Intra-cavity active optics in lasers W. Lubeigt, A. Kelly, V. Savitsky, D. Burns Institute of Photonics, University of Strathclyde Wolfson Centre,106 Rottenrow Glasgow G4 0NW, UK J. Gomes, G. Brown, D. Uttamchandani Centre for Microsystems and Photonics, EEE Department, University of Strathclyde, 204 George Street, Glasgow G1 1XW,UK M. Griffith and L. Laycock BAE Systems Advanced Technology Centre West Hanningfield Rd, Great Baddow Chelmsford, CM2 8HN, UK
Introduction Adaptive optics techniques primarily developed for improving the performance of astronomical telescopes by reducing the effects of (rapidly varying) atmospheric aberration Adaptive optics now seeing applications in many other areas, for instance: free-space optical communications retinal imaging / microscopy laser aberration correction In the recent years we have applied adaptive optics elements and techniques for aberration correction in laser resonators in: the steady-state, and transient domain Currently, apart from aiding the industrial application of AO in lasers and direct deployment in laboratory systems, we are moving more towards the laser applications of MEMS micro-mirror technology
Outline Why use adaptive optics in lasers? Steady state brightness optimisation of lasers Transient optimisation of lasers Introduction to MEMS micro-mirrors from the Centre for Microsystems and Photonics The use of MEMS micro-mirrors in solid-state lasers CW performance Q-switched performance Controlled array lasers
Why use adaptive optics in lasers? The main limitation to the performance of high power laser systems is the onset of thermally-induced aberrations In the simplest case as the pump power is increased the rod behaves like a lens and the cavity stability is affected this does not necessarily affect the performance, however, does limit the operational parameters e.g. dynamic stability can often be introduced Higher-order thermal aberration will ultimately limit the output brightness
Steady state adaptive control Unlike in astronomical systems, the hot laser is better suited to an optimisational aberration correction scheme Mirror shape change Intracavity laser mode is modified Thermal lens is modified Thermal distribution of the gain medium is modified In effect, we are constantly chasing our tail So, methodical optimisation is therefore required
Closed-loop feedback network Closed-loop AO-laser control Control PC + optimisation algorithm Mirror control hardware Deformable mirror (AO) X6 telescope LFS Laser output Laser 15mm KTP F=45mm Harmonic separator To photodiode
Example steady-state optimisation results End-pumped Nd:YVO4 laser End-pumped Nd:YVO4 laser Hill-climbing-based optimisation Side-pumped Nd:YAlO laser Genetic algorithm-based optimisation See Lubeigt et al. Optics Express 16, pp. 10943-10955 (2008)
Transient control of lasers Aim: reduce the time taken by the laser to reach its full brightness Temperature at a point in the pumped region of the gain medium Brightness Before optimisation After optimisation Time Time t Temperature build-up and brightness variation during transient period Solution: to vary the resonator parameters to track thermal lensing over time such that the size of the laser mode within the rod remains constant in effect, here we are only compensating for the first order of the thermal lens
Control elements used Mechanical translation stage (integrated to the control software) 31-element bimorph mirror Single element bimorph mirror
Example of transient optimisation of a Nd:YAG laser using the single element mirror Transverse mode profiles during the transient optimisation Fixed shape Time in ms 0 90 180 270 360 450 540 630 Change of mirror shape Warm-up time reduced by a factor of 3 here See Lubeigt et al. Optics Express 17, pp. 12057-12069 (2009)
Transition to the use of MEMS micro-mirrors inside laser cavities These demonstrations led to the development of a new robust bimorph mirror technology and its exploitation in industrial laser systems as a part of the DTI-funded INCAO programme see poster Recently, this expertise has been disseminated into other systems: Ultra-fast lasers at St Andrews University (W. Sibbett s group) Thin disk lasers at the IOP (A. Kemp) Also, a logical progression of this work is the implementation of MEMS micro-mirrors inside laser cavities Project in its early days MEMS micro-mirrors used were primarily developed for other applications
MEMS micro-mirrors Built using Silicon-On-Insulator foundry process from MEMSCAP Ltd and developed at the Centre for Microsystems and Photonics Scanning micro-mirrors with adjustable angular positioning on one or two dimensions according to the voltage applied Dimensions ranges from 0.3x0.3mm to 3x3mm Potential for micro-mirror arrays Low-cost technology Can be dielectrically or gold coated in order to ensure optimum reflection at wavelength desired (8 pairs of SiO 2 /Nb 2 O 5 to ensure R>99% at λ=1064nm) 2 types: Based on electro-thermal actuation Based on electro-static actuation
Electro-thermal MEMS mirror Mini tip-tilt mirror where the angular position varies with the voltage applied Maximum deflection 4.5º for 15V applied Has a DC response (the angular position can be kept constant over time)
Electro-static MEMS mirror Has no appreciable DC response: the angular position follows the frequency applied to the mirror (can range from 6 to 40kHz) This frequency applied must be reasonably close to a multiple of its main frequency response Maximum deflection +/- 10º for 200V applied 1mm
Temporal control of a Nd:YLF laser using an intra-cavity electro-thermal MEMS mirror 2-mirror Nd:YLF laser Stable CW operation of 20mW output power for a side-pump power of 18W (the power density on the micro-mirror surface was calculated at 50W/cm 2 ) However, fluctuation of the laser output power when the pump power is stronger. This is due to the strong surface distortion of the MEMS mirror
Temporal control of a Nd:YLF laser using an intra-cavity electro-thermal MEMS mirror The pump power is linearly increased as a function of time Output power (arbitrary units) Time (s) CW operation until 50W/cm 2 Rapid modulation of the laser oscillation up until a certain pump power (25W) No laser oscillation at all above thereafter
Temporal modulation of a Nd:YLF laser The MEMS mirror was inserted at the focus of a Nd:YLF laser cavity (beam size~ 100μm) The laser cavity stability was less sensitive to any thermallyinduced surface curvature of the MEMS micro-mirror Stable CW oscillation delivering 300mW of output power The micro-mirror sustained 20kW/cm 2 without damage
The routes to power scaling Use a wavelength where silicon is transparent (>1.2μm) Develop athermal MEMS mirrors Designing be-spoke coatings to reduce the bimorph effect Hybrid Au/dielectric coating Same dielectric coatings on both faces of the silicon chip
Remark Can this heat-induced deformation be used to our advantage? With a hot surface, we have (within a few ms) a curved mirror (ROC~few cms) Deformable mirror correcting for defocus here: Can reduce the laser warm-up time Potential for optical control of the laser operating at λ>1.2μm By shining a third part laser beam operating at a λ absorbed by the silicon (HeNe for instance)
Q-switching of a Nd:YLF laser cavity Voltage input: sinusoidal wave with amplitude ranging from 0 to 200V frequency varying from 5 to 40kHz At different pulse repetition frequency (ranging from 6kHz to 40kHz: 6, 10, 15, 30 and 40kHz ) resulting pulse duration ranging from 200ns to 1μs
Q-switching of a Nd:YLF laser cavity intensity in arbitrary units 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0-4 -3-2 -1 0 1 2 3 4 time in us Input signal: V p-p =200V f = 10kHz, P average =25mW Pulse duration: 200ns
Controlled array lasers Potential for pixellated lasers (i.e. lasers with multiple output beam with independent control)
Conclusion Successful steady-state and transient optimisations of solid-state lasers using intracavity deformable mirrors (for further details see our poster) Successful temporal modulation of Nd:YLF laser cavities using an intracavity MEMS micro-mirror from the Centre for Microsystems and Photonics 300mW CW Nd:YLF laser Nd:YLF laser Q-switched Output power limited by the heat-induced surface distortion of the MEMS micro-mirror The MEMS mirror sustained about 20kW/cm 2 without damage Potential for pixellated lasers i.e. lasers with multiple beam with independent temporal and spatial controls Future work: Use the MEMS in a laser cavity operating above 1.2μm Develop athermal MEMS micro-mirrors
Acknowledgement The work reported was partly funded: by the EMRS DTC established by the UK Ministry of Defence and run by a consortium of SELEX Sensors and Airborne Systems, Thales UK and Roke Manor Research and the DTI programme INCAO Thanks for your attention!