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Histological analysis is widely used to evaluate injured tendons; however, it has the limitation of being semi-quantitative. Hence, we developed a quantification method to objectively evaluate the fibrous structure of tendons, exhibiting the optical property of birefringence, using polarization-sensitive optical coherence tomography (PS-OCT). We used a partial-rupture rat model in which the middle 0.75 cm of the Achilles tendon was cut with a blade. Rats were sacrificed at 2, 4 or 6 weeks after the injury, and PS-OCT and histological analyzes were performed. The PS-OCT phase retardation images and score well represented the structural changes of the injured tendon according to the wound healing state. Therefore, the proposed novel quantification method using PS-OCT can be used to evaluate the fibrous structural status of tendons.

In microscopic imaging of biological tissues, particularly real-time visualization of neuronal activities, rapid acquisition of volumetric images poses a prominent challenge. Typically, two-dimensional (2D) microscopy can be devised into an imaging system with 3D capability using any varifocal lens. Despite the conceptual simplicity, such an upgrade yet requires additional, complicated device components and suffers a reduced acquisition rate, which is critical to document neuronal dynamics properly. In this study, we implemented an electro-tunable lens (ETL) in the line-scan confocal microscopy, enabling the volumetric acquisition at the rate of 20 frames per second with the maximum volume of interest of 315 × 315 × 80 μm3. The axial extent of point-spread-function (PSF) was 17.6 ± 1.6 μm and 90.4 ± 2.1 μm with the ETL operating in either stationary or resonant mode, respectively, revealing significant depth elongation by the resonant mode ETL microscopy. We further demonstrated the utilities of the ETL system by volume imaging of cleared mouse brain ex vivo samples and in vivo brains. The current study foregrounds the successful application of resonant ETL for constructing a basis for a high-performance 3D line-scan confocal microscopy system, which will enhance our understanding of various dynamic biological processes.

Swept-source optical coherence tomography (SS-OCT) is an attractive high-speed imaging technique for retinal angiography. However, conventional swept lasers vary the cavity length of the laser mechanically to tune the output wavelength. This causes sweep-timing jitter and hence low phase stability in OCT angiography. Here, we improve an earlier phase-stabilized, akinetic, SS-OCT angiography (OCTA) method by introducing coherent averaging. We develop an active mode-locking (AML) laser as a high phase-stable akinetic swept source for the OCTA system. The phase stability of the improved system was analyzed, and the effects of coherent averaging were validated using a retina phantom. The effectiveness of the coherent averaging method was further confirmed by comparing coherently and conventionally averaged en face images of human retinal vasculature for their contrast-to-noise ratio, signal-to-noise ratio, and vasculature connectivity. The contrast-to-noise ratio was approximately 1.3 times larger when applying the coherent averaging method in the human retinal experiment. Our coherent averaging method with the high phase-stability AML laser source for OCTA provides a valuable tool for studying healthy and diseased retinas.

We developed a structured illumination-based optical inspection system to inspect metallic nanostructures in real time. To address this, we used post-image-processing techniques to enhance the image resolution. To examine the fabricated metallic nanostructures in real time, a compact and highly resolved optical inspection system was designed for practical industrial use. Structured illumination microscopy yields multiple images with various linear illumination patterns, which can be used to reconstruct resolution-enhanced images. Images of nanosized posts and complex structures reflected in the structured illumination were reconstructed into images with improved resolution. A comparison with wide-field images demonstrates that the optical inspection system exhibits high performance and is available as a real-time nanostructure inspection platform. Because it does not require special environmental conditions and enables multiple systems to be covered in arrays, the developed system is expected to provide real-time and noninvasive inspections during the production of large-area nanostructured components.

Optical coherence tomography (OCT) is a noninvasive imaging modality that can provide high-resolution, cross-sectional images of tissues. Especially in retinal imaging, OCT has become one of the most valuable imaging tools for diagnosing eye diseases. Considering the scattering and absorption properties of the eye, the 1000-nm OCT system is preferred for retinal imaging. In this study, we describe the use of an akinetic swept-source OCT system based on a pulse-modulated active mode locking (AML) fiber laser at a 1080-nm wavelength for in-vivo human retinal imaging. The akinetic AML wavelength-swept fiber laser was constructed with polarization-maintaining fiber that has an average linewidth of 0.625 nm, a spectral bandwidth of 81.15 nm, and duty ratio of 90% without the buffering method. We successfully obtained in-vivo human retinal images using the proposed OCT system without the additional k-clock and the frequency shifter that provides a wide field of view of 43.1°. The main retina layers, such as the retinal pigment epithelium, can be distinguished from the OCT image with an axial resolution of 6.3 μm with this OCT system.

In microscopic imaging of biological tissues, particularly real-time visualization of neuronal activities, rapid acquisition of volumetric images poses a prominent challenge. Typically, two-dimensional (2D) microscopy can be devised into an imaging system with 3D capability using any varifocal lens. Despite the conceptual simplicity, such an upgrade yet requires additional, complicated device components and usually suffers from a reduced acquisition rate, which is critical to properly document rapid neurophysiological dynamics. In this study, we implemented an electrically tunable lens (ETL) in the line-scan confocal microscopy (LSCM), enabling the volumetric acquisition at the rate of 20 frames per second with a maximum volume of interest of 315 × 315 × 80 µm3. The axial extent of point-spread-function (PSF) was 17.6 ± 1.6 µm and 90.4 ± 2.1 µm with the ETL operating in either stationary or resonant mode, respectively, revealing significant depth axial penetration by the resonant mode ETL microscopy. We further demonstrated the utilities of the ETL system by volume imaging of both cleared mouse brain ex vivo samples and in vivo brains. The current study showed a successful application of resonant ETL for constructing a high-performance 3D axially scanning LSCM (asLSCM) system. Such advances in rapid volumetric imaging would significantly enhance our understanding of various dynamic biological processes.

A method allows the extraction of the recovery factor that maximizes the image contrast of OCT (optical coherence tomography) and/or OCTA (OCT angiography) of a living subject is proposed in this study. Due to the finite depth of focus in imaging optics, the volume OCT imaging suffers from blurriness in the lateral resolution. By utilizing the digital hologram method or angular spectrum method, the blurred image can be refocused. However, for in vivo OCT imaging, evaluation of the image focus is not easy, owing to the cloudy structure of the brain. In the proposed method, the blood flow signals were used as a guiding star to find the recovery factor. The propagation distance to a focal plane was automatically determined by evaluating the contrast of a cross-sectional OCTA image. The performance was examined though in vivo mouse brain OCT/OCTA imaging. The image singularity of the blood flow in OCTA was very effective at evaluating the contrast of the image.

The numerical refocusing feature of digital holography enables the reconstruction of a well-focused image from a digital hologram captured at an arbitrary out-of-focus plane without the supervision of end users. However, in general, the autofocusing process for getting a highly focused image requires a considerable computational cost. In this study, to reconstruct a better-focused image, we propose the zero padding technique implemented in the frequency domain. Zero padding in the frequency domain enhances the visibility or numerical resolution of the image, which allows one to measure the degree of focus with more accuracy. A coarse-to-fine search algorithm is used to reduce the computing load, and a graphics processing unit (GPU) is employed to accelerate the process. The performance of the proposed scheme is evaluated with simulation and experiment, and the possibility of obtaining a well-refocused image with an enhanced accuracy and speed are presented.

Optical coherence tomography (OCT) is a noninvasive imaging modality which can provide cross-sectional imaging of the tissues in high-resolution. Especially in retina imaging, the OCT becomes one of the most valuable imaging tools for the diagnostics of the eye diseases. Considering the scattering and absorption properties of the eye, the 1000 nm OCT system is preferred for the retina image. In this study, we describe an akinetic swept source OCT system based on pulse-modulated active mode locking (AML) fiber laser at the 1080 nm wavelength region for in-vivo human retina imaging. The akinetic AML wavelength swept fiber laser is constructed with polarization maintaining fiber which has average linewidth of 0.625 nm, a spectral bandwidth of 81.15 nm and a duty ratio of 90 % without buffering method. We successfully obtained in-vivo human retina images using proposed OCT system without the additional k-clock and the frequency shifter providing wide field of view of 43.1o. The main retina layers such as RPE can be distinguished through the OCT image with axial resolution of 6.3 m.

Significance: Collagen is a basic component of many tissues such as tendons, muscles, and skin, and its imaging helps diagnose and monitor treatments in a variety of fields, including orthopedics. However, due to the overlapping peaks of the absorption spectrum with water in the short-wave infrared region (SWIR), it is difficult to select an optimal wavelength and obtain the photoacoustic (PA) image for collagen-based tissues. Therefore, an additional approach to selecting the proper wavelength is needed. Aim: The aim of this study is to derive an effective PA absorption spectrum of collagen to select the optimal wavelength for high-sensitive PA imaging (PAI). Approach: We measure the absorption spectrum by acquiring the PA signal from various collagen-based samples. To derive an effective PA absorption spectrum in the SWIR band, the following two parameters should be considered: (1) the laser excitation for generating the PA signal and (2) the absorption spectrum for water in the SWIR band. This molecular intrinsic property suggests the optimal wavelength for high-sensitive PAI of collagen-based samples. Results: PA absorption spectral peaks of collagen were found at wavelengths of 1200, 1550, and 1700 nm. Thereby, the PA signal increased by up to five times compared with the wavelength commonly used in collagen PAI. We applied a pulsed fiber laser with a center wavelength of 1560 nm, and the three-dimensional PA image of a collagen patch was obtained. Conclusions: The effective PA absorption spectrum contributes to the improvement of the PA image sensitivity by presenting the optimal wavelength of the target samples.

The hippocampus is associated with memory and navigation, and the rodent hippocampus provides a useful model system for studying neurophysiology such as neural plasticity. Vascular changes at this site are closely related to brain diseases, such as Alzheimer’s disease, dementia, and epilepsy. Vascular imaging around the hippocampus in mice may help to further elucidate the mechanisms underlying these diseases. Optical coherence tomography angiography (OCTA) is an emerging technology that can provide label-free blood flow information. As the hippocampus is a deep structure in the mouse brain, direct in vivo visualisation of the vascular network using OCTA and other microscopic imaging modalities has been challenging. Imaging of blood vessels in the hippocampus has been performed using multiphoton microscopy; however, labelling with fluorescence probes is necessary when using this technique. Here, we report the use of label-free and noninvasive microvascular imaging in the hippocampal formation of mice using a 1.7-μm swept-source OCT system. The imaging results demonstrate that the proposed system can visualise blood flow at different locations of the hippocampus corresponding with deep brain areas.

We demonstrated a linearized wavelength interrogation system of fiber Bragg grating (FBG) strain sensors, based on a wavelength-swept active mode locking (AML) fiber laser. Since the wavelength sweeping mechanism of the AML fiber laser does not depend on the wavelength selecting filter, we can easily demonstrate the wavelength-linear sweeping by applying a programmable electrical signal to the laser cavity. Both static and dynamic strain measurements of FBG sensors were characterized with high linearity, which has an R-square value of 0.9999 at a sweep rate of 50 kHz.

Tendons are tough, flexible, and ubiquitous tissues that connect muscle to bone. Tendon injuries are a common musculoskeletal injury, which affect 7% of all patients and are involved in up to 50% of sports-related injuries in the United States. Various imaging modalities are used to evaluate tendons, and both magnetic resonance imaging and sonography are used clinically to evaluate tendons with non-invasive and non-ionizing radiation. However, these modalities cannot provide 3-dimensional (3D) structural images and are limited by angle dependency. In addition, anisotropy is an artifact that is unique to the musculoskeletal system. Thus, great care should be taken during tendon imaging. The present study evaluated a functional photoacoustic microscopy system for in-vivo tendon imaging without labeling. Tendons have a higher density of type 1 collagen in a cross-linked triple-helical formation (65–80% dry-weight collagen and 1–2% elastin in a proteoglycan-water matrix) than other tissues, which provides clear endogenous absorption contrast in the near-infrared spectrum. Therefore, photoacoustic imaging with a high sensitivity to absorption contrast is a powerful tool for label-free imaging of tendons. A pulsed near-infrared fiber-based laser with a centered wavelength of 780 nm was used for the imaging, and this system successfully provided a 3D image of mouse tendons with a wide field of view (5 × 5 mm2).

The strong optical scattering of biological tissue confounds our ability to focus light deeply into the brain beyond depths of a few hundred microns. This challenge can be potentially overcome by exploiting wavefront shaping techniques which allow light to be focused through or inside scattering media. However, these techniques require the scattering medium to be static, as changes in the arrangement of the scatterers between the wavefront recording and playback steps reduce the fidelity of the focus that is formed. Furthermore, as the thickness of the scattering medium increases, the influence of the dynamic nature becomes more severe due to the growing number of scattering events experienced by each photon. In this paper, by examining the scattering dynamics in the mouse brain in vivo via multispeckle diffusing wave spectroscopy (MSDWS) using a custom fiber probe that simulates a point-like source within the brain, we investigate the relationship between this decorrelation time and the depth of the point-like light source inside the living mouse brain at depths up to 3.2 mm.

We demonstrate a highly linearly wavenumber-swept active mode locking (AML) fiber laser in the 1.3 ${\mu}$ m region for in-vivo imaging in optical coherence tomography (OCT) without wavenumber space resampling. In this all-electric AML wavenumber-swept mechanism, the conventional wavelength selection filter is eliminated, and instead a suitable programmed electric modulation signal is applied directly to the gain medium. For a high sweep rate (up to 1 MHz) along the wavenumber, the fiber cavity structure is made as short as possible (0.88 m in air). A 15 ps/nm chirped fiber Bragg grating and a circulator are used for a shorter ring cavity configuration. A linewidth of 0.1 nm and tuning range of 42 nm are obtained under the mode-locking condition. Various types of wavenumber (or wavelength) tunings can be implemented because of the filterless cavity configuration. Therefore, we successfully demonstrate a linearly wavenumber-swept AML fiber laser with 26.5 mW of output power for obtaining in-vivo OCT images at a sweep rate of 100 kHz.

Herein we propose a method to mitigate a position mismatch problem for a spectral-domain polarization-sensitive optical coherence tomography (SD-PS-OCT) system that uses a single line-scan detection scheme. A single detector-based PS-OCT detects two orthogonal polarization components as two adjacent A-scan signals in turns. Thus, two adjacent A-scan signals are not scattered at a fixed point in time (position mismatch problem), resulting in uncorrelated signals between them. To achieve sequential detection of simultaneously scattered light, a buffering single-mode fiber was connected to one of the two ports coming out of the optical switch, provided a proper time delay. A single-mode optical fiber of 2.69 km in length was used to buffer, and its length was determined by a frame rate of the spectrometer used as a detector. With the proposed SD-PS-OCT scheme, the PS-OCT system with a simple configuration, and the minimized position mismatch problem between two polarization components can be set.

We have developed a surgical microscope-integrated optical coherence tomography (MI-OCT) system based on an active feedback method to obtain uniform optimal OCT image contrast along the depth of focus (DOF) of a surgical microscope. Conventional MI-OCT systems use a shorter DOF objective lens than those of surgical microscopes for OCT imaging. The existing MI-OCT system was developed to overcome sensitivity roll-off by using an electrically tunable lens (ETL). However, active change in the focus position through the ETL cannot cope with the sensitivity decrease due to optical path length difference (OPD) mismatch. The proposed active feedback method was able to maintain high sensitivity by actively performing OPD matching using a linear motor in the reference arm while tuning the focal position in the sample arm using the ETL. The optical system designed to maintain the OCT resolution and a retroreflector used for ensuring regular reflection intensity in the reference arm during OPD compensation contributed to the uniform sensitivity and stable OCT imaging performance. The simultaneous and automatic actuation of the ETL and linear motor provided sensitivity variation of 3 dB from 17 dB for 10-mm sample displacement corresponding to the DOF of the surgical microscope used in the MI-OCT system. By using an infrared detection card and a mouse brain tumor model, it was demonstrated that the proposed MI-OCT system could acquire OCT images with optimal sensitivity without the limitations due to short OCT DOF.

Purpose To elucidate the anatomic details of gland dropout detected on two-dimensional infrared (IR) meibography in cases of dry eye associated with meibomian gland dysfunction (MGD) by using three-dimensional optical coherence tomography (OCT) meibography. Methods In this cross-sectional, observational case series, we enrolled gland dropout detected on IR meibography; the condition was then examined using a real-time swept-source OCT system. Accordingly, a series of 500 raster B-scan OCT images, with the gland dropout site (observed on IR imaging) at the center, were obtained and rendered as three-dimensional volume images. The OCT images were classified based on the anatomic details, including acini and ducts, at the meibomian glands (Group I, constricted acini; II, atrophic acini; III, no acini). Results The percentage of disagreement between IR and OCT images for dropout detected on IR imaging was 49.45% (43 and 93 cases in group I and II, respectively). Loss of the meibomian glands on both IR and OCT imaging (Group III) was observed in 50.55% cases (133 and 6 cases of gland dropout at the partial and whole eyelid on IR imaging, respectively). The proportion of disagreement between IR and OCT images (Group I and II) was higher in the middle area (63/119, 53.39%), as compared to that in the nasal (34/73, 46.58%) or temporal areas of the eyelid (26/65, 40%). Conclusions The loss of the meibomian glands, as observed on IR imaging, should be carefully interpreted, and OCT images may be useful to confirm the anatomic details of the meibomian glands.

We provide an approach to improve the quality of image reconstruction in wide-field imaging through turbid media (WITM). In WITM, a calibration stage which measures the transmission matrix (TM), the set of responses of turbid medium to a set of plane waves with different incident angles, is preceded to the image recovery. Then, the TM is used for estimation of object image in image recovery stage. In this work, we aim to estimate highly resolved angular spectrum and use it for high quality image reconstruction. To this end, we propose to perform a dense sampling for TM measurement in calibration stage with finer incident angle spacing. In conventional approaches, incident angle spacing is made to be large enough so that the columns in TM are out of memory effect of turbid media. Otherwise, the columns in TM are correlated and the inversion becomes difficult. We employ compressed sensing (CS) for a successful high resolution angular spectrum recovery with dense sampled TM. CS is a relatively new information acquisition and reconstruction framework and has shown to provide superb performance in ill-conditioned inverse problems. We observe that the image quality metrics such as contrast-to-noise ratio and mean squared error are improved and the perceptual image quality is improved with reduced speckle noise in the reconstructed image. This results shows that the WITM performance can be improved only by executing dense sampling in the calibration stage and with an efficient signal reconstruction framework without elaborating the overall optical imaging systems.

We previously reported the potential of using vascular reactivity during respiratory challenges as a marker to predict the response of breast tumor to chemotherapy in a rat model by using a continuous wave near-infrared spectroscopy. However, it cannot visualize how the vascular reactivity from tumor vessel can predict the tumor response to its treatment. In this study, we utilized a spectral domain optical coherence tomography (SD-OCT) system to visualize vascular reactivity of both tumor and normal vasculature during respiratory challenges in a mouse model. We adapted intensity based Doppler variance algorithm to draw angiogram from the ear of mouse (8-week-old Balb/c nu/nu). Animals were anesthetized using 1.5% isoflurane, and the body temperature was maintained by a heating pad. Inhalational gas was switched from air (10min) to 100% oxygen (10min), and a pulse oximeter was used to monitor arterial oxygen saturation and heart rate. OCT angiograms were acquired 5 min after the onset of each gas. The vasoconstriction effect of hyperoxic gas on vasculature was shown by subtracting an en-face image acquired during 100% oxygen from the image acquired during air inhalation. The quantitative change in the vessel diameter was measured from the en-face OCT images of the individual blood vessels. The percentage of blood vessel diameter reduction varied from 1% to 12% depending on arterial, capillary, or venous blood vessel. The vascular reactivity change during breast tumor progression and post chemotherapy will be monitored by OCT angiography.

Variable light focusing is the ability to flexibly select the focal distance of a lens. This feature presents technical challenges, but is significant for optical interrogation of three-dimensional objects. Numerous lens designs have been proposed to provide flexible light focusing, including zoom, fluid, and liquid-crystal lenses. Although these lenses are useful for macroscale applications, they have limited utility in micron-scale applications due to restricted modulation range and exacting requirements for fabrication and control. Here, we present a holographic focusing method that enables variable light focusing without any physical modification to the lens element. In this method, a scattering layer couples low-angle (transverse wave vector) components into a full angular spectrum, and a digital optical phase conjugation (DOPC) system characterizes and plays back the wavefront that focuses through the scattering layer. We demonstrate micron-scale light focusing and patterning over a wide range of focal distances of 22-51 mm. The interferometric nature of the focusing scheme also enables an aberration-free scattering lens. The proposed method provides a unique variable focusing capability for imaging thick specimens or selective photoactivation of neuronal networks.

We demonstrate fiber-optic sensor applications to full-range complex optical coherence tomography (OCT). To extend imaging range in OCT, real value or interferogram measured from an interferometer is needed to convert into complex value. For the purpose, various treatments such as mechanical, electro-optical, optical and programming based methods have been exploited in the interferometer. To make complex signal in fiber-optic interferometer, we propose vibrationbased optical phase shifting method. The proposed method utilizes optical fiber sensors that are for the detection of vibration using optical fiber. When coiled fiber was exposed to vibration, interferogram presents fringe shift without periodicity variations, which means that vibration induces phase shift in the interferometer. Therefore, intentionally generated vibration could be applicable to controlling of the optical phase shift and retrieval of the complex signal. As a result, the vibrations applied to coiled fiber were able to remove mirror image in Fourier domain. This result proved the feasibility of the proposed method on the extending of optical imaging range.

The real time and high-resolution optical imaging were enhanced image processing quality and contribute for analyzing and diagnosis guidelines. A high speed OCT imaging system and an image processing modality was introduced for several ophthalmology applications to obtain quantified information.

We demonstrate a k-domain linearization using a fiber Bragg grating (FBG) array for Fourier domain optical coherence tomography based on a wavelength swept laser. The k-domain linearization is carried out with an interpolation method using a FBG array with five FBGs. The measured signal-to-noise ratio from the point spread function after k-domain linearization is 12 dB improved over that of without k-domain linearization at the 1 mm depth of the sample. Clear OCT imaging of the slide glass with k-domain linearization could be obtained.

We developed the optical coherence tomography (OCT) image referenced microsurgical system for ocular anterior segment. The system combined a swept-source OCT and a femto-second laser and afford the 2D and 3D structure information to increase the efficiency and safety of the cataract procedure. The OCT imaging range was extended to achieve the 3D image from the cornea to lens posterior. The surgeons can plan the laser illumination range for the nuclear division and segmentation, and monitor the whole cataract surgery procedure using the real time OCT. The surgery system was demonstrated with an extracted pig eye and in vivo rabbit eye to verify the system performance and stability.

We demonstrated a MHz speed wavelength-swept fiber laser based on the active mode locking (AML) technique and applied to interrogation system of an array of fiber Bragg grating (FBG) sensors. MHz speed wavelength sweeping of wavelength-swept fiber laser can be obtained by programmable frequency modulation of the semiconductor optical amplifier (SOA) without any wavelength tunable filter. Both static and dynamic strain measurement of FBG sensors were successfully characterized with high linearity of an R-square value of 0.9999 at sweeping speed of 50 kHz.

Purpose The dysfunction of meibomian glands which secrete components of lipid layer in tears is currently pointed out as one of the main causes occurring dry eye. The distribution of that is more than 70% in Asian, especially. This brought out the importance for the dysfunction of meibomian glands. Our study was aimed to confirm the efficacy of 3D meibography to evaluate the structures of meibomian glands. Methods This study is a cross sectional study for patients who had diagnosed as dry eye disease associated with the dysfunction of meibomian glands at Seoul Saint Mary's Hospital from July to October, 2014. To confirm the structure of dry eye patients, 3D images using 3D OCT (optical coherence tomography) and 2D images using infrared camera were obtained. Patients who had the drop-out lesion in 3D and 2D images were divided as two groups, and differences between them were analyzed. At the same time, to find the clinical signification for structural changes of meibomian glands, all patients had an ocular surface and a tear function examination to define the degree of dry eye. Results As compared between 3D and 2D images for dry eye patients who had the drop-out lesion on meibomian glands, 3D images was more useful for diagnosis of dry eye than 2D, especially in dry eye related with mild meibomian gland disease. Conclusions Our study confirmed that the structural change of meibomian glands was reflected in optical coherence tomography 3D images. Especially, 3D meibography was more powerful than 2D infrared camera to find out the real state of drop-out lesion on meibomian glands. But, there was no statistical significance between the location of drop-out lesions; such as near lid margin, middle area, near superior conjunctival fornix, and clinical features in these study.

We propose a spectral domain optical coherence tomography (SD-OCT) system that uses a single line-scan detection scheme for balanced detection. Two phase-opposed spectra, generated by two optical fiber couplers, were detected by using a spectrometer with fast optical switching. A 2.69 km optical fiber was introduced to provide a proper time delay to prevent phase errors caused by the difference in measurement time between the two opposing spectra and unstable output voltages for controlling the galvano-scanner. Hence, a phase difference of pi was obtained between the spectra over the sample depth without a phase error, which improved sensitivity by approximately 6 dB compared to that of conventional SD-OCT. We directly showed and compared the OCT images before and after applying the proposed balanced detection method in a phantom and in vivo sample. (C) 2015 Optical Society of America

Eye is an optical system composed briefly of cornea, lens, and retina. Ophthalmologists can diagnose status of patient’s eye from information provided by optical sensors or images as well as from history taking or physical examinations. Recently, we developed a prototype of optical coherence tomography (OCT) image guided femtosecond laser cataract surgery system. The system combined a swept-source OCT and a femtosecond (fs) laser and afford the 2D and 3D structure information to increase the efficiency and safety of the cataract procedure. The OCT imaging range was extended to achieve the 3D image from the cornea to lens posterior. A prototype of OCT image guided fs laser cataract surgery system. The surgeons can plan the laser illumination range for the nuclear division and segmentation, and monitor the whole cataract surgery procedure using the real time OCT. The surgery system was demonstrated with an extracted pig eye and in vivo rabbit eye to verify the system performance and stability.

We have imaged human corneal nerve bundles by using real-time Fourier-domain OCT (FD-OCT). Corneal nerves contribute to the maintenance of healthy ocular surface owing to their trophic influences on the corneal epithelium. The FD-OCT system was based on a swept laser of a 50 kHz sweeping rate and 1.31 μm center wavelength. At the area including sclera, limbus, and cornea, we could successfully get the in-vivo tomograms of the corneal nerve bundles. The scan range was 5 x 5mm. In this study, the A-scan images in each B-scan were realigned to have a flat air-surface boundary in the final B-scan image. With this effort, we could align corneal nerve bundle in a same depth and get the 3D image showing the branched and threadlike corneal nerve bundles.

Optical-resolution photoacoustic microscopy (OR-PAM) has been studied to improve its imaging resolution and functional imaging modality without labeling on biology sample. However the use of high numerical aperture (NA) objective lens confines the field of view or the axial imaging range of OR-PAM. In order to obtain images at different layers, one needs to change either the sample position or th e focusing position by mechanical scanning. This mechanical movement of the sample or the objective lens limits the scanning speed and the positioning precision. In this study, we propose a multi-depth PAM with a focus tunable lens. We electri cally adjusted the focal length in the depth direction of the sample, and twice extended the axial imaging range up to 660 m with the objective lens (20X, NA 0.4). The proposed approach can increase scanning speed and avoid step motor induced distortions during PA signal acquisitions without mechanical scanning in the depth direction. To in vestigate the performance of the multi-depth PAM system, we scanned a black human hair and the ear of a living nude mouse (BALB/c Nude). The obtained PAM images presented the volumetric rendering of black hair and the vasculature of the nude mouse. Keywords: photoacoustic imaging, focus tunable le ns, optical imaging, biomedical imaging

Refractive index variation in rare-earth doped specialty fiber can be possible through resonantly enhanced optical nonlinearity with the assistance of an optical pumping. The quantity of the variation under low-power optical pumping is enough to induce phase shift of 2π. By using this nonlinear effect in the specialty fiber, optical imaging system can perform phase shift-based optical imaging without mechanically controlled phase stepping. The pump-induced refractive index from the specialty fiber in reference arm of interferometer can produce optical delay depending on applied optical pumping power. At low optical power under few hundred mW, optical delay corresponding to 2π can be yielded in the reference arm efficiently. Contrast to the conventional mechanical phase stepping method, optically actuated phase stepping with the specialty optical fiber can avoid drawback of mechanical hysteresis and requirement of high voltage controllable electronics. The feasibility of the proposed method on optical imaging is suggested with demonstrating full range imaging in optical coherence tomography. Extended imaging range under optical phase stepping was successfully presented. The proposed method could be applied for detailed control of phase shift-based interferometry.

This paper propose an optical structure to generate trigger signals for optical coherence tomography (OCT) using backward light which is usually disposed. The backward light is called backward amplified stimulated emission generated from semiconductor optical amplifier (SOA) when using swept wavelength tunable laser (SWTL). A circulator is applied to block undesirable lights in the SWTL instead of an isolator in common SWTL. The circulator also diverts backward amplified spontaneous lights, which finally bring out trigger signals for a high speed digitizer. The spectra of the forward lights at SOA and the waveform of the backward lights were measured to check the procedure of the trigger formation in the experiment. The results showed that the trigger signals from the proposed SWTL with the circulator was quite usable in OCT.

We have implemented a distributed sensor system based on an array of fiber Bragg gratings (FBGs), which can measure up to 1000 points with a single piece of fiber. The system consists of FBGs having the same resonant wavelengths and small reflectivities (∼0.1 dB), and a wave-length tunable optical time-domain reflectometer (OTDR). To interrogate the distributed grating sensors and to address the event locations simultaneously, we have utilized the tunable OTDR. A thermoelectric temperature controller was used to tune the emission wavelength of the OTDR. The operating temperature of the laser diode was changed. By tuning the pulse wavelength of the OTDR, we could identify the FBGs whose resonant wavelengths were under change within the operating wavelength range of the DFB LD. A novel sensor cable with dry core structure and tensile cable was fabricated to realize significant construction savings at an industrial field and in-door and out-door applications. For experiments, a sensor cable having 52 gratings with 10m separations was fabricated. To prevent confusion with unexpected signals from the front-panel connector zone of the OTDR, a 1 km buffer cable was installed in front of the OTDR. The proposed system could distinguish and locate the gratings that were under temperature variation from 20°C to 70°C.

This paper proposes and demonstrates a technique for repetition-rate multiplication of an optical pulse train having an arbitrary period or pattern and the optical temporal encoding/decoding for the optical-code-division multiple-access (O-CDMA) system. The technique exploits the difference in the propagation speeds between the core and copropagating cladding modes of a fiber to obtain true-time-delay between the modes traveling in the core mode and the cladding mode, which can be used to achieve pulse multiplication. For the coupling to the cladding mode, long-period fiber gratings (LPGs) were used. A series of cascaded LPGs imprinted in a fiber with a specific separation has been employed to obtain a specific rate of pulse multiplication with a single input pulse. Second, by controlling the separations among the gratings, the temporal encoder/decoder for O-CDMA could be implemented. The principle and the applications of the proposed device are investigated in detail. The effect of the birefringence of fiber and fiber gratings on the system performance in the time and spectral domains is presented. The sensitivity of the cladding modes in a conventional fiber to the perturbations at the cladding has been overcome by replacing the conventional fiber with inner-cladding fiber. The properties and the benefits of using the inner-cladding mode are investigated.

Optical fiber connectors that had a long working distance and wide alignment tolerance were implemented by utilizing long-period fiber gratings (LPGs) written in double-cladding fiber. The power coupling between two separated pieces of fiber was accomplished through the inner cladding modes of both fibers. Assisted by the LPGs, approximately 450 /spl mu/m was achieved for a 1-dB working distance, as well as about 3 mm of 3-dB working distance. When the separation between two fiber tips was 1.5 mm, the 3-dB lateral tolerance was measured to be about 30 /spl mu/m, which was five times better than was the case when not utilizing the LPGs. By using the inner cladding mode of a dispersion-compensating fiber, it was possible to implement a fiber connector that was insensitive to the perturbations at the cladding surface of the fiber. The total insertion loss of the device at the maximum coupling was 2.06 dB, from which the loss purely induced by inserting the fiber gratings was analyzed to be smaller than 0.9 dB. The experimental results show strong feasibility for implementing a simple and cost-effective fiber connector that does not require any bulk optic elements.

We propose and demonstrate a novel optical pulse multiplier applicable to OTDM (Optical Time Division Multiplexing) systems using cascaded long-period fiber gratings. We have exploited the fact that each mode in a fiber has a different propagation constant to obtain time delays among optical pulses. The proposed scheme could realize high-frequency optical pulse multiplication for optical short pulse trains. We have successfully implemented two, four, and eight times multiplications with the maximum repetition rate of 416.7 ㎓. The obtained pulse delays are well matched with the simulated ones.

We correct values listed in the main text of our recent paper: [Biomed. Opt. Express 8(11), 4855 (2017)].