City Research Online

Abstract

Compact and low-cost semiconductor laser sources have significant potential for use in applications that are currently dominated by expensive solid-state lasers. The direct application of high-power semiconductor lasers for free-space and satellite communications, visual displays, biomedical applications and remote sensing, optical recording, spectroscopy, optical data storage, laser printers, laser radar and also for materials processing is becoming increasingly attractive due to the remarkable improvement in performance of high-power laser diodes. In addition, high-power spatially and spectrally coherent sources are required for the efficient pumping of solid-state and fiber lasers and efficient nonlinear frequency conversion to the short-wavelength part of the visible spectrum, which is not readily available with semiconductor sources directly.

The early development of the semiconductor amplifier had initially been assisted by the use of the semi-analytical and numerical approaches, which has been extended to include segmented sections to allow for lateral variations of the optical and electronic parameters. In this work, a vectorial finite element beam propagation method (FEB PM), which is numerically efficient and has incorporated a wide-angle approach to tackle rapid axial variations and the perfectly matched boundary condition, to avoid reflections from the orthodox computational window, has been employed to study and design the guided-wave photonic devices. The evolution of the optical beam profile along a high power tapered semiconductor amplifier has been demonstrated by employing this method. Numerically simulated results indicate the generation of many higher order modes, and their interference with the fundamental mode causes a variation of the optical beam, both along the transverse and the axial directions, which could significantly modify the output beam quality, which also leads to beam filamentation.

In this thesis, the FEBPM approach has also been utilized to study rigorously the complex refractive index profiles, which provide modal gain in the semiconductor structures. The power gain in an active photonic device, such as a laser or an amplifier is due to the presence of the imaginary part of the complex refractive index in the core. The injected current generates carrier density and when the density is above the transparent carrier density then the optical field can be amplified. In case of a highpower tapered semiconductor optical amplifier (SOA), the width of the SOA changes continuously, which reduces the power density to improve the total gain. The modal gain properties and field expansion have therefore been examined in this work. The effect of gain reduction along the transverse directions due to non-uniform transverse field profile is also demonstrated. Furthermore, the effect of gain saturation on the total optical gain of the amplifier is studied by considering both the transverse and axial variation of the local gain coefficient.

Finally, the study of the far field profiles and birefringence for various tapered waveguide structures, with particular interest to the very wide width SOA structures is carried out.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Departments:	Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses School of Science & Technology

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