APPLICATION SPECIFIC MULTILAYER OPTICAL STRUCTURES

Abstract

The contemporary physics revolution based on discovery of electromagnetic waves has led science and technology into new era. Development of modern technologies like wireless communication systems, high energy particle accelerators, electron microscopy, high precision devices and systems, active and passive microwave and optical devices, high speed internet, sensing systems and telecommunication infrastructure have great impact on modern society. This has been possible because of discovery of electromagnetic waves. Electromagnetic waves in open space can propagate in all the directions. The efficient transfer of optical energy from one point to another can be performed using a specially designed electromagnetic structure called optical waveguide. On the basis of geometry of the structure, waveguides one can be divided into following two main categories. 1. Integrated optic waveguide 2. Cylindrical waveguide ( Optical Fiber ) Optical waveguides are used in many applications in integrated optics, fiber optics and biomedical optics. Integrated optics mainly consists of planar waveguide and rectangular waveguide and is used in optical-integrated circuits while optical fiber is basically used in optical communication for data transmission at high bit rate over long distances. In integrated optics rectangular waveguides have used for applications in optical modulator, optical amplifier, optical switches, wavelength filters, laser, and refractive index sensors. Optical fibers are used for applications which include optical telecommunication, optical amplifiers, fiber sensors, fiber lasers, delivery of high power laser pulses, refractive index sensors and fiber optic devices and components such as directional coupler, wavelength filter, dispersion compensating fibers and dispersion flattened fibers. All these applications require designing waveguide in some specific geometry and refractive index profile. The structures used in general have several layers of materials of different refractive indices and specific thicknesses. This thesis presents various multilayer novel designs in cylindrical and rectangular geometries for specific applications. In particular we present multilayer designs for dispersion management, refractive index sensing and improving light extraction in organic light emitting diodes. All these waveguide structures have been designed by tailoring refractive index of ii waveguides in core and cladding regions to achieve specific characteristics according to the requirement of application. Segmented core fibers having non-zero positive, non-zero negative and near-zero ultra-flattened dispersion with small dispersion slope and ultra-large effective area over a wide spectral range has been presented. Transfer matrix method has been used to analyze the structure. The designs consist of a concentric multilayer segmented core followed by a trench assisted cladding and a thin secondary core. The central segmented core helps in maintaining desired dispersion over a wide range of wavelength. The second core of the fiber helps in achieving ultra-large effective area and trench assisted cladding reduces the bending loss. Designed fibers show very small bending loss. We report breakthrough in the mode area of the single mode optical fiber with ultra flattened dispersion and low dispersion slope. These characteristics of the fiber make it an attractive candidate for DWDM optical communication system. The idea of multilayer structure is also extended to design fibers for delivery of ultrashort laser pulses. Three layered fiber structure and segmented cladding fiber have been designed for delivery of femtosecond laser pulses. Effective index and field profile of modes of the fibers have been calculated using transfer matrix method. Pulse propagation through the fibers has been studied using split-step Fourier method. Distortion-free propagation of the pulse has been achieved by keeping ratio of dispersion to nonlinear length close to 1 in both the fibers. The modal purity in the fiber structures have been maintained by leaking out HOM except LP11. Mode stability has been ensured by sufficient mode spacing between LP01 and LP11 modes ensure mode stability. Design of the fibers ensures no intermodal coupling, low bending loss, and high fabrication tolerances while maintaining large-mode-area. Fibers can be fabricated using modified chemical vapor deposition technique. Whereas for fabrication of segmented cladding fiber in silica glass a technique has been developed and is presented in thesis. We have optimized design of large mode area segmented cladding fiber with effective single mode operation. Fiber has been fabricated using modified chemical vapor deposition technique at CGCRI Kolkata. Fiber has been fabricated with 45 m core diameter and 125 m as the outer diameter. Characterization of the fiber at IIT Roorkee has shown mode filtering effect. Another application of multilayer design as a refractive index sensor in cylindrical geometry has been presented. The design of a refractive index sensor is based on depressed index clad fiber. We utilize variation of leakage loss with variation iii in refractive index of outermost layer of waveguide (analyte) to design a simple, compact, low cost and ultra high sensitive refractive index sensor. Different regions of leakage loss curve to have been used to design sensors in different ranges, with different index resolutions, and for specific applications. The practical realization of the sensor requires etching of some portion of the cladding of the fiber and does not require any costly methods of wavelength or polarization interrogation. Fiber can be easily fabricated using standard modified chemical vapor deposition technique. However, fiber is too fragile to handle and the quantity of analyte to sense refractive index is also larger. We extended the idea to design a miniaturized sensor using planar geometry of the waveguide. We have presented a simple, compact, low cost and highly sensitive refractive index sensor based on single mode leaky planar waveguide. An optical waveguide deposited on glass substrate has been proposed to construct the sensor. In this structure cytop works as guiding layer while teflon forms the cladding layers. Propagation of light in cytop layer is strongly affected by the refractive index of external medium placed on top of teflon layer. If the refractive index of the medium is close to or greater than that of cytop then there is leakage of power from cytop layer to external medium which affects the transmittance of the waveguide. The structure can be designed to have strong variation in transmittance with the refractive index of external medium, and thus, can be utilized as highly sensitive refractive index sensor for biomedical applications. In this chapter we have carried out the design strategies of such a refractive index sensor for various biological and chemical applications. We have designed a sensor with index resolution of the order of 105. Fabrication of the sensor can be carried out by using cost effective spin coating technique. Long range, high sensitivity, simple manufacturing process and compactness make the sensor attractive for diverse applications. We have extended the applications of multilayer structures to organic light emitting diode to increase its efficiency. Extraction efficiency of conventional organic light emitting diode structure has been increased by inserting a low index layer of polymer Teflon between glass substrate and ITO. This low refractive index layer helps in breaking wave guidance of modes. Such a structure helps in leaking out more power into the glass substrate from organic and ITO layer. We show around 15% increment in extracted power at 442-nm wavelength and 29% increment at 539-nm wavelength, and iv 3 % at 620-nm wavelength. Fabrication of the structure can be done using very cost effective spin-coating technique. Future work coming out of this thesis is also discussed in brief.