Thulium Doped Fiber Laser and Its Applications

Abstract

Laser as one of the core components of the photonics industry plays a crucial role in keeping the compound annual growth rate (CAGR) of this industry much higher than the average of global GDP growth. The exceptional performance of fiber laser and its advantages, including robustness, excellent heat dissipation capability, compactness, high beam quality, free from alignment, low cost, etc. are the reasons that fiber laser shares about 50% of laser market revenue. The extensive applications of fiber lasers in industry, medicine, research, defense and other areas make them demanding for the future of our lives. Thulium doped fiber laser has been demonstrated to be able to operate from ultraviolet to mid infrared. The work presented in this dissertation, includes demonstration of single frequency blue laser fiber amplifier using thulium doped fiber; study of thulium doped fiber laser operating at 785 nm; investigation of 2 µm thulium doped fiber laser and magneto optical glasses for all fiber optical isolator and circulator in 2 µm region. In chapter 1, the performance of blue laser fiber amplifier was studied. A single frequency blue laser all-fiber amplifier was demonstrated for the first time. Over 150 mW continuous-wave single transverse-mode, and single longitudinal mode blue laser output was obtained with a 10-m 1000 ppm thulium-doped fluoride fiber pumped by a 1125 nm fiber laser at a power of 2 W. The output power was limited due to the onset of the competitive lasing at 784 nm. The concept of blue laser MOPA for underwater optical communication system was also demonstrated by using a GaN diode laser at 478 nm as the seed laser and a 7.5 m Tm3+-doped ZBLAN fiber amplifier as the power amplifier. A maximum output power of 235 mW was obtained. Photo-darkening and photo-curing of the thulium-doped fiber amplifier were also studied and analyzed. In chapter 2, all-fiber single-transverse-mode laser oscillators operating at 785 nm were demonstrated by splicing a 0.1 mol% Tm3+-doped fluoride fiber with a core diameter of 4 µm and a numerical aperture of 0.07 to a pair of silica fiber Bragg gratings. About 500 mW of continuous-wave single transverse mode laser output at 784.5 nm with a 3-dB spectral bandwidth of 0.2 nm was obtained by upconversion pumping a 3-m-long gain fiber at 1125 nm. Our experiments show that the ground-state absorption of Tm3+ at 785 nm is the origin of low efficiency in previous reports. The efficiency of this all-fiber laser can be improved by using a gain fiber with optimized overlap between the laser, the pump and the fiber core, and employing new pumping schemes that deplete the ground state sufficiently. In chapter 3, thulium doped fiber laser operating in CW regime with several watts output power in 2 µm region was demonstrated. 793 nm high power diode pump was used to pump the gain fiber and the slope efficiency of 35% was obtained. Ring cavity design and linear cavity design of thulium doped fiber Q-switched laser operating in 2 µm region were also demonstrated. In chapter 4, Dy3+ doped glass as a promising paramagnetic glass operating in mid IR region for magneto optical applications was studied. Due to their large effective magnetic moment, Dy3+-doped materials have attracted much interest for magneto-optical applications. Highly Dy3+-doped glasses with concentrations from 40 wt.% to 75 wt.% were fabricated and their magneto-optical properties were investigated. A Verdet constant of -7.94 rad/T/m (-455 deg/T/m) at 1950 nm was measured with the 75 wt.% Dy3+-doped glass. This is the highest reported Verdet constant around 2 µm for a paramagnetic glass. Our experimental results show that highly Dy3+-doped glasses are promising isotropic magneto-optical materials for applications in the 2 μm wavelength region. In chapter 5, magneto optical properties of chalcogenide glasses operating in 2 μm wavelength region were studied. A Verdet constant of 870 deg/T/m at 1950 nm with the figure of merit of more than 500 deg/T, which is the highest value reported in glass materials at this wavelength, was measured for tellurium-arsenic-selenium glass (Te₂₀As₃₀Se₅₀). Compared to other chalcogenide glasses, such as Ge₁₀Se₉₀ and Ge₂₅As₁₅S₆₀, Te₂₀As₃₀Se₅₀ glass exhibits higher Verdet constants, broader mid-infrared transparency window, and longer infrared absorption edge, making it a very promising material to fabricate magneto-optical devices for mid-infrared applications.