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Distributed acoustic sensing (DAS) systems are a recent technological development for seismic observations over a broad range of frequencies with a wide variety of applications. Typically, fibre-optic cables are buried underground or cemented into well casings where the cables are well-coupled to the ground. Quick and temporary surface deployment of cables has great potential utility in areas where rapid surveying and minimal disturbance of the subsurface are desired. However, proper mechanical coupling between the fibre and the ground is still a challenge for temporary surface deployments. Here we test four different coupling strategies for a DAS system deployed in a grassy field, including uncoupled, pinned under tension to the ground, weighted down by carpeting, and weighted down by a sandbag. We compare the DAS data to vertical component geophone data and estimated horizontal geophone data to assess the fidelity of DAS ground motion recordings. We find a completely uncoupled fibre is capable of recording seismic energy up to ∼10 m away from the source, while the pinned and weighted fibre record signals over several tens of metres. The DAS recordings compare favourably with the estimated horizontal displacement records from the multi-channel seismic system. There is a good agreement between the phase of the signals acquired by the DAS system with that of the geophones, but there is a mismatch of up to a factor of two in the absolute amplitude at some frequencies. We perform several standard analysis techniques, including refraction and multi-channel analysis of surface waves, on the coupled DAS data. Finally, the instrument response of the coupled DAS data to ground motions is determined using the estimated horizontal component from the multi-channel seismic system. Surface deployments of DAS systems provide a complementary set of observations to standard vertical geophone deployments, for instance, if multi-component geophones are not available. Also, there are some advantages in speed and ease of deployment of DAS in comparison to geophones depending on the coupling strategy used.

We demonstrate a polarization analyser based on processing of speckle patterns generated by a scattering medium. Each speckle pattern at a given wavelength and polarization state is unique and deterministic, and thus the polarization angle alters the speckle pattern motif. The polarization state of a given input light is obtained using reconstructive linear algebra methods. The system consists of a femtosecond laser written scattering chip and a CMOS sensor and contains no moving parts, making the proposed solution is low-cost and compact. The linear polarization angle was accurately reconstructed over a 0-20° test range, with 6 arcminutes (1/10° ) standard error. To demonstrate an application as a polarimeter, we used the system to measure Faraday rotation in a SF59 lead silicate glass within an electromagnet. The magnetic field was successfully traced by determining the induced changes in the input beam’s linear polarization angle in the range 0-80 mT with 10 mT standard error.

An optical fiber humidity sensor with a microfiber knot resonator (MKR) nested U-shaped Mach-Zehnder (MZ) interferometer based on the Vernier effect is proposed. While the reference arm is protected by cured ultraviolet glue, the sensing arm is coated by agarose. The Vernier effect is realized by superimposing the comb-like transmission spectra of the MKR and MZ interfernce, which are designed to have slightly different free spectral ranges. Experimental results indicate that the proposed sensor can provide a maximum humidity sensitivity of 2.442 nm/%RH with a measurement range from 60% RH to 95% RH. The sensor temperature response, reversibility and repeatability are also investigated. The response time is found to be 102 ms for a change of 30% RH. This sensor not only retains key features and merits of the MKR, including low cost and small footprint, but also has a simpler preparation process compared with conventional sensors based on the Vernier effect.

Fiber Bragg gratings are the most popular type of optical fiber sensor. However, its commercial use is frequently limited by high cost and complexity of the interrogation unit. Here, an interrogator based on a femtosecond laser written silica scattering chip is designed and implemented. Such device can directly reconstruct strain, from the scattering speckle patterns, with a resolution of 70 μϵ (microstrain) within the range of 180-700 μϵ, limited by the slippage of the fiber coating, with the potential to be reduced with the system improvements.

A novel optical sensor for ammonia concentration in water is demonstrated using up-conversion (UC) fluorescence intensity ratio (FIR). The sensor element consists of (i) an Er 3+ -Yb 3+ co-doped tellurite glass microsphere integrated inside a section of suspended tri-core hollow fiber (STCHF) and (ii) the pH indicator phenol red. When 980 nm pump is coupled into the microsphere, the Er 3+ ions produce green and red UC emissions. Exposure to ammonia results in a strong increase in the 560 nm absorption of the phenol red, which acts as a filter for the green emission, while the red emission is unaffected. A simple linear relation between the FIR and the ammonia aqueous concentration has been established.

This feature issue of Optics Express highlights contributions from authors who presented their latest research at the OPTICA Optical Sensors and Sensing Congress, held in Vancouver, British Columbia, Canada from 11–15 July 2022. The feature issue comprises 9 contributed papers, which expand upon their respective conference proceedings. The published papers introduced here cover a range of timely research topics in optics and photonics for chip-based sensing, open-path and remote sensing and fiber devices.

In this work, a distributed acoustic sensor (DAS) with 10 cm spatial resolution is demonstrated. Such a high resolution is achieved by employing an ultra low-loss enhanced backscattering (ULEB) fibre as a sensing element. A conventional DAS system is modified to interrogate the ULEB fibre comprised of 50 discrete reflectors with an average reflectance of −56 dB. The sensing arrangement exhibits a phase noise of 1.9 nε/ √ Hz over 1 km sensing range.

In this article, ultra-broadband photoluminescence in near-infrared is achieved in KZnF3 glass ceramics doped with Er3+and Ni2+ ions. Er3+-Ni2+ co-doped fluorosilicate glass and glass ceramics embedded with perovskite nanocrystals were fabricated and efficient energy transfer (ET) from Er3+ to Ni2+ ions was confirmed by luminescence spectra and decay curves. In the glass ceramic samples, Ni2+ ions were effectively sensitized by Er3+, and ultra-broadband photoluminescence from 1400 to 2300 nm was observed when a 980 nm laser was used as a pump, as shown in Fig. 1. The temperature and humidity stability of the glass ceramic samples was characterized from the measured transmittance. These results demonstrated that Ni2+-Er3+ co-doped glass ceramics have significant potential for application in optical communication and broadband amplifiers.

Monitoring the dynamic humidity requires sensors with fast response and anti-electromagnetic interference, especially for human respiration. Here, an ultrafast fiber-optic breath sensor based on the humidity-sensitive characteristics of gelatin film is proposed and experimentally demonstrated. The sensor consists of a microknot resonator superimposed on a Mach-Zehnder (MZ) interferometer produced by a tapered single-mode fiber, which has an ultrafast response (84 ms) and recovery time (29 ms) and a large dynamic transmission range. The humidity in dynamic ambient causes changes in the refractive index of gelatin coating, which could trigger spectral intensity transients that can be explicitly distinguished between the two states. The sensing principle is analyzed using the traditional transfer-matrix analysis method. The influence of coating thickness on the sensor's trigger threshold is further investigated. Experiments on monitoring breath patterns indicate that the proposed breath sensor has high repeatability, reliability, and validity, which enable many other potential applications such as food processing, health monitoring, and other biomedical applications.

Ho3+ doped and Ho3+/Tm3+ co-doped fluoroindate glass samples were prepared and their emission properties were compared. Under 888 nm laser excitation, the emission at 2 μm of Ho3+ ions with co-doping 2 mol% Tm3+ ions had a 2.9-fold improvement compared with that of Ho3+ doped. The absorption and emission spectra, and energy level lifetime of Ho3+/Tm3+ co-doped glass samples were measured to analyze the energy transfer processes and enhancement mechanisms. The luminescence intensity at 2 μm can be greatly increased due to the bidirectional energy transfer between Tm3+ and Ho3+ ions.

Laser irradiation of silica glass is shown to trigger redistribution of material resulting in accumulation of stress and refractive index modification, and the rearrangement of the glass network has a significant impact on the quality of laser written optical components. We propose an alternative laser writing approach for achieving the desired refractive index and optical phase profiles through improved material stress control, demonstrated using both Gaussian and Bessel writing beams. The new material processing strategy is successfully adapted for implementing photonic circuits and diffractive elements with greater efficiency due to improved uniformity and symmetry of the induced index modification.

The common challenge for reconstructive spectrometers is achieving high spectral resolution without sacrificing device stability, size and costs. Here a fully integrated scattering chip-based spectrometer build on Raspberry Pi platform is designed and implemented. It exhibits no dependence on temperature and humidity (22.7-23.8 °C and 39.5-41 %), is confined in small space (box 50x35x35 mm) and it can reconstruct spectra with resolution up to 0.05 nm (50 pm). The only instability – gravitational mechanical micromovements were compensated by applying pixel binning and device can still reconstruct spectra from binned pictures as small as 32 x 24 pixels.

Femtosecond laser irradiation allows to modify the optical properties of transparent materials with high accuracy. In many applications, optical scattering produced by the laser irradiation is one of the major limiting factors. However,there are situations when the scattering is responsible for the basic principle of operation of the optical element. This report reviews two research directions where laser-induced scattering can be successfully exploited.First, spectroscopic measurements can be performed by analyzing the speckle patterns created by the scattering medium.The measurements are made possible by the strong dependence of the speckle pattern on the wavelength of light. A scattering chip created thanks to a femtosecond laser makes allows addressing the stability problem faced by manyscattering spectrometers. The volumetric scattering centers are induced in silica substrate via micro-explosions caused by the focused laser beam. Such a spectrometer can be successfully used for interrogating fiber Bragg gratings orinterferometers.Second example is found in optical reflectometry. This technology allows turning an optical fiber into a distributed microphone or thermometer. A single optical fiber can monitor a stretch of several tens of kilometers with an accuracyof several meters. Such systems have wide range of applications in civil engineering, geosciences and other fields.Reflectometry measurements are performed by observing the back-scattered light produced by the glass medium of the optical fiber. Femtosecond laser writing allows effectively increase backscattered light whilst introducing minimaladditional losses. In this way, the sensitivity of reflectometric systems can be increased or their range can be extended.

In this Letter, we report the fabrication of fiber Bragg gratings (FBGs) in home-made Ho

In this study, a high spatial resolution distributed acoustic sensor (DAS) with 8 cm gauge length based on ultra low-loss enhanced backscattering (ULEB) fiber with 1 km sensing range is demonstrated.

In this study, an interferometric distributed acoustic sensor with multi-frequency acquisition capability based on a conventional electro optic modulator is developed with 100pε noise floor at 10Hz.

An optical gauge based on coupled multi-core fiber is proposed and experimentally demonstrated. By using direct laser writing to selectively break the fiber index profile symmetry, asymmetric mode coupling between cores is introduced. This allows to fabricate optical gauges with the ability to detect and differentiate different types of deformation in structures using just a single sensor. The fabricated optical gauges are compared with calibrated commercial gauges and fiber Bragg gratings as strain, vibration and curvature gauge. The tests indicate superior performance of this novel optical gauge over the commercially available sensors and offer the highest sensitivities. The proposed technology could be key to the fabrication of new fiber-based sensing devices with more capabilities and better features than previously achieved. This work has been funded in part by the Fondo Europeo de Desarrollo Regional (FEDER) , in part by the Ministerio de Ciencia, Innovacion y Universidades under projects RTI2018-094669-B-C31 in part by the GobiernoVasco/Eusko Jaurlaritza ELKARTEK KK-2021/00092 and ELKARTEK KK-2021/00082. The authors also acknowledge UKRI for partial support through research grants EP/S013776/1; NE/S012877/1; NE/T005890/1

In this study, an ultra low-loss enhanced backscattering (ULEB) fiber with 50 point reflectors, spaced 10 cm apart, is used to demonstrate a high spatial resolution distributed acoustic sensor.

In this Letter, we present a compact scattering spectrometer system based on fluorosilicate glass ceramics. By the algorithmic spectral calibration and reconstruction, we achieve wavelength detection with a resolution of 0.1 nm. Numerous nanocrystals embedded in the glass host in the glass ceramics result in a significant natural multilayer scattering medium, which can provide a 60% scattering efficiency for incident light while increasing the optical path of incident light transmitting in the medium. The glass ceramics scattering medium with a rather compact physical size is integrated with a low-cost camera to compose an optical spectral system, which has potential application in lab-on-a-chip optical spectroscopy.