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

Abstract

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.

Optical coherence tomography endoscopic probe based on a tilted MEMS mirror

“…With a larger fiber tip-lens distance, the lateral resolution will be enhanced while the working distance will be decreased. The fiber tip-lens distance should be controlled within 0.1 mm to obtain the working distance larger than 3.5 mm and lateral resolution of around 25 μm [18]. The GRIN lens (ILW-070, GoFoton, NJ) employed has a pitch of 0.27 with a length of 1.955 mm, a diameter of 0.7 mm and a focal length of 5 mm.…”

“…The distance between the GRIN lens and the mirror plate center is designed as 1 mm to achieve a working distance of 3 mm with a lateral resolution around 20 μm in air. The effects of the distance between the fiber and GRIN lens, the curvature of the MEMS mirror plate and the cylindrical tubing can be found in [18]. Electrical connections to the four actuators of the MEMS mirror are required.…”

“…Several types of planar electrothermal MEMS mirrors with 1-axis or 2-axis scan capability have been extensively employed in side-viewing endoscopic OCT probes [15,[17][18][19]. A typical side-viewing endoscopic OCT probe is shown in Fig.…”

An endoscopic forward-viewing OCT imaging probe based on a two-axis scanning mems mirror

“…However, these approaches suffer from several drawbacks including slow scan speed limited by proximal fiber end scanning, and hysteresis problem as well as scan nonuniformity when using piezoelectric cantilevers at resonance [8]. As an alternative technology, micro-electro-mechanical-system (MEMS) mirrors have been employed as the scan engines for endoscopic probes [7][8][9][10] thanks to their advantages of small size, fast speed and low cost. Side-viewing probes are most popular by employing MEMS optical scanners or micromotors [3,[11][12][13].…”

“…Another common factor that hinders electrostatic, electromagnetic and piezoelectric micromirrors for the miniaturization of endoscopic probes is the small fill factors of the devices, usually less than 10%. Alternatively, electro-thermal MEMS mirrors have been investigated to further extend the optical scanning range with low drive voltage smaller than 10 V [8,10]. In addition, electro-thermal MEMS mirrors have good scan linearity and high fill factor (~25%).…”