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Research Papers

Electromechanical Model-Based Design and Testing of Fiber Scanners for Endoscopy

[+] Author and Article Information
Abhijith Rajiv

Human Photonics Laboratory,
Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195
e-mail: abhijith@uw.edu

Yaxuan Zhou

Human Photonics Laboratory,
Department of Electrical Engineering,
University of Washington,
Seattle, WA 98195
e-mail: yaxuanzh@uw.edu

Jeremy Ridge

Human Photonics Laboratory,
Department of Electrical Engineering,
University of Washington,
Seattle, WA 98195

Per G. Reinhall

Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195

Eric J. Seibel

Human Photonics Laboratory,
Department of Mechanical Engineering,
University of Washington,
Seattle, WA 98195
e-mail: eseibel@uw.edu

Manuscript received January 17, 2018; final manuscript received May 3, 2018; published online September 21, 2018. Assoc. Editor: Yaling Liu.

J. Med. Devices 12(4), 041003 (Sep 21, 2018) (8 pages) Paper No: MED-18-1012; doi: 10.1115/1.4040271 History: Received January 17, 2018; Revised May 03, 2018

Forward-viewing catheters and scopes for diagnosing disease and guiding interventions in small ducts (less than 3 mm diameter) require wide-field high-quality imaging since scope tip bending is difficult and ineffective. A high-fidelity electromechanically coupled finite element (FE) model of a piezoelectric actuated resonant fiber scanner is presented, which enables improvement on the general design of fiber-optic scanner geometry to increase scan frequency and field of view (FOV). Using the proposed model, parametric sweeps on the specific design variables achieved by acid etching of glass fiber are analyzed to identify their effect on scanner performance and to choose improved designs. The resulting complex fiber scanner design requires development of unique microfabrication techniques. Comparison of three model simulations and their experimental testing show that our proposed coupled model has prediction error of ≤12% with respect to experimental data, while other uncoupled models have up to 39% error. The model and microfabrication techniques presented in this paper have significance for fiber scanning-based systems in that they demonstrate reliability for model-driven design and also flexibility for fiber scanner design of complex geometries, allowing for improvement on medical imaging performance.

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References

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Figures

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Fig. 1

Functional diagram of the SFE illustrates that at the tip of a flexible shaft, a short rigid tip contains a base excited fiberoptic cantilever that is driven in a spiral pattern. The backscattered light from the forward scanned laser illumination is used to generate wide-FOV images at video rates, such as the SFE image of a stent placed in a straight arterial phantom.

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Fig. 2

Image from an SFE showing FOV limitation in a 10-mm diameter by 40-mm long stent in a curved channel phantom

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Fig. 3

Three-dimensional computer-aided design model of the FE electromechanical coupled model (bottom) and a zoomed in cross section view of the joint between the optical fiber and the piezoelectric tube (top)

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Fig. 4

Schematic representation of the data acquisition setup. The fiber to test is mounted on a brass mount, which is clamped on microscope stage so that horizontal projection of fiber tip deflection xh can be measured; as the cantilever vibrates when driven by user-defined drive signal, the output laser light is collimated through lens assembly and falls onto the PSD so that the angle α between fiber deflection direction and horizontal axis can be measured.

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Fig. 5

Image of a step etched fiber. The top inset shows the tip of the cleaved fiber; bottom inset shows the tapered part where the etching started and is caused by etching solution climbing up along the fiber. Note that all images are at different scale.

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Fig. 6

Automated etching station: (A) the optical lenses and apertures convert the laser light into a ring shape and then focus it on the fiber being etched in (B) the sapphire cuvette, which is fixed on teflon stage. Scattered light from the fiber is collected by (C) camera, which then sends the video into (F) a personal computer for fiber localization and etching monitoring. Fiber mounting is driven by two-axis motors (D) and also allows manual adjustment of lateral position of fiber, (E) is a detachable fiber mounting block.

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Fig. 7

Customized setup for measuring and cleaving is composed of a commercial cleaver, a BioRad shear microscope and a video monitor. On the monitor, it shows horizontally oriented optical fiber which is bright in the center and dark on the borders due to the cylindrical geometry. The two vertical lines indicate the location where intensity is measured (right) and the intensity profile (left). The intensity profile makes the shearing measurement of fiber diameter more accurate.

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Fig. 8

Variation in resonance frequency of the scanner with the fiber cantilevered length showing that the coupled model introduced in this paper is more accurate at estimating resonance frequency than an uncoupled model of just the fiber with resonance frequency calculated according to Eq. (6), especially at short fiber lengths.

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Fig. 9

Variation in resonance frequency of the scanner with etch fraction ζ. The result shows a positive correlation at etch fraction smaller than 30% and a negative correlation at larger etch fraction.

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Fig. 10

Series of microscope photographs of a prototype fiber scanner. From left to right, rectangular metal base holding 0.4 mm diameter tube piezoelectric actuator, 80 μm diameter optical fiber, and step-etched fiber-optic cantilever with distal end being driven near its first mode of vibratory resonance (4.3 kHz). The drive voltage is well above model testing parameters to illustrate the maximum amplitude of near-resonance vibration, and the high tip deflection angle with respect to the optical axis where light exits the fiber and enters the lens assembly (not shown).

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