Research On A Tunable Optical Filter With Narrow-band And Flat-top

In the nodes of all optical network based on reconfigurable optical add/drop multiplexer(ROADM), the dense wavelength division multiplexing(DWDM) signals can be switched and added/dropped in wavelength granular. As an important device in the dropping module of a ROADM node, tunable optical filter(TOF) is one of the focuses in the field of optical communication devices. This paper researches a novel TOF with flat passband and the main work includes,The development of ROADM technologies and the applications of TOF are reviewed. Different TOF technologies are extensively analyzed in comparison. Based on these, a TOF proposal with narrow-band and flat-top is presented. The broadband optical signals are first dispersed by a grating, and then the signal corresponding to the passband of the TOF is selectively reflected to the output by a tilting mirror. In order to realize narrow-band filtering, a beam-expanding lens is employed to expand the optical beam incident on the grating. In order to realize flat-top filtering, a single mode fiber(SMF) is employed as the input and a multimode fiber(MMF) or few mode fiber(FMF) is employed as the output.The Gaussian optical field emitting from the SMF remains as a Gaussian field after passing through the elements in the TOF and incident on the surface of the MMF or FMF finally. Due to dispersion by the grating and control by the tilting mirror, the Gaussian filed of different wavelength incidents on the output fiber surface in different eccentricity. The characteristics of the guiding modes in the MMF and FMF are studied. The coupling efficiency between the Gaussian field with wavelength-dependent eccentricity and the guiding modes is simulated and thus the filtering spectrum of the TOF is obtained. With MMF/FMF as the output, the 0.5dB and 25 dB bandwidth are 0.75nm/0.27 nm and 1.16nm/0.66 nm, which meets the requirement by DWDM system of 100 GHz and 50 GHz channel spacing, respectively.An experimental system is constructed and above TOF design is verified. The experimental results show that the wavelength tuning range covers the C-band 1530-1570 nm of optical fiber communication, the insertion loss is 3.87 dB. The 0.5dB, 3dB and 25 dB bandwidth of the TOF with MMF output is measured as 0.44 nm, 1.02 nm and 4.7nm respectively. The 0.5dB and 3dB bandwidth meet the simulation results well, while the 25dB-bandwidth is too wider than the simulation result, which is due to the instability of the high-order modes in the MMF and system noise. The measured results of the TOF with FMF output is far from the simulation results, which is to be further studied.