Laser power and brightness scaling, in “eye safe” atmospheric transmission windows, is driving laser system research and development. High power lasers with good beam quality, at wavelength around 2.1 µm, are necessary for optical countermeasure applications. For such applications, focusing on efficiency and compactness of the system is mandatory. In order to cope with these requirements, one must consider the use of laser diodes which emit directly in the desired spectral region. The challenge for these diodes is to maintain a good beam quality factor as the output power increases. 2 µm diodes with excellent beam quality in both axes are available with output powers of ~100 mW. Therefore, in order to reach multi-watt of average output power, broad-area single emitters and beam combining becomes relevant. Different solutions have been implemented in the 1.9 to 2 µm wavelength range, one of which is to stack multiple emitter bars reaching more than one hundred watt, while another is a fibre coupled diode module. The beam propagation factor of these systems is too high for long atmospheric propagation applications.
Here we describe preliminary results on non-coherent beam combining of 2.1 µm high power Fabry-Perot GaSb laser diodes supplied by Brolis Semiconductors Ltd. First we evaluated single mode diodes (143 mW) with good beam quality (M2 < 1.5 for slow axis and < 1.1 for fast axis). Then we characterized broad-area single emitter diodes (808 mW) with an electrical-to-optical efficiency of 19 %. The emitter width was 90 µm with a cavity length of 1.5 mm. In our experiments we found that the slow axis multimode output beam consisted of two symmetric lobes with a total full width at half maximum (FWHM) divergence angle of 25 degrees, corresponding to a calculated beam quality factor of M2 = 25. The fast axis divergence was specified to be 44 degrees, with an expected beam quality factor close to the diffraction limit, which informed our selection of collimation lenses used in the experiment. We evaluated two broadband (1.8 - 3 µm) AR coated Geltech aspheric lenses with focal lengths of 1.87 mm and 4 mm, with numerical apertures of 0.85 and 0.56, respectively, as an initial collimation lens, followed by an additional cylindrical lens of focal length 100 mm for fully collimating the slow axis. Using D-shaped gold-coated mirrors, multiple single emitter beams are stacked in the fast axis direction with the objective that the combined beam has a beam propagation factor in the stacking direction close to the beam propagation factor of the slow axis of a single emitter, e.g. M2 of 20 to 25 in both axes. We further found that the output beam of a single emitter is highly linearly polarized along the slow axis, making it feasible to implement polarization beam combining techniques to increase the beam power by a factor two while maintaining the same beam quality.
Along with full beam characterization, a power scaling strategy towards a multi-watt output power beam combining laser system will be presented.
We report on recent progress in developing an industrially relevant, robust technique to bond dissimilar materials through ultra-fast microwelding. This technique is based on the use of a 5.9ps, 400kHz Trumpf laser operating at 1030nm. Tight focusing of the laser radiation at, or around, the interface between two materials allows for simultaneous absorption in both. This absorption rapidly, and locally, heats the material forming plasma from both materials. With suitable surface preparation this plasma can be confined to the interface region where it mixes, cools and forms a weld between the two materials.<p> </p>The use of ps pulses results in a short interaction time. This enables a bond to form whilst limiting the heat affected zone (HAZ) to a region of only a few hundred micrometres across. This small scale allows for the bonding of materials with highly dissimilar thermal properties, and in particular coefficients of thermal expansion e.g. glass-metal bonding.<p> </p>We report on our results for a range of material combinations including, Al-Bk7, Al-SiO2 and Nd:YAG-AlSi. Emphasis will be laid on the technical requirements for bonding including the required surface preparation of the two materials and on the laser parameters required. The quality of the resultant bonds are characterized through shear force measurements (where strengths equal to and exceeding equivalent adhesives will be presented). The lifetime of the welds is also discussed, paying particular attention to the results of thermal cycling tests.
Hollow waveguide technology is a route to efficient beam combining of multiple laser sources in a compact footprint. It is a technology appropriate for combining free-space or fibre-coupled beams generated by semiconductor, fibre or solidstate laser sources. This paper will present results of a breadboard mid-IR system comprising four laser sources combined using a hollow waveguide optical circuit. In this approach the individual dichroic beam combiner components are held in precision alignment slots in the hollow waveguide circuit and the different input wavelengths are guided between the components to a common output port. The hollow waveguide circuit is formed in the surface of a Macor (machinable glass-ceramic) substrate using precision CNC machining techniques. The hollow waveguides have fundamentally different propagation characteristics to solid core waveguides leading to transmission characteristics close to those of the atmosphere while still providing useful light guidance properties. The transmission efficiency and power handling of the hollow waveguide circuit can be designed to be very high across a broad waveband range. Three of the sources are quantum cascade lasers (QCLs), a semiconductor laser technology providing direct generation of midwave IR output. The combined beams provide 4.2 W of near diffraction-limited output co-boresighted to better than 20 µrad. High coupling efficiency into the waveguides is demonstrated, with negligible waveguide transmission losses. The overall transmission of the hollow waveguide beam combining optical circuit, weighted by the laser power at each wavelength, is 93%. This loss is dominated by the performance of the dichroic optics used to combine the beams.
We report on practical, industrially relevant, welding of optical components to themselves and aluminum alloy components. Weld formation is achieved through the tight focusing of a 5.9ps, 400kHz Trumpf laser operating at 1030nm. By selecting suitable surface preparation, clamping and laser parameters, the plasma can be confined, even with comparatively rough surfaces, by exploiting the melt properties of the glass. The short interaction time allows for a permanent weld to form between the two materials with heating limited to a region ~300 µm across. Practical application of these weld structures is typically limited due to the induced stress within the glass and, critically, the issues surrounding post-weld thermal expansion. We report on the measured strength of the weld, with a particular emphasis on laser parameters and surface preparation.
A simple scheme for efficient generation of two micron laser radiation is reported. Using a thulium fibre laser to pump a Q-switched Ho:YAG laser 31.7 W of average output power is achieved with an M<sup>2</sup> of 1.5 (an optical-to-optical conversion efficiency of 67% in terms of absorbed pump power) at a wavelength of 2.1 μm. A single-pass pump geometry is used, eliminating the risk of feedback of unabsorbed light into the fibre laser.
We investigate the use of volume Bragg gratings (VBGs) to achieve single longitudinal mode operation in a simple 3
mirror, diode pumped Nd:YVO4 Laser cavity at both 1064nm and 1342nm. A double VBG configuration is constructed
and shown to achieve single longitudinal mode operation with output powers of 2.3W and 2W at 1342nm and 1064nm respectively, the spectral performance of the dual VBG setup is also analyzed. In another configuration, we investigate the use of VBGs as wavelength selective elements in a dual-wavelength laser. Using this method we were able to achieve a combined output power of 3.55W at 1342 and 1064nm emission simultaneously, with a conversion efficiency of 18%.
We describe a beam combiner based on a cascade of dichroic components implemented in a hollow waveguide
integrated optic format. The approach results in a compact, rugged, optically robust solution for providing simultaneous
multi-wavelength emission in the near and mid-IR atmospheric transmission windows. In the approach the individual
dichroic beam combiner components are held in precision alignment slots in the hollow waveguide circuit and the
different input wavelengths are guided between the components to a common output port. The hollow waveguide circuit
and the alignment slots for the components are formed in the surface of a Macor (machinable glass-ceramic) substrate
using precision CNC machining techniques. The hollow waveguides have fundamentally different propagation
characteristics to solid core waveguides leading to transmission characteristics close to those of the atmosphere while
still providing useful light guidance properties. Because of the hollow nature of the core, the transmission efficiency and
power handling of the hollow waveguide circuit can be designed to be very high across a broad waveband range. The
specifications for the dichroic beam combiners and the design of the hollow waveguide circuit are described in relation
to a beam combiner for combining three QCLs emitting at 3.95 μm, 4.05 μm and 4.6 μm and a Ho:YAG laser emitting at
2.1 μm. Details of the design, modeling and manufacturing work, and preliminary assessments of the associated
components, are described.
Results are presented for generation of 1064 nm and 1573 nm outputs using a common ring resonator for a laser diodeside-pumped zig-zag geometry Nd:YAG laser slab and three NCPM (non-critically phase-matched) KTP crystals. The performance of the resonator at each wavelength is reported, for various configurations. First a common resonator was tested, then a separate resonator was used to gain an understanding of the performance variation with resonator length and OPO output coupling. A second common resonator was then tested, which had an optimized configuration to improve its efficiency. The conversion efficiency of the final design was 35% with 29 mJ output at 1573 nm wavelength for 83 mJ at 1064 nm.
Performance requirements for laser sources, operating in the mid-IR, providing protection of airborne platforms from
heat-seeking missiles are reviewed. The critical performance characteristic for a countermeasures laser is 'useful energy
on target', which requires the laser to generate high brightness output in the appropriate spectral bands with rapid turn-on
time. Integration with a compact beam director places an upper limit on the beam quality of the laser output. The key
driver for the detailed laser design is to maximise the overall wallplug efficiency in order to minimise the complexity and
volume, in turn maximising the reliability and reducing the cost. In particular routes to reduce the thermal management
system for the laser produce the single largest improvement in overall wallplug efficiency, with the ultimate goal of
realising a truly athermal laser. Candidate technologies for IR countermeasures lasers are briefly reviewed.
Results are presented for generation of visible and mid-IR output using a common fibre-based laser pump source. This
source comprised a master oscillator power amplifier (MOPA) configuration incorporating a semiconductor seed source.
Operation in the nanosecond and picosecond range is possible via use of the appropriate seed source. The MOPA is
capable of generating 100 W average power in an output beam with an M2 of 1.1. Here the MOPA was operated in the
nanosecond regime, using 100 ns seed pulses at a pulse repetition frequency of 100 kHz. 40 W each of pump power was
available for a frequency doubling and an OPO stage. 9.8 W of green light was generated in an output beam with an M<sup>2</sup>
of 1.2; using a degenerate PPLN OPO 12.7 W of broadband mid-IR output, with a FWHM linewidth in excess of 170
nm, was generated.
A simple scheme for efficient generation of mid-IR laser radiation is reported. Using a 50 W thulium fibre laser to pump
a Q-switched Ho:YAG laser 27.3 W of average output power is achieved with an M<sup>2</sup> of 1.5 (optical to optical conversion
efficiency 65%) at a wavelength of 2.1 μm. The holmium laser is used to pump a ZGP OPO generating 12.6 W of
average output power in the 3-5 μm waveband (optical to optical conversion efficiency 52%) with a worst case M<sup>2</sup> of
2.7. These efficiencies were maintained at 25% and 50% duty cycle operation.
Deformable bimorph mirrors with high damage threshold dielectric coatings are demonstrated as both intra- and extracavity
components with a pulsed energy diode-pumped Nd:YAG zigzag slab laser operating at the 150 mJ level. Two
resonator configurations were tested for intra-cavity operation; with a plane-plane resonator the far-field brightness was
enhanced by up to a factor of 1.5, whilst with a cross-Porro resonator an enhancement of up to 2.2 was achieved. As an
extracavity component far-field beam steering of ±2 mrad was demonstrated.
A simple scheme for efficient generation of mid-IR laser radiation is reported. Using a 50 W thulium fibre laser to pump a Q-switched Ho:YAG laser 27.3 W of average output power is achieved with an M<sup>2</sup> of 1.5 (optical to optical conversion efficiency 65%) at a wavelength of 2.1 μm. The holmium laser is used to pump a ZGP OPO generating 12.6 W of average output power in the 3-5 μm waveband (optical to optical conversion efficiency 52%) with a worst case M<sup>2</sup> of 2.7. These efficiencies were maintained at 25% and 50% duty cycle operation.
We describe the development of a time dependent thulium laser model. The model is used to predict both the CW and temporal behaviour of a Tm:YAG laser. Experimental results from a diode-pumped Tm:YAG laser are obtained and the model is used to obtain good agreement with these observations for both the CW and temporal behaviour of the laser. Particular results relate to switch-on time delays and the effect of pump diode modulation on Tm laser efficiency. The laser model has been extended to the case of the Ho:YAG laser where other important effects due to ground state depletion and self re-absorption must be taken into account. The holmium laser model has recently been used to predict reported experimental results from a thulium fibre laser pumped Ho:YAG laser.
Efficient generation of high average power mid-IR laser radiation is reported. Using a 50 W thulium fibre laser to pump
a Q-switched Ho:YAG laser 27.3 W of average output power is achieved with an M<sup>2</sup> of 1.5. The holmium laser is used
to pump a ZGP OPO generating 12.6 W of average output power in the 3-5 μm waveband with a worst case M<sup>2</sup> of 2.7.
A simple scheme for efficient generation of mid-IR laser radiation is demonstrated. Using a 25 W thulium fibre laser to pump a Q-switched Ho:YAG laser 15.3 W of average output power is achieved with an M<sup>2</sup> of 1.3. The holmium laser is used to pump a ZGP OPO generating 7.2 W of average output power in the 3-5 μm waveband with an M<sup>2</sup> of 1.8.
A laser diode end-pumped Q-switched Nd:YVO<sub>4</sub> laser operating over a 25% to 100% duty cycle range has been built and characterised. A maximum average output power of 24.5 W has been achieved at an optical-to-optical conversion efficiency of 43%, with a beam quality factor M<sup>2</sup> of 1.1.