Asrul Izam Azmi

Performance Enhancement of Vibration Sensing Employing Multiple Phase-Shifted Fiber Bragg Grating

Multiple phase-shifted fiber Bragg gratings (MPS-FBGs) are proposed for a novel intensity-type vibration sensing scheme with broadband source based interrogation. In this scheme, the collective and simultaneous operation of the subchannels of MPS-FBG multiplies the total acquired signal power change at a particular channel, considerably enhancing sensitivity. Compared to a standard FBG scheme, our experiment scheme attained significant sensitivity enhancement of nearly 20 dB using 17 phase-shifted FBG. These experimental results agree with the theoretical prediction based on cross-correlation relations. The sensitivity enhancement while retaining the system simplicity would be an attractive option for an economical coarse wavelength division multiplexed (CWDM) vibration sensing system based on a single broadband source.

Acoustic emission techniques for failure characterisation in composite top-hat stiffeners:

Modern day aerospace, automotive, marine, mechanical and civil structures rely on the advanced composites for their added benefits over conventional metallic structures. The complex damage process in composites involves various failure modes such as matrix cracking, fibre-matrix debonding, fibre fracture and delamination. This paper presents an acoustic emission technique for the failure characterisation of top-hat stiffener specimens using conventional piezoelectric acoustic emission sensors and modern fibre Bragg Gratings. Strain insensitive fibre Bragg Grating sensors are used in this experimental investigation to compare the performance of piezoelectric sensors for composite structures. This work has contributed to the development of an acoustic emission sensing system based on fibre Bragg Gratings. Main objectives of this work are to compare the sensing characteristics simultaneously in a composite structure with a surface-bonded acoustic emission–fibre Bragg Grating sensor and acoustic emission–piezoelectric sensor. The feasibility of the system is demonstrated in typical applications of in-situ structural health monitoring based on acoustic emission techniques.

Sensitivity Enhancement in Composite Cavity Fiber Laser Hydrophone

We report an application of composite cavity fiber laser (CCFL) for hydrophone sensitivity enhancement. While most of the sensitivity enhancement methods rely on amplification of acoustic signal by the coating design, our proposed scheme exploits the inherently nonlinear phase condition of the CCFL. A particular point of interest for CCFL hydrophone application is the proposed partial cavity sensing scheme that, when only one cavity is made responsive, a substantial sensitivity enhancement can be achieved. Theoretical analysis shows that this scheme can significantly enhance sensitivity, achieving as high as 40 dB excess to the standard response of a bare fiber. In experiment, this scheme produced a mean responsivity of -41.1 dB re rad Pa-1, representing an improvement in sensitivity by 14 dB compared to the standard response. Evidently, this CCFL sensing scheme provides an additional means for sensitivity improvement in conjunction to the conventional packaging technique.

High Sensitivity of Balloon-Like Bent MMI Fiber Low-Temperature Sensor

A novel low cost and simple temperature sensor based on multimode interference (MMI) is formed by a successive singlemode fiber (SMF)-bent multimode fiber (MMF)-SMF structure. Due to the small curvature radius of bent MMF section mimicking a balloon shape, MMI effect from the core is spreading into coating. The high temperature sensitivity is mainly due to the large thermo-optical coefficient of the existing acrylate coating of the MMF fiber. A temperature sensitivity of up to -2060 pm/°C and -25.1 nW/°C for wavelength- and intensity-based interrogation, respectively, have been achieved in the range between 27 °C and 31 °C. This sensor is most suitable in high sensitivity and low-temperature application such as the measurement of human body temperature.

Output power and threshold gain of apodized DFB fiber laser

Effects of apodization on distributed feedback fiber laser (DFB FL) output power and threshold gain are theoretically investigated by employing the transfer matrix method. Three distinct types of profile are investigated: the gaussian, flat or nonapodize, and sigmoid profile. The gaussian and sigmoid profiles are the two extreme cases examined; the former has a strong profile around a centrally located phase shift, while the latter is with a weaker profile. Findings indicate that the tradeoff between output power and higher order mode threshold performance are resulting from the interplay between these profile shapes. The comprehensive results presented in this paper should assist the development of high performance DFB FLs.

Temperature-insensitive photonic crystal fiber interferometer for relative humidity sensing without hygroscopic coating

A novel photonic crystal fiber (PCF) interferometer RH (relative humidity) sensor is proposed and demonstrated in this paper. The sensor is made from a piece of PCF spliced to standard fibers in transmission mode, which has the advantage that it does not require the use of any hygroscopic thin film or layer. In addition, the all-silica nature of the sensing element means the sensor is insensitive to temperature. The sensor has a humidity sensitivity of 60.3 pm/%RH over the range of 60–80% RH and 188.3 pm/%RH in the region from 80 to 95% RH. A response time around ~1 s is observed for a ~2 cm long section of photonic crystal fiber interferometer (PCFI). The sensor exhibits a very low cross temperature sensitivity of around 0.5 pm/°C. The proposed sensor has potential for high humidity monitoring applications, and temperature compensation is not necessary if such a sensor is operated in normal surrounding atmosphere.

Incident Angle Approach to Sensitivity Enhancement for Ozone Sensor

The design and mathematical model of a reflective type optical gas sensor is presented. Light source is radiated at an incident angle for 10 cm gas cell with an internal diameter of 0.4 cm. At an incident angle of 1o, optical path length obtained is 342.7886 cm, at 27o incident angle, optical path length is 10.4926 cm and at an incident angle of 28o, optical path length is 9.9631 cm. The model is most efficient at lower incident angles, precisely between (1o and 27o). Effects of variation in diameter and length of gas cell are also demonstrated.

Pressure Dependence of Ozone Absorption Cross Section

Accurate value of absorption cross section is required for correct measurement of ozone concentration. Measurement of ozone has been done at different altitude and pressure. However, previous work has failed to establish significant relation between pressure and ozone absorption cross section. Therefore, this work aims to establish relation between pressure and maximum ozone absorption cross section via spectralcalc.com gas cell simulator. Simulation results show maximum absorption cross section 1.148×10-21 m2 molecule-1 and maximum absorption wavelength 255.442 nm are independent of pressure changes from 0.1 atm to 3.0 atm. Thus, measurement of ozone concentration at maximum absorption wavelength is strongly recommended due to negligible pressure dependence.

Application of fiber grating-based acoustic sensor in progressive failure testing of e-glass/vinylester curve composites

This paper reports an application of phase shifted fiber Bragg grating (PS-FBG) intensity-type acoustic sensor in a continuous and in-situ failure testing of an E-glass/vinylester top hat stiffener (THS). The narrow transmission channel of the PS-FBG is highly sensitive to small perturbation, hence suitable to be used in an effective acoustic emission (AE) assessment technique. The progressive failure of THS was tested under transverse loading to experimentally simulate the actual loading in practice. Our experimental tests have demonstrated, in good agreement with the commercial piezoelectric sensors, that the important failures information of the THS was successfully recorded by the simple intensity-type PS-FBG sensor.

Progressive failure monitoring of E-glass/vinylester curve composites using embedded FBG sensors

In this paper, we report our recent work in an application of fiber Bragg grating (FBG) sensors in progressive failure monitoring of E-glass/vinylester top-hat stiffener (THS) composites. FBG sensor arrays were embedded at strategic points within the THS to monitor the onset and progress of failure modes as the THS undergone a transverse loading. Techniques to embed FBGs in-situ during composite structure fabrication are developed. Our experiments demonstrated that key structural failure information can be obtained from the analysis and assessment of data, such as average strain, strain gradient and full spectrum measurements, collected by the embedded FBG sensors.