High Power Tm: Fiber Lasers And Application In Fiber-bulk Hybrid Laser Technology

Cladding-pumped Tm:fibre laser sources operating in the eye-safe 2μm spectral region have attracted growing interest in recent years owing to their numerous applications in areas such as atmospheric monitoring, range finding, medicine and as the pump source for generation of mid-infrared (3～5μm) radiation through nonlinear frequency conversion. Also, operating at 1.7–2.1μm, this technology falls into the eye-safer category of lasers giving it potential advantages over 1μm lasers for industrial and military directed energy applications. In this paper, we investigated output characteristics of Tm: fiber lasers both theoretically and experimentally.For theoretical study, a numerical model that describes the operation behaviors and output characteristics of stable gain-switched Tm: fiber lasers resonantly pumped at 1.56μm is presented for the first time to the best of our knowledge. Effects of some important laser parameters, such as repetition rates, pump power levels, transmission of the laser output coupler, fiber core sizes, fiber lengths and Tm concentrations etc., on the laser output were investigated. The model and simulating results were verified by preliminary experiments and good agreements have been achieved, suggesting that the model is useful for design and optimization of stable gain-switched Tm: fiber lasers at 2μm wavelength region.For experimental study, first, a free-running Tm: fiber laser double-end pumped by a high power 792 nm diode laser was built up, output powers in excess of 177 W was produced with a high slope efficiency of 61.3%. Operation behaviors of the laser have been investigated with fibers of different Tm³⁺ concentration.Second, we report a cladding-pumped high power Tm-doped silica fiber laser using a volume Bragg grating for wavelength-selection and spectral narrowing. Over 119 W of nearly diffraction-limited continuous-wave output was generated with slope efficiency of 43.4%. The operating wavelength was tunable from 1943 to 1998 nm, with spectrum linewidth of～10 pm over the whole tuning range. Performance characteristics of the laser have been compared to that using a broadband dielectric mirror, instead of VBG, as external reflector, and the two laser set-ups performed very similarly in terms of threshold, slope efficiency and output power. Additionally, a further reduction in linewidth was achieved by using two VBGs in a Tm: fiber laser, and a maximum output power of 113 W was obtained with spectrum linewidth of～2 pm. This work is of great significance as the first demonstration of two VBG locked Tm: fiber laser especially with such high output power and such narrow linewidth.Third, efficient tunable dual-wavelength Tm: fiber laser was demonstrated for continuous wave operation with a simple cavity configuration. The maximum total output power of 118 W was obtained with simultaneously wavelength emission at 1989.7 nm and 1999.7 nm, for launched pump power of 279 W, corresponding to a slope efficiency of 45%. The wavelength splitting range is continuously tuned from 1 nm to 50 nm (0.1-3.8 THz) with total output power of two wavelengths between 118 W and 107 W. With all the merits of simple cavity configuration and easy tunability, the dual-wavelength Tm: fiber laser will sure find its way in applications such as distance measurements, imaging systems, spectroscopy, and terahertz difference signal generation.Finally, experiments was also carried out for Tm: fiber laser applications, for which a novel hybrid fiber-bulk laser was demonstrated with a cladding-pumped tunable Tm-doped silica fiber laser as the pump source and Ho:YLF as the laser crystal. With this configuration, most of the heat generated via quantum defect heating in this fiber-bulk laser scheme is deposited in the fiber, and thermal effects in the bulk laser was dramatically reduced leading to the prospect of much improved efficiency, beam quality and higher output power in the 2μm wavelength regime. Using a simple two-mirror resonator configuration, we obtained 5.3 W of polarized output at 2066 nm in a diffraction-limited TEM00 beam (with M²ï½ž1.1) for 9.4 W of incident pump power, corresponding to a slope efficiency of 70% with respect to incident pump power.