A route to high power continuous-wave generation around 1.7 µm has been explored using an Er-only doped LMA fiber laser for high brightness core pumping of a thulium-doped fiber laser (TDFL). The maximum output power of 47 W with a slope efficiency of 80% was achieved at 1726 nm which, to the author’s best knowledge, is the highest recorded power in this wavelength region from a TDFL. The motivation for an Er-only pump source is scalability; these results show that this configuration has significant potential for further power scaling towards hundred-watt class systems at 1.7 µm and beyond.
The automated selection and stabilization of the transverse mode of a radially polarized Ho:YAG laser is reported. A convolutional neural network (CNN) was developed to analyze the modal composition of the laser output in real-time. Calculated error signals from the CNN are compared to the desired mode, allowing a PID control algorithm to dynamically optimize the position of an intracavity lens and therefore maintain desired modal content over pump power changes. This CNN based diagnostic system provides a fast method for selection and stabilization of transverse modes in order to advance radially polarized sources for applications such as laser processing.
Laguerre-Gaussian (LG) modes have properties that make them well suited to many applications, particularly laser processing when scaled to high-power. Here we present an approach for generating high-purity LG01 vortex beams in a Nd:YVO4 laser which overcomes the common problems of degenerate handedness and low damage thresholds in previous methods. The obtained modes are scaled in power by application of a novel Thermally-Guiding Fiber-Rod Amplifier (TGFRA).
Our approach is based on a novel end-pumping arrangement for efficiently generating the Hermite-Gaussian TEM01 mode in a 1064 nm Nd:YVO4 laser. A fiber-coupled laser diode was spliced to a 50:50 fiber splitter to give two equal outputs. These outputs were bonded to a bulk optic with a small separation for spatially matching the TEM01 mode. An astigmatic mode converter made using two concave mirrors was used to obtain a LG01 mode with controlled handedness.
The obtained LG01 mode was propagated through the 300 µm core of a 10 cm long sample of triple-clad Yb-doped silica fiber by utilizing thermal lensing as a waveguiding mechanism. The fiber was pumped using a high-power 915 nm diode laser. This amplifier geometry ensures preservation of the mode while the inheriting good thermal management from a fiber geometry and the large mode area common in rod geometries.
The 0.89 W LG01 seed-source was amplified with gain of 2.7 dB. The gain was limited by available pump power and the emission cross-section at 1064 nm. This result provides an avenue to high-power LG01 modes.
There are a number of spectral features around 1700 nm which are very attractive to the laser community. Strong C-H bond absorption and moderate water absorption lend this region to many applications such as polymer processing and laser surgery. Despite being a growing area of interest, development of high-power, laser sources in this region is quite challenging. Here we present preliminary results of in-band pumping a thulium-doped fiber laser (TDFL) with erbium-only doped fiber laser (EDFL) to generate 1700 nm
Erbium-only was chosen to avoid the power scaling limitations of co-doping with ytterbium. Parasitic lasing on the ytterbium band at 1030 nm results in efficiency roll-off and self-pulsing. Because of this, single-mode Er/Yb laser systems are generally limited to moderate powers of ~20W and reliability remains an issue.
An in-house double-clad large mode-area fiber, with a fundamental mode-field diameter of 20 um, was cladding pumped at 975 nm. Maximum power at 90 W launched was 31 W and the slope efficiency was 44.2% (35.4%) with respect to absorbed (launched) pump power. Suitability for high-brightness core pumping of a TDFL was confirmed with an M2 measurement of 1.1 ± 0.1.
For 1700 nm generation, the 1580 nm pump light was free-space coupled into an in-house TDF (0.2 wt.% dopant concentration). For a maximum launched power of 23 W, 15 W of 1726 nm output was generated with a slope efficiency of 67.1% (64.7%) with respect to absorbed (launched). To the author’s best knowledge, this is the highest recorded power in this wavelength region from a TDFL.
Laser and optical amplifier geometries may be split into categories such as rod and fiber. Rod gain media are susceptible to thermal effects at high power, whereas fiber suffer from detrimental non-linear effects due to their long length and small mode areas. Here we present an application of a hybrid architecture between the two geometries – the Thermally-Guiding Fiber-Rod (TGFR). The TGFR inherits the large mode area of the rod amplifier, the high surface area of a fiber, and exploits thermal lensing to guide modes.
We present a successful demonstration of amplification of a radially polarized mode using the TGFR. A 1030 nm continuous-wave radially polarized seed source of high purity and beam quality (M2=1.9±0.1) was constructed using thermal bifocussing in a Yb:YAG crystal to provide mode selection. This seed source was carefully focussed into the 300 µm core of a 10 cm long sample of commercially available triple-clad Yb-doped silica fiber in order to satisfy the thermal guidance condition and avoid waveguiding due to the refractive index step. The TGFR was pumped using a high power 915 nm diode laser.
The radially polarized mode was preserved through transmission of the TGFR. The output beam polarization was maintained at 99.1% purity while the M2 factor was measured to be 2.1±0.1. The maximum output power was 12.6 W of radially polarized light, corresponding to a gain of 7.0 dB limited by available pump power. This promising geometry the potential for further power scaling of radially-polarized beams for application in laser processing.
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. In this paper, we investigate an alternative laser geometry occupying a domain that lies between traditional fiber and bulk laser systems. This geometry comprises a fiber-based thin-rod structure, with a diameter on the order of several hundred microns, and with length on the order of several centimetres. The motivation is to combine the advantages of the fiber geometry for excellent thermal management and the bulk geometry for greater immunity to non-linear effects and optical damage, whilst elegantly controlling the laser mode profile using thermally-induced waveguiding.
Rare earth ion-doped silica is an excellent candidate to demonstrate the thermally-guided fiber-rod laser (TGFRL) due to its high fracture limit, positive thermo-optic coefficient and well-established fabrication which can produce high purity material with exceptionally low loss. A 300μm core diameter, triple-clad Yb-doped fiber is used to demonstrate the TGFRL. Thermally-induced waveguides can be tailored to have significantly larger transverse dimensions than conventional ‘engineered’ waveguides yielding potential performance benefits, especially in pulsed mode.
We will present results covering thermally-induced waveguiding, amplification performance and cw laser performance at 976nm and 1030nm, with >10W achieved at 1030nm with excellent beam quality, M2 < 1.1, and slope efficiencies approaching 50% with respect to absorbed pump power. We will also present preliminary results for amplification of radially-polarised beams, highlighting the potential of these devices in a range of applications.
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