The production of a variable array of optical point sources from a single point source can be achieved through the self-imaging properties inherent in a rectangular waveguide. Two prototype devices, based upon this concept, were designed and constructed. The resulting output patterns are discussed along with future design considerations and applications

Alarie, J. P.

Allison, S. W.

Earl, D. D.

Gillies, G. T.

Hueber, D.

Landis, D. A.

Vo Dinh, T.

UNT Digital Library

Beamsplitting Using Self-Imaging By: D. D. Earl, S. W. Allison Engineering Technology Division Oak Ridge National Laboratory T. Vo Dinh, J. P. Alarie, D. Hueber, D. A. Landis Health Sciences Research Division Oak Ridge National Laboratory and G. T. Gillies University of Virginia August 7,1996 Prepared by the Oak Ridge National Laboratory, Oak Ridge, Tennessee, 3783 1-8058. managed by Lockheed Martin Energy Research Corporation, for the U. S. Department of Energy under contract DE-AC05-960R22464. The submitted manuscript has been authored by a contractor ofthe U. S. Government under contract No. DE-AC05-960R22464. Accordingly, the U. S. Government retains a nonexclusive. royalty-free license to publish or reproduce the published form ofthis contribution, or allow others to do SO, for U. S. Government purposes. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process d i s c i d ,  or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, proms, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. . Beamsplitting using self-imaging D. D. Earl, S. W. Allison, T. Vo Dinh, J. P. Alarie, D. Hueber, G. T. Gillies, D. A. Landis Oak Ridge National Laboratory, Oak Ridge, TN 3783 1 ABSTRACT The production of a variable array of optical point sources from a single point source can be achieved through the self-imaging properties inherent in a rectangular waveguide. Two prototype devices, based upon this concept, were designed and constructed. The resulting output patterns are discussed along with future design considerations and applications. Keywords: self-imaging, rectangular waveguides, beamsplitting, optical arrays. 1 .  INTRODUCTION The production of multiple beams from a single beam can be accomplished through the self-imaging properties inherent in a rectangular waveguide. The multiple virtual images produced as a result of reflections within the rectangular waveguide' can be collected and focused into a compact bundle of diverging beams. With the cooperation of Tuan Vo Dinh, David Landis, and Oak Ridge National Laboratories, this technology is being applied to the field of Health Physics. Assisting their research with DNA sequencing', self-imaging beamsplitters have been employed to replace the optical masks often required in multiple sample DNA fluorescence testing. These beamsplitters provide a simplified, precise, and efficient means of illumination3. What follows is a brief mathematical explanation of how these multiple beams are formed along with a discussion of the experimental results and their application to an improved self-imaging beamsplitter design. 2 .  MATHEMATICAL DEVELOPMENT As light enters a rectangular waveguide from a diverging source(see Fig. l), the number of internal reflections experienced is dependent on the waveguide dimensions as well as the angle of divergence for the source.' Fig. 1. SeIJrimaging schematic. As is illustrated in Fig. t , each internal reflection creates a virtual image which can be focused to a point with the aid of a lens. The total number of points produced is (2n + 1)*, where n is the number of internal reflections. This can quickly be deduced if one considers the rectangular waveguide to be a set of two orthogonal pairs of mirrors, each producing 2n virtual images. Since we have two sets of mirror pairs, then we will produce a total of (2n)2 virtual images for the rectangular configuration. In addition, a central spot resulting from unreflected rays, and 4n spots produced due to the divergence occurring at the corners of the waveguide, must be included in our calculations, giving (4n2 + 4n + 1) imaged points. Consequently, factoring this equation we arrive at (2n +- 1)2 focused spots. The spacing between each focused spot is dependent upon the waveguide width, the distance of the source to the focusing lens, and the magmfication of the focusing iens. It can easily be seen that the number of reflections occurring within the waveguide can be altered by adjusting either the dimensions of the waveguide or the distance between the waveguide and the source’. 3. EXPERIMENTAL RESULTS The output beams produced by a variety of self-imaging rectangular waveguides were measured. First, a nine beam pattern was produced using a red HeNe laser focused with a microscope objective into the end of a 25 x 2.36 x 2.47mm rectangular quartz wavepde and collected via a 25mm focal length spherical lens. What resulted was a pattern with a large central beam intensity surrounded by eight slightly lesser beams (Fig. 2a). Each beam was capable of being focused to a fine point within an area measuring roughly 5mm by 5mm. This area could easily be changed by either altering the collecting lens-to-waveguide distance or by changmg the lenses focal length. This system, however, required precise alignment to produce the expected number of output beams with misalignment resulting in an uneven intensity distributed and/or “double vision.” Intensity Profile for Self-imaging Beamsplitter bnrilyfto(l* fwsewhplllng O e a n l s p U b r l D l l h a C  sanw) (a) (6) Fig. 2. Graphs reveal the dgfferences in intensity profiles for an uncoated rectangular waveguide with focused source (a) and a silver coated rectangular waveguide with a d@se source (3). In an attempt to reduce the critical alignment required of the first system, a .5mm pinhole mask was centered and attached to the optical entrance of the waveguide. A diffuser was then brought into contact with the mask and illuminated by the source (see Fig. 3). What resulted was a system which was always aligned regardless of the direction or location of the illuminating source and was, therefore, alignment free. In addition, the input Beam .... WNcguide -.... ____) Pkhnholc Fig. 3. Alignment-pee design waveguide was coated with silver to reduce the amount of light lost upon reflection. This produced a pattern which had a 30% lower peak intensity then the first system’s pattern and with a slightly more Fig. 4. Pattern producedkorn a silvered waveguide with difuser uniform intensity distribution over its nine beams (Fig. 2b & 4). However, since the diffuse source was an extended object (.5mm diameter) as opposed to the focused point source of the first system, it was noted that the beams could not be focused to as small of a point as had been done previously. Experiments were also performed regarding the possible variations in waveguide dmensions. As expected, it was found that smaller cross section rectangular waveguides produced a larger amount of output beams for a given input angle. The best choice of collecting lenses was also qualitatively analyzed, It was observed that spherical lenses produced a noticeable distortion in the focused beams, due to a variety of optical aberrations. This effect was greatly reduced by using shorter focal length aspherical lenses. 4. DESIGN DETAILS Possible designs for the production of a standard multiple output beamsplitter, which could be easily interfaced with a variety of light sources, were considered. A prototype system was developed which used a 1 mm silica gel fiber optic cable as the input source for a silvered rectangular quartz waveguide. The optical fiber as well as the waveguide were mounted in two holes centered with one another which had been drilled into a 2” Iong plastic cylinder. An aspherical lens of short focal length was also mounted in a larger hole, also centered with the waveguide, and used to collect and focus the output Fig. . Compact self-contained beamsplitter (proto ype f: 1. beams. The resulting design was a device which was unaffected by alignment, efficient, simple and inexpensive to produce, and easily customizable (see Fig. 5). This compact beamsplitter was 97% efficient in splitting the beam diverging from the 1 mm core fiber. As expected, changing the core diameter, or the numerical aperture, of the optical fiber produced a change in the diameter of the focused spots. Therefore, a second device was constructed which allowed the optical fiber to be connected via an SMA connector, thus allowing the optical fiber’s core diameter to be easily varied. This second device was constructed out of aluminum (1.5” length, .5” diameter) and was designed so that the output lens was adjustable. A smaller uncoated quartz waveguide (25 x 1.5 x 1.5mm) was used to produce a total of 49 output beams with high efficiency. 5. FUTUREGOALS Future plans involve designing a third prototype which is capable of an even greater number of output beams with an increased uniformity in intensity. Experiments involving birefringent materials and intentional misalignment will also be conducted to determine advantages of “double vision.” An ongoing attempt will also be made in every design, to create systems which result in as flexible and versatile a device as is possible. In addition, the possibility of using the self-imaging rectangular waveguide beamsplitter as a four Preliminary experiments indicate that it is faced surface plasmon biosensor will be investigated. possible to coat each face of a rectangular waveguide with a thin metal island film, which when used as a beamsplitter, produces a 3 x 3 array with intensities dependent upon the opto-electronic coupling at the surface of each face. Therefore, each face would act as an individual sensor with a corresponding focused spot in the 3 x 3 array. The central spot, which does not correspond to any surface, would serve as a reference for this four-sensors-in-one device. These possibilities and others will be considered as possible applications of the self-imagmg beamsplitter. 6 .  ACKNOWLEDGMENTS This research was supported in part by an appointment of D. A. Landis to the Oak Ridge National Laboratory Postdoctoral Research Associates Program, administered jointly by the Oak Ridge Institute for Science and Education and Oak Ridge National Laboratory. 7. REFERENCES 1 .  S. W. Allison, G. T. Gillies, “Observations of and applications for self-imaging in opticai fibers,” Appl. Opt. 33, 1802-1805 (1994) 2. T. Vo D i d ,  N. Isola, J. P. Alarie, D. A. Landis, D. Hueber, “Development and Applications of Non-Radiative Gene Probes: A Biosensor Microclup” Invited manuscript for Trends in Analytical Chemistry. 3. R. Ishikawa, H. Nishimoto, K. Minemura, and S. Matsushita, “Kaleidoscope micro-optic star coupler,” Electron. Lett. 16,249-251 (1980) 4. 0. Bryngdahl, “Image formation using self-imaging techniques,” J. Opt. SOC. Am. 63, 416-419 ( 1973) 5 .  J. H. Myer, “Zoomable kaleidoscopic mirror tunnel,” Appl. Opt. 10,2179-2182 (1971) 

Beamsplitting using self-imaging

https://digital.library.unt.edu/ark:/67531/metadc667502/m2/1/high_res_d/288963.pdf

Beamsplitting using self-imaging

Abstract

Similar works

Full text

Available Versions

UNT Digital Library