Engineering & Laboratory Notes

Beam Expander, Pinhole, and Crosshair Alignment to Laser Beams By Paul R. Spyak, Remote Sensing Group, Optical Sciences Center, University of Arizona, Tucson, Ariz.

Abstract Simple methods are presented fo r a l i gn ing beam expanders, pinholes, and crosshairs to laser beams. The reliability of these methods illustrates that often these pro­cedures can be greatly simplif ied by observing the laser beam downstream from the element being aligned.

Introduction For years I have observed students and colleagues, includ­ing many w i t h M.S. and Ph.D. degrees, struggle whi le attempting to align a beam expander, pinhole, or crosshair (on a glass substrate) to a laser beam. In the case of the beam expander, it can be difficult to determine when colli-mation is achieved, but most often it is the pinhole align­ment that is the most troublesome. Many f ind it to be dif­ficult, tedious, and time-consuming, and the beam quality exiting the pinhole is often poor. It is not uncommon to hear that someone has spent two hours aligning such a sys­tem.

Aligning a crosshair on a transparent plate of glass to a laser beam can be a relatively quick procedure. The com­plaint here usually pertains to the difficulty of determining when "good" alignment is accomplished as multiple reflec­tions and the small beam size complicate the procedure. In this paper, methods to ease these alignment difficulties are presented for visible lasers, although similar procedures can apply for infrared wavelengths. It is left to the reader to identify and follow proper safety procedures.

Beam expander and pinhole alignment to a laser beam Aligning a pinhole to a laser beam usually implies passing the beam through a beam expander. I w i l l present the alignment procedure within this context. Figure 1 shows a typical, properly aligned, laser-beam expander. A micro­scope objective focuses the laser beam into the plane of a pinhole that "cleans," or spatially filters, the laser beam by blocking the scattered light leaving the laser. The beam expands until it is collimated by a collimating lens.

A l i gnmen t of th is system should begin w i t h the microscope objective and collimating lens. Remove all the elements and allow the laser beam to illuminate an obser­va t ion plane at least a few feet away f r o m the beam expander location. Create an observation plane by placing

a piece of paper in the beam; mark the beam's vertical and horizontal locations for future reference. Do the same for an observation plane a few inches downstream f rom where the microscope objective w i l l be posit ioned. Visually posit ion the microscope objective so the beam passes th rough its center. The alignment can be checked by observ ing the

FIGURE 1. Schematic of atypical beam expander.

expanding beam in the observation plane several inches beyond the microscope objective, or by viewing the light reflected from the lens back towards the laser. The beam in the observation plane should be circularly symmetric and should not deviate f rom the original path. The reflected light should return to the laser aperture.

Now place the collimating lens in the beam. Determine its position along the optical path by measuring the beam size at the collimating lens and in the distant observation plane a few feet away. When the lens is in its proper position, the beam size wil l be identical in both locations; the beam is said to be collimated. The vertical and horizontal alignment can be accomplished by centering the laser beam on the dis­tant reference point. Ensure that the beam sizes are the same at the collimating lens and at the distant observation plane. If they are not, repeat the above procedure. The final image in the observation plane should be circularly symmetric and centered on the original reference mark. At this point in the alignment, some stray radiation may be observed beyond the central bright lobe due to laser scattering, but this is nor­mal. The pinhole wil l eliminate the scattered light.

The above method for checking collimation works fair­ly well, but a more accurate check of col l imation can be obtained by placing a wedged shearing plate after the colli­mator and observing the interference fringes. The basic concepts of this lateral shearing interferometry technique can be found in the literature.1,2 Wi th the wedge situated vertically in the beam and tilted wi th respect to the vertical, interference fringes are produced f rom reflections off the front and back sides of the plate. A defocused collimator wil l produce vertical fringes (parallel to the optical axis). As col l imation is approached, the fringes wi l l widen, spread out, and rotate to a horizontal position (perpendicular to the optical axis). Horizontal fringes indicate collimation.

Now to align the part that many f ind di f f icul t—the pinhole. The primary reason this presents problems is that many try to visually place it as near to the proper location as possible. Then, when performing fine-tuning alignment, it is difficult to get any light through the hole, which may be only a few microns in diameter. Instead, place the pinhole between the two lenses, but downstream from the focused beam (see Fig. 2). By starting the process wi th a poorly mis­aligned pinhole, light wi l l easily pass through the pinhole and the alignment process wil l be greatly simplified. Align the pinhole vertically and horizontally so that the image in the observation plane is symmetric and centered on the mark. The image wil l probably appear as a bright circle sur­rounded by narrow circu­lar-di f f ract ion rings. Now position the pinhole closer to the focal point, and hori-zon ta l l y and ve r t i ca l l y rea l ign the p i nho le as described above. Continue this procedure unt i l all of the circular rings have dis­appeared and only a bright central disk exists. Check to ensure tha t the image is FIGURE 2. Misaligned pinhole.

Engineering & Laboratory Notes

symmetrical and centered on the mark. If it is, the beam expander and pinhole are aligned.

Crosshair alignment to a laser beam The most obvious method for aligning a crosshair to a laser beam is to simply observe when the beam is centered on the crosshair. Because a crosshair is composed of th in lines, very narrow when compared to the laser beam diameter, it is often uncomfortable to view and diff icult to determine when good alignment is actually achieved. This is particu­larly true when the crosshair is on glass, as reflections from both sides of the glass can make these observations difficult. The simplest and most straight forward method that I have found is described below. Here I discuss a method for align­ing a roughly 100-μm thick crosshair on a parallel-surface, glass substrate to an unexpanded 1-mm diameter, HeNe (0.6328 μm) laser beam. Although this example is very spe­cific the method should apply similarly to most situations.

Begin by adjusting the tilt and rotation of the crosshair so that a single beam reflects back onto the laser output port. To do this well, the laser should be at least a few feet away. Adjust the crosshair position vertically and horizontally so

that the laser visually illuminates the crosshair. Then hold a sheet of paper behind the crosshair, perhaps one to six inches away, and observe the diffraction pattern. The brightest por­tion of the diffraction pattern will be a relatively thick, frag­mented cross. A shadow of the crosshair can be observed in the diffraction pattern and alignment can be easily achieved by centering this in the diffraction pattern cross.

Conclusion Simple and quick methods for aligning a pinhole, beam expander, and crosshair to a laser beam have been present­ed. In each case, it was found that the procedures were greatly simplif ied by observing the laser beam in distant planes. The lesson here is that alignments are many times simplif ied by making observations downstream f rom the element being aligned.

References 1. M.V.R.K. Murty, Optical Shop Testing, D. Malacara ed.

(John Wiley & Sons, New York, N.Y., 1978), Chap. 4. 2. P. Hariharan, Optical Interferometry, (Academic Press,

San Diego, Calif., 1985), pp. 134-137.