Chapter 5.5.2.3 - The four beam SBS loop phase conjugate mirror

5.5.2.3 The four beamSBS loop phase conjugate mirror As described
in the introduction, we believe that previous attempts to reliably phase-lock
multiple amplifier apertures using SBS were not entirely successful because of
the disruption caused by temporal phase drift and random phase jumps that have

Figure 5.29. (a) Top view of our first version of the four-beam SBS loop geometry. As shown in (b), the beams are reflected out of the plane of the four beams to form the loop. In this configuration (c), it is generally beneficial to block all but one beamlet before the third SBS pass for maximum phase-locking stability.

been observed in the Stokes output. For this reason, we adopted a variation of the
SBS loop architecture in which all four beams were focused into the same
volume of the nonlinear medium. In previous work, we had shown that the SBS
loop design stabilized the temporal phase for the single-beam laser amplifier
system. Based on the successful operation of the single beam 500-ns laser system,
we chose to use nitrogen at 90 atm as the nonlinear medium.

We took two different approaches to the details of the SBS loop phase-locking
architecture as shown in Figs. 5.29 and 5.30. In both cases, the four beams were
brought into the system, side by side, in a 1 × 4 linear array. For the first approach,
shown in Fig. 5.29, the four beams were reflected out of the plane of the 1 × 4 array
to form the SBS loop. In other words, the plane of the array was oriented at 90^o to the
plane of the SBS loop. Initial tests with this optical geometry did not exhibit the
stability of the temporal phase-locking that was needed. We found that the best
results were often achieved when three of the beamlets were blocked after the
second pass through the SBS cell and only the fourth was allowed to reflect back
through the cell and form the SBS loop, as shown in Fig. 5.29c. In this case the
fourth beamlet formed the reference wave to which the remaining three beams were
locked, in a manner similar to the architecture proposed by Ridley and Scott [50].

This approach was ultimately not satisfactory, however, since we observed that only
the reference beam received the full benefit of the threshold reduction provided by
the SBS loop four-wave interaction. Furthermore, optimal phase-locking
performance often required that a different beam of the four be used as the
reference from one operating period to another. It became a somewhat cumbersome
“shell game” to experimentally identify the preferred reference beam for each run.
Although our observations led us to believe that this was related to subtle differences
in the co-alignment of the beams through the cells and small changes in integrated
energy between the four channels, we did not determine with certainty the source of
this fluctuating behavior.

Our second approach to the SBS loop beam combiner, shown in Fig. 5.30,
completely eliminates the random behavior observed with the previous setup. Rather
than orient the SBS loop path and the plane containing the four input beams
orthogonally, they are made to overlap. In other words, all four input beams trace
their way through the loop architecture in a single plane. This requires a specially
shaped mirror that we have termed a “comb” mirror, shown in Fig. 5.30c. Vertical
slots are ultrasonically milled in previously polished and coated HR mirrors. This
alignment scheme provides not only beam overlap near focus, but also substantial
overlap between adjacent beam paths away from focus. Although the optical
architecture appears initially complex, its alignment is very straightforward. After
one beam path is established through the SBS loop, the additional three beams need
only to be pointed through the system using a far-field diagnostic looking at the
transmitted beams to ensure collinearity. In practice, this system is very insensitive
to relative beam alignment and provides consistent, long-term phase-locking
performance between the four beams. Optimal phase-locking performance requires
that the input beams only fall within 4–5 spot diameters between each other at focus
in the SBS cell.

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Chapter 5.5.2.3 - The four beam SBS loop phase conjugate mirror

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