Fiber-based Optical Coherence Tomography Research Articles

Digital ocular swept source optical coherence aberrometry.

Ocular aberrometry is an essential technique in vision science and ophthalmology. We demonstrate how a phase-sensitive single mode fiber-based swept source optical coherence tomography (SS-OCT) setup can be employed for quantitative ocular aberrometry with digital adaptive optics (DAO). The system records the volumetric point spread function at the retina in a de-scanning geometry using a guide star pencil beam. Succeeding test-retest repeatability assessment with defocus and astigmatism analysis on a model eye within ± 3 D dynamic range, the feasibility of technique is demonstrated in-vivo at a B-scan rate of >1 kHz in comparison with a commercially available aberrometer.

Long-term in vivo monitoring of gliotic sheathing of ultrathin entropic coated brain microprobes with fiber-based optical coherence tomography

Objective. Microfabricated neuroprosthetic devices have made possible important observations on neuron activity; however, long-term high-fidelity recording performance of these devices has yet to be realized. Tissue-device interactions appear to be a primary source of lost recording performance. The current state of the art for visualizing the tissue response surrounding brain implants in animals is immunohistochemistry + confocal microscopy, which is mainly performed after sacrificing the animal. Monitoring the tissue response as it develops could reveal important features of the response which may inform improvements in electrode design. Approach. Optical coherence tomography (OCT), an imaging technique commonly used in ophthalmology, has already been adapted for imaging of brain tissue. Here, we use OCT to achieve real-time, in vivo monitoring of the tissue response surrounding chronically implanted neural devices. The employed tissue-response-provoking implants are coated with a plasma-deposited nanofilm, which has been demonstrated as a biocompatible and anti-inflammatory interface for indwelling devices. We evaluate the method by comparing the OCT results to traditional histology qualitatively and quantitatively. Main results. The differences in OCT signal across the implantation period between the plasma group and the control reveal that the plasma-type coating of otherwise rigid brain probes (glass) only slightly improve the glial encapsulation in the brain parenchyma indicating that geometrical or mechanical influences are dominating the encapsulation process. Significance. Our approach can long-term monitor and compare the tissue-response to chronically-implanted neural probes with and withour plasma coating in living animal models. Our findings provide valuable insigh to the well acknowledged yet not solved challenge.

Utilization potential of intraluminal optical coherence tomography for the Eustachian tube

Imaging the Eustachian tube is challenging because of its complex anatomy and limited accessibility. This study fabricated a fiber-based optical coherence tomography (OCT) catheter and investigated its potential for assessing the Eustachian tube anatomy. A customized OCT system and an imaging catheter, termed the Eustachian OCT, were developed for visualizing the Eustachian tube. Three male swine cadaver heads were used to study OCT image acquisition and for subsequent histologic correlation. The imaging catheter was introduced through the nasopharyngeal opening and reached toward the middle ear. The OCT images were acquired from the superior to the nasopharyngeal opening before and after Eustachian tube balloon dilatation. The histological anatomy of the Eustachian tube was compared with corresponding OCT images, The new, Eustachian OCT catheter was successfully inserted in the tubal lumen without damage. Cross-sectional images of the tube were successfully obtained, and the margins of the anatomical structures including cartilage, mucosa lining, and fat could be successfully delineated. After balloon dilatation, the expansion of the cross-sectional area could be identified from the OCT images. Using the OCT technique to assess the Eustachian tube anatomy was shown to be feasible, and the fabricated OCT image catheter was determined to be suitable for Eustachian tube assessment.

Towards real time assessment of intramuscular fat content in meat using optical fiber-based optical coherence tomography

We consider the use of optical coherence tomography (OCT) imaging to predict the quality of meat. We find that intramuscular fat (IMF) absorbs infrared light about nine times stronger than muscle, which enables us to estimate fat content in intact meat samples. The method is made very efficient by extracting relevant information from the three-dimensional high-resolution images generated by OCT using principal component analysis (PCA). The principal components are then used as regressors into a support vector regression (SVR) prediction model. The SVR model is found to predict IMF content stably and accurately, with an R2 value of 0.94. Our study paves the way for automated, contact-less, non-destructive, real time classification of the quality of meat samples.

Polarization state tracing method to map local birefringent properties in samples using polarization sensitive optical coherence tomography.

We propose a method that utilizes the trajectory of output polarization states on the Poincaré sphere to derive depth-resolved birefringent information within samples using a fiber-based polarization sensitive optical coherence tomography. The apparent (or intermediate) optic axis and the local phase retardation are first obtained by fitting a plane to the adjacent output polarization states along depths in the Poincare sphere. A sequence of 3D rotation operation determined by the local birefringent property of the upper layers is then applied to the apparent axis to finally determine the local optic axis. This method requires only one input polarization state and is compatible with both free-space and fiber-based PSOCT systems, simplifying the imaging system setup. The theoretical framework is presented to derive the local phase retardation and optic axis from the output polarization states and then demonstrated by mapping local birefringent information of the mouse thigh tissue in vitro.

Etching-enabled extreme miniaturization of graded-index fiber-based optical coherence tomography probes.

.We introduced and validated a method to miniaturize graded-index (GRIN) fiber-based optical coherence tomography (OCT) probes down to in diameter. The probes consist in an assembly of single-mode (SM), coreless (CL), and graded-index (GRIN) fibers. We opted for a probe design enabling controlled size reduction by hydrogen fluoride etching. The fabrication approach prevents nonuniform etching for both the GRIN and SM fiber components, while it requires no probe polishing postetching. We found that the miniaturized probes present insignificant loss of sensitivity () compared to their thicker () counterparts. We also showed that their focusing capabilities remain tunable and highly predictable. The fabrication process is simple and can be carried out by using inexpensive telecom equipment. Both the fabrication process and the developed probes can benefit the prototyping of minimally invasive endoscopic tools.

Long-range common-path spectral domain optical coherence tomography.

Fiber-based common-path spectral domain optical coherence tomography (SD-OCT) is compact and polarization insensitive, which is usually used in endoscopic biomedical imaging. In this study, we investigate a method to extend the working distance of a common-path SD-OCT system. Common-path OCT light, which consisting of sample and reference light signal, is directed into a free space optical interferometer. The OCT light is split spatially into two beam segments by a wavefront-splitting mirror, and the two parallel beams interfere noncollinearly in the interferometer. Distance between the end of the probing fiber, which serves as the reference plane of our OCT system, and the OCT sample is about 140 mm. The OCT performance is demonstrated by imaging biological samples. The proposed method can be used to develop polarization insensitive OCT probe for biomedical imaging applications.

Polarization management to mitigate misalignment-induced fringe fading in fiber-based optical coherence tomography.

In fiber-based optical coherence tomography (OCT), the interference fringes suffer from the fading effect due to misalignment of the light polarization states between the reference and sample arms, resulting in sensitivity degradation and image intensity variation. We theoretically and experimentally analyzed the relation between the misalignment and the fading coefficient. Assuming that the variation of the light polarization in single-mode fiber (SMF) was a random process, we statistically quantified the fading effect. Furthermore, in OCT configuration based on the Michelson interferometer, we reported an interesting observation that the polarization states of light traveling a round-trip in SMF are not evenly distributed on the Poincare sphere. Based on this observation, we demonstrated the existence of an optimal output polarization state of the reference arm to mitigate the fading effect. We demonstrated that in an optimal setup, the statistical average signal-to-noise ratio could be 3.5 dB higher than a setup without proper polarization management.

Colposcopic imaging using visible-light optical coherence tomography.

High-resolution colposcopic optical coherence tomography (OCT) provides key anatomical measures, such as thickness and minor traumatic injury of vaginal epithelium, of the female reproductive tract noninvasively. This information can be helpful in both fundamental investigations in animal models and disease screenings in humans. We present a fiber-based visible-light OCT and two probe designs for colposcopic application. One probe conducts circular scanning using a DC motor, and the other probe is capable of three-dimensional imaging over a 4.6 × 4.6 - mm 2 area using a pair of galvo scanners. Using this colposcopic vis-OCT with both probes, we acquired high-resolution images from whole isolated macaque vaginal samples and identified biopsy lesions.

Statistical properties of dynamic speckles from flowing Brownian scatterers in the vicinity of the image plane in optical coherence tomography.

A closed-form analytical expression is obtained for the spatio-temporal correlation function of the scattered radiation detected in fiber-based optical coherence tomography (OCT), assuming a clean optical system arrangement in the OCT sample arm. It is shown that the transverse flow component causes purely translational speckle motion with the predicted speckle velocity 2x higher than the velocity of the flowing particles as would be observed in the image plane under incoherent illumination. It is also shown that both speckle velocity and speckle radius do not depend on the position of the scattering volume relative to the focal plane, hence the derived correlation function is independent of the position of the scattering volume relative to the focal plane. Although the analytical results are obtained for a clean optical system arrangement, they can be used with high accuracy in most practical implementations of fiber based OCT. Validation experiments in control scattering phantoms with varying liquid viscosities show excellent agreement with the developed theoretical model, under both no-flow and flow conditions. Accurate viscosity determinations enabled by this methodology may have applications to non-invasive glucose measurements in medicine.

Advances of some critical technologies in discrete and distributed optical fiber sensing research

With the superiority of anti-electromagnetic interference, corrosion resistance, light quality, small size and so on, optical fiber sensing technology is widely used in aerospace industry, petrochemical engineering, power electronics, civil engineering and biological medicine. It can be divided as discrete and distributed. Discrete optical fiber sensing utilizes fiber sensitive element as sensors to detect the quantity to be measured. Optical spectrum, light intensity and polarization are usually used as the sensitivity parameter because they can be modulated by parameter such as rotation, acceleration, electromagnetic field, temperature, pressure, stress, stress, vibration, humidity, viscosity, refractive index and so on. Fiber works as the channel and links the fiber sensor and demodulating equipment. After a long period of research, the discrete optical fiber sensing technology stretch out many branches, we discussed the most representative ones as follows, the fiber grating sensing technique, the fiber fabry perot sensing technique, the fiber gyroscope sensing technique, the fiber intracavity sensing technique, the fiber surface plasma sensing technique, hollow-core fiber whispering gallery mode sensing technique, magnetic fluid fiber sensing technique and fiber-based optical coherence tomography sensing technique. Based on optical effect as rayleigh scattering, Raman scattering and Brillouin scattering, distributed fiber sensing system uses fiber itself as a sensor, when the vibration, stress, voice or temperature acts on the fiber changes, the optical signal transfers inside the fiber will change accordingly. The fiber distributes in a large range and a long distance, then the signal can be located at different positions and realize the multi-position measurement. We discussed the main distributed fiber sensing technologies as follows, the interferometric disturbance fiber sensing technology, the optical frequency domain reflectometry fiber sensing technology, the -optical time domain reflectometer fiber sensing technology, the optical fiber Brillouin sensing technology and the optical fiber Raman sensing technology. The development of technology is promoting the integration and network of optical fiber sensing, now it also becomes a research hotspot. Fiber optic smart sensor network is formed by various discrete and discrete optical fiber sensors in certain topological structure with the function of self-diagnosis and self-healing. Current research concentrates in the following areas, the increase of the multiplex sensor number, the topological structure with higher robustness and the intelligent control of sensing network. In this paper, we discuss the origination, development and research progress of discrete, distributed optical fiber sensing technologies and optical fiber sensing network technology, and the future research direction is also prospected.

Structural and functional human retinal imaging with a fiber-based visible light OCT ophthalmoscope.

The design of a multi-functional fiber-based Optical Coherence Tomography (OCT) system for human retinal imaging with < 2 micron axial resolution in tissue is described. A detailed noise characterization of two supercontinuum light sources with different pulse repetition rates is presented. The higher repetition rate and lower noise source is found to enable a sensitivity of 96 dB with 0.15 mW light power at the cornea and a 98 microsecond exposure time. Using a broadband (560 ± 50 nm), 90/10, fused single-mode fiber coupler designed for visible wavelengths, the sample arm is integrated into an ophthalmoscope platform, similar to current clinical OCT systems. To demonstrate the instrument's range of operation, in vivo structural retinal imaging is also shown at 0.15 mW exposure with 10,000 and 70,000 axial scans per second (the latter comparable to commercial OCT systems), and at 0.03 mW exposure and 10,000 axial scans per second (below maximum permissible continuous exposure levels). Lastly, in vivo spectroscopic imaging of anatomy, saturation, and hemoglobin content in the human retina is also demonstrated.

An All-Fiber-Optic Combined System of Noncontact Photoacoustic Tomography and Optical Coherence Tomography.

We propose an all-fiber-based dual-modal imaging system that combines noncontact photoacoustic tomography (PAT) and optical coherence tomography (OCT). The PAT remotely measures photoacoustic (PA) signals with a 1550-nm laser on the surface of a sample by utilizing a fiber interferometer as an ultrasound detector. The fiber-based OCT, employing a swept-source laser centered at 1310 nm, shares the sample arm of the PAT system. The fiber-optic probe for the combined system was homemade with a lensed single-mode fiber (SMF) and a large-core multimode fiber (MMF). The compact and robust common probe is capable of obtaining both the PA and the OCT signals at the same position without any physical contact. Additionally, the MMF of the probe delivers the short pulses of a Nd:YAG laser to efficiently excite the PA signals. We experimentally demonstrate the feasibility of the proposed dual-modal system with a phantom made of a fishing line and a black polyethylene terephthalate fiber in a tissue mimicking solution. The all-fiber-optic system, capable of providing complementary information about absorption and scattering, has a promising potential in minimally invasive and endoscopic imaging.

Dynamic light scattering by flowing Brownian particles measured with optical coherence tomography: impact of the optical system

The study of flowing Brownian particles finds numerous biomedical applications, ranging from blood flow analysis to diffusion research. A mathematical model for the correlation function of laser radiation scattered by flowing Brownian particles measured with fiber-based optical coherence tomography (OCT), which accounts for the effects of sample arm optics, is presented. It is shown that the parameters of an OCT optical system of any complexity can be taken into account by using the ABCD ray tracing matrix approach. Specifically, the impact of any optical system can be characterized by the changes in the effective beam radius, which replaces the Gaussian beam radius in the existing mathematical models of scattered radiation. It is shown that the validity of the developed ABCD matrix formalism is governed by the condition that the source coherence length is much smaller than the Rayleigh range in the sample. The predictions of the developed model are compared with previously published theories and with experimental data and agree well with the latter.

Ultrahigh-resolution optical coherence tomography at 1.3 μm central wavelength by using a supercontinuum source pumped by noise-like pulses

We report on the ultrahigh-resolution optical coherence tomography (OCT) with a novel high-power supercontinuum (SC) light source generated by noise-like pulses from an Yb-doped fiber laser. The SC spectrum is flat with a bandwidth of 420 nm centered around ~1.3 μm. The light source is successfully employed in a time-domain OCT (TD-OCT), achieving an axial resolution of 2.3 μm. High resolution fiber-based spectral-domain OCT (SD-OCT) imaging of bio-tissue was also demonstrated.

Hybrid Reflective Interferometric System Combining Wide-Field and Single-Point Phase Measurements

We present a hybrid scan-free reflective interferometric system, which combines a wide-field phase measurement, together with a single-point phase measurement, for optical inspection of thin reflective elements. The wide-field interferometric system is composed of a compact portable off-axis interferometer and is illuminated by either a highly coherence source or a narrowband low-coherence source. This is a free-space time-domain self-phase-referenced interferometric setup that can be attached to the output port of an existing reflection microscope. It records a spatial off-axis interferogram, which yields the wide-field phase map of the reflective sample. The other part of the hybrid system is a fiber-based phase-sensitive spectral-domain optical coherence tomography setup, which is illuminated by a boarder-band low-coherence source. It records an on-axis common-path spectral interferogram, which yields a single-point phase measurement of the reflective sample. In this case, since the reference beam does not interact with the sample, the phase is not self-referenced, and slow phase variations are measured as well. None of the setups contains scanning elements. The combination of these systems allows simultaneous wide-field and single-point phase measurements without co-calibration problems. By measuring thin reflective models with these external interferometers, we experimentally illustrate the ability to discriminate between refractive index changes from height changes in the sample.

Dual Modality Noncontact Photoacoustic and Spectral Domain OCT Imaging.

We developed a multimodal imaging system, combining noncontact photoacoustic imaging and optical coherence tomography (OCT). Photoacoustic signals are recorded without contact to the specimens’ surface by using an interferometric technique. The interferometer is realized within a fiber-optic network using a fiber laser at 1550 nm as source. The fiber-optic network allows the integration of a fiber-based OCT system operating at a wavelength region around 1310 nm. Light from the fiber laser and the OCT source are multiplexed into one fiber using wavelength-division multiplexing. The same focusing optics is used for both modalities. Back-reflected light from the sample is demultiplexed and guided to the respective imaging systems. As the same optical components are used for OCT and photoacoustic imaging, the obtained images are co-registered intrinsically in lateral direction. Three-dimensional imaging is implemented by hybrid galvanometer and mechanical scanning. To allow fast B-scan measurements, scanning of the interrogation beam along one dimension is executed by a galvanometer scanner. Slow-axis scanning, perpendicular to the fast axis, is performed utilizing a linear translational stage. We demonstrate two-dimensional and three-dimensional imaging on agarose phantoms.

Ultra-high resolution Fourier domain optical coherence tomography for old master paintings.

In the last 10 years, Optical Coherence Tomography (OCT) has been successfully applied to art conservation, history and archaeology. OCT has the potential to become a routine non-invasive tool in museums allowing cross-section imaging anywhere on an intact object where there are no other methods of obtaining subsurface information. While current commercial OCTs have shown potential in this field, they are still limited in depth resolution (> 4 μm in paint and varnish) compared to conventional microscopic examination of sampled paint cross-sections (~1 μm). An ultra-high resolution fiber-based Fourier domain optical coherence tomography system with a constant axial resolution of 1.2 μm in varnish or paint throughout a depth range of 1.5 mm has been developed. While Fourier domain OCT of similar resolution has been demonstrated recently, the sensitivity roll-off of some of these systems are still significant. In contrast, this current system achieved a sensitivity roll-off that is less than 2 dB over a 1.2 mm depth range with an incident power of ~1 mW on the sample. The high resolution and sensitivity of the system makes it convenient to image thin varnish and glaze layers with unprecedented contrast. The non-invasive 'virtual' cross-section images obtained with the system show the thin varnish layers with similar resolution in the depth direction but superior clarity in the layer interfaces when compared with conventional optical microscope images of actual paint sample cross-sections obtained micro-destructively.

Characterization of Optical Coherence Tomography Images Acquired at Large Distances With Large-Diameter Beams

We tested the imaging capabilities for variants of a 1550-nm swept-source fiber-based optical coherence tomography system with a telecentric system incorporated at the end of its sample arm. The system was designed for in vivo imaging of burns; therefore, we acquired images from samples located at distances greater than 24 cm from the exit of the telecentric system. Each system variation had a specific combination of diameters for the reference and sample beams. In the reference arm, we used, alternately, two collimated beams with diameters of 1.5 and 14 mm, respectively. In the sample arm, we tested collimated beams with the following diameters: 1.5, 3.5, 5.7, 8.4, and 14 mm. A galvanometric mirror system scanned the collimated sample beam across the entrance pupil of the telecentric system. The sample beam exited the telecentric system parallel with its optical axis and convergent onto the sample. Depending on the collimator used in the sample arm, images were acquired with beams focused to waist diameters ranging from 40 to 240 μm. We acquired images with the sample at different locations within a ±30 mm range centered about the sample beam waist. Furthermore, we used the signal-to-noise ratio, the detected signal intensity, and the visual appearance to compare images acquired with different sample/reference beam configurations.

Simultaneously measuring the refractive index and thickness of an optical sample by using improved fiber-based optical coherence tomography

An improved structure for the optical coherence tomography (OCT) scheme based on a 4×4 fiber coupler for simultaneously measuring the refractive index and thickness of optical samples is presented. The proposed structure incorporates the optical path length difference of interference light and is used to calculate the refractive index and thickness of an unknown sample without any prior knowledge of the sample parameters. Two methods (time-domain and Fourier-domain OCT) of obtaining information about an unknown sample are proposed, and a high-speed high-resolution OCT system was developed.