<title>Fiber array optical-coupling design issues for photonic beam formers</title>

Abstract

Novel two-dimensional (2-D) optical polarization switching array-based photonic time delay units (PTDUs) have been introduced for phased array antenna and wideband signal processing applications [1]. The use of low loss optical fibers allows remoting of the photonic beamformer, along with providing a compact, lightweight, and low electromagnetic interference (EMI) microwave frequency signal interconnection and distribution method, such as needed for very large aperture wide instantaneous bandwidth phased array antennas/radars. However, there are losses associated with multiple fiber interconnects that limit the maximum number of array channels in these systems. Thus, accurate analysis of such losses is crucial to the design of an optimal photonic fiber-based system. In this paper, we will present theoretical design and simulation results on optical fiber array interconnects for our 2-D N bit M channel photonic beamformer for wideband phased array antennas. In addition, we will discuss an alignment technique for the large channel count fiber arrays proposed for our beamformer that uses V-grooved silicon wafers. Note that these precise V-groove structures are fabricated via crystallographic perfection of the substrate, accurate alignment of the etch pattern with respect to the crystal planes, and optimized etch conditions. This paper will discuss these and other fiber array issues.Keywords: fiber arrays, optical coupling, optical loss, phased array antennas INTRODUCTIONIn recent years, photonically controlled phased array antennas have been developed and studied in the area of radars and wireless communications. Key advantages of photonic beamformers are light weight, low loss, low crosstalk, potentially low cost, and high speed parallel processing. Photonic beamformer systems for phased arrays such as the bulk-optical controller proposed by Riza [1,2] require a large (>100) number of fiber interconnects that should be assembled with the lowest possible loss, and with the least assembly complexity and expense. By utilizing fiber interconnects, delay lines with very low crosstalk can be formed and further remoting of the phased array antenna is possible. The fiber array can be aligned to an microlens array. Thus, guided wave in the fiber is coupled in the air through microlens or graded-index (GRIN) rod lens collimators. The collimated parallel beams are processed through the PTDUs in our 2-D N bit M channel photonic beamformer for wideband phased array antennas [3]. The collimated beams can be re-coupled into fibers through the GRIN rod lenses to realize the long time delays or interconnects. However, there are losses associated with multiple fiber interconnects that limit the maximum number of array channels in these systems. Thus, accurate analysis of such losses is crucial to the design of an optimal photonic fiber-based system. One of the most important operations involved in a fiber optic system is the fiber core alignment to reduce power loss. Manufacturing tolerances on core eccentricity and fiber outer diam...

<title>Advances in three-dimensional reversible photonic modules for phased-array control</title>

“…Earlier, we have characterized the issues dealing with fiber array coupling optics such as fiber GRiN lensbased coupling for phased array applications [18]. We experimentally showed that for direct GRIN--free space/solid optics path-GRIN coupling, optical losses are very small (e.g., <0.2 dB) if the free-space path is < 5 cm.…”

Synchronous amplitude and time control for an optimum dynamic range variable photonic delay line

“…Single-mode fiber-coupling efficiency with a FO collimator approach is highly dependent on the wavefront characteristics of the incident wave. 24,25 Thus the 2-D phase perturbation can affect the coupling efficiency of the beam into the FO collimator. Hence an attenuated optical signal can be obtained by proper adjustment of the phase perturbation distribution across the incident beam.…”