Light intensity-based wearable polymer optical fiber (POF) sensors : design, fabrication and characterization

Pao Yue-kong Library Electronic Theses Database

Light intensity-based wearable polymer optical fiber (POF) sensors : design, fabrication and characterization

 

Author: Zheng, Wei
Title: Light intensity-based wearable polymer optical fiber (POF) sensors : design, fabrication and characterization
Degree: Ph.D.
Year: 2014
Subject: Plastic optical fibers.
Optical fiber detectors
Data transmission systems
Hong Kong Polytechnic University -- Dissertations
Department: Institute of Textiles and Clothing
Pages: xxiii, 182 leaves : ill. ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2747310
URI: http://theses.lib.polyu.edu.hk/handle/200/7450
Abstract: Light intensity-based wearable polymer optical fiber (POF) sensors have attracted a considerable amount of interests due to their lightweight, small size, flexible structure, high sensitivity, low cost, simple equipment required and immunity to electromagnetic interference. However, as strain sensor, the strain measurement range of POFs is small thus not suitable for applications like flexible wearable devices. Most common POFs are made from polymethyl methacrylate (PMMA), which get degraded in sun-light or chemicals, such as alcohol or other solvents thus limiting their applications in healthcare or in hash environments. Therefore, the thesis is focused on the study of fabric strain sensors integrated with PMMA POF for large repeated strain measurement and alternative POF sensors made from silicones. POF fabric strain sensor system was designed and set up for small dimension and easy operation. POF with 500 μm diameter was selected as sensing element. LED with central wavelength of 650nm was chosen as light source. The Fc multimode bare fiber adapter, the linear stage and the software LabVIEW(R) were used for coupling, strain measurement and data collection, respectively. To increase the sensitivity of the sensor, POF surface ablation was conducted by the CO₂ laser. The results showed that expected notch configuration was obtained by the laser with scanning speeds equal or over 9.16mm/ms. To detect repeated large strain, three types of POF fabric strain sensor including S-shaped, ∞-shaped and four-loop shaped were designed and compared. Four-loop was selected as the basic structure of the sensor for its small size and uniform notch. The strain results illustrated that higher sensitivity of the four-loop POF fabric sensor was achieved by the outer arc notch, smaller loops' radius and lower scanning speed of laser on the top arc of loops. This type of sensor demonstrated its capacity to measure repeated large strain up to 21% with average sensitivity almost 3, average hysteresis about 4% and average repeatability approximate 3%. The performance of the four-loop POF fabric strain sensor was evaluated experimentally. The fatigue life of the sensor was more than 10,000 cycles. In the same temperature and testing condition, the zero output, rated output and sensitivity of four-loop POF fabric strain sensors were little affected by the relative humidity variations. With the increment of temperature, both the zero output and rated output of the sensor were increased, and the fluctuation of sensor's sensitivity was little. A theoretical model was built to simulate the light power variation during the sensor was strained. The sensitivities of the sensors were decreased with the increment of the laser scanning speed, which were proved both theoretically and experimentally. It found that all angles of notches on the sensor’s loop were larger than 72°, and no light refracted into air at core/air interface of the notch. Hence, less light power was blocked by the side of notches and more power passed through the fiber during the strain, so the normalized output power was increased.
The four-loop fabric POF sensor was integrated into a woven belt for human respiration monitoring. Optical fiber cable with Fc and Sc connectors for the O-breath system connection was designed and commissioned. The portable light source, the breath-belt, the optical power meter and a computer composed the O-breath system. The human respiration signals were collected by the system when the breath belt was either on the thorax or abdomen. The experimental results exhibited that the curves of normal breath, hold breath and deep breath were obtained by the O-breath system timely and effectively. Two types of silicone with different refractive index were used for silicone optical fiber (SOF) fabrication by dip coating method. To get more uniform SOF, the petroleum ether with 30wt% silicone rubber was used as solution for dip coating. Characteristics of SOF were investigated experimentally. The attenuation of as-made SOF was 13.4dB/m at the wavelength of 650nm. The coupling efficiencies of the FC/SMA coupler and SMA/FC coupler were 88.6% and 1.1% by using this coupling method. The tensile strain at break of SOF was around 70% and Young's modulus was about 1.81 MPa. Hysteresis of the fiber was decreased with the increment of the strain range in the cyclic test. The residual strain of fiber in the first cycle was larger than other cycles in 10 cyclic 2% elongation. The transmission window for silicone was between 600-750nm. Macro-bending losses of SOF were increased dramatically when the bending diameter was less than 20mm. SOF had a better thermal stability than its PMMA counterpart. U-shaped refractive index sensor was developed by SOF in various bending radius. Fructose solutions in various concentrations were measured by an optical measuring system based on the U-shaped SOF sensor. The results showed that the output light power was decreased with the bending radius decreased in the solution with specific refractive index. A theoretical model was performed on the relationship between the output power and the various solution concentrations. The simulation results are in correspondence with the experimental results qualitatively.

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