Eun Seo Choi

Miniature fiber-optic high temperature sensor based on a hybrid structured Fabry–Perot interferometer

A miniature Fabry-Perot (FP) interferometric fiber-optic sensor suitable for high-temperature sensing is proposed and demonstrated. The sensor head consists of two FP cavities formed by fusion splicing a short hollow-core fiber and a piece of single-mode fiber at a photonic crystal fiber in series. The reflection spectra of an implemented sensor are measured at several temperatures and analyzed in the spatial frequency domain. The experiment shows that the thermal-optic effect of the cavity material is much more appreciable than its thermal expansion. The temperature measurements up to 1000 degrees C with a step of 50 degrees C confirm that it could be applicable as a high-temperature sensor.

Combined system of optical coherence tomography and fluorescence spectroscopy based on double-cladding fiber

We report the development of an all-fiber multimodal system, based on a double-cladding fiber (DCF) and related devices, suitable for simultaneous measurements of optical coherence tomography (OCT) and fluorescence spectroscopy (FS). The DCF together with a DCF coupler and a single-body DCF lens has assisted in the realization of a multimodal but single-unit probe for the combined system. The DCF lens allowed simultaneous focusing of input beams for OCT and FS and also the effective collection of both signal beams from a sample. The DCF coupler could extract the OCT signal via the core channel and the FS signal through the cladding channel. The OCT image and the fluorescence spectra of a plant tissue were then simultaneously measured to validate the performance of the proposed multimodal system.

Self-referenced spectral interferometry for simultaneous measurements of thickness and refractive index

We present a method for simultaneously measuring the thickness and the group refractive index of a specimen using self-referenced spectral-domain fiber-based interferometry. By removing the scanning part and using the fiber-based configuration, the system complexity and stability could be significantly improved. To minimize the system drift, we utilized the signals originated from the fiber ends of both arms. Implementing in a self-referenced configuration, we could improve the measurement accuracy down to a decimal place. Experimental measurements were made with a 1.555 mm thick fused silica plate. At 814 nm the thickness was measured as 1.5546 +/- 0.0002 mm, and at the same time, the group index was obtained as 1.4627 +/- 0.0002.

Image restoration method based on Hilbert transform for full-field optical coherence tomography

A full-field optical coherence tomography (FF-OCT) system utilizing a simple but novel image restoration method suitable for a high-speed system is demonstrated. An en-face image is retrieved from only two phase-shifted interference fringe images through using the mathematical Hilbert transform. With a thermal light source, a high-resolution FF-OCT system having axial and transverse resolutions of 1 and 2.2 microm, respectively, was implemented. The feasibility of the proposed scheme is confirmed by presenting the obtained en-face images of biological samples such as a piece of garlic and a gold beetle. The proposed method is robust to the error in the amount of the phase shift and does not leave residual fringes. The use of just two interference images and the strong immunity to phase errors provide great advantages in the imaging speed and the system design flexibility of a high-speed high-resolution FF-OCT system.

VCSEL-based swept source for low-cost optical coherence tomography

We present a novel wavelength-swept laser source for optical coherence tomography (OCT) which is based on the conventional laser diode technology of the vertical-cavity surface-emitting laser (VCSEL). In our self-heating sweep VCSEL (SS-VCSEL), a VCSEL device is simply driven by ramped pulses of currents in direct intensity modulation. The intrinsic property of VCSEL produces a frequency-swept output through the self-heating effect. By the injected current, the temperature of the active region is gradually increased in this effect. Consequently, it changes the wavelength of the laser output by itself. In this study, various characteristics of our SS-VCSEL were experimentally investigated for low-cost instrumentation of a swept source OCT system. A low-cost SS-VCSEL-based OCT system was demonstrated in this research that provided an axial resolution of 135 μm in air, sensitivity of -91 dB and a maximum imaging range longer than 10 cm when our source was operated at a sweep repetition rate of 5 kHz with an output power of 0.41 mW. Based on the experimental observations, we believe that our SS-VCSEL swept source can be an economic alternative in some of low-cost or long-range applications of OCT.

Ultrawideband photonic crystal fiber coupler for multiband optical imaging system

We report a photonic crystal fiber (PCF) coupler having an ultrawide spectral bandwidth keeping single mode operation. The use of the PCF coupler in a fiber-based optical coherence tomography (OCT) system enables us to handle the wide spectral bands of various light sources, including superluminescent diodes (SLDs) at 1300 nm and 820 nm, Ti:sapphire lasers, and white-light sources. The multiband imaging performances of the PCF-based OCT system are demonstrated by obtaining dental images at 1300 nm and 820 nm with the same setup. In addition, we show that the PCF coupler could cover the spectrum over a one octave span and guide both the fundamental wave (1030 nm) and the second harmonic wave (515 nm) simultaneously.

Thickness and Refractive Index Measurements by Full-Field Optical Coherence Microscopy

We present the noble sensing method that can simultaneously measure the physical thickness and the refractive index of a transparent specimen based on full-field optical coherence microscopy. As a sample, a small drop of epoxy was placed on a flat glass plate and high-resolution depth resolved en-face images were taken. With adopting the reference plane from a cross-sectional image, the physical thickness, and the refractive index distribution could be obtained.

Narrowband wavelength selective detector applicable SD-OCT based on Fabry-Perot tunable filter and balanced photoreceiver

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.

Optical coherence tomography in material deformation by using short pulse laser irradiation

We demonstrate the feasibility of OCT imaging for the investigation of samples, which are processed by the short pulse laser. The use of short pulse lasers in various material processing have provided the advantages such as a high peak power and a small heat affected zone over conventional methods based on mechanical treatment. However, due to the improper application of the lasers, the unwanted surface or structural deformation of materials and the thermal damages around an irradiation spot can be caused. Thus, the real-time monitoring/evaluation of laser processing performance in-situ is needed to prevent the excessive deformation of the material and to determine optimal processing conditions. As a standard method to investigation of the material processing by using the lasers, the scanning electron microscopy (SEM) or the transmission electron microscopy (TEM) observation of a physically cleaved surface is used although sample damages are given during the cleaving and polishing process. In this paper, we utilized the OCT advantages such as high resolution and non-invasive investigation to evaluate the laser processing performance. OCT images for the deformation monitoring of the ABS plastic present correlation with images obtained from conventional investigation methods. OCT images of the maxillary bone clearly show the difference in the pit formation of the biological sample at different irradiation conditions. We prove the potential of OCT for the evaluation of laser-processed various samples. Integrating OCT system into a laser processing system, we can visualize the effect of laser-based treatments in clinical and industrial fields.

Wavelength tunable broadband source based on Ti:Sapphire femtosecond laser applicable to FD-OCT system

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 &mgr;m 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.