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.
Radially-polarised beams are attracting growing interest owing to their unique properties and numerous applications. Power-scaling whilst preserving the polarisation-purity of radially-polarised beams is challenging, with efforts predominantly focused on cylindrically-symmetric systems.
We explore an alternative strategy for power-scaling radially-polarised beams using a thin-slab amplifier geometry, which, to the best of our knowledge, has not been previously investigated. We show that very high radial polarisation-purity can be maintained in an architecture that can be operated at high powers.
A radially-polarised seed-source was constructed using an Yb:YAG rod in a plane-parallel configuration, pumped by a capillary delivery-fiber which provided effective overlap with the LG01 mode. By tuning the cavity length and utilising thermally-induced birefringence, a robust multi-Watt LG01 mode was generated with an excellent radial polarisation-purity of 15dB and good beam quality M2=2.2.
The Yb:YAG slab was pumped by a diode-bar producing a highly-elongated inversion region. The seed was amplified in a double-pass configuration, using a cylindrical lens to spatially-match the inversion. The output beam was re-collimated by the cylindrical lens, and compensation for the Gouy phase-shift was made using a half-waveplate.
At 50W of incident pump power we obtained a small-signal gain of 7.5dB and a power gain of 4.5dB for 1.45W seed power. At maximum pump power the radial polarisation-purity was maintained at 15dB, and the beam quality only degraded slightly to M2=2.3. Further optimisation of slab design and pump geometry will be discussed in addition to power-scaling the system to higher output powers necessary for a range of applications.