Chapter 5.5 - A 100-J Laser System Usinf Four Phase-Locked Amplifiers

5.5 A 100-J LASER SYSTEM USING FOUR PHASE-LOCKED
AMPLIFIERS

An effective way of scaling the average power from a laser system is to coherently
combine the outputs from a number of separate amplifiers that are operated in
parallel. This allows the pulse energy to be increased without the need to increase
the size of individual gain elements. The technique is particularly attractive in the
case of solid-state laser systems since increasing the size of the amplifiers can
greatly reduce the ability to efficiently pump and cool the gain medium. For this
approach to be effective, the individual beams must be phase conjugated in such a
way as to maintain the correct relative temporal phase relationships in the high-
power output beam. This cannot be accomplished by using independent SBS phase
conjugate mirrors for each beam since the output beams would then have arbitrary
temporal phase alignment that would randomly vary from pulse to pulse. Each beam
could individually have a well-corrected output wavefront with the effective
correction of thermally induced distortions in the amplifiers. However, they could
not be coherently combined into a single beam composed of a phase-locked array of
beamlets that would propagate as if it had been amplified in a single optical aperture.
For this reason, a method of individually referencing the absolute phases of each of
the individually amplified beams must be achieved.

A number of approaches have been taken to the phase-locking of individual
beams using SBS. Basov first dealt with the problem when addressing a technique to
correct depolarization by dividing the two orthogonal polarization components into
separate beams and then referencing them together in a single SBS cell [51]. In order
to maintain sufficient overlap and mixing between the two beams, random phase
plates were used in front of the cell. Unfortunately, the strong aberrations from the
phase plates could not be completed, conjugated and removed from the output beam.
Other researchers have tried to overlap multiple beams at the focus of a simple SBS
cell, without using phase plates. This approach, however, is very sensitive to the
degree of beam overlap achieved, and phase-locking with adequate accuracy and
stability cannot be attained. An example of the latter approach is work in which
sections of a multi-segment flashlamp-pumped Cr:Nd:GSGG slab amplifier were
phase-locked [52]. Although the divergence from four segments of the amplifier was
shown to be significantly reduced from that expected from a single segment alone,
the overall beam quality of the phase-locked array was limited to 2.5 times the
diffraction limit.

The major concern in previous attempts to use SBS phase conjugation to
successfully phase-lock multiple amplifier aperture was to improve the overlap,
and hence nonlinear coupling, between the individual beams in the SBS cell.
However, based on our previously discussed experience with extending the
coherence length of the SBS Stokes output, we believed that the shortcoming of
these attempts were primarily caused by temporal phase fluctuations in the SBS
output which served to disrupt the phase-locking process. Based on this, the
incorporation of the SBS loop architecture into the multiple beam phase conjugate
mirror should significantly enhance the phase-locking peformance. This was also
recognized by other researchers who have demonstrated the very accurate and
stable low-power phase-locking of two laser beams [50]. The approach in that
work, however, included the use of a third phase conjugated reference beam. To
avoid the additional complexity and expense of a separate reference laser system,
we have devised an approach that uses only the beams to be phase-locked in a
modified SBS loop arrangement [53].

5.5.1 Design considerations for the phase-locked system

The requirements for the phase-locked laser system are 100 J / pulse in the long
500-ns FWHM pulse format. Just as with the single-beam, 500-ns laser system, the
pulses are required to have near-transform-limited temporal bandwidth and need to
be frequency-converted to the green (527 nm) with a goal of 50 J / pulse. Our
approach to this laser design was to scale the single-beam, long-pulse laser system
by adding an additional three parallel laser amplifier channels, thus increasing the
1053-nm output from 25 J / pulse to 100 J / pulse.

Figure 5.26. Conceptual design of the four-beam SBS phase-locked amplifier system. A

Figure 5.27. Detailed design of the phase-locked amplifier system. The four flashlamp-pumped Nd:glass amplifiers are placed on an optical bench separate from that for the lowpower injection laser and the second harmonic converter. This provides mechanical isolation from flashlamp percussion and vibration from the amplifier cooling water.

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