Can Duan

Miniature probe combining optical-resolution photoacoustic microscopy and optical coherence tomography for in vivo microcirculation study

Photoacoustic microscopy (PAM) is sensitive to optical absorption, while optical coherence tomography (OCT) is based on optical backscattering. Combining PAM and OCT can provide complementary information about biological tissue. Here we present a combined optical-resolution PAM (ORPAM) and OCT system that is integrated through a miniature probe with an overall diameter of 2.3 mm, suitable for insertion through a standard endoscopic or laparoscopic port during minimally invasive surgery or endoscopic exam. The hybrid probe consists of a common optical path for OCT (light delivery/detection) and ORPAM (light excitation) and a 10 MHz unfocused ultrasound transducer for photoacoustic detection. The combined system yields a lateral resolution of 15 μm for both ORPAM and OCT.

Endoscopic swept-source optical coherence tomography based on a two-axis microelectromechanical system mirror

Abstract. A microelectromechanical system (MEMS) mirror based endoscopic swept-source optical coherence tomography (SS-OCT) system that can perform three-dimensional (3-D) imaging at high speed is reported. The key component enabling 3-D endoscopic imaging is a two-axis MEMS scanning mirror which has a 0.8×0.8  mm2 mirror plate and a 1.6×1.4  mm2 device footprint. The diameter of the endoscopic probe is only 3.5 mm. The imaging rate of the SS-OCT system is 50  frames/s. OCT images of both human suspicious oral leukoplakia tissue and normal buccal mucosa were taken in vivo and compared. The OCT imaging result agrees well with the histopathological analysis.

Probe alignment and design issues of microelectromechanical system based optical coherence tomography endoscopic imaging

Endoscopic optical coherence tomography (OCT) imaging has been demonstrated using microelectromechanical system (MEMS) technology by several research groups. The focus of this work is to study how the OCT imaging performance is affected by the radius of curvature of MEMS mirrors as well as the optical alignment accuracy inside small imaging probes. The goal of this study is to provide guidance for assembly tolerance and design optimization of OCT endoscopic probes. Gaussian beam propagation is used for theoretical analysis which is confirmed by optical simulation and verified experimentally with a time-domain OCT system as well. It has been found that the OCT imaging is very sensitive to the distance from the fiber end to the gradient-index (GRIN) lens, which needs to be controlled within 0.1 mm to achieve working distance (WD) longer than 3.5 mm and lateral resolution around 25 μm. The impact on image quality of the MEMS mirror is negligible if the radius of curvature of the mirror surface is greater than 200 mm. In addition, we studied the astigmatism introduced by cylindrical plastic tubing; the maximum astigmatism ratio is 1.1 when the WD is around 2.5 mm.

A silicon based Fourier transform spectrometer base on an open-loop controlled electrothermal MEMS mirror

This paper reports a compact Fourier transform spectrometer system with a large-stroke electrothermal MEMS mirror and other optical components all integrated on a micro-machined silicon base with the dimension of 2cm×2cm. The overall size of the system is reduced dramatically from the prior work [1]. The linear optical path difference (OPD) scan range is increased to 440μm and the mirror plate tilting is compensated down to ±0.002° during the full OPD scan using a new open-loop control method. A spectral resolution of 1.1nm at 532nm is achieved.

A 45°-tilted 2-axis scanning micromirror integrated on a silicon optical bench for 3D endoscopic optical imaging

This paper presents a 2-axis electrothermal single-crystal-silicon (SCS) micromirror that is tilted 45° out of plane on a silicon optical bench (SiOB). The SiOB provides mechanical support and electrical wiring to the tilted 2-axis mirror as well as an aligned trench for assembling other optical components such as optical fibers. The tilt of the mirror is achieved with the bending of a set of stressed bimorph beams and the stop is provided by the silicon sidewall. The tilt angle can be precisely controlled by properly choosing the distance from the mirror frame to the silicon sidewall and the flexure bimorph length. The mirror plate is 0.72 mm × 0.72 mm and the footprint of the entire MEMS device is 2.22 mm × 1.25 mm. The measured maximum optical scan angles of the mirror are 40.0° in both x- and y-axis.

Optical coherence tomography endoscopic probe based on a tilted MEMS mirror.

This paper reports a compact microendoscopic OCT probe with an outer diameter of only 2.7 mm. The small diameter is enabled by a novel 2-axis scanning MEMS mirror with a preset 45° tilted angle. The tilted MEMS mirror is directly integrated on a silicon optical bench (SiOB). The SiOB provides mechanical support and electrical wiring to the mirror plate via a set of bimorph flexure, enabling a compact probe mount design without the requirement of a 45° slope, which is capable to dramatically reduce the probe size and ease the assembly process. Additionally, the SiOB also provides trenches with properly-designed opening widths for automatic alignment of the MEMS mirror, GRIN lens and optical fiber. The 45°-tilted MEMS mirror plate is actuated by four electrothermal bimorph actuators. The packaged 2.7 mm-diameter probe offers 2-axis side-view optical scanning with a large optical scan range of 40° at a low drive voltage of 5.5 Vdc in both axes, allowing a lateral scan area of 2.2 mm × 2.2 mm at a 3 mm working distance. High-resolution 2D and 3D OCT images of the IR card, ex vivo imaging of meniscus specimens and rat brain slices, in vivo imaging of the human finger and nail have been obtained with a TDOCT system.

Swept-source common-path optical coherence tomography with a MEMS endoscopic imaging probe

A MEMS-based common-path endoscopic imaging probe for 3D swept-source optical coherence tomography (SSOCT) has been developed. The common path is achieved by setting the reference plane at the rear surface of the GRIN lens inside the probe. MEMS devices have the advantages of low cost, small size and fast speed, which are suitable for miniaturizing endoscopic probes. The aperture size of the two-axis MEMS mirror employed in this endoscopic probe is 1 mm by 1 mm and the footprint of the MEMS chip is 1.55 mm by 1.7 mm. The MEMS mirror achieves large two dimensional optical scan angles up to 34° at 4.0 V. The endoscopic probe using the MEMS mirror as the scan engine is only 4.0 mm in diameter. Additionally, an optimum length of the GRIN lens is established to remove the artifacts in the SSOCT images generated from the multiple interfaces inside the endoscopic imaging probe. The MEMS based commonpath probe demonstrates real time 3D OCT images of human finger with 10.6 μm axial resolution, 17.5 μm lateral resolution and 1.0 mm depth range at a frame rate of 50 frames per second.

A 2-axis electrothermal MEMS micro-scanner with torsional beam

In this paper we introduce a 2-axis MEMS micro-scanner with large scanning range of frame (32°) and mirror (22°) using a compact and optimized electrothermal bimorph actuator and torsional beams to tip and tilt a dual-reflective mirror plate.

A Self-Aligned 45°-Tilted Two-Axis Scanning Micromirror for Side-View Imaging

This paper presents a two-axis electrothermal single-crystal-silicon micromirror that is tilted 45° out of plane on a silicon optical bench (SiOB). The SiOB provides mechanical support and electrical wiring to the tilted two-axis scanning mirror, as well as aligned trenches for assembling other optical components, such as optical fibers and GRIN lens. The preset tilting of the mirror plate is achieved via the bending of a set of stressed bimorph cantilevers. A stopper connected on the bending bimorphs reaches and is stopped by the adjacent silicon sidewall. The tilt angle can be precisely controlled by properly choosing the distance from the stopper to the silicon sidewall and the flexure bimorph length. The fabricated mirror plate is 0.72 mm × 0.72 mm and the footprint of the entire MEMS device is 2.22 mm × 1.25 mm. The measured maximum total optical scan angle of the mirror is approximately 40° in both x-axis and y-axis at only 5.5 Vdc. This technology will enable a new class of more compact microendoscopic optical imaging probes for in vivo early cancer detection.

Circumferential-scanning endoscopic optical coherence tomography probe based on a circular array of six 2-axis MEMS mirrors

We present a novel circumferential-scan endoscopic optical coherence tomography (OCT) probe by using a circular array of six electrothermal microelectromechanical (MEMS) mirrors and six C-lenses. The MEMS mirrors have a 0.5 mm × 0.5 mm mirror plate and a chip size of 1.5 mm × 1.3 mm. Each MEMS mirror can scan up to 45° at a voltage of less than 12 V. Six of those mirrors have been successfully packaged to a probe head; full circumferential scans have been demonstrated. Furthermore, each scan unit is composed of a MEMS mirror and a C-lens and the six scan units can be designed with different focal lengths to adapt for lesions with uneven surfaces. Configured with a swept source OCT system, this MEMS array-based circumferential scanning probe has been applied to image a swines small intestine wrapped on a 20 mm-diameter glass tube. The OCT imaging result shows that this new MEMS endoscopic OCT has promising applications in large tubular organs.