The Research Of Key Optical Devices In Mode Division Multiplexing System

Optical fiber communication has been developing at an astonishing rate since it was proposed.With the rapid growth of data demand on optical communication,the communication system based on single-mode fibers is rapidly approaching its capacity limit.To meet the exponentially increasing bandwidth demand and improve the data capacity of communication system from a new dimension,mode division multiplexing(MDM)technology is proposed.MDM,in which different spatial modes in a few-mode fiber are exploited as independent channels of signals,is an effective way to significantly expand the fiber communication capacity.The MDM system is composed of some basic optical devices,such as mode(de)multiplexer,mode switch,mode converter and so on.These devices play different roles in MDM system and cooperate with each other to realize the function of increasing the transmission capacity.In this paper,a series of mode optical devices are theoretically designed and experimentally demonstrated.The paper mainly includes the following parts:1.In this paper,the developed history and current faced challenges of optical fiber communication system are introduced.And the research background and significance of the mode division multiplexing technology are clarified,the structure and basic functional devices of the mode division multiplexing system are explained.Some material platforms for MDM photonic integration are briefly introduced,especially SOI and polymer.At last,the latest research progress of the mode division multiplexing devices in recent years is introduced.2.One of the most important components in MDM system is mode(de)multiplexer,which serves to realize the conversion and(de)multiplexing between the fundamental mode and the high-order mode.In this paper,two silicon based mode(de)multiplexers are theoretically designed.One is a mode(de)multiplexer based on cascaded asymmetric Y-junctions.This structure can be extended with increasing the number of cascaded Y-junctions arms.The structure is described in this paper by taking 4-mode and 5-mode(de)multiplexers as examples.The device is designed based on effective refractive index matching,combined with the beam propagation method to optimize the device geometry parameters.The performance of the device is theoretically calculated,the overall length of the 4/5 mode(de)multiplexers is 23/150?m,the operating bandwidth is 165/200 nm,and the fabrication tolerance is about±30/±65 nm,respectively.The other one is a TM mode(de)multiplexer,which is used to realize the(de)multiplexing function of six modes of TM₀₀,TM₀₁,TM₁₀,TM₁₁,TM₂₀,and TM₂₁.The device consists of cascaded 2-arm asymmetric Y-junctions,trenched waveguide mode rotators and 3-arm asymmetric Y-junctions.The proposed design doctrine can be expanded to handle higher-order modes(de)multiplexing.The total length of the device is only 124?m,and it can achieve an insertion loss of less than 0.96 d B and a crosstalk of less than-24 d B at 1550 nm.3.Mode switch is usually used to realize switching between different modes or channels,which is of great significance for improving the flexibility of the MDM system.In this paper,four thermo-optic mode switches are proposed to realize different functions.The first work is a polymer thermo-optic mode switch based on an asymmetric Mach-Zehnder interferometer(MZI)structure,and theoretically calculated and experimental results have been given.A metal heater is deposited on one arm of the asymmetric MZI to implement switching between the E₀₀ and E₁₀ modes.The fabricated device shows a mode extinction ratio of 10.5 d B with a power consumption of 15.4 m W at 1550 nm.The operation bandwidth ranges from 1525 to 1615 nm.The rise time and fall time of mode switching are 71.5 and 140.3?s,respectively.The second work is a polymer waveguide mode-selective switch(MSS)based on cascaded multimode interferometers(MMI),which can switch different data channels over E₀₀and E₁₀ modes.The proposed MSS can be applied in MDM system by cascading with other optical devices.The mode switch cascaded mode selector switch with the asymmetric directional couplers(ADCs)mode multiplexer is fabricated and measured.At 1550 nm,the insertion losses of three output ports are 17.2 d B,12.8 d B and 16.7 d B,respectively.The switching powers for two different channels are 8.5 m W and 20.8 m W,respectively.The third work is a mode-insensitive switch,which is used to realize 2×2reconfigurable channel switching of E₀₀,E₁₀,and E₀₁ modes.The device is a symmetric MZI structure composed of two 2×2 MMIs and arm waveguides,with a total length of about 1.26 cm.The calculated results show that driving powers for E₀₀,E₁₀,and E₀₁modes is all about 12 m W.The corresponding extinction ratios of E₀₀,E₁₀,and E₀₁modes are 41.6 d B,39.8 d B and 35.6 d B,respectively.The fourth work is a broadband1×2 mode switch based on cascaded MZI and asymmetric Y-branch mode demultiplexer.The input E₀₀/E₁₀mode can be converted to E₀₀ mode and switched between two output ports by changing the working state of metal heater.At 1550 nm,the measured IL is 3.3 d B(off-state)/3.4 d B(on-state)and CT is-18.8 d B(off-state)/-16.1 d B(on-state),respectively,when E₀₀ mode is launched into the switch.The corresponding switching power is about 10.3 m W.And experimental results proved that the device is insensitive to wavelength in the range of 1500 to 1630 nm.4.Mode converter is used to realize the conversion between different order modes in the MDM system.Three mode converters are proposed in this paper.The first is a polymer E₀₀/E₁₀mode converter.The device has an ultra-broad operating bandwidth,and calculated results show that it can achieve a low crosstalk of less than-22.6 d B within the wavelength range from 1470 nm to 1690 nm.The fabricated device is only measured from 1500 nm to 1630 nm,which is limited by the output bandwidth of the laser source in the laboratory.Within this bandwidth,the mode crosstalk and insertion loss of the device were less than-15.3 d B and 9.2 d B,respectively.In particular,in C-band,the insertion loss and crosstalk of the mode converter are less than 5.6 d B and-20.1 d B,respectively.Then,a reconfigurable polymer thermo-optic mode converter is proposed to realize the conversion between arbitrary two modes among E₀₀,E₁₀,and E₂₀.The mode converter is composed of asymmetric Y-branch and directional couplers.The total length of the device is about 2.17 cm,and the driving power required for a single electrode is 97.5 m W.Finally,a silicon-based TE₀/TE₁ mode converter is designed using the direct binary search(DBS)algorithm,which realizes the mode conversion while changes the propagation direction of input mode(from the+y direction to the+x direction).The footprint of the device is only 2?m×2?m,and it exhibits low mode crosstalk of less than-22.6 d B within the wavelength range from1520 nm to 1580 nm.