Analysis on FM-to-AM conversion induced by spatial filter in a high-power laser system

Abstract

In order to avoid stimulated Brillouin scattering and smooth the final focal spot that reaches the D-T capsule, spectral broadening is essential. However, the modulation of spectral structure might result in frequency-to-amplitude modulation (FM-to-AM) conversion. The spatial filter pinhole is used to cut off the high-frequency transmission laser, and to ensure the desired pass. Improper parameter of pinhole would lead a negative impact. Using the spatial filter pinhole, we analyze the characteristic of intensity modulation using several pinholes of improper parameters. We then compare these results with the intensity modulation obtained from an experiment. Experimental diagnosis and design of an appropriate pinhole parameter would be highly beneficial to the field of high-power lasers.OCIS codes: 050.5080, 070.2615050.5080, 070. , 220.4840, 300.1030 To ensure the stability of implosion experiment in ICF, a high-quality temporal pulse shape laser is necessary. Some problems, such as how to control temporal profile and frequency-to-amplitude modulation (FMto-AM) conversion [1] , must be studied in detail. In the high-power laser system, the laser spectrum is broadened for two reasons to avoid stimulated Brillouin scattering [2] (SBS) and to "smooth" the final focal spot that reaches the D-T capsule. First, without spectral broadening, spectral power density of ignition laser pulses would exceed the transverse Brillouin threshold in optics at the end of the amplifier chain, thereby severely damaging the optics components. Spectral broadening spreads energy to different wavelengths to reduce the spectral power density maximum level below the SBS threshold. Second, nonuniformities in the laser irradiation may seed the Rayleigh-Taylor hydrodynamic instability, which degrades target performance. Therefore, high spatial homogeneity [3] is desired for focal spots. In fact, the focal spot of the laser beam is shaped by a phase plate and appears as a speckle pattern with hot spots. The intensity is time-averaged through spectral broadening and a dispersion system (grating) that moves the hot spots during the pulse. This scheme is called smoothing by spectral dispersion [4] . Although the required spectral broadening adopted by the aforementioned techniques are generated by a pure phase modulation that would not impact the spatial intensity distribution directly, some optical components, including fibers, amplifier media, and spatial filters, could slightly affect the spectrum. The consequences of such filters are a partial conversion of FM into intensity modulation called FM-to-AM conversion. Hocquet and Yang analyzed the factors resulting in FM-to-AM conversion in detail, including group velocity dispersion, gain narrowing, frequency doubling, grating, and so on [5−7] . Unfortunately, to the best of our knowledge, the conversation induced by the sideband cutoff of a spatial filter pinhole has not yet been discussed. Spatial filter is a necessary element in high-power laser systems, undertaking beam expanding, image tran...

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Theoretical analysis of frequency modulation-to-amplitude modulation on the final optics and target of the SG II-Up laser facility

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FM-to-AM modulations induced by a weak residual reflection stack of sine-modulated pulses in inertial confinement fusion laser systems

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