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The range of imaging depth in optical resolution photoacoustic microscopy (PAM) is limited by the short depth of focus of high-numerical aperture objective lenses. In this paper, focus tunable lens modulation has been employed at the resonant frequency of focus tunable lenses in order to enhance both the range of imaging depth and the scanning speed. By electrically controlling the focal length in the axial direction of the sample, the range of imaging depth was extended approximately 1.22 times and the scanning speed was enhanced by approximately 7.40 times, in comparison to corresponding values of conventional PAM systems.

We have employed Fourier-domain optical coherence tomography (FD-OCT) to achieve corneal nerve imaging, which could be useful in surgical planning and refractive surgery. Because the three-dimensional (3-D) images of the corneal nerves were acquired in vivo, unintentional movement of the subject during the measurement led to imaging artifacts. These artifacts were compensated for with a series of signal processing techniques, namely realigning A-scan images to flatten the boundary and cross-correlating adjacent B-scan images. To overcome the undesirably large signal from scattering at the corneal surface and iris, volume rendering and maximum intensity projections were performed with only the data taken in the stromal region of the cornea, which is located between 200 and 500???m from the corneal surface. The 3-D volume imaging of a 10×10??mm2 area took 9.8 s, which is slightly shorter than the normal tear breakup time. This allowed us to image the branched and threadlike corneal nerve bundles within the human eye. The experimental results show that FD-OCT systems have the potential to be useful in clinical investigations of corneal nerves and by minimizing nerve injury during clinical or surgical procedures.

We demonstrate a highly linear wavenumber- swept active mode locking (AML) fiber laser for optical sensing and imaging without any wavenumber-space resampling process. In this all-electric AML wavenumber-swept mechanism, a conventional wavelength selection filter is eliminated and, instead, the suitable programmed electric modulation signal is directly applied to the gain medium. Various types of wavenumber (or wavelength) tunings can be implemented because of the filter-less cavity configuration. Therefore, we successfully demonstrate a linearly wavenumber-swept AML fiber laser with 26.5 mW of output power to obtain an in-vivo OCT image at the 100 kHz swept rate.

We demonstrate segmentation of human MGs based on several image processing technic. 3D volumetric data of upper eyelid was acquired from real-time FD-OCT, and its acini area of MGs was segmented. Three dimensional volume informations of meibomian glands should be helpful to diagnose meibomian gland related disease. In order to reveal boundary between tarsal plate and acini, each B-scan images were obtained before averaged three times. Imaging area was 10x10mm and 700x1000x500 voxels. The acquisition time was 60ms for B-scan and 30sec for C-scan. The 3D data was flattened to remove curvature and axial vibration, and resized to reduce computational costs, and filtered to minimize speckles, and segmented. Marker based watershed transform was employed to segment each acini area of meibomian gland.

A wavelength swept broadband source based on a sub-10-femtosecond (fs) pulse laser and a spatial filtering method was developed for the application to frequency-domain optical coherence tomography (FD-OCT). The use of the sub-10-fs laser with a galvano scanner operating at a high repetition rate provided a fast tunable broadband light source, which tuned the spectral component across the spectral bandwidth of 110 nm at 1-kHz repetition rate. The spectrally filtered light beam had 0.5-nm instantaneous spectral linewidths at 1-mW average output power. The implemented FD-OCT system clearly imaged a narrow air gap between glass plates with an axial resolution of 5 mum, which proved a potential for the application of a short pulse laser source to the FD-OCT system. The proposed wavelength swept source scheme can secure mode-hopping-free operation at fast tuning speed, which could be applicable for the FD-OCT system properly.

We report an all-fiber bandpass filter (BPF) based on a null core hollow optical fiber serially concatenated between a pair of helicoidal long-period fiber gratings (HLPFGs) with flexible transmission control capability. With novel spectral functionality of HLPFGs that can be varied by the torsional stress to the helix, and core mode blocking in a hollow core fiber, the proposed device showed unique BPF characteristics whose transmission can be flexible and altered along with a low polarization-dependent loss and low thermal variation, which have not observed in prior photorefractive grating devices.

We have demonstrated the waveguide-type tunable wavelength filtering (Sole filter) characteristics in an 80-mm-long periodically poled Ti: LiNbO3 (Ti: PPLN) channel waveguide which has a domain period of 16.6 mum. At room temperature (25degC), the center wavelength and the full-width at half-maximum of the filter were about 1273.3 and 0.21 nm, respectively. By changing the temperature of the Ti: PPLN waveguide, we have tuned the center wavelength of the filter with a wavelength tuning rate of -0.683 nm/degC. Also, the characteristics of the waveguide-type Sole filter based on a Ti: PPLN waveguide were compared with those of a bulk PPLN device.

We report a tunable all-fiber bandpass filter based on a short hollow optical fiber serially concatenated between a pair of long-period fiber gratings. With novel core mode blocking in the hollow core fiber and its optimal design, the device showed a low insertion loss of 1.5 dB and broad-band passband tuning range of 84.3 nm covering both S- and C-band.

We present a single-channel detection-based polarization-sensitive full-field optical coherence tomography (PS-FF-OCT) for simultaneous acquisition of high-resolution OCT and linear retardance images. A linearly polarized sub-10-fs laser was used as a broadband light source for the OCT system. A bi-stable polarization switching device, composed of a ferroelectric liquid crystal cell and an analyzer, was employed to get the horizontal and the vertical polarization components of a full-field interference signal. The time-switched two perpendicular interference signals were sequentially recorded by a single charge-coupled device camera, then processed to extract en-face functional images of a biological sample. The rat tail tendon was imaged ex vivo to confirm the imaging feasibility of the proposed system, which showed axial and transverse resolutions of 2 and 1.3 μm, respectively.

We present polarization-sensitive full-field optical coherence tomography (PS-FF-OCT), which is based on a bi-stable polarization switch (BSPS) device. The proposed PS-FF-OCT is a Linnik type interferometer, and allows getting both the birefringence-induced phase retardation and the intensity images of specimens with high resolutions using a pair of micro objectives and a BSPS device. Two orthogonal polarization states are formed with a regular time interval by the BSPS device that changes the polarization direction of light in a short time by switching its optic axes. Therefore, both the horizontally polarized light signal and the vertically polarized light signal from the sample can be detected with a single CCD camera. For getting a phase retardation image in real-time, the BSPS device is phase-locked with the CCD camera. The proposed method makes easy implementation of the PS- FF-OCT system without the needs of complex alignment process of using two identical CCD cameras. The experimental results confirm the feasibility of the system.

This paper presents an algorithm for reducing speckle noise from optical coherence tomography (OCT) images using an artificial neural network (ANN) algorithm. The noise is modeled using Rayleigh distribution with a noise parameter, sigma, estimated by the ANN. The input to the ANN is a set of intensity and wavelet features computed from the image to be processed, and the output is an estimated sigma value. This is then used along with a numerical method to solve the inverse Rayleigh function to reduce the noise in the image. The algorithm is tested successfully on OCT images of Drosophila larvae. It is demonstrated that the signal-to-noise ratio and the contrast-to-noise ratio of the processed images are increased by the application of the ANN algorithm in comparison with the respective values of the original images.

We propose a label-free depth-resolved tomographic scheme for imaging a single live cell in fluid. This approach utilizes a modified time-domain full-field optical coherence tomography (FF-OCT) system combined with an optical tweezer technique. The optical trap for holding a moving specimen is made by tightly focusing a 1064 nm Q-switching pulsed laser beam with a 1.0 NA microscope objective in the sample arm of the FF-OCT part. By cosharing the probe for both systems, the optical actions of trapping and cellular resolution tomographic imaging could be achieved simultaneously. Feasibility of the combined system is demonstrated by imaging micron-sized polystyrene beads and a living suspension cell in medium.

We present an optical tweezer technique assisted full-field optical coherence tomography (FF-OCT) system. The proposed scheme enables ultrahigh-resolution OCT imaging of a floating object optically trapped by single-beam gradient force in medium. The set up consists of a Linnik type of white light interference microscope combined with an optical tweezer system. The optical trap is formed by tightly focusing a 1064 nm Q-switching pulsed laser beam with a microscope objective lens of high numerical aperture (1.0 NA) in sample arm of the OCT interferometer. This co-sharing of probe channel between two of systems enables concurrent actions of trapping and OCT imaging for the sample. OCT imaging of the sample in depth can achieve by positioning the coherence gating with displacement of reference arm in the OCT interferometer. To demonstrate the efficacy of the system, micron-sized dielectric particles and living cells in solution are simultaneously trapped and optically sliced with cellular resolution.

We report an in-line all-fiber bandpass filter based on a hollow optical fiber (HOF) core mode blocker along with a long-period fiber grating (LPG) pair. It is theoretically and experimentally confirmed that selective core mode blocking at the HOF region located in the middle of the LPG pair could provide efficient passband channels corresponding to the resonant wavelengths of the LPGs. The filter characteristics in passbands and the nearfield patterns of guided modes in the proposed device are discussed.

We demonstrate a real-time display of processed OCT images using multi-thread parallel computing with a quad-core CPU of a personal computer. The data of each A-line are treated as one vector to maximize the data translation rate between the cores of the CPU and RAM stored image data. A display rate of 29.9 frames/sec for processed OCT data (4096 FFT-size x 500 A-scans) is achieved in our system using a wavelength swept source with 52-kHz swept frequency. The data processing times of the OCT image and a Doppler OCT image with a 4-time average are 23.8 msec and 91.4 msec.

We propose a new method for k -domain linearization using fiber Bragg gratings (FBGs) in a wavelength-swept source for optical coherence tomography (OCT). A wavelength-swept source with a scanning fiber Fabry-Perot tunable filter is constructed using a conventional ring laser cavity. Five FBGs are used to recalibrate the nonlinear response from the wavelength-swept source. We achieved good quality sample imaging using the k -domain linearization algorithm based on FBGs. The sensitivity at 2 mm is improved by more than 10 dB after k -domain linearization.

We present a calibration protocol to get the alignment factors of a custom-made spectrometer and the nonlinear fitting function between the measured CCD pixel domain and the wavelength domain to apply the Fourier-domain optical coherence tomography (FD-OCT) using optical fiber gratings. We have used 5 different center wavelength gratings covered the broadband source spectral range with a narrow spectral bandwidth ( 92 %) to calibrate and align the custom-made spectrometer. The implemented SD-OCT system following the proposed protocol showed the alignment factors as 44.37o incident angle, 53.11o diffraction angle, and 70.0 mm focal length. The spectral resolution of 0.187 nm was recalculated from the alignment factors.

We demonstrate new wavelength tunable OTDR based on wavelength swept fiber-laser by deploying digital micro-mirror array device with 20m resolution and fast wavelength setting speed in conventional band.

We demonstrate a novel implementation of spectral domain OCT by using a proposed sweeping detector at 1320 nm wavelength range. A fiber pigtailed Fabry-Perot tunable filter is newly adapted to receive spectral interferometer information using a photo-receiver instead of using charged couple detector arrays. In order to show a possibility of the scheme in other view point, we have changed the position of the Fabry-Perot tunable filter of the interferometer. The combination of a super luminescent LED and a semiconductor optical amplifier was used as an optical source. Its output power is about 10 mW and the spectral bandwidth is about 60 nm. The filtered light after passing thorough the Fabry- Perot tunable filter has 0.15 nm instantaneous spectral linewidth with 1.3 mW average output power. The system with an axial resolution of 12 μm performed OCT imaging of a cornea of a rat eye proving potential about the application of the proposed sweeping detector OCT.

There have been several technologies to enable high resolution cross-sectional images of biological tissues in optical coherence tomography (OCT) method. Optical frequency comb (OFC) source has been proposed to overcome the crosstalk problem among the CCD detector pixels of the continuous spectrum of light source. Recently, a passive-type OFC is demonstrated simply placing a Fabry-Perot interferometer filter right after the broadband light source, but it shows a high loss of output light power and limited tenability of channel spacing of multi-wavelength. In this work, we experimentally demonstrate a spectral comparison of a novel multi-wavelength source based on a fiber Sagnac interferometer. The channel spacing is flexibly tuned by the effective length control of polarization-maintaining fiber (PMF). The uniform and stable multi-wavelength spectral distribution is also helpful to obtain the higher sensitivity from the lower exposure intensity source to get a better quality spectral OCT image.

We report a tunable multiwavelength erbium-doped fiber laser incorporating dispersion-shifted fiber and digital micromirror device. The flexible allocation of wavelengths is achieved by controlling micromirrors. Up to 13 channels with 0.8 nm spacing are generated.

We have analyzed the Solc filtering characteristics in a periodically poled Ti:LiNbO3 (Ti:PPLN) multimode waveguide. The single- and dual-wavelength filtering were achieved under the optimized guiding condition for the TEM(00)-like mode and two mode (TEM(00)- and TEM(01)-like mode), respectively. The full width at half-maximum of the filter was about 0.21 nm at both guiding conditions. We found that the origin of two peaks of the dual-wavelength Solc filter in the two-mode guiding condition is the different effective refractive index between the TEM(00)- and TEM(01)-like modes. The wavelength difference of two peaks is about 0.8 nm at room temperature.

We demonstrate a novel implementation of spectral domain OCT using sweeping detector. A fiber pigtailed Fabry-Perot tunable filter is newly adapted to receive spectral interferometer information using a photo-receiver instead of using charged couple detector arrays.

We have analyzed the wavelength filtering characteristics in a periodically poled Ti:LiNbO3 (TkPPLN) waveguide depending on wave-guiding mode. In single-mode guiding condition, we have observed a single wavelength filtering which has about a 0.21 nm bandwidth. We also have observed a dual wavelength filtering in the case of multi-mode guiding condition.

We demonstrated a multiwavelength switchable erbium-doped fiber ring laser using digital micromirror device. By controlling the tilt states of micromirrors, we achieved some flexible allocations of wavelength channels and multiwavelength switching operations. We generated up to 8 wavelength channels with 0.8 nm spacing at room temperature.

A further insight into the prior concept of polarization sensitive optical coherence tomography system intended for non-laboratory conditions is brought forward and an experimental proof-of-concept is presented. A phenomenological model is adopted from the theory of light depolarization in crystalline polymers and modified to yield a simplified algorithm for mapping depolarization ratio in dermis. The algorithm could distinguish between dermal layers with depleted collagen content and normal dermis of normal perilesional skin. Dermis is simulated by bireringent lamellae of collagen arranged chaotically in multiple layers parallel to the skin surface. Both the design concept and the model imply the sub-millimeter tumor thickness as a proofed prognostic factor and an important criterion for complementary functional diagnostics of skin cancers at their early phase of vertical growth. Choice of the model is inspired by similarity of structural and optical properties between liquid-crystal collagen fibers in dermis and birefringent crystalline lamellae in polymer materials. The numerical computation based on the model allowing for real characteristics of dermis gives plausible interpreting of depolarization peculiarities caused by collagen depletion. Feasibility is discussed of exploiting fiber optic analogs of achromatic retarders. Fabrication of the fiber retarders is shown to be realistic by making use of the photonics technology possessed by the authors.

A challenge is accepted to identify depth resolved optical birefringence of turbid media, particularly human skin using polarization sensitive optical coherence tomography under external environmental perturbations, e.g. under application conditions in clinics and hospitals. A novel concept is advanced and respective design is elaborated for a system which comprises a source of low-coherence light, a tandem of a readout interferometer and a sensing interferometer with reference and sample arms, and a handheld probe integrated into the sensing interferometer through polarization-maintaining fibers. The system provides fast electro-optic switching over orthogonal polarization components of a measurand light beam at the same output from the sensing interferometer. At said output either of the aforementioned components remains perpendicular to the polarization plane of light in the reference arm. For either of the components the readout interferometer compensates optical path length differences in the sensing interferometer within a given interval of depth in the sample, and evolves spatial fringes which are read out by optoelectronic means without any mechanical scanning in depth. The fringes are devoid of cross-correlation artifacts inasmuch as the readout interferometer recombines coherently the measurand beam and the reference beam having mutually orthogonal polarizations only.

We report a new method of fabrication of helicoidal long-period fiber gratings by twisting a single mode fiber with CO2 laser beam. The transmission characteristics of helicoidal long-period fiber gratings were experimentally demonstrated.

A novel wavelength swept broadband source based on an ultrashort pulse laser and an external tunable filter was proposed for application of frequency domain-optical coherence tomography (FD-OCT). The laser beam coupled into the single mode fiber, which provided 0.5-nm instantaneous spectral linewidth with 1-mW average output power, was tuned from 740 nm to 850 at a 1 kHz repetition rate. The system with an axial resolution of 5 mm performed OCT imaging of air-gap between glass plates proving potential about the application of pulse laser source to FD-OCT system. The proposed swept source scheme could be applied for the implementation of ultra-high resolution FD-OCT system based on a supercontinuum source with an ultra-short pulse laser and a high nonlinear optical fiber.

This work reports that the ablation volume and rate of porcine skin changed significantly with the change of skin water content. Under the same laser irradiation conditions (532 nm Nd:YAG laser, pulse width = 11.5 ns, pulse energ y = 1.54 J, beam radius = 0.54 mm ), the ablation volume dropped by a factor of 4 as the skin water content decreased from 40 wt. % (native) to 19 wt. % with a change in the ablation rate below and above around 25 wt. %. Based on the ablation characteristics observed by in situ shadowgraph images and the calculated tissue temperatures, it is considered that an explosive rupture by rapid volumetric vaporization of water is responsible for the ablation of the high water content of skin, whereas thermal disintegration of directly irradiated surface layer is responsible for the low water content of skin.

A long period fiber grating imprinted in a pure-silica PCF (photonic crystal fiber) by using CO/sub 2/ laser beams is presented. The sensitivities of the resonant wavelength under bending, strain, and temperature were measured to be +16.4 nm-m, -0.95 pm//spl mu//spl epsiv/, and +9 pm//spl deg/C, respectively.

A 2/spl times/10 Gb/s OCDMA system using cascaded long-period fiber gratings, formed in dispersion compensating fiber with an inner-cladding structure, is presented. Two encoder/decoder pairs are fabricated and the coding performances between them are verified with BER measurements.

In this study, polarization-sensitive optical coherence tomography (PS-OCT) capable of providing polarization contrasts such as phase retardation and degree of polarization uniformity (DOPU) was used for visualizing human meibomian glands (MGs) and investigating morphological characteristics of them. Especially, with the help of the DOPU contrast, MGs were exclusively extracted from the volumetric OCT image. In vivo PS-OCT measurements were performed on the upper eyelids of different age groups. From these measurements, different age-dependent aspects of the MG structure were also observed. Based on these observations, it can be inferred that the PS-OCT system has the potential for clinical diagnosis and investigation of MG-related dry eye diseases like MG dysfunction (MGD) and acinar atrophy.

Recently, we reported obtaining tomograms of meibomian glands from healthy volunteers using commercial anterior segment optical coherence tomography (AS-OCT), which is widely employed in clinics for examination of the anterior segment. However, we could not create 3D images of the meibomian glands, because the commercial OCT does not have a 3D reconstruction function. In this study we report the creation of 3D images of the meibomian glands by reconstructing the tomograms of these glands using high speed Fourier-Domain OCT (FD-OCT) developed in our laboratory. This research was jointly undertaken at the Department of Ophthalmology, Seoul St. Mary's Hospital (Seoul, Korea) and the Advanced Photonics Research Institute of Gwangju Institute of Science and Technology (Gwangju, Korea) with two healthy volunteers and seven patients with meibomian gland dysfunction. A real time imaging FD-OCT system based on a high-speed wavelength swept laser was developed that had a spectral bandwidth of 100 nm at the 1310 nm center wavelength. The axial resolution was 5 mu m and the lateral resolution was 13 mu m in air. Using this device, the meibomian glands of nine subjects were examined. A series of tomograms from the upper eyelid measuring 5 mm (from left to right, B-scan) x 2 mm (from upper part to lower part, C-scan) were collected. Three-D images of the meibomian glands were then reconstructed using 3D "data visualization, analysis, and modeling software''. Established infrared meibography was also performed for comparison. The 3D images of healthy subjects clearly showed the meibomian glands, which looked similar to bunches of grapes. These results were consistent with previous infrared meibography results. The meibomian glands were parallel to each other, and the saccular acini were clearly visible. Here we report the successful production of 3D images of human meibomian glands by reconstructing tomograms of these glands with high speed FD-OCT.

Although high optical illumination power is favored in optical coherence tomography (OCT) for better signal-to-noise ratio, optical power is often limited by a damaged threshold for biomedical living tissues and autocorrelation signals observed in tomograms. In order to improve signal sensitivity without increasing the optical illumination power, a spectrally sampled multi-wavelength light source is proposed for the OCT system. A fiber Sagnac comb filter was used to spectrally sample the output of a continuous spectral light source. Point spread function analysis shows that the spectrally sampled OCT has an almost 50% dynamic range improvement in comparison with a conventional continuous spectral light source OCT for the same average optical power of 6 mW.

All-optical wavelength tuning in a waveguide-type Solc filter based on a Ti:PPLN waveguide has been demonstrated for the first time by ultraviolet illumination. The measured wavelength tuning rate as a function of the UV intensity was about -26.42 nm/W cm -2 .

Experimental proof-of-concept is presented for a quasi-holographic solution to polarization-sensitive optical coherence tomography (PS OCT). Due to decoupling between the reference and sample beams by polarization, the solution seems acceptable to acquisition and communication of optical data in the nonlaboratory environment. The nonlab environment implies uncontrollable disturbances, e.g., temperature changes and mechanical effects happening under shop testing in industry or routine examinations in common clinics and hospitals. For mapping the collagen-related depolarization ratio of light backscattered from the human dermis, a phenomenological model is evolved from the theory of light depolarization in crystalline polymers. The model yielded a simplified intensity-based estimation algorithm. The design concept and the model rely on a submillimeter tumor thickness as a proofed prognostic factor and an important criterion for complementary functional diagnostics of skin cancers in their early phase. Choice of the model is inspired by similarity of structural and optical properties between liquid-crystal collagen fibers in the dermis and birefringent crystalline lamellae in some polymer materials. The model gives a plausible interpretation of a peculiarity of cumulative birefringence in the abnormal skin dermis. Following a top-down approach to design, the authors attempt to contribute to bridging the gap between practitioners' concerns and academic studies.

All-optical AND and NAND gates have been demonstrated in a Ti-diffused periodically poled LiNbO(3) channel waveguide which has two second-harmonic phase-matching peaks by cascaded sum-frequency-generation/difference-frequency-generation (cSFG/DFG) and sum-frequency-generation (SFG) processes. The conversion efficiency of signal to idler (AND gate signal) was approximately 0 dB in cSFG/DFG process. In the second SFG process, more than 15 dB extinction ratio between signal and dropped signal (NAND gate signal) has been observed.

We propose and demonstrate a tunable Q-switched erbium doped fiber laser with a digitally controlled micro-mirror array device. The tunable and pulsed output of the laser was achieved by the pixelated spatial modulation of the micro-mirror array. The wavelength tuning from 1530 nm to 1555 nm was shown with wavelength selectivity of ~0.1 nm and the pulsed operation was accomplished with 130 Hz repetition rate.

A two-user, 10-Gbits/s optical code-division multiple-access system implemented by using cascaded long-period fiber gratings formed in a dispersion-compensating fiber (DCF) is demonstrated. Our results show that the sensitivity of cladding modes to the refractive index change on the cladding surface is greatly reduced by utilizing the inner-cladding mode of the DCF. Two pairs of encoder/decoder are constructed and the performance is evaluated by measuring bit error rate (BER). With an interferer, a BER of 1.5×10–12 is measured at a received optical power of –6 dBm.

We present an optical pulse multiplication and temporal coding method for OCDMA systems. The true time delay among the pulses was obtained by utilizing the difference in the propagation speeds of the core mode and a co-propagating cladding mode coupled by a long-period fiber grating. By cascading gratings we could get an equally spaced 40-GHz pulse train from a 10-GHz input train. The dispersion compensating fiber having an inner cladding structure enabled us to have the gratings that were not sensitive to the polymer jacket on the cladding surface, and also allowed shortening the device length. Various coding and decoding of a pulse train are possible by controlling the separations among the gratings.

We propose a new type of all-fiber bandwidth tunable rejection filter using cascaded helicoidal long-period fiber gratings in single-mode optical fiber and report controllable broadband rejection characteristic with low insertion loss and polarization dependent loss.

We propose an all-fiber wavelength tunable bandpass filter with multi-mode to single-mode converting property via serially concatenated structure of a helicoidal long-period fiber grating (HLPFG), a hollow optical fiber (HOF), and a multimode fiber (MMF).