We have previously presented a very low size, weight and power (SWaP) rugged coherent beam combiner assembly designed for LDEW systems. This module used our monolithic, freeform array optics to create a collimated, shaped and tiled array that allowed for greatly increased power-in-bucket (PiB) compared to traditional gaussian beam systems. We now present a design solution increasing the channel count from 7 to 61, with detailed modelling of the return loss and thermal effects when the entire module is subject to a total input optical power of 183kW. Furthermore, we propose the design of a high-power endcap fiber array to support the complete assembly which will allow for >3kW power level per channel.
Laser-Induced Periodic Surface Structures (LIPSS) can be produced using ultra-short (fs) pulsed lasers. The typical Gaussian profile of a single mode ultrafast fiber laser is not ideal, as the fluence across a spot varies. Laser beam shaping can reform the output spot into a square or rectangular shape with a flat-top profile, tailored to the fluence requirements of the application. We present initial results, surface nanotexturing of stainless steel at a rate of 6.8 cm2 s -1 and efficiency of 0.38 cm2 s −1 W−1 with a 20 W average power laser. Further optimisation will allow the full output power range of the laser to be used to maximise throughput and efficiency. Beam shaping also provides improvements to black marking throughput, where the typical processing speed with a Gaussian spot is less than 0.04 cm2 s −1 while shaped beam increases the productivity up to 0.44 cm2 s −1 .
We present design, simulation and benchtop demonstration of a beam combining system for use in coherently combined fiber arrays with >1kW per channel. A beam combiner assembly using laser-smoothed, monolithic freeform beam shaping and phase correction optics is designed and manufactured to meet the low-SWaP and high efficiency targets set for deployable LDEW systems. We report on achievable power-in-the bucket in coherently combined system, perchannel power handling capability and scalability to a larger number of channels.
Laser beam shaping is increasingly used in laser material processing, additive manufacture, and biomedical fields. Output profiles can tailored to the requirements of the application, and do not need to be limited to typical Gaussian or super Gaussian appearances. Beam shaping can be achieved through refractive or diffractive optical elements (ROEs / DOEs), or more complex approaches such as coherent beam combination, and multiplane light conversion [3]. ROEs can provide beam shaping solutions with high transmission and shaping efficiency in a single optical element, without the need for bulky or complex systems.
Freeform optics offer many advantages when performing complex beam shaping for directed energy applications. A number of design methods are presented utilizing radial basis functions, the Monge Ampere solution to optimal transport theory, as well as a novel phase wrapping approach. These methods allow for the precise manufacturing of singular optical elements such as collimation arrays and phase flatteners. This design study also outlines a modular solution in which multiple of these optics can be combined into a single, low-SWaP housing producing an array of aligned and collimated beams with a simulated power in the bucket of more than 80%.
Laser directed energy effectors combine the beams from several singlemode optical fiber amplifiers into a single beam with near diffraction-limited divergence. Coherent beam combination achieves this by tiling an aperture with individual beams and co-phasing these beams. Deployment on mobile platforms requires a rugged effector with low size, weight and power consumption. These constraints challenge beam combiner architectures based on discrete optics as power is scaled via channel count. We describe how monolithic arrays of freeform optics solve these problems by providing collimation, beamshaping and high fill-factor aperture tiling for large numbers of fiber channels in a rugged low-SWaP configuration.
The goal of deploying a high-power laser directed energy effector on a mobile platform creates several challenges beyond the primary requirement of high laser power. The pump sources and beam conditioning optics that are used in industrial lasers do not provide the low volume and mass required for deployment on a mobile platform. Similarly, use of conventional discrete spherical and aspherical optical elements does not provide the level of beam control and efficiency required to achieve the necessary on-target power and spot size. We describe how freeform optics are used to realize pump sources and beam combining systems with the high levels of optical performance and efficiency, coupled with low mass and volume, required to meet the low-SWaP targets set for deployable LDEW systems.
The native shape of the single-mode laser beam used for high power material processing applications is circular with a Gaussian intensity profile. Manufacturers are now demanding the ability to transform the intensity profile and shape to be compatible with a new generation of advanced processing applications that require much higher precision and control. We describe the design, fabrication and application of a dual-optic, beam-shaping system for single-mode laser sources, that transforms a Gaussian laser beam by remapping – hence field mapping - the intensity profile to create a wide variety of spot shapes including discs, donuts, XY separable and rotationally symmetric. The pair of optics transform the intensity distribution and subsequently flatten the phase of the beam, with spot sizes and depth of focus close to that of a diffraction limited beam. The field mapping approach to beam-shaping is a refractive solution that does not add speckle to the beam, making it ideal for use with single mode laser sources, moving beyond the limits of conventional field mapping in terms of spot size and achievable shapes. We describe a manufacturing process for refractive optics in fused silica that uses a freeform direct-write process that is especially suited for the fabrication of this type of freeform optic. The beam-shaper described above was manufactured in conventional UV-fused silica using this process. The fabrication process generates a smooth surface (<1nm RMS), leading to laser damage thresholds of greater than 100J/cm2, which is well matched to high power laser sources. Experimental verification of the dual-optic filed mapper is presented.
High power laser beamshapers based on lens arrays are widely used to generate square, rectangular or hexagonal flat-top far-field beam profiles. These devices can provide high efficiency and excellent brightness preservation, but offer a limited range of far-field profiles and can suffer from diffraction-related artefacts when used with spatially-coherent beams. Diffractive optical elements (DOE) offer a far wider range of far-field profiles, and better speckle behavior, but bring performance trade-offs in terms of brightness, efficiency, scattered power and residual zeroth-order power. Freeform refractive optics offer additional choices in the design of high power laser beamshapers. Freeform lens arrays offer a wider range of beam profile options than that available from catalogue lens array parts. Freeform field mapping beamshapers can generate a wide range of application-specific beam profiles with high efficiency and, where required, minimal reduction in brightness. More complex quasi-random freeform surfaces can act as a pseudorandom refractive intensity mapping element (PRIME), providing a level of beamshaper design flexibility closer to that of DOEs, but without the related high-order scatter and zeroth order leakage. We describe the design and implementation of these different types of refractive beam shaper in fused silica, using PowerPhotonic’s direct-write freeform fabrication process. This is ideal for use in high-power laser systems, where high damage threshold and low loss are essential. We compare and contrast the performance, benefits and limitations of these types of beamshaper, and describe how to select the ideal beamshaper type based on source coherence properties and application beam profile requirements.
Commercially-available QCW diode laser stacks with bar pitch below 0.5mm can now deliver source power densities exceeding 10kW/cm2. An increasing number of applications for these sources also specify high brightness, with collimation requirements ranging from equalization of fast and slow axis divergence to achieving fast-axis divergence within a small multiple of the diffraction limit. While collimation can be achieved by mounting an array of rod lenses in a frame with a suitable v-groove array, the resulting optical assembly has a large number of elements and associated adhesive bonds, and the size of the mounting frame limits the density at which stacks can be packed together. We present results exploiting an alternative approach using monolithic fast-axis collimator arrays. This approach greatly reduces the component count and minimizes the number of adhesive bonds required, providing a compact and rugged assembly well-suited to demanding applications. The monolithic collimator array also simplifies package design, and maximizes the achievable device stack packing density. Lens array properties may be tailored to generate applicationspecific divergence profiles or to match the geometry of individual stacks in order to achieve low divergence. Directwrite fabrication of these components allows mass-customization, offering a scalable, low-cost route to high volume collimation for fusion applications.
We describe the successful use of wavefront compensator phaseplates to extend the locking range of VHG-stabilized
diode laser bars by correcting the effects of imperfect source collimation. We first show that smile values of greater than
1μm peak to valley typically limit the achievable wavelength locking range, and that using wavefront compensation to
reduce the effective smile to below 0.5μm allows all emitters to be simultaneously locked, even for bars with standard
facet coatings, operating under conditions where the bar's natural lasing wavelength is over 9nm from the VHG locking
wavelength. We then show that, even under conditions of low smile, wavefront errors can limit the locking range and
locking efficiency, and that these limits can again be overcome by wavefront compensation. This allows wavelength
lock to be maintained over an increased range of diode temperature and drive current, without incurring the efficiency
loss that would be incurred by increasing grating strength. By integrating wavefront compensation into the slow-axis
collimator, we can achieve this high-brightness VHG-optimized beam in a compact optical system.
We present our work on a single optical component that combines fast-axis smile and lens error correction with slowaxis
collimation, produced with a laser-cut phase-plate technique. Customized focal length of slow-axis lenses allows
optimization of fill-factor of diode laser bars. In this paper we report on excellent beam properties obtained for a 49-
single mode emitter array, 975 nm bar providing 30W cw output, with RMS pointing error of 3% and 6% of far-field
divergence in fast- and slow-axis, respectively. We observed a pitch mismatch of 0.03% between the slow-axis lens
array, with emitter pointing clearly affected by chip thermal expansion.
A planar waveguide laser consisting of a 13mm x 12mm x 150μm Yb:YAG core with 1mm high sapphire claddings is
edge pumped using two 450W diode stacks with custom aberration correcting phase-plates. A plano-concave resonator
gives 400W average power in a low-order transverse, multi-longitudinal mode beam with 75% slope efficiency,
comparable to other thin disk and slab lasers. Transverse beam quality is improved through use of novel mode selective
toroidal laser-cut resonator mirrors, whilst lateral beam quality is improved through the use of an unstable resonator.
Uniform gain with an amplification of 3-4 per pass shows promise for amplifier operation.
We report the realisation of a high power, picosecond pulse source at 530 nm pumped by an all-fiber, single mode,
single polarisation, Yb-doped MOPA. The pump MOPA comprised of a gain switched seed source generating 20 ps
pulse source at a repetition frequency of 910 MHz followed by three amplification stages. Output power in excess of 100
W was obtained at 85% slope efficiency with respect to launched pump power at 975 nm. A 15mm long LBO crystal
was used to frequency double the single mode, single polarisation output of the fiber MOPA. To satisfy the phase
matching condition, the internal temperature of the LBO crystal was maintained at 1550C. Frequency doubled power in
excess of 55 W was obtained at 56% optical-to-optical conversion efficiency. Output power at 530 nm started to roll-off
after 50 W due to self-phase modulation (SPM) assisted spectral broadening of the fundamental light within the final
stage amplifier. Measured spectral bandwidth of the frequency doubled signal remained at ~0.4 nm with the increase in
fundamental power even though that of the fundamental increased steadily with output power and reached to a value of
0.9 nm at 100 W output power.
A waveguide with 150 μm core height of 2% Yb:YAG between sapphire claddings is core-pumped at 480W by diode
bars coupled into the 13 mm long edge-facet. The pump unit has custom correction of collimation errors and lens
aberrations. Using a 6mm width and 7° edge-facet angle, power is limited by competing ASE loss or parasitic oscillation
along TIR-trapped internal paths, giving 40 W output for stable and 25 W for unstable resonators. Ray-tracing shows a
20° facet angle is necessary to successfully out-couple ASE from the core. Preliminary operation at 90W and an
increased threshold for the parasitic oscillation are obtained.
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