TECHNICAL NOTES

Method for splitting low power laser beams Barbara K. Pierscionek

University of Melbourne, Optometry Department, Park-ville, Victoria 3052, Australia. Received 3 April 1989. 0003-6935/90/101406-01$02.00/0. © 1990 Optical Society of America.

A new method for producing parallel rays from a laser beam using a cylindrical lens and pinholes is presented. This method can produce a greater number of emergent rays than using a beam splitter.

In recent years several studies have utilized ray tracing techniques to investigate optical properties of the crystalline eye lens: to derive data for determination of refractive index variations1–3 and to assess the level of spherical aberration.4,5

Since the tissue is of varying index it is advantageous in such studies to have as narrow a beam as possible to detect, so that for each ray path the tissue region sampled is very small. It is also desirable, particularly for studies of spherical aberra­tion, to pass several rays simultaneously through the lens. Sivak and Dovrat5 described a technique in which they used a beam splitter consisting of a partially silvered and full mirror to split the incoming beam. This severely limits the number of rays into which the initial beam can be split because at every partial reflection the intensity of the beam is decreased by a constant factor depending on the property of the beam splitter.

In this Technical Note another method of splitting laser beams which allows for a greater number of emergent rays is described. The laser beam is refracted by a negative cylin­drical lens oriented with its axis vertical, which elongates the intensity profile of the beam in the specified direction. The resulting elliptical image then passes through a vertical aper­ture (slit) to limit the ray bundle in the horizontal direction. This diverges onto a plate consisting of a line of pinholes of equal size and spacing on one side and a neutral density filter (ND 0.6) on the other. The beam is subsequently broken into component rays, and the filter attenuates the intensity of the rays so that the effect of diffracted light is sufficiently reduced. Finally the divergent rays are collimated by a low power positive lens (Fig. 1).

Using this method the number of components is governed by the size of the pinholes and the initial power of the negative cylindrical lens. With a –12-D cylindrical lens set at a 90° axis placed at a distance of 5 cm from a 5-mW He–Ne source with beam diameter of 0.68 mm, the size of the beam falling on the pinholes placed 90 cm beyond the cylindrical lens is magnified 18.6× in the vertical direction. If the pinholes are of 0.25-mm radius and separated at 1 mm be­tween their centers, up to twelve emergent rays can be pro­duced. In Fig. 2 is shown a series of such rays passing through the sagittal plane of a bovine lens.

1406 APPLIED OPTICS / Vol. 29, No. 10 / 1 April 1990

Fig. 1. Diagrammatic representation of schema used to split laser beams into component rays.

Fig. 2. Ray paths through the sagittal plane of a bovine lens.

References 1. M. C. W. Campbell and A. Hughes, "An Analytic Gradient Index

Schematic Lens and Eye for the Rat which Predicts Aberrations for Finite Pupils," Vision Res. 21, 1129–1148 (1981).

2. M. C. W. Campbell, "Measurement of Refractive Index in an Intact Crystalline Lens," Vision Res. 24, 409–415 (1984).

3. B. K. Pierscionek, D. Y. C. Chan, J. P. Ennis, G. Smith, and R. C. Augusteyn, "A Nondestructive Method of Constructing Three-Dimensional Gradient Index Models for Crystalline Lenses: 1. Theory and Experiment," Am. J. Optom. Physiol. Opt. 65, 481– 491 (1988).

4. J. G. Sivak and A. Dovrat, "Early Postnatal Development of the Rat Lens," Exp. Biol. 43, 57–65 (1984).

5. J. G. Sivak and A. Dovrat, "Embryonic Lens of the Human Eye as an Optical Structure," Am. J. Optom. Physiol. Opt. 64, 599–603 (1987).