Summary

Based on the post-EOM IMC scan results showing significant mode mismatch, we performed a detailed mode-matching analysis using the as-built HAM1 layout. Optics positions were identified from chamber photos, beam traces were reconstructed, and the expected IMC mode-matching and higher-order mode fractions were calculated. The analysis shows that the dominant source of the mode-mismatch degradation is a longitudinal shift of JM2 introduced by the EOM installation, and that moving JM2 can effectively recover good mode matching.

Procedure

• The positions of the HAM1 optics were identified from chamber photos.
  • The identified optics locations are marked with purple circles in the attached layout figure.
  • Since the JM2 position differs before and after the EOM installation, two circles are shown for JM2 (pre- and post-EOM positions).

• Using these as-built positions, we reconstructed the beam trace and calculated the mode matching into the IMC and the expected 2nd-order mode fraction.

  • This prediction is shown in the first plot.

  • The lensing effect of the EOM was included using the test results from the actually installed crystal (see p12 of slides).

• The measured 2nd-order mode fractions obtained from the IMC scan (reported in the previous post) are overlaid on the same plot for direct comparison.

Notes

• From the reconstructed layout, JM2 is shifted by approximately 2 inches between the pre- and post-EOM configurations.
  • Due to the reflection geometry, this corresponds to roughly 4 inches of effective beam path length change.
  • This shift is identified as the primary cause of the observed degradation in mode matching after the EOM installation.
  • The calculated 2nd-order mode fraction agrees well with the independently measured values from the IMC scan, validating the beam-trace model.
• The model indicates that shortening the total path length by approximately 6.3 inches optimizes the mode matching for 100 W input power as shown in the second plot.
  • This corresponds to moving JM2 by about 3 inches, which is feasible within the current layout.
  • Around the optimum, a JM2 placement error of ±1 inch degrades the mode matching by only ~1%, suggesting that achieving <1% mismatch is realistic from an installation-accuracy standpoint.
• The third plot shows that, when JM2 is placed at the optimal position, the expected change in mode matching over 0–100 W input power variation is at the ~0.1% level, indicating low sensitivity to power-dependent effects.

Result

• The dominant contributor to the post-EOM mode mismatch is the longitudinal shift of JM2 associated with the EOM installation.
• Moving JM2 is expected to be highly effective in recovering good mode matching to the IMC.
• If JM3 is replaced with a tip-tilt mount in the future, keeping its longitudinal position within ±0.5 inch would be desirable to avoid introducing additional mode mismatch.

Proposal (Practical Implementation)

1. Install one iris in the reflected beam immediately after JM3, and place another iris on the IOT2L table.
2. Change the longitudinal position of JM2.
   • The target position will be documented in a follow-up figure.
   • After moving JM2 and roughly passing the beam through the irises, fix the mount using dog clamps.
   • Replace the JM2 mirror at this stage.
3. Perform alignment using the iris just after JM3 and the iris after the periscope.
   • Use JM2 to center the beam on the first iris and JM3 to center on the output iris.
   • This procedure is expected to provide sufficient alignment accuracy; if needed, the IOT2 table can be used for further refinement.
4. Repeat the IMC scan measurement to confirm the improvement in mode matching.