Research/Wavelength Tunable Light Source for the Next Generation OCT System by High-speed MEMS Optical Scanner

2019-02-17 (日) 23:45:38 (243d)

OCT Imaging using Wavelength Tunable Laser with Fast MEMS Optical Scanner

光干渉画像法 (OCT = Optical Coherence Tomography) とは干渉によって断層画像を得る手法であり、眼底検査等の医療機器に応用されている光学原理です。従来のOCTでは、試料表面からの深さ計測に低コヒーレンス光源による干渉光学系を用いていましたが、深さ方向のスキャンが低速であるため動画像の取得が困難でした。そこで本研究では、高速光MEMSスキャナを用いて、波長を走引する方式のOCT光源を新たに開発しました。

We present an electrostatic vertical comb-drive MEMS optical scanner with the angle magnifier mechanism developed for a 140-kHz wavelength tunable-laser in the swept-source OCT system of a 2 ms/frame visualization capability.




Optical coherent tomography (OCT) is a novel human-friendly medical inspection method to visualize the living tissue by using the interference of light, and it has found an expanding range of applications including ophthalmology, gastroenterology, and plaque inspection. Due to the escalating demands for faster frame the conventional 10 ms/frame to 2 ms/frame, OCT system has been evolutionally advanced from the conventional time-domain type to the swept-source OCT (SS-OCT) that uses wavelength tunable laser as a light source. One of our previous swept light sources uses a motor-driven polygon mirror inserted within the external cavity laser; the maximum scan speed was, however, limited to only 20 kHz to 30 kHz due to the bulky mass of the rotating mechanism. For this reason, the MEMS type micro scanner is expected to replace them to realize scan rate of 100 kHz or faster. This paper reports a new MEMS optical scanner for a 140-kHz SS-OCT implementation.

Operation Principle of MEMS Optical Scanner




Figure on the left compares two different schemes for the vertical-comb equipped MEMS optical scanner. In the conventional design, the mirror angle is identical to that of the actuator plate, as they are made to be one piece suspended with a pair of torsion bars. Our new design, on the other hand, has the inner hinges between the actuator plates and the mirror; the moment of inertia and the spring constants are designed such that the mirror’s out-of-plane motion at the 2nd resonance becomes larger than that of the actuator plate by the resonant coupling between the two bodies. We typically use the FEM (finite element method) numerical analysis to tune the suspension dimensions to make the leverage factor of over 5.

Fabrication Proces



We used an SOI wafer of a 60-µm-thick active layer and a 4-µm-thick BOX to microfabricate the scanner with the double-side deep RIE processes with only two photolithography steps. Apart from the conventional two-step DRIE process, we used the process stiction of the surface tension force to self-assemble the vertical comb-drive electrodes with an initial offset angle. Figure on the left shows the SEM images of the developed device. The mirror diameter was 1 mm, and the fundamental resonance was found at 69.7 kHz to yield the optical angle of 6.5 degrees with 70 Vdc +/- 70 Vac operation; the leverage factor to the mirror with respect to the actuator plate was found to be 5.

Performance Demonstration



We used the round trip of the scanner’s motion to sweep the wavelength at 140 kHz, which was double the fundamental resonant frequency of the scanner. As shown in the Figure, maximum power intensity of 20 mW was obtained within the wavelength span of 100 nm around the 1.3 µm center wavelength with a 0.25 nm FWHM, corresponding to a coherence length of 3 mm. The scanner was assembled in the swept-source OCT system to visualize the strawberry tissue at 2 ms/frame as shown in Figure 8. Compared with the conventional 35 kHz scan, the improved results at 140 kHz was successful to give a 100 µm resolution for more than 1 mm in depth under such fast scan condition.




This work has been performed in collaboration with Santec Corp.

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