Hirohisa Yokota

Optical Irradiation Method for Fiber Coupler Fabrications

An optical irradiation method for high-speed fiber coupler fabrications without contamination is proposed. In optical fiber taper fabrications we found an interesting effect that fiber elongation is self-arrested although laser irradiation is continued. This is a typical effect in the optical irradiation method, but is not seen in the flame method or the thermal heating method. The “self-controlled fiber taper shape effect” is theoretically explained by considering thermal energy flows, and optical fiber couplers are fabricated using this effect. From our experiments, we show that optical fiber coupler fabrications at high-speed without contamination are possible using the optical irradiation method.

Designs and Fabrications of Photonic Crystal Fiber Couplers with Air Hole Controlled Tapers

For the construction of photonic crystal fiber (PCF) systems using their unique properties, a PCF coupler (PCFC) is one of the key components of the systems. The characteristics of the PCFC depend on the state of air holes in the tapered region of the PCFC because the state of air holes in the tapered region affects light propagation in the PCFC taper. In this paper, coupling characteristics of PCFCs were theoretically investigated. In PCFCs with air hole remaining tapers, we found that a smaller elongation ratio i.e. a stronger elongation is required to obtain optical coupling as an air hole pitch or a ratio of air hole diameter to pitch is larger. In PCFCs with air hole collapsed tapers, it was clarified that a dependence of extinction ratio on air hole collapsed elongation ratio is higher for smaller elongation ratio. It was also clarified that an air hole remaining PCFC has slow wavelength characteristics in extinction ratio compared to an air hole collapsed PCFC. Air hole remaining PCFCs and air hole collapsed PCFCs were fabricated using a CO 2 ; laser irradiation technique. We could successfully control whether air holes in the PCFC taper were remaining or collapsed by adjusting the irradiated laser power in the elongation process of the PCFC fabrication. It was experimentally clarified that the air hole remaining PCFC has slow wavelength characteristics in extinction ratio compared to the air hole collapsed PCFC. The tendencies of the measured wavelength characteristics of PCFCs agree with those of numerical results.

Light propagation characteristics in photonic crystal fiber with graded air hole diameters

Light propagation characteristics in a photonic crystal fiber (PCF) with graded air hole diameters were theoretically investigated. The PCF has a square-law distribution of air hole diameters. It was clarified that the intensity peak varied periodically in the beam propagation for Gaussian beam incident. We found that the attenuation of the total guided power in the PCF could be reduced compared to that in a PCF with uniform air holes.

SNR improvement in EDFA repeaters using cascaded optical fiber grating couplers

A configuration for SNR improvement by reducing ASE noise in EDFA repeaters for WDM signals using cascaded optical fiber grating couplers (FGCs) is proposed. The effectiveness of the configuration is experimentally demonstrated and discussed.

A low loss butt-joint connection by using a graded-index photonic crystal fiber

A low loss butt-joint connection by using a graded-index photonic crystal fiber (GI-PCF) is proposed. The mode extent in the GI-PCF varies periodically along the fiber when the LP01 mode of the PCF is launched into the GI-PCF. It is found that the connection loss can be reduced by adjusting the length of the GI-PCF for an air gap given.

Determination of stereoisomer in sugars by THz-TDS

Transmission-type terahertz time-domain spectroscopy (THz-TDS) using dipole-antennas on LT-GaAs layers as THzemitter and detector with fs-fiber laser as the excitation light source was applied to determine the stereoisomer of sugars in carefully controlled environment with low humidity below 5% at 20°C. Commercially available α-D-lactose including about 4% anomer and β-D-lactose including below 30% anomer were used as stereoisomer samples. The absorption spectra of compounds by the α-D- and β-D-lactose powders with various ratios were successively deconvoluted to four spectra by Lorentzian and dependent on the composition ratio. Integrated absorptions due to α-D-lactose and β-D-lactose determined the decreasing rate (rα) and the increasing rate (rβ) for the composition ratio of the β-D-lactose powder, because the intensities were linearly dependent on the composition ratio. The net-composition ratio of α-lactose and β-lactose in the compounds was precisely evaluated by the ratio of the integrated intensities and the value of rβ / rα, for example, the anomer-contents in commercially available α-D-lactose and β-D-lactose powders were disclosed as 3.9% and 29.1%, respectively. The demonstrated results indicate that THz-TDS is so useful not only for precise qualitativeanalysis but also for precise quantitative-analysis of stereoisomer in sugars with partially different molecular structure such as lactose.

Light propagation characteristics in photonic crystal fibers with α-power profiles of air hole diameter distributions and their application to fiber collimator

Light propagation characteristics in photonic crystal fibers (PCFs) with α-power profiles of air hole diameter distributions were theoretically investigated. It was clarified that the intensity peak of the beam propagating in the PCF with Gaussian beam excitation varied periodically with little power attenuation. It was found that the envelope of the periodic intensity variation depended on α. We theoretically demonstrated that the PCF with the α-power profile of the air hole diameter distribution could be applied to a collimator for a conventional PCF with uniform air holes in Gaussian beam excitation to reduce coupling loss, where a PCF of appropriate length with the α-power air hole diameter distribution was spliced to a conventional PCF. It was also found that the coupling efficiency was higher for a larger α.

Coupling Characteristics of Fused Optical Fiber Coupler Formed with Single-Mode Fiber and Photonic Crystal Fiber Having Air Hole Collapsed Taper

Fused coupler forming with a single-mode fiber (SMF) and a photonic crystal fiber (PCF) is one of the solutions for optical coupling from a light source to a PCF. In this paper, we presented coupling characteristics of a fused fiber coupler formed with an ordinary SMF and a PCF having air hole collapsed taper. A prototype of SMF-PCF coupler with air hole collapsed taper was fabricated using CO2 laser irradiation. The coupling efficiency from SMF to PCF was −6.2 dB at 1554 nm wavelength in the fabricated coupler. The structure of the SMF-PCF coupler to obtain high coupling efficiency was theoretically clarified by beam propagation analysis using an equivalent model of the coupler with simplification. It was clarified that appropriately choosing the prestretched or etched SMF diameter and the length of air hole collapsed region was effective to obtain high coupling efficiency that was a result of high extinction ratio at cross port and low excess loss. We also demonstrated that the diameter of prestretched SMF to obtain high coupling efficiency was insensitive to the air hole diameter ratio to pitch of the PCF in the air hole collapsed SMF-PCF coupler.

Terahertz fiber using polymer tube bundle

Recently, much attention has been paid for low loss waveguide for terahertz wave. Many types of waveguide structure using polymers have been investigated. A bundle of polymer tubes is a powerful candidate for the terahertz fiber. In this paper, we proposed and analyzed a new terahertz fiber using polymer tubes. Optimum tube parameters in bundle structure for low loss waveguide are clarified.

Estimation of fiber parameters by using OFRR nonlinear dynamics

In this paper, a new method for measuring the nonlinear refractive index by using the nonlinear dynamics generated in optical fiber ring resonator (OFRR) is proposed. In OFRRs, input light and circulated light components are added in succession, and then, result in forming an interference output. The OFRR output exhibits nonlinear dynamics such as a chaotic state due to the optical Kerr effect, as the input light power increases. In the bifurcation diagram, the output power indicates peaks in the input power range lower than the bifurcation point at which the output changes from stable to periodic state. It is found that the input power at the peak shifts, dependent on the nonlinear refractive index. The nonlinear refractive index can be estimated by applying the measured input power giving the peak point to the numerical relationship between the input power and the nonlinear refractive index. In experiments the nonlinear refractive index of the two pure silica core fibers with different cutoff wavelength was estimated as n2=0.97×10-22[m2/V2] and n2=1.06×10-22[m2/V2] which are in good agreement with those reported previously.