Performance Characteristics of 5-bit Optical Receive-Beamformer at 1550nm

Binary and ternary 5-bit programmable dispersion matrix is built to control a two-channel receives beamformer at 1550 nm. Phase measurements for the delay configurations along with beam-patterns at RF frequencies 0.2-1 GHz are presented. 1INTRODUCTION Many RF and microwave systems, such as high-resolution phased-array antennas and signal processing electronics, require true-time delay (TTD) phase shifters. In such systems the individual T/R-element control allows the implementation of beam steering and shaping. In conventional RF systems, TTD is achieved by switching to different lengths of electrical cable. However, these implementations tend to be bulky, heavy and susceptible to electromagnetic interference. The fiber-optic control systems provide benefits in the above areas. A variety of optical techniques have been proposed for obtaining TTD capability using fiber-optic systems [1]. In particular, systems using fiber Bragg reflectors for providing time delays have been proposed and demonstrated [2,3]. In Ref. [4] a 2-bit transmit/receive module using a fiber Bragg grating matrix has been demonstrated. In this paper we design and experimentally demonstrate a binary and ternary version of a two-channel truetime delay programmable matrix for controlling a phase array antenna. The wideband processor has a resolution of 5bit. 2SYSTEM OVERVIEW The schematic drawing for a receive-mode beamformer is shown in Figure 1. An incoming RF signal from a target is received by the two antenna array elements. The phase difference at the antenna elements depends on the target position. The received RF signal at each element separately modulates two individual optical carriers, which are provided by laser diodes with wavelength λ1 and λ2. Figure 1: Beamformer setup configuration for receive mode. The modulation is performed by two Mach-Zender electro-optic modulators (EOM). The multiplexed optical carriers feed a programmable dispersion matrix, which performs the true-time delay processing. The time delay between optical channels is corrected by the PDM and detected with a single photodetector. The output power of the photodetector is a function of the corrected phase difference between the two RF signals ( ) φ ∆ + = cos log 10 ) ( 2 1 K K dB P , (1) where φ ∆ is the phase difference and K1, K2 are the proportionality constants. Thus, the output power is related to the target angular position via this phase difference. When the PDM corrects for the phase difference at the antenna elements a maximum power will be detected for the target position. The architecture of a 5-bit programmable dispersion matrix (PMD), which is based on fiber Bragg grating (FBG) arrays, is shown in Figure 2. The N-bit version of two-channel architecture consists of an array of N delay lines. Each delay line is constructed by splicing two FBGs. The center wavelength of each FBG matches one of the multiplexed optical channels. The separation between Bragg reflectors is different for each delay line. Thus, the time delay(s) between channels are proportional to these FBG separations. The separation of two adjacent gratings for the th i line is given by 1 1 2 L L i i ∆ = ∆ − , (2) where 1 L ∆ is the minimum separation between gratings that corresponds to line 1. Using Eq. (2) the time delay provided by the th i line can be written as c L n i eff i ∆ = 2 τ , (3) where neff is the effective refraction index of the fiber and c is the speed of light. Each of the N 2 delay configurations of the PDM is an integer multiple, m, of minimum time delay 1 τ . The minimum time delay, associated with line 1, is directly related with the angular resolution and the minimum steering angle of any beamformer [5]. The steered angle m θ is related to a characteristic parameter of the PDM, that is 1 τ , and a geometrical parameter of the antenna, the T/R element spacing Λ , by     Λ = 1 arcsin τ θ m c