High-power lasers at 2 µm are required for optical countermeasure applications, but can also be used for processing of materials where the mid-infrared laser wavelength provides an advantage. These applications can benefit from power scaling the 2 µm output, with the requirement to maintain a compact footprint. In particular, a Tm-doped slab laser design can be very compact, as an alternative to high power Tm:fiber lasers at 1.9 µm. It can be used directly for modulated continuous-wave output or for pumping Ho-doped lasers and amplifiers that emit at 2.1 µm.
We have directly compared Tm:YLF and Tm:LLF slab crystals (1.5 mm x 11 mm x 20 mm), in an otherwise identical diode end-pumped laser configuration, to evaluate the power scaling to 150 W of these two related materials. We will present the analysis of the thermal lens behaviour of that could not be fully supressed for Tm:LLF in the slab architecture when pumped at 450 W of incident pump power from the high-brightness 793 nm laser diode stack (Lasertel T6 Diode). Further power scaling to the 300 W output power level of Tm:YLF in a dual-end-pumped slab laser configuration will be presented, in which parasitic internal lasing has been supressed through careful consideration of the slab geometry.
The improved Tm:YLF laser will be used to pump a Ho:YAG slab (1.5 mm x 10 mm x 55 mm) to amplify seed pulses from a nanosecond Q-switched oscillator. A spatially and temporally resolved model has been developed to determine the optimal pump configuration and crystal dimensions to amplify seed pulses from 7 W average power at 10 kHz repetition rate, to upwards of 150 W at 2.1 µm. The model is based on rate equations and determines the distribution of thermal load throughout the crystal, permitting accurate prediction of saturation- and thermal-induced aberrations in the amplifier.
A simulation method is proposed which approximates the atmospheric beam path as an extremely large aperture hollow waveguide containing a numerically sufficient subset of linearly polarized lossless propagation modes. The proposed method is shown to agree numerically with the standard angular spectrum of plane waves method in a single transverse dimension simulation and is readily expandable into two transverse dimensions but currently limited by available hardware. The method is expanded to involve coupling matrices describing the transition through each phase screen and the intermediate free-space portions. The coupling matrices are combined into a single multimode coupling matrix describing the propagation for one instance of atmosphere. The proposed matrix method has potential to evaluate multiple input beams simultaneously or condense high turbulence simulations requiring many phase screens. The input beam can be constructed by multiplying the decomposition of the desired output profile with the pseudo-inverse of the coupling matrix; however, the matrix cannot be realized in experiment. Therefore, the opportunity for beam shaping to compensate optical turbulence is evaluated by principal component analysis of the compound coupling matrix. It is shown that an average of the lowest order eigenmode across multiple simulations produces a super-Gaussian-like beam with improved power delivery and stability. The implication for optical countermeasure beam control is the potential to create any desired beam shape at the target plane.
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.
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 tunable optically pumped HBr laser has been demonstrated for the first time. As pump source for the HBr oscillator,
we developed a single-frequency Ho:YLF laser- amplifier system which was locked to the 2064 nm absorption line of
HBr. Through the implementation of an intra-cavity diffraction grating, laser oscillation was demonstrated on nineteen
molecular transition lines including both the R-branch (3870 nm to 4015 nm) and the P-branch (4070 nm to 4453 nm).
The highest output energy for the given input energy was 2.4 mJ at 4133 nm.
The second moment method of laser beam propagation allows for the calculation of the beam quality factor for any
laser beam, or combination of laser beams. When several laser beams are added, their effective beam quality factor is
not simply the sum of the individual beam quality factors, that is, it does not act as a linear operator. In this paper we
derive an analytical expression for the beam quality factor of incoherently added laser beams whose centroids are not
collinear. We illustrate the versatility of the final result by showing how this may be applied to the problem of the laser
beam propagation characteristics of high power diode bar stacks.