Fabrication of Chirped Fiber Grating with Small Dispersion and Its Dispersion Measurement Method
Objective Chirped fiber Bragg gratings(CFBGs),which feature a large dispersion range,low insertion loss,and variable positive and negative dispersions,are used as core devices in chirped pulse amplification systems to achieve high-power femtosecond pulses in fiber lasers.The spectral quality and intra-cavity dispersion compensation provided by small-dispersion CFBGs are related to the pulse width and quality of the femtosecond laser output.Achieving high reflectivity,spectral optimization,and accurate and effective dispersion measurements of CFBGs in a limit-length short grating length is key to the application of small-dispersion CFBGs to femtosecond laser oscillators.In this study,a method for the fabrication and measurement of small dispersions and large CFBGs is proposed.The fabrication of a small-dispersion CFBG with high reflectance and a flat reflection spectrum can be used to achieve the accurate dispersion matching of oscillator cavities in high-power femtosecond laser systems,which is of great significance for the performance improvement and development of fiber lasers.Methods In this study,a large chirped phase mask technology combined with high-order Gaussian apodization function and a refractive index modulation depth optimization method is used to fabricate a small dispersion CFBG,and the nonlinear curve fitting optimization method based on Michelson white light interference is used to achieve an accurate measurement of the dispersion.Results and Discussions Figure 3 shows the CFBG reflection spectrum obtained by simulating the effects of the grating area length,apodization function,and refractive index modulation depth on the reflection spectrum and adjusting the relationship among the three.According to the simulation results,when the CFBG chirp rate is determined,the change in the grating length changes the reflection spectrum bandwidth,different apodization functions affect the spectral shape,and the change in the refractive index modulation depth changes the reflectivity.Moreover,the CFBG is fabricated with a center wavelength of 1031 nm,bandwidth of 12 nm,and reflectivity of approximately 36.9％.Figure 4 shows the transmission and reflection spectra.Figure 5 shows the dispersion data processing method,and Fig.5(c)shows the phase-fitting result extracted from the interference spectrum,with a correlation coefficient of 0.9999.The dispersion of the CFBG solved using the fitted phase is 1.1088 ps/nm at 1031 nm.Figure 6 shows the dispersion results and errors at different wavelengths,among which the standard deviation is the largest at 1027 nm,with a value of 0.00294.Figure 7(b)shows the influence of the different strengths of the two arms of the interferometer on the dispersion measurement results,and Fig.8 shows the effect of the optical fiber connection loss in the dispersion measurement system on the dispersion measurement.The experimental results show that the strengths of the two arms of the interferometer as well as the optical fiber fusion connection loss influence the dispersion measurement experiment and that the dispersion value is more stable when the strengths of the two arms of the interferometer are close to each other.Conclusions In this study,a method for fabricating CFBGs based on a large-chirped phase mask combined with high-order Gaussian apodization function and refractive index modulation depth optimization is proposed.For a limited grating length,a small-dispersion CFBG with high reflectivity and spectral optimization is produced.A nonlinear curve-fitting optimization method based on Michelson white light interference is used to measure the dispersion.The experimental results demonstrate that the dispersion measurement method is accurate and effective.Small-dispersion CFBGs are expected to be used for the accurate dispersion matching of high-quality femtosecond fiber oscillators,which is of great significance for the improvement and development of fiber laser performance.
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