Mode theory of plane Fabry-Pérot resonator with inhomogeneous active medium

A new method of the eigenvalue problem solution for a resonator with inhomogeneous active medium is given. The approach is based on Maxwell's wave equation and impedance boundary conditions of resonance-type at open resonator ends. A resonator equipped with mirrors in the form of infinite long strips is studied as an example. A rigorous solutions for the cases of stepped and bounded parabolic active medium profiles are obtained. Transcendental eigenvalue equations are investigated, distributions of field amplitude of active resonator modes are found. Asymptotic behavior of rigorous solutions is investigated. A multilayer approximation method is proposed for the eigenvalue problem solution for a resonator with an arbitrary gradient profile of active medium. The testing of this method was carried out with the rigorous solutions for the bounded parabolic profile.     

A simple fibre Fabry-Perot sensor

A simple fibry Fabry-Perot sensor is described in which the uncoated cleaved ends of a length of single mode fibre are employed as the mirror surfaces to produce high visibility cosine squared fringes in the back reflected light. In an accelerometer configuration the device displayed a below resonance sensitivity of ?200 rads/g.     

Multi-wavelength Fabry-Pérot Brillouin-Erbium fibre laser

Multi-wavelength Brillouin-Erbium fibre laser utilising fibre Fabry-Pérot resonator is presented here. The proposed laser cavity used only a 980 nm laser diode and a coil of Erbium-doped fibre to produce cascaded Brillouin Stokes. The highest number of Stokes up to 43 lines with a spacing of 0.088 nm was obtained experimentally.     

Error analysis of dual wavelength quadrature phase demodulation for low-finesse Fabry-Pérot cavity based fibre optic sensor

An analysis of the errors in the dual wavelength quadrature phase demodulation technique for low-finesse Fabry-Pérot cavity based fibre optic sensor is presented. A simple numeric model for calculating the error is established. For sensors that experience only small cavity length changes during measurement, a simplified analytical expression of relative error is also derived. Errors for various sets of interrogation wavelengths are calculated and analyzed. The results show that two wavelengths with very slight difference should be chosen to avoid significant errors.     

Laser self-mixing interference fibre sensor

Fibre sensors exhibit a number of advantages over other sensors such as high sensitivity, electric insulation, corrosion resistance, interference rejection and so on. And laser self-mixing interference can accurately detect the phase difference of feedback light. In this paper, a novel laser self-mixing interference fibre sensor that combines the advantages of fibre sensors with those of laser self-mixing interference is presented. Experimental configurations are set up to study the relationship between laser power output and phase of laser feedback light when the fibre trembles or when the fibre is stretched or pressed. The theoretical analysis of pressure sensors based on laser self-mixing interference is indicated to accord with the experimental results.     

一种新型的NH_3气敏光纤传感器

本文作者成功地研究一种二氧化锡与氧化锌混合物薄膜,用作光纤传感器,对氨气具有气敏特性,在一定的浓度范围内,具有良好的线性灵敏度。     

A simple temperature-insensitive fiber Bragg grating displacement sensor

The study on fiber Bragg grating (FBG) displacement sensor based on monitoring the back-reflected power from an array of novelly embedded FBG sensors is presented. The sensor is a uniform FBG with three sections that are embedded in different layer in a composite lamina. Its bandwidth is displacement dependent and hence its reflected power varies almost linearly with displacement and it is insensitive to temperature variation. Thus, only low-cost photodetector (PD) is required to monitor displacement. This study demonstrates a novel fiber sensor, a method of fabricated the same, and a method to achieve simultaneous multi-sensor measurement.     

Single-mode fibre coupler as refractometer sensor

We report a simple, non-intrusive fibre-optic refractometer sensor for measuring the refractive index of liquid and optically transparent solid medium. Sensing principle of the proposed sensor is based on monitoring the back-reflected light signal through the second input port of a 2 × 1 single-mode fibre coupler when light signal from the output port is focussed at the interface of air and a liquid or solid medium and back-reflected exactly along the same path. Depending on the refractive index of the medium, the amount of back-reflected intensity would vary and in the present work we exploit this principle to measure the refractive index of an optically transparent medium. Variation of refractive index as small as 0.001 RIU can be measured with our proposed sensor.     

A simple empirical model of polyester fibre materials for acoustical applications

A new empirical model has been developed by the authors to predict the flow resistivity, acoustic impedance and sound absorption coefficient of polyester fibre materials. The parameters of the model have been adjusted to best fit the values of airflow resistivity and sound absorption coefficient measured over a set of 38 samples. Calculated results are compared with normal incidence measurements carried out using two different techniques: the transfer-function method in an impedance tube (ISO 10534-2) and the free-field impulse response method (ISO 13472-1). Measurements performed on polyester fibre materials with different density and thickness values, and diameter ranging from 18 to 48 μm, are in good agreement with the predictions of the new model. It is concluded that the new model can predict the basic acoustic properties of common polyester fibre materials with any practical combination of thickness and density².     

A simple thermal phase stabilizer for real-time optical fibre holographic interferometry

An optical fibre interferometer can be made to phase-stabilize by simply attaching a short length of one arm of the interferometer to a straight copper track and heating the track with an electric current. This feedback thermal stabilization scheme works well even for relatively short fibre lengths and there is no need to treat or unjacket the fibre. A fibre-optic Michelson interferometer stabilized by this method successfully provided object and reference beams for real-time holographic interferometric measurements of the distortion of heated test objects.     

A simple model for the pore size distribution in random fibre networks

A heuristic approach based upon excluded volume arguments is developed for modelling the distribution of pore sizes in isotropic networks of randomly distributed cylindrical fibres. Our formalism accounts for the finite hard core diameters of the fibres, and leads to compact, analytically tractable expressions that span the complete range of volume fractions. Results are presented for the mean and mean-squared pore radii as functions of the fibre volume fraction, and for the partition coefficient of a spherical tracer particle into such a network under conditions such that steric effects are dominant.     

A laser diode with feedback using a fibre delay line as a stable-frequency signal generator and potential fibre sensor

An optical fibre communication line (OFCL) closed-circuited into a ring network is considered as a controlled automodulated oscillation source. Theoretical and experimental investigations of the amplitude and frequency of automodulation for stationary self-consistent automodulation and external synchronization regimes in such a system are given on the basis of a balance equations analysis. It is shown that this OFCL ring can be used as an optical fibre tester and sensor.     

Optical fibre temperature sensor in the cryogenic range

The realization of an optical fibre sensor applied to cryogenic temperature measurement in harsh environments is presented. The measurement principle is based on the analysis of the decay-time of the fluorescence emitted by special doped crystals, the excited state lifetimes of which are greatly dependent on temperature. The sensor is intended to work in the presence of strong perturbations encountered on the testing benches of the liquid hydrogen and liquid oxygen turbo-pumps of the Ariane 5 Vulcan engine developed by ‘la Societé Européenne de Propulsion (SEP)’.     

A simple fiber-optic sensor for use over a large displacement range

An intensity-based fiber-optic displacement sensor has been developed and tested for static and dynamic response. The sensor incorporates an extremely simple design, light source and detector. Testing was done using quasi-static extension, a simple oscillating cantilever beam and a small shaker capable of frequencies up to 10 kHz. The sensor shows a response over a wide range of 410 mm. The response has two distinct linear regions with a central nonlinear region. For small displacements, the sensor shows excellent frequency response up to 10 kHz. At this stage, no attempt has been made to theoretically predict the light-loss behavior associated with this macrobend sensor [Marcuse, D., Journal of the Optics Society of America, 66(3) (1976) 216–220; Lagakos, N., Cole, J. H. & Bucaro, Applied Optics, 26(11) (1987) 2171–2180];^1,2 rather this paper serves to demonstrate the basic function of the novel geometry of the sensing element.     

An optical fibre sensor for the measurement of pressure

This paper describes the design, construction and testing of a fibre optic pressure sensor based on a reflecting Fabry-Perot etalon. The etalon comprised one fixed mirror and a second mirror designed to flex under the action of the pressure being monitored. A single multimode fibre was used to connect the passive, remote sensor to the transmitter/receiver section, and dual wavelength referencing was used to eliminate the effects of bending-induced attenuation in the fibre.     

An optical fibre sensor for the measurement of pressure

This paper describes the design, construction and testing of a fibre optic pressure sensor based on a reflecting Fabry-Perot etalon. The etalon comprised one fixed mirror and a second mirror designed to flex under the action of the pressure being monitored. A single multimode fibre was used to connect the passive, remote sensor to the transmitter/receiver section, and dual wavelength referencing was used to eliminate the effects of bending-induced attenuation in the fibre.     

Micro-displacement or bending measurement using a long-period fibre grating in a self-referenced fibre optic intensity sensor

The measurement of micro-displacement or bending using a long-period fibre grating (LPG) in a self-referenced fibre optic intensity sensor is presented. The sensing head is based on a LPG whose attenuation peak changes as a function of its micro-displacement or bending and the fibre optic intensity sensor is referenced in frequency. Two other LPGs were used as rejection filters, located at the output of the optical source to shorten its spectral width. The implemented experimental set-up is described and, the results are presented being considered the measurement range and the intrinsic resolution.     

Temperature-compensated fibre Bragg grating ‐based sensor with variable sensitivity

In this paper a Fibre Bragg Grating (FBG)-based sensor device for strain measurement with adjustable full-scale sensitivity is proposed. Installation flanges of the sensor can be moved with respect to the internal fixed FBG sensing length in order to adjust the overall strain sensitivity and the full scale measurement range of the device. Thermal drift is compensated using a technique based on the thermal expansion of a solid block connected to the fibre, in the pre-stressed region outside the grating. Typical calibration curves are reported to illustrate the sensor sensitivity variation with the layout and temperature.     

Thin film stress measurement by instrumented optical fibre displacement sensor

Residual stress can adversely affect the mechanical, electronic, optical and magnetic properties of thin films. This work describes a simple stress measurement instrument based on the bending beam method together with a sensitive non-contact fibre optical displacement sensor. The fibre optical displacement sensor is interfaced to a computer and a Labview programme enables film stress to be determined from changes in the radius of curvature of the film-substrate system. The stress measurement instrument was tested for two different kinds of thin film, hard amorphous carbon nitride (CN) and soft copper (Cu) films on silicon substrates deposited by RF magnetron sputtering. Residual stress developed in 500 nm thick CN thin films deposited at substrate temperatures in the range 50-550 °C was examined and it was found that stress in CN films decreased from 0.83 to 0.44 GPa compressive with increase of substrate temperature. Residual stress was found to be tensile (121 MPa) for Cu films of thickness 1500 nm deposited at room temperature.