For the accurate cavity-length demodulation of fiber-optic Fabry–Perot (FP) sensors, a combined correlation method based on the fundamental cross-correlation and a higher-order one is proposed, simulated, and experimentally verified. By extending the reflection spectrum eightfold through continuous frequency-doubling three times, cross-correlation using both the original and eightfold spectra, and determination of the main peak of the fundamental cross-correlation coefficient function with the assistance of the eighth-order cross-correlation coefficient, the cavity-length demodulation resolution for fiber-optic FP sensors can be significantly improved even when the spectral bandwidth of the source is limited or the cavity length is relatively short. A cavity length resolution better than 1.8 nm is achieved for an FP sensor with a cavity length of ∼162 μm. The proposed demodulation method can effectively reduce the bandwidth requirement of the light source for the cavity-length extraction of fiber-optic FP sensors, particularly those with relatively short cavity lengths.
In order to ensure the consistency of the images in different waveband, a common aperture ultraviolet and mid wave infrared dual band microscope is designed. Based on the analysis of the factors that affect the image consistency, the condition of the equal focal length is derived. Fitting the curve of the focal length difference changing with the wavelength, and making the focal length of the two wavebands equal by reasonable matching of the focal power, thus ensuring the good consistency of different band images.The working range of the optical system is 0.24~0.28μm and 3~5μm; the field is 0.8mm, the numerical aperture is 0.25,the focal length is 13.86mm, and the magnification is 10X. The transfer function value of UV is higher than 0.75 at the spatial frequency of 37 lp/mm, The transfer function value of MWIR is higher than 0.35 at the spatial frequency of 30lp/mm
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