Bahrampour, Abolfazl (2013) New hollow core fiber design and porphyrin thin film deposition method towards enhanced optical fiber sensors. [Tesi di dottorato]

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Abstract

In last decades, optoelectronic components and in particular fiber optic based devices have received great attention for their advantages in several fields. For instance, fiber optic technology is with a short history but with very rapid development in the past and now its impact in all aspects of communication field is well evident whereas fiber–optic sensors (FOSs) provide the potential to create very sensitive and selective measurements. Enhanced FOSs are typical achievement by combining the versatility of the optical transducer and an intelligent design of the active materials. An appropriate sensitive material permits to reach a high sensitivity and selectivity towards specific chemicals [1] while an optimized transducer enhances the interaction between guided light and sensitive materials itself. In this scenario, the present PhD work is focused on two main aspects. a) In the first two years we experimentally investigated simple method to integrate an appealing class of functional material with optical fiber transducer. Due to strong multidisciplinary nature of this activity, it was principally provided at Institute for Composite and Biomedical Materials (IMCB) of the CNR, Napoli, in cooperation with Giovanna De Luca and Michele Giordano (both with IMCB-CNR). b) In the third year of this thesis, during my collaboration with photonic research group in Sharif University of Technology, Tehran, the research activity was principally focused on the design of novel hollow core quasi-crystal fiber via theoretical/numerical analysis for their appealing features in sensing and communications fields. In following, please permit us to briefly introduce both topics. Among different functional materials as possible candidates to realize novel sensors, porphyrin derivatives have received great attentions [2]. Therefore recently a novel UV-induced technique for deposition of porphyrin molecules on planar substrates/devices has been widely investigated [3]. The main aim of this work is to integrate porphyrin molecules with non-conventional substrate such optical fiber end via UV-assisted procedure. We experimentally demonstrated that different porphyrin molecules such as TpyP (4) (meso-tetra (4-pyridyl) porphyrin) and tetrakis (4-sulfonatophenyl) (TPPS) present their compatibility to this technique. Finally an extent of stated method represents ability to integrate silver on optical fiber, taking the photophysical benefits of silver salts. In all results of mentioned coating method a thin film completely aligned in the core of fiber has been obtained. Additionally sensing potentialities of TPPS coated optical fiber for environmental monitoring in the detection of acid vapor was experimentally investigated by spectral measurements of the reflected signal. TPPS self-assembled film can switch from H to J aggregate as a response to presence of base and acid gases and thus different spectral features can be observed in the reflected spectrum as consequence of probe exposure. In the base (deprotonated) form of TPPS film four Q-bands is observable according to H-form of aggregation whereas in acid treated film very narrow and intense peak (bandwidth of ~14 nm), around 490 nm is visible. This particular feature indicate the presence of very ordered J-aggregates [3]. In this work spectral analysis and dynamic time response are investigated and reported [4] demonstrating very appealing sensing features in the detection of acid and basic vapors. During the last year coinciding with the staying in Photonic research group of Sharif university of technology in Tehran (Iran) and keeping direct and frequent contact with prof. Iadicicco (Univ. Naples Parthenope), the research activity was principally focused on the design of novel quasi-crystal hollow core fiber via numerical analysis based on FEM analysis. Photonic crystal fibers (PCFs) is refering to a new class of optical fibers that have wavelength-scale morphological microstructure running down their length [5]. They, according to their guiding mechanisms, may be divided into index-guiding PCFs (IG-PCFs) and photonic band-gap fibers (PBFs). In light of their composite nature, PCFs enable a plenty of possibilities and functionalities hitherto not possible. In the last decade, particular attention has been focused on PBFs due to the lattice assisted light propagation within the hollow core (HC) [6] and thus named hollow core PCFs (HC-PCFs). As matter of fact in an HC-PCF, light is confined and guided by a PBG in the air core. This particular feature has a number of advantages such as lower Rayleigh scattering, reduced nonlinearity, novel dispersion characteristics, and potentially lower loss compared to conventional optical fibers [7, 8]. In addition, the HC-PCFs also enable enhanced light/material interaction (in the core region), thus providing a valuable technological platform for ultra-sensitive and distributed biochemical sensors [9, 10]. For instance the ability to host material in the hollow core permits to integrate HC-PCFs with fluorescent molecules [10]. Hence the HC-PCF integration with functional material can lead to promising high sensitivity sensors. It is worth noting that in past all HC-PCFs and IG-PCFs focused on the periodic structure (such as the PBGs generated by holes of various shapes arranged on a triangular, square, honeycomb, or Kagome periodic arrays lattice) [7,11,12]. Recently a few works are focused on new fiber design based on photonic quasi crystals (PQs) [13-14]. PQs are a special class of aperiodic crystals having long-range order but lacking periodicity. Several research demonstrates that PQs can produce interesting photonic properties [15-17]. For example, the PQs can offer higher rotational symmetries, more isotropic Brillion zone, and hence potentially can open more uniform PBGs at a lower dielectric contrast [15]. On this concept IG-PQFs based on PQs are proposed since 2010 [14]. Sun et al. proposed a HC-PQF based on 12-fold symmetric PQs [14]. It is shown that this type of fiber provides two PBGs enabling simultaneous guidance in two wavelength regions. Here our attention relies on the design of novel hollow core photonic quasi crystal fibers via theoretical/numerical analysis employing finite element method (FEM). The FEM allows the PCF cross-section in the transverse x – y plane to be divided into a patchwork of triangular elements, which can be of different sizes, shapes, and refractive indices. In particular, we first studied HC-PCF with triangular lattice, and then we focused quasi-crystal fiber involving 12-fold, and 8-fold structures. Successively a modified 8-fold structure was designed and investigated to improve guided band gaps and dispersion properties of pristine 8-fold. In conclusion this work is organized in two sections, each one including three chapters. Here we briefly present the chapter’s contents of both sections. Section I: Chapter 1. An introduction to chemical sensing and chemical optical fiber sensor is provided, and we present fiber's abilities and its advantages as chemical sensor. Followed, by an approach to optical fiber modification and employing functional material to enhance sensing characteristic of optical fiber sensors. Chapter 2. A UV-Induced deposition technique for coating of different porphyrin molecules is introduced. The results for adoption this technique on the optical fiber technology with aim to fabricate new class of fiber devices is presented and analyses. We studied different Porphyrin molecules and the extension of technique to molecules such as silver in this chapter. Chapter 3. The integration of a tetrakis (4-sulfonatophenyl) porphyrin (TPPS) thin film with optical fiber technology is obtained via UV-induced deposition. This leads to the growth of TPPS thin films the potential of porphyrins as functional materials, enable the design of novel active and passive in-fiber devices. Here, the presence of intense and narrow band in the reflected spectra is successfully exploited to detect corrosive or toxic vapors, such as strong volatile acids and bases, at room temperature and pressure. Florescence analysis and reflectometry for detection of volatile acid and base is presented and stability and dynamic response of new device as chemical optical fiber sensor is discussed. Section II : Chapter 4. The fundamental principles of operation of Photonic crystal fibers (PCF) are discussed. First of all a general introduction to PCF is given. Then the attention is focused on the derivation and the physical interpretation of the band structure and guiding mode. The concept of loss will be discussed completely and finally fabrication process for the new class of optical fiber is presented. Chapter 5. We introduce Photonic quasicrystal structures, several optical properties and novel applications of quasicrystals will be discussed. Different methods of quasicrystal analysis will be named. Finally, we study band structure and field distribution of 2 dimensional, 8-fold (Ammann-Beenker) and 12-fold symmetry with employing finite element method. We came to results of appearing band gap in such a low refractive index contrast of 1:1.46 with increasing the out of plane wave vector. Chapter 6. The fundamental principles to introduce Hollow core photonic Quasicrystal fibers is presented. Comparative study of different symmetries for study of Hollow core Photonic quasicrystal fiber (HC-PQF) is obtainable. Band gap for different structures of 12-fold and 8-fold will be calculated .we propose a new geometry which is called modified 8-fold with idea of attaining enhanced hollow core photonic quasi crystal. Expectedly Modified 8-fold represents extraordinary gap profile at permitting simultaneous propagation in two spectral windows. The results of bandgap are in agreement with fundamental mode dispersion relation and loss of the proposed HC-PQF. Conclusion, conclude this work with a discussion of achievement and ongoing activities and an outline of the main conclusions.

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