Gregory Kriehn

Optical BEAMTAP beam-forming and jammer-nulling system for broadband phased-array antennas.

We present an approach to receive-mode broadband beam forming and jammer nulling for large adaptive antenna arrays as well as its efficient and compact optical implementation. This broadband efficient adaptive method for true-time-delay array processing (BEAMTAP) algorithm decreases the number of tapped delay lines required for processing an N-element phased-array antenna from N to only 2, producing an enormous savings in delay-line hardware (especially for large broadband arrays) while still providing the full NM degrees of freedom of a conventional N-element time-delay-and-sum beam former that requires N tapped delay lines with M taps each. This allows the system to adapt fully and optimally to an arbitrarily complex spatiotemporal signal environment that can contain broadband signals of interest, as well as interference sources and narrow-band and broadband jammers--all of which can arrive from arbitrary angles onto an arbitrarily shaped array--thus enabling a variety of applications in radar, sonar, and communication. This algorithm is an excellent match with the capabilities of radio frequency (rf) photonic systems, as it uses a coherent optically modulated fiber-optic feed network, gratings in a photorefractive crystal as adaptive weights, a traveling-wave detector for generating time delay, and an acousto-optic device to control weight adaptation. Because the number of available adaptive coefficients in a photorefractive crystal is as large as 10(9), these photonic systems can adaptively control arbitrarily large one- or two-dimensional antenna arrays that are well beyond the capabilities of conventional rf and real-time digital signal processing techniques or alternative photonic techniques.

Spatio-temporal operator formalism for holographic recording and diffraction in a photorefractive-based true-time-delay phased-array processor

We present a spatio-temporal operator formalism and beam propagation simulations that describe the broadband efficient adaptive method for a true-time-delay array processing (BEAMTAP) algorithm for an optical beamformer by use of a photorefractive crystal. The optical system consists of a tapped-delay line implemented with an acoustooptic Bragg cell, an accumulating scrolling time-delay detector achieved with a traveling-fringes detector, and a photorefractive crystal to store the adaptive spatio-temporal weights as volume holographic gratings. In this analysis, linear shift-invariant integral operators are used to describe the propagation, interference, grating accumulation, and volume holographic diffraction of the spatio-temporally modulated optical fields in the system to compute the adaptive array processing operation. In addition, it is shown that the random fluctuation in time and phase delays of the optically modulated and transmitted array signals produced by fiber perturbations (temperature fluctuations, vibrations, or bending) are dynamically compensated for through the process of holographic wavefront reconstruction as a byproduct of the adaptive beam-forming and jammer-excision operation. The complexity of the cascaded spatial-temporal integrals describing the holographic formation, and subsequent readout processes, is shown to collapse to a simple imaging condition through standard operator manipulation. We also present spatio-temporal beam propagation simulation results as an illustrative demonstration of our analysis and the operation of a BEAMTAP beamformer.

Holographic method of cohering fiber tapped delay lines.

We propose, analyze, and demonstrate the use of a holographic method for cohering the output of a fiber tapped delay line (FTDL) that enables the use of fiber-remote optical modulators in coherent optical processing systems. We perform a theoretical examination of the phase-cohering process and show experimental results for a radio frequency (RF) spectrum analyzer that uses a lens to spatially Fourier transform the output of a holographically phase-cohered FTDL providing 50 MHz resolution and bandwidths approaching 3 GHz. Substantial improvements in bandwidth should be achievable with better fiber length-trimming accuracy and improvements in resolution can be obtained with longer fiber delay lines. We also analyze and demonstrate the use of a parallel holographic technique that compensates for polarization state scrambling induced by propagation through an array of single-mode fibers. Both the phase-cohering holography and the polarization fluctuation compensation can operate on hundreds of fibers in parallel, enabling both coherent optical signal processing with FTDLs and coherent fiber remoting of optically modulated RF signals from antenna arrays.

Detection and time delay of a broadband rf signal using a traveling fringes detector

This paper presents an optical system which enables a broadband RF signal to be detected and delayed by a traveling fringes detector (TFD) using an acousto-optic deflector (AOD) and a 4f imaging system. The TFD is based upon the synchronous drift of photo-generated carriers with a moving interference pattern; the moving interference pattern is generated by interfering two coherent beams of light at different frequencies. Light which is incident on the photoconductive layer of the detector will generate photocarriers with a specific drift velocity proportional to the applied bias voltage. For a fixed angle between the two beams, a resonance peak occurs when the drift velocity equals the fringe velocity of the moving interference pattern. Detection of a broadband signal, therefore, is difficult since each frequency component produces a different fringe velocity and thus has a different resonance peak associated with the detector. Broadband detection of a signal is allowed by forcing each of the detected moving interference patterns, each corresponding to a specific temporal RF frequency, to have the same velocity as the electron drift velocity. This can be accomplished by using an AOD to linearly deflect each frequency component of the RF signal at the appropriate angle in order to maintain a constant overall fringe velocity at the TFD.

All-optical antenna array adaptive processing system

We present an all-optical architecture for a fully adaptive antenna array processor capable of optimally processing the signals from very large arrays in the presence of high frequency and wideband signals. A modified version of the least mean square algorithm is employed using the BEAMTAP (Broadband and Efficient Adaptive Method for True-time-delay Array Processing) architecture. A dynamic photorefractive volume hologram is used for the adaptive weights and two cohered fiber arrays are used as tapped-delay-lines at the output and feedback paths, allowing for the processing of signals at bandwidths exceeding 10 GHz. The optical cohering of the fiber arrays is discussed and simulations are shown which describe the performance of the proposed architecture in the presence of broadband signals and multiple broadband jammers.

Photorefractive phased array beamforming with true-time-delay processing

We introduce a new optical approach to the problem of implementing true-time-delay (TDD) beamforming for a large N-element adaptive array that requires only 2 tapped delay lines (TDLs) instead of the conventional N TDLs. This new algorithm, BEAMTAP (Broadband and Efficient Adaptive Method for TTD Array processing) is capable of processing very large phased array radar systems with thousands of receiving elements with GHz of bandwidth. The corresponding digital processing load that would be required to solve such large, broadband adaptive array problems exceeds 10/sup 15/ multiply-accumulate operations per second: well beyond the capability of current electronic systems.

Experimental adaptive beam forming with polarization read-write multiplexing using BEAMTAP

In this paper, we present experimental results demonstrating broadband beam forming on simulated signals from a phased-array using a true-time-delay RF photonic processor implementing the BEAMTAP (Broadband Efficient Adaptive Method for True-time-delay Array Processing) algorithm.

Wideband all-optical BEAMTAP

A holographically-cohered approach to the all-optical implementation of time-delay-and-sum array processing for large, wideband adaptive-beamforming and jammer-nulling antenna arrays that requires only two coherent-fiber tapped-delay-lines with 20 GHz squint-free bandwidth capability is presented.

BEAMTAP beam forming using a traveling fringes detector

Demonstration of traveling-fringes-detector operation for beam forming on simulated signals from a fiber-remoted phased array using a true-time-delay RF-photonic processor implementing the BEAMTAP (Broadband Efficient Adaptive Method for True-time-delay Array Processing) algorithm is presented.

Adaptive beamforming with TDI CCD-based true-time-delay processing

We present an adaptation of the BEAMTAP (Broadband and Efficient Adaptive Method for True-time-delay Array Processing) algorithm, previously developed for wideband phased array radars, to lower bandwidth applications such as sonar. This system utilizes the emerging time or wavelength multiplexed optical hydro-phone sensors and processes the cohered array of signals in the optical domain without conversion to the electronic domain or digitization. Modulated signals from an optical hydro-phone array are pre- processed then imaged through a photorefractive crystal where they interfere with a reference signal and its delayed replicas. The diffraction of the sonar signals off these adaptive weight gratings and detection on a linear time- delay-and-integrate charge coupled device (TDI CCD) completes the true-time-delay (TTD) beamforming process. Optical signals focused on different regions of the TDI CCD accumulate the appropriate delays necessary to synchronize and coherently sum the acoustic signals arriving at various angles on the hydro-phone array. In this paper, we present an experimental demonstration of TTD processing of low frequency signals (in the KHz sonar regime) using a TDI CCD tapped delay line. Simulations demonstrating the performance of the overall system are also presented.