S.K. Sudheer

Effect of thermal annealing on the phase evolution of silver tungstate in Ag/WO3 films

Silver/tungsten oxide multi-layer films are deposited over quartz substrates by RF magnetron sputtering technique and the films are annealed at temperatures 200, 400 and 600°C. The effect of thermal annealing on the phase evolution of silver tungstate phase in Ag/WO3 films is studied extensively using techniques like X-ray diffraction, micro-Raman analysis, atomic force microscopy and photoluminescence studies. The XRD pattern of the as-deposited film shows only the peaks of cubic phase of silver. The film annealed at 200°C shows the presence of XRD peaks corresponding to orthorhombic phase of Ag2WO4 and peaks corresponding to cubic phase of silver with reduced intensity. It is found that, as annealing temperature increases, the volume fraction of Ag decreases and that of Ag2WO4 phase increases and becomes highest at a temperature of 400°C. When the temperature increases beyond 400°C, the volume fraction of Ag2WO4 decreases, due to its decomposition into silver and oxygen deficient phase Ag2W4O13. The micro-Raman spectra of the annealed films show the characteristic bands of tungstate phase which is in agreement with XRD analysis. The surface morphology of the films studied by atomic force microscopy reveals that the particle size and r.m.s roughness are highest for the sample annealed at 400°C. In the photoluminescence study, the films with silver tungstate phase show an emission peak in blue region centered around the wavelength 441 nm (excitation wavelength 256 nm).

Processing of Diamond for Integrated Optic Devices Using Q-Switched Nd:YAG Laser at Different Wavelengths

In the present investigation, a Q-switched Nd:YAG laser is used to study the various aspects of diamond processing for fabricating integrated optic and UV optoelectronic devices. Diamond is a better choice of substrate compared to silicon and gallium arsenide for the fabrication of waveguides to perform operations such as modulation, switching, multiplexing, and filtering, particularly in the ultraviolet spectrum. The experimental setup of the present investigation consists of two Q-Switched Nd:YAG lasers capable of operating at wavelengths of 1064 nm and 532 nm. The diamond cutting is performed using these two wavelengths by making the “V”-shaped groove with various opening angle. The variation of material loss of diamond during cutting is noted for the two wavelengths. The cut surface morphology and elemental and structural analysis of graphite formed during processing in both cases are compared using scanning electron microscopy (SEM) and laser Raman spectroscopy. Both the Q-Switched Nd:YAG laser systems (at 1064 nm and 532 nm) show very good performance in terms of peak-to-peak output stability, minimal spot diameter, smaller divergence angle, higher peak power in Q-switched mode, and good fundamental TEM 00 mode quality for processing natural diamond stones. Less material loss and minimal micro cracks are achieved with wavelength 532 nm whereas a better diamond cut surface is achieved with processing at 1064 nm with minimum roughness.

Design and simulation of highly nonlinear and low loss square Photonic Crystal Fiber

Here, we designed a highly nonlinear and low loss Photonic Crystal Fiber (PCF) structure with square lattice using COMSOL multiphysics. Propagation properties of proposed PCF has been analysed. The analysis has been carried out using finite element method. The proposed fiber obtained low confinement loss, minimum dispersion for wide range of wavelength. Results shows that the proposed PCF gives large negative dispersion for a wide range of wavelength and this makes it suitable for dispersion compensating applications and also it gives high nonlinear coefficient of 117 W-1Km-1. The square lattice PCF structure with elliptical holes is also designed and its birefringence property is compared with PCF with circular holes. Results shows that PCF with elliptical holes shows high birefringence as compared to other and such high birefringent PCF can be used for polarization maintaining applications.

Design and analysis of a non linear, low confinement loss Photonic Crystal Fiber with Liquid Crystal and air filled holes

Design and analysis of a Nonlinear Photonic Crystal Fiber having low confinement loss, low birefringence and low effective mode area has been presented using COMSOL Multiphysics software. The suggested design has a central hole filled with Nematic Liquid Crystal (NLC) surrounded by air holes and a Perfectly Matched Layer (PML). Results show that by using circular holes of small diameter and materials with considerable refractive index difference, the nonlinearity and confinement increases, but the effective area and birefringence properties of the fiber decreases.

Investigation of optical nonlinearity in Photonic Crystal Fibers and supercontinuum generation

Photonic Crystal Fibers (PCFs) are a new class of optical waveguides which offer exceptional light guiding mechanism and deliver significantly improved performance compared to conventional optical fibers. In the present investigation, we propose an solid core PCF for three different values of air filling fraction. The proposed PCF is simulated by using COMSOL Multiphysics. The nonlinearity of proposed PCF is found to be varying with respect to its air filling fraction and a nonlinear coefficient of 148 W-1Km-1 is obtained for PCF with air filling fraction of 0.7. The variation of other parameters like effective refractive index, confinement loss and effective area with respect to wavelength and air filling fraction is also investigated. The supercontinuum spectrum is generated using the designed highly nonlinear fiber and the effect of pump power on the generated output spectrum is studied and results shows that the bandwidth of output spectrum increases with pump power provided its pulse width remains constant.

Discrete Gyrator Transform based optical image encryption and decryption using Double Random Phase Mask

A new method for Discrete Gyrator Transform (DGT) based image encryption and decryption using Double Random Phase Mask (RPM) is proposed. In the present work encryption and decryption has been done using Gyrator Transform (GT) a kind of discrete algorithm using convolution operation. The proposed method uses DGT to reduce the computational load. The robustness of the proposed algorithm to blind decryption in terms of different key values has been calculated. Numerical simulations and some analysis for security have been presented to verify its validity and efficiency.

Modeling and simulation of photonic crystal fiber structure for single mode polarization for fiber laser applications

Photonic crystal fibers have attracted increasing interest over the past few years because of their ability to provide manipulation in optical properties of light. High birefringence can be easily achieved in PCFS based on design flexibility and the large index contrast. Amongst several designs high birefringence exceeding 10−3 has been shown. Birefringence of the PCF can be further improved by employing elliptical air holes in the fiber cladding. In this category of PCFS high birefringence is achieved when the bulk of the mode energy is in the fiber cladding thus high birefringence is often accompanied with poor energy confinement. In this paper an ultrahigh birefringent PCF with ultra low confinement loss is proposed by employing elliptical holes in the fiber core [2] to induce the birefringence but circular holes in the fiber cladding to reduce the confinement loss. Such a design is able to offer is able to offer a perfect solution to the tradeoff between the high birefringence and the confinement loss in elliptical-hole PCFS. MATLAB and COMSOL softwares have been used for the coding and simulation of the problem.

Fractional thermal loading and effective stimulated-emission cross section of diode double-end-pumped Nd:YVO4 crystal

Studies on a diode double-end-pumped Nd:YVO4 laser operating in the fundamental mode with a maximum cw output power of 14.8 W are reported. Measurements were taken with an a-axis-cut crystal in the form of a rectangular bar of size 4×4×10 mm with doping concentration 1.1 at.%, using a diode double-end-pumped resonator configuration operating in the cw TEM00 mode. The pumping arrangement mainly consists of two fiber-coupled diode-laser arrays with a maximum output power of 30 W and output wavelength range of 807 to 810 nm at 25°C set temperature. Since the quality of the pump beam and the actual focused spot size are important parameters for overlap optimization of the pump and cavity modes, these two parameters are calculated for this set up. The fractional thermal loading and effective stimulated emission cross section for 4F3/24I11/2 transition of 1.1-at.% doped Nd:YVO4 crystal are calculated using the planar resonator configuration stabilized by pump-power-induced thermal lensing. The focal length of the effective lens and its variation with the absorbed pump power are determined from the far-field divergence angle of the output beam.