We present a method for an intraoperative microscope-mounted Fourier-domain optical coherence tomography (FD-OCT) system to maintain high image contrast while dynamic adjusting focal planes. Because two imaging system with different imaging depth are integrated into one system, active control of OCT imaging conditions is indispensible for functioning high quality imaging modality. For the purpose of active adjustment of the focal plane, an electrically focus tunable lens (FTL) was used in the sample arm of the OCT system. Because the OCT image contrast at a depth is given by roll-off characteristics of the FD-OCT that is a function of difference in OPL between the sample and reference arm, we should compensate the difference in the OPL to enhance image contrast. We proposed the use of a piezoelectric actuator (PZT) attached to a reflection optic to actively control the OPL in the reference arm. With active controlling the FTL and PZT simultaneously, we can optimize and keep the OCT image contrast while maintaining image depth positions. From a surface position in the OCT image, the focal length variations with the FTL are calculated and the focal length of the FTL is tuned to match on the sample surface. Contrast optimization with the PZT is performed with compensating the optical path length difference from the additional focal length of the FTL. We integrated the OCT to a conventional surgical microscope and demonstrate feasible observation of OCT image with high contrast at constant imaging depth under the change of focal plane of the microscope.
We present a balanced detection method for spectral domain optical coherence tomography (SD-OCT) using a fiberoptic phase shifter. SD-OCT systems typically use a single line scan camera to detect the spectrogram, and noninterfered signals are not excluded from the recorded signal. That limits dynamic range and reduces image quality. Balanced detection methods are used to overcome these problems in swept-source OCT system. Detection using two line scan cameras or multi line camera was proposed to perform the balanced detection in SD-OCT systems. Time delayed replica generated by long optical fiber have been made for that purpose. To induce a phase shift in interference signal, we used a phase shifting interferometry based on an optically tunable phase shifter. The proposed phase shifter can control refractive index variations using the optical power of the pumping beam incident on the rare-earth doped optical fiber. Phase shifts on the optical power represent non-linear behavior and require optimized tuning for proper phase shift. We measured the spectrogram and analyzed the phase change characteristics. By subtracting the phase-shifted replica from the original interference signal, the amplitude of the interference signal is doubled and sensitivity is improved. We prove balanced detection performance based on a phase shifting interferometer.
We proposed a cell-counting method using optical fiber interferometer and demonstrated the performance of the proposed method. The cell counting means the counting or the quantification of individual cells. Its application ranges from the biological research to practical disease diagnosis. As a conventional approach for cell counting, various methods are employed. Among them, flow cytometry is quite accurate and exact method but it uses bulk and expensive optical equipment. When image-based methods are exploited, the limited field of view obtained by microscope is considered for cell counting. From this reason, problem of time consuming for whole cell counting is to be solved. The proposed method utilized single-mode optical fiber and high-speed spectrometer. Light beam having broad spectral bandwidth over 100 nm at 850-nm central wavelength is irradiated to a flow channel through fiber from top to bottom. Different optical path length differences are made whether the cell is passing though the flow channel across the beam area or not. The difference of optical path lengths in the beam area due to the cell induces interference signal depending on optical thickness of the cell. By measuring a series of interferences, the number of cells can be analyzed. The proposed system can be implemented without any expensive and perform the cell counting in the absence of complex image analysis. Interferometer-based cell counting can be a good alternative to the reported cell-counting methods.
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.
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.
We have presented full-field optical coherence tomography(FF-OCT) system implemented with fiber optics. Usually FFOCT
system illuminates large area at once while conventional OCT system irradiates light at single focal point. From
these reason, light guidance with single fiber waveguide is not proper in FF-OCT system and fiber-optic components is
not dealt in the system implementation. In this paper, we demonstrate FF-OCT system implemented with fiber-optics,
where fiber coupler and fiber-optic circulator were used to perform the function of beam splitting and optical delay line.
Each arm of fiber coupler acts as reference arm and sample arm. Fiber-optic collimator and metal-coated mirror mounted
on translator in the reference arm could adjust optical path length properly. Separated beam after the fiber coupler was
combined after bulk beam combiner, where beam size at fiber end is expanded by large fiber-optic collimator and then
illuminated to sample. The larger size beam reflected from sample was interfered with reference beam, which
experienced optical delay in the reference arm. The utilization of fiber-optic components could provide merits such as
easiness in optical alignment and reduction of sensitiveness to external vibration and perturbation.
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