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High energy laser sources operating around 2 µm are of interest in a variety of applications: LIDAR, material processing, medicine and pump sources for optical parametric oscillators (OPOs) for high-energy mid-IR nonlinear conversion. The OPO approach using ZGP is a scalable route for generating mid-IR radiation. In order to achieve high energy output, a pump source with wavelengths above 2.05 µm and excellent beam quality is required. Different laser configurations using Ho:YLF and Ho:YAG at room temperature have been used, but with thermal issues such as thermo-optical and mechanical properties being the major limitations. Cryogenic cooling results in drastic improvements in the thermos-optical and mechanical properties of YLF and YAG. Additionally, changes in the holmium ion population distribution creates a near four-level material, enabling larger mode volumes to be pumped and a significant reduction of the laser threshold in comparison to room temperature operation [1] , [2] . The value of cryogenic cooling was demonstrated as early as the 1980s, where Ho 3+ in YAG and YLF flash-pumped in co-doped scenario were reported [3] , [4] . High energy resonantly pumped Ho:YLF operating at cryogenic temperatures were reported by H. Fonnum et. al [5] , with

A theoretical model to describe in-band pumped holmium doped silica fiber lasers is presented. The model is established based upon theory and parameters obtained from published research works. Both core-pumped system and cladding-pumped system are simulated with the model. The simulation results agree well with most experimental results, and for the cases in which the simulation results show discrepancy with the experimental results the disagreement can be reasonably explained. Through numerical analysis it is found that besides common negative factors such as non-radiative decay, fiber loss, and non-optimized resonator configuration, energy transfer upconversion plays a deleterious role in the performance of in-band pumped holmium doped silica fiber lasers. In particular, inhomogeneous upconversion associated with ion clustering is found to cause significant degradation of slope efficiencies of lasers, a fact that is useful to understand the unsolved problem regarding the lower-than-expected slope efficiencies observed in previous studies of in-band pumped holmium doped fiber lasers.

The authors report on the performance and scalability of a Q-switched, cryogenically cooled Ho:YAG laser. The Ho:YAG slab was resonantly pumped using a continuous wave thulium fibre laser, and the output energy from the oscillator was extracted using a confocal resonator with a single-pass pump geometry, resulting in 135 mJ at 200 Hz with a beam quality of M2

We have investigated the power scaling of a Ho:YAG laser at room temperature. The laser is in-band pumped by two 1907 nm Tm-fiber lasers. A near confocal resonator design allows for 142 W power extraction.

We have investigated spectral broadening of a 2070 nm discrete mode diode laser by both direct modulation of the diode injection current, and external modulation of an electro-optical phase modulator. © Commonwealth of Australia

Consolidation of flame hydrolysis deposited silica soot with a 9.3 μm CO2 laser has been demonstrated. A range of laser parameters were investigated and the surface roughness of the resulting silica layers were characterised with a stylus profiler and white light interferometer. The surface roughness parameters were Ra = 68.9 nm, Rq = 83.8 nm perpendicular to the trajectory of the translated laser beam for a speed of 300 mm s-1 and an average laser power of 42.5 W, and Ra = 29.5 nm, Rq = 36.18 nm along the trajectory of the translated laser beam for a speed of 250 mm s-1 and an average laser power of 35.8 W.

There is a problem associated with various natural and man-made emergencies. The consequences of which can be colossal destruction in densely populated territories. Therefore, USAR teams, which include rescue robots, need to act quickly. However, due to the unpredictability of the situation, rescuers are faced with a number of difficulties due to which expensive equipment can suffer. To prevent the latter and optimize the work of robots, it is necessary to conduct testing in an environment close to reality. For many reasons, this cannot be done in real life, so there is a need to recreate the danger situations in format USAR in a simulation environment, designed to develop algorithms for the interaction of robots with a simulated environment.

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. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Many countries suffer from various natural disasters, including heavy rains, that are associated with further flood and landslide disasters. Based on our experiences of different disasters response, we develop a joint international operation framework for a disaster site management with distributed heterogeneous robotic teams that consist of unmanned aerial, ground, surface, and underwater vehicles. The artificial intelligence-based information collection system, which is targeting to become a worldwide standard, contains interaction protocols, thematic mapping approaches, and map fusion processes. The project provides a new working framework and control strategies for heterogeneous robotic teams’ cooperative behavior in sensing, monitoring, and mapping of flood and landslide disaster areas. In this paper, we present an overview of the system and a first stage toward robot interaction protocols development and the system modeling within robot operating system’s Gazebo environment.

Laser sources operating in the atmospheric transmission window between 2.1 and 2.25 μm are of interest for a range of applications (e.g. remote sensing, free-space communication and defence). Holmium fibre lasers provide good coverage of the lower part of this spectral region [1], but operation towards 2.2 μm is much more challenging due to a reduced emission cross-section and higher propagation losses associated with the IR absorption edge of fused silica. Furthermore, OH−-contamination can cause additional absorption around 2.2 μm. Here we investigate the limits of long-wavelength tuning of a holmium-doped fibre laser, and report, to the best of our knowledge, the first watt-level (4.1 W) output from a silica fibre laser operating at 2.2 μm.

In this paper, we describe a thermally guided fiber-rod amplifier and laser oscillator, which exploit thermal guiding to achieve mode control. This power scalable approach operates with mode sizes that are typically an order of magnitude larger than traditional single-mode fibers. Operating as an amplifier, we achieve a small-signal gain of 5 dB at 1030 nm whilst maintaining excellent beam quality (M2

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.

We report on a very highly doped and very highly efficient cladding pumped single mode Tm3+ silica fiber laser operating at 1.985 microns. The preform was fabricated using a proprietary all halide vapor phase modification of the MCVD process. The preform core has a maximum Tm2O3 doping concentration of greater than 8.5 wt%. A laser based on the resulting double clad fiber was pumped at 793 nm and demonstrated a record slope efficiency of 74.5 % wrt absorbed pump power.

We describe the first power scalable, Q-switched, cryogenically cooled, resonantly pumped, Ho:YAG laser oscillator. The output energy was extracted from a confocal resonator with a single-pass pump geometry. Pulsed operation at higher average powers will be reported.

In this paper we present a review of holmium-doped silica fibre based sources. We discuss recent demonstrations of an efficient cladding-pumped fibre geometry and the impact it has made on the power scaling of these sources. We discuss the wavelength region that is addressable by holmium-doped silica based devices and highlight the advantage over thulium-doped fibres in terms of atmospheric transmission. Finally we review the development and current status of the pulsed and CW operation of holmium fibre sources and discuss the future development potential of sources in the ns-fs pulse-width range.

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.

High energy sources operating around 2 μm are of interest in applications such as LIDAR and material processing and have also been used effectively as pump sources for high-energy mid-lR non-linear conversion, an established approach for generating mid-IR lasers. Commonly used bulk laser materials are Ho:YLF and Ho:YAG. However, at room temperature the quasi-three-level nature of these materials is a fundamental limitation, as it requires high pump power focused into small areas in order to reach transparency, leading to wave front distortions and optical damage.

We present a simple, low cost, technique for the rapid heating and fine control of the longitudinal temperature profile of an active optical fibre with metal outer coating. The novel combination of resistive (Joule) heating and a selective coating etching procedure allow near arbitrary thermal profiles to be imparted along the length of the active fibre. The low thermal mass of the active fibre results in very rapid heat-up times. Using this technique, fibre temperatures in excess of 400°C can be reached in seconds and efficiently applied directly to the active fibre.

Holmium fibre lasers are required for remote sensing, LIDAR and some medical applications [1]. In addition, pulsed Holmium fibre lasers also offer an attractive power scalable alternative to Ho:YAG and Ho:YLF lasers for pumping mid-infrared optical parametric oscillators (OPOs). In these applications it is necessary to operate at a high peak power (>20 kW) with minimal spectral broadening (

Custom spatial laser profiles such as annular beams have advantages over Gaussian beam profiles for applications including materials processing [1-3], fibre optic communications [4], reduction of nonlinear effects in fibre lasers [5] and optical trapping [6, 7]. Conversion to a desired higher order mode can be done using bulk optics [1-6], however, for some applications a monolithic, integrated solution is desirable. Existing techniques for fabricating structures with nanometer depth resolution onto the output facet of a fibre laser that are capable of high power operation can be slow [7] and sometimes difficult to implement due to critical alignment restrictions [8].

Fiber and bulk lasers form two distinct classes of solid-state laser, both of which have achieved tremendous success in various arenas, but they are not without their limitations. The long, thin geometry of a fiber allows excellent heat dissipation, which combined with a waveguiding structure provides stronger resilience to thermally-induced mode distortions than in bulk lasers, allowing diffraction-limited single-mode operation at multi-kW power levels in continuous-wave mode. However, the threshold for deleterious non-linear effects and laser-induced damage in fibers is generally much lower than bulk systems, owing to very tight beam confinement over a long interaction length, placing stringent limitations on pulsed performance in fibers.

We investigate the effects of coating thickness on the scattering losses of metal coated active fibre. A range of low numerical aperture metal coated optical fibres are placed in etchant solutions whilst measuring propagation loss as a function of time. By utilising concurrent coating diameter measurements, we are able to correlate propagation losses with coating thickness. Experimentally we find a monotonic dependence on coating thickness and scattering loss. We present the results of this work, providing useful parameters for high power metal coated fibre laser designs.

A significant issue for holmium-doped fiber lasers (HoDFLs) operating near 2 μm is multiphonon quenching due to the high phonon energy ~1100 cm-1 of the silica host, which complicates power scaling due to reduced lifetimes and increased heating. Nanoparticle (NP) doping is a new technique where the structure surrounding the Ho ions is developed chemically prior to doping into the silica core. We have incorporated Ho3+ ions into various NPs, such as LaF3, Al2O3 and Lu2O3, to shield them from the silica glass matrix. Results indicate slightly longer lifetimes with Ho:LaF3 NPs and the possibility of further improvement with oxide NPs. We report the first of lasing in a Ho:Lu2O3 NP-doped fiber pumped at 1.95 μm and operating at 2.09 μm with a record slope efficiency of 85.2%.

We present the coherent beam combination of four 2100 nm holmium amplifiers using direct digital synthesizer chip controlled acousto-optic modulators and a stochastic parallel gradient descent algorithm.

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.

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.

CO2 laser processing facilitates contamination free, rapid, precise and reproducible fabrication of devices for high power fibre laser applications. We present recent progress in fibre end-face preparation and cladding surface modification techniques. We demonstrate a fine feature CO2 laser process that yields topography significantly smaller than that achieved with typical mechanical cleaving processes. We also investigate the side processing of optical fibres for the fabrication of all-glass cladding light strippers and demonstrate extremely efficient cladding mode removal. We apply both techniques to fibres with complex designs containing multiple layers of doped and un-doped silica as well as shaped and circularly symmetric structures. Finally, we discuss the challenges and approaches to working with various fibre and glass-types.

For the first time, an electronically-controlled, wavelength-agile tuneable holmium-doped fibre laser is presented. A narrow-band acousto-optic tuneable filter was characterized and used as the wavelength selective element to avoid any inertial effects associated with opto-mechanical tuning mechanisms. We demonstrate operation over a 90 nm wavelength range spanning 2040 – 2130 nm. The laser produced >150 mW over this entire range with a signal-to-noise ratio of >45 dB and line-width of ~0.16 nm. Switching times of ~35 μs and sweep rates of up to 9 nm/ms were also demonstrated.

We present a novel metal coated triple clad active fibre design, utilising an all glass inner cladding structure and aluminium outer coating. This metal coated active fibre enables a number of benefits to high power laser design, such as increase robustness and extended operating temperature range. As a demonstration of the advantages of this design a passively cooled ytterbium fibre laser is presented. A 20 m length of active fibre was coiled into a planar arrangement and mounted onto a high emissivity heatsink. Up to 405 W of output power was achieved without the need for active water or forced air cooling. The slope efficiency of this source was 74 % and maximum outer heat sink temperature was ~140°C. This arrangement allowed for significant weight and size savings to be achieved with the active fibre laser head weighing less than 100 g. We will discuss the design choices and trade-offs of metal coated active fibre on high power fibre laser systems as well as the prospects for further power scaling to the kW level.

We investigate the potential for power scaling of 975 nm cw radiation from frequency doubling a monolithic, polarized thulium fiber laser in a periodically poled non-linear material.

We will review recent progress in the area of average power and peak power scaling of sources based around thulium and holmium doped fibre lasers.

We present the first high-resolution cryogenic absorption measurements of Ho:YAG. The results reveal previously unresolved structure in the absorption spectra. These measurements provide important data for the design of efficient resonantly pumped, cryogenically cooled Ho:YAG lasers.

A rapid, contamination free and highly precise optical fibre processing technique is presented. Based on CO2laser processing, this technique enables precision fibre cleaving, near arbitrary end face processing and fabrication of high power devices.

We introduce a novel metal coated active fibre design for use in high power fibre laser systems. Comparisons with polymer coated fibres as well as the prospects of kW level power scaling will be discussed.

We present a single-mode, 2.09 μm holmium-doped silica fibre laser resonantly pumped by a 1.95 μm thulium-doped fibre laser. The slope efficiency of 87% versus absorbed power is to our knowledge the highest reported to date.

There is currently no laser source that can provide both efficient excitation of mesospheric sodium and allow removal of the effects of guide-star elongation, as required for multi-conjugate adaptive optics in next-generation extremely large telescopes. We describe a Q-switched 1319 nm Nd:YAG laser that is mode-locked by injecting a short pulse into the Q-switched laser. It produces Q-switched mode-locked pulses that are synchronized to an external source. We also discuss how such pulses at 1064 nm and 1319 nm can be used to generate pulsed sodium-resonant guide stars via sum frequency generation, and thus satisfy the above requirements.

We have improved significantly the brightness of cryogenic Ho:YAG, reporting up to 65 W output power with a beam quality of M2

Ultra-short wavelength operation of a thulium fibre laser is investigated. Through use of core pumping and high feedback efficiency wavelength selection, a continuously-tunable fibre laser source operating from 1660 nm to 1720 nm is demonstrated in a silica host. We discuss the range of applications within this important wavelength band such as polymer materials processing and medical applications targeting characteristic C-H bond resonance peaks. As a demonstration of the power scalability of thulium fibre lasers in this band, fixed wavelength operation at 1726 nm with output power up 12.6 W and with slope efficiency > 60% is also shown.

Very high temperature operation of a rare earth doped fibre laser is investigated. An all-glass ytterbium-doped fibre laser is demonstrated, operating at heat sink temperatures ranging from 25°C to 400°C and output powers of > 40 W. Over this temperature range only a small reduction in laser output power was observed. The emission spectrum of this source was also measured for heat sink temperatures up to 400°C. We discuss the advantages of operating at such extended temperatures such as weight savings from reduced heat sink size, extended long wavelength operation and suppression of detrimental non-linear effects.

We will present recent results on thulium fibre lasers and their application to pumping fibre and solid-state based holmium-doped lasers and amplifiers. Resonantly pumped holmium-doped silica fibre devices utilising these pump sources will be discussed.