Thulium-doped fibre laser in the 2 μm wavelength region for gas sensing

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

Thulium-doped fibre laser in the 2 μm wavelength region for gas sensing

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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Tm-Yb doped optical fiber performance with variation of host-glass composition

The fabrication process of Thulium-Ytterbium doped optical fiber comprising different host glass through the Modified Chemical Vapor Deposition (MCVD) coupled with solution doping technique is presented. The material and optical performance of different fibers are compared with special emphasis on their lasing efficiency for 2 µm application

Ytterbium-sensitized Thulium-doped fiber laser in the near-IR with 980 nm pumping

The use of an unidirectional auxiliary pump at approximately 1600 nm in conjunction with a 980 nm primary pump for Ytterbium (Yb3+)-sensitized-Thulium (Tm3+)-doped single mode silica fiber (YTDF) is found to be very effective to activate the most significant resonance energy transfer from Yb3+ to Tm3+, in order to obtain significant emission in the near-infrared. The resulting laser performance of the YTDF at 1874 nm is reported here. The influence of the Tm3+/Yb3+ concentration, their relative proportions and the host glass composition on the lasing efficiency has also been investigated to optimize the fiber parameters for maximum laser output power

Rare Earth Doped Fibers for Fiber Laser: Opportunities and Challenges

The advancement in the design and fabrication technology of rare earth doped fibers opens up exciting opportunities for realization of laser systems in a range of wavelengths with designed configurations and power levels for diverse applications

Gain Switched All-fiber Holmium Laser at 2.1 Micron

Gain switched all-fiber holmium laser has been designed by using Q-switched thulium fiber laser as pump source. The output power of 1 W at 2.1 micron with slope efficiency of 31% has been achieved. The spectral line width is 0.3 nm with SNR of more than 25 dB

Vapor-phase doping of ytterbium in high power laser fiber

A 'state-of-the-art' facility for vapor-phase deposition of rare-earth compounds has been established and process technology optimized for fabricating large core preforms/fibers doped with Yb- and Al- oxides with excellent longitudinal and radial uniformity

Gain Switched All-Fiber Holmium Laser at 2.1 Micron

Gain switched all-fiber holmium laser has been designed by using Q-switched thulium fiber laser as pump source. The output power of 1 W at 2.1 micron with slope efficiency of 31 has been achieved. The spectral line width is 0.3 nm with SNR of more than 25 dB

All-Fiber Laser at 1.94 mu m: Effect on Soft Tissue

A focused laser beam at wavelength of strong water absorption at 1.94 mu m can be a good scalpel for precision soft tissue surgery. A fiber Bragg grating-based, all-fiber, continuous-wave as well as modulated, cladding pumped, thulium-doped fiber laser at 1.94 mu m has been configured to deliver up to 10 W of laser power under pumping at 793 nm having an efficiency of 32 %. The laser was exposed to freshly sacrificed chicken breast at different power level and exposure time. The formalin-fixed samples were examined by microscopy to identify the ablation region, carbonization and necrosis region for laser parameter optimization

Fiber Laser at 2 mu m for Soft Tissue Surgery

Strong water absorption at 2 mu m generated recent interest in lasers at this wavelength for soft tissue surgery. A fiber Bragg grating-based, all-fiber, continuous-wave, cladding pumped, thulium-doped fiber laser at 1.95 mu m is configured. The thulium-doped active fiber with octagonal-shaped inner cladding is pumped at 808 nm (total power of 17 W) with six laser diodes through a combiner. The laser power of 3.3 W (after elimination of unabsorbed pump power through a passive fiber) with slope efficiency of 23% (against launched pump power) is achieved. The linear variation of laser power with pump offers scope of further power scaling