Source: https://www.osapublishing.org/ao/abstract.cfm?uri=ao-54-17-5495
Timestamp: 2019-04-24 14:42:18+00:00

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We study experimentally the effective duty cycle of galvanometer-based scanners (GSs) with regard to three main parameters of the scanning process: theoretical/imposed duty cycle (of the input signal), scan frequency, and scan amplitude. Sawtooth and triangular input signals for the device are considered. The effects of the mechanical inertia of the oscillatory element of the GS are analyzed and their consequences are discussed in the context of optical coherence tomography (OCT) imaging. When the theoretical duty cycle and the scan amplitude are increased to the limit, the saturation of the device is demonstrated for a useful range of scan frequencies by direct measurement of the position of the galvomirror. Investigations of OCT imaging of large samples also validate this saturation, as examplified by the gaps/blurred portions obtained between neighboring images when using both triangular and sawtooth scanning at high scan frequencies. For this latter aspect, the necessary overlap between neighboring B-scans, and therefore between the corresponding volumetric reconstructions of the sample, are evaluated and implemented with regard to the same parameters of the scanning process. OCT images that are free of these artifacts are thus obtained.
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Fig. 1. Schematic illustrating the operating principle of a galvanometer-based scanner (GS), with constructive parameters: J , moment of inertia of the mobile element (with galvomirror); c , damping coefficient; k , elastic coefficient of the torsion springs that support the mobile element.
Fig. 2. Study of sawtooth scanning functions/output signals (i.e., angular positions of the galvomirror) of a GS for input signals with different theoretical duty cycles η t equal to (a1) 50% (triangular function), (a2) 75%, and (a3) 87.5%; (b1)–(b3) output signals (positions of the galvomirror) for a scan frequency f s = 50 Hz ; (c1)–(c3) output signals for f s = 500 Hz ; (d1)–(d3) effective duty cycle of the GS with regard to f s for three different driving duty cycles and different scan amplitudes θ m of 0.2, 0.4, 0.8, 1.6, and 3.2 V (where 1 V corresponds to about 7.6 deg of optical scan angle).
Fig. 3. Study of the effective duty cycle of a GS with regard to the theoretical/programmed duty cycle, for different scan amplitudes θ m of 0.2, 0.4, 0.8,1.6, and 3.2 V (1 V corresponds to 7.6 deg optically in terms of angular scan amplitude)—each study being made for a certain scan frequency f s : (a) 10 Hz; (b) 50 Hz; (c) 100 Hz; (d) 200 Hz; (e) 300 Hz; (f) 500 Hz.
Fig. 4. (a) Triangular and (b1), (b2) sawtooth scanning regimes of the GS: ideal/input signals versus scanning functions/output signals of the GS. For the latter, the current positions of the galvomirror are those determined experimentally and shown in Fig. 2.
Fig. 5. GD-OCM setup .
Fig. 6. OCT imaging of a sample with a regular structure [frontal view of 3D OCT images shown as examples in Fig. 7(b)] with two scanning regimes: (a) triangular, (b) sawtooth with a theoretical duty cycle η t equals 75%, and (c) sawtooth with η t equals 90%. All images considered the same scan amplitude ( θ m equals 1.6 V, where 1 V stands for about 7.6 deg optically) and three different scan frequencies ( f s ): 100 Hz (a1), (b1), (c1); 300 Hz (a2), (b2), (c2); and 500 Hz (a3), (b3), (c3). For the images in column 3, the necessary overlaps are applied (Fig. 7) and the corrected images, without the previous artifacts, are shown in column 4. The dimensions of the images are 0.73 mm × 0.73 mm .
Fig. 7. (a) Principle of the overlapping of the adjacent images for distortion correction. (b) The same study as in Fig. 6, but only for the sawtooth scanning with the maximum reachable (i.e., 90%) theoretical duty cycle, with GD-OCM volumetric images taken with fast scan, with the scan frequency f s equal to 500 Hz: (b1) original and (b2) corrected image—the latter obtained by overlapping two individual adjacent images. The dimensions of the images are 0.73 mm × 0.73 mm × 0.35 mm .

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