Polymer-based Guided Wave True-time Delay
 Module for Phased Array Antenna

This program is sponsored by MDA and AFOSR.Phased array antennas (PAAs) are more and more important for human society due to the following properties.  Phased array antennas can transmit/receive electromagnetic field at/from any direction without any mechanical movement of the antenna body.  Phased array antennas usually have broad bandwidth, so that they can carry more information.  Phased array antennas have low visibility, the difficulty to detect due to the fact that there are no moving parts.  Phased array antennas have the character of quick steering, either electrical or optical controlled.  Furthermore, phased array antennas can be applied to a multi-mode operation.  Having so many advantages over a single antenna, phased array antennas are widely used in both civilian operations, such as in air traffic, smart antennas, and broadcast satellite communications, and military areas, such as in marine radar, airborne radar, missile guidance, trajectory determination, and satellite communications.

True time delay (TTD) feeding network, which can be categorized into electrical true time delay techniques and optical true time delay techniques, is important for broadband phased array antennas to obtain squint free steering.  Electrical true time delay techniques have longer histories and are more mature compared to optical counterparts. However, these techniques are intrinsic narrowband.  Optical true time delay techniques are promising for squint-free beam steering of phased array antennas with features of wide bandwidth, compact size, reduced weights and low electromagnetic interference.  Recently, there have been researches on acoustic-optic (AO) integrated circuit technique and Fourier optical technique, and broad band techniques including bulky optics techniques, wavelength-division-multiplexing technique (WDM), planar waveguide techniques, fan-out technique, etc.

In our group, we developed both digital and analog TTD module based on holographic gratings and substrate-guided-wave.  The digital TTD module is composed of eight sub-units.  The optical signal, carrying an encoded microwave signal, is distributed among the eight sub-units using a 1-to-8 splitter. Each of the sub-units has a wedge of 21.5 degrees as indicated in Figure 1.  The wedges are coated with total reflect material to ensure that all the optical power is coupled into the substrate. The wedge angle introduces a bounce angle that is larger then the total internal reflection angle (41.8 degrees) at the interface of the BK-7 substrate and air. Adjacent to each wedge, the height of each sub-unit is maintained at the same value at 2.6 mm.  The height of each sub-unit varies after one zigzag bouncing and maintains at a fixed value for the rest of the sub-unit. Heights of the eight sub-units after the first zigzag bouncing are from h₁ to h₈, with a difference of  between adjacent sub-units. The difference  is pre-selected to satisfy the required delay combinations. The input signals from single-mode optical fibers are coupled into the module using graded index (GRIN) lenses. The substrate-guided wave zigzags within the substrate through total internal reflection. A portion of the substrate guided wave is extracted out each time the wave encounters the output holographic-grating coupler. The extracted optical waves are focused back into optical fibers using GRIN lenses.

Figure 1 Diagram of the structure of the 6-bit TTD module based on substrate-guided-wave and holographic-grating couplers.

The position of delay signals in the delay matrix is depicted by (i, j), with i for the row number, j for the column number. Assuming the wedge angle is , the introduced time delay between signals at (i, j) and (k, l) is given by

where c is the velocity of light in free space, n the refractive index of the substrate, h_i the height of the ith substrate, and h_k the height of the kth substrate.

            The module we designed have the ability to provide delay intervals for 0, ±4.5⁰, ±9.1⁰, ±13.7⁰, ±18.5⁰, ±23.3⁰, ±28.3⁰, and ±33.6⁰ steering.

We designed an analog TTD module employing the dispersion effect of the holographic gratings.  Figure 2 illustrates the architecture of the continuously variable TTD module. The substrate, made of BK7 glass, is divided into several sub-areas.  Each sub-area has an optical input signal with the microwave signal impressed on it.  The wavelengths of all input optical carriers have a fixed relationship: the wavelength differences between adjacent linear fan out array are the same.  The wavelengths can be expressed as , , Ö,, , , and , where  is the central wavelength, the wavelength pitch and  the total number of wavelengths.  Collimators are used to convert the signals in optical fibers into parallel beams, which are coupled into the substrate using the input holographic optical couplers.  The diffraction angle of the holographic optical couplers is designed to be greater than the critical angle at glass and air surface.  After several zigzags through total internal reflection within the substrate, the delayed parallel beams are coupled out vertically using output holographic optical couplers. The coupled out parallel beams are focused back into optical fibers through focusers.

Figure 2  Architecture of the continuously variable TTD module

If the wavelength difference is , the time delay step can be calculated as:

            Special design of these parameters can produce time delays from 0 to 90⁰ steering.

Two-dimensional TTD structures based on these modules are under investigation. Please refer to the published papers for detail TTD characters and system performances, and keep updated.