Circle To Point Converters

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A Simple How To

See the paper on Circle-to-point conversion and Optical rotary joints for more information.

Film Specs Exposure Processing Cap & Grind
Mixture: 10-30-350

Spin Speed: 88 rpm
Relative Humidity: 75% +
Incubation: 3 hrs
Temp: 96 deg F +

Wavelength: 488 nm

Copy Angle: -2 deg off normal
Exposure time: 55 sec for each ring
Energy in mj: 30
Room Humidity: 58%
Room Temp: 67 deg F

Development: 45 sec.

Hot Alcohol Baths: 10 sec in each
Drying: Slow pull
Soaking: none required

Glass: 7059 1/2 mm AR visible

Glue: NOA 61
Grinding notes: ground to 2" dia

Circle to Point Converter Setup and Alignment Procedure

Preliminary Inspection

  • Verify that all of the stepper motors and controllers are connected properly and that the appropriate relays are connected to their respective devises.
  • Verify that when using toggling of relays in the script, that no

toggles in the script are missed. A toggle is used to close the relay, but another toggle is required to open it again.


To align the mask, you must choose a reference point and align it to that reference. The reference is the stage that the mask is clamped against. The two stages that make up the X and Y axes must be orthogonal. If the laser is focused on the mask and a detector is placed under the mask, a measurement of the power from the laser can be obtained. By moving the Y stage in small increments, you can cut the laser power with the mask. Continue to move the Y stage until the detector is in the 50% power range. Any fluctuation in the power can the be monitored as you move the X stage back and forth. When the mask is aligned, minimal power fluctuation should be seen along the entire length of the mask. This is observed by simply moving the mask with the X stage control and watching the power monitor. The mask must be in it's resting position, therefor the film holder must be out of the way so as not to scratch the bottom of the mask.

An alternate method of aligning the mask is to use the laser "knife test". This involves a similar method of focusing the laser to one of the X edges of the mask. The preliminary adjustment is to make the laser orthogonal to the surface of the glass. This is easily done with a mirror placed on the surface of the mask for reflection. When the beam is reflected directly back into the laser, normal incidence with the mask has been achieved. Once the laser's focal point is close to the surface of the mask, which is at the bottom of the glass, the Y axis can be moved to count the number of micron steps are required to block out the laser. By adjusting the laser in the Z direction, you can then continue this process until you are at the focal point of the laser, which is at the point where the laser can be shut out and allowed to transmit with the smallest number of steps by the Y stage. When at the focal point, the laser should be able to be completely shuttered with approximately 30 microns of movement of the Y stage. The stage controllers display the magnitude of the movement in an obvious fashion. Also, the laser should always be oriented in a manner that allows the smallest portion of the beam to be orthogonal with the edge that is being aligned. By simply moving the laser to another edge, orthogonality with the stages can also be measured. This same method is used in making the stages orthogonal in the preliminary portion of this entire setup.


The stage top, the mask, film holder, and master hologram must be parallel. In order to achieve this, you must use a spatial filter, a collimating mirror, and several other mirrors. A mirror should be placed on each surface that needs to be parallelized. The light must be collimated with a collimating mirror in line after the spatial filter. The mirror on the surface of the reference plane is the first used. By directing the beam down to this reference mirror and adjusting the deflecting mirror, the beam can achieve normal incidence with the reference surface. By placing a white board next to the spatial filter, you can then adjust the same deflecting mirror such that the reflected beam is off axis. This gives you a reference point for all of the other surfaces. Now, with a large collimated beam, you can see the reflected points of all of the surfaces, and simply shim them until all beams lie in the same spot on the white board. This verifies the parallelism of the different surfaces.

Master alignment

Whereas the master is in place over the mask, and must be swung out of position between each piece of film that is being exposed, it is difficult to make it exactly parallel with the masks Y edge. A square can be used to get this as close as possible as far as alignment is concerned. As far a parallelism with the surface of the mask and film, the section on parallelism explains the process to achieve this.

Making the Mask Lie Flat Against the Film

A feeler gauge should be used to test the width of the mask and film separation on all sides of the film. The mask should be moved to all extremes and the test should be repeated until all places on the mask have similar tolerance. The mask must make contact with the film or shadow will occur. The film holder has a rotational adjustment to move the film in the Z direction. After the surface of the film and the mask are parallel, this adjustment will allow the mask to lie flat against the film in all positions of the exposure.

DCG (DiChromated Gelatin) Film

With DCG film, an exposure energy of approximately 20-25 mJ should be used. Older film may require more energy to be exposed properly. An index matching material such as Decahydranaptholine should also be used. This not only acts as an index match, but it also holds the film in place by creating a vacuum effect between the film holder and the film.

The exposure is followed with a development time of the following:

  • Developer: 45 sec.
  • Rinse #1: dip
  • Rinse #2: dip
  • Rinse #3: dip
  • Alcohol #1: 10 sec.
  • Alcohol #2: 10 sec.
  • Alcohol #3: 10 sec.

Optical Setup for Exposure

In order to expose the rings on the mask, the beam needs to be larger than the largest ring and centered on the ring. The power of the beam should be uniformly distributed also. First the beam should be at normal incidence with the mask. This is accomplished by adjusting the reflected beam to lie directly on the beam of origin. Once this is achieved, the 2.5 degree adjustment in the angle of incidence is easily obtained. A mirror with an angular measure on it's adjustment is used. The mirror is then adjusted such that the beam has a negative 2.5 degree incidence with the mask. At this point the mirror may need to be adjusted toward the beam that is coming from the collimating mirror so that the spot is centered on the rings. Any portion of the beam that is larger than necessary should be masked. The optical path from the laser is as follows: the beam from the laser is sent though a spatial filter and into a collimating mirror. The beam is then directed down to the mask with the mirror that has the angular adjustment measure. Between the collimating mirror and the mirror that directs the beam down to the mask should be the extra mask that makes the spot the correct shape and size for the exposure.

Removal of Mask

The mask holder should have the mask attached to it in a secure manner and aligned with the rest of the system as stated in the alignment section of the instructions. Now, between exposures, the mask holder with the mask can be removed and replaced in a precise way. Whereas the mask holder is fastened to the base with a single spring bolt, it can be easily removed. The spring bolt holds the balls on the mask holder into the sockets in the base. After removing the mask holder, the film can be placed on the film holder and the mask holder is then replaced. After tightening the screw slightly, enough to hold the balls securely in the sockets, the mask holder should be cycled up and down to ensure that it is in place. A realignment is not necessary if the mask holder is removed and replaced in this fashion. The mask holder should be handled with much care because if the mask is moved even slightly within the holder, a realignment would be necessary. To cycle the mask up and down, simply pull the pin into the solenoid manually and then let the mask lie back down gently. The mask should be cycled several times each time the mask holder is removed from the base.

Using the Scripting Language

Script Format

  • The script must be written in ASCII DOS text without comments or any extra spacing.
  • Line 1: Number 1-5 that represents the current number of stages to control.
  • Line 2: Unique address, number 0-31, for each stage. Addresses

should be separated by a single space only.

  • Line 3: Number of times to repeat the entire instruction set. Lines 4-200: These are the lines that control the stage movement and the relay actuator.
  • Each line is either a relay control, or a stage control. For a stage control, the fields represent stage address (previously specified), number of steps, and a delay time or pause (number in milliseconds or "p" for pause). These fields are also separated by a space.
  • For a relay control, the fields represent relay control ("r"), relay number (0-7), and time in milliseconds for relay to be closed.

Sample Script

The data will be entered in this manner (without comments), starting on line one.

3 This is the number of stages to be controlled.
6 9 10 These are the unique addresses of the stages.
2 This is the number of times to repeat the entire script that follows.
9 100 p Move stage at address 9 100 steps and wait for user input "c" to continue.
r 2 2000 Control Relay number 2, close for 2 seconds.
6 100 1000 Move stage at address 6 100 steps and wait 1 second before continuing.
r 7 2000 Control Relay number 7, close for 2 seconds.
10 31000 2000 Move stage 10 31000 steps and wait 2 seconds before continuing.
r 0 30000 Control Relay number 0, close for 30 seconds.

Software Limitations

  • Currently only five stages can be controlled at the same time with the program.
  • As specified by the IEEE 488, the addresses of the stages must be unique and be within the range of 0-31.
  • The program will only accommodate 200 lines of code in the script. This infers that only 197 control instructions may be loaded by the program.
  • The number of repetitions of the execution of the entire script is limited to 32000 times.
  • A line is limited to 80 characters.
  • All fields must be separated by only a single space.
  • Time must be entered in milliseconds.
  • The character "r" must start the line of any relay controlling command.
  • Currently only 8 relays can be controlled with the program, numbered from 0 - 7.
  • When "p" is entered in the delay time field of the stage controlling command, the key "c" must be pressed in order to continue with the script.
  • Steps are limited to the range from -32000 to 32000.
  • Most of the stages have a one micron resolution, giving steps of one micron. For other stages with different resolutions, the step size will change. The user must modify the script for any differences in stage resolution.
  • The relay control portion of this program is only accurate down to approximately ten milliseconds. This constraint is a result of the parallel port communication with the relay actuator.

Hot Alcohol Bath (HAB) Specs

# Temp in F Specific Gravity


135 deg

135 deg
135 deg



Table Setup


Stepper Setup




Last modified on 5/12/00