Coherent beam combination of four holmium amplifiers with phase control via a direct digital synthesizer chip.

We present the coherent beam combination of four 2100 nm holmium amplifiers with their phase controlled through acousto-optic modulators driven by the RF output of direct digital synthesizer chips. Phase alignment was achieved through the use of a field programmable gate array based stochastic parallel gradient descent algorithm.

High power cryogenic Ho:YAG laser.

We have improved significantly the brightness of cryogenic Ho:YAG, reporting up to 65 W output power with a beam quality of M2 <1.3 and a slope efficiency of 71%. The laser emission was ~2 nm wide and centered at 2097.5 nm. This result demonstrates the scalability of both the narrow-line thulium fibre pump laser and the cryogenic laser head.

Nanoparticle doping for high power fiber lasers at eye-safer wavelengths.

A nanoparticle (NP) doping technique was developed for fabricating erbium (Er)- and holmium (Ho)-doped silica-based optical fibers for high energy lasers. Slope efficiencies in excess of 74% were realized for Er NP doping in a single mode fiber based master oscillator power amplifier (MOPA) and 53% with multi-Watt-level output in a resonantly cladding-pumped power oscillator laser configuration based on a double-clad fiber. Cores comprising Ho doped LaF3 and Lu2O3 nanoparticles exhibited slope efficiencies as high as 85% at 2.09 µm in a laser configuration. To the best of the authors' knowledge, this is the first report of a holmium nanoparticle doped fiber laser as well as the highest efficiency and power output reported from an erbium nanoparticle doped fiber laser.

Metal clad active fibres for power scaling and thermal management at kW power levels.

We present a new approach to high power fibre laser design, consisting of a polymer-free all-glass optical fibre waveguide directly overclad with a high thermal conductivity metal coating. This metal clad active fibre allows a significant reduction in thermal resistance between the active fibre and the laser heat-sink as well as a significant increase in the operating temperature range. In this paper we show the results of a detailed thermal analysis of both polymer and metal coated active fibres under thermal loads typical of kW fibre laser systems. Through several different experiments we present the first demonstration of a cladding pumped aluminium-coated fibre laser and the first demonstration of efficient operation of a cladding-pumped fibre laser at temperatures of greater than 400 °C. Finally, we highlight the versatility of this approach through operation of a passively (radiatively) cooled ytterbium fibre laser head at an output power of 405 W in a compact and ultralight package weighing less than 100 g.

Efficient, low threshold, cryogenic Ho:YAG laser.

We report the development of an efficient, liquid-nitrogen conduction cooled Ho:YAG slab laser with good beam quality. Detailed measurements resolving the structure of the 1900-1911 nm absorption band in Ho:YAG at 77 K are presented. Stress-free conduction cooled mounting of the Ho:YAG slab was demonstrated and the resulting laser operated with a large mode volume of 42 mm3, a slope efficiency of 75% and a threshold of 0.84 W. To our knowledge this corresponds to the lowest reported threshold intensity for a Ho:YAG laser.

CO2 laser-fabricated cladding light strippers for high-power fiber lasers and amplifiers.

We present and characterize a simple CO2 laser processing technique for the fabrication of compact all-glass optical fiber cladding light strippers. We investigate the cladding light loss as a function of radiation angle of incidence and demonstrate devices in a 400 µm diameter fiber with cladding losses of greater than 20 dB for a 7 cm device length. The core losses are also measured giving a loss of <0.008±0.006 dB/cm. Finally we demonstrate the successful cladding light stripping of a 300 W laser diode with minimal heating of the fiber coating and packaging adhesives.

High precision 9.6 µm CO(2) laser end-face processing of optical fibres.

Reproducible, precise cleaving of optical fibres is of great importance to the fibre laser and telecommunications industries. We present a novel approach to the end-face processing of optical fibres using a 9.6 µm CO2 laser to produce flat, smooth and symmetric fibre end-face profiles with no rounding or melting at the edges of the fibre. As a demonstration, precision cleaving of a 400 µm diameter optical fibre is reported. For this fibre a topographical profile height of <400 nm (0.06°) and a reproducibility better than 200 nm (0.03°) was achieved. To the best of our knowledge this is the first demonstration of a CO2 process that has generated a fibre end-face topography substantially smaller than a typical mechanical cleave. Highlighting the flexibility of this system, we have also demonstrated the generation of near arbitrary fiber end-face profiles such as discrete phase steps and non-spherical surface profiles.

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.

Pulsed operation of a resonantly pumped, linearly polarised, large mode area holmium-doped fibre amplifier.

We present the design of a polarisation maintaining, resonantly and cladding pumped, large mode area holmium-doped fibre. We describe the operation of this fibre in a pulsed amplifier configuration. The amplifier produced pulses with energy of 2.25 mJ and duration of 20 ns at a repetition rate of 20 kHz and average power of 45 W. The holmium-doped fibre provided >10 dB of gain at 2.09 µm and the polarisation extinction ratio was >11.5 dB. The beam quality of the output (M2) was 1.6. To the best of our knowledge this is the highest pulse energy achieved from any singly holmium-doped fibre by an order of magnitude.

99 W mid-IR operation of a ZGP OPO at 25% duty cycle.

We have demonstrated the highest reported output power from a mid-IR ZGP OPO. The laser is a cascaded hybrid system consisting of a thulium fibre laser, Ho:YAG solid state laser and a Zinc Germanium Phosphide parametric oscillator. The system produces 27 W of output power in the 3-5 µm wavelength range with an M(2) = 4.0 when operating in a repetitively q-switched mode, and a modulated peak output power of 99 W at a reduced duty cycle of 25%.

High power operation of cladding pumped holmium-doped silica fibre lasers.

We report the highest power operation of a resonantly cladding-pumped, holmium-doped silica fibre laser. The cladding pumped all-glass fibre utilises a fluorine doped glass layer to provide low loss cladding guidance of the 1.95 µm pump radiation. The operation of both single mode and large-mode area fibre lasers was demonstrated, with up to 140 W of output power achieved. A slope efficiency of 59% versus launched pump power was demonstrated. The free running emission was measured to be 2.12-2.15 µm demonstrating the potential of this architecture to address the long wavelength operation of silica based fibre lasers with high efficiency.

A cladding-pumped, tunable holmium doped fiber laser.

We present a tunable, high power cladding-pumped holmium doped fiber laser. The laser generated >15 W CW average power across a wavelength range of 2.043 - 2.171 µm, with a maximum output power of 29.7 W at 2.120 µm. The laser also produced 18.2 W when operating at 2.171 µm. To the best of our knowledge this is the highest power operation of a holmium doped laser at a wavelength >2.15 µm. We discuss the significance of background losses and fiber design for achieving efficient operation in holmium doped fibers.

Efficient, polarised, gain-switched operation of a Tm-doped fibre laser.

We present a monolithic, robustly polarised thulium fibre laser which produces pulses with 25 ns duration and energy of up to 35 µJ. A pulsed 1.55 µm source was used to gain switch the laser at repetition rates of up to 300 kHz, producing average powers of up to 8 W at 2.044 µm. To the best of our knowledge this is the highest power gain-switched thulium fibre laser and it is also the first with a linearly polarised output. The large repetition rate, pulse energy and average power will enable efficient energy extraction from a large mode-area, polarisation maintaining thulium fibre amplifier.