The multi-pass cell (MPC) is an effective approach to extend the interaction path between laser radiation and gas molecules, thereby improving the detection sensitivity of laser-based gas sensors. However, an inaccurate determination of the MPC optical path length can introduce significant errors in direct absorption spectroscopy. In this work, we proposed a fast and precise technique—optical frequency domain interferometry (OFDI)—for measuring the MPC length, particularly suitable for configurations where the laser entrance and exit share the same window. Both the conventional absorption-based method and the proposed OFDI method were implemented and compared. Experimental results show that for a 295.4 m-long MPC, the absorption method yielded an uncertainty of approximately 1 m, while the OFDI approach reduced this to about 5 cm. Using the corrected MPC length, a carbon dioxide sensor was developed, achieving a detection sensitivity of 1.2 ppm with an integration time of 60 s. The proposed OFDI method provides a simple and highly effective solution for accurately measuring the optical path length of multi-pass cells and other optical systems.
In the context of digital holographic microscopic quantitative phase imaging, a microscope is employed in the experimental setup to magnify both the object light and the reference light beam. The three-dimensional phase information within the hologram is influenced by the curvature of the microscope, resulting in significant distortion of the object phase. Additionally, off-axis tilting introduces further phase distortions. In this study, we propose a two-wavelength phase subtraction method to mitigate such distortions in digital holographic microscopy. The procedure involves capturing phase images at distinct wavelengths using filters that are highly transparent to the 671 nm band and highly reflective to the 532 nm band. By subtracting the phase image containing both object information and distortion from the phase image containing only distortion, a distortion-compensated phase image is obtained. Through both theoretical analysis and experimental validation, we demonstrate that the proposed method offers substantial potential for effectively compensating distortions in digital holographic microscopy.
Billions of people worldwide suffer from sleep disorders and it is important to investigate how the lack of sleep or poor sleep quality affects the visual system. The primary aim of this study was to evaluate the influence of 1 night without sleep on the near point of convergence (NPC) and accommodation parameters in young adults. Twenty three young (mean age: 23.3 +/- 1.68 years) adult subjects were recruited. NPC, accommodative facility, and amplitude of accommodation were measured after normal habitual sleep and after 1 night of sleep deprivation. Both the mean amplitude of accommodation and accommodative facility were significantly reduced after one night of sleep deprivation. With respect to NPC, both the break and recovery points of fusion were significantly receded after a sleep-deprived night. Our results showed that even 1 night without sleep may affect visual function associated with near vision tasks. Our findings suggest that sleep habits should be considered during optometric/ophthalmic examinations, especially when convergence and/or accommodation disorders are observed.
This work introduces a novel iterative reconstruction algorithm that enhances low-resolution optical spectrum. The proposed method effectively recovers spectral features that are otherwise undetectable using low-to moderate-resolution interrogators. An adaptive low-pass filter is integrated into error feedback to effectively suppress noise while selectively preserving high-frequency spectral components during iterative reconstruction process. The algorithm's performance was validated using fiber grating Fabry-Perot interferometer (FPI) sensor spectrum with varying cavity lengths (4, 19, and 25 mm). Using high-resolution spectrum as references, the reconstructed spectrum demonstrated notable improvements, with reductions in Delta MAE (4 mm: -2.42%, 19 mm: -9.37%, and 25 mm: -32.51%) and Delta MSE (4 mm: -4.23%, 19 mm: -17.96%, and 25 mm: -61.22%) compared to the original low-resolution spectrum. Amongst filter methods, an "equiripple" low-pass filter is effective in enhancing the reconstructed spectrum by suppressing noise, resulting in reduced spectral errors (Delta MAE = -6.49%; Delta MSE = -12.97%). The proposed algorithm offers aprac-tical and computationally efficient solution for recovering lost spectral details, particularly in the high-frequency region with minimal error. This capability is especially valuable for enhancing the performance of low-resolution interrogators, allowing their reconstructed spectrum to exhibit characteristics comparable to those obtained with high-resolution systems.
To enhance the early warning rate of pipeline leak events and reduce the false alarm rate in Phi-OTDR -based DAS systems, this paper proposes a new method based on FMD. The introduced vibration event types include not only pipeline leaks but also hammer strikes, steel pipe hits, and noise. A 160-meter pipeline setup was constructed, and the sensing optical fiber wound around it was monitored to collect 2 240 samples. The results after denoising demonstrate the effectiveness of the proposed denoising method. Subsequently, features were extracted using overlapping frames, dimensionality reduction was performed with t-SNE, and after training with NRBO-XGBoost, the leak event recognition rate reached 99.74%, the average classification accuracy reached 98.71%, and the false alarm rate was reduced to 0.19%.
