LARGE-MODE-AREA RECTANGULAR OPTICAL WAVEGUIDES AND FIBRES

Abstract

Continuous efforts have been made to increase the power level of the lasers by various approaches, since the invention of laser in 1960 by T. H. Maiman. At present, kilowatt ranges of the lasers (gas and solid state) are commercially available in the market. However, compactness and the thermal management are the main issues with the gas and solid state lasers. The effective solution of this problem is to develop the fibre lasers of the comparative power level. A fibre doped with rare-earth elements can serve as a gain medium for the laser and amplifier applications and can make a compact device. The inherent structural property of the fibre also solves the thermal problem. Although, the first fibre laser was invented in 1961 by Snitzer yet a remarkable interest in the development of the fibre laser was only shown after the invention of erbium doped fibre amplifier (EDFA) in 1987. Suitable gain medium, efficient pump power and the appropriate core size play an important role to scale up the power level of the fibre lasers. Detailed analysis of the amplification characteristics of the rare-earth doping elements and the double clad pumping scheme have already been analyzed in recent years. In scaling-up the power level of a fibre laser core size of the fibre plays an important role. Good beam quality from a fibre laser can be achieved by using a single-mode (SM) fibre, which requires a small core-size in conventional optical waveguide. However, in small core waveguides, the tight light confinement can reduce optical damage threshold and at the same time, can give rise to significant unwanted nonlinear optical effects. Nonlinear effects (such as four wave mixing, stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS)) in a fibre are the major factors, which reduce its applicability in optical communication and in high power applications. A fibre with large core area can reduce these effects. Such fibres can be used to achieve the fibre laser and 'amplifier with high power output. Such fibres also find applications in optical communication systems employing dense-wavelength-division-multiplexing (DWDM). A lot of interest has been shown to increase the mode area of fibre for applications in optical communication and high-power laser and amplifier. Abstract There have been few reports to suppress the higher orders modes in a multimode fibre by tapering the fibre ends, by introducing special cavity configuration, by optimizing the seed launch condition, and by bending the fibre at right bending radius. A significant work has been done to increase the core area by tailoring the refractive index profile in rare-earth doped fibre to achieve the high power output in fibre lasers and amplifiers. The modal field patterns in these fibres, however, deviated from the Gaussian. Another way to increase the modal field area is to control the relative index difference between the core and cladding. The lack of freedom in controlling the core-cladding index difference because of practical difficulties restricts the use of this method in achieving large-mode area (LMA) fibres. Kawakami et al. have used double clad fibre geometry to enlarge the core diameter up to twice of that of the standard single-mode step index fibre. Another double clad design was used to fabricate an extremely high power fibre laser in Ytterbium doped fibres. A photonic crystal fibre or a holey fibre, which is characterized by distribution of air holes in cladding region running through the entire length of the fibre, is another class of fibre for single-mode operation with a large core. Recently, some designs, where the refractive index profile of the cladding was tailored in radial or angular direction have shown LMA operation. These fibres work on the principle of higher order mode discrimination. A high differential leakage loss between LI301 and L1311 mode and a nominal loss to fundamental mode ensures the effective single-mode operation in these fibres. A leaky graded-index cladding fibre, in which the refractive index of cladding gradually increases as one moves away from core in the radial direction, has shown the possibility of single-mode operation with core area as large as 1000 ilm2. Segmented cladding fibre (SCF), in which the refractive index is tailored in angular direction, has shown extended single-mode operation with a large-mode-area. The SCF offers low polarization mode dispersion as compared to the PCF. SCF has already been fabricated in the polymers and the silver halide by some groups because of its unique applications. It is experimentally demonstrated that SCF leaky fibre is able to show SM with good beam quality. However it is difficult to fabricate this fibre in silica by modified chemical vapour deposition (MCVD) technique. LMA-SM operation with good beam quality of leaky fibre motivated us to look for the few more possible leaky designs for LMA-SM operation in silica which can be fabricated by MCVD technique. ii Abstract Apart from fibre lasers, there are activities to scale up the power level in slab waveguide lasers as they are well suited for high-power diode-array pumping and can make very compact laser systems. The use of slab structures also allows for integration of various optical components on a single substrate and thus simplifies the optical alignment problem. The LMA designs presented above either can not fabricated using MCVD at all or are cumbersome to fabricate with MCVD. The designs can be fabricated by MCVD but either have small NA or the modal field deviates from the Gaussian profile. In this thesis we present some novel LMA designs in optical fibres which can be fabricated with ease using MCVD, large NA and good beam quality. We also propose some such designs in rectangular geometry and explore their possible applications in high power lasers and amplifiers. This thesis presents various novel designs in rectangular and cylindrical geometry to achieve LMA-SM operation that would be possible to fabricate by usual fabrication techniques. All these designs have been achieved by tailoring the refractive index of the waveguide in the cladding region such that all the modes supported by the waveguide become leaky. The basic principle behind all the designed profiles is to introduce the high leakage loss to higher order modes while nominal loss to the fundamental mode, which makes the waveguide effectively single-moded. Leakage loss and dispersion of the modes have been calculated by solving the structure by well established numerical methods such as transfer matrix method (TMM), effective index method (EIM), radial effective index method (REIM) and finite element method (FEM). Leaky planar waveguide presented in this thesis have been characterized by either graded-index or multi-layer leaky cladding. These waveguides offer — 2.25 times larger core in comparison to conventional SM waveguides with a few millimeter length of the waveguide. Proposed waveguides have clean Gaussian mode profile and have shown insensitivity to bending for bending radii larger than 3 cm. Waveguides are easier to fabricate and can be good candidate for designing the high power waveguide lasers and amplifiers. The idea of mode filtering is also extended to 2-dimentional rectangular waveguide and a new class of channel waveguides, where a wing like geometrically shaped cladding of uniform refractive index is used for LMA-SM operation. Such a geometrically shaped cladding is highly dispersive and show SM operation in the wavelength range 750-1600 nm with a 100 [mi2 core area. This waveguide is the first iii Abstract all-solid channel waveguide reported for the realization of extended single-mode operation. After successful demonstration of LMA-SM of these leaky designs in rectangular geometry we have implemented these in fibre. We found that multi-layer cladding (MLC) leaky fibre is able to show LMA-SM operation with 30 .im core diameter and 0.16 numerical aperture. To validate the working principle of MLC fibre to a simplified multi-clad fibre design with only two depressed cladding has been fabricated Universite de Nice-Sophia Antipolis, France. The fabricated fibre showed SM operation with a Gaussian-like mode with 22±2 1.1,m mode field diameter (MFD) after 3-m propagation in agreement with modeling (Modeled MFD = 25 vtm). We have also utilized the concept of resonant coupling to show LMA-SM operation in fibre with only two depressed layers cladding. This fibre is known as co-axial dual-core resonant leaky fibre and is able to demonstrate effective SM by leaking all the higher-order modes with only 36 cm length of fibre with mode area as large as — 706 1.1.m2. Apart from LMA operation, we have also explored the possibility of this fibre in gain equalization. With small core, this fibre has shown a 20 dB inherent flat gain in S-band EDFA. Earlier proposed segmented cladding fibre (SCF) has also been investigated by full vectorial finite element method (FEM). It is shown that one can control birefringence of SCF by the segmented cladding which can be helpful in realizing the fibre-based polarization controlling components and single-polarization devices. Future work coming out of this thesis is also discussed in brief. iv