Page 423 - High Power Laser Handbook

390 So l i d - S t at e La s e r s The National Ignition Facility Laser 391

assumption. Figures 14.30 and 14.31 show that the modeled and
measured focal spots are in good agreement. The encircled energies
and the FOPAI also agree well for both the 1-MJ and the 1.8-MJ CPP
spots.
The smallest speckle size in the patterns seen in Fig. 14.30 is the
diffraction limit of the final focusing lens: 2λ f/D = 15.4 μm. Although
the contrast of an ideal speckle pattern is unity, the measured focal
spots show contrast of 0.79 ± 0.02. We account for this lower value by
noting the presence of the SBS-suppression modulation (3-GHz mod-
ulation frequency, 30-GHz full-width, half-maximum [FWHM] band-
width at 1ω, 90 GHz at 3ω) and the chromatic dispersion in the
wedged final focusing lens. The lateral displacement in the focal
plane due to the lens chromatic dispersion is about 0.045 μm/GHz at
the third harmonic. When averaged over the pulse length, the shifted
speckle patterns add incoherently and reduce the contrast, in a pro-
cess analogous to SSD. This effect predicts a decrease in contrast to
0.84, which is in reasonably good agreement with the measurement.

14.6.2 Temporal Beam Conditioning with
One-Dimensional SSD
SSD consists of phase modulating the laser pulse and angularly dis-
persing its spectral content sufficiently to displace individual FM side
bands in the focal plane by at least half the speckle size, a condition
generally referred to as critical dispersion. 48,49 On NIF, the SSD modu-
lator runs at 17 GHz (ν mod ), and the 3ω lateral spectral displacement
at the target is 0.58 μm/GHz, which is comfortably beyond the criti-
cal dispersion value of 0.45 μm/GHz. This dispersion is provided by
a Littrow grating in the PAM, which is oriented so that the dispersion
direction is aligned along the short axis of the elliptical focal spot.
SSD bandwidths of up to ~150 GHz (1ω) can be produced by adjust-
ing the modulation index (δ) of the modulator (∆ν = 2δν mod ). The
1ω
maximum 1ω bandwidth in the tests reported here was measured to
be 95 ± 5 GHz.
Figure 14.32 compares the measured and calculated focal spots
with both CPP and SSD for the 1-MJ PQ shot and for one of the 1.8-MJ
ignition pulses. The time-averaged SSD focal spot was calculated by
performing a spectrally weighted incoherent sum of spatially trans-
lated non-SSD focal spots (Fig. 14.30), using the measured 3ω spec-
trum (Fig. 14.33). This spectrum includes both the 3-GHz-modulated
SBS-suppression bandwidth and the 17-GHz-modulated SSD band-
width. The calculations include both the lens chromatic dispersion
(horizontal in the figure) and the grating SSD dispersion (vertical).
The observed reduction in speckle contrast from 0.79 to 0.19 for the
1-MJ focal spot is equivalent to an incoherent average of ~28 speckles.
The speckle contrast for the 1.8-MJ spot decreased from 0.79 to 0.24,
which compares well with the calculated decrease to 0.26 and