Development of a Double-Sided Micro Lens Array
for Micro Laser Projector Application
Fong-Zhi CHEN, Cheng-Huan CHEN1, Cheng-Hsien WU2, Ching-Hsiang KUO, Jyh-Rou SZE, Wei-Yao HSU, and Yuan-Chieh CHENG
Instrument Technology Research Center, National Applied Research Laboratories, No. 20 R&D Rd. VI, Hsinchu Science Park, Hsinchu 300, Taiwan
1Department of Power Mechanical Engineering, National Tsing Hua University,
No. 101, Section 2, Kuang-Fu Road, Hsinchu 300, Taiwan
2Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences,
No. 415 Chien Kung Road, Kaohsiung 807, Taiwan (Received December 27, 2010; Accepted May 10, 2012)
This paper introduces the development of a double-sided micro lens array (DSMLA) for application in micro laser projectors. For commercial mass production, it is necessary to investigate the concurrent engineering of optical design, mold fabrication, and plastic injection molding at once. This experiment based the design of the micro lens array on the scalar diffraction theory. The proposed DSMLA can simultaneously shape red, green, and blue laser beams into a uniform projection pattern. An ultra precision diamond turning machine using a slow tool servo method fabricated the mold. The study considered optical design constraints from the feedback of mold fabrication and plastic injection molding, measuring and comparing fabricated samples with calculated results. Experimental results show that the fabricated DSMLAs achieve the desired function and application feasibility for micro laser projectors.
# 2012 The Japan Society of Applied Physics
Keywords:double-sided micro lens array, micro laser projector
1. Introduction
Lens arrays have found wide use in optics products such as projectors. Because the miniaturization of projectors is a common trend, applying light-emitting diode (LED)1) and lasers as the light source of a projector can reduce weight and size substantially. This study investigated using a micro lens array in micro laser projectors.
The uniform illumination and rectangular shape of laser beams are essential for laser projectors. In the past few years, holography, machining, heat treatments, and so on, have used a laser beam with uniform illumination. Sze et al. proposed a single refractive element to synthesize a circular symmetric flattop beam.2)Deng et al. presented the results using the lens array for uniform illumination of large targets.3)Wippermann et al. applied the double-sided micro lens array (DSMLA) with a tilt angle to improve the uniformity.4)Pan et al. successfully applied DSMLAs to an LED uniform illumination design for the micro projector.5) Few studies attempted using the lateral position adjustment of the DSMLA double-sided surfaces to improve illumina-tion uniformity of the laser output pattern. This paper experimentally introduces the proposed DSMLAs with a lateral position adjustment of double-sided surfaces.
Because fabrication techniques are critical in commercial application of micro lens arrays, researchers have studied various approaches. Generating a micro convex lens array using photolithography and a photo-resist reflow process,6)
an electron beam lithography on sol–gel material,7)a coding gray-tone mask method,8)and a focused ion beam9)are a few
of the various approaches. A study5)also discussed using the micro lens array, composed of a transmissible glass substrate covered with two hot embossing transparent thermoplastic poly(methyl methacrylate) (PMMA) layers. Chen et al. proposed the slow tool servo method on a diamond turning machine to fabricate the aspherical micro lens array.10)This paper verifies that a mold fabricated by the slow tool servo method and plastic injection molding for DSMLAs meets the requirements of commercial mass production.
2. Design and Fabrication of DSMLA
LEDs and lasers have supplanted high intensity discharge (HID) lamps as the light source in projectors for superior color efficiency and miniaturization effect. Figure 1 shows laser light sources combined with a DSMLA to homogenize illumination. The micro lens array consisted of lenses with an identical radius R arrayed with pitch P. The surfaces of the micro lens arrays were arranged by shifting the displacement S on different sides.
The illumination analysis was based on the scalar diffraction theory, because the aperture of each lens was larger than the wavelength of the laser light used.11) As shown in Fig. 2, the input optical field behind the DSMLA is denoted by uð; Þ. The input optical field propagates distance z, and forms the output field Uðx; yÞ in the output plane: Uðx; yÞ ¼ ie ikz z exp ik 2zðx2þ y2Þ ZZ uð; Þ exp i2 zðx þ yÞ d d ð1Þ
E-mail address: chen@itrc.narl.org.tw
OPTICAL REVIEW Vol. 19, No. 4 (2012) 238–241
DSMLA with a high frequency actuator to reduce the obscure speckles. The uniformity and speckle pattern are improved substantially.12)
For micro laser projector applications, the rectangle uniform illumination pattern of a liquid crystal display (LCD) is essential. This study attached a cylindrical lens to implement the rectangle illumination pattern and Fig. 10(a) shows the test schematic arrangement. The test also simultaneously overlapped the red, green, and blue laser illuminations. Figure 10(b) shows the test results. The aspect
ratio of the illumination pattern with a diagonal distance of 12.9 mm is approximately 16 : 9. The modification of the aspect ratio with a cylindrical lens does not degenerate illumination uniformity.
4. Conclusion
Experimental verification reveals that a DSMLA with an appropriate lateral shift can be a novel and feasible design to homogenize laser beams in a micro laser projector. Shifting two micro lens arrays on opposite sides produces a square flattop illumination pattern. The cylindrical lens can reform the aspect ratio of the illumination area and maintain its uniformity. A slow tool servo on an ultra precision diamond turning machine fabricated the mold properly, which is suitable for mass production. Results show that the slow tool servo technology can fabricate an optical mold with good form accuracy and surface roughness. Plastic injection molding successfully generated a prototype that represented good replication ability. To reduce the thickness and to improve the misalignment from different sides of DSMLA, the optimal injection molding parameters requires further investigation. This study implemented concurrent engineer-ing coverengineer-ing the design, fabrication, and application of the DSMLA to micro laser projectors.
Acknowledgements
The National Science Council, Taiwan, ROC, supported this work under grants: NSC 492-001, and NSC 98-2221-E-492-002. The authors highly appreciate and acknowledge the facilities and experimental data provided by Instrument Technology Research Center of National Applied Research Laboratories, Micro Optics and Display System Laboratory of National Tsing Hua University, and Polymer Processing Laboratory of Kaohsiung University of Applied Sciences.
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11.2 mm 11.2 mm 11.2 mm
Red 650 nm Green 532 nm
Blue 473 nm
Fig. 9. (Color online) Measured illumination distributions with R, G, and B lasers.
(a)
Dichroic Plate (Long Pass)
Dichroic Plate (Short Pass)
DSMLA Diode laser G B R Microdisplay (b) 11.2 mm diagonal= 12.9 mm 6.4 mm
Fig. 10. (Color online) (a) Schematic of illumination test with arranging a cylindrical lens. (b) Illumination pattern at 60 mm with overlapping R, G, and B lasers.
