| Under the background of the period of big data,integrated optics achieves fast development because of its advantages on the communication rate and bandwidth.Silicon photonics is considered to be one of the most potential direction with its CMOS compatibility,high integration,small volume,etc.Micro ring optical filter is one of the basic devices in optical communication system and is widely applied to all-optical signal processing and WDM technology.But the minimum line width of this kind of structure is often less than 200 nm,bringing great difficulty to manufacture.This article is dedicated to improve the condition of small minimum line width from the aspect of design.This article has done research and review of the development of silicon photonics and optical filter at first and then illustrates the focus part of the study,then introduces waveguide theory,coupling theory and micro ring theory.With the support of the theories above,transmission curves of all-pass and add-drop filter based on micro ring structure have been studied.During this process,we found that the output curve of a single micro ring is lorentz curve with small 3 dB bandwidth and not very sharp rise-fall edge.In order to get transmission curve with flat-top characteristics,a cascade of multiple micro rings is needed.Next,calculation method of transmission curve of cascaded micro ring filter and design steps are introduced,and the influence factors of transmission curve are discussed.Explore the conditions for second order micro ring filter to achieve flat-top and calculate flat-top condition for third order,fourth order and fifth order with matlab.Get the corresponding waveguide spacing with 3D FDTD simulation.The analysis of the effect of rings number on transmission curve shows that extinction ratio of drop port and slope of rise-fall edge increase while the number of rings becomes bigger.In addition,the analysis of the influence of the asymmetry of coupling coefficients and waveguide loss on fifth order micro rings has been done.It shows that output power of drop port is further from 100%and extinction ratio of through port falls faster with coupling coefficients becoming more asymmetrical.In addition,the micro ring in the more central position of the whole device has greater influence on the performance of the filter.We can also find that big loss coefficient makes the maximum output power of the resonant wavelength fall while it has less influence on through port.This article choose to optimize third order micro ring filter.We compare several common coupling ways first and determine to use the racetrack resonator structure.Then calculate the relationship between gap and waveguide width to achieve third order micro rings coupling coefficients of 0.45,0.09,0.09,0.45.Eventually we determine to use waveguide with width of 350 nm and the minimum line width can reach 300 nm,achieving the purpose of reducing processing difficulty.Using FDTD method to simulate the whole device.The result shows that the filter can meet the requirements of flat-top and its free spectral range is about 13 nm.Besides,its insertion loss at drop port is about 4 dB and extinction ratio is about 22 dB at 1550 nm.Finally,we layout the design and introduce the processing technology and testing platform.The experiment result demonstrates the properties of this filter.There has been a flat-top property at drop port,with fluctuations of nearly 1.5 dB.The free spectral range is about 13.3 nm and 3-dB bandwidth is about 3 nm.Insertion loss at drop port is around 5 dB and extinction ratio is 14.2 dB.Experiment result and simulation result has been compared.As can be seen from the comparison result,manufacturing error of the central ring(waveguide width slightly smaller than design value,gap width slightly bigger than the designed value)leads to produce ripples of nearly 1.5 dB and extinction ratio lower than the simulation value at the same time,but the whole filter can still meet the design purpose and the filtering performance. |
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