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Chapter Two: Background
Fiber Optic Sensors
A fiber optic sensor is a broad term referring to a strand of fiber optic used to measure environmental stimulation by utilizing the properties of the light propagation within the fiber. Generally speaking there are two types of fiber optic sensors, these being categorized as extrinsic and intrinsic. In extrinsic sensors the light travels through the fiber to an external sensing mechanism, and the modulated light is reflected back into the fiber and analyzed. In intrinsic sensors, the light is modulated by the environmental stimuli as it passes through the fiber. In both, the input and output signals are detected by optoelectronic devices, analyzed, and the environmental parameters determined.
Further classification of fiber optic sensors delineates how the altered light traveling in the fiber is used to obtain the corresponding environmental information. Several methods include intensity, interference, frequency, polarization and spectral content modulation. Many different types of light modulation schemes can be used to measure any given environmental parameter. The fiber configurations introduced below can be used in conjunction with the different types of light modulation schemes to produce dozens of fibers capable of measuring numerous environmental parameters
Fiber Optic Structures
This section provides a brief introduction to the different types of fiber configurations used in sensing applications. A review of the literature relevant to each type of fiber configuration and pertaining to fiber optic sensing will be provided in order to map the evolution of the porous fiber sensors studied in this research.
Solid Cladding
The first fiber optic sensors to be developed utilized a typical optical fiber configuration consisting of a solid core and cladding as shown in Figure 1. In general, these fibers guide light through total internal reflection, where the refractive index of the core is larger than that of the cladding. The refractive index difference between the core and cladding in these fibers is typically introduced through the use of dopant materials such as GeO2, P2O5, and Al2O3, to the parent glass[3].
These types of fibers are capable of sensing parameters such as pressure, temperature, chemical composition, and displacement both intrinsically and extrinsically, as well as with and without the use of special coating materials [2, 4]. However, these solid clad fibers often are not suitable for use in high temperature environments due to dopant migration within the cladding of the fiber. Dopant migration changes the optical properties in the fiber, thus decreasing the sensitivity and reliability of its measurements[5]. The disadvantages of solid clad fiber sensors have spurred the development of fibers which do not contain dopants in the cladding of the fiber and can sense environmental parameters without the use of special coatings.
Microstructured Optical Fibers
Microstructured optical fibers (MOF’s), commonly termed holey fibers, are relatively new types of fibers that consist of patterned arrays of air holes surrounding either a solid or hollow core as shown in Figure 2. Two varieties of MOF’s guide light based on two different principles. The first is that of modified total internal reflection where the presence of air holes in the cladding surrounding the solid core lowers the average index of refraction in that region and effectively confines the light along the length of the fiber. The second is what is referred to as a photonic bandgap fiber (PBGF). The photonic bandgap effect typically occurs in hollow core MOF’s and allows light propagation by limiting the frequencies and wavelengths of radiation that can penetrate the air hole structure[6, 7].
MOFs have been demonstrated as chemical, gas [8-11], temperature, elongation [12], and pressure sensors for use in various industrial applications. Many of the chemical and gas sensors base on MOFs are very effective because of the light interaction with the evanescent waves propagating through the fiber. Evanescent waves are modes of light that are not confine to the fiber core through either the photonic bandgap effect or total internal reflection The air holes allow for either chemical or gaseous species to penetrate closer to the core and increase the interaction with the evanescent waves [10, 11].
Advantages of using MOFs as sensors include the ability to tailor the properties with the number, size, shape, and spatial location of air holes, and the lack of dopant elements needed for light propagation [8, 13]. However, despite their obvious advantages over conventional fiber optic sensors, MOFs become increasingly difficult to manufacture as the number of air holes needed increases [5]. This problem has been overcome by the introduction of new types of holey fibers termed “Random Hole Optical Fibers” or RHOFs.
Random Hole Fibers
Random hole optical fibers (RHOF) consist of a solid core surrounded by a cladding of randomly sized air holes running parallel to the fiber axis as shown in Figure The air holes are produced by either sol-gel techniques or the incorporation of gas producing agents surrounding the core in the fiber preform [5, 14, 15]. These types of fibers confine light to the core region using the modified total internal reflection method as described for the MOFs. They also utilize the evanescent waves propagating through the fiber in order to sense chemical and gas media present in the environment [5, 16]. The advantages of using this type of fiber for gas sensing is the ease of fabrication over the MOFs, as well as the ability to tailor the light guiding and sensing properties of the fiber by altering the size, number, and distribution of holes in the cladding.
In addition, like the MOFs the RHOF do not require dopant materials to alter the index of refraction of the cladding, thus enabling high temperature use. However there are some disadvantages to having the holes running along the length of the fiber. The characteristics that give both RHOFs and MOFs the ability to sense chemical and gas media also require that the gas or chemicals be fully distributed along the fiber length. The response time of these sensors is thus limited by the diffusion kinetics of the gases penetrating the cladding region[5].
The problem of diffusion length restricted response time has been alleviated by the development of what is termed a porous clad optical fiber. These fibers consist of a three dimensionally interconnected pore structure surrounding a solid glass core[17].
Porous Clad Fibers
Porous clad fibers refer to fibers that contain a three-dimensionally interconnected network of air holes (pores) that penetrate both axially and longitudinally throughout the cladding region, from the surface of the fiber to the core [16]. The advantages of these porous clad optical fibers is that the diffusion region exists along the entire length of the fiber, and the diffusion distance from surface to core is much smaller than the lengths of tubes within the MOFs. This has the potential to decrease the response time of gas and chemical sensors[17].
The porous structure around the solid cladding has been produced by means of sol-gel processing and phase separation of borosilicate glasses. Sol-gel processing produces the porous glass structure through hydrolysis and condensation polymerization of metal alkoxide precursors such as tetraethylorthosilicate. The claddings are typically dip-coated onto a solid glass core, followed by specific heat treatments that evaporate the gel precursor leaving an interwoven network of silica. Fibers produced by sol-gel techniques have been shown capable of sensing environmental parameters with and without the aid of active coating materials. For example, sol-gel glass porous optical fibers with fluorescent dyes trapped in the pore structure have been used as evanescent wave pH sensors[18], while a plain sol-gel glass optical fiber has been shown to measure the critical micelle concentration in surfactant solutions by means of optical transmission changes[19].
Phase separation offers an alternative to creating a porous silica structure for use with optical fibers. The phase separation process does not require the complex chemistry associated with sol-gel process, but instead takes advantage of the spinodal phase separation characteristics of various borosilicate glasses. Spinodal phase separation of glasses has been researched thoroughly, and the details of the phase separation process can be found in numerous texts[20, 21].
Fibers have been produced by leaching the alkaline borate phase from a purely phase separable glass fiber with HF and HCl solutions for a given period of time. This process leaves either a fully porous fiber (complete dissolution of alkaline borate phase), or a solid core surrounded by a porous cladding (partial dissolution of alkaline borate phase). The fibers produced in this manner have been demonstrated as an end of service life indicator for respiratory cartridges [22]. A second method for producing porous clad fibers using phase separating glasses is by collapsing a tube of the phase separable glass around a core of different composition glass to create a fiber preform. Fibers pulled from the preform then contain a non-phase separating glass core surrounded by the phase separable glass. In this way the core and cladding remain proportional to the original dimensions of the preform. By adjusting the diameters of the core and cladding glass in the preform, the size of each can be tailored accordingly.
ABSTRACT
ACKNOWLEDGEMENTS
CHAPTER 1: INTRODUCTION
1. OUTLINE
CHAPTER 2: BACKGROUND
1. FIBER OPTIC SENSORS
2. FIBER OPTIC STRUCTURES
3. FIBER OPTIC COATINGS
4. MATERIALS AND DEPOSITION METHODS
5. PHOTOVOLTAICS DEVICES
CHAPTER 3: EXPERIMENTAL PROCEDURE
1. FIBER FABRICATION
2. ELECTROLESS DEPOSITION OF POROUS CLAD FIBERS
3. CHEMICAL VAPOR DEPOSITION
4. PHOTOVOLTAIC DEVICE CONSTRUCTION
5. QUANTUM DOT DEPOSITION
6. ELECTRICAL CHARACTERIZATION
7. SEM AND EDS ANALYSIS
CHAPTER 4: RESULTS AND DISCUSSION
1. POROUS CLAD FIBERS
2. STOCHASTIC ORDERED HOLE FIBERS
3. ELECTROLESS PLATING
4. CHEMICAL VAPOR DEPOSITION
5. PHOTOVOLTAIC FIBERS
6. QUANTUM DOT DOPED STOCHASTIC ORDERED HOLE FIBERS
CHAPETER 5: CONCLUSION AND FUTURE WORK
REFERENCES
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Increased Functionality Porous Optical Fiber Structures
