Alignment

Basic procedure for laser alignment

1. Set the height of the diode laser, the center of the polarizing cube (beam splitter), the two-inch mirror, the quarter wave plates, the lens, and the three-inch mirror to be at the same height.
2. Viewing along the direction of the cube, the mirror and the wave plates, you should be able to see the target. Here target refers to the 4" mirror on top of the oven (for longitudinal laser beam path), or the side window on target (for transverse laser beam path). You might need to adjust the position of each post to compromise between transverse and longitudinal lasers.
3. Place diode alignment laser such that it passes through the center of the polarizing cube and the center of the target.
4. Rotate the cube and change the angle such that the back reflection hits the alignment laser and such that the reflection hits the diode laser simultaneously.
5. Place the focusing lens in its mount with the flat side away from the diode laser. Adjust it so that the back reflection goes back on itself.
6. Turn the diode laser on at LOW POWER.
7. Adjust the head of the diode laser such that the light goes through the center of the lens.
8. Rotate the cube mount such that the light passes through the center. Be sure that the cube is mounted so that the light enters the marked side and is reflected towards the two-inch mirror.
9. Turn off the diode laser.
10. Place the two-inch mirror into its mount and move such that the alignment laser hits the center.
11. Place a quarter wave plate between the cube and the two-inch mirror.
12. Turn the laser on at LOW POWER.
13. Adjust the position of quarter wave plate so that the laser passes through the center.
14. Adjust the two-inch mirror so that the reflected light hits the target.
15. If you know the axises of quarter wave plate, rotate it according to Figure . If not,
    • Rotate the quarter wave plate so that the light which passes through the cube from the two-inch mirror is a minimum.
    • Rotate the quarter wave plate forty-five degrees.
16. Adjust the two-inch mirror so that the light hits the target.
17. Check to see where the back reflection of the diode is hitting. It should be near the head of the diode laser but not directly on top of it. If it is then rotate the cube slightly and realign the two-inch mirror. Repeat if necessary. It is important to keep the back reflection away from the fiber since a small amount of back reflected laser could reach the diode through fiber and will damage the laser.
18. Place the three-inch mirror in its mount.
19. Check to see that the diode light is hitting the mirror.
20. Adjust the three-inch mirror so that the light is hitting the target.
21. Center a quarter wave plate in the path of each beam.
22. If you know the axises of quarter wave plate, rotate it according to Figure . If not, you need to do bench test first to find the axises of each quarter wave plate.
23. Repeat for all beams heading towards the target.
24. Be sure that the helicity of each beam line is the same.
25. Place the half wave plates in mounts, adjust the position of the post and the mounts such that all either (or six) laser beams are able to hit the center of the corresponding half wave plates simutanuously.
26. If you know the axises of half wave plate, rotate each one according to Figure . If not, you need to do bench test first to find the axises of each half wave plate.

Testing quarter (half) wave plates

• Find the direction of the axes:
  1. Use the laser beam which passes through the polarizing cube (beam splitter).
  2. Place the unknown wave plate in the beam.
  3. Place a polarizing cube after the unknown wave plate.
  4. Place a power meter perpendicular to the polarizing cube such that it measures the power of reflected light.
  5. Rotate the unknown wave plate such that the reflected light coming from the second cube is at a minimum.
  6. Now one of the axes is in horizontal direction and the other is in vertical direction.

• Make sure the axes of all quarter wave plates are identical (either fast or slow):
  1. Use the laser beam which passes through the polarizing cube (beam splitter).
  2. Measure the laser beam power P.
  3. Place the 1st quarter wave plate in the beam, rotate it such that one of the axes (marked axis) is in vertical direction.
  4. Rotate the 1st quarter wave plate by 45$^\circ$ clockwise.
  5. Place a 2nd quarter wave plate after the 1st one, rotate it such that one of the axes (marked axis) is in vertical direction.
  6. Rotate the 2nd quarter wave plate by 45$^\circ$ clockwise.
  7. Place a polarizing cube after the 2nd quarter wave plate.
  8. If the reflected light coming from the cube is at a minimum, then the marked axes of these two quarter wave plates are opposite (one is fast and the other is slow);
  9. If the reflected light coming from the cube is at a maximum and equals roughtly the full power P, then the marked axes of these two quarter wave plates are identical (both are fast or both are slow);
  10. If the reflected light coming from the cube is at a maximum and equals roughtly the half power P/2, one of the two wave plates is a half wave plate.

Some technical details

• One needs to be careful about backscattering light since it may damage the laser FAP (diode) if it is right on top of the fiber. However, if the backscattering light is far away from the fiber, it indicates that the system is not optimized and will affect the quality (intensity and polarization) of laser light to the target. The major part of backscattering light comes from the P light reflected from the 2'' mirror and the beam-splitter. To check position of backscattering light, one should use a backscattering test plate. Never use paper or any flammable material around fiber. Always turn off the laser when putting on or taking off the test plate.

• Adjust position of back-scattering spot vertically: Probablly the reflecting surfact of beam-splitter is not exactly perpendicular to the laser beam. Make sure the fiber, focuing lens and cube(beam-splitter) are at the same level, then rotate cube, at the same time adjust the 2'' mirror if necessary.

• Adjust position of back-scattering spot horizontally:
  1. Moving spot to the left (viewing towards fiber): Rotate cube holder clockwisely (viewing from the top), then rotate cube (relative to its holder) anticlockwisely. Keep the spot at the center of 2'' mirror. See Figure

  2. Moving spot to the right (viewing towards fiber): Rotate cube holder anticlockwisely, then rotate cube anticlockwisely. Keep the spot at the center of 2'' mirror.

  Figure: Adjusting back-scattering light