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.
We report a new route to obtaining custom freeform micro-optical components that is free from symmetry restrictions,
offering drastically lower cost and delivery times than what is required by other freeform manufacturing methods. We
describe how this process can be used to realize a complex custom optic using data generated directly from a design in
Zemax. This surface is then extracted from Zemax and fabricated using the LightForge service before being measured. A
quantitative analysis of the real optic is carried out both numerically and with the design source in Zemax, and we
present a comparison between design and fabricated part performance.
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 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.
This work relates to combining a phase corrected array of tapered laser diodes, emitting at λ = 975 nm, coherently
using the Talbot effect. Diffractive coupling of semiconductor lasers by use of the Talbot effect provides a means for
coherent beam addition of multiple elements in laser diode arrays and makes possible a very compact external
cavity. We have used, in this work, fully index guided tapered laser diodes. They contain a
ridge waveguide, which acts as a
modal filter, and a tapered section of increasing width, which provides high power. We have realized arrays of
several emitters (N=10), which are not optically coupled to each other. First, to improve the beam quality of the array, a phase correcting micro system, achieving collimation in the fast
axis, correction of the wave front tilts in both directions and also a slow axis collimation, was added. The FWHM
divergences of the array were reduced from 34 ° to 0.17 ° in the fast-axis and from 3.5 ° to 0.7 ° in the slow-axis at
6A, 3.7 W. Then, to be close to diffraction limit, we have combined this corrected array coherently using the Talbot effect. We
have obtained quasi-monolobe slow axis far field profile for the in phase mode with a central peak divergence of
only 0.27 ° at 1.5 A, 315 mW under CW operation and of only 0.20 ° at 2.5 A, 787 mW under pulsed operation.
We demonstrate a technique for collimating conduction-cooled QCW diode laser stacks that achieves very high
brightness in a compact and robust package. First we collimate the bars in the fast-axis using a pre-aligned array of
Doric GRIN cylindrical lenses, where each lens is oriented to correct the gross positioning errors of each bar. We then
measure the residual beam errors using a proprietary wavefront mapping system, and fabricate a refractive wavefront
correction phaseplate to effectively flatten the wavefront. The type of lens used exhibits particularly low aberration in
the presence of misalignment, ensuring that the resultant wavefront can be effectively corrected. We applied the
technique to two 0.5 mm pitch, 12-bar stacks operating at 1.2 kW. By this method, we repeatably obtained a 10-fold
increase in stack brightness, reducing fast-axis beam divergence for the entire stack to below 0.3°, close to the theoretical
limit. The result is an extremely compact, high brightness source optimised for side-pumping thin slab lasers.
A 1.8kW diode laser source made up of 100 W diode bars is designed for fibre delivery, using two new techniques to
enhance the delivered brightness and equalise the beam-parameter product. Custom corrective phase plates in laser-cut
silica are attached permanently to the two stacks of ten bars, correcting bar smile and restoring a factor of 2.5 in lost
brightness. The two units are beam-compacted and polarisation coupled to a single array beam. As a final step, a novel
confocal beam-slicer produces five segments from the slow-axis beam profile and stacks the segments in the fast-axis
direction.
This work relates to combining a phase corrected array of tapered laser diodes, emitting at λ = 975 nm, coherently
using the Talbot effect. Diffractive coupling of semiconductor lasers by use of the Talbot effect provides a means for
coherent beam addition of multiple elements in laser diode arrays and makes possible a very compact external
cavity. We have used, in this work, fully index guided tapered laser diodes. They contain a
ridge waveguide, which acts as a
modal filter, and a tapered section of increasing width, which provides high power. We have realized arrays of
several emitters (N=10), which are not optically coupled to each other. First, to improve the beam quality of the array, a phase correcting micro system, achieving collimation in the fast
axis, correction of the wave front tilts in both directions and also a slow axis collimation, was added. The FWHM
divergences of the array were reduced from 34° to 0.17° in the fast-axis and from 3.5° to 0.7° in the slow-axis at
6A, 3.7 W. Then, to be close to diffraction limit, we have combined this corrected array coherently using the Talbot effect. We
have obtained quasi-monolobe slow axis far field profile for the in phase mode with a central peak divergence of
only 0.27° at 1.5 A, 315 mW under CW operation and of only 0.20° at 2.5 A, 787 mW under pulsed operation.
We describe an extension of the active homodyne technique to produce stabilised, (pi) /2 radian phase steps in a full-field interferometer. We have implemented the technique in a single mode fibre optic fringe- projector for shape measurement. The interference intensity (and hence phase) is maintained constant by feedback control to a phase modulator in one of the fibre arms. The feedback system has been modified to produce (pi) /2 radian phase steps in the projected fringes. The phase stability and accuracy of each step was measured to be 17 milliradians in a 50 Hz bandwidth. The technique can be used to produce stabilised phase-stepped images in any fibre or bulk optic interferometer where active homodyne feedback control can be implemented.
We describe the use of a four-core optical fibre as the basis of a sensor capable of measuring the angle through which the fibre is bent in two dimensions. The intended application of the sensor is in measuring the shape of flexible structures.
Optical fibre interferometric strain sensors embedded into structures offer a very accurate and robust method for shape measurement [1]. Many schemes have been demonstrated in which strain and/or temperature in a structure are inferred from monochromatic optical phase delay [2].
We demonstrate temperature-insensitive strain measurement in a carbon fiber composite panel using a sensor based on broad-band interferometry in highly-birefringent optical fiber. The sensing element forms an unbalanced Fabry-Perot cavity in the measurement arm of a tandem interferometer. This is interrogated using an LED source and a scanning Michelson interferometer, producing three distinct interferograms, two of which relate to the group delay (GD) of the eigenmodes of the sensing element, the other providing a zero-OPD reference in the scanning interferometer. We measure the GD of each interferogram by dispersive Fourier-transform spectroscopy. Changes in strain and temperature in the measurement fiber affect the group delays of the sensing interferograms, but do not affect the zero-OPD interferograms, which is therefore used as the origin for group delay measurements. We determine a linear transformation relating the measured group delays to strain and temperature. Inverting this transformation then provides a means of recovering strain and temperature from measurements of group delay. We apply this technique to the simultaneous measurement of strain and temperature in the composite panel. Typical measurement errors are 7 microsecond(s) train and 0.7 K. The measured values are independent, and the strain values show no evidence of thermal-apparent strain.
Optical fibre interferometers have been widely used to measure strain. Most of the interferometers used are also sensitive to temperature. These two parameters can be distinguished by measuring phase for two optical modes or for two wavelengths. A linear transformation converts the two values of phase into independent values for strain and temperature [1], but this transformation is usually ill-conditioned, and so magnifies measurement errors.
A novel intrinsic optical fiber sensor for detection of acoustic emission (AE) has been designed, built, and tested. This sensor is intended for use in monitoring machine tool wear. An optical fiber is held between the transducer backing and the surface being probed. The coupling of AE waves through the fiber into the backing causes a phase change in the light transmitted by the fiber. This phase change is detected using a Mach-Zehnder interferometer, locked in quadrature with a phase servo. Acoustic emission during cutting is conveyed to the sensing fiber on the machine bed via either the tool or workpiece. Sensing of AE may then yield information on the wear state of the tool.
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