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SB Transmission



SB Reflection

MB Transmission

MB Reflection



Testing for Vibrations

I strongly recommend that you read this whole sub-section on Testing for Vibrations before you proceed with setting up the interferometer. The first optical arrangement you will set up is a Michelson interferometer as shown in Figure 46. The interferometer produces a bulls-eye pattern on the screen S called an interference fringe pattern. Its purpose is to visibly detect, with your eyes, any vibrations that occur on the table surface that are emanating from the floor and any movements that can occur in an optical setup's components (the black fringes, as shown in Figure 46 and Figure 53, move if there are vibrations and/or component movements present). Once you understand what causes the interference fringe patterns to move, you will be able to avoid these vibrations and movements during your exposures. In a multi-beam optical setup, all components from the beamsplitter to the object scene and plate holder (including the beamsplitter, object scene, and plate holder), cannot move relative to each other during the exposure. In a single beam optical setup, no movement can occur between the object scene and the plate holder. The bottom line: if the fringe pattern is not moving, you'll have a successful hologram. Because of limited table space, having an interferometer set up during your exposures is not practical. You will now set up the Michelson interferometer.

Michelson interferometer setup image
  Figure 46: Michelson interferometer: laser (L), diverging lens (DL),
            beamsplitter (BS), mirrors (M1 & M2), and screen (S).

Your first objective is to make sure the laser's beam is parallel with the table surface as it travels around the table and through all the components to the screen. Viewing the optical arrangement in Figure 46, position the laser L at one end of the table's longest length and about 3 to 4 inches (7.6 - 10 cm) left of center on the table's width (the reason for this offset will be explained later). Turn the laser on. Allow the laser to warm up for 60 minutes. Adjust the height of the center of the laser's output aperture to 9 inches above the table surface by raising or lowering the laser on its mounting poles. You want to next make sure the height of the output beam at the aperture and at the end of the table furthest from the aperture are at the same height (9 inches) so that the beam is parallel with the table surface. Using Figure 46, place mirror M2 with its table mount at this far distance with the front surface of the mirror facing towards the laser and tie a piece of string around its mounting pole at 9 inches high. Move the mirror's mounting pole into the laser beam and then loosen the connector on the cross bar between the two laser table mounts to swivel the laser's front up or down until the beam hits the 9 inch high string on the mirror's pole. You may have to raise or lower and swivel the whole laser to get the aperture back to 9 inches. When you finally get the laser aperture and the laser beam at mirror M2's string location on its pole at 9 inches, make sure the laser's mounting connectors are all re-tightened. This all has to do with maintaining the laser beam parallel with the table surface which will maintain the direction of polarization of the beam throughout its journey all over the table. Re-check that the laser aperture and the beam dot on mirror M2 are at 9 inches above the table surface.

Now, again viewing Figure 46, place the beamsplitter (with a 50/50 fixed ratio) on the table so that its reflective surface is facing the incident laser beam and at a 45 degree angle that reflects part of the beam to where mirror M1 will be placed, then place mirror M1 in its position. The centers of the beamsplitter and two mirrors should be approximately 9 inches above the table and vertically perpendicular to their incident beam. Place the screen in its location with its center at approximately 9 inches and perpendicular to the incident beam. Leave the diverging lens DL out of the setup for now. You will be making some fine tuning adjustments to the three optical components (BS, M2, M1) in a moment.

As Figure 46 shows, one beam is transmitted through the beamsplitter to mirror M2 and the other beam is reflected at 90 degrees to the right to mirror M1.. The distance, or path length as it is called, between each mirror and the beamsplitter should be almost the same, to within 1/8 inch. If you make them both exactly the same, you're bulls-eye pattern may be distorted (this distortion can also be caused by the mirrors not being at right angles to the beamsplitter). These path lengths can be determined using a metal tape measure and should be as long as possible for the table size you're using. Mirror M1 is obviously the shortest path length in Figure 46 and you previously offset the position of the laser to the left of center on the table's width to allow M1's path length to be as long as possible. The interferometer's sensitivity increases the further the mirrors are from the beamsplitter.

Both mirrors should then reflect their beams back to the beamsplitter and strike the beamsplitter close to the original incident beam's position. Part of mirror M2's reflected beam will then be reflected by the beamsplitter to the screen S and part of mirror M1's reflected beam will be transmitted through the beamsplitter to the screen S. Two beam dots should be visible on the screen as shown in Figures 47 and 48. The screen S is a piece of 4 inch x 5 inch x 1/16 inch white mounting board placed in the plate holder.

Two beam dots on screen image    Two beam dots on screen image
    Figure 47: Two beam dots on screen                Figure 48: Two beam dots on screen
                  with room lights on.                                              with room lights off.

Before you fine tune and finish this setup, you need to apply the retro-reflection technique. This technique will insure that all the beams are parallel with the table surface as they travel the table from one component to the next.

Viewing Figure 49, you need to make sure the vertical plane of the beamsplitter is perpendicular with the table. Turn the beamsplitter's table mount so it reflects the beam back towards the laser aperture just 4-5 mm to its right or left (but not back down the laser tube) as shown in Figure 49 (retro-reflection). Adjust the beamsplitter in its holder so that its reflected beam is level with the horizontal central position of the laser aperture. Once you've done this, you can turn the beamsplitter back towards mirror M1.

Moving bulls-eye pattern up or down image
               Figure 49: Retro-reflecting laser beam
                             back to laser aperture.

You also need to apply the retro-reflection technique to mirrors M2 and M1 after you've used the technique on the beamsplitter. Adjust the reflected beam from M2 back through the beamsplitter to hit the laser housing at the same location shown in Figure 49. Do the same with mirror M1.

There are two reasons for doing this technique:

Once you're finished with the retro-reflection of mirrors M2 and M1, re-check their path lengths from the beamsplitter. Now, back to finishing your interferometer setup.

Your goal is to superimpose (overlap) these two dots so that the center of the interference fringe pattern (center of the bulls-eye) is visible as shown in Figure 53. The dots can be closely superimposed by grossly moving mirror M1 slightly up and down by adjusting the mirror mount in its connector and/or moving the lead base of the table mount (sideways). It is best at this point to have just one beam dot barely overlapping the other beam dot or having their edges touching. Try to get the two beams as horizontally level to each other as you can. In Figure 47, the left dot is slightly higher than the right dot. They should have been more horizontally level. This is not critical, but it will make your final adjustments a bit easier. You'll make these final adjustments using a fine adjustment technique described later in this sub-section Testing for Vibrations labeled Note: Fine Adjustments. (I strongly suggest you read this section now before proceeding to the next paragraph).

Once you get the two dots somewhat overlapped, you will now need to magnify the dots to see the fringe pattern. You do this by using a diverging lens (DL). A plano-concave lens with a -9 mm focal length and 9 mm diameter or a 20x microscope objective with a 8.33 mm focal length is suitable for this purpose. There are two possible positions in the optical setup where you can place the lens or objective: either between the beamsplitter and the screen, or between the laser and the beamsplitter. You will start by placing the diverging lens between the beamsplitter and the screen. This position will enlarge the dots significantly on the screen S and allow you to see the fringe pattern easily as shown in Figures 50 and 51. By enlarging the dots this way, you can easily adjust mirror M1 to help you completely overlap the two dots (later on, the final position of the diverging lens will be placed between the laser and beamsplitter to de-magnify the fringe pattern so you can see the whole, centered bulls-eye fringe pattern).

Magnified beam dots on screen image    Magnified beam dots on screen image
    Figure 50: Magnified beam dots with              Figure 51: Magnified beam dots with
        fringe patterns, room lights on.                        fringe patterns, room lights off.

Because of the huge magnification of the fringes with the lens at this position, it is not possible to see the whole bulls-eye pattern (which actually is not present yet). The fringe patterns you’re seeing in the above figures are the outer edges of the bulls-eye pattern, not the center of the bulls-eye pattern. Again, adjust mirror M1, using the fine adjustments technique, to get these two dots more overlapped. As you get them more overlapped and approach the center of the bulls-eye pattern, the fringes become fatter and less numerous as shown in Figure 52. Once you reach this point, let the table and components settle down for a few seconds so the fringe pattern is not moving or barely moving, then touch the table and watch the fringes move. You'll notice how very sensitive the interferometer is when the table is touched.

Large interference fringe patterns image
  Figure 52: Fringe patterns closer to the
        center of the bulls-eye pattern.

To see the whole bulls-eye pattern, you need to place the diverging lens between the laser and the beamsplitter, making sure the diverging beam passes through the beamsplitter and is reflected back to the beamsplitter from both mirrors to the screen. You can move the lens around until you see the divergent beam surrounding the beamsplitter on the screen as shown in Figure 53. You should now see the whole bulls-eye pattern within the beamsplitter's shadow on the screen. If you don’t, adjust mirror M1 until the bulls-eye pattern is centered using the fine adjustments technique. With the whole bulls-eye pattern visible, it is easier to analysis what is causing the fringe pattern to move, as discussed next.

Bulls-eye pattern centered image
           Figure 53: Bulls-eye interference fringe pattern centered.

These interference fringe patterns can be used to analyze any vibrations occurring on the table surface and/or any movement of the optical components. There are three types of fringe movement:

By studying the movements of your fringe patterns and relating the movements to causes in your environment and components, you'll get a good feel for what the most favorable conditions will be when making your exposures. The only thing you can't control is traffic driving past your house. This is why you should make your exposures in the evening after 10 pm. Bottom line: the quieter the table surface, the brighter the hologram.

Note: Fine Adjustments

To get the center of the bulls-eye pattern exactly in the center of the beamsplitter's shadow on the screen takes some practice and fine tuning. Seeing the center of the bulls-eye pattern in the center of the beamsplitter's shadow on the screen means you have both dots exactly overlapped. Make sure the diverging lens has been placed between the laser and beamsplitter when doing these fine adjustments which you will do after you have made your gross adjustments with the diverging lens between the beamsplitter and screen. Figure 53 shows how the bulls-eye pattern is no longer magnified with the lens in this position and you can see all of the bulls-eye pattern.

To get the bulls-eye centered, you should work with just one of the mirrors and its mount. Referring again to Figure 46, mirror M1 is your best choice. This is because if you move mirror M1 right or left, the pattern on the screen moves the same direction. This is also true when rotating the mirror up or down. The reason for this is because the reflected laser light from mirror M1 is transmitted through the beamsplitter to the screen. Because the reflected beam from mirror M2 is reflected (not transmitted) to the screen from the beamsplitter, right and left positions switch. Moving M2 right, the screen pattern moves left. Using M1 removes any possible confusion.

Before you start to move mirror M1, first look at the curvature of fringe pattern on the screen. The center of the bulls-eye is in the direction of the fringe pattern's inside concave curvature. As shown in Figure 54, if the concave curvature is on the right side of the fringes, so is the center of the bulls-eye. Additionally, the width of the fringes gets thicker in that direction.

Concave curvature image
    Figure 54: Concave curvature of the fringe pattern is to the right.

To move the center of the bulls-eye to the left, you need to move mirror M1 to the left. Move the mirror gently and slightly by tapping the front right side of the lead weight slightly with your index finger as shown in Figure 55.

Tapping lead weight image
                Figure 55: Moving mirror M1 left with index finger.

Repeat this tapping gently until you see the center, or portion of the center, of the bulls-eye as shown in Figure 56. Continue tapping until the bulls-eye is completely centered as shown in Figure 53. If the concave curvature started out to the left, then gently tap the front left side of the lead weight to move the bulls-eye pattern to the right and center.

Center of bulls-eye pattern moving left image
             Figure 56: Center of bulls-eye moving left with tapping.

If the concave curvature is facing upward, you need to rotate mirror M1 itself downward (clockwise in relation to its short rod) to move the center of the bulls-eye downward and get it vertically centered within the beamsplitter's shadow. To achieve this, hold the optical mount's base with one hand so it won't move (don't accidently move it yourself ! ), then grab the acrylic base of the mirror mount by its ends with your thumb and index finger, as shown in Figure 57, and rotate the whole mirror mount clockwise downwards around the 8-32 short rod bolt. This is a very tiny, slight rotation and is actually further tightening the mirror mount on the 8-32 bolt. This also assumes that mirror M1's table mount is on the left side of the mirror towards the laser. This also assumes that mirror M1's table mount is on the left side of the mirror towards the laser as shown in Figures 55 and 46.

If the curvature is facing downward, you need to rotate the mirror mount upward (counterclockwise in relation to its short rod). Again, this is a very slight rotation. Since you're rotating the mirror mount counterclockwise, you're actually loosing the mount from the 8-32 bolt in the short rod. You don't want to loosen the mount from the bolt so much that it is no longer rigidly attached to the bolt. If this starts to happen, move the mirror mount base clockwise until it is tight, then slightly loosen the short rod enough to slightly rotate the mirror upward (counterclockwise) until you can see some of the center of the bulls-eye pattern, then tighten the short rod. Now rotate the mirror mount base upward slightly until the bulls-eye pattern is centered horizontally.

Moving bulls-eye pattern up or down image
Figure 57: Rotating mirror up or down.

Sometimes the concave curvature faces a diagonal direction towards a corner of the beamsplitter shadow. In this case, you would have to make both left or right and up or down movements of the mirror.