Center Wavelength Calibration of Fabry-Perot Filter Based on Dual-Wavelength Intensity Modulation
Objective The Fabry-Perot(F-P)filter,with the advantages of narrow bandwidth,high transmittance,and large aperture,is an essential component in solar two-dimensional imaging spectrometers.It is also a crucial optical element in the next-generation two-dimensional imaging spectrometer for the New Vacuum Solar Telescope(NVST).To achieve high-precision solar spectrum observation,the F-P filter must have an accurate center wavelength.However,in practical applications,the filter can only provide controller values and cannot obtain wavelength information directly.Additionally,the accuracy and stability of cavity length are affected by material defects,component fatigue,and environmental changes,resulting in center wavelength drift.Therefore,the center wavelength of the filter must be calibrated.Traditional calibration methods require a stable continuous spectrum light source and a spectrometer or interferometer with a resolution higher than that of the filter,which are large and expensive.Currently,the common calibration method uses the sun as a standard light source,but this is limited by variations in the solar spectrum.We propose a novel method for calibrating the center wavelength of the F-P filter.This method is simple in structure and does not require additional spectrometers or interferometers,avoiding the problem of single-wavelength 2π entanglement and overcoming the limitations of solar spectral calibration.The calibration accuracy is better than 0.01 Å,fully meeting the requirements of the NVST two-dimensional imaging spectrometer.Methods Our method for calibrating the center wavelength of the F-P filter is based on the periodicity of the intensity modulation curve of the F-P filter.First,we measure the unit step of the controller using the adjacent peaks of the single-wavelength intensity modulation curve.The 2π entanglement problem is then solved using the dual-wavelength intensity modulation curve,and the corresponding relationship between cavity length and the controller value is established according to the positions between adjacent peaks of the dual-wavelength intensity modulation curve.Using this relationship,we adjust the cavity length of the F-P filter to the position of the center wavelength and calibrate the wavelength accurately.Finally,we conduct a calibration test on the F-P filter and verify the feasibility of this calibration method.Results and Discussions We carry out the calibration test of the ET100-FS-100 F-P filter produced by IC Optical Systems Ltd.The test results show that when observing the Ha solar spectral line,adjusting the controller to the-101st step,and with a corresponding cavity length is 1001.81121 μm,we obtain a center wavelength of 6562.8 Å with an accuracy of 0.0093 Å.Analysis reveals that the errors mainly arise from two sources:random errors,including laser intensity stability,detector photon noise,electronic noise of F-P filter,digital-to-analog conversion quantization error,and non-linear error,resulting in a wavelength drift of 0.0079 Å;and the numerical quantization error of the controller,resulting in a wavelength drift of 0.0014 Å.The average unit step of the controller measured by the calibration test is 429.8 pm,compared to the theoretical value of 488.4 pm provided by the manufacturer,with a difference of 58.6 pm.The discrepancy may be due to differences between the laboratory calibration environment and the manufacturer's test environment.The changes of environmental factors(such as temperature,humidity,and pressure)cause the filter material to expand and contract,electronic components to become unstable,and the refractive index of air in the cavity to change,resulting in differences between measured and theoretical values.This indicates that the measured unit step of the controller is only valid for the environment at the time of calibration.Once the environment changes,recalibration is necessary,or temperature control of the F-P filter is required to ensure its stability.Conclusions We propose a novel method for calibrating the center wavelength of the F-P filter based on the periodicity of the intensity modulation curve.By using the dual-wavelength intensity modulation curve of the filter,we measure the unit step of the controller,establish the corresponding relationship between cavity length and the controller value,and accurately calibrate the wavelength of the filter.A calibration system is built in the laboratory for testing the ET100-FS-100 F-P filter produced by IC Optical Systems Ltd.The results show that the unit step of controller is 429.8 pm.When observing the Hα solar spectral line,adjusting the controller to the-101st step,and with a corresponding cavity length of 1001.81121 μm,the center wavelength of the F-P filter is 6562.8 Å,with a calibration accuracy of 0.0093 Å,fully meeting the requirements of the NVST two-dimensional imaging spectrometer.
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