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.

47 W continuous-wave 1726 nm thulium fiber laser core-pumped by an erbium fiber laser.

The high-power, short-wavelength operation of a thulium-doped silica fiber laser at 1726 nm has been demonstrated in a core-pumped monolithic (all-fiber) resonator configuration, in-band pumped by a high-power erbium-only fiber laser operating at 1580 nm. The thulium fiber laser yielded 47 W in a single-spatial-mode output beam for 60-W absorbed pump power. The corresponding slope efficiency, with respect to an absorbed pump power of 80%, compares favorably with the theoretical maximum (Stokes) efficiency of 91.5%. The prospects for further scaling of single-mode power in this wavelength regime to >100 W are considered, as well as the potential applications for high-power lasers operating in this difficult-to-reach wavelength band.

Tunable holmium-doped fiber laser with multiwatt operation from 2025 nm to 2200 nm.

The emission band of holmium-doped silica fibers extends beyond 2200 nm, which means these lasers have the potential of covering considerable parts of the atmospheric transmission window between ∼2100 nm and 2250 nm. However, efficient operation toward 2200 nm is challenging due to absorption in fused silica at the laser wavelength. Here we present a holmium-doped fiber laser specifically targeting long-wavelength operation. The laser is implemented as a high-feedback wavelength selective ring cavity and is tunable from 2025 nm to 2200 nm. A maximum slope efficiency of 58% is obtained at 2050 nm and a slope of 27% is obtained at 2200 nm. A power of 5.5 W from a single aperture (8.9 W total) is demonstrated at 2200 nm. Our results represent extended coverage of the 2 µm spectral band with multiwatt-level silica fiber lasers.

Approach for power scaling solid-state lasers with intracavity motion.

Solid-state lasers are typically limited by adverse thermal effects within the gain medium. In this Letter we describe a new method for dramatically reducing thermal effects in an end-pumped solid-state laser by incorporating a rotating intracavity periscope in the resonator to spatially separate the lasing and thermal processes. In contrast with previous examples of moving solid-state lasers, our approach keeps the gain medium stationary, simplifying the heat removal arrangement. This scheme has been applied to an Nd:YAG laser, yielding an output power of 120 W at 1.064 µm, limited by available pump power. Analysis suggests that scaling to much higher power is feasible with the appropriate laser design.

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.

High gain holmium-doped fibre amplifiers.

We investigate the operation of holmium-doped fibre amplifiers (HDFAs) in the 2.1 µm spectral region. For the first time we demonstrate a diode-pumped HDFA. This amplifier provides a peak gain of 25 dB at 2040 nm with a 15 dB gain window spanning the wavelength range 2030 - 2100 nm with an external noise figure (NF) of 4-6 dB. We also compare the operation of HDFAs when pumped at 1950 nm and 2008 nm. The 1950 nm pumped HDFA provides 41 dB peak gain at 2060 nm with 15 dB of gain spanning the wavelength range 2050 - 2120 nm and an external NF of 7-10 dB. By pumping at the longer wavelength of 2008 nm the gain bandwidth of the amplifier is shifted to longer wavelengths and using this architecture a HDFA was demonstrated with a peak gain of 39 dB at 2090 nm and 15 dB of gain spanning the wavelength range 2050 - 2150 nm. The external NF over this wavelength range was 8-14 dB.

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.

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.