Chemically Sensitive Cladding Materials for Distributed Optical Fibre Sensors

• Main Author: Noreña-Franco, Luis Enrique
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1997
• Subjects: Chemical engineering
• ISBN: 9798352905104

The aim of the research work was to prepare and evaluate the performance of new sensing coatings based on near-infrared dyes in polymer matrices for the fibre optic distributed detection and the remote detection of leaks of hazardous industrial chemicals. The experimental work was focused on the detection of vapours of ammonia, hydrochloric acid and acetic acid.In a distributed sensor the specially designed sensing coating covers the whole length of the fibre forming a sensing cladding. In order to transmit light efficiently, the refractive index of the cladding material must be lower than the refractive index of the core material, however, most of common polymers have a refractive index larger than the refractive index of the fibre silica core. Polymer materials under study for cladding optical fibres and entrapping the sensing reagents were silicone, cellulose acetate-butyrate, fluoropolymers, and silica-silicone sol-gel processed composites. The structure of a commercial fluoropolymer was modified to prepare a new fluoropolymer of lower refractive index, and it is the first time the silica-silicone sol-gel composites are employed in sensing applications. Commercial optical fibres for long distance communications have a minimum optical attenuation in the near-infrared spectral region (NIR). Therefore, the sensing reagents under study were commercial cyanine, phthalocyanine, quinone and nitroso NIR dyes, not used in sensing applications before, and specially synthesised metal-complex and squarilium NIR dyes.A commercial laboratory spectrophotometer was used for recording the absorption spectra of the NIR dyes in liquid solution and entrapped within polymer matrices, before and after reaction with ammonia and acids. Some NIR dye-polymer sensing coatings had the right properties and performance to effectively work as sensing claddings for optical fibres. 4 m-long distributed sensing fibres with cresyl violet in silicone and in silica-silicone sol-gel composite claddings detected ammonia vapours in the visible spectral region. Distributed sensing fibres with the NIR dyes DDI and ProJet 830NP in silicone claddings were employed for the detection of acetic acid vapours, whereas distributed sensing fibres with the NIR dyes ProJet 860NP and IR-140 in the new fluoropolymer claddings were employed for the detection of hydrochloric acid and ammonia vapours respectively. The results confirm the feasibility of the preparation of dye-based distributed chemical sensors working in the NIR spectral region.