Photosensitive polymer optical fibers and gratings

Pao Yue-kong Library Electronic Theses Database

Photosensitive polymer optical fibers and gratings


Author: Yu, Jian-ming
Title: Photosensitive polymer optical fibers and gratings
Degree: Ph.D.
Year: 2005
Subject: Hong Kong Polytechnic University -- Dissertations.
Optical fibers.
Glass, Photosensitive.
Fiber optics.
Department: Institute of Textiles and Clothing
Pages: xxiv, 181 leaves : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: The objective of this thesis is to develop ultraviolet (UV) photosensitive single-mode polymer optical fibers (POFs) which exhibit a photo-induced change in refractive index (commonly referred to as photosensitivity) and can be applied to fabricate fiber optic gratings with a low energy and small dosage of UV irradiation. POFs are flexible with a large extension capacity and thermal expansion as compared to silica based optical fibers. Therefore, polymer fiber Bragg gratings (FBG) have a large tuning capacity in Bragg wavelength and may be applied to telecommunication and sensing technology. The target optical fiber is expected to possess a novel photosensitivity and can be processed into FBGs upon UV irradiation with minimal degradation. POFs were fabricated with poly(methyl methacrylate) (PMMA) and methyl mechacrylate (MMA) copolymers as the cladding and core, respectively. The fabrication processes for single-mode fibers by means of preform drawing method were investigated. Thermal polymerization conditions were optimized, including the concentrations of initiator, the concentrations of chain transfer, and the thermal processes. The preforms fabricated using these conditions exhibit good heat drawing property, very low content of residual monomers, and free of physical defects such as inner or surface cavities. POFs were made with a good concentric core-cladding structure and a fine core diameter of ~8um or less. The effect of the core copolymer composition on its refractive index was studied. It is revealed that changing the composition in the core polymers can finely modulate their refractive index, which provides a tunable difference in refractive index between the core and cladding to satisfy the single-mode condition at different operation wavelengths. In addition, Lorentz-Lorenz equation, a relation between the chemical structure and the refractive index, was used to predict the index values of the core polymers with different copolymer compositions and the core with photosensitive dopants. The evaluation values were in good agreement with the measured results. PMMA is transparent to UV light when the wavelength is larger than ~250 nm. Below this wavelength, it has a very strong absorption and an evident photochemical reaction occurs, exhibiting a significant photo-induced change in refractive index accordingly. In the range of 260 to 300 nm, the degradation due to the photochemical reactions had been reported. But the significant photorefractive effect had not been observed. The photosensitive mechanism of PMMA was studied by examining FT-IR, UV absorption, refractive index, thickness and mass loss of PMMA thin films irradiated by the UV light from a high pressure mercury lamp. The change in FT-IR after irradiation confirms the elimination of side chain (ester group) from PMMA main chain. Refractive index increase, thickness reduction, and mass loss of irradiated PMMA films were detected. The agreement between the thickness reduction and mass loss supports the suggestion that the thickness reduction results from the mass loss and the materials density of irradiated PMMA films is unchanged. The calculation by Lorentz-Lorenz equation indicates that the photo-induced increase in refractive index may result from the formation of aliphatic chain structures in the irradiated PMMA during the photochemical reactions. In a single-mode fiber, the fiber core is surrounded by a cladding layer (with a thickness of ~60 um for a fiber with 125 um outer diameter). Therefore, a suitable material design for photosensitivity is that the fiber core exhibits photo-induced change in refractive index at wavelengths which are not absorbed by the cladding (for PMMA cladding larger than 250 nm). When considering the PMMA degradation due to the photochemical reactions, the photosensitivity to UV light longer than 300 nm is preferable. Accordingly, three core systems with the enhanced photosensitivity were proposed: (1) MMA copolymer with rich co-monomer benzyl methacrylate (BzMA), (2) MMA copolymer containing benzophenone (BP), a photoinitiator, (3) MMA copolymer containing trans-4- stilbenemethanol (TS), a photochromatic compound. BzMA is a phenyl-ring containing monomer and has a strong absorption at ~260 nm. BP can generate free radicals upon UV irradiation at wavelengths larger than 300 nm; these radicals may initiate the chemical reactions among polymer chains. TS conducts a trans-cis isomerization upon UV irradiation near 320 nm and the change in refractive index occurs due to the different index values between its two isomers. Experiments demonstrated that the TS-doped core system displays an evident photosensitivity with irradiation at ~320 nm UV, low intensity, and short exposure time. The photosensitive mechanism of TS-doped core was ascribed to the trans-cis photoisomerization of the dopant, and confirmed by UV absorption spectra and 1H NMR measurement. In TS-doped core system, the photoisomerization process can be initiated by UV irradiation (325 nm) with an intensity as low as 0.62 mW/cm2. The modulation magnitude in refractive index is about 0.0005 per wt% doping concentration. The UV spectra of irradiated film samples were measured after storing at ambient temperature for more than one year and a high temperature of 80'C for 4 hours. Negligible change is observed, implying that this photo-induced effect exhibits good thermal stability and a long shelf life under normal conditions. Using the phase mask technique, fiber Bragg gratings (FBGs) were fabricated in TS-doped fibers by a pulse laser system at 325 nm (from Sirah CSTR-G-18 pulsed dye laser which was pumped at 532 nm, the second harmonics from Nd:YAG pulse laser). With the TS dopant, FBGs can be inscribed in the fibers using a lower irradiation energy and smaller exposure dosage than those in the fibers of a similar core but no TS dopant. Using an amplitude mask, long period gratings (LPGs) were also inscribed in TS-doped fibers by 325 nm lasers and in TS-doped PMMA films by 297 nm UV light from a lamp. The generation of diffraction pattern confirmed the existence of LPG structure. In the film samples, LPG images were viewed under an optical microscopy, which had the same pitch as the applied mask.

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