Kazuro Kikuchi

Wavelength tuning of fiber Bragg gratings over 90 nm using a simple tuning package

We demonstrate a silica-based tunable fiber Bragg grating filter with a wavelength tuning range over 90 nm. A bend-tuning technique in a simple tuning package is employed to obtain a wide wavelength tuning range from 1634 to 1544 nm, covering the entire L-band and a half of the C-band. The polarization-mode dispersion and polarization-dependent loss of the device show little variations over the whole operating region.

Bismuth-oxide-based nonlinear fiber with a high SBS threshold and its application to four-wave-mixing wavelength conversion using a pure continuous-wave pump

The unique and practical benefits of the use of bismuth-oxide-based nonlinear fiber (Bi-NLF) in implementing a four-wave-mixing (FWM)-based wavelength converter for fiber-optic-communication-system applications are experimentally demonstrated. First, the Kerr-nonlinearity and stimulated-Brillouin-scattering (SBS) characteristics of our fabricated Bi-NLF are experimentally investigated. The Bi-NLF is found to have the superior advantage of a significantly high SBS threshold in addition to its ultrahigh Kerr nonlinearity /spl gamma/ of /spl sim/1100 W/sup -1//spl middot/km/sup -1/, compared to the conventional silica-based highly nonlinear fiber. Next, the authors perform an experiment for the FWM-based wavelength conversion of a non-return-to-zero (NRZ) signal within a 40-cm length of the Bi-NLF fusion spliced to standard silica fibers by using a continuous-wave (CW) high-power pump beam. Error-free tunable wavelength conversion over a 10-nm bandwidth is readily achieved. No SBS-suppression scheme is employed for the pump due to the high SBS threshold, which simplifies the system configuration and improves the quality of the wavelength-converted signal.

Widely tunable optical filters based on fiber Bragg gratings

The authors design and demonstrate a widely tunable wavelength filter, which is realized by lateral bending of a fiber Bragg grating mounted on a substrate. The 40-nm tunable range of the device covers the entire C-band of erbium-doped fiber amplifiers with very small changes in a spectral width and a reflectivity across the operation bandwidth. Furthermore, employment of a hybrid material as a substrate alleviates the stringent requirement for the bending radius. With this hybrid substrate structure, a tuning range as wide as 50 nm is achieved.

All-optical regenerator using wavelength shift induced by cross-phase modulation in highly nonlinear dispersion-shifted fiber

We propose and demonstrate a novel fiber-based all-optical regenerator, where the wavelength shift induced by cross-phase modulation is employed for level discrimination. Regeneration of a 10-Gb/s signal using the proposed scheme improves the Q-factor by 1.8 dB. The scheme is stable and robust against changes in environmental conditions.

Nonlinearly strain-chirped fiber Bragg grating with an adjustable dispersion slope

In this letter, we demonstrate an adjustable dispersion slope compensator for waveform distortion compensation based on a nonlinearly strain-chirped fiber Bragg grating. The dispersion slope of the device can be tuned up to -18.9 ps/nm/sup 2/ with a bandwidth >2.4 nm. After transmission through a 120-km-long dispersion-shifted fiber, the resulting waveform distortion for a 2.65-ps pulse is successfully compensated by using this device.

Polarization-insensitive all-optical wavelength conversion using cross-phase modulation in twisted fiber and optical filtering

Polarization-independent all-optical wavelength conversion is demonstrated by using cross-phase modulation in a twisted fiber and subsequent optical filtering. By twisting a 1-km-long fiber at a rate of 15 turns/m and aligning the probe light to circular polarization, we reduce the polarization sensitivity successfully from 3.5 to 0.3 dB. Error-free operation with only 1-dB penalty is realized at 40 Gb/s with the input signal polarization scrambled. The demonstrated scheme also offers a novel function of restoring the degree-of-polarization of the signal light.

Unified analysis of modulational instability induced by cross-phase modulation in optical fibers

Cross-phase modulation in an optical fiber can lead to various types of modulational instability, such as polarization instability in a highly birefringent fiber and two-pump optical parametric amplification. We present unified analyses of such instabilities and clarify the general mechanisms behind them. By solving the generalized eigenvalue equation, we indicate the explicit conditions of inducing modulational instability. The eigenvector is also calculated, which allows us to explain underlying physics of instabilities using a phasor diagram. As a result, all types of cross-phase modulation-induced modulational instability are classified into three types in terms of their mechanisms.

Design and fabrication of a tunable dispersion-slope compensating module based on strain-chirped fiber Bragg gratings

We report the design and fabrication of a tunable dispersion-slope compensating module based on a concatenation of a linearly strain-chirped fiber Bragg grating (FBG) and a nonlinearly strain-chirped FBG. By bending a substrate in an S shape, on which each FBG is mounted, we can adjust only the dispersion slope while maintaining the group-velocity dispersion as well as the center wavelength. The dispersion slope can be tuned up to -58.4 ps/nm/sup 2/ in this module for a bandwidth larger than 1.65 nm.

Wide-band tunable wavelength conversion of 10-gb/s nonreturn-to-zero signal using cross-phase-Modulation-induced polarization rotation in 1-m bismuth oxide-based nonlinear optical fiber

We experimentally demonstrate that the use of only 1-m length of our fabricated Bismuth oxide-based nonlinear fiber readily allows for implementing a high-performance 10-Gb/s nonreturn-to-zero signal wavelength converter based on cross-phase-modulation-induced polarization rotation. No stimulated Brillouin scattering (SBS) suppression scheme was employed owing to a high SBS threshold of the fiber. Our wavelength converter is shown to provide a pattern-inverted or a noninverted output signal in the same configuration depending on the relative polarization directions between the probe and the polarizer. Error-free wavelength conversion over a 30-nm bandwidth is readily achieved for both cases, i.e., noninverted and inverted patterns.

Polarization-insensitive asymmetric four-wave mixing using circularly polarized pumps in a twisted fiber

We show theoretically and experimentally that the polarization sensitivity of asymmetric nondegenerate fiber four-wave mixing can be eliminated by using circularly polarized pump waves in a twisted fiber. By twisting a fiber at 15 turns/m and aligning the pump waves to a circular state of polarization, we successfully reduce the polarization sensitivity from 5.8 dB to 0.9 dB. Although the polarization-mode dispersion (PMD) of the twisted fiber sets the limitation to the conversion bandwidth, its effect is relatively small owing to the small PMD of the twisted fiber. The demonstrated scheme should be a simple and efficient way of realizing all-optical tunable wavelength converters and wavelength-exchange devices without polarization dependence.