Photonic Microwave True-Time Delay Based on a Tapered Fiber Bragg Grating With Resistive Coating

Abstract

A photonic microwave true-time delay based on a tapered fiber Bragg grating (FBG) with resistive coating has been proposed and demonstrated. It features voltage-controlled continuous operation, requiring no moving parts and no mechanical perturbation. The time delay of a microwave signal carried over an optical signal is obtained by electrically controlling the reflection positions of it from the tapered FBG via the thermooptic effect. The achieved time delay was about 105 ps with the electrical power consumption of 48 mW. Index Terms-Gratings, microwave antennas, optical fiber applications, phased arrays, time delay.O PTICAL beamforming networks based on a photonic microwave true-time delay (TTD) have attracted much attention due to their applications to a phased-array antenna for modern radar and communication systems [1]. They have advantages over the traditional electronic steering systems including the immunity to electromagnetic interference, the squint-free wide instantaneous bandwidth, and the light weight and small volume, which is especially important in airborne applications. A continuous TTD is preferred to a discrete type one for achieving higer beam steering accuracy. So far various types of photonic TTDs, which mostly utilize a fiber Bragg grating (FBG), have been reported based on the static strain perturbation [2], the moving strain perturbation [3], and the wavelength scanning [4]. The approaches in [2] and [3] may suffer from reliability and accuracy, involving mechanical moving parts and deformation. And for the method mentioned in [3], an expensive tunable light source is required. In this letter, a continuous photonic microwave TTD is proposed and demonstrated using a tapered FBG with resistive coating. It features voltage controlled continuous operation-desirable for high precision and good reliability-requiring no moving parts and no mechanical perturbation. Only a single wavelength light source is needed.For a photonic microwave TTD, an optical signal is modulated by a microwave signal, delayed appropriately by the device, and detected to provide a time-delayed microwave signal. Fig. 1(a) shows the schematic configuration of the proposed photonic TTD. It consists of a tapered uniform FBG Manuscript Fig. 1. Proposed photonic microwave TTD based on a tapered FBG (a) schematic configuration (b) distribution of reflection wavelength with the control voltage.and a heating electrode. With no control voltage applied to the electrode, the FBG exhibits ideally a single reflection wavelength. As the control voltage is applied to the electrode, the temperature of the FBG region under the electrode is elevated and accordingly its refractive index is increased through the thermooptic effect. At a certain position on the tapered FBG, the temperature change caused by the resistive heating is inversely proportional to the squared value of the fiber diameter because the resistance of the electrode decreases with it. Therefore, a temperature gradient is introduced to the tapered FBG so that the change in t...

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