A raster scan is often achieved by using torsional silicon micromirrors with a gimbal mount or fiber scanners. The frame rate is subject to the slow scanning frequency, which should be increased up to the persistence of human vision for real-time imaging. A raster scan provides a rectangular FOV by using two different driving signals of fast and slow scanning frequencies in integer multiple relationship. A spiral scan is often achieved by using a resonant bare-fiber scanner 24. Besides, high illumination density at the center region readily causes photo-damage or photo-bleaching 23. However, the non-uniform illumination densities over the field-of-view (FOV) occur as the scanning speed increase along the scan radius 12. The frame rate of spiral scanning is simply determined by the amplitude modulation frequency. A spiral scan is obtained by using two driving signals of the same frequency and 90° phase-shift on the orthogonal axes under the amplitude modulation along the radial direction. Laser scanning patterns such as spiral, raster, and Lissajous scans substantially affect both the image resolution and the frame rate of scanned images. However, attaining both the requirements are still challenging, particularly for laser microscanners. In addition, high frame rate apparently reduces motion artifacts 13 as well as laser exposure dose on a tissue sample for less photo-bleaching and photo damages of live cells during in-vivo fluorescence imaging 23. High definition allows diffraction-limited endomicroscopic imaging of high resolution for visualizing morphological characteristics of tumor cells 22. Recently, the microscanners move to clinical in-vivo endomicroscopic imaging for optical biopsy, where laser scanning of high definition and high frame rate within persistence of human vision is imperative in achieving clinically acceptable image resolution and real-time imaging 20, 21. In contrast, the resonant fiber scanners mounted inside a quadrupole PZT allow the facile compact packaging. MEMS mirrors have high design flexibility for the scanning speed as well as the batch fabrication but they still have some intrinsic limitations in realizing compact packaging schemes for forward-viewing applications. Laser microscanners such as microelectromechanical systems (MEMS) mirror scanners 18 and resonant fiber piezoelectric tube (PZT) scanners 14, 19 are actively utilized for compact optical applications. Recently, compact laser scanners become actively engaged in miniaturized imaging 11, 12, 13, 14, 15 or pico-projection display 16, 17 systems. This selection rule provides a new guideline for HDHF Lissajous scanning in compact laser scanning systems.Ĭontrolled steering of laser beams attracts assorted optical applications such as advanced optical microscopy 1, 2, 3D material processing 3, 4 including 3D printing 5, 6, single-pixel camera 7, or screen-less display 8, 9, 10. Based on this selection rule, the experimental results clearly demonstrate that conventional Lissajous scanners substantially increase both FF and FPS by slightly modulating the scanning frequencies at near the resonance within the resonance bandwidth of a Lissajous scanner. HDHF Lissajous scanning is achieved at the bi-axial scanning frequencies, where the GCD has the maximum value among various sets of the scanning frequencies satisfying the total lobe number for a target FF. The frames per second (FPS), called the pattern repeated rate or the frame rate, linearly increases with GCD. The fill factor (FF) monotonically increases with the total lobe number of a Lissajous curve, i.e., the sum of scanning frequencies divided by the great common divisor (GCD) of bi-axial scanning frequencies. Here we report the selection rule of scanning frequencies that can realize high definition and high frame-rate (HDHF) full-repeated Lissajous scanning imaging. The scanning frequency serves as a critical factor for determining the scanning imaging quality. Lissajous microscanners are very attractive in compact laser scanning applications such as endomicroscopy or pro-projection display owing to high mechanical stability and low operating voltages.
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