The Application Of Optical Filter Based On Multimode Fiber In Carrier Recovery

The common optical filters mainly include the filters using the optical thin film interference,the optical fiber based filters formed by the cascades of single-mode fiber(SMF),multimode fiber(MMF),grating and other related devices and the waveguide based filters formed by the interference in the waveguide structures according to their different structures.Because they are wavelength selective,the narrower the bandwidth is,the more sensitive the filters are to the wavelength,which limits their application.An all-optical integrator was achieved in our laboratory in 2012 and a 1.5ns integral interval was achieved in the time domain through the system consist of lens and MMF.One of its important features is that the filter is not sensitive to the changes of wavelength.At the same time,it is cost effective because of its simple structure and cheap components.In the experiment,the integrator proved the effect of temporal filtering,and its principle is different from the above optical filters.Moreover,since the filter is in the time domain and not very sensitive to wavelengths,it may be applied in many different situations.Based on this system,this thesis analyzed its frequency domain theory and proposed a new method of the optical carrier recovery.The technology can be used to convert phase-modulated optical signals into intensity modulation without the local oscillator laser.The method was tested by simulations.The main content of this thesis is:(1)After introducing the theoretical background,with the difference of the integral intervals in the time domain,we took the square wave integral interval,triangle wave integral interval and Gaussian wave integral interval as examples,and the frequency domain characteristics of the filters were theoretically analyzed respectively.Moreover,the actual 3dB bandwidths were calculated and the corresponding infinite impulse response(?R)and finite impulse response(FIR)filters were designed.In the case of multiple integral intervals,which were divided into single waveform and multi-waveform according to whether they had the same waveforms,and the theoretical analyses were carried out respectively.(2)This thesis proposed the method of optical carrier recovery based on the temporal filtering,which can convert phase-modulated optical signals into intensity modulation without the local oscillator laser.This method can reduce the requirements of laser line-width and stability in coherent optical communications.Therefore,it can significantly reduce the cost of coherent optical communications.(3)With filters having different shapes,this thesis compared the Q factor of the signals recovered from the carrier.The simulation verified the method we proposed based on the temporal filtering.