Optical sensors, 2022: introduction to the feature issue.

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.

Fast, selective, and sensitive detection of 2,4-dichlorophenol by an imprinted polymer functionalized core-offset U-shaped fiber sensor.

Pollution monitoring in waterways and oceans is often performed in a laboratory on samples previously taken from the environment. The integration of molecular imprinting polymer nanoparticles (MIP-NPs) with a novel, to the best of our knowledge, fiber optic interferometer allowed a fast and selective detection of water pollutant 2,4-Dichlorophenol (2,4-DCP). The proposed sensor with an increased surface-to-volume ratio of MIP-NPs provided an enhanced sensitivity of 17.1 nm/µM and a wide operating range of 0.1-100 µM. It showed a highly repeatable performance and potential to measure up to nM concentrations. This integrated technique is suitable for the development of compact, stable, precise, and sensitive biosensors for online monitoring and remote chemical sensing applications.

High resolution compact spectrometer system based on scattering and spectral reconstruction.

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.

Variable ratio blue/red upconversion in Yb3+-Tm3+ codoped fluorosilicate glasses.

Simultaneous blue-red emission in a fiber pumped by a single wavelength source is perceived as a great challenge because of the large energy difference of the emitted photons. This Letter reports the dependence of the blue-to-red upconversion (UC) emission ratio in Yb3+-Tm3+ codoped fluorosilicate glasses (FSGs) under the excitation of a 980-nm laser on the host glass silica content. Photoluminescence spectra and SEM-EDS are used to clarify the UC mechanism, indicating that the probability of the cross-relaxation (CR) process 1G4 + 3F2→3H6 + 3F4 is key to the dominance of the blue or red emissions. This research can provide a new platform for variable UC luminescence.

Super-long-range distributed vibration sensor based on the polarimetric forward-transmission of light.

Undersea earthquake-triggered giant tsunamis pose significant threats to coastal areas, spanning thousands of kilometers and affecting populations, ecosystems, and infrastructure. To mitigate their impact, monitoring seismic activity in underwater environments is crucial. In this study, we propose a new, to the best of our knowledge, approach for monitoring vibrations in submarine optical cables. By detecting vibration-induced polarization rotation, our dual-wavelength fiber-optic sensing system enables precise measurement of acoustic/vibration amplitude, frequency, and position. As a proof of concept, a double-ended forward-transmission distributed fiber-optic vibration sensor was demonstrated with a single vibration source with a sensitivity of 3.4 mrad/µÎµ at 100 Hz (20 m fiber on PZT), limit of detection of 1.7 pε/Hz1/2 at 100 Hz, sensing range of 121.5 km without an optical amplifier, spatial resolution of 5 m, and position error as small as 34 m. The vibration frequency range tested is from 0.01 to 100 Hz. The sensing system has several advantages, including elegant setup, noise mitigation, and super-long sensing distance.

Extruded TOPAS hollow-core anti-resonant fiber optimized for THz guidance at 0.9THz.

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.

Polarization fading mitigation in distributed acoustic sensors based on a high-speed polarization rotator.

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 <4 m and <7 nɛ, respectively.

Direct femtosecond laser inscription of Bragg gratings in Ho3+/Pr3+ co-doped AlF3-based glass fibers for a 2.86†µm laser.

In this Letter, we report the fabrication of fiber Bragg gratings (FBGs) in home-made Ho3+/Pr3+ co-doped single-cladding fluoroaluminate (AlF3) glass fibers and its application in watt-level lasing at the mid-infrared (MIR) wavelength of 2.86 µm. The FBGs were inscribed using an 800 nm femtosecond (fs) laser direct-writing technique. The FBG properties were investigated for different pulse energies, inscription speeds, grating orders, and transversal lengths. A second-order FBG with a high reflectivity of 99% was obtained at one end of a 16.5-cm-long gain fiber. Under 1150 nm laser pumping, this fiber yielded a power exceeding 1 W at 2863.9 nm with an overall laser efficiency of 17.7%. The fiber laser showed a FWHM bandwidth of 0.46 nm and long-term spectral stability.

Long Range Raman-Amplified Distributed Acoustic Sensor Based on Spontaneous Brillouin Scattering for Large Strain Sensing.

A Brillouin distributed acoustic sensor (DAS) based on optical time-domain refractometry exhibiting a maximum detectible strain of 8.7 mε and a low signal fading is developed. Strain waves with frequencies of up to 120 Hz are measured with an accuracy of 12 µÎµ at a sampling rate of 1.2 kHz and a spatial resolution of 4 m over a sensing range of 8.5 km. The sensing range is further extended by using a modified inline Raman amplifier configuration. Using 80 ns Raman pump pulses, the signal-to-noise ratio is improved by 3.5 dB, while the accuracy of the measurement is enhanced by a factor of 2.5 to 62 µÎµ at the far-end of a 20 km fiber.

High-sensitivity, fast-response ethanol gas optical sensor based on a dual microfiber coupler structure with the Vernier effect.

A high-sensitivity ethanol gas sensor based on two microfiber couplers and the Vernier effect is examined in this Letter using the unique variation rate conversion point characteristics. The output spectrum of the two couplers connected in parallel are superimposed to form a symmetrical envelope curve, showing high responsivity to variations in the external environment. Ethanol sensitivity was achieved by coating the waist region of the coupler with a mixture of Nile red and polymethyl methacrylate. When the concentration of ethanol gas changes, the envelope spectrum shifts. Experimental results show that a high responsivity of 160 pm/ppm can be obtained by tracing the reference peaks in the envelope curve and that the response and recovery times are on the order of seconds.

152 km-range single-ended distributed acoustic sensor based on inline optical amplification and a micromachined enhanced-backscattering fiber.

In this Letter, a distributed acoustic sensor (DAS) with a sensing range in excess of 150 km is reported. This extended sensing range is achieved by adding a low-loss enhanced-backscattering fiber at the far end of a standard single-mode fiber. A conventional DAS system along with inline optical amplifiers are used to interrogate the sensing fiber. The combined system exhibits a minimum detectable strain of 40 nε at 1 Hz over a spatial resolution of 5 m.

Enhanced 3.9 µm emission from diode pumped Ho3+/Eu3+ codoped fluoroindate glasses.

The use of Eu3+ codoping for enhancing the Ho3+:5I5→5I6 emission in fluoroindate glasses shows that Eu3+ could depopulate the lower laser state Ho3+:5I6 while having little effect on the upper state Ho3+:5I5, resulting in greater population inversion. The Ho3+/Eu3+ codoped glass has high spontaneous transition probability (6.31s-1) together with large emission cross section (7.68×10-21cm2). This study indicates that codoping of Ho3+ with Eu3+ is a feasible alternative to quench the lower energy level of the 3.9 µm emission and the Ho3+/Eu3+ codoped fluoroindate glass is a promising material for efficient 3.9 µm fiber lasers.

Room-temperature watt-level and tunable ∼3 µm lasers in Ho3+/Pr3+ co-doped AlF3-based glass fiber.

A room-temperature watt-level continuous-wave-output power mid-infrared fiber laser operating at $\lambda\sim 3\; \unicode{x00B5}{\rm m}$ is demonstrated using a ${{\rm Ho}^{3 +}}/{{\rm Pr}^{3 +}}$ co-doped ${{\rm AlF} _3}$ based glass fiber as a gain fiber. This fixed-wavelength laser had maximum output power of 1.13 W with a slope efficiency of 10.3% and a long-term operating stability of ${\gt}{40}\;{\min }$ without any additional packaging or active thermal management. A fiber laser with tunability from 2.842 to 2.938 µm showed a maximum output power of 110 mW.

Numerical Modelling of a Distributed Acoustic Sensor Based on Ultra-Low Loss-Enhanced Backscattering Fibers.

In this study, a distributed acoustic sensor (DAS) was numerically modeled based on the non-ideal optical components with their noises and imperfections. This model is used to compare the response of DAS systems to standard single-mode fibers and ultra-low loss-enhanced backscattering (ULEB) fibers, a fiber with an array of high reflective points equally spaced along its length. It is shown that using ULEB fibers with highly reflective points improves the signal-to-noise ratio and linearity of the measurement, compared with the measurement based on standard single-mode fibers.

Curvature sensing with a D-shaped multicore fibre and Brillouin optical time-domain reflectometry.

A distributed curvature sensor based on Brillouin optical time-domain reflectometry interrogation technique in a D-shaped 7-core fibre is presented. By comparing the relative Brillouin frequency shift between the central core and three of the outer cores of the 7-core fibre, the curvature of various spools with different diameters is measured with a deviation from the actual value ranging between 9% and 15%. The analysis and results presented in this study show the first demonstration of distributed bend sensing using a specially designed multicore D-shaped fibre, paving the way for fully distributed 3D shape sensing.

Low-noise distributed acoustic sensing using enhanced backscattering fiber with ultra-low-loss point reflectors.

We present a low-noise distributed acoustic sensor using enhanced backscattering fiber with a series of localized reflectors. The point reflectors were inscribed in a standard telecom fiber in a fully automated system by focusing an ultra-fast laser through the fiber cladding. The inscribed reflectors provided a reflectance of -53 dB, significantly higher than the Rayleigh backscattering level of -70 dB/m, despite adding only 0.01 dB of loss per 100 reflection points. We constructed a coherent φ-OTDR system using a double-pulse architecture to probe the enhanced backscattering fiber. Using this system, we found that the point reflectors enabled an average phase noise of -91 dB (re rad2/Hz), 20 dB lower than sensors formed using Rayleigh backscattering in the same fiber. The sensors are immune to interference fading, exhibit a high degree of linearity, and demonstrate excellent non-local signal suppression (>50 dB). This work illustrates the potential for low-cost enhanced backscattering fiber to enable low-noise, long-range distributed acoustic sensing.

2.9 µm lasing from a Ho3+/Pr3+ co-doped AlF3-based glass fiber pumped by a 1150 nm laser: publisher's note.

This publisher's note contains corrections to Opt. Lett. 45, 1216 (2020).

Coherently enhanced third-harmonic generation in cascaded microfibers.

A scheme using cascaded silica microfibers is proposed for efficient third-harmonic (TH) generation. By tuning the phase difference via input pump power, the TH from the microfibers could overlap coherently, yielding great output enhancement. Conversion efficiency ∼20% is demonstrated analytically and numerically. Moreover, as the TH output features are dominated by behavior analogous to optical interference, the influence of random diameter deviation of each microfiber is reduced, and the conversion process could be well controlled.

2.9 µm lasing from a Ho3+/Pr3+ co-doped AlF3-based glass fiber pumped by a 1150 nm laser.

Ho3+/Pr3+ co-doped AlF3-based glass fibers were fabricated by using a rod-in-tube method based on the matrix glass composition of AlF3-BaF2-CaF2-YF3-SrF2-MgF2-LiF-ZrF4-PbF2. Under the pump of a 1150 mW Raman fiber laser, a 2.9 µm laser was observed in a 19 cm long Ho3+/Pr3+ co-doped AlF3-based glass fiber with an output power of 173 mW and a slope efficiency of 10.4%. Ho3+/Pr3+ co-doped AlF3-based glasses were fabricated to investigate the deactivation effects of Pr3+ ions on the Ho3+:5I7 level. Our results showed that the Ho3+/Pr3+ co-doped AlF3-based glass fibers are potential gain media for ∼2.9µm lasers.

Ultra-high-sensitivity refractive index sensor based on dual-microfiber coupler structure with the Vernier effect.

We demonstrate a novel, to the best of our knowledge, refractive index (RI) sensor based on the Vernier effect in dual-microfiber coupler (MFC) structures. The sensor sensitivity was studied both theoretically and experimentally. The numerical results show that by tracing the wavelength shifts of the envelope formed by the Vernier effect, the sensitivity can be improved by several times compared to that obtained for normal coupler-based sensors. In this Letter, two MFCs with a width and free spectral range (FSR) of $\sim{3.5}\;{\unicode{x00B5} \rm m}$∼3.5µm and 6 nm, respectively, were fabricated. Based on the sensitivity of 5820 nm/RIU for a single coupler, we experimentally achieved an ultra-high sensitivity of 126,540 nm/RIU using dual MFCs by the Vernier effect in the RI range of 1.3350 to 1.3455, which shows good agreement with numerical simulations. The proposed all-fiber RI sensor has the advantages of high sensitivity and low cost and can find applications in chemical and biological detection as well as electronic/magnetic field measurement.