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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.

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.

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.

Ho3+ and Nd3+/Ho3+ doped fluoroaluminate glass samples were fabricated by the melt-quenching method and showed high transmittance and a wide transparency window. Under 808 nm pumping, emission at 3.9 µm, corresponding to the Ho: 5I5→5I6 transition, was observed. The presence of Nd3+ strongly enhances the Ho3+ emission. The emission and absorption cross-sections were calculated using the Judd-Ofelt, Füchtbauer-Ladenburg and McCumber theories and the energy transfer mechanism between Nd3+ and Ho3+ was analyzed. The most efficient 3.9 µm emission was obtained when the concentration ratio of Nd3+: Ho3+ is 1:2.

Because of their intrinsic geometrical features and extraordinary transparency, optical fibers allow for a continuous and dynamic monitoring of strain along all their length. In state of the art systems, 100 000 points can be resolved over a range of the order of 50 km, providing an extremely cost effective solution per sensing unit. In its basic implementation, a light pulse is injected into the sensing optical fibre and the Rayleigh backscattered light is continuously monitored, in a process similar to a radar system. While the phase relation between different point along the fiber is used to reconstruct the strain (thus the acoustic information), the time of flight (e.g., delay between injected pulse and received backscattered light) provides information about the physical location along the fiber. In the last decade, research on novel detection schemes and fibers has allowed to extend the detection range to over 170km, to decrease the spatial resolution below 10 cm and significantly increase the signal-to-noise-ratio. Because of this, distributed acoustic optical fiber sensing (DAS) has found applications in the oil and gas industry for oil well monitoring and geoseismics and more widely for the monitoring of border and perimeter security, resulting in market of the order of $1 billion. More recent applications include monitoring of railways and road traffic, structural health, earthquakes and other geophysical events.

Random scattering of light in disordered media can be used for highly sensitive speckle-based wavemeters and spectrometers. However, the multiple scattering events that fold long optical paths within a compact space also make such devices exceedingly sensitive to vibrations and small disturbances to the disordered media. Here, we show how scattering can be engineered so that it can be used for a compact computational spectrometer that is largely insensitive to environmental factors. We designed and fabricated a three-dimensional pseudo-random nano-void pattern with 62% scattering efficiency. The controlled amount of multiple scattering ensured a sufficiently long optical path for the target resolution of 100 pm, with optimal long-term stability. The 200-μm-thick scattering silica substrate was integrated in a compact assembly with a low-cost camera sensor. The target resolution was achieved for full spectrum measurements while single wavelengths could be determined with 50 pm resolution. Such tailored scattering systems can improve the trade-off among cost, size, stability, and spectral resolution in computational spectrometers.

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) fiber as a sensing element. A conventional DAS system is modified to interrogate the ULEB fiber comprised of 50 discrete reflectors with an average reflectance of −56 dB. The ULEB fiber was fabricated with an automated reel-to-reel inscription machine, modified to create more uniform reflectors with similar reflectivity in the core of the fiber. The sensing arrangement exhibits a phase noise of 1.9 n ε / H z over 1 km sensing range.

A periodic optical filter based on nested knots resonator structure is proposed. The structure consists of a knot resonator with a number of smaller coupled-knot-resonators nested in it, similarly to a dual-bus coupled ring resonator, where the end bus terminations are connected to form a Mach–Zehnder interferometer. The variation of periodical transmission output is shown to be mainly due to the change of field components that results in constructive or destructive interference between the nested multiple coupled-knots and the outer loop. The coupling matrix formalism is used to investigate the influence of the number of coupled-knots and their coupling coefficient on the transmission spectrum. The theoretical result shows that the temperature effect can be eliminated by immersing the resonator in ethanol solution. The output spectrum from a fabricated sample with high-Q is also presented. The device is capable of generating a periodical spectral filtering response and has potential applications as a filter or interleaver.

A hollow-core anti-resonant fiber for the THz regime is proposed and demonstrated. The proposed fiber is the hexagonal core shape which is directly extruded using a conventional 3D printer. Experimental results show that by using cyclic olefin copolymer (COC), the proposed fiber design provides a low attenuation of ∼3 dB∕m at ∼ 0.86 THz and ∼15 dB∕m at ∼ 0.42 THz.

A distributed optical fiber acoustic sensor based on interferometric demodulation technique with no polarization fading is demonstrated. A polarization diversity scheme based on a high-speed polarization rotator is used to eliminate signal fading due to polarization mismatch in the Rayleigh backscattered signal between adjacent points on the sensing fiber. This technique yields a spatially uniform response to the applied strain. The sensor exhibited spatial and strain resolutions of