Reference beam setup/alignment

  1. If pellicule beamsplitter is out, put it in between the CCD and the last lens of the 4f system. The engraved side should be facing away from the CCD. It should be at as much of a 45 degree angle as possible, given the space constraints. Make sure the beamsplitter is not clipping the sample beam.
  2. Direct the reference beam towards the CCD using reference mirrors 1 and 2.
  3. With all ND filters out, adjust reference mirror 1 so that the reference beam transmitted through the pellicule and the sample beam reflected off of it hit the same spot in the near field (close to the pellicule).
  4. Adjust reference mirror 2 so that the two spots overlap in the far field (farther away from the pellicule).
    1. When the spots are very close to overlapping, it may be beneficial to stop down both beams so that the spot sizes are small. Just make sure the iris’s used are centered!
  5. Put the ND filters back in, and make final adjustments to mirror 1 so that the two beams overlap perfectly.
    1. If this part of the alignment is correct, you should have circular fringes with good contrast.
  6. Put a sample of fixed beads into the system (larger beads make alignment easier).
  7. Place a bead on the edge of the elastic excitation beam, diagonal from the center. Adjust BS1 and reference mirror 1 so that the tilt fringes are nulled in the interferogram.
  8. Move the bead to the center of the excitation beam. Adjust the BS and mirror until the frequency of the tilt fringes approximately doubles in both x and y directions.
  9. Put in two pinholes along the reference beam path to make future alignments easier.

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