Laser scanners with oscillatory elements: Design and optimization of 1D and 2D scanning functions

Abstract

• Multi-parameter analysis (with optical, mechanical, and electrical aspects) of laser scanners with oscillatory elements. • Analysis of common scanning functions (sinusoidal, sawtooth, and triangular) from the point of view of their duty cycle. • Development of 1D optimal scanning functions with the maximum possible duty cycle (and minimum distortions). • Trade-off between the duty cycle and the voltage peaks of the command functions of scanners with oscillatory elements. • Development and optimization of 2D scanning functions of galvanometer scanners or Micro-Electro-Mechanical Systems (MEMS). Optomechatronic laser scanners with oscillatory mirrors, built as galvanometer scanners (GSs) or as Micro-Electro-Mechanical Systems (MEMS) are approached from the point of view of their scanning functions. The non-linearity of the 1D scanning functions which are the output signals of such scanners (i.e., the current angular position of their oscillatory mirrors) are first discussed for the three most common input signals (triangular, sawtooth, and sinusoidal), for which the effective duty cycle/time efficiency (of the output) was modeled using Optical Coherence Tomography (OCT) imaging, in contrast to the theoretical duty cycle (of the input). Second, optimal linear plus non-linear 1D scanning functions are designed, with specially introduced parabolic portions, in order to maximize the duty cycle of the scanning process and to provide the most distortion-free scanning, for example for OCT. A trade-off is also discussed between this duty cycle and the peaks of the voltage that has to be applied to the motor of the scanner, in order to minimize these peaks and thus to protect the system from an electrical point of view. Finally, using all the results above, both angular and linear 2D scanning functions are designed and analyzed, for their possible variants. For the fast scan axis the triangular 1D functions obtained above (with or without non-linear portions introduced between the constant speed, useful scan) are utilized. For the slow scan axis the two most utilized scanning algorithms are employed: with a step-by-step or with a continuous angular movement. The characteristic parameters of their different variants are analyzed in order to optimize these scanning functions. We demonstrate that the step-by-step angular movement is the best one, in order to avoid producing acceleration (therefore inertia torque and voltage) spikes. Equations that avoid such issues are proposed. These developments are placed in the context of the requirements of high-end applications like OCT, both for biomedical imaging and for Non-Destructive Testing (NDT) in industry, including for hot topics, such as handheld or endoscope scanning probes.