This paper presents experimental results demonstrating adaptive beam forming and jammer nulling for phased-array antenna applications using the Broadband Efficient Adaptive Method for True-time-delay Array Processing (BEAMTAP) algorithm. The BEAMTAP algorithm has the advantage of mapping efficiently into an opto-electronic architecture that minimizes the required number of tapped-delay lines and simultaneously allows for the signals to be processed coherently, assuming that phase stabilization has been achieved. The architecture also utilizes a unique polarization-angle, read-write multiplexing system that allows for 45 dB of total jammer suppression at the output. Successful narrowband and broadband adaptive beam forming and jammer nulling results are provided in the worst-case scenario of co-site interference where both the jamming signal's angle of incidence and spectral content overlap with that of the signal of interest.
We propose, analyze, and demonstrate the use of a holographic method for cohering the output of a fiber tapped-delay-line (FTDL). We perform a theoretical examination of the phase-cohering process and show experimental results for an RF spectrum analyzer based on a phase-cohered FTDL that shows 50 MHz resolution and bandwidths in excess of 2 GHz. Phase-cohering holography can operate on thousands of fibers in parallel, enabling both fiber tapped-delay-lines and the coherent fiber remoting of optically-modulated RF signals from antenna arrays.
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.
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.
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.
We present the analytical description of a photorefractive phased array beamforming system using the BEAMTAP (Broadband and Efficient Adaptive Method for True-Time-Delay Array Processing) algorithm for a large N-element array that requires only 2 tapped delay lines (TDLs) instead of the conventional N TDLs. Simulation results indicate that the processor is able to adapt to a broadband signal of interest at a specific angle of arrival. We show that the system produces a coherent sum of the desired signals from the phased array, with the corresponding time delays appropriately compensated for in an adaptive fashion without prior knowledge of the angle-of-arrival.