Experimental measurements of frequency transfer function due to smoothing by spectral dispersion

Jacques Luce; Denis Penninckx

doi:10.1117/12.2003991

18 February 2013 Experimental measurements of frequency transfer function due to smoothing by spectral dispersion

Jacques Luce, Denis Penninckx

Proceedings Volume 8602, High Power Lasers for Fusion Research II; 860209 (2013) https://doi.org/10.1117/12.2003991
Event: SPIE LASE, 2013, San Francisco, California, United States

Abstract

In order to avoid propagation nonlinearities (Kerr effect, Raman and Brillouin scattering) and optical damage, nanosecond high power lasers such as the Laser MegaJoule (LMJ) amplify quasi-monochromatic pulses. But they generate a static speckle pattern in the focal spot. This speckle pattern needs to be smoothed in order to lower high intensity peaks which are detrimental during the propagation and the interaction with the plasma in the target. Different techniques are implemented to smooth the intensity nevertheless all high power lasers carry at least smoothing by spectral dispersion. It consists in broadening the spectrum through a phase modulator and focusing the different wavelengths at slightly different positions using a diffractive element such as a grating. In the temporal domain, it has been theoretically shown that the pulse power is thus filtered between near field and far field [1, 2]. The filtering allows techniques such as “picket fence” to increase conversion efficiency [1] and reduces detrimental effects of unwanted intensity distortions called FM-AM conversion [2, 3]. Here, to the best of our knowledge we show the first experimental measurement of the frequency transfer function of this filtering. Measurements are in perfect agreement with the numerical calculations.

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Jacques Luce and Denis Penninckx "Experimental measurements of frequency transfer function due to smoothing by spectral dispersion", Proc. SPIE 8602, High Power Lasers for Fusion Research II, 860209 (18 February 2013); https://doi.org/10.1117/12.2003991

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KEYWORDS

Modulation

Near field

Optical filters

Picosecond phenomena

Speckle pattern

Near field optics

Phase modulation

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