Brillouin Distributed Optical Fiber Sensor Based on a Closed-Loop Configuration

A Brillouin optical time-domain analysis (BOTDA) method based on a closed-loop control system is proposed to track fast variations of the Brillouin frequency shift along the sensing fiber. Whilst the method eliminates the gain spectral scanning, the exact distributed Brillouin frequency profile is retrieved directly from the output of a closed-loop controller with no need of post-processing. Moreover, as the operating frequency is being continuously updated to follow the Brillouin frequency change, an unlimited temperature or strain measurement range can be achieved. Both theoretical analysis and experimental results validate that the closed-loop controlled BOTDA acts as a low-pass filter that considerably rejects the noise from photodetector, with an efficiency which fundamentally outperforms basic averaging. By optimizing the closed-loop parameters, the measurement time is reduced from a few minutes to a couple of seconds compared with standard BOTDA, i.e., two orders of magnitude improvement in terms of measurement speed, while keeping the same accuracy and measurement conditions. If the sampling time interval that is limited by our instrument can be further reduced, the method offers the potentiality of km-range sensing with sub-second measurement time, with an unmatched favorable trade-off between measurand accuracy and closed-loop delay

High-precision intermode beating electro-optic distance measurement for mitigation of atmospheric delays

High-precision electro-optic distance measurement (EDM) is essential for deformation monitoring. Although sub-ppm instrumental accuracy is already feasible with state-of-the-art commercial technology, the practically attainable accuracy on distances over more than a few hundred meters is limited by uncertainties in estimating the integral refractive index along the propagation path, which often results in measurement errors of several ppm. This paper presents a new instrumental basis for high-accuracy multispectral EDM using an optical supercontinuum to enable dispersion-based inline refractivity compensation. Initial experiments performed on two spectrally filtered bands of 590 and 890 nm from the supercontinuum show measurement precision better than 0.05 mm over 50 m for an acquisition time of around 3 ms on the individual bands. This represents a comparable performance to our previously reported results on 5 cm by over a range of 3 orders of magnitude longer, which can still be improved by increasing the acquisition time. The preliminary results indicate a relative accuracy of about 0.1 mm at 50 m on each wavelength. Improvement is possible by calibration and by implementing a self-reference scheme that mitigates slow drifts caused by power-to-phase coupling. The results reported herein thus indicate that the presented approach can be further developed for achieving sub-ppm accuracy of refractivity compensated distance measurements on practically useful ranges and under outdoor conditions.ISSN:1862-9016ISSN:1862-902

High-precision intermode-beating EDM for mitigation of atmospheric delays

[EN] High-precision electro-optical distance measurement (EDM) is essential for deformation monitoring. Although sub-ppm instrumental accuracy is already feasible with state-of-the-art commercial technology, the practically attainable accuracy on distances over more than a few hundred meters is limited by uncertainties in estimating the integral refractive index along the propagation path, which often results in measurement errors of several ppm. This paper presents a new instrumental basis for high-accuracy multispectral EDM using an optical supercontinuum to enable dispersion-based inline refractivity compensation. Initial experiments performed on two spectrally filtered bands of 590 and 890 nm from the supercontinuum show measurement precision better than 0.05 mm over 50 m for an acquisition time of around 3 ms on the individual bands. This represents a comparable performance to our previously reported results on 5 cm by over a range of 3 orders of magnitude longer, which can still be improved by increasing the acquisition time. The preliminary results indicate a relative accuracy of about 0.1 mm at 50 m on each wavelength. Improvement is possible by calibration and by implementing a self-reference scheme that mitigates slow drifts caused by power-to-phase coupling. The results reported herein thus indicate that the presented approach can be further developed for achieving sub-ppm accuracy of refractivity compensated distance measurements on practically useful ranges and under outdoor conditions.The present research was co-funded by the Swiss National Science Foundation (SNSF) through research grant 200021_184988.Ray, P.; Salido-Monzú, D.; Wieser, A. (2023). High-precision intermode-beating EDM for mitigation of atmospheric delays. Editorial Universitat Politècnica de València. http://hdl.handle.net/10251/19224

Supercontinuum-based hyperspectral laser scanning: towards enhanced 3D surface reconstruction and its benefits for remote sensing

We demonstrate a supercontinuum-based hyperspectral laser scanning technique that provides high-precision distance measurements of natural surfaces along with their reflectance signature over the broad spectral range of the supercontinuum (SC) output. The SC used in our experiments is spectrally broadened to 570-970 nm from a 780 nm mode-locked femtosecond laser. Distance measurements are carried out by monitoring the differential phase delay of the intermode beat notes obtained from direct photodetection of the SC, while the backscattered reflection spectrum is acquired using a commercial spectrometer. We achieve a single-point range precision below 10 μm on natural targets (gypsum board and leaves of a plant used herein) placed at a stand-off distance of 5 m. Our results demonstrate the acquisition of hyperspectral point clouds together with sub-mm range noise on the scanned surface. This range performance is comparable to commercial state-of-the-art terrestrial laser scanners which traditionally employ a monochromatic laser source.We show the benefit of enhanced range precision toward correctly estimating the surface orientation and for radiometric calibration of the acquired intensities. Initial results illustrate the direct 3D mapping of spectral data of plant leaves with a reduced angle of incidence-related bias, highlighting new opportunities for future research into remote sensing of vegetation

Supercontinuum-based hyperspectral LiDAR for precision laser scanning

Hyperspectral LiDAR enables non-contact mapping of the 3D surface geometry of an object along with its spectral reflectance signature and has proved to be effective for automated point cloud segmentation in various remote sensing applications. The established hyperspectral LiDAR methods offer a range precision of a few mm to a few cm for distances exceeding several meters. We propose a novel approach to hyperspectral LiDAR scanning based on a supercontinuum (SC) coherently broadened from a 780 nm frequency comb. It provides high precision distance measurements along with target reflectance over the 570–970 nm range of the SC output. The distance measurements are carried out by monitoring the differential phase delay of intermode beat notes generated by direct photodetection, while the backscattered light spectrum is acquired using a commercial CCD spectrometer with 0.16 nm resolution across the 400 nm bandwidth of the SC output. We demonstrate a measurement precision below 0.1 mm for a stand-off range up to 50 m on a diffuse target with around 89% reflectance. The measured relative accuracy as compared to a reference interferometer is on the order of 10−5 for distances up to 50 m. Initial results also indicate spectrum-based material classification within a 3D point cloud using a linear support vector machine. The results highlight the potential of this approach for joint high-precision laser scanning and automated material classification.ISSN:1094-408

Supplementary document for Supercontinuum-based hyperspectral LiDAR for precision laser scanning - 6583671.pdf

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