IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 9, MAY 1, 2015 947 A Wideband Tunable Optoelectronic Oscillator Based on a Spectral-Subtraction-Induced MPF Yi Wang, Xiaofeng Jin, Yanhong Zhu, Xianmin Zhang, Shilie Zheng, and Hao Chi Abstract—We propose and demonstrate a frequency tunable optoelectronic oscillator (OEO) based on a spectral-subtraction- induced microwave photonic filter (MPF), which is implemented by a subtraction operation of transmission responses between two high-pass filters of different cutoff frequency. By changing the bandwidth of a tunable optical filter or the wavelengths of two laser sources, the central frequency of the synthesized single- pass MPF is shifted and the oscillation frequency of OEO can be tuned. In the experiment, frequency-tunable oscillation signals from 7.8 to 14.2 GHz at a step of ∼1.25 GHz are generated and its performance is investigated. Index Terms—Optoelectronic oscillator, microwave photonic filter, microwave generation, phase noise. I. I NTRODUCTION O PTOELECTRONIC oscillator (OEO) has been attracting significant interest for its capability in generating high frequency and low phase noise microwave signals [1], [2]. To ensure single frequency oscillation of OEO, a high-Q electrical band-pass filter is generally employed, while its frequency tuning range is restricted. With the advantages of reconfigurability and wideband tunability, the microwave photonic filter (MPF) is a reasonable candidate to its elec- trical counterpart [3]. MPFs can be constructed by using a wavelength-dependent component such as a high-finesse Fabry-Perot etalon [4], [5], an injection-locked Fabry-Perot laser diode [6], or fiber Bragg gratings [7], [8], the frequency tuning of an OEO can then be realized by adjusting a variable optical delay line, changing the wavelength of light wave, or controlling a tunable optical filter (TOF) [14]. A transversal MPF, implemented by a sliced broadband optical source and a dispersive element, can also be used to perform frequency selection of OEO. However its performance is limited by the carrier suppression effect and the dispersion-induced radio frequency (RF) distortion in high frequency [9], [10]. The frequency tuning of OEO based on dispersion-induced MPF can be realized in a variety of methods including tuning the laser wavelength or the chromatic dispersion of a linear chirped fiber Bragg grating (LCFBG) [11], a polarization Manuscript received August 21, 2014; revised January 6, 2015; accepted February 11, 2015. Date of publication February 13, 2015; date of current version April 6, 2015. This work was supported in part by the National Basic Research Program (973 Program) of China under Grant 2012CB315703 and in part by the Natural Science Foundation of China under Grant 61371029 and Grant 60971060. The authors are with the Department of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, China (e-mail: jinxf00@zju.edu.cn). 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.2015.2403838 Fig. 1. Schematic diagram of the proposed frequency-tunable OEO. modulator incorporation with a LCFBG [12], and adjust- ing the DC bias voltage of a dual-parallel Mach-Zehnder modulator (DPMZM) [13], while its relatively low tuning sensitivity by changing the group delay leads to a limited tuning range of the oscillation frequency. In this letter, a wideband frequency-tunable OEO based on a synthesized single-pass MPF is introduced. The MPF is simply implemented by a subtraction operation of trans- mission responses between two high-pass filters (HPFs) of different cut-off frequency. Frequency tuning of the OEO can be achieved by changing either the bandwidth of a tunable optical filter (TOF) or the wavelengths of two laser sources. Theoretically, the oscillation frequency can be tuned up to tens of GHz. II. OPERATION PRINCIPLE The schematic of the proposed OEO is shown in Fig. 1. Two optical carriers from two laser diodes (LDs) with different wavelength are combined by a 3 dB optical coupler (OC), and then sent to a phase modulator (PM) followed by a TOF, which act as a stopband filter. The optical output from the TOF is amplified by an Erbium-doped fiber amplifier (EDFA), and converted back to an electrical signal by a photo detector (PD). The converted electrical signal is amplified and fed back to the PM to complete the OEO loop. A length of dispersion-shifted fiber (DSF) is used to provide optical loop delay and make sure the loop delay of the OEO seen by two light waves from two LDs is almost identical. The mechanism to perform the subtraction operation of transmission responses between two HPFs is to construct two anti-phase HPFs with different cut-off frequency, so that their over-lapping pass-bands would cancel each other out when combined together, thus forming a synthesized single-pass MPF to select the oscillation mode of the OEO. The output of the PM contains two carriers f 1 and f 2 , and their two 1041-1135 © 2015 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.