[IEEE 2006 Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science...
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Transverse Spatial Structures and OPCPA C. Tsangaris, P. Kinsler, G.H.C. New. Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BW, United Kingdom. [email protected] +44-20-7594-7520; Fax: +44-20-7594-7714; http://www.qols.ph.ic.ac.uk/ Abstract: A numerical model of OPCPA incorporating transverse effects indicates that spatial phenom- ena are of critical importance. The results differ radically from those of plane-wave codes, revealing complex beam profiles and significantly altered conversion efficiencies. c 2006 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers (230.4320) Nonlinear optical devices We have developeda numerical model of Optical Parametric Chirped Pulse Amplification (OPCPA) [1, 2] that takes transverse effects fully into account. In typical situations, the results reveal complex spatial behaviour that seriously impairs the efficiency of the process, and indicates that plane-wave codes cannot provide reliable predictions of per- formance in the laboratory. In view of the enormous potential of OPCPA for the generation of high-intensity optical pulses in the femtosecond and attosecond regimes, codes of this kind are likely to be of immense practical value. The efficiency of the code is enhanced using a “local”approximation in which all temporal points on the pump and (chirped) signal pulses evolve separately on the basis of their initial frequencies and the corresponding phase-matching conditions. This assumption, which amounts to ignoring group velocity dispersion, has been shown to be an excellent approximationin plane-wave OPCPA computations [3]. Our simulations typically evolve using a 50 50 spatial grid, with each of the diffraction, phase mismatch and walk-off processes being taken fully into account in the spatial frequency domain. Fig. 1. Transverse OPCPA simulation as described in the text, showing the transverse profiles for the case of a strong initial pump pulse interacting with a slightly angled signal pulse. The frames show (a) the initial pump profile, (b) the final pump profile, (c) the final signal profile, and (d) the final idler profile. The horizontal and vertical directions are covered by a 50 50 spatial grid, covering an area 8 10 4 m square. In the typical result presented in fig. 1, the pump beam undergoes strong asymmetric depletion during the amplification process, resulting in significant violation of a plane-wave model. The simulation models a 900 μJ 750 ps pump pulse at 547 nm which propagates as an extraordinary wave through 1.82 cm of LBO near the 12.22 phase matching angle for down conversion to 1054 nm. A 1 nJ 300 ps signal pulse chirped at 52.5 GHz/ps propagates as an ordinary wave in a direction close to the Poynting vector of the pump, being slanted at about 0.5 to the pump wavefront. Non-collinear phase matching then ensures that the idler beam is generated at 0.5 on the opposite side of the pump to the signal. The pump beam starts in the centre of the display (frame (a)), but it walks off to the left, and its heavily distorted profile at the end of the interaction is shown in frame (b). The signal beam, which is initially coincident with the pump, starts to follow the same path, but it is pulled to the right by the idler, and its intensity cross section at the exit face of the LBO is shown in frame (c). Initially, the idler grows quickly because of the near-perfect overlap of the signal with the pump. JThC59.pdf 1-55752-813-6/06/$25.00 ©2006 IEEE
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Transverse Spatial Structures and OPCPAC. Tsangaris, P. Kinsler, G.H.C. New.
Blackett Laboratory, Imperial College London, Prince Consort Road,London SW7 2BW, United Kingdom.
[email protected] +44-20-7594-7520; Fax: +44-20-7594-7714;http://www.qols.ph.ic.ac.uk/
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However, the idler wants to move to the right to satisfy the phase-matching conditions, and the consequent tendencyof the signal and idler to separate distorts their spatial profiles and reduces the efficiency of the interaction. Althoughthe final idler profile in frame (d) is clearly biased to the right, it has been held to the left of the frame by the pump andthe signal; indeed the signal and idler profiles appear as mirror images of each other, and this is the main observablefeature that reflects their opposite angular directions. Although the overall conversion efficiency in the configurationdescribed is better than for collinear propagation, the beam angling effects ensure that the process is still about fivetimes less efficient than under comparable plane wave conditions.
A wide range of results generated by the code will be presented. The detailed characteristics of the OPCPA process willbe explored under different conditions, and ways to optimise the system in both single and multi-state configurationswill be described.
References
1. A. Dubietis, G. Jonusauskas, A. Piskarskas, Opt. Comm. 88, 437 (1992).2. I.N. Ross, P. Matousek, K. Osvay, G.H.C. New, J. Opt. Soc. Am. B 19, 2945 (2002).3. E. J. Grace, C. L. Tsangaris, G. H. C. New, Opt. Comm., in press.
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