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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

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.

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.

Current sensing based on the Faraday effect in optical fibres is a wellestablished 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 Refereed/Peer-reviewed

An optical fibre sensor system for humidity and temperature, comprising two Bragg gratings recorded in silica and polymer fibre, has been characterised. The response of this system is very well conditioned (2-norm condition number = 8.8) and consequently uncertainties in wavelength measurement do not lead to large errors in the recovered humidity and temperature.

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.

A flexible technique to periodically perturb the evolution of differential phase in birefringent optical microfiber is proposed. This conceptual demonstration offers a simple yet effective solution to rectify non-ideal microfiber coil sensor heads for high-performance current sensing.Introduction:The rise of optical microfiber technology [1] has opened up a new lineage of current sensing. Optical current sensors employing optical microfibers (OM) based on the Faraday Effect have shown their potential for ultra-fast current detection in ultra-small geometries [2–4]. However, as with conventional current sensors, the problem of birefringence [5,6] still persists. Spun optical microfiber was previously proposed and demonstrated [7]. It offered improvements in the current responsivity at the expense of increased fabrication time and resources. In this Letter, we propose a post-fabrication technique to achieve the same goal, but with considerably lower fabrication complexity.

The advent of multiplexing interferometric schemes for fibre Bragg grating (FBG) sensor interrogation has enabled the combination of large-scale multiplexing and high-resolution measurement. In this letter, we describe a fast and simple technique to add absolute wavelength measurement capabilities to any passive-interferometric multiplexer. Through simulation we demonstrate a wavelength accuracy of

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.).

A study is presented of the effects of temperature non-uniformity in a distributed temperature sensor based on the temperature dependence of Brillouin gain in an optical fibre. The form of the Brillouin gain profile for a given temperature distribution is measured and successfully modelled.

A technique for the generation of quadrature phase components at the output of a fibre interferometer using input polarisation modulation is described. This approach allows a passive path-balanced sensor interferometer to be remotely interrogated using a simple concept based on polarisation switching of the input state of polarisation coupled to the interferometer.

The paper demonstrates a technique that adds absolute FBG reflected wavelength determination to any passive phase demodulation technique based FBG multiplexing system. Their technique and wider system could find applications in areas from smart grids to nuclear power via hydrophones.

The spontaneous length fluctuation of optical fibres caused by mechanical dissipation is analysed using a one-dimensional model based on the Fluctuation-Dissipation Theorem. Scale estimate shows evidence that the 1/f thermal noise originated from this fluctuation dominates at low frequencies in fibre interferometers and fibre cavities.

A new method to guide and control microsurgical tools using optical coherence tomography (OCT) is being developed by researchers at Johns Hopkins University and the US Food and Drug Administration Center for Devices and Radiological Health. In their recent work, which appears in this issue of Electronics Letters, the team have demonstrated a fibre nerve stimulation probe integrated with an OCT distance sensor which can be controlled much more precisely; therefore significantly improving the safety of the technique.

A simple and effective common-path optical coherence tomography guided fibre probe for optical nerve stimulation has been developed and tested. The probe is capable of real-time monitoring of the fibre-to-nerve distance up to more than 1 mm at an axial resolution of approximately 3 m, thus improving the precision and safety of stimulation laser power delivery.

A singlemode–multimode–singlemode (SMS) fibre structure for temperature measurement that utilises a simple intensity-based interrogation system is proposed. The temperature dependence of the SMS fibre structure utilised as a sensor is investigated numerically and experimentally. It is found that a strong temperature dependence for the SMS fibre structure exists at selected wavelengths. The temperature characteristic at such wavelengths is linear in nature and can be used for temperature measurements. The proposed temperature sensor offers a high resolution and accuracy, and also benefits from a simple configuration and low cost when compared to other fibre-optic temperature sensors.

A new scheme is presented for simultaneous measurement of strain and temperature using a sampled fibre Bragg grating based on a long period structure written using the electric arc technique. The temperature and strain measurement resolutions are estimated to be±0.50°C/√Hz and ±3.38 µɛ/√Hz, respectively.

An undersea-deployed, remotely-operated, interferometric fibre optic sensor system consisting of two three-axis magnetometers is described. During undersea testing, the system demonstrated 0.6 mu rad/ square root (Hz) phase resolution and 150pT/ square root (Hz) magnetic field resolution at 0.1 Hz.

The characteristics of fibre-grating-type sensing tuned by applying torsion stress are studied for both the solid column beam and air-core column beam. The wavelength shift is linear with respect to the torsional angle and torque in the range between –45 and +45°. The tuning ranges are no less than 7 nm.

A novel method for simultaneously measuring displacement and temperature using an in-fibre-Bragg-grating (FBG) based extrinsic Fizeau interferometric sensor is presented. The Fizeau cavity is formed by an FBG with low reflectivity and a highly reflective surface. The FBG acts as both a reflector for the Fizeau interferometer and a temperature sensor, at the same time. Experimental results are provided which demonstrate the feasibility of this sensor design.

The dense wavelength division multiplexing of fibre Bragg grating sensors is demonstrated by combining code division and wavelength division multiplexing schemes. This approach allows close spectral spacing among Bragg grating sensors without sensor spectral dynamic range limitation.

A novel fibre Bragg grating displacement sensor is presented. Using a uniform density and thickness overlapped dual-isosceles triangular cantilever beam as a strain agent, it is demonstrated both experimentally and theoretically that this sensor is able to automatically compensate for wavelength shifts induced by temperature. The measured sensitivity of this sensor is 0.1051 nm/mm within the working range of –18 – 18 mm.

The authors describe self-interference fringes created in a single long-period fibre grating (LPG) formed in a double-cladding fibre, one end of which is coated with a metal film. The temperature-induced fringe-shift in the reflection spectrum of the proposed LPG device was measured to be 0.055 nm//spl deg/C with 0.9/spl deg/C RMS deviation. Applications of such devices as filters and fine temperature sensors are expected.

Rocking filters have been successfully fabricated in side-hole fibre by exposure to UV light and the filters have been tested for pressure sensitivity. Sensitivities up to 20000 times higher than those reported previously using Bragg gratings were obtained.

A fibre optic sensor structure is proposed for recovering the most relevant physiological parameters aiming at identifying the different sleep phases. The device comprises a multimode loss fibre optic sensor built into a watch casing. The sensor structure generates micro-deformations and allows a sensor to be made which is sensitive enough to detect the composite signal from the patient's wrist.

A method for compensating intensity variations in passive in-fibre Fabry-Perot sensors is studied. To achieve passive Fabry-Perot sensors for intensity compensations, three read-out cavities are used. Two path-balanced Fabry-Perot read-out cavities are designed for two quadrature signals, and the other path-imbalanced cavity is for eliminating the effect of intensity variations via subtraction from two quadrature signals. Consequently, a compensation method proposed to eliminate intensity variations in the interferometry shows good a result in highly disturbed environments.

A strategy for suppressing the perturbations of an intensity-based optical fibre sensor is presented by introducing an adaptive noise cancelling method. The time-varying offset between the sensing fibre arm and the reference fibre arm can be significantly reduced by the proposed strategy compared to direct subtraction. Experimental results are provided to demonstrate the performance of the perturbation suppression.

Anomalous dispersion of a medium surrounding a fused singlemode fibre optic coupler is analysed. The coupler output is shown to respond to Kramers-Kronig dispersion. This effect can be used to develop chemical sensors.

For monitoring temporal modulation of a laser-generated asymmetric (a0) Lamb wave, a non-contact fibre optic sensor has been used. Simultaneously, an electromagnetic acoustic transducer (EMAT) sensed in-plane acoustic transients of the symmetric (s0) Lamb wave. Measurements in 75 µm thick aluminium film show that a0 amplitude modulation is associated with a perturbation from the symmetric wave.

An optical sensor incorporating near infrared opto-electronics which measures pH in the alkaline region is reported. Contaminants within the aqueous solutions whose pH is being measured interfered with the evanescent wave sensing technique and a method to overcome this problem is demonstrated.

Results from the first measurement of an arbitrary strain field within a fibre Bragg grating are presented. Low coherence reflectometry has been used, together with a wavelength-tunable reference grating, to obtain strain against distance measurements.

The classical technique of dual wavelength interferometry is used to determine the absolute OPD of very short cavities using a parametric plot of the relative interferometric intensifies. A network of eight sensors is demonstrated with phase resolution of 1.5 mrad corresponding to 0.2 nm with an unambiguous range of 114 µm.

A fibre ring resonator composed of two portions of twisted singlemode fibre is proposed. Each portion is twisted in the opposite direction with an appropriate length difference. This resonator realises the stable circular eigenstate of polarisations, which makes it possible to excite only one eigenstate selectively and stably. Experimental demonstration is successfully carried out.

A new technique for the simultaneous, independent measurement of temperature and strain using in-fibre Bragg grating sensors is proposed and demonstrated. Two Bragg gratings with closely spaced centre wavelengths are written either side of a splice between two fibres of different diameter. The gratings show similar temperature sensitivities but different strain responses to an applied stress. Maximum errors of /spl plusmn/17 /spl mu//spl epsiv/ and /spl plusmn/1/spl deg/ are reported over 2500 /spl mu//spl epsiv/ and 120/spl deg/C measurement ranges.

A method for reducing the Shupe effect in fibre-optic gyros is described, and verified experimentally. The unbalanced phase shifts in the gyro Sagnac intereferometer that are produced by a traversing temperature gradient are randomised by winding the coil in such a way as to scramble the relative positions of the turns.

Low reflectance optical fibre mirrors were fabricated using mechanical splices and pieces of fibre coated at the cleaved end with a TiO2 dielectric film. When fibre mirrors of reflectance of ~0.1% were produced, the insertion loss ranged from 0.055 to 0.3 dB and the average insertion loss was 0.15 dB.

The near field of a step-index singlemode fibre is calculated using the weakly guiding approximation. It is found that there are oscillations in intensity along the axis, the beam coming to a focus at /spl sim/10 /spl mu/m from the fibre end.

A Brillouin-gain based distributed temperature sensor has been investigated experimentally and theoretically. The relation between Brillouin gain, input probe power and sensing length have been studied. The study shows that there is an optimum probe power providing a maximum Brillouin gain signal for a given sensing length.

The optical detection of methane gas usually requires an open cell with light propagating through it as the detection system This implies the need for precise and, therefore, often expensive and delicate alignment optics. The Letter reports, for the first time, the use of a continuous optical fibre link as the absorption system, thereby obviating the need entirely for precision optics. The system uses a suitably designed section of D-fibre as the interaction mechanism. Initial experimental results indicate that resolutions in the region of 100 parts per million of methane should be achievable.

Remote detection of ethylene has been demonstrated using infra-red hollow glass waveguides and a CO2 laser. By a method of intrinsic waveguide sensing, gas detection at ppm levels can be achieved.

Improvements to a conventional double-ended birefringent fibre-optic sensor system are presented in which a single end of the fibre is used to launch and recover the light signals. This overlooked and unexploited sensor configuration is shown to enhance performance by increasing the sensitivity and depth of modulation while simplifying the optical arrangement when compared to its double-ended counterpart.

Demodulation methods are presented that allow measurement of the amplitude and the direction of dynamically varying environmental parameters with two-mode, elliptical-core fibre sensors. The wavelength dependence of the differential phase-shift between the LP01 and the LP11even modes is used to obtain quadrature-phase-shifted signals at the sensor output. It is also shown that a two-fibre coupler attached to the output of the sensing fibre enables stable demodulation.

Micromechanical resonators have been fabricated and driven into mechanical vibration by application of a time-dependent potential difference of less than 1 V. A laser beam guided via a fibre optic coupler was used to detect the mechanical vibration.

The continuously stable selfoscillation of silicon cantilever microresonators (SCMRs) has been observed. A theoretical analysis has been presented which explains how this phenomenon may be used for developing a practical sensor system. It has been pointed out that SCMRs have a much greater optical-mechanical transformation coefficient compared with silicon bridge microresonators, resulting in a considerable increase in the time selfoscillation is sustained and substantially greater misalignment error tolerance between the fibre end and the SCMR surface.

A Fourier transform optical spectrum analyser is described which uses the stretching of a polarisation-maintaining optical fibre as the scanning element. Delay scans of 15 mm have been achieved in 50 ms allowing a resolution bandwidth of 10 GHz. Future systems with greatly improved resolution are envisaged.

A practical tactile sensor using micromachined techniques has been implemented and evaluated. Optical fibres laid on the pedestals of micromachined silicon showed outstanding improvement in sensitivity, mechanical decoupling between fibres and excellent repeatability. The upper cover made from plastic and Cu-Be foils, not only provides a protective membrane to the sensor but also acts as the force concentrator, allowing better force point resolution. >