Rose-Hulman/NSWC-Crane Optical Radar Project
Project Advisor: Dr. Keith Williams
Program Officer: Dr. Daniel Purdy
Program Advisor: Dr. Harold Szu
Project Manager: Kenneth Johnson 
Project Engineers: Jeff Chestnut, Kevin Kellar and Charles Pagel
Project PI: Dr. Azad Siahmakoun

Wideband Optically Multiplexed Beamformer Architecture

Rose-Hulman Institute of Technology, in collaboration with NSWC-Crane, demonstrated a wideband optically multiplexed beamforming architecture (WOMBAt).  The system possess a minimum of 2 GHz instantaneous bandwidth at 70 scan. The beamformer has achieved high receive/transmit performance in terms of dynamic range and noise figure. WDM MUX and DEMUX are used to combine and separate the optical channels for the parallel delay processor while fiber Bragg grating arrays provide the coarse delay to each 1x4 subarray . 

WOMBAt Summer 2000 Research Team

WOMBAt Summer 2001 Research Team

WOMBAt Summer 2002 Research Team

WOMBAt Summer 2003 Research Team


            Rose-Hulman  has also proposed and demonstrated a high-performance transmit/receive two RF-beams two-optical channel beamformer prototype based on wavelength division multiplexing (WDM) technology capable of controlling a phased-array antenna. The processor can be programmed to sweep the antenna aperture through 60o following an independent angular sequence for each RF beam. The optical beamformer processes two independent RF-beams, for eight different angular directions, and it is based on a binary array of three delay lines. Each delay line is composed of four fiber-Bragg gratings (FBG) whose center wavelengths are channels 30 to 33 of the ITU grid. All FBG arrays with multiple gratings on a single-strand of photosensitive-fiber have been successfully fabricated for this project using an Excimer laser. Measurements are performed for both receive and transmit modes and for RF values between 0.5 GHz to 1.5 GHz. Beampattern results show the squint-free performance of the beamformer within this frequency range. In the transmit/receive mode two independent-simultaneous RF beams are steered and characterized for a broadside target position.


The WDM optical beamformer architecture is designed to be scalable to a large system consisting of multiple RF-beams and 10,000 or more antenna elements. Note that no additional optical components such as circulators, switches or attenuators will be necessary to scale up this proposed PDM for phased array antennas with large number of T/R elements. This upgrade only requires an increase in the number of FBG in each delay line. However, the total insertion loss (IL) of the PDM will increase significantly as the number of RF-beams and delay lines increase. This total loss is the sum of contribution from the circulators, couplers, the optical switches and the interleavers. The total number of required optical channels is proportional to the number of RF-beams and antenna elements. Consequently, the number of available WDM channels limits the number of RF-beams that can be processed by antennas with large number of T/R elements. However, stable multi-wavelength broadband lasers sources combined with reduction in the inter-channel spacing of DWDM technology to 25 GHz or less can partially overcome these limitations.


A novel high-speed, cost-effective and programmable true-time delay (TTD) processor for optical phased-array antennas based on a fiber-optic recirculating-loop has also been proposed. In comparison to the other methods this architecture reduces significantly the number of true-time delay components. An experimental feasibility study of this architecture has been completed.



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