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It is shown experimentally a new digital optical decoding scheme based on the transmission or polarized light at p polarization planes using a K‐Nearest Neighbour (KNN) algorithm through a single‐mode optical fibre at 633 nm. The optical power signal is sent at p polarization planes which constitute p classes required for signal bit recognition. Results show that it is possible to recognize 32 polarizations planes, 5 bits, using 4 features corresponding to the measurement of optical power at 4 different angles at the photodetector side with an average assertiveness of 99.02%. [ABSTRACT FROM AUTHOR]

This letter proposes a novel dynamic range extension method using an instantaneous frequency (IF)‐tracking method for heterodyne fibre optic hydrophones (FOHs). IFs are used to compensate for large signal amplitudes in the demodulator. Theoretical analysis and simulation prove the effectiveness of the proposed algorithm. Simulation results achieve at least 19.76 dB dynamic range improvement, demonstrating great potential for extending FOHs for hydroacoustic detection and communication applications.This letter proposes an instantaneous frequency‐tracking method to extend the dynamic range for heterodyne fibre optic hydrophones (FOHs). They are used directly to compensate for large signal amplitudes. The working principle is discussed, and simulations are conducted. The simulations achieve at least 19.76 dB dynamic range improvement. [ABSTRACT FROM AUTHOR]

In this Letter, the authors present a first measurement of Brillouin gain spectrum with dual-frequency solid-state laser. This allows a coherent detection of the Brillouin backscattered spectrum in a frequency range below the hundreds of MHz by fine-tuning of the difference between the two cross-polarised fields emitted by the laser. The photodetection at low-frequency benefits from the balanced detection technique where high dynamic range measurements are largely available. The narrow line-width of the cross-polarised field permits also the direct measurement of the Brillouin spectrum line-width. The system is demonstrated by measuring the Brillouin frequency shift with temperature when standard single-mode fibre-28 (SMF-28) and large effective area fibre (LEAF) are fully heated. This gives temperature coefficients close to theoretical expectations with values of 1.16 MHz/°C for SMF-28 and 1.15 MHz/°C for LEAF. [ABSTRACT FROM AUTHOR]

The radiation recovery effect on sensing fibre in Brillouin optical time‐domain reflectometer (BOTDR) has been experimentally investigated. In the experiment, three kinds of commercial single‐mode G.652.D fibres: Yangtze Optical Fibre and Cable, Corning and Fujikura have been used as sensing fibre in BOTDR system. After 200 Gy dose gamma‐ray radiation absorbed by sensing fibre, the performance of BOTDR at different time intervals has been tested. Moreover, by analysing Brillouin gain spectra along with fibre sample, the recovery in radiation‐induced attenuation, the variation of Brillouin frequency shift and the variation of stress coefficient are characterised. The results will be helpful for the error correction in the simulation experiment and are a reliable basis for the introduction of BOTDR into the space station. [ABSTRACT FROM AUTHOR]

Brillouin optical correlation‐domain reflectometry (BOCDR) is one of the fibre‐optic distributed strain and temperature sensing systems. Conventionally, BOCDR has been implemented using three erbium‐doped fibre amplifiers (EDFAs) to enhance the signal‐to‐noise ratio (SNR) of the measurement. However, the use of multiple EDFAs has restricted the size and cost efficiency of the system. In this work, the authors show that, among the three EDFAs previously employed, the one in the Stokes path plays the most important role in enhancing the SNR. Subsequently, they implement BOCDR using only one EDFA and demonstrate distributed strain sensing. The measurement error is also investigated as a function of the output power of a light source. [ABSTRACT FROM AUTHOR]

The authors demonstrate distributed temperature sensing based on slope‐assisted Brillouin optical correlation‐domain reflectometry (BOCDR) with a long measurement range of >10 km. They find that to achieve such a long‐range measurement, a delay line in a reference path needs to be at least four times longer than the sensing fibre. In addition, they show that the use of such a long delay line induces forward‐propagating Brillouin‐scattered light in the reference path, which deteriorates the signal‐to‐noise ratio (SNR) of the system and should be suppressed. Finally, by exploiting a beyond‐nominal‐resolution effect of the slope‐assisted BOCDR, they detect a 3‐m‐long heated section in a 13‐km‐long silica fibre in a distributed manner; the reason for its low SNR is also discussed. [ABSTRACT FROM AUTHOR]

The authors report a one‐piece flexible liquid level optical fibre sensing probe in this Letter, which is meant to measure the liquid level inside the cryogenic propellant tanks of spacecraft and rockets. The ultra‐lightweight sensing probe can be used in both aerospace vehicles and in any type of cryogenic fluid storage system. In contrast to current liquid gauging technologies such as Delta‐P pressure sensor, capacitance probe, ultrasonic sensor, and silicon diode liquid level sensing probe, the fibre optic continuous liquid level sensor has unique advantages in terms of gauging accuracy, reliability, simplicity, and maintenance. The sensor uses one single optical fibre probe to measure the liquid level and the liquid distribution of cryogenic propellant. Every point of the sensing fibre is a 'point sensor' that is able to distinguish liquid and vapour. The physical location of each 'point sensor' can be determined with 1 mm spatial resolution or better. The optical fibre probe can be installed in a propellant tank with the same manner as currently used silicon diode liquid level sensing probe via only one single feed‐through to connect to an optical signal interrogation unit located outside the tank. Experiments on measuring liquid nitrogen level are demonstrated. [ABSTRACT FROM AUTHOR]

Fibre Bragg grating inscription in the crystalline silicon (Si)‐core fibre is described using visible light femtosecond laser pulses. The femtosecond pulses at 517 nm propagate through the transparent silica glass cladding and are fully absorbed by the Si core generating locally high temperatures and stress fields in forming the grating periods. The Si FBGs were characterised in reflection, calibrated for a first time as strain and temperature sensors and compared with that of FBGs inscribed in standard silica fibres. [ABSTRACT FROM AUTHOR]

Owing to unstable multi‐wavelength lasing, robust measurement is difficult with a fibre Bragg grating (FBG) laser sensor system that uses FBGs as laser cavity mirrors as well as sensors. A stabilising technique is essential for proper data acquisition. Here, it is demonstrated that polarisation scrambling (PS) stabilises the output of laser sensor system in combination with a spectrometer demodulation. The state of polarisation (SOP) of a fibre‐optic laser cavity is changed by a 1 MHz PS. A 5 kHz spectrometer traces the distribution of output laser wavelengths. The rapid changes in SOP make all the FBG wavelengths lase as long as necessary to determine the Bragg wavelength shifts in the sampling windows of a photo‐diode array. A laser sensor system with eight FBG sensors is used to demonstrate robust measurement and data collection under environmental perturbations. [ABSTRACT FROM AUTHOR]

To explore fibre refractive index (RI) sensing based on radiation modes, the influences of ambient RI on radiation modes are investigated by a leaky mode approximation in a two‐layer fibre. Based on the analytical analysis and simulation results, it is identified that the change of ambient RI has no effect on the real part of the complex propagation constant of a leaky mode, but a remarkable effect on the imaginary part. More importantly, the influence of the medium's absorption loss on the imaginary part is almost negligible. If the medium's absorption loss is lower than 3.521 × 103 dB/cm, the induced average deviation rate of the imaginary part is <1.91%. [ABSTRACT FROM AUTHOR]

A photonic technique for microwave frequency measurement based on stimulated Brillouin scattering (SBS) with low measurement error and large measurement range is demonstrated. In the scheme, an optical phase modulator is employed and the intensity balance between the two sidebands of the phase‐modulated signal is broken by use of SBS in a fibre. The frequency measurement is achieved by detecting the existence of a microwave signal after photoelectric detection of the phase modulated signal. The measurement error is experimentally shown to be 50 MHz in a narrow measuring range of ∼1 GHz and 100 MHz in a wide range of 20 GHz. [ABSTRACT FROM AUTHOR]

Resonant optical strain sensors, obtained by closing in loop coupled π‐shifted Bragg gratings (π‐FBGs), are reported. Despite of a conventional π‐FBG, showing a transmission Lorentzian resonance in the reflection band, the coupled π‐shifted Bragg grating structure is characterised by a spectral response formed by a split doublet of the primary transmission Lorentzian mode of π‐FBG. By closing in loop this structure, the coupled π‐shifted Bragg grating ring resonators show better performance than π‐shifted Bragg grating ring resonators in terms of strain sensitivity (i.e. 2.86 pm/με) is demonstrated. Thus, the coupled π‐FBGRR is suitable for enhanced sensitivity strain sensing applications. [ABSTRACT FROM AUTHOR]

The ultrasonic welding of polymer optical fibres (POFs) onto carbon‐fibre‐reinforced polymers for advanced fibre‐optic sensing is demonstrated. The relationships among the welding time, preload, optical loss, and adhesive force are fully evaluated. A high‐speed camera to monitor changes in the cross section of the POF during the ultrasonic welding is also used. [ABSTRACT FROM AUTHOR]

The development and preliminary characterisation of a novel fibre probe for laser thermotherapy of solid tumours are presented. The probe introduces two innovative features: the tailoring of the laser irradiation pattern to adapt it also for larger tumour applications, and the all‐optical real time evaluation of the induced temperature increase. These features are simultaneously obtained by integrating in a single capillary tube a laser delivery fibre with properly micro‐structured tip surface together with some fibre Bragg gratings. Preliminary validation examples in human liver phantoms have demonstrated the viability of the proposed approach for the development of a whole set of new probes for laser ablation in medical applications. [ABSTRACT FROM AUTHOR]

A high‐speed quasi‐distributed fibre optic sensing technique for strain is proposed and a proof‐of‐concept experiment is performed. The sensing link is formed by multiple fibre Bragg gratings and interrogated by a pulsed broadband source. A dispersive unit is used to translate wavelength variations to time‐delay changes to enable high‐speed sensing capability. As the gratings can be weakly reflecting and nominally identical in their reflection spectra, this technique has the inherent capability of high‐capacity multiplexing. Linear and repeatable static strain response was achieved with a strain–delay conversion factor of −3.37 ps/με and a resolution of 4.73 με. Dynamic strain was successfully measured at a sampling rate of 7 kHz and dynamic resolution of 0.2 με/√Hz was obtained. [ABSTRACT FROM AUTHOR]

A surface plasmon resonance (SPR) sensor based on hollow‐core photonic crystal fibre (HC‐PCF) filled with silver nanowires is designed. The analyte and silver nanowires are full filled in the air holes of the HC‐PCF to realise the SPR sensing, which is more convenient than silver coated in operation. The designed sensor is analysed through numerical simulations and demonstrated by experiments. All the results show that a blue‐shift is obtained with increase of the analyte refractive index (RI), and the silver nanowires concentration has no effect on spectral sensitivity. The highest average spectral sensitivity of 14 240 nm/RIU is obtained by experiments, which is higher than that previously reported for the same type of sensors. The sensor is useful for detecting small analyte RI changes, and can also provide a reference for the implementation and application of PCF‐SPR sensors with high sensitivity. [ABSTRACT FROM AUTHOR]

A new design of a fibre‐optic probe system is presented which can be used to simultaneously detect scattering signals from multiple channels for spatially offset Raman spectroscopy and Raman tomography in reflection mode. The fibre setup contained 32 source fibres with an auto‐controlled optical switch and 40 detector fibres, which were deployed in a large‐area planar interface. Phantom experiments were conducted using a prototype of the Raman instrument, and have demonstrated the practical applicability of the system design. The presented area‐detection instrument provides an efficient platform for various reflectance applications of Raman modalities over a large area. It could also be potentially adopted for other diffuse optical‐based spectroscopic and tomographic applications. [ABSTRACT FROM AUTHOR]

The use of an artificial neural network (ANN) for extraction of a temperature profile from a local Brillouin gain spectrum in a Brillouin optical time‐domain analysis fibre sensor system is proposed and demonstrated. An ANN is applied to process the Brillouin time‐domain trace in order to extract the temperature information along the fibre after the data acquisition process. The results show that the ANN provides higher accuracy and larger tolerance to measurement error than Lorentzian curve fitting does, especially for a large frequency scanning step. Hence the measurement time can be greatly reduced by adopting a larger frequency scanning step without sacrificing accuracy. [ABSTRACT FROM AUTHOR]

A novel technique to perform dynamic strain measurement with a Brillouin optical time‐domain reflectometry (BOTDR) sensing system based on short‐time Fourier transform (STFT) is proposed. Unlike traditional frequency scanning methods, the proposed system samples the Brillouin backscattering signal directly after it has been downconverted to the intermediated frequency (IF) region. The Brillouin frequency shift (BFS) of the sensing fibre was reconstructed from the recorded time‐domain data continuously without dead‐time, which provided a much higher refreshing rate of the strain measurement. Therefore, dynamic strain variation over an optical fibre can be measured with a single‐end structure that is more flexible and reliable. A 16.7 Hz strain variation on a 12 m section was successfully detected at the end of a 270 m sensing fibre with 4 m spatial resolution and 45 με uncertainty. [ABSTRACT FROM AUTHOR]

A sensor array employing the distributed Bragg reflector (DBR) fibre laser is presented. This array could be utilised as a multi‐channel multi‐functional detector for bending radius, transverse load and axial strain by simultaneously monitoring the lasing wavelength and the output beat note frequency. The measurement theory of the DBR fibre laser is explained, and it is then experimentally verified with a three‐probe array. The system's detection sensitivities for curvature, transverse load and axial strain are found to be 600 MHz/cm−2, 1.3 MHz/g and 1.263 pm/με, respectively. [ABSTRACT FROM AUTHOR]

The leakage light of an electro‐optic modulator induced by its finite extinction ratio (ER) may degrade the performance of the Brillouin optical time domain reflectometer (BOTDR) sensing system, especially for long distance measurement. A novel configuration of a modulator based on a synchronous optical switch is presented. Experimental results have shown that the performance of the sensing system can be obviously improved with the proposed high ER modulator. At the end of a 48.5 km sensing fibre, the maximum uncertainty of temperature measurement has been reduced from 5.2 to 0.8°C with a 25 m spatial resolution after making an improvement in the ER from 35 to 65 dB. [ABSTRACT FROM AUTHOR]

A fibre Bragg grating (FBG) array‐based sensing probe has been installed on a device for radiofrequency thermal ablation (RFTA). The probe is made of five FBGs with 0.5 cm active area and 1 cm spacing, to provide quasi‐distributed thermal pattern measurements. Multiple experiments have been conducted on porcine liver, reporting the temperature pattern along the ablation axis. Thermal maps allow an estimation of the RFTA efficiency and spatial extension in the liver. [ABSTRACT FROM AUTHOR]

Although haptic feedback is critical for many surgical tasks, surgeons cannot currently obtain haptic feedback from teleoperated surgical robots because it is difficult to attach force sensitive sensors to the surgical robot in the surgery environment. Therefore, force sensitive forceps with an optical fibre Bragg grating sensor attached on the grasper are proposed. The proposed prototype forceps confirmed that the sensitive grasping force in the teleoperated grasper is comparable to the providing force and measuring force. [ABSTRACT FROM AUTHOR]

A method for enhancing the sensitivity of a fibre Bragg grating (FBG) sensor by all‐optical signal processing is demonstrated. Degenerated four‐wave mixing (FWM) has been used to perform frequency chirp magnification, which can be used for magnifying the wavelength drift of the FBG sensor. Ultra‐highly sensitive measurement of the static strain has been experimentally demonstrated with strain sensitivity of 5.40 pm/μɛ. The sensitivity has been enhanced by a factor of five. [ABSTRACT FROM AUTHOR]

A fibre bundle has been developed for coherent anti‐Stokes Raman scattering (CARS) microendoscopy. The polarisation scheme was employed to suppress the four‐wave mixing (FWM) effect in the fibre. High collection efficiency of epi‐CARS signals and isolation of excitation lasers are achieved. Epi‐CARS and epi‐FWM images were compared in different polarisation schemes. The use of a broadband dual wavelength waveplate offers spectroscopic imaging capability to the system, which was shown by using ex vivo imaging mouse tissues. [ABSTRACT FROM AUTHOR]

The first practical demonstration of readily useable fibre Bragg gratings in long lengths (>20 m) of polymer optical fibres (POFs) is reported; this is an increase in fibre length that exceeds two orders of magnitude for use in the C‐band. A low loss perfluorinated fibre and a femtosecond‐laser, direct‐write method to inscribe gratings at two different wavelengths are used. Gratings are fabricated to operate at ∼800 nm, where the optical loss is minimal, and in the C‐band, commonly used with traditional optical fibres for the majority of telecommunication applications. The development of POF Bragg gratings inscribed in long fibre lengths is the first important step for the integration of practical sensing devices and could promote the use of multiplexed POF sensors. The sensors were tested and characterised and demonstrate their compatibility with a commercial Bragg grating demodulator. [ABSTRACT FROM AUTHOR]

This Letter reports a lead (II) ion sensor composed of a tapered multimode fibre (MMF) coated with chitosan. The diameter and length of the tapered fibre region is 20 µm and 1 cm, respectively. Tapered MMF is used because it is more flexible and compact compared with plastic clad silica. Chitosan is chosen as the sensing layer because it can absorb heavy metal ions due to its unique properties as polyelectrolyte and chelating agent. The refractive index of the chitosan layer changes as it absorbs lead (II) ion and this leads to an increase in the absorbance measured by the spectrophotometer. This is tested by immersing the sensor in a solution that contains lead (II) ion with different concentrations, from 0.2 to 1 ppm. The results show that the absorbance measured by the spectrophotometer increased as the concentration of the lead (II) ion increased. The sensor's average sensitivity is measured as 40.554 abs/ppm. This sensor is simpler compared with surface plasmon resonance‐based lead (II) ion sensor because it does not require metal coating prior to the sensing layer coating. [ABSTRACT FROM AUTHOR]

A surface plasmon resonance (SPR) sensor based on exposed‐core microstructured optical fibres (EC‐MOFs) is presented and numerically characterised. The exposed section (analyte channel) of the EC‐MOF is coated with a SPR supporting thin film of gold. The asymmetrically coated fibre can support two separate resonance peaks, with orthogonal polarisations (x‐ and y‐polarised peaks). Although the sensitivity of both the polarised peaks is very similar, the narrower linewidth of the y‐polarised peak provides a much enhanced signal‐to‐noise ratio. The SPR sensing performance for a large refractive index (RI) range from 1.33 to 1.42 is evaluated. The sensor has higher sensitivity for detecting the high analyte RI. The feasibility of using EC‐MOFs to develop a fast‐response, real‐time and distributed SPR sensor is determined. [ABSTRACT FROM AUTHOR]

A buffer stripped high birefringent photonic crystal fibre based polarimetric sensor is developed for monitoring the curing process of magnetorheological elastomer (MRE) smart composite material. Using the developed sensor, different phases of the MRE curing process are clearly visible from the phase shift variation of the embedded polarisation maintaining photonic crystal fibre (PM‐PCF) sensor. During the curing process, the buffer stripped PM‐PCF exhibits a stress/strain induced phase shift variation from 0 to 1.98 rad. This is a significantly large phase change, which can be used to very clearly identify the different stages in the curing process. For comparison, a fibre Bragg grating sensor is also used for monitoring the internal strain during the curing process and its response does not allow one to reliably distinguish all the curing stages. The present investigation offers a simple non‐destructive method to monitor the curing process of MRE smart composite material. [ABSTRACT FROM AUTHOR]

A tip‐sensitive fibre‐optic Bragg grating ultrasonic hydrophone (TSFBGUH) with high spatial resolution for measuring high‐intensity focused ultrasound (HIFU) fields is reported. When measuring a HIFU field, the sensitive axis of the TSFBGUH with a flexible fibre‐optic sensor holder should be in parallel with the acoustic axis of the HIFU transducer to ensure the measured ultrasound impinges on the tip of the TSFBGUH. The TSFBGUH system has been established and tested. The experimental results show that the acoustic pressure sensitivity of the TSFBGUH system is about 31.3 mV/MPa within the measurement range of 10 MHz and the noise equivalent pressure is about 10.4 kPa. [ABSTRACT FROM AUTHOR]

Current sensing based on the Faraday effect in optical fibres is a well‐established area in the landscape of sensor technologies. However, the optical behaviour that sets an upper limit on the detection bandwidth is often overlooked. The underlying mechanisms are explored in this reported analysis to raise awareness about their impact on the measurement results. The findings show that the Faraday effect cancellation and pulse broadening grow with increasing signal frequency, which result in suppression and distortion of the optical response. A correction factor is proposed for alternating and pulsed signals when using the simplified equation, to maintain an accurate measure of the peak current. [ABSTRACT FROM AUTHOR]

A novel signal processing‐based approach for the real‐time estimation of air‐gap length in fibre optic extrinsic Fabry‐Pérot pressure sensors is presented; the algorithm is based on eigendecomposition of the spectrum mutual correlation matrix. A pressure accuracy of 6.1 Pa (0.045 mmHg) is achieved in a pressure probe with 1.13 nm/kPa sensitivity, highlighting a 6.4 times improvement over standard quadrature point amplitude tracking by just acting on signal processing. [ABSTRACT FROM AUTHOR]

A temperature‐ and strain‐independent torsion sensor based on analysis of polarisation‐dependent loss (PDL) characteristics of high‐birefringent (Hi‐Bi) fibre Bragg gratings (FBGs), is proposed and demonstrated. Unlike the amplitude spectra, the PDL response has higher sensitivity to torsion. Experimetal results demonstrate that the difference between the two peaks in the PDL spectra has a linear relationship with the twist rate, but remains invariant with changes in temperature and strain. In addition, it was observed that the PDL responses to the clockwise twist are just opposite of the responses to the couterclockwise twist. [ABSTRACT FROM AUTHOR]

A flexible technique to periodically perturb the evolution of differential phase in birefringent optical microfibre is proposed. This conceptual demonstration offers a simple yet effective solution to rectify non‐ideal microfibre coil sensor heads for high‐performance current sensing. [ABSTRACT FROM AUTHOR]

The advent of multiplexing interferometric schemes for fibre Bragg grating sensor interrogation has enabled the combination of large‐scale multiplexing and high‐resolution measurement. Described is a simple technique to add absolute wavelength measurement capabilities to any passive‐interferometric multiplexer. Through simulation a wavelength accuracy of < 10 pm using common system specifications is demonstrated. The technique allows high‐resolution interferometric interrogation to be applied to many key industrial applications requiring precise knowledge of absolute values, such as static temperature or strain, without trading‐off high interrogation rate or resolution. [ABSTRACT FROM AUTHOR]

Based on the pump‐probe technique (PPT), stimulated Brillouin scattering (SBS) was observed in a 100 m‐long silica graded‐index multimode optical fibre with the same core diameter as that of a perfluorinated graded‐index polymer optical fibre (PFGI‐POF) (=50 µm). The SBS threshold was measured to be ∼50 mW, which was much lower than 5.08 W, the SBS threshold obtained for the same fibre without the PPT. This result indicates that the PPT is useful even under multimode conditions and that the difficulty in observing SBS in PFGI‐POFs consists not in their physical problems but in their material problems (high propagation loss etc.). [ABSTRACT FROM AUTHOR]