Numerical analysis of astigmatism correction in gradient refractive index lens based optical coherence tomography catheters

Astigmatism analysis and correction method introduced by an inclined plate in a convergent optical path

A plate beam splitter can simultaneously increase the transmittances and reflectances of different split spectrum segments. The splitter has been widely used in optical systems. However, when a beam passes through a beam splitter in a convergent optical path, the image quality may be degraded severely because the tilted plate may introduce aberrations such as astigmatism at the on-axis point. The formula for calculating this astigmatism is established. The calculated results are consistent with the simulated results, and the validity of the formula is verified through experiments. A cylindrical lens curvature calculation formula is derived to correct the on-axis astigmatism, which reduces the aberrations introduced by the tilted plate and improves the imaging performance. The imaging characteristics of the off-axis field of view and the reasons for the changes in image quality are analyzed. The F-number range of the converging light path is calculated, which provides a reference for tilted applications of flat-panel optical elements in converging light paths.

Astigmatism-corrected endoscopic imaging probe for optical coherence tomography using soft lithography

In endoscopic optical coherence tomography, a transparent protective sheath is used to protect the optics and tissue. However, the sheath causes astigmatism, which degrades transverse resolution and signal-to-noise ratio due to the cylindrical lens effect. Generally used methods for correcting this astigmatism are complex, difficult to control precisely, high-cost, and increase the dimensions of the imaging probe. To overcome these problems, we have developed an astigmatism-corrected imaging probe with an epoxy window. The astigmatism is precisely and cost-effectively adjusted controlling the curvature radius of the epoxy window, which is produced by soft lithography. Using the fiber optic fusion splicing, the fabrication process is simple. The fabricated imaging probe is almost monolithic, so its diameter is similar to that of a standard single-mode fiber. We demonstrate its astigmatism-correcting performance using focal spot analysis, imaging micro-beads and a biological sample.

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 swine's 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.

Astigmatism corrected common path probe for optical coherence tomography

BACKGROUND AND OBJECTIVES

Optical coherence tomography (OCT) catheters for intraluminal imaging are subject to various artifacts due to reference-sample arm dispersion imbalances and sample arm beam astigmatism. The goal of this work was to develop a probe that minimizes such artifacts.

MATERIALS AND METHODS

Our probe was fabricated using a single mode fiber at the tip of which a glass spacer and graded index objective lens were spliced to achieve the desired focal distance. The signal was reflected using a curved reflector to correct for astigmatism caused by the thin, protective, transparent sheath that surrounds the optics. The probe design was optimized using Zemax, a commercially available optical design software. Common path interferometric operation was achieved using Fresnel reflection from the tip of the focusing graded index objective lens. The performance of the probe was tested using a custom designed spectrometer-based OCT system.

RESULTS

The probe achieved an axial resolution of 15.6 μm in air, a lateral resolution 33 μm, and a sensitivity of 103 dB. A scattering tissue phantom was imaged to test the performance of the probe for astigmatism correction. Images of the phantom confirmed that this common-path, astigmatism-corrected OCT imaging probe had minimal artifacts in the axial, and lateral dimensions.

CONCLUSIONS

In this work, we developed an astigmatism-corrected, common path probe that minimizes artifacts associated with standard OCT probes. This design may be useful for OCT applications that require high axial and lateral resolutions. Lasers Surg. Med. 49:312-318, 2017. © 2016 Wiley Periodicals, Inc.