Photonic microwave bandpass filter based on optical single-sideband polarization modulation for longreach radio over fiber applications Jianbin Fu, Shilong Pan*, Menghao Huang and Ronghui Guo Microwave Photonics Research Laboratory, College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China *e-mail address:
[email protected] Abstract—A two-tap dispersion-insensitive photonic microwave bandpass filter based on optical single-sideband (SSB) polarization modulation is proposed for long-reach radio over fiber applications. The optical SSB polarization modulation, which generates two complementary intensity-modulated signals along two orthogonal polarization directions, is implemented by a polarization modulator (PolM) followed by an optical filter. With a polarization maintaining fiber (PMF) to introduce a time delay, a two-tap dispersion-insensitive photonic microwave bandpass filter is obtained. The proposed filter is used to shape a Gaussian pulse to an ultra-wideband (UWB) signal. After 40-km fiber transmission, no distortion in the waveform of the UWB signal is observed. Keywords—single-sideband; polarization modulation; radio over fiber; ultra-wideband
I. INTRODUCTION Since microwave photonic signal processing was proposed in 1976 [1], it has received increasing interests in the past three decades thanks to the advantages such as wide bandwidth, low loss and immunity to electromagnetic interference [2-5]. Because of the inherent compatibility with radio-over-fiber (RoF) system, a lot of efforts have been devoted to apply the photonic microwave filters to the RoF networks for processing the microwave signals directly in the optical domain [2-6]. However, most of the reported photonic microwave filters operate in the incoherent regime, which have all positive coefficients and offer low pass filtering only. This largely limits the application of the photonic microwave filter in the ROF systems since most of the undesirable frequency components such as noises and DC component are located in the low frequency regime. In addition, multiple wavelengths are usually used to construct the photonic microwave filter [2,3], which is undesirable for the RoF applications since optical wavelength is an important resource in the ROF communication system. To achieve the photonic microwave bandpass filtering This work was supported in part by the Program for New Century Excellent Talents in University (NCET-10-0072), the Ph.D. Programs Foundation of the Ministry of Education of China under grant of 20113218120018, the National Basic Research Program of China (973 Program, 2012CB315705), the Fok Ying Tung Education Foundation, and the Fundamental Research Funds for the Central Universities (NE2012002)
using single-wavelength laser source [4], one of the simplest schemes is a two-tap bandpass filter based on a polarization modulator (PolM) [7]. Since the PolM is a special phase modulator (PM) supporting both TE and TM modes with opposite phase modulation indices, it is actually a type of double-sideband (DSB) modulator [8]. The major problem associate with the photonic microwave filter based on DSB modulation is that the signal after filtering tends to be distorted by fiber dispersion [9,10]. Therefore, the photonic microwave filter based on the PolM could not be applied in a long-reach ROF system. As measured in [11], when a 40-km single mode fiber (SMF) is incorporated, the power penalty of the 7.5-GHz component exceeds 2 dB and the frequency component at 9.5 GHz is almost reduced to zero. This distortion will degrade severely the performance of an ROF system, especially when the system applies correlation receivers. In addition, in the previously reported PolM-based microwave photonic bandpass filter, the principal axes of the PMF should be aligned to have a 45 degree to the principal axes of the PolM, to convert the complementary phase modulations to the complementary intensity modulations, which not only creates difficulties for the splicing of the polarization-maintaining pigtail of the PolM and the PMF, but also makes the intensity-modulated signals sensitive to the phase difference between signals along the principal axes of the PolM (similar to the phase difference of the two arms in the Mach-Zehnder modulator). This phase difference is always guaranteed by an external polarization controller (PC), making the PC unremovable from the configuration. In this paper, we propose and demonstrate a novel photonic microwave bandpass filter with high tolerance to fiber dispersion based on a single-wavelength laser source and optical single-sideband (SSB) polarization modulation. The optical SSB polarization modulation, which is proposed for the first time to the best of our knowledge, is implemented by a polarization modulator (PolM) followed by an optical filter. Driven by an RF signal, the PolM generates two orthogonally polarized DSB optical signals with complementary phase modulations. The optical filter removes one of the two sidebands, converting the DSB phase-modulated signals into two complementary SSB intensity-modulated signals. With a polarization maintaining fiber (PMF) to introduce a time delay to the two signals, a two-tap dispersion-insensitive photonic
microwave bandpass filter is obtained. Since only the optical carrier and one sideband present in the SSB signal, no adjustment of the phase difference between the two principal axes of the PolM is required. Therefore, the PC between the PolM and PMF is removable, making the scheme simple and compact. The frequency response of the proposed bandpass filter with fiber transmission is analyzed by theoretical analysis and measured by an experiment. The application of the filter to shape a Gaussian pulse train to an ultra-wideband (UWB) signal in a 40-km UWB over fiber link is also experimentally verified.
⎡ E x' (t ) ⎤ ⎡ jJ1e − jωmt + J 0 +kjJ1e jωmt ⎤ ⎢ ' ⎥∝⎢ jωm ( t +τ ) ⎥ − jωm ( t +τ ) +J 0 − kjJ1e ⎦ ⎣⎢ E y (t ) ⎦⎥ ⎣ − jJ1e
(2)
where τ is the differential group delay (DGD) of the PMF. When the signal in (2) is directly sent to a PD, the generated electrical signal can be expressed as 2
I ∝ Ex' (t ) + Ey' (t )
2
= 2J02 + 2(1 + k 2 ) J12 + 4kJ12[cos ωm (2t + τ )cos ωmτ ]
(3)
+ 2(1 − k ) J0 J1 [sin ωmt − sin ωm (t + τ )] For small signal modulation, J1(γ)
