Integrated Reconfigurable Photonic Filters Based On All-Optical Fractional Hilbert Transform

Integrated photonic devices have great advantages in the applications of all-optical signal processing,optical communication,microwave photonics,etc.Compared with traditional photoelectric optical conversion processes,they have larger operation bandwidth,faster processing speed and reconfigurable capability.Therefore,integrated photonic devices have attracted the attention of global researchers,as the essential topic of this paper.The basic content in this paper is planar integrated reconfigurable photonic filters based on all-optical fractional Hilbert transform(FrHT).The device changes and controls the optical functions by writing FrHTs gratings together with the X-shaped waveguide structure as a coupler on a Silica-onSilicon chip.To control the optical phase difference between the arms of the X-coupler,two identical thermal tuning modules are located on the straight waveguides.Optical filters of different functions can be implemented using reflection spectra of the device with different thermal tuning scenarios.These optical filters have great potential in all-optical signal processing and microwave photonics.This paper mainly elaborates contents as follows:(1)Based on the math definition and operation principle of the optical FrHT,the transfer function based on Bragg grating implementation is introduced.The principle analysis of the FrHT grating reflection spectrum is carried out and the mathematical model is established by numerical calculation and simulation according to the coupling mode theory.(2)The simulation model of the all-optical FrHT device is designed and established.The reflected light from the two identical FrHTs on the chip are coupled into the waveguide.The reflected spectrum of the device will be different if the phase difference caused by the thermal tuning modules changes.When the phase difference between the two arms of the X-coupler changes periodically,the reflection spectrum will also vary periodically.Therefore it is able to obtain the reconfigurable characteristic and the application flexibility of the device.(3)A Silica-on-Silicon substrate is fabricated by the FHD method.The waveguide structure and gratings are written on the planar chip by the direct-UV-writing technique.The optical and RF measurement platforms were established.The spectral responses of the fabricated device under different thermal tuning conditions were experimentally tested and analyzed,also compared with the simulation data.(4)Numerical simulation and experimental analysis of filtering functions of the device are conducted and discussed.When the optical phase difference is 0,the device can be used as an optical notch filter,allowing an full operation bandwidth of 200 GHz and the full width at half maximum(FWHM)of the central notch of 0.5 GHz.When the phase difference is 0.75? or 1.25?,the device can be used to implement a microwave photonic single sideband filter,achieving a single sideband suppression ratio of 12 dB for a 6 GHz RF signal.The performance in an instantaneous frequency measurement(IFM)system is also analyzed.The simulation results show that the device can be applied to an IFM system with a bandwidth of about 50 GHz together with advanced measurement accuracy and lower measurement error.