712 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 11, JUNE 1, 2011 Photonic Generation of Phase-Coded Microwave Signal With Large Frequency Tunability Ze Li, Student Member, IEEE, Wangzhe Li, Student Member, IEEE, Hao Chi, Xianmin Zhang, and Jianping Yao, Senior Member, IEEE Abstract—A photonic approach to realizing phase-coded microwave signal generation with large frequency tunability is proposed and demonstrated. Two coherent optical wave- lengths are generated based on external modulation by biasing a Mach–Zehnder modulator (MZM) at the minimum transmission point to generate -order sidebands while suppressing the optical carrier. The two -order sidebands are then sent to a fiber Sagnac interferometer (SI) incorporating an optical phase modulator (PM) and a broadband flat-top fiber Bragg grating (FBG), with one of the sidebands being phase modulated at the PM. A frequency tunable phase-coded microwave signal is gener- ated by beating the two sidebands at a photodetector (PD). The proposed technique is experimentally investigated. The generation of a frequency tunable phase-coded microwave signal at 22 and 27 GHz is demonstrated. Index Terms—Microwave signal generation, phase coding, radar pulse compression. I. INTRODUCTION I N modern radar systems, pulse compression has been widely used to increase the range resolution [1]. Usually, pulse compression is implemented in a radar receiver by com- pressing a frequency-chirped or phase-coded pulse using a matched filter. Chirped or phase-coded pulses can be generated in the electrical domain using electronic circuitry. The main limitation of the electrical techniques is the small time-band- width product (TBWP), which limits the pulse compression ratio. A solution to generate a chirped or phase-coded pulse with a large TBWP is to use photonics techniques [2]–[8]. In [2], [3], a chirped or phase-coded microwave pulse was generated using a pulse shaping system in which a spatial light modulator (SLM) was employed. However, since an SLM-based system involves the use of free-space optics, the system is usually bulky and lossy. A chirped or phase-coded microwave pulse can also be generated using an all-fiber-based system. In [4], Manuscript received October 21, 2010; revised February 06, 2011; accepted March 16, 2011. Date of publication March 24, 2011; date of current version May 13, 2011. This work was supported by the Natural Sciences and Engi- neering Research Council of Canada (NSERC). The work of Z. Li was sup- ported by a scholarship from the China Scholarship Council. Z. Li is with the Microwave Photonics Research Laboratory, School of Infor- mation Technology and Engineering, University of Ottawa, Ottawa, ON K1N 6N5, Canada, and also with the Department of Information Science and Elec- tronic Engineering, Zhejiang University, Hangzhou, 310027 China. W. Li and J. Yao are with the Microwave Photonics Research Laboratory, School of Information Technology and Engineering, University of Ottawa, Ot- tawa, ON K1N 6N5, Canada (e-mail: jpyao@site.uOttawa.ca). H. Chi and X. Zhang are with the Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, 310027 China. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2011.2132121 a linearly chirped microwave pulse was generated by beating two linearly chirped optical pulses with different chirp rates at a photodetector (PD). In [5], a high-frequency chirped elec- trical pulse was generated based on optical spectral shaping and frequency-to-time mapping in a dispersive element. In [6], a technique to generate a phase-coded microwave pulse was demonstrated. However, the stability of the system was poor due to the use of a Mach–Zehnder interferometer [6]. To achieve a stable operation, a technique based on a photonic microwave delay-line filter was proposed [7]. The major lim- itation of the approach in [7] is the narrow bandwidth of the delay-line filter, which limits the bandwidth of the generated pulse. A technique to generate a phase-coded microwave pulse with a large bandwidth was recently demonstrated using a polarization modulator (PolM) [8]. The major limitation of the technique is that the microwave frequency is not tunable as the orthogonality of the two light waves to the PolM is dependent on the wavelength spacing. In this letter, we propose and demonstrate a novel approach to realizing phase-coded microwave signal generation with large frequency tunability. In the approach, two coherent op- tical wavelengths are generated based on external modulation by biasing a Mach–Zehnder modulator (MZM) at the minimum transmission point (MITP) to generate -order sidebands while suppressing the optical carrier. The two sidebands are then sent to a fiber Sagnac interferometer (SI), incorporating a phase modulator (PM) and a broadband flat-top fiber Bragg grating (FBG), with one of the sidebands being phase mod- ulated at the PM. By beating the two sidebands at a PD, a phase-coded microwave signal is generated. The frequency of the phase-coded microwave signal can be tuned by tuning the frequency of the microwave driving signal. The proposed technique is experimentally investigated. A phase-coded mi- crowave signal at 22 and 27 GHz is generated. II. PRINCIPLE The schematic of the proposed system is shown in Fig. 1. A CW light wave from a tunable laser source (TLS) is sent to an MZM through a polarization controller (PC1). A sinusoidal mi- crowave driving signal is applied to the MZM, which is biased at the MITP to generate two -order sidebands while sup- pressing the optical carrier. The two sidebands are then sent to an SI through PC2. The SI consists of a 3-dB optical cou- pler (OC), an optical isolator (OI), an FBG and a PM. The bidi- rectional operation of the PM is employed. Due to the velocity match in the PM, the light wave that is copropagating with the microwave signal is efficiently modulated. When the light wave 1041-1135/$26.00 © 2011 IEEE