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A platinum-coated singlemode-multimode structure is investigated in this paper as an optical fibre sensor (OFS) to monitor the phase transition of a phase change material (PCM). Paraffin wax has been used as an example to demonstrate the sensor's performance and operation. Most materials have the same temperature but different thermal energy levels during the phase change process, therefore, sole dependency on temperature measurement may lead to an incorrect estimation of the stored energy in PCM. The output spectrum of the reflected light from the OFS is very sensitive to the bend introduced by the PCM where both liquid and solid states exist during the phase transition. The measurement of strain experienced by the OFS during the phase change of the PCM is utilized for identifying the phase transition of paraffin wax between the solid and liquid states. The experimental results presented in this paper show that the OFS can measure the phase change point of paraffin wax and the sensor with a multimode fibre length of 10 mm measured the phase transition temperature range from 37.8 °C to 57.7 °C.

An all optical fiber Fresnel reflection sensor for detecting the solid–liquid phase change in n-octadecane is proposed and experimentally demonstrated. The sensor probe consists of a single-mode fiber with a cleaved end which is immersed in the material sample under test. As the n-octadecane changes from solid to liquid or from liquid to solid, its refractive index (RI) changes resulting in changes of the light power ratio reflected by the probe. Since the RI of n-octadecane during its phase transition changes discontinuously, inflection in the reflected light power ratio dependence indicates the phase change of the sample in the vicinity of the sensor probe. The results of this work suggest that such a simple optical fiber sensor can be used for detection of liquid–solid phase changes in other materials with similar to n-octadecane thermo-optic properties.

A macrobending standard single-mode fibre (SMF28) is optimized as an edge filter, with the optimal bending radius and length determined according to the bend loss and polarization sensitivity of the bending fibre. An all-fibre ratiometric wavelength system is built and calibrated. The system's accuracy and resolution is discussed with the influence of noise of input signal and photodetectors. Finally, the ratiometric system is employed for the demodulation of FBG strain sensing.

Bandpass filters based on birefringent plates employed in optical communications and sensing for channel dropping are considered. A filter with a flat-top passband is designed based on the configuration of the Lyot birefringent filter. A genetic algorithm is employed for the optimization of the flat-top passband. Compared to the traditional Lyot birefringent filter, the designed filter has a wider bandwidth, sharp transition and low crosstalk. The fabrication tolerance is also discussed.

A breathing sensor has been experimentally demonstrated based on a singlemode–multimode– singlemode (SMS) fiber structure which is attached to a thin plastic film in an oxygen mask. By detecting power variations due to the macro bending applied to the SMS section by each inhalation and exhalation process, the breath state can be monitored. The proposed sensor is capable of distinguishing different types of breathing conditions including regular and irregular breath state. The sensor can be used in a strong electric/magnetic field and radioactive testing systems such as magnetic resonance imaging (MRI) systems and computed tomography (CT) examinations where electrical sensors are restricted.

Glass fiber reinforced composite material beams embedded with two types of polarimetric sensors are fabricated by the hand layup method and characterized. Two types of polarimetric sensors, a high strain sensitive Panda type fiber and a low strain sensitive polarization maintaining photonic crystal fiber (PM-PCF), are compared for low frequency vibration measurements from 0 to 100 Hz. Different lengths of glass fiber reinforced composite samples with embedded polarimetric sensors are fabricated and compared for different vibration amplitudes and vibration frequencies. The influence of the vibration amplitude of the composite beams on the accuracy of vibration measurements using the two types of polarimetric sensors is investigated. At high amplitude vibrations the low strain sensitive PM-PCF polarimetric sensors offer a wider linear range and thus reproduce the vibration frequency and vibration amplitude accurately. However for high amplitude vibrations the high sensitivity and low dynamic strain range of Panda type fibers result in a multiple-peak intensity pattern within one vibration cycle which leads to inaccurate vibration frequency and vibration amplitude measurements. The experimental results show that the strain sensitivity of polarimetric sensors limits the vibration measurements to a certain range of vibration amplitudes. The vibration amplitude range over which the polarimetric sensors provide accurate information about the vibration frequency is experimentally investigated and the results are presented. Also, for a composite beam undergoing deflections in the 'simply–simply supported' configuration, a theoretical method to predict the allowable maximum measurable vibration amplitude for any type of polarimetric sensor, is derived in this paper. It is envisaged that the results from the studies will provide significant information, which can be used in composite material applications such as marine and aerospace for selecting an appropriate type and length of polarimetric sensor for vibration measurements in composite materials.

A comprehensive theoretical model for an SMF28–small-core SMF (SCSMF)–SMF28 structure based refractometer is developed based on the modal propagation analysis (MPA) method. The simulation result shows that the wavelength shift of this refractometer changes exponentially as the surrounding refractive index (SRI) varies. The core diameter of SCSMF does not have a significant influence on the sensitivity of the refractometer but cladding diameter does have. The simulation results are verified experimentally and it is also experimentally demonstrated that there is a maximum sensitivity of 1808 nm/RIU (refractive index unit) for an SRI range from 1.324 to 1.431 and that, as expected, the wavelength shift response is an exponential function of SRI.

Some of the most critical issues of the influence of the thermal expansion of composite materials on embedded polarimetric sensors for measurements of strain and temperature are studied in this paper. A composite material sample with polarimetric fiber sensors embedded in two distinct layers of a multi-layer composite structure is fabricated and characterized. The polarimetric fiber sensors used in this study are based on panda type fiber and polarization maintaining photonic crystal fiber (PM-PCF). The temperature sensitivities of polarimetric fiber sensors with acrylate buffer coated and buffer stripped polarization maintaining optical fibers are measured in free space and compared to those for embedded in the composite material. It is found that a polarimetric fiber sensor with an acrylate coating embedded in the composite material shows the same response as the one in free space while the coating-stripped fiber polarimetric sensor shows significant temperature sensitivity when embedded in the composite material. This is due to the stress-induced change in birefringence created by the thermal expansion of the composite material, while in the case of a buffer coated fiber, the effect is considerably reduced as the thermal stress is largely eliminated by the buffer coating. The results obtained in this study demonstrated that thermal expansion of the composite material are the main source of error in strain and temperature measurement using embedded polarimetric fiber sensors and that more accurate strain and temperature measurements can be obtained with buffer coated polarimetric fiber sensors.

This paper investigated the application of FBG sensors in the measurement of scissor blade–tissue interaction forces. Sensor placement at the blade–tissue interaction site provides the basis for increased force measurement accuracy, without compromising the functionality of the instrument. This increased accuracy provided the foundation for in-depth analysis of the force components generated during typical scissor cutting cycles. Theoretical analysis and experimental investigation explored the decoupling of the major forces present during cutting. The isolation of tissue-fracture forces from inter-blade friction forces was carried out. This is useful as knowledge of the fracture properties of biological tissues can be diff cult to obtain. Moreover, the force information obtained can be ref ected to the user in a telerobotic application ensuring a greater sense of user immersion. Scissor cutting force data is of particular relevance to those involved in the development of soft tissue models for medical simulation systems. The motivation behind this work is to develop a smart surgical instrument capable of unobtrusively, and with minimal impact on instrument functionality, detecting tissue– instrument interaction forces. Future work will involve the miniaturization of the current technique enabling the development of a laparoscopic prototype instrument capable of acquiring in vivo force and material fracture properties

A simple displacement sensor based on a bent single-mode–multimode–single-mode (SMS) fiber structure is proposed and experimentally investigated. The sensor offers a wider displacement range, not limited by the risk of fiber breakage, as well as a three-fold increase in displacement sensitivity by comparison with a straight SMS structure sensor. This sensor can be interrogated by either an optical spectral analyzer (OSA) or a ratiometric interrogation system: (1) if interrogated by an OSA assuming a resolution of 1 pm, it has a sensitivity of 28.2 nm for a displacement measurement range from 0 to 280 µm; (2) if interrogated by a ratiometric interrogation system, it has worst and best case resolutions of 556 and 38 nm, respectively, for a displacement measurement range from 0 to 520 µm.

An experimental investigation on liquid crystal coated tapered photonic crystal fiber (PCF) interferometer is presented in this paper. The interferometer is fabricated by tapering a small section of a PCF by collapsing the air holes and thinning down the air hole collapsed region to a micron size. Several interferometers are fabricated from different types of photonic crystal fibers and the temperature dependence studies are carried out to select an intrinsically temperature insensitive interferometer that can be used with liquid crystal (LC) materials. The properties of the LC materials to be used with the tapered PCF interferometers are also discussed. Temperature tuning of the spectral response of the tapered interferometers with different waist diameters coated with high index and low index LC materials are carried out and the characteristics are presented.

The effect of the position of an edge filter within a ratiometric wavelength measurement system was investigated based on three cases: (1) the reflected fibre Bragg grating (FBG) signal passes through both the reference arm and the edge filter arm, (2) the reflected FBG signal is connected directly to the edge filter arm and does not pass through the reference arm, (3) the edge filter sits in line with the FBG and thus the source power is filtered prior to reaching the FBG. Both numerical simulations and experimental results show that cases 1 and 2 have similar system performance whilst case 3 is the best arrangement which offers the highest wavelength resolution.