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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.

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.

In this work, a novel 3D-printed biaxial sensor system for tilt measurement, based primarily on the use of four Fiber Bragg Grating (FBG) devices, has been developed and its performance characterized. The tilt sensor system created is of a compact design and relatively small dimensions, making it ideally suited to a variety of industrial applications. In the system developed, the four FBGs used were spliced in a serial formation and attached to four different sides of the sensor structure designed, to allow biaxial measurements to be made. The wavelengths' shift of the FBGs used were monitored as a function of the tilt of the device, using an Optical Spectrum Analyzer (OSA) for this development work. In the sensor, an average FBG-based responsivity of 0.01 nm/° of tilt was measured for each of the different FBGs used. To provide compensation for temperature changes in the system itself, a further FBG-based approach was used (in which they were configured to be insensitive to the effect of the tilt). They were thus calibrated by being exposed to a range of operational temperatures for the system, showing, as a result, a calibration of 0.011 nm/°C. Prior work on the sensor system had proved it to be highly linear in response, over the tilt range of 0° ± 90°. The experimental results obtained from the performance characterization indicate that the small, compact design of this type yields excellent responsivity, compared to other larger and more complex designs discussed in the literature. The sensor system was also relatively easy to fabricate using the 3D-printing method, creating in that way an inexpensive, temperature-compensated tilt monitoring device that had a wide variety of potential industrial applications.

A long-period fiber grating (LPFG) was fabricated in a home-made fluoroindate fiber using a femtosecond laser, and attenuation peaks as strong as −17 dB, with sharp and predictable spectral resonances, were obtained over the transmission spectrum. A strain up to 2000 μe applied on the LPFG yielded a resonant wavelength redshift of circa 8.46 nm. The strain sensitivity of this LPFG in fluoroindate fibers could pave the way for a new range of applications in the mid-infrared wavelength regions, such as tunable optical rejection band filters in fiber amplifiers and sensing devices.

This work reports a microsphere laser at ∼2.0 μm in a Tm3+-doped ZrF4-BaF2-YF3-AlF3 (ZBYA) fluoride glass. The ZBYA glass microsphere was fabricated by reflowing the tip of a 1.0 mol% Tm3+-doped ZBYA glass fiber with a CO2 laser. By using a fiber taper for input and output coupling in the Tm3+-doped ZBYA microsphere, single and multi-mode laser outputs at around 2.0 μm were observed under 808 nm laser excitation. The radiation lifetime and emission cross-section of Tm3+ ions in ZBYA glass were calculated to be 6.42 ms and 5.86 × 10−21 cm2, respectively. Our results indicate that the Tm3+-doped ZBYA fluoride glass has potential applications for 2.0 μm lasers.

The use of E u 3 + codoping for enhancing the H o 3 + : 5 I 5 → 5 I 6 emission in fluoroindate glasses shows that E u 3 + could depopulate the lower laser state H o 3 + : 5 I 6 while having little effect on the upper state H o 3 + : 5 I 5 , resulting in greater population inversion. The H o 3 + / E u 3 + codoped glass has high spontaneous transition probability ( 6.31 s − 1 ) together with large emission cross section ( 7.68 × 10 − 21 c m 2 ). This study indicates that codoping of H o 3 + with E u 3 + is a feasible alternative to quench the lower energy level of the 3.9 µm emission and the H o 3 + / E u 3 + codoped fluoroindate glass is a promising material for efficient 3.9 µm fiber lasers.

So far, Distribute Acoustic Sensor (DAS) systems have been used for railway condition monitoring using the dark fiber laid next to the track. In this study, the fiber was attached on the rail for in-depth analysis of the rail deflection and the load per sleeper.

A D-shaped 7-core fibre is used for curvature sensing with a Brillouin Optical Time-Domain Reflectometry setup. The preferential bending of the D-shaped fibre, com- pensates for its twist, eliminating the need for manually correcting the fibre’s position.

A single-ended distributed acoustic sensor based on remotely pumped Er-doped fibre amplifier is demonstrated using a 1480nm Raman fibre laser as pump at the front-end of the fibre to achieve a 100km sensing range and 2.6m spatial resolution.

Acoustics can be sensed with high spatial resolution by analyzing fiber Rayleigh scatter with optical frequency domain reflectometry, but signal-to-noise levels are low. Writing an array of discrete reflectors in fiber with a fs-pulsed laser increases returned light and leads to significant improvements in signal-to-noise levels and dynamic range.

A 3D-printed tilt fiber sensor using a two-mode fiber interferometer (TMFI) as the sensing mechanism has been demonstrated. The TMFI was constructed by splicing single-mode fibers (SMF-28s) at both ends of a two-mode fiber (TMF) to generate a comb-like interference spectrum that is sensitive to bending. A 3D-printed cantilever with a weight attached to one end was used to induce bending as the structure was tilted and by embedding the TMFI onto the 3D printed cantilever, different tilt angles can be measured, as a result of the bending of the fiber. The TMFI exhibited a linear response towards the tilt angles, $\theta $ , with a responsivity of $1\times 10^{-2}$ nm/deg at negative $\theta$ (0 to −90°) and $5 \times 10^{-3}$ nm/deg at positive $\theta$ (0 to 90°), respectively. The sensor was able to detect small angle changes in increments as small as 1° and it performs better than embedded FBG sensors. The tilt sensor proposed has a small formfactor and simple design, as well as being cost-effective and light-weight, thus showing significant potential for a variety of civil engineering applications.

We present a distributed acoustic sensor using micro-machined fiber with localized reflectors. The enhanced reflectors enable a phase noise of −91.5 dB (re rad2/Hz), 20 dB below the noise achieved using the Rayleigh backscattered light.

Fibre optic technology is rapidly evolving, driven mainly by telecommunication and sensing applications. Excellent reliability of the manufacturing processes and low cost have drawn ever increasing attention to fibre-based sensors, e.g. for studying mechanical response/limitations of aerospace composite structures. Here, a vector bending and orientation distinguishing curvature sensor, based on asymmetric coupled multi-core fibre, is proposed and experimentally demonstrated. By optimising the mode coupling effect of a seven core multi-core fibre, we have achieved a sensitivity of -1.4nm/° as a vector bending sensor and -17.5nm/m-1 as a curvature sensor. These are the highest sensitivities reported so far, to the best of our knowledge. In addition, our sensor offers several advantages such as repeatability of fabrication, wide operating range and small size and weight which benefit its sensing applications. This work has been funded in part by the Fondo Europeo de Desarrollo Regional (FEDER), in part by the Min-isterio de Ciencia, Innovación y Universidades under projects RTI2018-094669-B-C31 and GC2018-101997-B-I00, in part by the GobiernoVasco/Eusko Jaurlaritza IT933-16, ELKARTEK KK-2019/00101 (μ4Indust) and ELKARTEK KK-2019/00051 (SMARTRESNAK) and in part by NE/S012877/1 and EP/S013776/1. We also are grateful to Aernnova for allowing us to test our sensor in a real environment.

In this Letter, we report a single-ended distributed vibration sensor with the 100 km sensing range. This sensing range is achieved by remotely pumping two pieces of Er-doped fibers incorporated along the sensing fiber with a 1480 nm Raman fiber laser at the front end. A strain resolution of 100 nϵ combined with a spatial resolution of 2.6 m is achieved at the far end of the fiber.

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.

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

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.

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 ∼ 3.5 µ m 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.

This work presents the fabrication and experimental evaluation of instrumentation designed to enable higher spatial resolution neutron radiography for those performing research at neutron scattering facilities. Herein, we describe a proof-of-concept array of microstructured silicate fibers with 6Li doped cores that shows progress towards a design for μ m resolution neutron radiography. The multicore fiber was fabricated by drawing stacked unit elements of Guardian Glass (Nucsafe Inc., Oak Ridge, TN, USA), a 6Li scintillating core glass, and a silicate cladding glass. These structured fibers function as an array of sub-10- μ m waveguides for scintillation light. Measurements have shown a significantly increased integrated charge distribution in response to neutrons, and the spatial resolution of the radiographs is described by edge response and line spread functions of 48 ± 4 μ m and 59 ± 8 μ m , respectively.

A D-shaped 7-core fibre is used for curvature sensing with a BOTDR technique. Due to the asymmetry in its geometry, the D-shaped fibre bends along a preferred plane, self-correcting for any twist. This renders it a good candidate for field applications.

Fiber optics represents a platform suitable for the monitoring of numerous physical properties. In biology and medicine, optical fibers have found a range of applications ranging from diagnosis to therapy such as cavitational and endoscopic laser surgery. This chapter reviews optical fibers and their wealth of applications in biomedical sensing: from cellular microenvironment to pH, gas, temperature, pressure, and blood flow.

An ultrasensitive optical nanofiber coupler (NFC) for thermal and refractive index sensing with nanoscale dimension is proposed and experimentally demonstrated. The NFC operating near the cut-off region of the higher order modes is fabricated by fusing and pulling two standard telecom fibers using the modified flame brushing technique. Due to the sub-micron diameter, the NFC exhibits an anomalous behavior in close proximity to the odd supermode cut-off wavelength, where the high extinction ratio spectral oscillations exhibit very slow oscillations. The sensitivity of this region is exploited for thermal sensing over a broad range of temperatures (84 °C–661 °C). An average temperature sensitivity of 55 pm/°C and a maximum sensitivity of 60 pm/°C at the highest temperature were demonstrated. Owing to its small size, the thermal sensor exhibits a fast response time of ~7.2 ms. The sensor capability for detecting refractive index changes is also examined with different ethanol concentrations. A record sensitivity of $4.80\times 10^{5}$ nm/RIU was achieved, which is the highest reported value in all optical fiber refractometers.

Relative performance and detection limit of conventional, amplified, and gain-clamped cavity ring-down techniques (CRDT) in all-fiber configurations are compared experimentally for the first time. Refractive index measurement using evanescent field in tapered fibers is used as a benchmark for the comparison. The systematic optimization of a nested-loop configuration in gain-clamped CRDT is also discussed, which is crucial for achieving a constant gain in a CRDT experiment. It is found that even though conventional CRDT has the lowest standard error in ring-down time ( Δ τ ), the value of ring-down time ( τ ) is very small, thus leading to poor detection limit. Amplified CRDT provides an improvement in τ , albeit with two orders of magnitude higher Δ τ due to amplifier noise. The nested-loop configuration in gain-clamped CRDT helps in reducing Δ τ by an order of magnitude as compared to amplified CRDT whilst retaining the improvement in τ . A detection limit of 1 . 03 × 1 0 − 4 RIU at refractive index of 1.322 with a 3 mm long and 4.5 μ m diameter tapered fiber is demonstrated with the gain-clamped CRDT.

Terahertz (THz) technology has witnessed a significant growth in a wide range of applications, including spectroscopy, bio-medical sensing, astronomical and space detection, THz tomography, and non-invasive imaging. Current THz microstructured fibers show a complex fabrication process and their flexibility is severely restricted by the relatively large cross-sections, which turn them into rigid rods. In this paper, we demonstrate a simple and novel method to fabricate low-cost THz microstructured fibers. A cyclic olefin copolymer (TOPAS) suspended-core fiber guiding in the THz is extruded from a structured 3D printer nozzle and directly drawn in a single step process. Spectrograms of broadband THz pulses propagated through different lengths of fiber clearly indicate guidance in the fiber core. Cladding mode stripping allow for the identification of the single mode in the spectrograms and the determination of the average propagation loss (~ 0.11 dB/mm) in the 0.5–1 THz frequency range. This work points towards single step manufacturing of microstructured fibers using a wide variety of materials and geometries using a 3D printer platform.

This erratum corrects the mistyped pump repetition rate in Opt. Lett.42, 4671 (2017)OPLEDP0146-959210.1364/OL.42.004671.

In this letter, an Nd3+-doped compound fluorosilicate glass was successfully fabricated with the method of melt-quenching. Under the excitation of a 808 nm laser, enhanced near-infrared photoluminescence emission at the range from 1,065 to 1,140 nm was observed in the glass sample. To characterize its stability and resistance to environmental effects, transmission spectra at the range of NIR-MIR were measured under different environments, including humidity and temperature. In addition, the obtained fluorosilicate glass was also developed as a microsphere resonator by using CO2 laser heating method. When the diameter of the microsphere was controlled at 61.5 µm, coupling with a tapered fiber, single-multimode lasing in the wavelength range ~1,056–1,071 nm was achieved with a low lasing threshold of 1.5 mW. Compared to silica and phosphate glasses, this fluorosilicate glasses have lower phonon energy, which can reduce the probability of non-radiative transitions and improve the photoluminescence efficiency. Therefore, using it as the raw material, the developed microsphere resonator offers a high transition temperature and with a low lasing threshold, which are promising it for high performance sensing and detection applications.

The influence of nonlinear phase modulations on third-harmonic (TH) conversion in silica microfibers is experimentally demonstrated. By utilizing such influence, enhanced narrow-bandwidth ultraviolet TH is generated at a high signal-to-noise ratio (33 dB) and an average power of several hundred nanowatts. Detailed trends of TH power against input pump power were characterized with peak pump power up to 2.5 kW, and the results agree with predicted features, confirming that harmonic output could be optimized with adaptive control of the phase mismatch.

Deterministic single soliton corresponding to a smooth Kerr combs is more favored in applications. However, precise and rapid control, stochastic solitons number and low conversion efficiency are great challenges in the microresonator (MRR) driven by continuous-wave (c.w.). In this paper, we show that the MRR driving by chirped optical pulse can effectively overcome these issues. Deterministic single soliton can be produced when the chirp parameter is larger than the threshold with proper final pump phase detuning. Once above the threshold, the chirp parameter and pulse duration have no significant effects on the soliton peak power and bandwidth indicating a flexible choice of driving pulse. Nevertheless, the soliton peak power and bandwidth can be simply controlled by the final detuning.

The femtosecond laser writing technique has been widely exploited for transparent materials processing. One of the useful features following laser irradiation is refractive index modification. The sign of refractive index change depends on irradiation conditions. Negative refractive index change can be 10−2, but it is accompanied with strong optical scattering. The laser induced positive refractive index change is about 10−3. For diffractive optics this is relatively small change and thus requires to modify a large physical thickness for inducing a significant optical phase delay. This can be achieved with Bessel beams [1]. However, an extended Bessel zone leads to higher stress around the laser affected zone, which can induce structural cracks with a high energy deposition level [2]. The large stressed region also distorts subsequent writing pulses which detriments the overall uniformity of the inscribed structure limiting the performance of the written diffractive element. Here, we report a method for controlling stress accumulated during the inscription with a femtosecond Bessel beam inside silica, and demonstrate its benefits for improving structural uniformity and preventing crack formation.

While recent advances in femtosecond writing have allowed rapid and flexible realization of lab-on-chip and integrated optics systems in various media, miniaturisation of such fs-written waveguide architectures remains hampered by high bend loss α b . Despite efforts to reduce bend loss, e.g. by annealing the waveguides [1] or writing stress structures [2], the minimum bend radius at telecommunications wavelengths remains large (> 16mm) in silica [1]. To overcome this, we demonstrate a novel bend loss reduction method by inducing stress to fabricate a microcrack on the outer bend edge of the waveguide. The large index difference at the core-microcrack interface (1.45 vs 1.0) enhances modal confinement and inhibits radiation loss. Laser induced stress cracking is often used for dicing glass sheets with excellent smoothness (roughness < 30nm) [3]; hence such cracks will not exacerbate scattering losses when integrated alongside a waveguide.

A high-peak-power tunable green source operating in the range of 544-552 nm is demonstrated through frequency doubling a diode-seeded, nanosecond, polarization-maintaining, Yb-doped fiber master oscillator power amplifier system. Type I second-harmonic generation was implemented in a lithium triborate crystal via noncritical phase matching by temperature tuning. Narrow-bandwidth green light of average output power of up to 9 W, with a pulse duration of 1.8 ns and a peak power of 25 kW, was obtained at a repetition rate of 200 kHz. The pump-to-green conversion efficiency reached 30%. This linearly polarized, narrow bandwidth, tunable source has great potential for applications, such as material processing, sensing, and medical research.

A polymer plasmonic waveguide for second harmonic generation (SHG) from mid-infrared (MIR) to near-infrared (NIR) is proposed. The required phase matching condition (PMC) is satisfied between the fundamental plasmonic mode at the fundamental frequency (FF) and the photonic mode at the second harmonic frequency (SHF). According to our simulations, the SHG conversion efficiency of 4.173% can be obtained in 92.3 μm length waveguide with 1 W pump power. Moreover, the conversion can be further enhanced by using a microring resonator (MRR). By optimizing the MRR, the SHG conversion efficiency can be increased to 26.1%. The proposed scheme can be a potential approach for MIR and NIR wavelength conversion and quantum signal processing.

Here we discuss several different pathways of exploiting optical fibers for sensing applications. We demonstrate that fused optical fiber couplers can operate as a sensitive refractometers and thermometers. Their potential for bio-sensing applications is also discussed. We also discuss application of plastic fibers for accessing mid-IR spectral region for optical sensing.

The improvement of neutron imaging towards and beyond the microscale is a well-documented need for the iterative characterization and modeling of numerous microstructured X-ray opaque materials. This work presents the recent progress in evaluating a SCIntillating FIber (SCIFI) proof-of-concept towards micron-level thermal neutron radiography. These SCIFIs are composed of 6Li-enriched silicate glass cores doped with a Ce activator. The cores possess ∼8.5 μm diameters and ∼10 μm pitch following fiber drawing with a cladding glass into an all-solid multicore fiber. A polished 5 mm × 5 mm array of 100 microstructured multicore SCIFI pixels was fabricated into a 1 mm thick faceplate. The neutron efficiency and light yield of the faceplate are characterized as functions of the 7.38 weight percent of Li2O, thickness, and the 70% active volume. It was determined that approximately 39% of a thermal neutron ( $\text{2 }$ A) beam can be absorbed by the faceplate. The $^6{\rm{Li}}({n,\; \alpha })t$ reaction is estimated to produce 7,700 ± 1,000 scintillation photons per event, referencing light collection from 241Am irradiation of the faceplate. Simulations suggest that on average 17.5 ± 1.4% of these photons will be transported to an end of the fiber array for a thermal beam, with at least 7.2% of that total scintillation light being confined into the fiber cores in which it originated. The SCIFI faceplate was integrated into the Neutron Microscope (NM) at the Pulse OverLap DIffractometer (POLDI) beamline located at the Paul Scherrer Institut to image a Siemens star test object. Processed neutron radiographs acquired with the proof-of-concept faceplate resolved features at a state-of-the-art resolution of 16.1 ± 0.5 μm. The potential for even high resolution designs having smaller pitch or different cladding material is discussed.

peer-reviewed In this Letter, a Tm-3(+)-Ho3+ codoped tellurite glass microsphere laser in the 1.47 mu m wavelength region is described. Using a traditional tapered microfiber-microsphere coupling method, multimode and single-mode lasing around the wavelength of 1.47 mu m is observed using an 802 nm laser diode as a pump source. This Tm3+-Ho3+ codoped tellurite glass microsphere laser can be used in near-infrared telecommunications, biomedical, and astrophysical applications. (C) 2019 Optical Society of America peer-reviewed

Cr3+-Yb3+ codoped bulk glass-ceramics containing KZnF3 nanocrystals are fabricated by thermal treatment of cast glass samples and characterized by X-ray diffraction and transmission electron microscopy. The luminescent properties of the glass and glass ceramic are investigated from the measured photoluminescence spectra and fluorescent lifetime. The measurement results demonstrate that Cr3+ and Yb3+ ions are both predominantly hosted in the KZnF3 nanocrystals, and the energy absorbed by Cr3+ ions is efficiently transferred to Yb3+ ions when excited at 450 nm. Compared to the glass, the near-infrared emission in the Cr3+-Yb3+ codoped glass ceramics is significantly enhanced when the excitation wavelength lies in the range λ~400–800 nm of a solar simulator. Results indicate that the Cr3+-Yb3+ codoped KZnF3 glass ceramic provides a promising material for spectral conversion from visible sunlight to near-infrared emission and a novel gain material for solar pumped fiber laser.

A room-temperature watt-level continuous-wave-output power mid-infrared fiber laser operating at λ ∼ 3 µ m is demonstrated using a H o 3 + / P r 3 + co-doped A l F 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 > 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.

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.

In this study, the sensitivity and the linearity of the un-coated and TiO2-coated microfiber knot resonator (MKR) have been analyzed. The MKR is very sensitive to humidity changes since its refractive index is strongly humidity dependent. As a result, shifts occur in the resonance wavelength and there are also changes in output power. The un-coated MKR showed a sensitivity of 1.3 pm/%RH, in terms of the resonance wavelength, and a sensitivity of 0.0626 dB/%RH for the transmitted output power. The sensitivity increased greatly after the deposition of a porous TiO2 nanoparticle coating on the MKR. The TiO2-coated MKR showed an improved sensitivity of 2.5 pm/%RH, with respect to the resonance wavelength, and 0.0836 dB/%RH for the transmitted output power. This MKR sensor has the potential for use in a variety of humidity sensing applications.

We propose an internal asymmetric plasmonic slot waveguide (IAPSW) containing two different materials in the slot region for third harmonic generation (THG) from 2.25 μm. In the IAPSW, the required phase matching condition is satisfied between the 0th-order mode at the fundamental frequency and 1st-order mode at the third harmonic frequency. By choosing an appropriate slot geometry and materials, the third harmonic electric field distribution can be engineered to significantly enhance the nonlinear overlap coefficient for THG. According to our simulation, a conversion efficiency up to 0.67 % with 1 W pump power is predicted within a ∼10 μm IAPSW. Additionally, the waveguide shows large fabrication tolerance in terms of geometry parameters. The proposed waveguide can find potential applications for high-speed all-optical signal processing.

In the last decade optical fibre tapers with micrometre diameter (often called microfibers) have been investigated for numerous applications ranging from sensing to wavelength convertors, telecom and optical manipulation. This paper reviews the various applications of microfibres.

The removal of polarization and modal degeneracy in the excitation of higher order modes is realised in all-fiber mode-selective weakly-fused-couplers by controlling the coupler geometry. Experiments carried out with short couplers show strong polarization dependency, while long couplers with a slow varying cross-section have shown to remove polarization and spatial degeneracy in the first high order mode.

All-optical modulation is experimentally demonstrated in a microfibre coil resonator (MCR) functionalized with black phosphorus (BP) nanosheets (BP-MCR). The BP nanosheets is deposited onto microfibre and then microfibre is wrapped on a PMMA rod covered with UV-curable low refractive index polycarbonate (PC) resin. The BP-MCR modulation properties are regulated based on BP absorption, photo-thermal and optical Kerr effects. The resonance wavelength and extinction ratio responsivity to incident pump power can attain ∼0.033 nm mW−1 and ∼0.180 dBm mW−1, while the averaged rising/falling response time is ∼9.58/8.53 ms. The device could find applications as all-optical modulator, optical switch or tunable optical filter.

Broadband emission at 2.7 µm is observed in an E r 3 + -doped P b O − P b F 2 − B i 2 O 3 − G a 2 O 3 glass. The measured emission band full-width-at-half-maximum (FWHM) is ∼ 184.4 n m , approximately 36 nm wider than that of fluoride glasses. The 2.7 µm emission intensity is almost twice as strong as that of fluoride glasses. The peak values of emission and absorption cross-sections are calculated to be 1.54 × 10 − 20 c m 2 and 1.19 × 10 − 20 c m 2 , respectively. This oxyfluoride heavy metal glass shows potential as broadband mid-infrared emission gain material.

In this Letter, we report lasing at 2.3 µm in Tm 3 + -single-doped and Tm 3 + / Ho 3 + -codoped fluorotellurite glass microsphere resonators. By employing a 793 nm diode laser as a pump and exploiting whispering gallery mode microresonators (WGMRs), dual-wavelength lasing at 1.9 and 2.3 µm and triple-wavelength lasing at 1.9, 2.07, and 2.3 µm are achieved in Tm 3 + -doped and Tm 3 + / Ho 3 + -codoped microspheres, respectively. The introduction of Ho 3 + ions significantly reduces the lasing threshold of Tm 3 + at 2.3 µm because of energy transfer.

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.

In this letter, high Q factor microsphere resonators with excellent stability and high damage threshold are fabricated from Tm3+ doped fluorosilicate glasses using the traditional melt-quenching and fiber heating techniques. Intense 1.88 μm photoluminescence and single mode laser emission are obtained under the excitation of an 808 nm laser. The Q factor of the microsphere resonator is measured to be as high as 1.4 × 106. Models for photoluminescence and lasing are proposed using spectroscopic data from the absorption and emission spectra. The excellent resistance to humidity and temperature are characterized from the transmittance spectra under different environmental conditions. Differential thermal analysis (DTA), measured in four different glass samples proves the high damage threshold. The proposed glass microsphere resonator laser can find application in optical sensors and micro lasers, especially in complex environments.

Simultaneous 1.50 μm, 1.84 μm and 2.08 μm laser emission is observed in a Ho3+/Tm3+ co-doped ZrF4–BaF2–YF3–AlF3 (ZBYA) fluoride glass microsphere when pumped using a 793 nm laser diode. The ZBYA fluoride glass microsphere was prepared using a CO2 laser heating method and whispering-gallery modes (WGM) were excited when the pump light was coupled using a tapered fiber. The introduction of Ho3+ ions reduced the population of the long-lived Tm3+:3F4 level and removed the 3H4→ 3F4 self-termination effect, resulting in the population of the Ho3+:5I7 level through a resonant energy transfer process. As the 793 nm laser pump power grows, lasing occurs initially at 2.08 μm, followed by the cascade outputs at 1.50 μm and 1.84 μm.

H o 3 + / P r 3 + co-doped A l F 3 -based glass fibers were fabricated by using a rod-in-tube method based on the matrix glass composition of A l F 3 − B a F 2 − C a F 2 − Y F 3 − S r F 2 − M g F 2 − L i F − Z r F 4 − P b F 2 . Under the pump of a 1150 mW Raman fiber laser, a 2.9 µm laser was observed in a 19 cm long H o 3 + / P r 3 + co-doped A l F 3 -based glass fiber with an output power of 173 mW and a slope efficiency of 10.4%. H o 3 + / P r 3 + co-doped A l F 3 -based glasses were fabricated to investigate the deactivation effects of P r 3 + ions on the H o 3 + : 5 I 7 level. Our results showed that the H o 3 + / P r 3 + co-doped A l F 3 -based glass fibers are potential gain media for ∼ 2.9 µ m lasers.

We propose a mid-infrared (MIR) waveguide in which two pure photonic fundamental modes are perfect phase-matched for third harmonic generation (THG) and triple photon generation (TPG). The waveguide is based on the germanium–silicon carbide–germanium (Ge–SiC–Ge) hybrid structure. Leveraging the anomalous dispersion of SiC near the resonance, the THG phase mismatch between two fundamental modes can be overcome. By carefully engineering the nonlinear mode overlap, the MIR THG performance is investigated in detail. In addition, a MIR TPG rate of ∼ 1 0 − 3 Hz is estimated. The proposed scheme provides an integrated platform for future MIR all-optical signal processing and quantum signal processing.