D. Y. Kim

Residual stresses in a doubly clad fiber with depressed inner cladding (DIC)

Thermal and mechanical stresses developed in concentric three-layered optical fiber-core, and inner and outer cladding, have been thoroughly studied for various concentrations of dopants and geometric structures. In order to examine the parametric results of thermal stresses in preforms, the stresses were measured with a polariscope. The results agreed well with the theoretical calculations. The thermal stresses were calculated for three temperature ranges in which the glass in each layer has a different thermal expansion coefficient. The mechanical stresses were studied considering the normal stress in the molten neck down region and its development with time. In order to include the time dependence of the stress below softening point, Maxwells one dimensional viscoelasticity was applied. In a parametric study, the analyzes were carried out based on the fiber parameters such as relative index difference, ratio of clad to core, and depressed relative index difference. With an increase of core index above the silica, the thermal stresses in core increased linearly, but the depressed inner clad does not affect the stresses in core. From the parametric studies and modeling it was found that when the depressed inner cladding (DIC) layer has a large cross-section or high dopant concentration, the mechanical stress in core change from compression to tension.

High-speed confocal fluorescence lifetime imaging microscopy (FLIM) with the analog mean delay (AMD) method

We demonstrate a high-speed confocal fluorescence lifetime imaging microscopy (FLIM) whose accuracy and photon economy are as good as that of a time-correlated single photon counting (TCSPC). It is based on a new lifetime determination scheme, the analog mean delay (AMD) method. Due to the technical advantages of multiple fluorescence photon detection capability, accurate lifetime determination scheme and high photon detection efficiency, the AMD method can be the most effective method for high-speed confocal FLIM. The feasibility of real-time confocal FLIM with the AMD method has been demonstrated by observing the dynamic reaction of calcium channels in a RBL-2H3 cell with respect to 4αPDD stimulus. We have achieved the photon detection rate of 125 times faster than a conventional TCSPC based system in this experiment.

High-resolution differential mode delay measurement for a multimode optical fiber using a modified optical frequency domain reflectometer.

A novel differential mode delay (DMD) measurement technique for a multimode optical fiber based on optical frequency domain reflectometry (OFDR) has been proposed. We have obtained a high-resolution DMD value of 0.054 ps/m for a commercial multimode optical fiber with length of 50 m by using a modified OFDR in a Mach-Zehnder interferometer structure with a tunable external cavity laser and a Mach-Zehnder interferometer instead of Michelson interferometer. We have also compared the OFDR measurement results with those obtained using a traditional time-domain measurement method. DMD resolution with our proposed OFDR technique is more than an order of magnitude better than a result obtainable with a conventional time-domain method.

Determination of the differential mode delay of a multimode fiber using Fourierdomain intermodal interference analysis.

We present a novel differential mode delay (DMD) measurement method for a multimode optical fiber (MMF) based on Fourier-domain lowcoherence interferometry (fLCI) for the first time. An optical spectrum analyzer and a mode scrambler are used to obtain the intermodal interference signal in spectral domain after light passes an MMF. The MMF is used as a common path interferometer by itself without using any reference arm in an interferometer. Instead of using an offset launching technique, we present an effective method to obtain all available intermodal interference signals at a single shot by using a mode scrambler. The measured results using the proposed method are compared with those obtained using a conventional time-domain DMD measurement method.

New optical frequency domain differential mode delay measurement method for a multimode optical fiber.

A novel mode analysis method and differential mode delay (DMD) measurement technique for a multimode optical fiber based on optical frequency domain reflectometry has been proposed for the first time. We have used a conventional OFDR with a tunable external cavity laser and a Michelson interferometer. A few-mode optical multimode fiber was prepared to test our proposed measurement technique. We have also compared the OFDR measurement results with those obtained using a traditional time-domain measurement method.

Novel phase-matching condition for a four wave mixing experiment in an optical fiber.

A new phase-matching condition for a four-wave-mixing (FWM) experiment in an optical fiber is proposed to simultaneously measure the linear and nonlinear optical properties of an optical fiber such as dispersion-zero wavelength, dispersion slop, and nonlinear refractive index. Several different dispersion shifted fibers (DSFs) and nonzero dispersion shifted fibers (NZDSFs) were tested to demonstrate the validity of our proposed method. We have also shown that experimental results are in good agreement with those obtained using a conventional measurement method. We believe that technique is a very powerful and efficient tool for zero-dispersion and dispersion slop mapping for already installed optical fibers.

Effect of OH impurity on residual stress development in optical fiber

The compressive residual stress was found to develop at the outer-cladding/jacketing boundary of the optical fibers during the fiber drawing process. The residual stress of the fiber drawn at 3.48 N was found to be -61 MPa and it was due to the viscosity decrease induced by a significant OH impurity of /spl sim/582 ppm.

WALK-OFF LENGTH LIMITED SPECTRAL BROADENING IN SUPERCONTINUUM GENERATION

Detailed spectrum broadening mechanisms in supercontinuum (SC) generation are studied with an all-fiber pulsed laser source and a highly nonlinear dispersion-shifted fiber (HN-DSF). Mode-locked fiber laser pulses are stretched to four different pulse widths, and SC spectra are measured with different propagation distances with these pulses. By observing the development of spectral width with distance, we have observed that self-phase modulation (SPM) rather than soliton fission is the dominant process at the beginning of SC generation in our case. We have also confirmed that four-wave mixing (FWM) and its walk-off problem associated with different spectral components are the major limiting factors in efficient wide-band SC generation.

Low coherent hybrid detection technique for differential mode delay in a multimode optical fiber

We present a low coherent hybrid detection technique for the differential model delay (DMD) measurement of a conventional optical multimode fiber with a modified Mach-Zehnder interferometer. A low coherent hybrid detection technique enhances mode coupling between guided modes and the reference mode to easily resolve the modal distribution. An optical spectrum analyzer and a broadband source were used to obtain time-resolved optical beat signals. The measured interference signal was Fourier transformed to obtain time-delay information. A scanning offset launching method was used to excite every available mode in a multimode fiber (MMF). Measurements of a conventional 8-m-long MMF demonstrated the validity of our proposed method. The experimental results of our proposed method agree well with results obtained using a conventional time domain measurement method. Our proposed method can be used to measure the DMD in a short length of optical MMF with a temporal resolution better than 0.72 ps when using an 8-m-long single-mode fiber as a reference.

Fabrication of a tapered holey optical fiber and its guiding properties

We have fabricated the tapered HF by using a fusion splicer and a fiber cleaver. We have experimentally observed the near-field mode profile of the HF and tapered HF. The mode field profile of the HF shows a good confinement of its field within the core diameter of the tapered HF. The mode pattern of the HF and tapered HF are all single mode. These properties can be potentially useful for improving the efficient coupling of light from an optical fiber into a planar waveguide.