Research On Nonlinear Waveguide Design And Applications

As one of the most important optical devices in the on-chip integrated optics,nonlinear waveguides have been extensively studied.Compared with highly nonlinear optical fibers,waveguides have the advantages of small size and low energy consumption.Its appearance makes traditional nonlinear optics come to a new life.The waveguide can well confine the mode field within the core region.The optical intensity will be improved.Therefore it can produce a nonlinear response at a lower pump energy level.The dispersion of the waveguide is sensitive to changes in the structural size of its cross-section,and it can be adjusted by changing the structural size of the cross-section.Thanks to the mature CMOS technology,people have successfully developed a variety of nonlinear waveguide platforms based on different materials.For example:silicon waveguide,chalcogenide waveguide,?-? waveguide,etc.At present,these different nonlinear waveguide platforms have been implied to the fields of on-chip optical communication,optical information processing,and biomedical imaging.Combining the advantages of different nonlinear waveguide platforms,different kind of waveguides will be designed and their applications in pulse pulse compression,spectral compression,supercontinuum generation,and optical frequency comb generation will be discussed.At the same time,an experimental platform is built up to study the generation of the optical frequency comb in the telecommunication bands.The results and innovations of this thesis are summarized as follows:1)The method for realizing self-similar pulse compression and supercontinuum generation based on silicon waveguides is studied.This scheme firstly compresses the 1-ps pulse through the self-similar pulse compression method for high-quality compression,and then the compressed ultrashort femtosecond pulse is used to generate a supercontinuum with high coherence characteristics and one octave bandwidth.According to the self-similar pulse compression theory,a suspended tapered silicon material waveguide with a length of 3.9 cm and the profile of ?2 exponentially increasing as the increment of the propagation distance is designed.A 1-ps pulse with a center wavelength of 2.8 ?m is successfully compressed to 47.06 fs,the compression factor is high up to 24.25.The peak power of the pulse increased from 1.67 W to 27.63 W.Subsequently,the compressed pulse is used to pump a strip silicon waveguide with two zero-dispersion points to generate one octave and coherent supercontinuum.Compared with the coherence characteristics of the narrow supercontinuum generated by 1-ps pulse,the coherence characteristics are improved by 2 orders of magnitude.This research lays the foundation for the generation of on-chip high-quality mid-infr-ared ultrashort femtosecond pulses and the use of long pulses to indirectly generate high coherent broadband supercontinuum.2)A scheme based on the theory of soliton spectral compression to achieve high-quality spectrum compression in the waveguide is proposed.A suspended tapered silicon waveguide with a length of 6 cm is designed,and its |?2| increases linearly as the increment of the propagation distance.Using this waveguide,a pulse with a center wavelength of 2.4?m is successfully compressed from the 121 nm to 11.1 nm.The spectral compression factor is high up to 10.9 times,and the corresponding spectral peak power is increased by 8 times.The shapes of the compressed spectrum in the spectral and time domain are good.This research provides a solution for the realization of on-chip coherent light sources with narrow bandwidth and high spectral brightness in the mid-infrared band.3)In the multi-mode chalcogenide waveguide with all-normal dispersion profiles,a scheme that octave-spanning and high coherent supercontinuum and optical frequency combs generated by three different mode is proposed.All the dispersion curves corresponding to the three quasi-TE modes(TE00,TE10,and TE20 modes)of the waveguide are in the normal dispersion region and have good flatness.Among them,the TE20 mode is closest to the zero dispersion,the TE00 mode deviates slightly from the zero dispersion,and the TE10 mode is in the middle of them.When the pulse pump parameters are selected as ?0=3?m,P0=3 kW and T0=80 fs,the bandwidth of the supercontinuum generated by these three modes in a 1cm-long waveguide is 1.82,1.93,and 2.33 octave,respectively.The corresponding first-order coherence degree value is close to 1 in the generated spectrum range.A pulse train with a repetition rate of 50 MHz is launched into the waveguide.Multi octave-spanning optical frequency combs are generated by these three modes.This research provides a suitable solution for realizing on-chip multi-octave and high coherent supercontinuum and optical frequency comb sources in the mid-infrared band.4)An experimental scheme for generating a high conversion efficiency optical frequency comb covering multiple telecommunication bands in a nonlinear waveguide based on AlGaAs is proposed,and a simulation scheme of mid-infrared broadband and high coherent optical frequency comb generation using multi-zero-dispersion waveguide is proposed.In the experiment work,5-mm long and 320-nm high AlGaAsOS waveguides were designed and fabricated.When 200-fs quasi-Gaussian pulses at 1560 nm with 8.9-W peak power are coupled into the waveguide which has 900-nm width and all-normal dispersion profiles,an optical frequency comb covering the S+C+L+U bands is generated.Its 20-dB bandwidth is 24.5 THz,where covering from 1480nm to 1683nm.The conversion efficiency reaches up to 47%.C+L-bands power variation is within 8 dB.The result provides an experimental reference for the realization of on-chip optical frequency combs with high conversion efficiency,wide bandwidth and high coherence.It can be considered as a comb-shaped multi-wavelength light source to be applied to high-speed optical communication systems to achieve high data transmission rate.In the simulation work,a 3-mm AlGaAs waveguide with three zerodispersion wavelengths was designed and pumped with suitable pulses(?0=4.9 ?m,PO=900 W,and To=100 fs).2.7 octave-spanning supercontinuum was generated.In the process of pulse propagation,the corresponding firstorder coherence value is close to 1.Based on the generated supercontinuum,a pulse train containing 50 pulses with repetition frequency of 100 MHz is used to generate a multi-octave and high-coherent optical frequency comb in the mid-infrared band.This research provides a theoretical guidance for using a waveguide with multiple zero-dispersion wavelengths to realize on-chip mid-infrared optical frequency combs.By combining the advantages of waveguide platforms of different materials and designing appropriate waveguide structure parameters,highquality on-chip pulse compression,spectrum compression,supercontinuum and optical frequency combs are generated by the nonlinear effects.The simulation and some experimental results not only enrich the nonlinear waveguide theory,but also have potential application value in the future onchip high-speed optical communications,information processing,medical imaging and so on.