54.3 / Q. Y. J. Smithwick SID 03 DIGEST  1455 54.3: Modeling and Control of the Resonant Fiber Scanner for Laser Scanning Display or Acquisition Quinn YJ Smithwick 1,2,3 , Juris Vagners 1 , Per G Reinhall 2 , Eric J Seibel 2,3 1 Department of Aeronautics and Astronautics, University of Washington, Seattle, Washington 2 Department of Mechanical Engineering, University of Washington, Seattle, Washington 3 Human Interface Technology Lab, University of Washington, Seattle, Washington Abstract The resonant fiber scanner produces a flying laser spot scan for display or image acquisition purposes. Dynamic nonlinearities during large amplitude vibrations of the resonant fiber scanner result in distortions in the two-dimensional scan pattern and the acquired images. A dynamic model which includes the fibers dynamic nonlinearities has been developed to understand the nonlinear behavior and as the basis of a controller to remove the scan distortion. A robust state-space controller has been implemented to force the resonant fiber scanner to follow a spiral scan pattern. Acquired images at 250x250 pixel resolution demonstrate improved image fidelity over previous images taken with open-loop scanning. Keywords - Nonlinear feedback control; optical scanning; resonant fiber scanner; laser display 1. Introduction At the University of Washington’s Human Interface Technology Lab, small high-speed laser scanners with wide fields of view (fov) are being developed for various display and image acquisition applications. Among them is a resonating single fiber scanner, which has demonstrated a scan of 20° fov at 15.8kHz. Its potential uses include a Laser Fiber Scanning Image Source (LFSIS)– a compact scanning laser display for head mounted display (HMD) applications with holographic optical elements (HOE). The fiber scanner consists of a light-carrying single mode optical fiber attached to a quadrant piezoelectric tube at a point close to the fiber’s distal end. The length of the fiber extending beyond the piezotube acts as base excited cantilever and is adjusted so the resonant frequency is the desired scan frequency. Opposite planar quadrants of the piezotube excite the fiber at the cantilever’s resonant frequency. The low damping and resonant behavior of the fiber amplifies small actuator motion into large fiber tip displacements. Laser light coupled into the fiber emanates from the vibrating tip, producing a large fov spot scan. See Figure 1. Figure 1. Resonant Scanner with Spiral Scan Pattern A 2-D scan pattern can be produced with a single actuator using a symmetric fiber with equal resonant frequencies in both axes. If the horizontal axes produce a constant amplitude sine wave, and the vertical axes produce a cosine wave of the same frequency and amplitude, a circle results. This is a 1:1 Lissajous pattern. This pattern, however, does not scan over an area; that is, it is not space filling. To produce a space filling scan from a circular scan, we can progressively decrease and increase the circular scan’s amplitude to produce a spiral scan. This is an amplitude modulated 1:1 Lissajous pattern. For an evenly spaced spiral, the horizontal vibration is a triangle amplitude modulated sinewave; the vertical vibration is a triangle amplitude modulated cosine wave. Once a spot scan is produced, the intensity of the light spot can be modulated for image display applications. For image acquisition applications, the intensity of the backscattered light from the spot scanned across a target can be collected [1]. In both these applications, high quality scans are required. However, due to the dynamics of the resonating fiber, the scan may not follow the assumed ideal reference scan pattern resulting in image distortion. Figure 2 shows an open-loop spiral scan with a 2.5kHz resonant carrier frequency and a 12.5Hz triangle amplitude modulation. The laser light is modulated in an on-off pattern at 20kHz or eight times per carrier cycle with the image intensity inverted to enhance contrast. The resulting light pattern was projected onto a white card and recorded using a digital camera. If the scan followed an ideal spiral, the modulated light pattern would appear to be a series of eight wedges converging toward the center of the scan. Figure 2. Open Loop Spiral Scan with Light Modulation Actual recorded image shows the wedges are wavy and do not converge to the center of the scan. An edge of a wedge may start at the top on the outer periphery of the spiral but will end near the left of the inner periphery of the spiral. The scan does not go to the center resulting in a toroidal scan rather than a spiral scan. A second pattern with less contrast is an outward scan starting near the inner periphery of the spiral and ending in the outer periphery. The wedges of this pattern do not exhibit as much waviness as the inward spiral but still exhibits other distortion. ISSN/0003-0966X/03/3402-1455-$1.00+.00 © 2003 SID