Optical amplifiers and lasers continue to play its crucial role and they have become an indispensable part of the every fiber optic communication systems being installed from optical network to ultra-long haul systems. It seems that they will keep on to be a promising future technology for high speed, long-distance fiber optic transmission systems. The numerical simulations of the model equations have been already commercialized by the photonic system designers to meet the future challenges. One of the challenging problems for designing Raman amplifiers or lasers is to develop a numerical method that meets all the requirements such as accuracy, robustness and speed. In the last few years, there have been much effort towards solving the coupled differential equations of Raman model with high accuracy and stability. The techniques applied in literature for solving propagation equations are mainly based on the finite differences, shooting or in some cases relaxation methods. We have described a new method to solve the nonlinear equations such as Newton-Krylov iteration and performed numerical simulations using spectral methods. A novel algorithm implementing spectral method (pseuodspectral) for solving the two-point boundary value problem of propagation equations is proposed, for the first time to the authors&#039 / knowledge in this thesis. Numerical results demonstrate that in a few iterations great accuracy is obtained using fewer grid points.

Reproduction de : Thèse de doctorat : Instrumentation et analyses avancées : Lille 1 : 2005. / N° d'ordre (Lille 1) : 3638. Titre provenant de la page de titre du document numérisé. Bibliogr. à la suite des chapitres. Liste des publications et communications.

This thesis describes a series of experiments that were undertaken to extend the limits of output power and wavelength coverage of optically pumped mid-infrared lasers. Initially, two new CW Raman lasers operating at wavelengths of 11.5 and 12.5 pm were developed. Maximum powers of 650 and 150 mW were produced, with pump powers of 11 and 3.3 W, respectively. The effect of the pump offset on the output power was then determined by measuring the efficiency of an NHg laser pumped at frequency offsets of 94 and 274 MHz. In lasers operating in pure NH3, the larger pump offset required a greater pump intensity to reach threshold, but efficiency increased with pump offset. Higher NH3 pressures could be used at larger pump offsets and the improved efficiency was attributed to reduced saturation effects at the higher operating pressures. Experiments carried out with NHg inversion lasers have greatly increased the output powers available at a large number of wavelengths in the 10 to.14 pm range. In a buffered NH3 mixture, the sR(5,0) transition was pumped on resonance. C^llis;o^r^s with either — or Ar buffer gases were effective in thermalizing the rotational populations in the v>2=1 vibrational level and producing gain on a wide range of frequencies. Output powers as large as 3.5 W on a single line and greater than 5 W multi-line were produced, at efficiencies of 20 and 30 % respectively. The number of lasing wavelengths increased substantially, as more than forty ortho-NH3 transitions were observed to lase in a grating-tuned cavity. The optical pumping technique was then used for the first time to produce line-tunable lasing on para-NH3 transitions. The sR(5,l) transition was pumped near resonance and 24 para-transst'lons were observed to lase. In total, lasing was achieved on 65 different transitions in 14NH3, with wavelengths of 10.3 to 13.8 pm. / Thesis / Master of Science (MSc)

Mid-infrared (IR) lasers (2-12 μm) have found tremendous applications in medical surgeries, spectroscopy, remote sensing, etc. Nowadays, mid-IR emissions are usually generated from semiconductor lasers, gas lasers, and solid-state lasers based on nonlinear wavelength conversion. However, they usually have disadvantages including poor beam quality, low efficiency, and complicated configurations. Mid-IR fiber lasers have the advantages of excellent beam quality, high efficiency, inherent simplicity, compactness, and outstanding heat-dissipating capability, and have attracted significant interest in recent years. In this dissertation, I have studied and investigated Q-switched and mode-locked fiber lasers in the mid-IR wavelength region. My dissertation includes six chapters: In Chapter 1, I review the background of mid-IR lasers and address my motivation on the research of mid-IR fiber lasers; In Chapter 2, I present the experimental results of microsecond and nanosecond Er³⁺-doped and Ho³⁺-doped fiber lasers in the 3 μm wavelength region Q-switched by Fe²⁺:ZnSe and graphene saturable absorbers. In Chapter 3, Q-switched 3 μm laser fiber amplifiers are investigated experimentally and theoretically and their power scaling are discussed. In Chapter 4, a graphene mode-locked Er³⁺-doped fiber lasers at 2.8 μm with a pulse width < 50 ps is presented. In Chapter 5, extending the spectral range of mid-IR fiber lasers by use of nonlinear wavelength conversion is addressed and discussed. I have proposed 10-watt-level 3-5 μm Raman lasers using tellurite fibers as the nonlinear gain medium and pumped by our Er³⁺-doped fiber lasers at 2.8 μm. In the last chapter, the prospect of mid-IR fiber laser is addressed and further research work is discussed.