K. Mochizuki

Novel chromatic dispersion measurement method over continuous Gigahertz tuning range

A method for the measurement of chromatic dispersion in a single-mode optical fiber is proposed and demonstrated. This method adopts the technique of external modulation of the distributed feedback (DFB) laser output and direct optical frequency sweep range monitoring by coherent heterodyne detection. The method is also applicable to the future optical communication system with narrowband optical elements such as optical amplifiers, whereas measurement is impossible given the present state of the art. >

Amplified spontaneous Raman scattering in fiber Raman amplifiers

Amplified spontaneous Raman scattered light power in forward and backward Raman amplifiers is theoretically and experimentally studied under the assumption of the constant Raman gain coefficient in the bandwidth of an optical filter. On the basis of the results, the power ratio of the amplified signal to amplified spontaneous scattering is discussed. As a result, it is clarified that the effect of amplified spontaneous Raman scattering on the signal to noise ratio for backward Raman scattering is larger than for forward Raman scattering.

Novel coherent heterodyne optical time domain reflectometry for fault localization of optical amplifier submarine cable systems

A coherent, heterodyne-detection, optical time-domain reflectometry technique suitable for an all-optical repeater system is proposed. Rayleigh backscatter measurement through a 309-km bidirectional optical transmission line with six cascaded in-line semiconductor laser amplifiers is demonstrated using this technique. The possibility of fault localization on an optical amplifier system with optical isolators is shown.<<ETX>>

Dynamic polarization fluctuation characteristics of optical fiber submarine cables under various environmental conditions

Polarization fluctuation characteristics of optical-fiber submarine cable under 8000-m-deep sea environmental conditions, optical-fiber submarine cable coupling under periodic variable tension, and cable performance during and after installation, are presented. As a worst case, it is demonstrated that the maximum frequency spectrum of polarization fluctuation is less than 200 Hz under dynamic composite stress states. It is found that the polarization fluctuation of the optical-fiber submarine cable coupling changes linearly with periodic variable tensile forces. It is confirmed that the power spectrum of polarization fluctuations are less than 50 approximately 60 Hz, and the maximum power spectra are between 5 and 10 Hz during the cable installation within a maximum cable tension of 18 kN (1.8 tons) and the cable laying speed of 3.5 knots, respectively. >

Polarization mode dispersion measurements in an installed optical fiber submarine cable

Highly accurate polarization mode dispersion (PMD) measurement using an interferometric detection technique with AC modulation is discussed. This technique, which searches for the interference fringe signal, was used to measure PMD in a 60 km long installed optical submarine cabled fiber 2500 m sea depth and to determine the fiber-length dependence of PMD. PMD measurement in 60-km-long installed optical submarine cabled fibers was confirmed to be less than about 0.7 ps. It was found that the fiber length dependence of the PMD measurement in the less than approximately 20-km-long cascaded uncabled fibers was proportional to square root L; however, the PMD data in the long-length (over approximately 50 km long) fibers were slightly scattered.<<ETX>>

Influence of hydrogen on optical fiber loss in submarine cables

Characteristics of the loss increase in optical fibers due to hydrogen permeation are described, and it is shown that the loss increase due to OH formation in GeO 2 -doped single-mode fibers is thought to be small compared to the loss increase due to the vibration of hydrogen molecules even in 20 years. Furthermore, the hydrogen generation due to electrochemical reaction in the optical cables is described. Finally, the countermeasure against the problem is discussed.

Estimation of long-term transmission loss increase in silica-based optical fibers under hydrogen atmosphere

The irreversible loss increase in silica-based optical fibers due to hydrogen is discussed on the basis of results of various high-temperature tests. The results show that germanium-doped-core fibers have different behavior with respect to irreversible loss increase, and that a pure-silica-core fiber fabricated under optimum conditions is very stable against irreversible loss increase. The estimation of long-term transmission loss stability is also discussed, and high-temperature testing is certified to be effective for estimating the long-term loss stability under low temperature. >

Polarization diversity techniques for the use of coherent optical fiber submarine cable systems

Recent research and development work on coherent transmission systems, with emphasis on its application to optical-fiber submarine cable systems, are described. The authors demonstrated the feasibility of a polarization diversity scheme in a laboratory environment as well as in the first sea trial of a coherent frequency-shift-keying (FSK) system. The authors propose a novel external cavity laser module and a polarization coupling/dividing module in order to improve the system performance. The polarization fluctuation speed in optical-fiber submarine cable reaches 50 Hz in the worst case, and the polarization diversity scheme is a simple and very powerful technique under such polarization fluctuation conditions. >

Influence of nondegenerate four-wave mixing on coherent transmission system using in-line semiconductor laser amplifiers

Three-channel coherent frequency-shift-keying (FSK) transmission experiments using in-line semiconductor laser amplifiers under the influence of nondegenerate four-wave mixing in laser amplifiers are discussed. The experiments show that the effect of four-wave mixing is reduced by applying modulation to the interfering carriers and enlarging the frequency separation of each carrier. However, transmission quality degradation becomes serious in multistage amplifier systems. >

Power penalty due to optical back reflection in semiconductor optical amplifier repeater systems

The effect of optical back-reflection on the bit-error-rate performance of a semiconductor laser amplifier repeater system is evaluated at 565 Mb/s. It is found that the degradation of the back-reflection is mainly caused by multiple reflection in the amplifier and depends on the internal gain of the optical amplifier and the reflectivity of the reflection points. It is concluded that in optical amplifier repeater systems with large gain, an optical isolator should be used to reduce the back-reflection penalty.<<ETX>>