Common-path Optical Coherence Tomography System Research Articles

Spatial Assessment of Heterogeneous Tissue Natural Frequency Using Micro-Force Optical Coherence Elastography.

Analysis of corneal tissue natural frequency was recently proposed as a biomarker for corneal biomechanics and has been performed using high-resolution optical coherence tomography (OCT)-based elastography (OCE). However, it remains unknown whether natural frequency analysis can resolve local variations in tissue structure. We measured heterogeneous samples to evaluate the correspondence between natural frequency distributions and regional structural variations. Sub-micrometer sample oscillations were induced point-wise by microliter air pulses (60–85 Pa, 3 ms) and detected correspondingly at each point using a 1,300 nm spectral domain common path OCT system with 0.44 nm phase detection sensitivity. The resulting oscillation frequency features were analyzed via fast Fourier transform and natural frequency was characterized using a single degree of freedom (SDOF) model. Oscillation features at each measurement point showed a complex frequency response with multiple frequency components that corresponded with global structural features; while the variation of frequency magnitude at each location reflected the local sample features. Silicone blocks (255.1 ± 11.0 Hz and 249.0 ± 4.6 Hz) embedded in an agar base (355.6 ± 0.8 Hz and 361.3 ± 5.5 Hz) were clearly distinguishable by natural frequency. In a beef shank sample, central fat and connective tissues had lower natural frequencies (91.7 ± 58.2 Hz) than muscle tissue (left side: 252.6 ± 52.3 Hz; right side: 161.5 ± 35.8 Hz). As a first step, we have shown the possibility of natural frequency OCE methods to characterize global and local features of heterogeneous samples. This method can provide additional information on corneal properties, complementary to current clinical biomechanical assessments, and could become a useful tool for clinical detection of ocular disease and evaluation of medical or surgical treatment outcomes.

Chip based common-path optical coherence tomography system with an on-chip microlens and multi-reference suppression algorithm.

We demonstrate an integrated optical probe including an on-chip microlens for a common-path swept-source optical coherence tomography system. This common-path design uses the end facet of the silicon oxynitride waveguide as the reference plane, thus eliminating the need of a space-consuming and dispersive on-chip loop reference arm, thereby obviating the need for dispersion compensation. The on-chip micro-ball lens eliminates the need of external optical elements for coupling the light between the chip and the sample. The use of this lens leads to a signal enhancement up to 37 dB compared to the chip without a lens. The light source, the common-path arm and the detector are connected by a symmetric Y junction having a wavelength independent splitting ratio (50/50) over a much larger bandwidth than can be obtained with a directional coupler. The signal-to-noise ratio of the system was measured to be 71 dB with 2.6 mW of power on a mirror sample at a distance of 0.3 mm from the waveguide end facet. Cross-sectional OCT images of a layered optical phantom sample are demonstrated with our system. A method, based on an extended Fourier-domain OCT model, for suppressing ghost images caused by additional parasitic reference planes is experimentally demonstrated.

Mode-filtered large-core fiber for optical coherence tomography

We have investigated the use of multimode fiber in optical coherence tomography (OCT) with a mode filter that selectively suppresses the power of the high-order modes (HOMs). A large-core fiber (LCF) that has a moderate number of guiding modes was found to be an attractive alternative to the conventional single-mode fiber for its large mode area and the consequentially wide Rayleigh range of the output beam if the HOMs of the LCF were efficiently filtered out by a mode filter installed in the middle. For this, a simple mode filtering scheme of a fiber-coil mode filter was developed in this study. The LCF was uniformly coiled by an optimal bend radius with a fiber winder, specially devised for making a low-loss mode filter. The feasibility of the mode-filtered LCF in OCT imaging was tested with a common-path OCT system. It has been successfully demonstrated that our mode-filtered LCF can provide a useful imaging or sensing probe without an objective lens that greatly simplifies the structure of the probing optics.

Assembled Singlemode and Graded Index Fibers Simulating Collimator for All Fiber Common-Path Optical Coherence Tomography

A single mode fiber assembled with a specified length graded index multimode fiber as a collimator, which is used in common-path optical coherence tomography (CPOCT) is described in this paper. Compared with conventional collimator lens, this assembled fibers structure possesses the advantages of small volume, easy to be fused or connected with other part of the CPOCT system. Furthermore, with this assembled structure an all fiber CPOCT system is constructed which has the potential to detect the internal bodily organs and tissues of human being. Numerical simulation and experiments are implemented to verify the effectiveness of this method.

Depth-Resolved Blood Oxygen Saturation Assessment Using Spectroscopic Common-Path Fourier Domain Optical Coherence Tomography

Although spectroscopic optical coherence tomography (OCT) has been shown to be a promising method for measuring blood oxygen saturation with high-spatial resolution and accuracy, there are several technical issues that need to be addressed before it could become a practical method. In this letter, we have attempted to address two issues that could significantly improve the quantitative assessment of blood oxygen saturation level. First, we have implemented a spectral normalization technique to eliminate the spectral modulation induced by the wavelength-distance-dependent point spread function (PSF) of OCT's. Second, to reduce the spectral speckle noise due to the highly scattering blood, we have implemented a spatial low-pass filter to the 2-D OCT dataset consisting of spectra obtained at different lateral positions. We have assessed the effectiveness of these methods using common-path OCT system. Results showed that we were able to extract unambiguous depth-resolved, SO(2)-dependent spectroscopic information from 1-D and 2-D OCT images, which could be used to accurately assess the SO(2) level.

Flexible Curled Optical Cord for Bending-Insensitive Optical Imaging Delivery

In the application of optical imaging technologies to the biomedical field, the flexible optical cord is one of the key components used to deliver the reflected optical signal from the internal biological tissue to the biomedical imaging system. However, the conventional optical fibers suffer from a critical wiring limitation, which results in severe power losses under a small bending radius that is less than a few centimeters. In this paper, we demonstrate that holey optical fibers with 18 air holes in cladding have negligible bending loss under a minimum bending radius of 3 mm. Excellent bending insensitivity of the holey fibers can effectively reduce the device size and space, which will be highly beneficial to the flexible endoscopic common-path optical coherence tomography system. A curled optical cord, similar to a curled telephone cord, is specially designed and fabricated to accommodate access of the imaging probe to the biological target at flexible distances. The curled optical cord displays excellent optical properties of a much lower bending loss and better optical imaging quality, but the attenuation and insertion losses are still the same as that of a conventional single-mode fiber-based optical cord.

Signal-to-noise ratio analysis of all-fiber common-path optical coherence tomography

We present theoretical analysis and experimental verification of the signal to noise ratio (SNR) of a common-path interferometer-based optical coherence tomography (OCT) system. Based on fully integrated all-fiber implementation of a common-path time-domain OCT system, we derived the SNR of the system including the effect of beat noise, which turns out to be twice as large as the excess noise term. We verified the theoretical SNR through a series of experiments, utilizing both controlled phantom and biological samples such as a rat brain with tumor and a frog retina. The results showed that the source power and the reference reflectivity can be easily controlled to optimize the SNR of OCT imaging. We have also analyzed the effect of the fiber delays and the offset in the fiber autocorrelator of the common-path OCT system on the overall SNR.

Common-path optical coherence tomography with side-viewing bare fiber probe for endoscopic optical coherence tomography

All-fiber-optic common-path optical coherence tomography (OCT) using a side-viewing bare fiber probe has been demonstrated and analyzed. A bare single mode fiber tip is angle cleaved at approximately 49 degrees to enable side illumination due to total internal reflection. The bare fiber probe was inserted in an arterial tissue and a circumferential OCT scan was obtained. The research is aimed at realizing highly miniaturized monolithic probes for possible applications in miniature endoscopic OCT or intravascular OCT. The effects of the angle of the cleaved fiber on reference reflection and the sensitivity of the common-path OCT system have been studied theoretically. The angle cleaved fiber probe is also used in series with a microlens to analyze and optimize its performance in a common-path OCT system. Our research aims to explore the combined advantages of common-path OCT and extremely simplified miniature probe design and to discuss how it may greatly simplify the endoscopic OCT instrumentation eventually.