EunSeo Choi

All-fiber variable optical delay line for applications in optical coherence tomography: feasibility study for a novel delay line

We have implemented an all-fiber optical delay line using two linearly chirped fiber Bragg gratings cascaded in reverse order and all-fiber optics components. The features of the proposed all-fiber based technique for variable delay line are discussed theoretically and demonstrated experimentally. The non-invasive cross-sectional images of biomedical samples as well as a transparent glass plate obtained with implemented all-fiber delay line having the axial resolution of 100 mum and the dynamic range of 50dB are presented to validates the imaging performance and demonstrate the feasibility of the delay line for optical coherence tomography.

Combined applications of iterative deconvolution methods and adaptive speckle filters for optical coherence tomography

Coherence of light commonly used in optical coherence tomography can induce axial resolution loss and speckle generation. Combined use of iterative deconvolution methods and adaptive speckle filters can significantly reduce it and give clearer images.

Implementation of an All-Fiber Variable Optical Delay Line with a Pair of Linearly Chirped Fiber Bragg Gratings

We implemented all-fiber delay line using linearly chirped fiber Bragg gratings (CFBG), which can be applicable for reflectometry or optical coherence tomography (OCT). Compared with the previously reported delay lines, the proposed fiber-based optical delay line has in principle novel advantages such as automatic dispersion cancellations without additional treatment and a gain in optical delay that is dependent on parameters of used CFBGs. Dispersion compensation in optical delay line (ODL), which is the indispensable problem in bulk optics based ODL, is demonstrated in fiber by using two identical but reversely ordered CFBGs. Amplified variable optical delay of around 2.5 mm can be obtained by applying small physical stretching of one of CFBGs in the proposed scheme. The operational principles of the all-fiber variable optical delay line, which are based on the distributed reflection characteristic of a CFBG employed, are described. Especially properties such as in-line automatic dispersion cancellation and amplified optical delay under strain are dealt. To demonstrate the properties of the proposed scheme, which is theoretical consequences under assumptions, an all-fiber optical delay line have been implemented using fiber optic components such as fiber couplers and fiber circulators. With the implanted ODL, the group delay and amplified optical delay length was measured with/without strain. The wavelength independent group delay measured within reflection bandwidth of the CFBG has proved the property of automatic dispersion cancellations in the proposed fiber delay line. Optical delay length of 2.5 mm was obtained when we apply small physical stretching to the CFBG by 100μm and this is expressed by the amplification factor of 25. Amplification factor 25, which is less than theoretical value of 34 due to slipping of fiber in the fiber holder, shows that the proposed scheme can provide large optical delay with applying small physical stretching to the CPBG. We measure slide glass thickness to check the performance of the fiber delay line and the good agreement in measured and physical thickness of slide glass (∼1 mm thick) validates the potential of proposed delay line in the applications of optical reflectometry and OCT. We also discuss the problem and the solution to improve the performance.

Fiber-based high-resolution OCT system with halogen light source

Fiber-based high resolution OCT system was achieved using white-light source with a halogen lamp, which has advantages of wide spectrum, compact size and low cost. The axial resolution measured without using objective lens in the sample arm was about 2.5 mm. The thickness of a thin film (about 7 mm thick) was measured to evaluate the high resolution performance. The measured interferogram showed two well-distinguished peaks corresponding to two interfaces of the thin film. The implemented OCT system was composed of fiber-optic Michelson interferometer instead of that of conventional bulk optics. To adapt a white-light source to the fiber based OCT system and providing high resolution, a wideband single mode fiber with a large mode field diameter for high coupling efficiency, a wideband fiber coupler with flat coupling response, a cascaded detector scheme for broadband detection and dispersion control are required. Dispersion mismatch due to introducing an objective lens in the sample arm can be controlled effectively by employing a proper optical component in the reference arm. After dispersion control, resolution of about 3.5 mm was enhanced to about 2.5 mm, which is similar to the objective lens-free resolution, and wide sidelobes was also well suppressed.

OCT resolution enhancement using long-period fiber grating and erbium-doped fiber

We proposed a technique to improve OCT resolution using LPG and EDF. The proposed technique improves the resolution of OCT by resphaping the spectrum of ASE source into Gaussian-like form. The ASE source has a strong characteristic peak due to erbium that is used for doping material in EDF. To reduce the peak-induced artifact to OCT image that may degrade image quality and distort inner structure information, LPG is used as a strong ASE peak rejection filter and EDF does a function of absorber when pumping power is not applied. Both of them can do spectral tailoring function properly so that modified source shape and sidelobe suppression can be performed. With the LPG-assisted reshaping, we have enhanced the spatial resolution up to 5 times (approximately 200 μm resolution was reduced to about 40 μm). With EDF absorber, we can obtain reshaped ASE source hence resolution was enhanced from 25 μm to less than 20 μm). The spatial resolution can be further enhanced by using cascaded LPGs or control of EDF parameters such as doping material, doping concentration, and the length of used EDF.

Temperature-dependent chromatic dispersion measurement using long-period fiber grating

A novel method for measuring the temperature-dependent chromatic dispersion of a fiber is proposed, which is based on the interference fringe formed by a pair of long-period fiber gratings (LPGs). The effective index difference between the fundamental core mode and a cladding mode was obtained from the interference fringe of the LPG pair. The order of the involved cladding mode and the core size could be obtained from the general parameters of the fiber; the numerical aperture and the cut-off wavelength. Using the obtained cladding mode order and the Sellmeier equation of silica material for the cladding index, the refractive index of the fiber core was obtained by comparing the measured effective index difference with the calculated one. Since the temperature-dependent Sellmeier equation of the cladding material is already known, the temperature-dependent refractive index of the core can be obtained. Measured refractive index of the core is fitted with a linear combination of the well-known Sellmeier equations of germanium and silica. The concentration of germanium is used as the fitting parameter. The calculated chromatic dispersion is well matched with the one measured with a conventional instrument within 0.2 ps/km/nm in a spectral range of from 1.3 mm to 1.6 mm at room temperature.

Axial resolution enhancement for an OCT system implemented by mechanically induced long-period fiber grating

We demonstrate a spectral filtering method that can enhance the axial resolution of an OCT system. A mechanically induced long-period grating was used to tailor the spectral shape of an in-house made Er-doped fiber amplified spontaneous emission source. The proposed method improved the visibility of the OCT image dramatically.

Implementation of all-fiber optical delay line and applications for optical coherence tomography

All-fiber optical delay line suitable for the applications of optical coherence tomography has been implemented. The distributed reflection property of an optical fiber grating was utilized in achieving true time delay.

Spatial Resolution Enhancement with Fiber - based Spectral Filtering for Optical Coherence Tomography

We report a technique that improves the spatial resolution of optical coherence tomography (OCT) by utilizing fiber-based spectral filtering. The proposed technique improves the resolution by filtering out the erbium’s characteristic peak from the amplified spontaneous emission (ASE) source spectrum, and reshaping the spectrum to Gaussian-like. We used a long period fiber grating (LPG) and an erbium doped fiber (EDF) absorber for the spectral filtering. An in-house made ASE source as well as a commercial ASE source [ASE-FL7002] was used as the OCT sources to study the proposed technique. The resolution of the OCT based on an in-house made ASE source is enhanced from 200 to 40 ㎛ with an LPG. While, the resolution of the OCT based on a commercial ASE source is enhanced from 25 to 19 ㎛ with the aid of an EDF absorber. However, sidelobes still exist in the interferogram due to imperfect spectral filtering, which limited the resolution. Further enhancement in the spatial resolution of the OCT system using the ASE source is possible with the aid of cascaded LPGs and/or carefully designed EDF absorber.