Nonlinear Transmission Characteristics Of Optical Pulse In Micro-nano Waveguides And Their Application

With the rapid development of technology,optical pulses play an important role in more and more fields,such as dense wavelength division multiplexing systems in telecommunications,laser weapons in the military,cell imaging in biomedical applications,and high-energy laser in the field of precision machining.It is expected that the time-frequency domain characteristics of the pulse can be flexibly processed in order to obtain ultrashort pulses,high energy pulses,broadband pulses,and pulses with narrow linewidth.These pulses with various performances can be used in broadband communication systems,ultrafast microscopy and biology,high energy pulsed lasers,high-speed signal transmission systems,high-resolution coherence tomography,high-coherence optical frequency comb,high-resolution molecular imaging,And all-optical signal processing.However,Optical pulses with the ultra-intense and ultra-fast response at the time-frequency domain can only occur with suitable media and large light intensities.The emergence of micro-nano waveguides(MNW)such as silicon waveguide and chalcogenide waveguide provides an opportunity to solve this problem.Due to the fast third-order nonlinear effects of these materials,the response time of the device can be decreased from nanoseconds to femtoseconds.In this thesis,the in-depth study of temporal pulse compression and shaping,spectral broadening and compression caused by the third-order nonlinear effects in MNW is carried out.The main research contents and achievements are as follows:1.A scheme of self-similar pulse compression in the anomalous dispersion region is proposed.The chalcogenide-silicon slotted waveguide taper and the anti-tapered silicon waveguide are designed in the telecommunication band and the mid-infrared band(2490 nm)respectively and the self-similar compression of picosecond pulse is performed.In the chalcogenide-silicon slotted waveguide taper with a length of 6 cm,the fundamental soliton with an incident pulse width of 1 ps is compressed to 81.5 fs,achieving a compression factor of 12.3.The pulse with the same width was compressed to 57.29 fs in an inversely tapered silicon waveguide within a length of 5.1 cm,and the compression factor was 17.46.These two compression ratios are much higher than that of reported GaInP and silicon photonic crystal waveguides.The inevitable pedestal in the pulse compression is solved by the proposed schemes and the pedestal-free pulse is realized theoretically.In conclusion,the effective compression of picosecond pulse realized in these two waveguides by using lower input power and shorter waveguide length is advantageous for integratable on-chip ultrashort pulse source with low-power consumption.2.The method of parabolic pulse(PP)generation in the normal dispersion region is proposed.The generations of PP in telecommunication band(1550 nm)and mid-infrared band(2150 nm)are studied by using tapered hydrogen-doped amorphous silicon waveguide.A high-quality PP was produced in a waveguide with a length of I cm.The mismatch coefficient at 1550 nm was as low as 1.19 x 10-3,and the mismatch coefficient at 2150 nm was as low as 9.17 × 10-4.The self-similarity theory of normal dispersion region is used to study the mechanism of PP generation under the condition of independently changing dispersion and nonlinear coefficient.The method of generating PP from the tapered waveguide is theoretically analyzed,and the application of self-similarity theory in nonlinear optics is developed.On the basis of this method,the bisection algorithm is used to design a tapered silicon waveguide with the simultaneous variation of dispersion and nonlinearity.A PP with a mismatch coefficient as low as 1.3 × 10-3 was produced in a tapered silicon waveguide in a length of 1 cm.The effects of input peak power and pulse width on PP generation in tapered silicon waveguides with different lengths are comprehensively studied,which provides theoretical guidance for the generation of on-chip PP.3.A method for self-similar transmission of PP in the normal dispersion region of tapered waveguide is proposed.According to the self-similarity theory(SST),the self-similar condition of nonlinear Schrodinger equation is derived,which is divided into three cases to explore the physical mechanisms in detail.Three kinds of tapered silicon waveguides with the same length of 1 cm were designed according to the three kinds of self-similar condition.The similarities and differences of self-similar transmission in the three kinds of waveguides were analyzed.The perfect agreements of pulse width and peak power between the numerical and analytical solutions are realized.So the correctness of the analytical model is verified.Three kinds of cascaded silicon waveguide structures were proposed to achieve PP generation and compression.The input Gaussian pulse with a pulse width of 300 fs was converted to the PP and then compressed to 35.6 fs,achieving a compression factor of 8.4.The scheme utilizes the characteristics of the positive and linear chirp of PP to generate femtosecond pulses in the cascaded waveguide and has the potential to realize on-chip ultrashort pulse sources with integrated,low power consumption and simple structure.4.The generation of mid-infrared ultra-wideband and high-coherence supercontinuum in III-V MNW is studied.Four kinds of Al0.18Ga0.82As strip waveguides with different sizes and materials are proposed respectively.Al0.2Ga0.8As and Al2O3 are used as the lower cladding layers respectively,and Al0.18Gao.82As is used as the core layer.Different widths and heights are proposed to produce highly coherent supercontinuum.The maximum spectral width was achieved from 2?m to 18?m,which is greater than that generated in the silicon and chalcogenide waveguides.Besides,by changing the pump peak power and central wavelength,the coherence characteristics of the supercontinuum broadening are analyzed in detail,which provides guidance for designing the integrated broadband source which has important applications in biomedical detection and imaging and high-precision measurement.5.A spectrum compression scheme for telecommunication bands in silicon nitride MNW is proposed.The chirped PP with an initial spectral width of 6.19 THz is compressed to 0.24 THz in a 4-cm long silicon nitride strip waveguide.The compression factor is up to 25.8 and the produced pulse is free of chirp.Besides,the pedestal is as low as-15.5 dB.For the first time,the scheme effectively achieves spectrum compression in silicon nitride waveguide.It has the advantages of large compression factor,low susceptibility,and integration,which can be used to implement an integrated on-chip spectrum compressor.Besides,a bent chalcogenide strip waveguide is proposed to achieve spectral compression of wavelength-shifted solitons.When the input peak power is 25 W,the 52.04-nm spectrum is compressed to 7.23 nm.The compression factor is 7.2,and the center wavelength is shifted by 17 nm.When the input peak power is 75 W,the spectral compression factor is only 4.9,but the wavelength shift is as high as 190 nm.This work achieves both large spectral compression and soliton self-frequency shift in the MNW and has important applications in tunable lasers and wideband imaging systems.Then the scheme is used in the all-optical quantization.As the quantization resolution is closely related to the spectral compression and soliton self-frequency shift,a 4-bit quantization resolution can be realized by using the proposed method of spectral compression.The scheme reduces the complexity of the all-optical quantification system,and is of great significance for implementing an on-chip all-optical signal processing system that is miniaturized,low-power,and simple in structure.The research work in this paper enriches the theory of nonlinear transmission of optical pulses in MNWs and explores the pulse mechanism in the time and frequency domain based on third-order nonlinear effects.MNWs with different sizes and configurations are designed to achieve pulse compression and shaping,spectral broadening,and compression,providing theoretical guidance for the integratable,low-power,low-cost,high-performance and on-chip light sources in biomedical applications.It also has important application in the fields of imaging and detection,high-speed communication and measurement.