Research On Second Harmonic Generation In Optical Fiber Nanowire Coupler

In recent years,optical devices are developing quickly towards miniaturization and integration.Optical fiber nanowires(OFNs),as the miniaturized products of common commercial optical fibers,have attracted the research interests of many people due to their excellent optical properties,such as low transmission loss,low insertion loss,strong evanescent field and so on.Especially,OFNs have rapidly become a research hotspot in nonlinear optics area because they can confine light in a very small area and greatly enhance the power density of the optical field.In the paper,we mainly focus on the phase matching method of second harmonic generation(SHG)in OFNs.To our best knowledge,it is the first time that the quasi-phase matching method based on coupling compensation(C-QPM)is introduced into the OFNs system.The C-QPM in both symmetric and asymmetric OFN couplers is studied through theory,simulation and experiment systematically.The outline of this paper is as follows:(1)SHG in symmetric OFN couplers.We solved the four waves coupling equations,which are used to describe the SHG in symmetric optical couplers,and obtained the phase matching conditions for efficient SHG.We found that the coupling coefficients of both fundamental wave(FW)and second harmonic wave(SHW)could compensate the phase mismatch.The relationship between phase-matching wavelengths and the diameters of OFNs constituting the couplers was researched through simulation.We found that the coupler with smaller size has larger coupling coefficients,and thus can compensate larger phase mismatch and correspond to a shorter phase matching wavelength.Besides,we proposed new crafts to fabricate OFN couplers.On this basis,a unique OFN coupler composed of two suspended parallel 650 nm-diameter OFNs is fabricated,and a distinct SHG peak is observed for 1014 nm pump light,which coincides with the calculated results by the theory.The efficiency of the SHG of the OFN coupler is four orders of magnitude larger than that of each individual OFN.Moreover,the polarization extinction ratio of the output SHG in the OFN coupler is as large as 24 dB,which implies that the SHG in the structure has good polarization tunability.(2)SHG in asymmetric OFN couplers.In asymmetric couplers,the waveguide mismatch can affect the period of the equivalent intensity grating in the coupler,so it can also compensate for the phase mismatch between FW and SHW.In this paper,the four-wave coupling equations of the second harmonic generation process in an asymmetric coupler are solved analytically,and 12 different C-QPM conditions are obtained.Among these 12 C-QPM conditions,6 of them can be simplified into the 3 C-QPM conditions derived in symmetric case,and the other 6 conditions can only be effective in asymmetric cases.Besides,we also studied the effect of the asymmetry of couplers on both phase matching and the efficiency of SHG under C-QPM.On one hand,the coupler with larger asymmetry can compensate for larger phase mismatch;on the one hand,it will be less efficient on SHG.Therefore,we have to balance the two factors when we design an asymmetric coupler for SHG.Finally,we carried out a verification experiment.An asymmetric coupler consisting of two 700 nm±20 nm-diameter OFNs was fabricated.The coupler can realize most eff-icient SHG when the wavelength of pump light was 1128 nm,which was consistent with the theoretical prediction.However,the wavelengths of other enhanced signal cannot correspond to the phase-matching wavelengths predicted by the theory.Therefore,more work on experiment still need to do.(3)Tuning the C-QPM wavelength of a coupler.Since the coupling coefficients play significant roles in the phase matching process,we can adjust the value of coupling coefficients to enable different wavelengths achieving C-QPM.For the maximum tuning range of C-QPM wavelengths,a method that the C-QPM wavelengths are tuned through changing the distance between two OFNs is proposed.Theoretically,this method can adjust the C-QPM wavelength from the largest C-QPM wavelength of the coupler to the modal phase matching wavelength of a single OFN.In an asymmetric coupler consisting of two OFNs with diameter of 700 nm±5 nm,the phase matching wavelength can be tuned from 886 nm to 1388 nm as the spacing between the two OFNs increases from zero to 450 nm.In this case,the tuning range of the C-QPM wavelengths is more than 500 nm.The larger phase-mismatch compensation ability of a coupler has,the larger difference between C-QPM wavelength of a coupler and modal phase matching wavelength of a single OFN is,and this method has a larger C-QPM wavelength tuning range.