Chapter 5.5.3.3 - Second harmonic conversion of the phase-locked array

5.5.3.3 Second harmonic conversion of the phase-locked array The
frequency doubler for the 100-J / pulse phase-locked laser system is a scaled version
of that used for the 30-J / pulse 500-ns laser. The width of the 45^o beam shaping
prisms and the KD*P doubler crystals was increased to accommodate the four phase-
locked beams, side by side. A dichroic mirror outside of the laser enclosure is used to
separate the unconverted 1053-nm light from the 527-nm beam. As shown in the
optical layout of Fig. 5.27, the unconverted light is transmitted through the dichroic
and emerges as a 25 × 100-mm² array of four beamlets. The 527-nm output is
reformatted to a square 50 × 50-mm² array using a final anamorphic beam-reshaping
telescope. This provides a circularly symmetric far field central lobe for
the array of four phased-locked 12 × 50-mm² beamlets.

A typical external conversion efficiency of >60% was obtained at up to 25 J /
pulse with 500-ns pulse durations. Figure 5.34 shows an external frequency
conversion efficiency curve measured for a single beamlet using one doubler crystal
and two crystals in the alternating-Z geometry [41]. The measured efficiency
includes the reflective losses from the prism and KD*P surfaces. Figure 5.35 shows a
measured near-field profile at 527 nm, 50 J / pulse, and 3 Hz after reshaping by the
final anamorphic telescope. Slightly less energy is typically present in the outside

Figure 5.34. Measured second harmonic frequency conversion for one and two KD*P doubler crystals using a single beamlet. A conversion efficiency of >60% is readily achievable for the 500-ns, 25-J pulses.

two beams due to the roll-off of injected energy to the outside edges of the spatially
truncated Gaussian oscillator beam profile.

Just as for the previously described laser designs, the second harmonic converter
is not located in a phase-conjugated path so that any wavefront errors introduced by
the doubler crystals or by the 527-nm anamorphic telescope are not corrected. Figure
5.36 shows the measured far-field profile for the 50-J pulse shown in Fig. 5.35. Small
errors introduced by the doubler and telescope have increased the beam divergence
by ~ 1.5× from the diffraction limit. Detailed measurements have confirmed a
Strehl of ~ 0.5 for the 527-nm four-beam phase-locked output measured in single-
shot mode. When the laser is taken from single shot up to 3-Hz steady-state
operation, there is no measured increase in divergence for the 1053-nm beam.
However, we discovered an unanticipated effect in the second harmonic converter.
Weak linear absorption at 1053 nm (1.5–2% total) is known to introduce small
wavefront distortions in the KD*P at full average power. As described earlier, this
caused a ~ 0.2λ focus term on the output of the long-pulse single-beam laser system
that was easily corrected in a subsequent beam expansion telescope. However, in the
case of the four-beam phase locked laser, the dark stripes that can be seen between
each beamlet in the near field (Fig. 5.35) cause a ~ 0.2λ wavefront ripple across the
beam array which serves to break the far-field pattern into multiple lobes as shown in
Fig. 5.37b, reducing the on axis Strehl by an additional 2×.We initially believed that
the effect might be due to nonuniform heating in the crystals, causing piston phase
errors from beamlet to beamlet. However, numerical simulation of the thermal
effects in the KD*P have successfully explained the source of the wavefront error. It

Figure 5.35. The near field distribution of the frequency converted, 527 nm phase-locked beamlet array. A final anamorphic relay telescope on the output of the laser system transforms the 25 x 100-mm2 array shown in Fig. 5.31 to this 50 x 50-mm2 square array in order to generate a round central lobe in the far-field.

can be effectively simulated, as shown in Fig. 5.37c by placing a 0.2λ focus term on
each individual beamlet. Although the 527-nm brightness for the phase-locked array
was sufficient to support the laser’s intended mission, the ultimate solution for this
problem may be a laser design that places the frequency doubler in a phase-conjugated
beam path [7].

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Chapter 5.5.3.3 - Second harmonic conversion of the phase-locked array

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