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Module 6-2 COMPONENT SUPPORTS ————————————————————————————————— ©Copyright 1987 by The Center for Occupational Research and Development All rights reserved. No part of this book may be reproduced in any form or by any means without permission in writing from the publisher. The Center for Occupational Research and Development Printed in the U.S.A. ISBN 1-55502-024-0
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Idea Bank (2) This module will help you to select, recognize, and properly use the many types of component mounts available. (3)You should have an understanding of algebra and of general laboratory safety. You also must be familiar with the use of low-power helium-neon lasers.
(4) When you complete this module, you should be able to:
Translator table Rotary tables Tilting trough Scissors jack Focusing holder Two-axis translator Two-axis mirror mount Laser cavity mirror mount Laser beam director Six types of lens holders Filter holders Prism holders Laser mounts Translator plus rotator Hold-down clamp Vertical translator Sine table
DISCUSSION (5) Component supports are used to hold and position optical devices. To attempt an optical experiment or alignment if the component supports aren’t stable enough, or if they don’t provide for smooth enough adjustment of a component’s position, can be very frustrating.
Linear Movement (6) The fine adjustment of position on a component support (whether linear or angular) generally is accomplished by the slow linear movement of a threaded rod as it’s screwed through or against a plate. A typical example is in Figure 1.
Fig. 1 (7) The precision, smooth adjustment is possible
because of the principle on which the screws work. As the threaded rod is turned, it
pushes the component table a proportionate distance. The springs keep the adjustment screw
and component support in close contact with one another. They cause the support to return
when the screw direction is reversed. The increment or resolution of linear movement is
limited by the least amount you can turn the screw and the pitch of the threads on the
screw. For example, if the screw had 40 threads per inch, each turn would
move the platform 1/40 inch per revolution or 0.025 inch/revolution.
If the knob on the screw is about one inch in diameter, the average
person can twist a screw of this diameter a minimum of about 2 degrees, therefore (8) You can get finer resolution by a device called the differential screw. This device is shown in Figure 2. The long adjustment screw passes first through a fixed block at the left. In this block, the screw has right-hand threads. The screw also penetrates a "movable" block attached to the component table. The screw in the movable block also has right-hand threads.
Fig. 2 (9) When you rotate the adjusting knob in a clockwise direction, the screw moves forward, pushing the movable block and component table to the right. At the same time, the movable block moves to the left on the rotating screw, offsetting some of the rightward motion of the screw. (10) Suppose that the pitch is 30 threads per inch in the fixed block and 40 threads per inch in the movable block. Then, when you turn the knob clockwise one complete revolution, the screw advancement through the fixed block pushes the component table l/30 inch = 0.0333 inch to the right, while the opposite motion of the movable blocks along the advancing screw pulls the table l/40 inch = 0.0250 inch to the left. The net movement for one revolution is 0.0333 inch - 0.0250 inch = 0.0083 inch to the right. (11) If the knob on the adjusting screw is turned two degrees at a time, the increment of movement will be:
(since 1 inch = 2.54 cm = 2.54 ´ 10–2 m = 2.54 ´ 104 mm, we see that
The distance 1.17 mm is about 2 wavelengths of green light! (12) You can get very small linear movements on components (such as mirrors) by using piezoelectric translators. A piezoelectric device is a special crystal (such as barium titanate) that changes its physical dimensions when an electric field is applied across it. Normally, a piezoelectric crystal has 3 sets of parallel faces. One is coated with metal electrodes. When a voltage is applied to the electrodes, one of the dimensions expands. An increase in voltage causes a corresponding increase in the lateral dimension of the crystal. A typical piezoelectric sensitivity is 2 ´ 10–9 meters/volt. This means that a change in electrode voltage of 100 volts will cause a change in crystal size of (100 volts) ´ (2 ´ 10–9 meters/volt) = 2 ´ 10–7 meters or 200 nanometers, about l /2 for violet light. (13) Piezoelectric translators are used to move a mirror that adjusts the length of a laser resonant cavity when "tuning" a longitudinal mode.
Angular Movement
(14) You can make angular adjustments with a threaded rod in a
configuration similar to the one in Figure 3. Movement of the screw causes the movable
plate to rotate about the pivot ball. The angular change D q is, for small values of q ,
approximately equal to the screw movement D S divided by
the pivot screw point distance L. D S is, of
course, the resolution of linear movement of the screw. The angular resolution is simply
that value divided by L. Use the 40-pitch screw (D S
= 1.38 ´ 10–4 inches) and a 2-inch distance
for L. Angular resolution (D q
) is:
Fig. 3 Possible Movements or Degrees of Freedom (15) The position or motion of a piece of optics can be described using six motions or six "degrees of freedom." Figure 4 shows the six possible motions or adjustments in position for a lens.
Fig. 4 (16) The six possible movements for the lens are:
all of which are classed as linear, and:
which are all rotational. (17) By combinations of these six basic motions, you can put the lens—or other component—in any position you want. One or more of these six motions usually are provided by component supports. (The words pitch, roll, and yaw are used commonly in describing the motion of a ship or aircraft. These motions are identical to those defined in movements b,d,e, respectively, above.)
Supports With One-Degree Freedom (18) The simplest type of adjustable component support is the one that provides one degree of freedom. This lets you move the component back and forth along, or rotate it about some line. (19) Figure 5 shows a translator table (translational stage) that supports components and provides one degree of freedom (linear). The translator table gives controlled linear movements. With the micrometer, it permits position readout. You can use it along or transverse to the optical axis to position prisms, crystals, lenses, and other components. The translator table is controlled by a micrometer pushing against a ball-bearing-supported, spring-loaded table.
Fig. 5 (20) Figure 6 shows examples of other supports that allow only one degree of freedom (rotational). The rotary table and rotational mount provide controlled rotational movements with high resolution and readout capabilities. They can be used to rotate prisms, polarizers, and crystals.
Fig. 6 (21) The rotary table (or "rotational stage") is controlled by a worm gear on; the knob shaft that drives a circular ring gear through a full 360°. A simpler, and less expensive, rotational stage is shown in Figure 7. In this device, rotation is accomplished by a round shaft (attached to the knob) that rolls against the circumference of a central circular platform. The circular degree scale is provided for noncritical angular measurements. The knob on the right is used to lock the position of the stage. This particular stage is 3.5" square.
Fig. 7 (22) The central platform usually is supported by ball bearings to allow smooth rotation. You can purchase the platform of the stage with small tapped mounting holes or a larger-diameter access hole, with or without internal mounting threads. (23) You can determine the resolution of such a device by noting that the arc length along the circumference through which both the central platform and the shaft move is the same (if no slippage occurs). Thus you can determine relative angular motion from a ratio of shaft to platform circumference.
Fig. 8 (24) A hold-down clamp like the one in Figure 8 often is used with a rotary table. But it is not limited to such usage. You can use it on the translator tables or any of the other mounts where you want to hold an irregular-shaped optical component in place. The Scissors Jack (Lab Jack) (25) Another component support with one degree of freedom is the scissors jack. It’s shown in Figure 9. This support looks like one type of auto jack. It’s useful in changing the height of heavy components above a work surface. Its basic design, including a number of pivots and relatively thin load-bearing members, makes it relatively unstable. So, it’s not very satisfactory when you need rigidity.
Fig. 9 (26) If you need very fine control of the vertical motion, you can use a vertical translator equipped with a micrometer. One is shown in Figure 10.
Fig. 10 The Sine Table (27) A controlled angular displacement about one axis is provided by the sine table shown in Figure 11.
Fig. 11 The Tilting Trough (28) Movement of one degree of freedom is provided by telescopes, collimators, and other cylindrically shaped objects by the tilting trough support. The tilting trough shown in Figure 12 has about 10 degrees of fine pitch adjustment by turning a spring-loaded screw. While it is listed here, essentially for completeness, it does not find wide or general use in the field.
Fig. 12 Focusing Holder (29) The last example of an optical support with one degree of freedom of motion is the focusing tube holder. This device (shown in Figure 13) is used to hold microscopes, telescopes, spectroscopes, and other cylindrical units. The one controllable adjustment is used to bring these instruments to their focus position.
Fig 13 Multiple Degrees of Freedom (30) The component supports providing one degree of freedom can be combined to provide adjustments in many directions. Some of the combinations used more often will be discussed in this section.
Two-Axis Translation (X-Y Translator) (31) You can stack two separate translator tables in a way that will allow controlled adjustment along two axes. Figure 14 shows a two-translator arrangement and the movements that it makes possible. An additional translator can be bolted at right angles to the upper table to produce motion along three orthogonal axes. Such an assembly often is called an "X-Y-Z translator."
Fig. 14 (32) If you combine a translation unit and a rotational unit, you’ll get a device similar to Figure 15, permitting the movements indicated. Such a device might be used to first center an object along the optical axis with the translation stage and then view it from all rotational angles with the rotary table.
Fig. 15 Mirror Mounts There are many mirror mounts. Three of the most common are the gimbal mirror mount, the kinernatic mirror mount, and the laser-cavity mirror mount. The kinernatic mount is less expensive than the gimbal mount but does experience some small linear movement when adjusted. The gimbal mount costs more but does not experience any significant linear movement. The laser cavity mirror mount is designed for those very fine adjustments required when alignng a laser cavity. The small adjustments are accomplished with set screws on the mount. Another mirror mount—The flexire mount—offers a translation in addition to the usual two-axis tilt. Gimbal Mount (33) You’ll need mounts with two rotational degrees of freedom to position a mirror for many uses. Figure 16a shows a common two-axis gimbal mirror mount where the mirror is supported by two adjustable rings. A mount like this might be used to point a laser beam in a certain direction without having to move the entire laser.
Fig. 16a
Kinernatic Mirror Mount Typical kinernatic mirror mounts are shown in Figures 16b, c. They serve equally well for mounting beamsplitters. They are an ideal, low cost solution for a two-axis tilt requirement. Figure 16b is a diagram of the lower cost mount with thumbscrew-drives. Figure 16c shows a higher cost, higher performance mount. It can hold mirrors from 25 mm - 100 mm diameter, with a backlash free movement of around 6° tilt. Drive styles can be either by thumbscrew or micrometer, depending on cost.
Flexure Mirror/Beamsplitter Mount Another low-cost mirror mount is referred to as the flexure mount. It is shown below in Figure 16d. This mount can accommodate mirror and beam-splitters with diameters of 16 mm – 50 mm. It provides two-axis tilt and one axis translation, controlled by adjustable thumbscrews.
Fig. 16d Flexure Mirror/Beamsplitter Mount (34) A variation of the two-axis mirror mounts is shown in Figure 17. This is the laser cavity mirror mounts. It functions similarly to the two-axis mount described above, but the adjustments are made with screws instead of micrometers. It’s available in several forms, differing in the type of pivots and loading that are used to prevent "backlash."
Fig. 17 Laser Beam Steering Device ("Beam Raiser") (35) In the laboratory you’ll often need to redirect the laser beam without moving the laser. Laser-beam directors are made to fill this need. As shown in Figure 18, the laser-beam steering device lets you move a mirror in three degrees of freedom so that a beam can be put anywhere in the field of view of that mirror. Both mirror carriages can be moved independently of the vertical support column so that you can position the laser beam to the desired height and direction easily. This use is shown in Figure 19.
Fig 18
Fig. 19 Individual Component Supports (36) Some optical components are used so often in optics work that special mounts are made for them. This section will discuss mounts for lenses, mirrors, prisms, and other commonly used components.
Lens Holders (37) Lenses are used in so many optical setups that there are many types of lens supports. The simplest type is the sliding-grip lens holder shown in Figure 20. The bottom and top supports have deep grooves for seating the lens. The top support slides on the two vertical rods and has a locking screw.
Fig. 20 (38) An inexpensive lens holder is the spring-grip lens holder shown in Figure 21. The lens is inserted between two jaws. The jaws are operated by pressing down two spring-loaded triggers.
Fig. 21 (39) The swing-arm lens holder is another variation of lens support. It’s shown by Figure 22. This type of lens holder has a swing-up, spring-loaded arm to hold the lens. Both the bottom support and the arm have grooves for seating the lens.
Fig. 22 (40) For permanent or semipermanent lens-mounting applications, you should use the threaded lens holder. This mount must be used with a hi red size of lens, which is held in place by threaded rings. A threaded lens holder is shown in Figure 23. Adapter rings are usually available to let you mount more than one size of lens in a given size of holder.
Fig. 23 (41) When an optical setup calls for using lenses of various diameters interchangeably, you can use the self-centering lens holder. Figure 24 illustrates such a lens holder and shows why it’s self-centering.
Fig. 24 (42) The three arms of this lens holder pivot on screws in a fixed ring. The outer ends of the arms have pins that ride in grooves in a spring-loaded, rotatable outer ring. When the top levers are pinched, the rotatable ring is moved and the jaws open. When the levers are released, the lens is gripped at three points by jaws with deep grooves, and the lens is automatically brought to the center of the holder. With this holder, the lenses can be changed quickly and automatically located in the optical axis without making further adjustments. (43) Another self-centering lens holder is the iris-diaphragm lens holder. It’s shown in Figure 25. The leaves of the iris diaphragm are strong. They will hold any lens that has flat edges. You merely place the lens in the center of the aperture and close the leaves on it. You operate the iris diaphragm by a large control knob that engages a gear drive. An added advantage of this type of lens mount is that no light is transmitted except through the lens.
Fig. 25 Filter Holder (44) The simplest filter holder is shown in Figure 26. This holder is a spring clip mounted on the stand. Besides filters, this type of mount can hold ground or mirrored glass, beam-splitters, targets, beam-blocks, or paper.
Fig. 26 (45) A slightly more complicated filter holder is shown in Figure 27. This one has two spring clips that push against a square frame. It’s used to hold filters or targets that are too flexible to be held in the single-clip holder. The two-clip holder can hold filters, frosted glass screens, opaque screens, mirrors, mounted graph paper, targets, and so on.
Fig. 27 (46) If you’re going to add filters one at a time until a desired density is reached, you’ll use a multiple filter holder. Figure 28 shows a rack-type holder for holding up to four filters. You can build holders like this inexpensively by cutting off sections of linear 35-mm slide trays and attaching mounting rods. One side of this holder is mounted with thumb screws and slotted holes so that you can move it to tilt the filters. With this feature, most internal reflections can be directed out of the optical path. In addition, after transmission it can displace the beam or change the angle or incidence.
Fig. 28 Prism Holder (47) A special holder for mounting prisms or beam-splitter cubes is shown in Figure 29. The rubber coated, spring-loaded arm on this mount holds the prism firmly against a flat table.
Fig. 29 Laser and Collimator Mounts (48) You can mount lasers, telescopes and collimators a lot of ways. Some of the ways were discussed in the sections on the tilting trough, the hold-down clamp, and the scissors jack. Many lasers, however, are too large and heavy for these types of supports. A custom-made support for the particular laser must be designed. One popular design for circular laser tubes and collimators is the V-block mount. It’s shown in Figure 30.
Fig. 30 (49) The V-block mount has three adjustable screws for feet. They let you level the laser and point the beam in the desired direction. Note here that three points are sufficient to level an instrument. You should avoid mounts with four points. (50) Another way to mount lasers is shown in Figure 31. In this case, the rubber feet normally supplied on the laser have been removed, and "T"-shaped bars have been attached. This lets you mount the laser to standard optical bench carriers. Perhaps the most convenient method of mounting small cylindrical HeNe laser heads for general alignment use in the laboratory is shown in Figure 32. Figure 32a shows a typical cylindrical laser mount that’s available with a variety of inner diameters. In Figure 32b a typical rod/clamp assembly is shown. A variety of component mounts can be bolted to the clamp assembly for vertical height adjustment.
Fig. 31
Fig. 32 (51) Mounts like these can be equipped with micrometers that can tilt the laser in two directions with considerable accuracy and repeatability. For additional convenience, the laser mount can be attached to a vertical rod/clamp assembly as shown in Figure 32b. The clamp assembly has mounting holes and moves vertically on a rack-and-pinion gear. You can attach the mounting rod directly to the table or bench with mounting bolts, or attach it to a magnetic base for easy repositioning. For even more flexibility, translational stages can be bolted between the clamp assembly and the laser mount to give micrometer-driven resolution in one or several directions. (52) When possible, it’s always a safe policy to buy your optical mounts from a single vendor. Experience shows that the mounting holes from various manufacturers seldom align. That means you would need to fabricate adapter plates to align the holes.
Design Your Own (53) You can see from all the types of components shown so far that there’s no single way to mount optical components. Many times you’ll have to custom-make part of the setup or rearrange it so that it will work right. In mounting optical hardware, you have to consider all aspects of the problem. One aspect is which supports are available and which components are to be mounted. You should consider the range over which you want the mounts to be adjustable, and the degree of rigidity required for the component weight. Advanced planning of the intended use is important. You’ll need to incorporate for positioning, translating, focusing, splitting, or reflecting the light through the required ranges.
An Example of Designing Your Own (54) A need arose during testing of the Apollo laser altimeter that called for a custom-designed mount. The design approach for this mount shows some of the things you should consider when you build or assemble component supports. (55) The laser altimeter was mounted on a table along with a pointing telescope. The arrangement is shown in Figure 33. Two major problems in mounting the two components had to be overcome.
Fig. 33 (56) The first problem was to mount the telescope on the table so that it could be adjusted to point in the same direction as the laser altimeter. The altimeter was fastened firmly to the table. It wasn’t adjustable. (57) The second problem was to mount the entire table so that it could point the telescope (and thus the laser) precisely at a small target 30 miles away. The elements considered in the solution of the first problem were:
(58) The pointing telescope mounting problem was solved by a custom-made mount shown in Figure 34. This design called for two custom-made stands with a commercially available translation stage mounted on each one. The micrometers on the translation stages could be locked in position when the desired adjustment was reached. The second mounting problem was solved by supporting the entire table by a second table, as shown in Figure 35.
Fig. 34
Fig. 35 (59) This way of supporting the laser and telescope was used to aim the laser (by looking through the telescope) at the small target 30 miles away. Therefore, care had to be taken to make the threads of the adjusting screws fine enough. They were made so that a full turn of one of the screws moved the laser one-fourth of the full beam width. The table and adjusting screws also were made strong enough that when alignment could be maintained, accurate aiming was achieved. (60) One final note. In this module we usually have limited our discussion to manually driven translators and rotators. Virtually any micrometer now can be replaced with a motorized version. Drive resolutions for single-step motions of 0.02 micrometer are available. The additional cost of motorize drives may be warranted if you need high repeatability or remote operation. Figure 36 shows a joy-stick controller and a variety of motorized micrometer heads. Controllers are also available that connect to microcomputers for data-acquisition and process-control applications. If even more resolution is required, piezoelectric drivers—which were discussed at the beginning of this module—can be attached to optical components to produce adjustments on the order of a small fraction of a wavelength of light.
Fig. 36
Fig. 37
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