Sunduck Kim

Investigation of an Arc-induced Long Period Fiber Grating Inscribed in a Photonic Crystal Fiber with Two Large Air Holes

A photonic crystal fiber with two large air holes outside the holey cladding region is fabricated to induce an effective long periodic grating (LPG) in the core by an electric arc discharge. We believe that the two large air holes lead to the asymmetric perturbation in the core under the electric arc discharge, thereby introducing the coupling to the first higher-order mode. The transmission characteristics of the PCF with the LPG for the external perturbation such as strain, curvature, and temperature are also investigated. It was found that the shift of resonance peak in the transmission spectrum depends on the bending direction. The curvature of 8.55 m-1 results in the center wavelength shifts of 1.8, 4.3, and 11 nm for a vertical, diagonal, and horizontal direction of the curvature to the large air-hole alignment, respectively.

Long distance fiber Bragg grating strain sensor interrogation using a high speed Raman-based Fourier domain mode-locked fiber laser with recycled residual Raman pump.

We propose a novel fiber Bragg grating (FBG) sensor interrogation using a Raman-based Fourier-domain mode locking (FDML) fiber laser for a high speed and long distance measurement. A residual Raman pump after the generation of the Raman-based FDML fiber laser is recycled for secondary signal amplification in a 2-m erbium-doped fiber (EDF) to further enhance the output power. The chromatic dispersion is precisely controlled to suppress the phase noise in the FDML laser cavity, resulting in the improvement of an R-number of 1.43 mm/dB. After recycling residual pump, we achieve the 40-km round trip transmission of the sensing probe signal with a high scan rate of 30.8 kHz. With 205-mW residual pump power, the bandwidth and the maximum gain are measured to be more than 50 nm, 10.3 dB at 1550 nm, respectively. The sensitivity of the proposed Raman-based FDML fiber laser to strain is also measured, which are 0.81 pm/μstrain in the spectral domain and 0.19 ns/μstrain in the time domain, respectively.

Tunable dispersion slope compensator using two uniform fiber Bragg gratings mounted on S-shape plate

We propose and experimentally demonstrate a novel method for tunable dispersion slope compensation. We use two uniform fiber Bragg gratings (FBGs), a spatially designed S-bending stage and 4-port circulator. Two FBGs are mounted on each surface of a metal plate along the calculated quadratic curve. The dispersion slope (DS) can be tuned by adjusting a nonlinear strain along two uniform FBGs without changing second order dispersion as well as the central wavelength. In the experiment, a DS tuning range from -13.9 to -54.8 ps/nm(2) is achieved with the bandwidth of larger than 2.0 nm.

Temperature-Insensitive Fiber Bragg Grating-Based Bending Sensor Using Radio-Freqeuncy-Modulated Reflective Semiconductor Optical Amplifier

A novel method for the temperature-insensitive measurement of the bending of a cantilever plate is proposed. It is based on the radio-frequency (RF) power measurement of the beat signal that has been generated from a dual fiber Bragg grating (FBG)-based sensing head. Another noticeable feature is that a sine-modulated amplified spontaneous emission from a directly modulated reflective semiconductor optical amplifier (RSOA) is employed as a broadband light source.

Optical and magneto‐optical properties of NdTbFeCo thin films

Three series of NdxTbyFe1−x−y−zCoz films have been fabricated for their potential application in magneto‐optical recording: (i) x=0–0.20, Tb was adjusted so that the coercivity at room temperature was 5–10 kOe; (ii) x+y=0.28–0.32 with x in the range of 0 to 0.25; and (iii) x+y=0.21–0.23 with x in the range of 0 to 0.17. These films have been characterized both optically and magneto‐optically by Kerr hysteresis trace, variable angle of incidence ellipsometry, and normal angle of incidence Kerr spectroscopy. From these measurements, the optical constants (n and k), magneto‐optical constants (Q1 and Q2), and maximum possible figure of merit (FOM) have been determined at wavelengths of 405, 546, and 633 nm. Results demonstrate that the magneto‐optical behavior of NdTbFeCo films is sensitive to the concentration of both Nd and Tb in the film. For samples in the series (i), the addition of Nd into TbFeCo alloys was shown not to enhance their magneto‐optic behavior at the short wavelengths. For samples in the se...

Fiber-bundle illumination: realizing high-degree time-multiplexed multifocal multiphoton microscopy with simplicity

High-degree time-multiplexed multifocal multiphoton microscopy was expected to provide a facile path to scanningless optical-sectioning and the fast imaging of dynamic three-dimensional biological systems. However, physical constraints on typical time multiplexing devices, arising from diffraction in the free-space propagation of light waves, lead to significant manufacturing difficulties and have prevented the experimental realization of high-degree time multiplexing. To resolve this issue, we have developed a novel method using optical fiber bundles of various lengths to confine the diffraction of propagating light waves and to create a time multiplexing effect. Through this method, we experimentally demonstrate the highest degree of time multiplexing ever achieved in multifocal multiphoton microscopy (~50 times larger than conventional approaches), and hence the potential of using simply-manufactured devices for scanningless optical sectioning of biological systems.

Real-time multi-monitoring interrogation based on Fourier domain mode-locked fiber laser for measurement of radiation dose and multipoint strain

We propose a real-time multi-monitoring interrogation technique based on Fourier domain mode-locked fiber laser for simultaneous measurement of radiation dose and multi-point strain. Radiation dose and multipoint strain can be monitored in real-time by measuring the variation of output power and detection time interval of the sensing signals. Since the operating wavelength of the FDML is continuously controlled as a function of time, it is possible to simultaneously measure the variation of radiation dose and multipoint strain in real time.

Absolute strain measurement based on a microfiber Mach-Zehnder interferometer

A microfiber-based Mach-Zehnder interferometer (MZI) for measurement of absolute strain is proposed and demonstrated experimentally. The diameter of the microfiber is optimized to induce the negative thermo-optic effect in the microfiber MZI and the temperature sensitivity of the microfiber MZI is successfully suppressed. When the diameter of the microfiber is ~5 μm, the temperature sensitivity of the microfiber-MZI is dramatically mitigated. We apply the proposed microfiber MZI for absolute strain measurement and its strain sensitivity is measured to be ~7.13×10-2 nm/ μЄ.

Interrogation of fiber Bragg grating sensor for long distance measurement using Raman-based Fourier domain mode-locked fiber laser with recycled residual Raman pump

We propose a fiber Bragg grating sensor interrogation system using a Raman-based Fourier domain mode locking fiber laser. To improve transmission efficiency of the signal over 20 km, the residual pump powers are recycled.

Long distance fiber Bragg grating strain sensor interrogation using high speed Raman-based Fourier domain mode-locked fiber laser with recycled residual Raman pump

We propose a novel fiber Bragg grating (FBG) sensor interrogation system using a Raman-based Fourier domain mode locking (FDML) fiber laser for a high speed and a long distance measurement. To improve transmission efficiency of the sensing probe signal over a long distance of 20-km, the residual pump powers are recycled. The external strain change can be measured by detecting the tine interval between two reflected signals from two FBGs as the Rama-based FDML is swept. The measured strain sensitivities with respect to the time are 0.19 ns/μstrain.