Application Study On Mode Field Control Based On Novel Waveguide Bragg Gratings

Internet of things,cloud compluting and artificial intelligence are rapidly developed in 5G era,which require explosive growth of transmission bandwidth.Nowadays,the fiber optical network is an important component of communication system.The wavelength division multiplexing(WDM)and mode division multiplexing(WDM)technologies enable the transmition of multiple lights with signals,which can increase the transmission bandwidth.The traditional network composed of the discrete optical devices with different functions has the disadvantages such as large volume,high cost and low efficiency.It cannot satisfy the demond of current fiber optical system.The Photonic integrated circuit(PIC)can integrate these optical devices on one single chip.It is becoming the mainstream option due to the advantages of compact size,low power consumption and high bandwidth.Waveguide Bragg grating(WBG)is one of the widely used structures in PICs,which can realize various spectral responses.It can be applied to distributed feedback(DFB)semiconductor lasers on active chips,as well as filters,demutiplexers and modulators on passive chips.In this work,we studied the application of WBG on the control of longitudinal and transverse mode fields,and proposed several optical devices serving for WDM and MDM systems.The control of longitudinal modes includes the improvements of single longitudinal mode characteristics and wavelength stabilities.The control of transverse modes includes the selective manipulation and coupling between two transverse modes.The main research results are as follows:(1)We proposed a novel cascaded tunable laser based on the step-wise multiphaseshift Bragg gratings.The tunable laser contains several grating sections with different periods,which leads to a step-wise grating period profile.In addition,the ? phaseshifts are inserted in the joint between the adjacent grating sections.The total cavity length of the cascaded tunable laser is significantly reduced compared with the traditional structure.As an example,a tunable laser with 5 channels are designed and simulated.Its total length is reduced by 40%.Furthermore,the apodized grating can improve the single longitudinal mode stability and fabrication tolerancewe.We think the proposed compact structure would benefit the practical applications to the low cost tunable lasers in WDM systems.(2)We proposed and fabricated an asymmetric phaseshifted DFB laser array.The two facets are coated with anti-reflection film and high reflection film,respectively.The ? phaseshift and the facet with high reflection film are one in five cavity length apart.It compromises the single longitudinal mode characteristics and wavelength stability,but significantly improves the output power.We designed and fabricated four channel laser array with 800-GHz channel spacing and 250-?m cavity length in 1310-nm region.The testing results indicates that the sampled grating based on reconstruction equivalent chirp(REC)technique can improve the wavelength accuracy.In addition,we designed and fabricated eight channel laser array with 200-GHz channel spacing and 750-?m cavity length in 1310-nm region.The output power reaches 100 m W with 300-m A injection current.(3)We proposed a hybrid transverse mode resonance DFB semiconductor laser which consists of one active section with ?-phase shifted antisymmetric Bragg grating and two passive sections with uniform Bragg grating.The emitted light from one facet with the TE0 or TE1 mode can be controlled by tuning the injection currents of the passive sections.As an example,a laser with ridge waveguide is designed and simulated.The transverse mode with the same wavelength is successfully tuned.The suppression ratio between the two transverse modes reaches 18.4 d B and 18.7 d B when the TE0 and TE1 modes are emitted,respectively.We think the proposed mode tunable laser structure will benefit the practical application of the MDM system due to its high integration and flexibility.(4)We proposed and fabricated a microresonator on the silicon-on-insulator(SOI)platform based on antisymmetric Bragg grating.The resonator is coupled to a single mode bus waveguide through the asymmetric directional coupler.Single and multiple wavelength resonance can be realized by inserting the ? phase shift and straight waveguide as phase shifter,respectively.The experimental results show that Q-factor of the ?-phase shifted resonator reaches 9888.In addition,multiple wavelength resonances with FSR of 1.0 nm is observed when the phase shifter length is 200 ?m.Furthermore,we proposed and fabricated two kinds of single wavelength(de)multiplexers based on the ?-phase shifted resonator.Compared with the microring resonator,the proposed resonator can provide single wavelength resonance.We believe the proposed resonator will find broad applications in WDM systems.(5)We proposed an all-optical method that the TE1 mode controls the TE0 mode.The ridge waveguide structure on SOI combined with two titanium(Ti)strips located on the two waveguide edges is designed to selectively absorb the TE1 mode.Thus the temperature of waveguide rises and the effective index of the TE0 mode is changed.In the simulation,the waveguide temperature shift of 18.01 K is achieved with the rise and fall time of both about 50 ?s.As an example,we designed an all-optical switch based on the waveguide structure and Bragg grating.The simulation result shows that it has a low power consumption of 5.0 m W to achieve the extinction ratio of 19.82 d B.The proposed all-optical control method only requires light source with single wavelength,and thus can be applied to on-chip MDM systems.