FM-to-AM conversion in high-power lasers

Abstract

FM-to-AM conversion is an important issue that could prevent fusion ignition with high-power lasers, such as the Laser MegaJoule (LMJ). We first overview the whole problem of FM-to-AM conversion in high-power lasers and we explain why AM spectral content of FM-to-AM conversion is important, although this information was not used in previous studies. We then propose simple analytical models to simulate FM-to-AM conversion in the LMJ frequency conversion system. We succeed in isolating every cause of spectrum distortion and give, for each of them, FM-to-AM predictions that are in very good agreement with simulations of a complex propagation code. Finally, we show how the last grating filters most of the FM-to-AM conversion. We conclude that the FM-to-AM conversion distortion criterion will be, on LMJ, below 40% in the last optics and 10% on the target.

Linear precompensation of FM-to-AM conversion due to frequency conversion system

“…In the KTP crystal, the main origin of FM-to-AM conversion is the nonlinear amplitude filter of the spectrum due to the group velocity difference between the fundamental and second harmonic waves. We have measured the resulting modulations due to the unique crystal, and we use the distortion criterion equal to twice the temporal contrast to quantify them [2]. The result is in excellent agreement with those obtained by numerically solving the nonlinear coupled wave equations [cf.…”

“…Intensity modulations due to linear spectral filters can be entirely and easily compensated with inverse transfer functions. Finally, for the LMJ, amplitude modulations are principally due to non-linear phenomena in the Frequency Conversion System (FCS) and thus the suppression of this FM-to-AM conversion seems to be particularly difficult but of major interest [2].…”

Research of beam smoothing technology on the technical integration line

“…(7) can also be deduced from Eq. (16) in reference [14] using the linear filter conception considering From fig.1, it is found as beam aperture increases, restriction on the free-space propagation distance is relaxed, which indicates inserting the dispersion grating in the image relayed planes with larger beam apertures. The cost and damage threshold of the dispersion grating are also important factors in considering the appropriate inserting place.…”

“…A purely frequencymodulated (FM) beam exhibits no amplitude modulation. However, if its spectrum is sheared, the frequency modulation can transfer to amplitude modulation; a process known as FM-to-AM conversion [13,14] . Herein, influence of SSD beam propagation on FM-to-AM conversion is investigated through theoretical analysis and experiment on the Technical Integration Line (TIL).…”

Progress on the suppression of FM-to-AM modulations in SG-III laser facility

“…Ideally, the process of pure phase modulation cannot involve the FM-to-AM modulations. Actually, because of many contributors, the FM converts to AM, which include group velocity dispersion (GVD), polarization mode dispersion (PMD), gain-narrowing effect [4] and weak etalon effect [5] and frequency conversion process [6], etc. The common basis for these effects caused FM-to-AM modulations is non-uniform spectral transmission in laser system.…”