Research On Mode Multiplexer Based On Planar Lightwave Circuit

Since 1966,when Chinese scientist K.Gao proposed optical fiber communication technology,the traffic capacity of a single-mode fiber?SMF?has improved by an order of magnitude because of the application of technologies like Wavelength Division Multiplexing?WDM?,Polarization Division Multiplexing?PDM?,and high order modulation and Orthogonal Frequency Division Multiplexing?OFDM?.However,the demands of society for more optical fiber communication system capacity are growing with the rapid development of internet technology for the likes of the internet of things,e-commerce,and high definition internet video.The traffic capacity of an SMF is not infinite;it is limited to about 100 Tb/s because of the Shannon limit,fiber nonlinear effects,and fiber adiabatic effects.However,the newest experimental results concerning the traffic capacity of an SMF approach such limit,so SMFs face a capacity crisis.To meet data traffic requirements,the next-generation optical communication technology needs to improve communication capacity with reduced transmission cost and energy consumption per bit.The technology of Space Division Multiplexing?SDM?is proposed against this background.There are two technical roadmaps for SDM:multi-core fiber?MCF?and Mode Division Multiplexing?MDM?.Between them,MDM is considered to be the most promising technology due to its higher information density.MDM is based on the dimension of the modes.It replaces traditional SMF with few-mode fiber?FMF?and thus expands the communication channels from the fundamental mode in SMF to a few lower order modes in FMF.Because these lower order modes have the same information transmission capability as the fundamental mode,the traffic capacity of FMF represents a great improvement.A mode multiplexer that performs functions like mode conversion and multiplexing/?DE?multiplexing is the key component in a fiber MDM system.Until now,mode multiplexers have been constructed based on three technology platforms:bulk-optic components,optical fiber and planar lightwave circuits?PLC?.Among them,PLC-based mode multiplexers attract more and more attention due to their compact structure,easy integration and multifunction.For MDM systems,controlling as many modes as possible to increase system capacity,possessing large bandwidth to be compatible with existing WDM systems and having reconfigurable functions to achieve a flexible and efficient network structure are important mode multiplexer development directions.Based on the above development requirements of MDM systems,a series of PLC-based mode multiplexers are studied in this dissertation.The main research work and achievements of this dissertation are summarized as follows:1.Proposing an uneven directional coupler?DC?that has two arms of different heights,making it very easy to achieve the coupling of two modes having different mode symmetry in the vertical direction from such an uneven DC.For validation,a mode multiplexer for E₁₁ and E₁₂ was designed and fabricated based on such a DC.The fabricated device containing the input and output waveguides has a length of 14 mm,a coupling efficiency larger than 95%in the wavelength range of 1530-1560 nm,and an insertion loss of 9.6 dB and 12.8 dB for the E₁₁ and E₁₂ modes,respectively.The proposed uneven DC effectively achieves the mode conversion of modes having different mode symmetry.2.Proposing a novel DC by combining a taper structure with the proposed uneven DC,giving the proposed DC a broad bandwidth and a larger fabrication tolerance.By cascading four such tapered uneven DCs,a mode multiplexer that can manipulate E₁₁,E₂₁,E₁₂,E₂₂,and E₃₁ modes was designed and fabricated.The fabricated device containing the input and output waveguides has a length of 21 mm.The measurement results show,for both polarizations,that all four of the cascaded DCs have a coupling efficiency higher than 94.5%in the C+L band?1530-1605 nm?.They have an insertion loss of 15.2 dB,11.6 dB,14.1 dB,19.1 dB and 10.6 dB for the five ports in the demultiplexing port,respectively.The experiment results prove the proposed tapered uneven DC can improve the bandwidth and fabrication tolerance of DCs and has the potential to manipulate more modes.3.Proposing a mode multiplexer that can manipulate the E₁₁,E₂₁,E₁₂,E₂₂,E₃₁ and E₁₃ modes.First,the differences and the conversion methods of the first six modes between FMF and PLC waveguide are discussed.Next,the characteristics of these six modes are analyzed,and a mode multiplexer that can manipulate these modes is designed by cascading five tapered uneven DCs.The resulting device is compact and has a length of 19 mm containing input and output waveguides;the device shows coupling efficiencies larger than 75%for five higher-order modes.Finally,a device was fabricated as designed,but the parameters of the fabricated device and the designed device were inconsistent due to fabrication error;the fabricated device was able to multiplex the E₁₁,E₂₁,E₁₂,and E₂₂ modes but failed to multiplex the E₃₁ and E₁₃ modes.4.Proposing an ultra-short embedded long-period waveguide grating?LPWG?.By placing the perturbation area in the inner of the waveguide where the two coupled modes have the largest mode overlap factor,the proposed grating achieves the largest coupling coefficient and thus an ultra-short length.In addition,the controlled dispersion characteristics of the LPWG give the device a broad bandwidth.A mode converter for the E₁₁ and E₂₁ modes was designed and fabricated based on the proposed grating.The grating region of the device is only 834?m long and is one-fourth the length of an ordinary grating.In the C+L band,the device has a coupling efficiency greater than 98.2%for both polarizations.The proposed grating clearly improves the grating bandwidth while having an ultra-short length.5.Proposing a reconfigurable mode multiplexer created by cascading a thermally induced LPWG with a Y branch;the design is based on the thermo-optic effect in polymer material.First,the method of design and simulation of the thermo-optic device is studied and a thermally induced LPWG is designed.Then,by controlling the dispersion characteristic of the LPWG,the LPWG has a broad bandwidth.The fabricated device with its input and output waveguides has a length of 14 mm and an operating power of 198 mW.For either the E₁₁ mode or E₂₁ mode,in the on or off state,the device shows extinction ratio higher than 13 dB.In addition,the switch response of the device is 0.55 ms for its rising edge and 0.75 ms for its falling edge.