Natalia Trela

Dual-axis beam correction for an array of single-mode diode laser emitters using a laser-written custom phase-plate

A single optical component for a diode laser bar combines fast-axis smile and lens error correction with slow-axis collimation. Produced by laser-machining/polishing, it provides 0.9 mm focal length, 200 microm pitch slow-axis collimation on the same surface that corrects fast-axis errors. Custom fabrication enables fill-factor optimization for the 49 single-mode beams and gives parallel collimation with rms pointing errors of 3% and 6% of the far-field divergence for the fast- and slow-axis array respectively. Sub-micron pitch mismatch between the slow-axis lens and emitter arrays, and beam pointing changes by thermal expansion of the laser bar are detected.

Locking and wavelength selection of an ultra-collimated single-mode diode laser bar by a volume holographic grating.

Wavelength-locking by a volume holographic grating (VHG) is reported for a diode laser bar with 49 single mode emitters, fitted with a dual-axis collimation phase-plate for smile elimination and excellent beam pointing correction. The much-improved VHG feedback with the ultra-collimated array beam gives 100% wavelength locking at 975 nm over a 17°C temperature range and external cavity lengths up to 110 mm. This enables a folded cavity configuration to provide a fully-locked array with wavelength selection into 200 pm channels over an 8 nm band, suitable for multi-bar dense wavelength-combining.

Over 55W of frequency doubled light at 530 nm pumped by an all-fiber diffraction limited picosecond fibre MOPA

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.

400W Yb:YAG planar waveguide laser using novel unstable resonators

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.

Development of edge pumped Yb:YAG planar waveguide lasers

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.

Field mappers for laser material processing

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.

Extending the locking range of VHG-stabilized diode laser bars using wavefront compensator phaseplates

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 bars 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.

Low-loss wavelength locking of a 49-element single mode diode laser bar with phase-plate beam correction

In previous work on laser diode optics, we have used laser-written phase-plates to correct the fabrication and fast-axis collimation errors of diode laser bars and produce a restoration of brightness of up to a factor of 10. Recently, we extended the technique to provide both fast-axis correction and slow-axis collimation in a single element, applied to an array of 10 tapered emitters with a 100µm pitch [1]. Here we report the use of dual-axis correction for a full-length diode bar from Bookham, with 49 single mode emitters and 30W cw output at 975 nm. Fast-axis smile and lens error correction is combined with laser-cut slow axis collimation to give highly parallel beams with far-field divergence of 2.6mrad (fast axis) and 13mrad (slow-axis). Fig 1 shows the improvement in fast-axis divergence, in the form of far-field pattern vs. emitter number.

Monolithic fast-axis collimation of diode laser stacks

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.

Reformatting linear beam arrays to hexagonal beam arrays using custom refractive micro-optics

Custom laser-cut refractive surfaces are used to reformat a 49-emitter single-mode laser-diode bar into seven groups of aperture-filled arrays of Gaussian beams. A circular far-field pattern was produced with near-symmetrical divergences and low M2 values.