Development and testing of a holographic projection system
Wu Jiang and David L. Shealy
Department of Physics
The University of Alabama at Birmingham
1300 University Boulevard, 310 Campbell Hall
Birmingham, AL 35294-1170
Kenneth M. Baker
Optimerix Company
13659 Victory Boulevard
Van Nuys, CA 91401-1735
Abstract
This paper describes the development and testing of a holographic projection system which is used to produce
micro-optical devices. The projector uses a two-dimensional two-phase-level diffraction g rating to produce
multiple coherent beams and an interferometric optical system behind the grating to recombine the beams to
produce interference patterns that have been recorded within a photosensitive substrate. The two different
substrates used in this work are a diazo imaging material and bisbenzocyclobutene (BCB) polymeric resin for
fabrication of surface relief microstructures. After the exposed photosensitive substrate is developed, the
recorded interference pattern forms a micro-optical device. The analysis and testing of these micro-optical
devices show promise that this technique can form patterns uniformly over a region of several centimeters in
diameter on flat or curved substrates. The experimental testing results of these micro-optical devices
demonstrate that this method is a simple and energy efficient system to produce micro-structures compared to
other methods. These devices may he used as a new generation of directional light filters or monolithic micro-lens arrays that may have applications in communications, display, and components technologies.
Keywords: Holographic projection system; micro-optical devices; interference; diffraction.
*Present address: Instruments S.A., Inc., 6 Olsen Avenue, Edison, NJ 08820
Introduction
Holographic projection processing of images can be used to record desired interference patterns on photosensitive substrates to form arrays of micro-structures over a large area. However, if a non-uniform laser beam profile is used, the intensity of the recorded pattern is regionally non-uniform since a uniform interference pattern is not obtained. Conventional holographic projection systems[1-4] based on dividing a laser into multiple beams utilize spatial filters to truncate a laser beam to achieve a uniform profile. Compared to these holographic projectors, the present projection system [5-8] consists of a UV Argon ion laser, a beam expander, a beam reshaping system which reshapes the Gaussian beam profile of the laser into a uniform beam profile, a two-dimensional two-phase-level diffraction grating used to produce multiple coherent beams, and an interferometric optical system behind the grating to recombine the beams. This system provides high energy transmission efficiency, since it utilizes a laser beam reshaping system.[9-11]
This beam profile reshaper consist of two plano-aspheric lenses, and it can be designed to convert an arbitrary radially symmetric beam profile into a uniform beam profile with high energy efficiency. Thus, a Gaussian beam can be converted into a uniform beam without truncating the incoming beam profile. The design of these plano-aspheric lenses is based on two principles: conversation of energy and constant optical path length. These two conditions allow almost all of the energy of the incoming beam to be transferred into the output beam and allow the output beam to have a plane wavefront. if the incoming beam wavefront is also a plane. Employing such a beam reshaping system gives this projector a high energy efficiency, which is very important in this particular application as well as in many other illumination applications.
Another unique feature of this system is that it utilizes a two-dimensional, two-phase-level diffraction
grating to generate multiple beams instead of using beam splitters and mirrors, which are difficult to align and
have low energy efficiency. Therefore, this projector is a very compact and simple system compared to other
holographic projectors.[1-4] These two distinctive features (beam profile reshaping system and diffraction
grating) of this projector make it possible to mass produce monolithic microlenslet arrays since large elements
can be produced with a uniform grid pattern. The next two sections of this paper present more details about
the theory and the testing of this projection processing system.
Theory
In this holographic projection system shown in Fig. 1, radiation of 0.364 u m wavelength from an Argon ion laser has been used, since the present substrates are sensitive at this wavelength. An expansion system is located behind the laser to enlarge the laser beam. After the laser beam is expanded, a beam intensity profile reshaper has been used to transform the, Gaussian beam profile into a uniform profile with a high energy transmission efficiency. Therefore, the interference pattern will have a uniform grid over the entire surface of the substrate. Otherwise, the interference pattern will have maximum intensity at the center and will trail off at the edges if a Gaussian beam profile is utilized, and the non-uniformity of the pattern will be revealed in the microstructure formed. The design of this reshaper[9-11] is based on the conservation of energy and a constant optical path length condition. As shown in Fig. 2, the collimated incoming beam has an energy per unit area p(r), which is incident upon the primary lens at a radius r from the optical axis, and the output beam that leaves the optical system as a collimated beam at a radius R has an energy per unit area u(R), which is desired to be uniform so that u(R) = constant. The aspheric surface of the primary lens deviates the plane wave Gaussian input beam so that the intensity is uniform at the second lens. The first lens is designed to satisfy the first condition, namely, the energy conservation condition. The second lens is designed to satisfy the second condition, namely, the constant optical path length condition. It allows all rays passing through this system to have the same optical path length so that the wavefront of the laser beam is retained. The second aspheric surface has no effect on the energy distribution but deviates the rays so that the outgoing wavefront is a plane wave if the input wavefront is also a plane wave. By applying Snell's Law to both surfaces, the slopes of the two surfaces can be solved. The surfaces of the two lenses can he obtained by integrating the corresponding slopes of radius r and R. Therefore, this two-plano-aspheric-lens system is solved, and it reshapes the Gaussian beam profile into a uniform beam profile without truncating the beam or distorting the wavefront. In other words, it has high energy throughput, which is very important in this application.
After the laser beam profile is reshaper, the binary grating is placed behind the uniform beam to divide the amplitude of the beam into multiple beams. This binary grating is a two-dimensional two-phase-level holographic diffraction grating as shown in Fig. 3. Because each cell of the diffraction grating is a square, four equiangular diverging symmetric spectral orders which have the exact size and shape of the original laser beam are produced. Because the phase difference between the two levels of this grating is designed to be pi, the zeroth diffraction order of the laser beam is eliminated, and the four first order beams which contain 65% of the total energy[12] are utilized to generate the interference pattern. These four diverging and symmetric beams are recombined by an interferometric system to produce the desired interference pattern within a substrate which may be a diazo imaging material or a photo-imageable bisbenzocyclobutene (BCB) polymeric resin. In this prototype, eight prisms were used as the interferometric optical system to recombine the four beams as shown in Fig. 4. Each pair of the prisms was assembled behind one beam face to face. When a pair of the prism was rotated, the beam angles would rotate accordingly so that the four diverging beams could be recombined at a focal plane. A multiple lens system [12] was designed for a production system to combine these multiple, coherent beams over a curved surface.
This holographic projection system has also been analyzed by a physical optics analysis code, GLAD.[13] A typical energy distribution of the interference pattern of these four beams is shown in Fig. 5. Since these four beams are symmetric, the interference pattern is a uniform two-dimensional grid.
The grid size of the interference patterns produced over a plane perpendicular to the symmetry axis
of this system can be calculated according to the following formula [14]:
D = L / Q..............................................................(1)
where D is the distance between two successive interference maxima or minima, L is the laser wavelength, and Q (phi in figure) is the angle between two of these four beams which are diagonally opposed to each other as illustrated in Fig. 6. By measuring the distance between the two centers of the prisms and the distance between the location where the interference occurs and the, location of the prisms, Q can be calculated so that D can be obtained.
Results
When the laser beam passes through the expansion system, the reshaper, and the two-dimensional two-phase-level diffraction grating, multiple divergent beams are generated. Each of the four first order diffraction beams
pass through a pair of prisms so that they can be recombined at a focal plane to produce an interferometric
pattern in the region of space where the beams overlap, as shown in Fig. 6. Two different photosensitive
substrates were used: diazo film and photo-imageable BCB (The Dow Chemical Company Cyclotene 4026-46)
resin which were perpendicular to the symmetry axis and were exposed to this uniform, two-dimensional
intensity grid.
The first photosensitive substrate was coated with an aromatic diazo compound-coupler system consisting of diazo, acid, and coupler. When the coating is exposed by a certain amount of energy, the diazo decomposes. Therefore, the energy maxima of the interference pattern extends through the substrate to decompose the diazo. After the exposure, the substrate was developed by moist ammonia gas to neutralize the acid so that the diazo in the area of destructive interference reacts with the coupler during the developing process to produce azo-dye. The azo-dye forms the walls of a micro-structure. Since diazo molecules have a certain cut off energy level, the micro-structure recorded in the substrate has a sharp edge instead of the sinusoidal-like distribution as the intensity interference pattern.[14] This was verified by the measurement of the microstructure using phase contrast microscopy as shown in Fig. 7. According to Eq. 1, the grid size of the pattern can be calculated by measuring the experimental setup. If the angle, (Fig. 6) between two beams is measured to be Q = 0.032 rad, and L = 0.354 um. Then, D = 11.43 um. After recording the interference pattern on the photosensitive substrate, a laser beam was passed through the substrate. The diffraction pattern generated by this substrate which contains the microstructure of the interference pattern was measured to determine the size of the micro-structure in the substrate. Figure 8 represents a schematic drawing to illustrate a laser beam passing through a micro-structure, The beam diffracts in certain directions according to the grating equation [14]:
|
d sin (theta) = m L ........................(2) |
where d is the grating constant, m is the diffraction order, (theta) is the angle of the m-th diffraction order,
L is the laser wavelength. By measuring the distance between the zeroth and the first order diffraction on the
observing screen and the distance between the substrate and the observing screen, can he calculated.
Therefore, d can be determined according Eq. 2. In order to observe the diffraction pattern in the visible
region, a HeCd laser with wavelength of 0.441 um was used to test this system. Therefore, in Eq. 2, L = 0.441
um, and (theta) is calculated to be 0.045 rad. The dimension of the micro-structure, or the grating constant
was obtained: d. = 9.7 um. This is consistent with the theoretical predication (Eq. 1) even through the testing
result (d) is 15% different from the theoretical calculation (D). The size of the microstructure of the substrate
was also verified by measuring the microstructure by phase contrast microscopy as shown in Fig. 7.
Based on the calculation (Eq. 1) and the experimental testing of the micro-structure of the photosensitive substrate (Fig. 7), we conclude that micro-optical structures with a diameter of approximately 10 um have been produced within a photosensitive substrate using the method of exposure and development. The structure of the micro-optical elements is analogous to the structure of (he intensity interference pattern produced by this holographic projection system.
The second photosensitive material, Cyclotene 4026-46 with a nominal thickness of 10 um, was also
used to record the interference pattern of this holographic projection system. The exposure was made through
the base of a coated substrate, Therefore, the energy intensity grid of the interference pattern generated by this
holographic projection system was recorded normal to the, surface of the coating, but in reverse direction.
After the exposure, the coating was developed (Advanced Development System DS2100) and thermally cured
according to the processing procedure.[15] Cyclotene 4026-46 is a negative acting resin formulated for
maximum photosensitivity at 0.365 um. After full processing it has a refractive index of 1.562 at 0.589 um
and has been used for the fabrication of optical channel waveguide and interconnects.[16] Because of
absorption, the sinusoidal contoured intensity wells shown in Fig. 5 become defined as the near-paraboloid
microstructures in the microlenslet array shown in the SEM picture of Fig. 9. The periodicity here is about
10 um. This result clearly demonstrates that fabrication of microlenslet arrays by this holographic projection
system is simple and efficient.
Conclusions
These experimental results have demonstrated that this holographic projection system can produce full beam, uniformly illuminated interference patterns on photosensitive substrates or holographic recording films to generate large arrays of micro-optical elements. These micro-optical devices can he used as a new generation of directional light filters and monolithic microlenslet arrays. This projector is a simple, energy efficient, and cost-effective system compared to conventional holographic projection systems. This system also can be used to produce these micro-optical elements on a curved surface, which may be used in many new and. interesting design applications.
The authors would like to express gratitude to NASA/Alabama Space Grant Consortium and NSF-ESI grant[NSF-OSR-9450570, 8/1/94-7/31/95, "Alabama Laser Research Initiative," C. M. Lawson (PI),
D. L. Shealy (Co-PI), and S. B. Mirov (Co-PI)] for partial funding of this research during 1994 and 1995.
The authors also would like to thank Rick Foster, Marketing Manager, Advanced Electronic Materials, The
Dow Chemical Company for his generous contribution of Cyclotene photo-imageable BCB resins and
developer, along with Mr. Brian Gaschen in the Biology Department of The University of Alabama at
Birmingham for his help in the characterization of the microstructure within the diazo film.
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