthermore, the electrically-switchable waveguide gratings have a very fast switching speed (<50 μm) that is at least 100 times faster than any existing commercial optical switching matrix. Photonic phased array antenna based on optical true-time delay lines offers improved performance and reduced weight and power consumption over existing parabolic dish antenna presently used for communications.
Wireless communications are rapidly becoming the means of data and information transfer for a broad range of applications. As wireless communication applications continue to expand, the information transfer rates are evolving toward the Gigabit per second data rate and, for some applications, there is even a need for terabit per second data rate transfer in the wireless network. In addition, wireless terminals often require instantaneous switching and communications between network members. For most applications directional antennas are needed to support the high data throughput requirements, and phased array antennas are the only high gain, directional antennas that can be rapidly switched to provide instantaneous communications among network members scattered geographically. Wireless terminal equipment is currently designed to operate in the 1 to 60 GHz frequency range and, traditionally, these equipment are designed with RF hardware. More recently, optics technology has been demonstrated to play an important role in RF systems as the True-Time-Delay in the phased array antenna, and, for some systems operating at high data rates, optical interconnects at the baseband level require E-O and O-E conversions. This paper discusses the considerations in using optics technology in the design of the wireless terminal network including optical signal processing, optical backplanes, optical networking, optical interconnects, and optical components. This paper also describes the architecture of an RF wireless communications network using a range of optical technologies.
Radio systems and, in particular, RF data link systems are evolving toward progressively more bandwidth and higher data rates. For many military RF data link applications the data transfer requirements exceed one Gigabit per second. Airborne collectors need to transfer sensor information and other large data files to ground locations and other airborne terminals, including the rel time transfer of files. It is a challenge to the system designer to provide a system design, which meets the RF link budget requirements for a one Gigabit per second data link; and there is a corresponding challenge in the development of the terminal architecture and hardware. The utilization of photonic circuitry and devices as a part of the terminal design offers the designer some alternatives to the conventional RF hardware design within the radio. Areas of consideration for the implementation of photonic technology include Gigabit per second baseband data interfaces with fiber along with the associated clocking rates and extending these Gigabit data rates into the radio for optical processing technology; optical interconnections within the individual circuit boards in the radio; and optical backplanes to allow the transfer of not only the Gigabit per second data rates and high speed clocks but other RF signals within the radio. True time delay using photonics in phased array antennas has been demonstrated and is an alternative to the conventional phase shifter designs used in phased array antennas, and remoting of phased array antennas from the terminal electronics in the Ku and Ka frequency bands using fiber optics as the carrier to minimize the RF losses, negate the use of the conventional waveguides, and allow the terminal equipment to be located with other electronic equipment in the aircraft suitable for controlled environment, ready access, and maintenance. The various photonics design alternatives will be discussed including specific photonic design approaches. Packaging, performance, and affordability of the various design alternatives will also be discussed.
Significant improvements in technology have made phased array antennas an attractive alternative to the traditional dish antenna for use on wide body airplanes. These improvements have resulted in reduced size, reduced cost, reduced losses in the transmit and receive channels (simplifying the design), a significant extension in the bandwidth capability, and an increase in the functional capability. Flush mounting (thus reduced drag) and rapid beam switching are among the evolving desirable features of phased array antennas. Beam scanning of phased array antennas is limited to +/-45 degrees at best and therefore multiple phased array antennas would need to be used to insure instantaneous communications with any ground station (stations located at different geographical locations on the ground) and with other airborne stations. The exact number of phased array antennas and the specific installation location of each antenna on the wide body airplane would need to be determined by the specific communication requirements, but it is conceivable as many as five phased array antennas may need to be used to provide the required coverage. Control and switching of these antennas would need to be accomplished at a centralized location on the airplane and since these antennas would be at different locations on the airplane an efficient scheme of remoting would need to be used. To save in cost and keep the phased array antennas as small as possible the design of the phased array antennas would need to be kept simple. A dish antenna and a blade antenna (small size) could also be used to augment the system. Generating the RF signals at the central location and then using RF cables or waveguide to get the signal to any given antenna could result in significant RF losses. This paper will evaluate a number of remoting alternatives to keep the system design simple, reduce system cost, and utilize the functional capability of networking multiple phased array antennas on a wide body airplane. Included in these alternatives will be the use of optical modules as the true time delay in the phased array antennas and using a fiber optic bus from the centralized control to drive the optical modules.
The design of some communication systems requires the implementation of time delays within the system. As a result, the system design often becomes more complex and/or the system size is increased when the delay is accomplished by digitizing the RF analog signal or using RF transmission lines cut to the length necessary to accomplish the time delay. This time delay can be accomplished with a variety of optics technologies, which could be readily fabricated and integrated into the communication system without significant impacts on the system design. This paper describes three different potential applications of optics designs, which could be implemented to accomplish the time delay requirements associated with communication system.
This paper describes a novel compact detector-switched polymeric waveguide true-time-delay (TTD) module for application in a wideband (18 - 26 GHz) phased array antenna. The use of photolithographically defined ultra-low-loss polymeric waveguide delay lines provides an attractive solution for achieving the necessary delay time over tens of nsec with ultra fine resolution of less than 1 ps. The two- dimensionally distributed waveguide grating couplers tap the optically encoded microwave signal, propagating along the polymeric channel waveguide, to high-speed photodetectors. Each photodetector is independently switched on and off to select the appropriate delay time. The optically encoded microwave signals are obtained by using a semiconductor-laser- based optical heterodyne technique. Such a monolithic integrated module could reduce the cost associated with optoelectronic packaging and also reduce the system complexity. This paper provides a description of the ongoing research activities in the development and integration of advanced polymeric circuits and packaging as a key building block of the next generation wide-bandwidth phased-array antenna system.
This paper reports our efforts to develop an optical True- Time-Delay line module for Phased Array Antenna applications using optical polymeric waveguides. We first give a brief description of a targeted phased array antenna, having chosen a 16-element sub-array as our demonstration system. Then we address the design considerations of the True-Time- Delay lines pattern based on the sub-array antenna's parameters, including simulations we have done to optimize the building blocks of the pattern: splitters, arcs' curvature, and crossings. Finally, we describe the steps of a modified fabrication process and present the primary result. Our experiment shows that the polyimide-based waveguide has a promising future because it has high fabrication precision and packaging density.
The switching characteristic of wide-band MSM and PIN photodetectors has been studied in theory and experiments. MSM detector has threshold bias voltage to activate its response and so has a better performance than PIN photodetectors when working as a photo-electronic switch. However, our study tells that through a suitable designed bias circuit, the PIN photodetector also can provide a switching operation with considerable performance. Especially for RF photonic signal, the extinction ratio can reach around 30dB. At different bias condition, the gain of PIN can be continually tuned and it has very important application in photonic phased-array antenna system.
CMOS compatible optical polyimide based thermo-optic switches have the potential use as low-power switches. These switches would have many advantages over other switches based on inorganic crystals. For one, they can be integrated into module-to-module systems using currently available VLSI fabrication techniques. Polyimide based, 1 by 2 thermo-optic switches are fabricated onto silicon wafers and tested. We report the properties and characteristics of digital thermo- optic switches designed to operate at 1.3 micrometers . Also, the switching characteristics at different heating electrode voltages are tested and compared.
We have demonstrated a polymeric electro-optic modulator based on a 1 X 2 Y-fed directional waveguide coupler. The symmetric geometry of the 1 X 2 Y-fed directional coupler provided the modulator unique characteristics of intrinsic 3 dB operating point and two complementary output ends. A low switching voltage of 3.6 V and a high extinction ratio of 26 dB were obtained with the modulator operating at a wavelength of 1.34 micrometers . The modulator was fabricated with a novel electro-optic polymer that was synthesized from polyurethane crosslinking with a chromophore.
A detailed design and fabrication procedure of high-speed traveling-wave electrodes for EO polymer-based modulator has been developed. Design consideration, thick photoresist deposition and electroplating are specially focused on. A lot of practical experiences are introduced as well. This kind of modulator can be used in satellite receiver systems, remote connection of cellular radio systems, and LANs.