Why phase modulating the lasers used in ICF

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Within the past few decades, several major Inertial Confinement Fusion (ICF) facilities using high power lasers have been built around the world. The two largest ICF lasers currently in operation, the Laser Mégajoule (in Bordeaux, France), and the National Ignition Facility (in Livermore, CA, USA), both generate enough energy (1015 Watts/cm2) to initiate the fusion reaction of two hydrogen isotopes, or to replicate the conditions of a nuclear explosion.

Figure1

Figure 1 : Inertial confinement fusion process

Courtesy of National Ignition Facility

When working under such harsh conditions, singular issues arise. Amongst them, the Stimulated Brillouin Scattering (SBS) is one of the most critical since it can induce catastrophic damage to the system at the end of the optical chain.

The SBS appears when an intense beam of light crosses a medium. The variation of the electric field creates acoustic phonons (by electrostriction or radiation pressure) which induce a Brillouin Scattering in the opposite direction of the incoming beam. As a result, above a given threshold most of the power of the incident beam is reflected back. Hence the inducing huge losses of optical power, as shown in Figure 2.

Figure2

Figure 2: Effect of SBS on the output power of a high power laser

The SBS threshold being proportional to the power spectral density, a spectral dispersion of the signal by sinusoidal phase modulation is performed to spread the energy over a broader spectral range.

Figure3

Figure 3: SBS threshold vs source linewidth for a given power

When the phase modulation is performed, the spacing between 2 neighboring wavelengths corresponds exactly to the phase modulation frequency. Therefore, it is required to use a higher phase modulation frequency than the SBS bandwidth (1 GHz) to mitigate this effect. This is the reason why most of the ICFs use a 3 GHz phase modulation frequency. For instance, the NIF and its former version, the NOVA laser use a  3 GHz phase modulation to spread the signal around 1053 nm, as can be seen in Figure 4.

Figure4

Figure 4: Signal at the output of the phase modulation as recorded by an ultra-high resolution ZOOM Spectra laser spectrum analyzer.

However as one can guess, performing a phase modulation, especially at such high power, is not straightforward. Hence, in a further article we will explain how to overcome this hurdle, and develop in more details the phase modulation theory.


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