DESIGN AND STUDY OF RECTANGULAR OPTICAL WAVEGUIDE COMPONENTS AND DEVICES

Abstract

The requirement for high bit rate transmission is increasing day by day due to the demand of high-bandwidth applications, such as video conference, internet, e-commerce, etc. To meet this explosive global demand wavelength division multiplexing networks have been developed which in turn require the different optical devices such as low cost transmitters, receivers, connectors, optical amplifiers, dispersion compensators, wavelength filters and polarizers. Such kind of optical components often need integration in a single substrate for improved efficiency and minaturization. Depending on their performance and advantages, integrated optical devices and components are attractive for local area networks, long haul communication, and sensing systems. A standard single-mode fiber cable installed for long haul optical communication has zero dispersion at 1.3 m wavelength and finite amount of dispersion at 1.55 m. In such a system dispersion leads to the degradation of optical pulses in single mode optical fibers and limits the data transmission rate and bandwidth of the system. To address the issue of dispersion, new categories of transmission fibers such as large effective area fiber (LEAF) and Truewave-RS fibers have been i ii developed. However, these fibers also have a finite amount of dispersion. In a communication link, such accumulated dispersion can be compensated by suitable dispersion compensators. One can use pre-compensation or post compensation schemes. A post compensator is advantageous when used in-line with the transmission fiber. However, a pre-compensator may prove useful if integrated with the source itself. Among various dispersion compensators, fiber-based ones are most common and there have been several reported studies on dispersion compensating fibers. Fiber Bragg gratings can also be designed to exhibit high group velocity dispersion (GVD) and can recompress pulses broadened by chromatic dispersion in an optical fiber link. There have also been reports on rectangular waveguide based dispersion compensators. In 1987, an attempt was made using linearly chirped grating filters in an optical waveguide to compensate the dispersion in optical fiber link. It exhibited a large dispersion in a small bandwidth region. An integrated ring resonator all-pass filters for dispersion and dispersion slope compensation, and planar lightwave circuits with asymmetric directional couplers have also been reported. A dispersion compensating high GVD design presented in silicon-on-insulator material system is promising as a compact dispersion compensator. Recently dispersion compensator design in a photonic crystal waveguide has also been demonstrated. Apart from dispersion compensation, the control of polarization plays a key role in coherent optical communication systems involving modulators and switches. A polarizer is also an important component in sensor systems. Recently optical fiber designs have been reported for single polarization operation, for polarization control, and for polarization preserving directional coupler. A significant work has also been reported in optical waveguides on polarization filters and polarizers. Spectral filters have significant interest in wavelength division multiplexing netiii works (WDM) and sensors. Long period fiber gratings have been widely used for wavelength filtering, for gain flattening filters for erbium doped fiber amplifier, and in sensors. Long period waveguide gratings (LPWGs) have been proposed to overcome the material and design constraints of LPFG. LPWGs have been designed for various applications including band-rejection filter, grating coupler, and temperature/pressure sensor. Erbium doped fiber amplifiers (EDFAs) are of significant interest in long haul optical communication. However, EDFAs are bulky and costly to be used in local area network application. In view of this, erbium doped waveguide amplifiers (EDWAs) have been developed. Also some other types of amplifiers such as counter propagating optical parametric amplifiers, Nd:LiTaO3 waveguide lasers have been proposed. In optical communication systems employing WDM it is required to have a flat gain spectrum in the entire conventional wavelength band (C-band) that lies from the 1530 to 1560 nm. Intensive research has been done to equalize the gain spectrum of an optical fiber amplifier. There are also several studies on doped waveguide amplifiers reported in literature. Some of these designs include erbium-ytterbium co-doped waveguide amplifier employing multilayer thin film filter and cascaded multilayer thin film filters. To flatten the gain spectra of waveguide amplifiers LPWG have been employed. In an LPWG the transmission spectrum of the device is utilized to flatten the gain spectrum of waveguide amplifier. This thesis presents various novel designs in planar waveguide to achieve the high GVD, single polarization single mode operation, and gain flattening filter for erbium doped waveguide amplifier. In addition, an LPWG in a novel trench assisted waveguide is proposed to achieve broadband rejection, refractive index sensing and gain flattening of EDWA gain spectrum. iv We have proposed dual core waveguide structure for dispersion compensation. The resonant coupling between the two cores of the waveguide results in the generation of symmetric and antisymmetric supermodes with a very high and opposite GVDs. Symmetric supermode possesses positive GVD and negative chromatic dispersion, and can be used to compensate the accumulated positive dispersion in an optical fiber. Such a design is particularly useful as a pre-compensator in view of the possibility of integration with the source. We present a planar waveguide structure to achieve single polarization single mode operation. Here, we use dual core leaky waveguide structure using In- GaAs/AlGaAs semiconductor material layers on glass substrate. The design is based on the mode discrimination. We show that suitable choice of waveguide parameters leads to a resonant coupling design, which is very efficient for mode discrimination. The structure can be designed as TE-pass or TM-pass polarizer. Further, the single-mode operation of the structure is ensured by having high leakage loss for all the higher-order modes. We have extended the application of dual core leaky waveguide for flattening the gain spectrum of erbium doped waveguide amplifier. In the present structure, spectral variation of leakage loss has been utilized to suppress the gain peak of the erbium doped waveguide amplifier and to achieve the flattened gain spectrum. We investigate the long-period-grating in a trench assisted planar optical waveguide for application in wavelength filtering and refractive index sensing. The proposed structure shows triple resonances between a set of guided and cladding modes and results in an ultra-wide-rejection band over a wavelength span of 528 nm in the transmission spectrum of the grating. Furthermore, the proposed LPWG shows v refractive index (RI) sensitivity of 2.3 nm/RIU. In addition we have also designed such an LPWG for gain flattening of erbium doped waveguide amplifier gain in C-band. The transmission spectrum of the long period waveguide grating has been utilized to suppress the gain peak of the EDWA. Such a design is able to show the gain equalization within 0.96 dB over the entire C-band.