Line of sight methods of alignment

Line of sight methods of alignment Robert E. Parks Optical Perspectives Group, LLC, 5130 N. Calle la Cima, Tucson, AZ 85718 College of Optical Sciences, University of Arizona, Tucson, AZ 85721 ABSTRACT Three methods of line of sight alignment are compared and contrasted for ease of use, accuracy of alignment and cost of equipment and fixtures. The three methods are the classical use of an alignment telescope, use of a laser beam to establish an axis, something impossible until the invention of the CW laser, and an autostigmatic microscope used in conjunction with mechanical means to establish an axis or points in space. While the method of choice is usually a pragmatic one of using the equipment at hand, this paper will demonstrate the flexibility of the autostigmatic microscope approach. Keywords: Optical alignment, autostigmatic microscope, optical axis, alignment telescope, pip generator

1. INTRODUCTION Alignment is an often overlooked way of optimizing optical system performance once a design has reached the hardware phase, whether it is a highly sophisticated system or a laboratory experiment that just moves a laser beam around to get it on target. In this paper we will look at three approaches to alignment and contrast them in terms of ease of use, accuracy and cost of equipment. Alignment for the purposes of this discussion is getting light through an optical system laid out on a table top as opposed to what might be called centering where elements are centered within a cell using a rotary table to establish an axis. For our purposes an axis is established using a line of sight that may make bends or may simply be a straight axis some specific height above an optical table. The line of sight or conjugates of an optical system may also be established mechanically with tooling and a CMM or laser tracker to define either a straight line or points in space.

2. THREE APPROACHES TO ALIGNMENT 2.1 Three approaches defined The three approaches to alignment that will be discussed are using a laser beam and targets, using an alignment telescope and targets, and using a mechanically established axis or points in space and bringing centers of curvature to that axis or points using an auto-stigmatic microscope as the transfer mechanism. The first method using a laser is an approach that could not be done forty years ago or more because there was no continuous laser so we cannot call this a classical method. That title would have to go to the alignment telescope. The method with the auto-stigmatic microscope was also pretty much out of the question as well because of a lack of sufficiently bright sources (lasers) and video imaging to make it practical. This is why it makes sense to talk about alignment in the first place; the available tools have changed in the last 40 years yet little has been written about the new versus the old. 2.2 Laser beam alignment Now that we have lasers it seems completely obvious that we should use them for alignment; the beam is visible, reasonably stable angle wise and certainly traces a straight line except for small perturbations due to air currents and instabilities within the laser. The concept is easy in the extreme; we align the laser in 4 degrees of freedom, two displacements and two tilts until it is the desired height off the table top and (usually) parallel to the edge or tapped holes in the top. This can be done sighting where the beam hits a target on a card a two positions along the beam.

Optical System Alignment and Tolerancing II, edited by José M. Sasian, Richard N. Youngworth, Proc. of SPIE Vol. 7068, 70680B, (2008) 0277-786X/08/$18 · doi: 10.1117/12.796911 Proc. of SPIE Vol. 7068 70680B-1 2008 SPIE Digital Library -- Subscriber Archive Copy

When the beam hits in the same lateral position at the two axial locations the beam is parallel to the table and can be used to bring other optics to that axis. The main problem with the initial alignment so far described is that the laser beam is usually so bright relative to the target that it is difficult to judge where the center of the beam is striking the target. In my experience it is difficult to center on a target to better than about 0.25 mm with any consistency. By making a target of three bearing balls about the diameter of the beam and held in a ring on something like a washer as shown in Fig. 1a, the beam passing through the target creates a central triangular core and three near linear “spokes”. The spokes are the narrow triangular areas beyond where the balls touch each other but inside the read circle, the nominal edge of the beam. It is easy to balance the size and intensity of the spokes on a white card held beyond the target by shifting the 3-ball target laterally with the repeatability of about 0.02 mm.

Fig. 1 Three bearing balls in contact to form an aperture (a), and a profile of the balls and same diameter circle showing the light path producing the central triangular patch and three “spokes” of light within the boundary of the beam and centered around the core (b).

If one wanted better centration on the laser beam a quad cell could be used as a detector to obtain a sensitively to position on the order of 0.001 mm. The limit to centering here is the stability of the laser and the environment. 2.3 The alignment telescope The classical method of optical alignment is to use an alignment telescope (AT), a telescope that can focus from about 0.5 m to infinity while maintaining a straight line of sight. In addition to a focusing capability there are micrometer adjustments to x-y crosshairs so that deviations from target centers can be measured with a resolution of 0.025 mm or better. An AT also serves at an autocollimator when focused at infinity or at the center of curvature of an optical element. An internally projected target is reflected back off the mirror or surface allowing the mirror to be made normal to or surface centered on the AT axis to about 0.5 arc seconds. While an AT can focus over a wide range the relatively small aperture limits the depth of focus so it cannot be relied upon for axial positioning. This must be done with inside micrometers or similar tooling. Similarly with return reflections from optical surfaces, if they are close to each other axially they may be difficult to separate unless one of the surfaces can be displaced slightly to see which return moves. The initial alignment of the AT is much the same as for the laser beam; a pair of targets with an axial spacing are sighted and the AT’s lateral position and angle of sight are adjusted until the center of both targets appear in the

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same position on the AT crosshairs. In both cases, laser beam or AT, the device should be positioned relative to the near target, then focused on the far target and tilted to center the far target. This process is repeated iteratively. If the process is tried in the reverse order the alignment diverges. A disadvantage of an AT is that it is a classical instrument and as such has an incandescent source. This source is dim enough that it is difficult to find return specular reflections under normal lighting conditions. Often the room must be completely darkened and occasionally a flashlight or fiber illuminator bundle has to be shown into the eyepiece to project a visible beam and track its return. Once the return is found an AT is very easy to use; the difficulty is often the initial capture of a return reflection, something that can be frustrating and time consuming. There is an attachment for alignment telescopes that somewhat relieves this problem and is particularly useful for aligning lenses using their centers of curvature, the “pip” generator.1 A useful example is given in Yoder.2 Another aspect of AT’s that make their use tedious is that one person is tied to the AT eyepiece while another makes adjustments at or near the target. Seldom is the target within arms reach of the eyepiece so the job is tedious for one person moving back and forth or two people are needed. One way around this situation would be to mount a video accessory on the eyepiece of the AT and put a monitor close to the element being adjusted but few to none are available. Recently video cameras3 have become available for AT’s and these make alignment less tedious. (www.brunson.us/Products/8415SY.asp) 2.4 Alignment with an autostigmatic microscope In one mode the autostigmatic microscope, or PSM4, can be used just as in the laser beam alignment mode because it uses infinity corrected objectives and removing the objective produces a collimated laser beam, see Fig. 2 for modes of operation. In its original design mode the PSM uses single ball targets that have been positioned mechanically to the conjugates of the optics to be aligned. The PSM then picks up the geometrical center of the ball with 1 µm or better sensitivity, the ball is removed and the optics is adjusted to bring its conjugate to the ball center, the PSM being used as the transfer device.

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Fig. 2 Three modes of operation of the PSM; an autostigmatic microscope (left), an electronic autocollimator (center), and a video imaging microscope (right)

There are several modes of implementing this approach. One is to build a fixture with accurately bored holes defining the optical system conjugates and locating a ball on top of a post that fits the holes and has a collar to set the height above the fixture. Pins locate the optical bench to the fixture so the conjugates can be transferred from fixture to optical assembly. For prototype systems or folded systems a laser tracker can be used to position the ball targets. Tracker ball nests are attached to x-y-z stages and the laser tracker is used to position the SMR tracker balls to the design conjugates positions. The tracker balls are removed from the nest and complete spheres of the same diameter replace them and the PSM is used to transfer the ball centers to the optics being aligned. The reason for replacing the tracker balls with grade 25 of better steel balls is that the finish of most tracker balls is not very good and the centers cannot be accurately located with the PSM.

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For optical systems where all the centers of curvature of the elements lie on a single straight axis the optical bench can be mounted on a long linear stage with a position scale. From a reference axial position the location of all the conjugates can be located. In one application the PSM was held in the tool collet while the optical assembly was clamped to the bed of a milling machine. Once the bench of the assembly was aligned parallel to the travel of the bed it was possible to move the assembly along the optical axis and align one element after another to the axis. Another way of using the PSM is to mount it on a CMM or robot and program the CMM to position the focus of the PSM from one conjugate to the next. The CMM may be most useful in a prototype situation while the robot is more applicable to a production environment. At first glance a robot would not appear to have sufficient accuracy but 4 degree of freedom SCARA type robots have remarkably good repeatability (±0.02 mm) over distances of about 1 m.

3. USE AND CONTRAST OF THREE METHODS OF ALIGNMENT 3.1 Laser beam alignment There are typically only two types of optical element to align, either a plane object such as a mirror or a powered element like a lens or mirror. In the case of a plane mirror the object is to change the direction of the light beam to follow a new axis. This axis must be defined by design and might be a 90º bend some distance along the initial beam. Two targets are needed to define this new axis and the mirror must then be adjusted in an axial position and two tilts in order to pass through the targets. This is all rather straight forward and the three ball apertures make fine targets. For powered elements the laser beam will pass through only a small area of the aperture of commonly used optics, say those about 25 mm in diameter. (If dealing with very small optics, on the order of the size of the laser beam, the use of a laser beam for alignment may not be a useful approach but the autostigmatic microscope is near perfect.) To first order over a small part of the aperture the lens will appear to the beam like a prism with a small wedge angle and deviate the beam. Thus the first correction to make to a lens inserted in the beam is to position it laterally in two directions until the beam is no longer deviated. This will center the lens. Although the lens is centered it may have tilt. The tilt can be removed by noting the direction of the beam reflected off the lens surfaces. This is most easily done by inserting a beamsplitter immediately in front of the laser source with which to view the reflected beam against a target. This target then acts as a centering indication for all reflected beams. Now tilt the lens in two directions until light reflected from the lens falls on the target beyond the beamsplitter. If the system being aligned is complex and has several folds it is best to set up the path through the system using plane mirrors only and getting the beam to follow the desired path through the system first. Insert a target just before the fold in each leg so that each leg can be aligned separately when time comes to insert the powered elements and then insert and align these as the last step. This method of centering has several advantages. The light beam is almost always bright enough to see even under normal room lighting. The cost of the needed equipment is modest and the method rather simple. 3.2 Alignment telescope The alignment telescope is set up much the same as the laser beam, two targets are used to define the axis to which the AT is first aligned, and then the AT is used to bring other optics onto its line of sight. For plane mirrors and surfaces the AT is focused at infinity and the mirror or surface is adjusted until it is perpendicular to the AT axis. This can be done to sub-second accuracy but the narrow field of view means the initial set up can be lengthy and tedious. For powered elements reflections are picked up from the two powered surfaces. If the center of curvature of a powered surface lies at or behind the AT a “pip” generator is needed to create an image that can be focused by the AT within its focus range. The focus knob of an AT is not calibrated in distance and thus care must be used to assure

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one is looking the desire axial when finding a surface or center of curvature. This can be frustrating if there are several reflections that focus within a small focus range. Again it may be difficult to get the surface reflection within the AT field of view. Observing with an AT is somewhat cumbersome because most have the eyepiece on the axis of observation so the eyepiece is often at an uncomfortable height. Further, most adjustments are not within the reach of the operator so the user must either move back and forth or alignment is a two person operation. Being reasonably large and heavy, an AT requires a custom four degree of freedom mount often placed on a sturdy tripod. When all the hardware to use an AT is considered there is an investment on the order of $20K. This is not to say that good used AT’s are not available these days for about $1K but they still need mounts and tripods. 3.3 Autostigmatic microscope For plane mirrors the autostigmatic microscope or PSM can be used just as with the laser beam mode of alignment because it projects a collimated laser beam about 6 mm in diameter when the objective is removed. The advantage over the bare laser beam is that plane mirrors, or even more usefully, transmissive elements with plane faces can be aligned perpendicular to the beam from the PSM with sub-arcsecond precision. Light is reflected back into the PSM and onto the detector where a centroiding algorithm can located the centroid to 1 or 2 µradians assuming the whole optical setup is stable. In practice in the presence of real world environments a practical number would be 1 to 2 seconds of arc unless great care is used. For powered elements the PSM can also be used in the autocollimating mode. The laser diode in the bright mode (about 0.2 mW exiting the PSM) is sufficiently coherent to produce interference and diffraction patterns from the surfaces of the powered elements. These circularly symmetric patterns can be used to align powered elements in both decenter and tilt as shown in Fig. 3. The patterns were purposefully misaligned so both could be seen. Although it does not show too well in the reproduced figure it is possible to see the central ring in each of these two superimposed but tilted patterns and the rings are about 20 µm apart in decenter. Moving the patterns during adjustment also makes the features of the patterns easier to see on the video screen than on the still reproduction.

Fig. 3 The video display of the two diffraction/interference patterns returned from a 200 mm efl singlet lens about 1 m from the PSM used in the autocollimator mode. The two patterns are purposefully misaligned so both are visible, the one with less contrast is to the right.

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In Fig. 3 the 200 mm efl singlet used was about 1 m from the PSM and it was possible to center the element to 10 µm and remove tilt to on the order of 10 seconds of arc. This is a particular example and every lens element will have a different sensitivity to alignment, some more, some less depending on the specific radii. However, the method is very simple to implement, the pattern can be viewed in real time directly adjacent to the element being centered by virtue of the laptop computer display and there is more than enough light returning to the PSM even from AR coated surfaces to perform a high degree of alignment. This is a perfect way to use the PSM for aligning bench top optical layouts where it is of interest to get maximum performance out of a system of catalog optics without going to lengthy measures for alignment. Other modes of use of the PSM have been described elsewhere for the alignment and centering of powered optical elements. In other than the autocollimator mode the PSM must be brought to the center of curvature or surface of whatever is being aligned. To do this an x-y-z stage is needed but any reasonably high quality set of stages is all that is needed for positioning the PSM. A particularly stable mount for the PSM consists of 1” travel x-y stages plus a wedge actuated vertical stage such as is available from Newport4 and is shown in Fig. 4. For micro optics the PSM is perfect for alignment because it is already a microscopic system that produces fast but perfect wavefronts due to the microscope objectives used. In addition, the PSM can be used in the imaging mode by a computer controlled switching of internal light sources so mechanical features of the micro optical element can be viewed. For example it is often advantageous to center molded optics using the residual diamond turning features imprinted in the surfaces as seen in Fig. 5. One final mode of operation combines the imaging and autostigmatic functions as illustrated in Fig. 6. Here it was necessary to measure the location of the corners of a CCD array relative to laser tracker reference spheres. Since precise stages with sufficient travel were not available tracker balls were also placed on the PSM. The motions of the PSM were followed with the tracker as the PSM first went to the corners of the CCD array and then went on to find the centers of the three balls in the tracker nests on the side of the camera. In this way the CCD is located precisely relative to the tacker balls on the camera. Later the tracker will be used to orient the camera precisely on an optical surface so that alignment markers from a null lens hologram will fall in a known position on the CCD. While this is a circuitous path we cannot think of another way to accomplish this sort of alignment. The autostigmatic PSM and suitable stages are available for about $23K including a laptop computer for controlling the video camera, light sources and displaying the images.

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Fig. 4 PSM (upper right on its side) mounted on x-y-z stages. The Newport MVN120 vertical stage shown here at the bottom of the stack is particularly stable.

Fig. 5 Although not easily seen in this reproduction but clearly visible on the video display of the PSM are diamond turning marks in the surface of this molded plastic micro lens and are useful for alignment purposes. Image is about 100 µm across.

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Fig. 6 PSM focused on a CCD array in a Mightex camera. Note the laser tracker ball nests and balls on both the PSM and the camera. The laser tracker serves as a CMM to measure the CCD relative to the tracker balls on the camera

CONCLUSION The contrast between the three methods of alignment can be best summarized in the Table below

Laser beam

Visibility of light and ease of initial setup Good for both visibility of beam and initial alignment of laser

Alignment telescope

Beam difficult to see. Need to darken room Most difficult part of use

Autostigmatic microscope or PSM

Beam easy to see. Psm is small and lightweight so easy to align initially

Precision in decenter and tilt

Cost of equipment

About 25 µm for decenter About 25 µradians for tilt but depends on power and distance About 20 µm for centering. Generally less than 5 µradians for reflections off surfaces

Cost is quite modest, laser, 4 DoF mount and beamsplitter cost maybe $3K

Depending on mode of use 20 to