City Research Online

Abstract

The transition 3F4->3H6 of trivalent Thulium is widely studied for generating lasers at wavelength near 2 μm. For decades, tuneable continuous wave narrow line-width sources in this wavelength region have been proved to be very useful as spectroscopic tools for trace gas detection. Semiconductor lasers are often not readily available at a reasonable cost with the specific wavelengths required to provide a close ‘match’ to the key absorption features of the gases of interest. Well-designed fibre laser-based systems, however, can overcome this limitation by offering potentially much wider wavelength ranges, coupled with their distinctive and valuable features such as stability, narrow linewidth and high tuneability at room temperature. In this work, a compact ‘all-fibre’ laser system has been specifically designed, developed and evaluated, as this type of laser systems is highly desirable for ‘in-the-field’ applications. This takes full advantages of the active fibres based on silica glass host compared to other non-oxide glass hosts in terms of their chemical durability, stability and crucial structural compatibility with readily available telecommunication optical fibres. Ideal host composition for Thulium and efficient pumping scheme posses major challenges restricting the production of commercially deployable efficient ‘all-fibre’ lasers in the 2 μm wavelength region. The aim of the thesis work is to address these challenges. The work presented in this thesis demonstrates a modulated Thulium-doped ‘all-fibre’ tuneable laser in the 2 μm wavelength region suitable for detection of a number of gases of interest. The scope of work includes the fabrication and optimization of the active fibre with the core composition suitable for the creation of an effective Thulium-doped fibre laser. Codoping of Ytterbium is explored to investigate the energy-transfer mechanism from Ytterbium to Thulium and thereby opening up the opportunity of using economic pump laser diodes emitting at around 0.98 μm. In this respect, both Thulium- and Thulium/Ytterbium-doped single-mode single-clad silica optical fibres are designed and fabricated for a systematic analysis before being used as laser gain media. The optical preforms having different host compositions, Thulium-ion concentrations and proportions of Ytterbium to Thulium are fabricated by using the Modified Chemical Vapour Deposition technique coupled with solution doping to enable the incorporation of rareearth ions into the preforms. A thorough investigation of the basic absorption and emission properties of Thulium-doped silica fibres has been performed. The step-wise energy-transfer parameters in Thulium/Ytterbium-doped silica fibre have been determined quantitatively from spectroscopic measurements along with migrationassisted energy-transfer model. A set of tuneable Thulium-doped ‘all-fibre’ lasers, offering a narrow line-width in the 2 μm wavelength region, is created by using fabricated Thulium-and Thulium/Ytterbium-doped fibres as gain media and fibre Bragg grating pairs under in-band pumping at 1.6 μm and/or pumping by an economical laser diode at 0.98 μm, utilizing Ytterbium to Thulium energy- transfer. The host composition and the dopnat concentration in the single-mode single-clad fibre configuration are optimized to achieve maximum lasing efficiency. The tuning of laser wavelength has been achieved by using relaxation/compression mechanism of the fibre Bragg grating pair used to confine the laser cavity. A new set of laser resonators has also been formed by using a combination of a high reflective fibre Bragg grating with a low reflective broadband mirror, fabricated at the end of the fibre through silver film deposition, to enable only one fibre Bragg grating to be tuned. The stability of the laser output power, line-width and shape have been monitored throughout the tuning range. This is followed by the design of a compact, high-Q, narrow line-width and low threshold microsphere laser resonator, operating in the 2 μm wavelength region, by coupling a Thulium-doped silica microsphere to a tapered fibre. In the microsphere, laser emission occurred at wavelengths over the range from 1.9 to 2.0 μm under excitation at a wavelength of around 1.6 μm. The designed modulated tuneable Thulium-doped ‘all-fibre’ laser, operating at a wavelength range centred at a wavelength of 1.995 μm, has been tested for CO2 gas detection. Both the modulation of the fibre laser, through pump source modulation and the ‘locking’ detection mechanism have been utilized to eliminate laser intensity noise and therefore to obtain a compact gas sensor with high sensitivity. The absorption spectrum, the line-strength and the concentration level of CO2, have been monitored using the absorption spectroscopic technique. The measured minimum detectable concentration of CO2 obtained using the system confirms the claim that it is capable of detecting trace gases at the ppm level. The stable laser performance achieved in the sensor system illustrates its potential for the development of practical, compact yet sensitive fibre laser based gas sensor systems.

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

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