3846 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 20, OCTOBER 15, 2014 Spurious Noises Analysis of Microwave Photonic Filters Based on Phase Modulation to Intensity Modulation Conversion Using an Optical Notch Filter Peixuan Li, Wei Pan, Xihua Zou, Member, IEEE, Wangzhe Li, Member, IEEE, Lianshan Yan, Senior Member, IEEE, and Bin Luo Abstract—The spurious noises of a narrow-bandpass microwave photonic filter (MPF) using phase modulation to intensity modu- lation (PM-IM) conversion and an optical notch filter are analyzed in theory and demonstrated experimentally. Due to the difference between the nonlinear filtering effect of PM-IM conversion and the linear one of finite impulse response or infinite impulse response, additional spurious noises will be generated in the former filter. Analysis models with two input microwave tones are developed for revealing the spurious noises in the MPF based on PM-IM conver- sion under small-signal and large-signal modulations, which are then verified by simulations and experiments, respectively. The re- sults show that, besides the target microwave signal to be selected, the inter-modulation noise and the harmonic noises also coexist at the output of the MPF, wherein the 2nd-order inter-modulation noises and the 2nd-order harmonic noise of the target signal are the dominant spurious ones. Therefore, an excellent agreement among the results from analysis models, the simulations, and the experiments is obtained. Index Terms—Microwave photonic filter (MPF), optical notch filter, phase modulation to intensity modulation (PM-IM) conver- sion, spurious noise. I. INTRODUCTION M ICROWAVE photonic filters (MPFs) have attracted much attention because of the advantages of large in- stantaneous bandwidth, flexible transmission response, light weight, and immunity to electromagnetic interference over tra- ditional electrical filters. An MPF is usually a finite impulse response (FIR)-based [1], [2] or infinite impulse response (IIR)- based [3], [4] structure. In the FIR-based MPF, linear phase Manuscript received January 14, 2014; revised April 2, 2014; accepted April 18, 2014. Date of publication April 24, 2014; date of current version September 1, 2014. This work was supported in part by the National Natural Science Foundation of China under Grant 61378008, by the “973” Project un- der Grant 2012CB315704, by the Research Fund for the Doctoral Program of Higher Education of China under Grant 20110184130003, and by the Program for New Century Excellent Talents in University of China under Grant NCET- 12-0940. P. Li, W. Pan, X. Zou, L. Yan, and B. Luo are with the Center for Infor- mation Photonics and Communications, School of Information Science and Technology, Southwest Jiaotong University, Chengdu 610031, China (e-mail: zouxihua@swjtu.edu.cn). W. Li is with the Microwave Photonics Research Laboratory, School of Elec- trical Engineering and Computer Science, University of Ottawa, Ottawa, ON K1N 6N5, Canada. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2014.2320257 spectral responses can be formed easily, regardless of the re- quirement of a large number of taps for frequency selectivity enhancement. While in the IIR-based MPF, a narrow bandwidth or steep transition band can be realized with a few taps, al- beit of the instability. On the other hand, both FIR-based and IIR-based MPFs have intrinsic periodic responses, rather than a single narrow passband desired in many application fields. An MPF using ultranarrow optical filters provides a promis- ing way to realize a single narrow passband without the limitations listed above. For instance, the stimulated Brillouin scattering (SBS) enables the MPF to have a narrow and central- frequency-tunable passband for its ultra-narrow bandwidth gain spectrum [5], where the SBS gain selectively amplifies the side- band of the single-sideband (SSB) modulated optical signal [6] or destroys the orthogonal condition between the optical carrier and sideband [7]. Here both carrier-suppressed double side- band and the SSB modulations should be implemented over a wide frequency range. When an optical narrow band-pass filter [e.g., a phase-shifted fiber Bragg (PS-FBG) with an ultranar- row passband in transmission response] is employed, the MPF can selectively filter out the sideband induced by the target mi- crowave signal under SSB [8], [9]. Here the SSB modulation is still needed. Also, phase modulation to intensity modulation (PM-IM) conversion has been employed to implement photonic mi- crowave signal filtering with a single passband, with additional advantages of phase modulation (PM) including low half-wave voltage and no dc bias. PM-IM conversion can be realized by us- ing dispersive device to eliminate the dc components [10]–[12], and to provide arbitrary flat-top transmission responses [13]. However, it still has a quasi-periodic response. To realize a single passband, optical notch filters are introduced to implement PM- IM conversion, by destroying the balanced relationship between the lower and the upper optical sidebands in amplitude or/and in phase [14]. Based on PM-IM conversion, an ultranarrow notch in the optical domain leads to the generation of a single passband in the microwave domain. For instance, in [15] the SBS gain se- lectively amplifies one sideband of the phase-modulated light wave, resulting in a narrow, single passband in the microwave transmission response. Also, a ring resonator or two uniform fiber Bragg gratings (FBGs) can be employed to remove one sideband of the phase-modulated light wave as well [16], [17]. By using a PS-FBG having a very narrow notch in its re- flection spectrum, a narrow single passband in the microwave field can be achieved for the MPF based on PM-IM conversion 0733-8724 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.