Nori Shibata

Phase-mismatch dependence of efficiency of wave generation through four-wave mixing in a single-mode optical fiber

The generation wave efficiency with respect to phase mismatch in the four-wave mixing process is clarified experimentally in a single-mode fiber transmission line at 825 nm wavelength. The generated power of approximately 20 pW is measured successfully for input signal powers below 1 mW by the technique utilizing a heterodyne receiver and lock-in detector. The calculated efficiency as a function of the equivalent frequency separation can well explain and reflect the results obtained experimentally. Furthermore, the efficiency at zero chromatic dispersion wavelengths of 1.3 and 1.55 μm is also discussed considering chromatic dispersion slope against wavelength.

Interferometric method for chromatic dispersion measurement in a single-mode optical fiber

Chromatic dispersion of a single-mode optical fiber only 1 m long is measured by a newly proposed interferometric method. Interference fringes are produced by two laser beams, one transmitting through the fiber and the other through air. Path length shifts in an interferometer for attaining the visibility maximum are measured by varying optical source wavelengths between 818 and 904 nm. Group delay time differences around 1 ps are measured. The measured results are in good agreement with those measured by the conventional pulse method for the identical 1.3 km long fiber.

A 100-channel optical FDM transmission/distribution at 622 Mb/s over 50 km

A 100-channel optical frequency-division multiplexing (FDM) transmission/distribution experiment at 622 Mb/s is demonstrated for a fiber length of 50 km. The feasibility of a polarization-insensitive waveguide frequency selection switch for 10-GHz intervals and a frequency-shift-keying (FSK) direct-detection scheme employing a Mach-Zehnder filter is verified. The demodulation circuit employs a Mach-Zehnder filter and a balanced receiver, which utilizes optical power more efficiently than the Fabry-Perot filter. No receiver sensitivity degradation is observed due to interchannel crosstalk of the 128-channel tunable waveguide frequency selection switch (FS-SW) or fiber four-wave mixing for transmissions over a 50-km-long nondispersion-shifted (NDS) fiber and a 26-km-long dispersion-shifted (DS) fiber. >

Longitudinal acoustic modes and Brillouin-gain spectra for GeO 2 -doped-core single-mode fibers

Guided longitudinal-acoustic modes, which give rise to Brillouin gain, are theoretically clarified for a single-mode fiber with a GeO2-doped core and pure-silica cladding. Longitudinal-acoustic L0m modes are found from the theoretical analysis to interact with the electromagnetic field of the HE11 mode. Brillouin-gain spectra are measured for clarifying the theory. A few gain peaks in the Brillouin-gain spectra are successfully explained by taking account of the phase-velocity characteristics of the L01, L02 and L03 modes guided in the GeO2-doped core region. Furthermore, Brillouin frequency shifts per unit dopant concentration for GeO2 and F are experimentally obtained to confirm the dispersion characteristics of the guided acoustic modes. The evaluated frequency shifts per unit dopant concentration are 107 and 356 MHz/mol% for GeO2 and F, respectively, at a wavelength of 1550 nm.

Brillouin-gain spectra for single-mode fibers having pure-silica, GeO(2)-doped, and P(2)O(5)-doped cores.

Brillouin-gain spectra are measured for pure-silica core, GeO(2)-doped core, and P(2)O(5)-doped core single-mode fibers with different index profiles. A narrow-linewidth semiconductor laser operating at 828 nm is used as the pump light source. The spectral shape of the Brillouin gain is found to be strongly related to the refractive-index profiles. The Brillouin linewidths evaluated experimentally are 90 and 215 MHz for a step-index single-mode fiber and a graded-index single-mode fiber, respectively. The Brillouin Stokes shifts depend on the core/clad-ding dopant materials and their concentrations and range from 20 GHz for the P(2)O(5)-doped core fiber to 21.6 GHz for the puresilica core fiber in the 0.8-microm-wavelength region.

Transmission limitations due to fiber nonlinearities in optical FDM systems

Transmission limitations due to stimulated Brillouin scattering and four-wave mixing processes are investigated for optical frequency division multiplexing (FDM) systems. The applicability of the dispersion-shifted (DS) and nondispersion-shifted (NDS) fibers is discussed, taking account of channel frequency separation, total channel numbers, input signal power, transmission length, and receiver sensitivity degradation. Experimental results on Brillouin gain spectra and the wave generation efficiency in four-wave mixing processes are also presented to discuss the applicability of the two types of single-node fiber. It was found that NDS fibers operated at a wavelength of 1550 nm can be widely deployed in multichannel systems both for the long-haul and information distribution transmissions, if the signal waveform distortion due to fiber chromatic dispersion is precluded. The delay equalizer will be useful for a high-speed system employing bit rates over 10 Gb/s and repeaterless spans over 300 km. For such an application, DS fiber is preferable. Concerning information distribution network applications, the NDS fiber should be more attractive as a transmission medium for FDM system applications. >

Identification of longitudinal acoustic modes guided in the core region of a single-mode optical fiber by Brillouin gain spectra measurements

Several resonance peaks owing to an interaction between light-wave and guided modes of longitudinal acoustic waves are observed in the Brillouin gain spectra for a single-mode fiber with a GeO2-doped core and pure silica cladding. Brillouin gain spectra measurements are carried out at 1286- and 1550-nm wavelengths. Longitudinal acoustic modes guided in the GeO2-doped core region are identified by applying the analysis of leaky modes in a fiber acoustic waveguide.

Polarization properties of single-polarization fibers.

A rigorous theoretical analysis has been presented of the stress birefringence B(s) and the polarization dispersion D(s) in single-polarization fibers. Both B(s) and D(s) vary considerably in dependence on normalized frequency nu, and the nu-value dependence of B(s) and D(s) is strongly affected by the anisotropic stress distribution in the fiber. It is shown that the experimental results agree well with the calculated ones.

Loss increase for optical fibers exposed to hydrogen atmosphere

Loss spectrum changes for optical fibers exposed to a hydrogen atmosphere in the 15-200\deg C temperature range are measured. Loss increase due to molecular hydrogen dissolved into fibers is investigated from the loss peak at 1.24 μm, and that due to hydroxyl group formation from the loss peak at 1.41 μm. The loss increase due to molecular hydrogen is fully explained by physical solubility theory and diffusion equation. The empirical formula for time, temperature, and hydrogen-pressure dependences of the loss increase due to hydroxyl group formation is evaluated from the experimental results. The loss increase at 1.3- and 1.5-μm wavelength band at room temperature are estimated.

Mode coupling effects in stress-applied single polarization fibers

Mode coupling effects caused by waveguide imperfections and ambient fluctuations in the stress-applied single polarization fibers have been investigated. Several origins of mode coupling, such as: 1) core deformations, 2) deformation of stress-applying parts, 3) micro-bending, and 4) temperature fluctuations, have been treated. The mode coupling parameters between the orthogonally polarized HE 11 modes are compared for various kinds of waveguide imperfections and ambient fluctuations. It was clarified that the deformation of the stress-applying parts is the dominant factor in the mode coupling effects in stress-applied single polarization fibers.