Design Optimization and Implementation of a Miniature Optical ...

Design Optimization and Implementation of a Miniature Optical Coherence Tomography Probe Based on a MEMS Mirror Donglin Wang1,2, Linlai Fu2, Jingjing Sun3, Hongzhi Jia1, and Huikai Xie2,3 1 University of Shanghai for Science and Technology, Shanghai, China 2 WiO Technology Ltd., Co., Wuxi, Jiangsu, China 3 University of Florida, Gainesville, Florida, USA ABSTRACT Optical coherence tomography (OCT) provides non-invasive cross-sectional imaging capability and high resolution, but it has very limited applications inside human body because of the stringent size requirements for accessing the internal organs. Micro-Electro-Mechanical Systems (MEMS) is an emerging technology that can make devices with small size and fast speed. This paper reports the design optimization of a MEMS mirror-based miniature OCT probe. The probe consists of three main parts: a GRIN lens module (1.3 mm in diameter), a MEMS mirror (1.7 mm x 1.55 mm), and a stainless steel mount. A special assembly holder is designed for easy placement of parts and accurate optical alignment and real-time monitoring of optical alignment and electrical characteristics is also used to the assembly process. Code V is used for the optical design and analysis. Simulation shows that the changes of the spot size and focal length are within the acceptable range when the distance between the optical fiber and the GRIN lens varies less than 0.1 mm. The fiber may tilt as much as 2.5 degrees without any considerable change of the spot size and working distance. The maximum tolerance to the lateral shift between the fiber and GRIN lens is about 0.1 mm. Key words: OCT, MEMS, Endoscope probe, Code V, Optical design,

Introduction Optical coherence tomography (OCT) has become a versatile practice in medical imaging applications due to its non-invasive cross-sectional imaging capability and high resolution [1]. Now, it is a common procedure for ophthalmological clinics to diagnose vision loss diseases by using OCT systems [2]. OCT has also widely applied in dermatology and dentistry [3] [4]. Recent research is emphasized on early cancer detection due to the real-time 3-D in vivo imaging capabilities of endoscopic OCT systems [5] [6]. However, OCT has very limited applications inside human body because of the stringent size requirements for accessing the internal organs; and optical scanning must be done at high speed. There are various methods to obtain fast optical scanning, some by rotating a fiber micro-prism [7], or by swinging the distal fiber tip [8], but further miniaturization of the imaging probes is difficult because motors are required. Micro-Electro-Mechanical System (MEMS) is an emerging technology that can make devices with small size and fast speed [9][10]. MEMS-based OCT for endoscopic applications has been explored by several research groups [11] [12]. In this paper, we present our latest work on a MEMS mirror-based miniature OCT probe design. The probe design is similar to the one reported in [13]. It is a side-viewing probe and contains four components, a single mode fiber to deliver an infrared light with the center wavelength of 1310nm, a International Symposium on Photoelectronic Detection and Imaging 2011: Sensor and Micromachined Optical Device Technologies, Yuelin Wang, Huikai Xie, Yufeng Jin, Eds., Proc. of SPIE Vol. 8191, 81910M · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.899645 Proc. of SPIE Vol. 8191 81910M-1 Downloaded From: http://www.spiedl.org/ on 05/29/2014 Terms of Use: http://spiedl.org/terms

gradient refractive index lens (GRIN) to focus the light into a ~20 micron spot at a 5mm distance, a MEMS mirror to direct the light for side-viewing and perform fast optical scanning, and a stainless steel mount to provide strength and ensure optical alignment. In order to simplify the assembly process, the probe is divided into three modules, i.e., a GRIN lens module, a MEMS module, and a mount module. Some tools are also designed to assist the assembly and testing.

Probe Design The probe design is shown in Fig. 1, where the GRIN lens part consists of a single-mode fiber with an FC/APC connector and a GRIN lens. Both the GRIN lens and the fiber are cut with an angle of 8 degree to minimize back reflection. The on-axis refractive index of GRIN lens is 1.59 and its gradient constant is 0.39 for λ=1310nm. CODE-V is used to optimize lens design. The length of the GRIN lens is set to 4.6mm. The fiber is inserted into a glass sleeve to match the diameter of the GRIN lens. A MEMS micromirror with a size of 1.55 x1.7 x0.5mm3 is employed and it is directly mounted on a flexible printed circuit board (FPCB). The mirror surface directs the light from the GRIN lens to the side of the probe to achieve 2-dimensional side-view scanning. The mount part is a stainless steel tube with slots, concentric holes and an inclined plane to hold the GRIN lens and MEMS parts.

GRIN lens Glass sleeve FPCB MEMs mirror Mount part Fiber

Fig.1 3D model of probe design

Effects of MEMS Mirror Curvature and Plastic Tube The mirror surface of MEMS micromirrors is not as flat as that of conventional bulk mirrors due to the inherent residual stresses from microfabricaiton. The effect of the MEMS mirror deformation on image quality is evaluated using Code-V. As shown in Fig. 2, a radius of curvature of 50mm of the MEMS mirror will change the spot size by 4%. If the probe is covered by a circular plastic tube, the light spot at the focal point will become an ellipse and has a 15% increase in size, as shown in Fig. 3.

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Fig.2a Side view scanning by an MEMS mirror

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Design Optimization In order to minimize probe size and reduce cost, all parts need to be modularly designed and the assembly process should be real-time monitored. In the optimized design, the GRIN lens is 4.4mm long with a 0.23 pitch and there is a space between the fiber and GRIN lens to achieve the same optical specifications. This space can also be used to compensate the assembly errors during the probe assembling. Table 1 lists the lens data after the optimization done by using CODE-V. Fig. 4 shows the Gaussian beam trace of the GRIN lens module, where the Gaussian beam radius is 0.009mm at imaging surface. Working distance and spot diameter at imaging plane are our mainly concerns about optical quality when fabricating this lens module. The tolerance analyses depicted in Fig. 5 show the effect of fiber misalignment with GRIN lens by axial separation, lateral displacement and angular tilt. Since the depth of penetration for infrared light in human organs was about 2mm and focusing plane must be located below the surface of tissue, decline of probe working distance by fabrication should not be more than 0.6mm.Spot diameter at imaging surface should not vary more than 5 microns to maintain a ideal focal depth. From Fig.5, we know that when axial separation between the fiber and GRIN lens varies by 0.1mm the focal distance will decrease about 0.5mm and the light spot size becomes 2 microns larger. Although later displacement will not cause any serious degrades in image, it prefer to be controlled in 0.1mm causes the radius of GRIN lens is only 0.5mm.The fiber may tilt as much as 2.5 degrees. In a word, all these tolerances are not tight and can be easily achieved during process of production. Table1 GRIN lens data

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Fig.4 Gaussian beam trace of the GRIN lens module.

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Real-Time Monitoring During Assembling A glass sleeve with an outer diameter of 1.3mm is used to ensure the optical alignment. As shown in Fig. 6, a holder mounted on a microstage is used to hold the fiber and precisely control the separation distance between the fiber end and the GRIN lens. A reflective mirror is placed in front of the GRIN lens. First, the mirror is set at 5mm away from the GRIN lens, and then we manipulate the microstage to obtain the maximum reflected light intensity. This maximum reflection corresponds to the focal point.

Mirror

Fiber holder

Micro stage GRIN lens module

Fig.6 Real-time monitored assembly process

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Conclusion MEMS-based OCT probe design has been optimized. The effects of the deformation of the MEMS mirror and the circular wall of the outer plastic tube have been analyzed. It is found that the MEMS mirror curvature has negligible effect on the light spot size and focal length. The circular plastic tube increases the spot size by 15%. Simulation shows that the tilt, distance and off-center shift of the optical fiber to the GRIN lens have small effect on the spot size and focal distance but all can be controlled easily in the acceptable range. Real-time monitoring is also introduced to improve the assembly process.

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