Efficiency dependence on the Tm3+ and Al3+ concentrations for Tm3+-doped silica double-clad fiber lasers

Inband-pumped, high-power thulium-doped fiber amplifiers for an ultrafast pulsed operation

We investigate the influence of the pump wavelength on the high-power amplification of large-mode area, thulium-doped fibers which are suitable for an ultrashort pulsed operation in the 2 µm wavelength region. By pumping a standard, commercially available photonic crystal fiber in an amplifier configuration at 1692 nm, a slope efficiency of 80 % at an average output power of 60 W could be shown. With the help of simulations we investigate the effect of cross-relaxations on the efficiency and the thermal behavior. We extend our investigations to a rod-type, large-pitch fiber with very large mode area, which is exceptionally suited for high-energy ultrafast operation. Pumping at 1692 nm leads to a slope efficiency of 74 % with a average output power of 67 W, instead of the 38 % slope efficiency obtained when pumping at 793 nm. These results pave the way to highly efficient 2 µm fiber-based CPA systems.

Transverse mode instability and thermal effects in thulium-doped fiber amplifiers under high thermal loads

We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 µm, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847 W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.

Nested-ring doping for highly efficient 1907 nm short-wavelength cladding-pumped thulium fiber lasers

Cladding-pumped Tm-doped fiber lasers operating below 1950 nm have difficulty matching the high-efficiency, power-scalable output that can be achieved at longer wavelengths. This challenge arises due to the strong three-level behavior at short wavelengths and strong competition from higher-gain long wavelength emission. In this Letter, we demonstrate a nested-ring fiber design in which a highly doped Tm ring is embedded within a larger undoped core. The fiber is specifically tailored for highly efficient and high power short-wavelength operation (<1950nm). The nested-ring Tm fiber laser has generated 62 W of single-mode 1907 nm output with up to 65% (70%) slope efficiency with respect to launched (absorbed) pump power.

Tm3+-doped silica-glass fiber for ∼2-μm fiber laser

Tm³⁺-doped silica-glass preforms were fabricated by the modified chemical vapor deposition process combined with the solution doping technique. Its spectroscopic properties were evaluated according to the detected absorption spectrum and the fluorescence spectrum. The calculated maximum emission cross section of Tm³⁺ ions in this core glass is 5.66×10^-21  cm², and the tested fluorescence lifetime is 544 μs at 1816 nm. According to the theory of Dexter and Forster, the cross-relaxation rate was calculated by the integral overlap of the absorption and emission cross sections. A double-cladding fiber with a core NA of 0.14 was prepared by high temperature drawing. A maximum 14.5-W fiber laser output centered at 2001 nm with a slope efficiency of 50.6% was obtained from a 4.5-m-length double-cladding fiber.

Highly efficient thulium-doped high-power laser fibers fabricated by MCVD

We report a hybrid process by combining both vapor-phase and solution-doping techniques of rare-earth doped preform fabrication in conjunction with the MCVD technique, in order to fabricate highly efficient Tm-doped laser fibers. The proposed fabrication route takes advantage of co-doping silica with high alumina content through the vapor-phase doping process, which is otherwise difficult to achieve using conventional solution doping technique. In addition, by employing the solution doping method, high-purity thulium halide precursors that have low vapor pressures up to several hundred degree Celsius. These high-purity thulium halide precursors can be used to dope the fiber core region with a high thulium concentration that is optimized for an efficient two-for-one cross-relaxation process for 79xnm diode pumped thulium-doped fiber laser. Fibers fabricated using the hybrid approach show more homogeneous and flat-top dopant profiles, compared with the conventional approach, where both aluminum and thulium are incorporated in the core through solution doping. This will ensure that more doped region will take part in the cross-relaxation process. Superior laser performance with a slope efficiency of >70% in the two-micron band has been demonstrated when diode pumped at ~790nm.

High-efficiency ultrafast Tm-doped fiber amplifier based on resonant pumping

We demonstrated a high-efficiency ultrafast Tm-doped fiber amplifier based on a resonant pumping technique. A continuous-wave fiber laser at 1940 nm was employed as the pump laser. The slope efficiency of the resonantly pumped pulsed Tm-doped fiber amplifier reached 87% with respect to the launched pump power. The maximum average output power reached 40 W when the launched pump power was 53 W. The repetition rate and the pulse duration of the output pulses from a fiber amplifier were 248 MHz and 129 ps, respectively. The corresponding peak power was 1.25 kW, and the pulse energy was 161.3 nJ. To the best of our knowledge, this is the first demonstration of a resonant pumping enabled high-power high-efficiency ultrafast fiber laser operating at a 2 μm band.

Continuous-wave and quasi-continuous wave thulium-doped all-fiber laser: implementation on kidney stone fragmentations

A continuous-wave (CW) as well as quasi-continuous wave (QCW) thulium-doped all-fiber laser at 1.94 μm has been designed for targeting applications in urology. The thulium-doped active fiber with an octagonal-shaped inner cladding is pumped at 793 nm to achieve stable CW laser power of 10 W with 32% lasing efficiency (against launched pump power). The linear variation of laser power with pump offers a scope of further power scaling. A QCW operation with variation of duty cycle from 0.5% to 90%, repetition rate from 0.1 Hz to 1 kHz, and pulse width from 40 μs to 2 s has been presented. Laser power of 9.5 W in CW mode of operation and average power of 5.2 W with energy range of 10.4-104 mJ in QCW mode of operation has been employed to fragment calcium oxalate monohydrate kidney stones (size of 1.5-4 cm) having different colors and composition. Dependence of ablation threshold, ablation rate, and average fragmented particle size on the average power and energy has been studied. One minute of laser exposure results in fragmentation of a stone surface with ablation rate of 8  mg/min having minimum particle size of 6.54 μm with an average size of 20-100 μm ensuring the natural removal of fragmented parts through the urethra.

Mode instability thresholds for Tm-doped fiber amplifiers pumped at 790 nm

We use a detailed numerical model of stimulated thermal Rayleigh scattering to compute mode instability thresholds in Tm(3+)-doped fiber amplifiers. The fiber amplifies 2040 nm light using a 790 nm pump. The cross-relaxation process is strong, permitting power efficiencies of 60%. The predicted instability thresholds are compared with those in similar Yb(3+)-doped fiber amplifiers with 976 nm pump and 1060 nm signal, and are found to be higher, even though the heat load is much higher in Tm-doped amplifiers. The higher threshold in the Tm-doped fiber is attributed to its longer signal wavelength, and to stronger gain saturation, due in part to cross-relaxation heating.

Design and experimental demonstration of a large pedestal thulium-doped fibre

We present a novel large-mode-area thulium-doped fibre with a large pedestal design. We discuss the advantages of this large pedestal fibre in the context of overcoming limitations imposed by cleaving and splicing tolerances. Finally we demonstrate the use of such a fibre in constructing monolithic fibre lasers operating at 1.95 µm with 170 W of output power, 0.1 nm line-width and a diffraction limited beam quality of M(2)(X,Y) = 1.02, 1.03.

High power resonant pumping of Tm-doped fiber amplifiers in core- and cladding-pumped configurations

We have demonstrated ultra-high efficiency amplification in Tm-doped fiber with both core- and cladding-pumped configurations using a resonant tandem-pumping approach. These Tm-doped fiber amplifiers are pumped in-band with a 1908 nm Tm-doped fiber laser and operate at 1993 nm with >90% slope efficiency. In a core-pumped configuration, we have achieved 92.1% slope efficiency and 88.4% optical efficiency at 41 W output power. In a cladding-pumped configuration, we have achieved 123.1 W of output power with 90.4% optical efficiency and a 91.6% slope efficiency. We believe these are the highest optical efficiencies achieved in a Tm-doped fiber amplifier operating in the 2-micron spectral region.

Semiconductor saturable absorber mirror passively Q-switched fiber laser near 2 μm

A passively Q-switched fiber laser near 2 μm is achieved with a semiconductor saturable absorber mirror (SESAM) as a saturable absorber. Stable Q-switched pulses are generated from an extremely compact setup with a central wavelength of 1958.2 nm. Under the bidirectional pump configuration, the repetition rate of the fiber laser can be widely tuned from 20 to 80 kHz by increasing the pump power at the same time the pulse width decreases from 1 μs to 490 ns. When the incident pump power is 1.3 W, the average output power, the pulse repetition rate, the pulse width, and the highest single pulse energy are 91 mW, 80 kHz, 490 ns, and 1.14 μJ, respectively. To further optimize the system configuration, the pulse width can be reduced to 362 ns when the cavity length is reduced.

Dissipative dispersion-managed soliton 2 μm thulium/holmium fiber laser

We report a first dissipative dispersive-managed soliton fiber laser operating at 2 μm. The cavity comprised of all-anomalous-dispersion fiber employs chirped fiber Bragg grating, which ensures net-normal cavity dispersion and semiconductor saturable absorber for mode-locking.

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 (Yb(3+))-sensitized-Thulium (Tm(3+))-doped single mode silica fiber (YTDF) is found to be very effective to activate the most significant resonance energy transfer from Yb(3+) to Tm(3+), 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 Tm(3+)/Yb(3+) 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.

High-power linearly-polarized single-frequency thulium-doped fiber Master-Oscillator Power-Amplifier

We report a high power narrow-linewidth source at approximately 2 microm based on a Tm-doped fiber distributed-feedback master-oscillator and three Tm fiber amplifier stages. The master-oscillator and first two amplifier stages were in-band pumped by Er,Yb fiber lasers operating at 1565 nm, and the final stage amplifier was cladding-pumped at 795 nm by two spatially-combined diode-stacks. The MOPA yielded 100 W of single frequency output at 1943 nm with a beam propagation factor (M(2)) of 1.25 and with a polarization extinction ratio of >94%. The output power was limited by thermally-induced damage in the final amplifier stage. The prospects for further power scaling are considered.

Design considerations for short-wavelength operation of 790-nm-pumped Tm-doped fibers

Tm-doped 790-nm-pumped silica fiber lasers are excellent candidates for producing emission at <1.95 microm, but achieving efficient operation at these wavelengths requires careful attention to fiber design because of the characteristic three-level reabsorption effects. We present a discussion of methods for mitigation of these effects and two high-efficiency systems that are capable of producing up to 70 W at <1.92 microm.

Visible and near infra-red up-conversion in Tm3+/Yb3+ co-doped silica fibers under 980 nm excitation

The spectroscopic properties of Tm(3+)/Yb(3+) co-doped silica fibers under excitation at 980 nm are reported. Three distinct up-conversion fluorescence bands were observed in the visible to near infra-red regions. The blue and red fluorescence bands at 475 and 650 nm, respectively, were found to originate from the (1)G(4) level of Tm(3+). A three step up-conversion process was established as the populating mechanism for these fluorescence bands. The fluorescence band at 800 nm was found to originate from two possible transitions in Tm(3+); one being the transition from the (3)H(4) to (3)H(6) manifold which was found to dominate at low pump powers; the other being the transition from the (1)G(4) to (3)H(6) level which dominates at higher pump powers. The fluorescence lifetime of the (3)H(4) and (3)F(4) levels of Tm(3+) and (2)F(5/2) level of Yb(3+) were studied as a function of Yb(3+) concentration, with no significant energy back transfer from Tm(3+) to Yb(3+) observed.

Highly efficient high-power thulium-doped germanate glass fiber laser

A 64 W fiber laser at 1.9 microm with a slope efficiency of 68% with respect to the launching pump power at 800 nm was demonstrated in a one-end pump configuration using a piece of 20 cm long newly developed thulium-doped germanate glass double-cladding single-mode fiber. A quantum efficiency of 1.8 was achieved. An output laser power of 104 W at 1.9 microm was demonstrated from a piece of 40 cm long fiber with a dual-end pump configuration.

Single-frequency thulium-doped distributed-feedback fiber laser

We have successfully demonstrated a single-frequency distributed-feedback (DFB) thulium-doped silica fiber laser emitting at a wavelength of 1735 nm. The laser cavity is less than 5 cm long and is formed by intracore UV-written Bragg gratings with a phase shift. The laser is pumped at 790 nm from a Ti:sapphire laser and has a threshold pump power of 59 mW. The laser has a maximum output power of 1 mW in a single-frequency, single-polarization radiation mode and is tunable over a few nanometers. To the best of the authors' knowledge, this is the first report of a single-frequency DFB fiber laser that uses thulium as the amplifying medium. The lasing wavelength is the longest demonstrated with DFB fiber lasers and yet is among the shortest obtained for thulium-doped silica fiber lasers.

Power scaling method for 2-microm diode-cladding-pumped Tm(3+)-doped silica fiber lasers that uses Yb(3+) codoping

Unidirectional dual-wavelength pumping at 805 and 975 nm of a Tm(3+)-doped silica double-clad fiber laser that has been codoped with Yb(3+) is demonstrated. With equal 2-wt. % concentrations of the Tm(3+) and Yb(3+) ions, slope efficiencies of 49% (57%) and 35% (40%) with respect to the launched (absorbed) pump power of the 805- and 975-nm pump wavelengths, respectively, were obtained. Efficient Yb(3+) --> Tm(3+) energy transfer and efficient cross relaxation combined with minimal upconversion losses were thus operating. At the maximum output power of 18 W, the fiber laser operated at a center wavelength of 2050 nm.