Tunable Filters Based On Cascaded Long Period Gratings In Polymer Waveguide

Optical filter is a device used for wavelength selection.It can filter out specific wavelengths from many wavelengths.It has many important applications in optical fiber sensing systems and optical fiber communication,such as wavelength selection and noise filtering of optical amplifiers,Gain equalization,optical multiplexing demultiplexing,etc.In recent years,with the rapid development of Dense Wavelength Division Multiplexing(DWDM)and Optical Frequency Division Multiplexing(OFDM)technologies,the research and development of tunable optical filters have flexible options for optical channels and it is of great significance to reduce the work of the system and the cost of backup.At present,researches on tunable optical filters are mainly divided into the following types: Mach-Zehnder optical filters,acousto-optic Tunable filter,distributed feedback(DFB)laser tunable optical filter,liquid crystal optical tunable filter,grating filter,tunable Fabry-Perot filter.Among them,filters based on long-period waveguide gratings can be based on a variety of materials and structures because of their flexible manufacturing.They have low back reflection and low insertion loss.Their excellent performance in terms of tuning range and temperature sensitivity has attracted attention.In this paper,for the first time,a two-segment polymer long-period waveguide grating is cascaded to implement a filter with a narrower bandwidth.The article first introduces the basic theory of optical waveguide and the working principle of long-period waveguide grating based on mode coupling theory,and analyzes the working principle of cascaded long-period waveguide grating.Then we introduced the first device structure we designed-a long-period grating filter based on the fundamental mode and cladding mode.First,based on the dispersion curve of the waveguide,we determined the size of the single-mode waveguide.After the effective refractive indexes of the two working modes were obtained by simulation,the period and other parameters of the long period grating were calculated by the phase matching equation;however,when they were produced in the laboratory,the sample works poorly.After analyzing the reasons,we have improved the device structure.The improved device works based on the two core modes in the multimode waveguide,and the manufacturing process is very simple.Finally,we built an optical test platform and measured the transmission spectrum of the device.And the temperature sensitivity of the device was measured by the heater.The device has a linear response to temperature so that it can be tuned by changing the temperature.Experimental results show that the device has a maximum extinction ratio of 24 dB at 1577 nm,and the 12 dB bandwidth is 10 nm.The temperature sensitivity of the device is 1.76 nm/?.