Photonic instantaneous measurement of microwave frequency using fiber Bragg grating Ze Li, Bo Yang, Hao Chi * , Xianmin Zhang, Shilie Zheng, Xiaofeng Jin Department of Information and Electronic Engineering, Zhejiang University, Hangzhou 310027, China article info Article history: Received 6 July 2009 Received in revised form 16 October 2009 Accepted 16 October 2009 Keywords: Microwave photonics Microwave frequency measurement Fiber Bragg grating (FBG) abstract A photonic approach to realizing instantaneous measurement of microwave frequency based on optical monitoring using a fiber Bragg grating (FBG) is proposed and demonstrated. In the approach, a frequency- unknown microwave signal is modulated on an optical carrier in a Mach–Zehnder modulator biased at the minimum transmission point. After detecting the transmission and reflection optical powers at the output of the FBG, the microwave frequency can be determined according to the value of transmis- sion-to-reflection power ratio, due to the fixed relationship between the microwave frequency and the power ratio. A proof-of-concept experiment has been performed, which demonstrates that a measure- ment resolution of ±0.08 GHz over a 10 GHz measurement bandwidth is achieved. The measurement per- formance in terms of resolution is better than previously reported results. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Due to the advantages of wide bandwidth, light weight, low loss and immunity to electromagnetic interference (EMI), the genera- tion, distribution, and processing of microwave signals using pho- tonic methods have attracted great interest for more than one decade [1–3]. In modern radar systems and other military applica- tions, it is required for a microwave receiver to estimate the fre- quency of a received unknown microwave signal over a large bandwidth. The conventional electronic techniques for the micro- wave frequency measurement are thought to be slow, bulky, lim- ited in bandwidth and vulnerable to EMI. Recently, photonic approaches for microwave frequency measurement have been intensively investigated [4–13]. Several photonic scanning receivers and channelizers for fre- quency measurement were proposed for the microwave frequency measurement [4–7]. A Fabry-Pérot (F–P) etalon-based temporal scanning receiver system was proposed in [4]. A high-resolution free-space optical diffraction grating-based channelizer was dem- onstrated in [5]. In [6], an array of ultra-narrow phase-shifted grat- ings was employed as wideband microwave channelized receiver. It was demonstrated that microwave channelizer could also be realized using an integrated optical Bragg-grating F–P together with an integrated hybrid Fresnel lens system [7]. Recently, a num- ber of photonic techniques for instantaneous microwave frequency measurement have been proposed and demonstrated [8–13]. In [8–12], the microwave frequency was estimated according to the mapping relationship between the frequency and the ratio of two dispersion-induced RF power-fading functions. The RF power fad- ing can be easily realized in an intensity-modulated or phase-mod- ulated dispersive radio over fiber (RoF) link. The limitation was that the expensive high-speed photodiodes (PD) should be em- ployed in the approach based on RF power monitoring and com- parison. Another approach was based on optical power monitoring and comparison [13], which can be realized using a low-frequency PD. The use of low-frequency PD reduces the sys- tem cost. However, the approach in [13] needs two laser sources and the employed Sagnac-loop optical filter in the approach has stability problem, which inevitably leads to a relatively low mea- surement resolution. In this paper, we propose and demonstrate a novel approach to the instantaneous measurement of microwave frequency with a higher resolution based on optical power monitoring, in which a well-packaged athermal fiber Bragg grating (FBG) is employed. Thanks to the complementary nature of the FBG’s transmission and reflection spectrum, only one laser source is needed and a fixed relationship between the microwave frequency and the opti- cal power ratio can be obtained. Since the optical power, not the RF power, is measured, low cost PD with low-frequency response can be employed in the approach. In addition, the achieved measure- ment resolution is much higher than previous approach due to the use of a well-packaged athermal FBG. It is experimentally dem- onstrated that measurement error less than ±0.08 GHz over a bandwidth larger than 10 GHz is realized. 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.10.036 * Corresponding author. Tel.: +86 571 87176827. E-mail address: chihao@zju.edu.cn (H. Chi). Optics Communications 283 (2010) 396–399 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom