OPTICAL SCANNING WITH MEMS IN-PLANE VIBRATORY GRATINGS AND ITS APPLICATIONS

Guangya Zhou*, Yu Du, Kelvin K.L. Cheo, Hongbin Yu, Fook Siong Chau Dept. of Mechanical Engineering, National University of Singapore

*Corresponding author: Guangya Zhou, mpezgy@nus.edu.sg

ABSTRACT MEMS optical scanners are highly desired due to their low-power, high-speed scanning. The in-plane vibratory grating scanner is a development in this area which possesses several unique features. The in-plane scanning minimizes the dynamic deformation, allowing for higher-resolution displays. The dispersive element permits splitting the incoming beam into its constituents for analysis and imaging. Coupling a grating platform to an in-plane moving structure is useful for real-time motion measurement which would otherwise be difficult to analyze. These applications are described including a recent development in the structural design of a double-layer layout which further improves the performance of the grating scanner. INTRODUCTION Microelectromechanical systems (MEMS) based micromirror laser scanners [1][2], which utilize out-of-plane deflection of a reflective surface to scan the laser beam, were mostly developed due to their outstanding advantages, such as having a miniaturized device size, a high scanning speed, low power consumption and a low per-unit cost, compared with macro laser scanners. The need to balance between maximizing scanning performance with minimizing the dynamic non-rigid body deformation of the reflective surface and significant aberration to the optical system during high speed scanning [3][4] has been a limiting factor in their development. MEMS vibratory grating scanners [5]-[7], which utilize in-plane rotation of a diffraction grating, have the potential to scan at a high scanning speed with enhanced mechanical stability and subjected to less dynamic deformation. Multi-wavelength collinear laser scanning, suitable for miniaturized raster-scanning color display applications, was demonstrated [8]. High optical efficiency was also achieved (close to that of a coated scanning mirror) using subwavelength gold-coated diffraction gratings [9] and large optical scan angle using a 2-DOF circular resonator design [10]. DOUBLE-LAYER MEMS GRATING SCANNER The initial designs had the actuation mechanism and

the grating platform in the same plane. Since the rotational angle of the grating platform is inversely proportional to the diameter of the diffraction grating when the maximum allowable deformation of platform suspension flexures is fixed, the aperture size and the optical scan angle cannot be increased simultaneously. This significantly limited both the aperture size and the scanning angle. Recent double-layer design overcomes this limitation. In this configuration, the driving actuators (Fig.1a) and the grating platform (Fig.1b) are fabricated separately. They are then post-assembled to form a double-layer design (Fig.1c & 1d). The size of the diffraction grating can be increased and the size of the connection platform reduced independently, thus increasing the aperture size and the scanning amplitude simultaneously. Moreover, the structural thickness of the grating platform and the actuation layer can be varied independently. Thinning the grating platform and increasing the actuators can lower the rotational inertia and increase the structural rigidity respectively. This is useful in maintaining the operating frequencies on the back of having a larger rotational inertia when the aperture size increases.

Figure 1: a) The actuation layer with connection platform, b) the bottom of grating platform with connection pillar, c) the assembled double-layer device, d) close-up showing the separated layers of the top grating and the bottom suspension flexures.

2010 International Conference on Optical MEMS & Nanophotonics 21 978-1-4244-8925-1/10/$26.00 ©2010 IEEE

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A prototype scanner with a 2mm diameter diffraction grating was fabricated. Fig. 2 shows the measured frequency response of the prototype grating scanner near the region of the first resonating mode. The inset shows a photograph of the projected laser scanning trajectory on a projection screen, which is located at a distance of 100 mm from the grating scanner. We were able to achieve scanning at a frequency of 23.391 kHz with an optical scan angle of 33º, resulting a θopticalD product of 66 deg·mm.

Figure 2: Frequency response of the grating scanner with inset of projected scan line on a projection screen.

APPLICATIONS The in-plane vibratory grating scanner mechanism opens up several developments. Fundamentally, it is a potential large-scan-amplitude and high-frequency optical scanner. With the double-layer design, high-resolution laser projection displays [11] are possible. The presence of a dispersive grating element can also be utilized to configure the grating scanner as a low-cost, miniature line hyperspectral imager [12]. As shown in Fig.3, we had previously demonstrated the viability to capture the spectral image of two different wavelength laser diodes positioned in a straight line 11mm apart.

Figure 3: Captured spectral image showing the location and corresponding wavelength of the two laser diode inputs.

Another novel application is in the real-time precision measurement of in-plane motion of micro devices. Similar to the geometrical signal amplification of an AFM tip for out-of-plane deflection, measurement of in-plane motion of microstructures can be augmented using a grating platform. With one end of the grating directly connected to the movable platform under test, while the other end is fixed to the substrate, the in-plane movement of the platform will be translated into the grating’s rotation. With a laser beam incident on the grating, the small rotation angle of the grating is magnified into a diffracted linear scan onto a PSD for measurement as in Fig.4.

Figure 4: Grating platform configured to measure in-plane displacement of a movable structure. REFERENCES [1] P.M. Hagelin and O. Solgaard, IEEE J. Sel. Topics

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