Performance Study Of Optical Triangular-shaped Waveform Generation With Adjustable Symmetric Coefficients And Wavelength Conversion

The generation of signals with special time-domain waveforms is an important research content in the field of microwave photonics,and triangular waveform signals have very wide applications in the field of all-optical signals due to their linear rising and falling edges in the time domain.In this paper,based on the previous research on the generation and application of triangular waveform signals,a series of theoretical and simulation analyses are carried out.This paper analyzes an optical approach for triangular waveform generation with adjustable symmetrical coefficient based on a single-drive Mach-Zehnder modulator.The fitting error is used to quantitatively evaluate the quality of the generated waveform,and simulation is performed through Optisystem software to verify the feasibility of the scheme.At the same time,using the self-phase modulation effect of the optical fiber,the wavelength conversion phenomenon of the generated triangular waveform signal is studied.The scheme in this paper mainly solves the three problems in the optical triangular waveform generation: 1)optical filter or dispersive fiber is an irreplaceable component;2)the only symmetric triangular waveform signals can be generated;3)the modulation index cannot be flexibly tuned.The paper first introduces the previous schemes of generating optical triangular waveform,and then analyzes the key device Mach-Zehnder modulator used in the scheme in detail,and then studies the scheme of this paper on this basis.In the scheme,a single-drive Mach-Zehnder modulator operated under the push-pull mode is firstly driven by a sinusoid signal.The lightwave is then split into two branches and converted by a balanced photodetector.Through theoretical analysis and simulation experiments,the main results obtained are as follows:(1)Research and analyze an optical approach for triangular waveform generation with adjustable symmetrical coefficient.By adjusting three variables(modulation index?,bias state ? and time delay ?),the generation of triangular waveform signals can be achieved.The ratios between the harmonic amplitudes of ideal triangular waveform with different symmetrical coefficients are obtained,and the relationship curves of the bias state ? and time delay ? with the modulation index ? are plotted.It can be found that the modulation index can be flexibly tuned.(2)Research and analyze the simulation realization of triangular waveform signal.Taking the two parameters of modulation index ?=1.4 and ?=1.6 as examples,thetriangular waveform signals with symmetry coefficients of 25%,35%,45% and 50%are generated by Optisystem,thus verifying the feasibility of the scheme.The relationship curve of the fitting error of the triangular waveform signal with the change of the symmetry coefficient is drawn.When the fitting error is less than 5%,the triangular waveform with the symmetry coefficient of 20% to 80% can be obtained.(3)Research and analyze the wavelength conversion phenomenon of the triangular waveform caused by the self-phase modulation effect of the optical fiber.Through Optisystem software,a single triangular waveform signal with a signal width of 0.1 ns is obtained from the periodic triangular waveform signal,which is transmitted through a highly nonlinear optical fiber,and the spectrum after the center frequency is changed is obtained.The influence of the input power of the signal,the length of the highly nonlinear optical fiber,the nonlinear coefficient and the rate of change of optical intensity over time on the wavelength shift of the symmetric and asymmetric triangular waveform signals is analyzed.It is found that the wavelength shift of the triangle waveform signal is approximately linear with these variables.At the same time,the relationship between wavelength shift and symmetrical coefficient of triangular waveform as well as that between conversion efficiency and symmetrical coefficient of triangular waveform is analyzed.It is found that the conversion efficiency of the asymmetric triangular waveform signal is higher than that of the symmetric triangular waveform signal.