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051896B.ENG  + Source:  ONR Asia +
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Contributory Categories:

Country: Japan

From:  NTT Review
       Vol. 7, no. 4, p. 70-74

KEYWORDS: Japan; Submarine Fiber Optic Transmission


A Fiber Submarine Transmission System Using
Optical Amplifiers

                              Yoshiaki SATO
                             Senior Engineer
                NTT Network Systems Development Department        
                     Makoto MURAKAMI
                            Research Engineer
                 NTT Optical Network Systems Laboratories
                              Masaki AMEMIYA
                         Senior Research Engineer
                 NTT Optical Network Systems Laboratories
                            Kazuhiro NISHIHATA
                              Project Leader
                NTT Network Systems Development Department        
     Very high-speed fiber submarine transmission systems in
Japan are described. The transmission bit-rate is flexible and is
available at 600 Mbit/s, 2.4 Gbit/s and 10 Gbit/s in the system,
because optical amplifiers are used as submarine repeaters.  A
submarine repeater is composed of six subsystems, therefore the
maximum transmission capacity is 60 Gbit/s per repeater.  To
operate this system easily, in-service performance monitoring of
submarine repeaters is suggested.

     NTT was the first to introduce synchronous digital hierarchy
(SDH) as specified by ITUT Recommendations in 1989.  Since then,
150 Mbit/s, 600 Mbit/s and 2.4 Gbit/s terrestrial transmission
systems and 1.8 Gbit/s submarine transmission systems have been
installed in Japan(1).  A 160 km long-span 600 Mbit/s
transmission system has been recently developed using optical
amplifiers and has been installed in 1994 in NTT SDH network(2).
Higher speed (larger capacity) and longer repeater spacing are
both important for an efflclent network.  Submarine transmission
systems are necessary since Japan is made up of many small
islands.  Figure 1 shows the development trend of submarine
transmission systems in Japan.
     This paper describes a very high-speed fiber submarine
transmission system "FSA"(3).  The transmission bit-rate of the
FSA is flexible and can be 600 Mbit/s, 2.4 Gbit/s, and 10 Gbit/s.
Upgradability can be achieved without replacing the optical
transmission line, because optical amplifiers are used as
submarine repeaters.  Optical amplifiers are a breakthrough in
achieving bit-rate transparency, low cost, and reliable submarine
transmission systems.  A submarine repeater is composed of six
subsystems, therefore the maximum transmission capacity is 60
Gbit/s per repeater.

     Figure 2 shows the FSA system configuration.  This system is
based on an optical transmission line and its operation system. 
The optical transmission line is constructed with submarine
optical cables and submarine repeaters.  The submarine repeaters
are optical amplifiers, which are Er-doped fiber amplifiers
(EDFAs).  The line signal is transmitted by direct amplification
in each EDFA to compensate for fiber losses.  The FSA has the ad-
vantage of upgradability from 600 Mbit/s to 10 Gbit/s, because
optical amplifiers are independent of the signal bit-rate.  If
the FSA was installed and commercially started as a 2.4 Gbit/s
transmission system, upgradability to 10 Gbit/s would be possible
without replacing the optical transmission line, only the
terminals need to be changed.  The repeater size of the FSA is
the same as the FS-1.8G, but the number of subsystems is double
in a submarine repeater.  A submarine repeater is composed of six
subsystems, therefore the maximum transmission capacity is 60
Gbit/s per repeater.
     Supervisory terminals (SV terminals) act as regenerators for
the line signal and add/ drop multiplexers for the supervisory
signal.  The optical parameters of the received line signal from
terrestrial SDH equipment is modified for submarine repeaters,
and factors such as optical power, wavelength allowance, and
chirping are adjusted.  The optical parameters of the received
line signal from the final submarine repeaterare modified for
terrestrial SDH equipment.  The supervisory signal for in-service
performance monitoring is transmitted by overmodulation of the
line signal.  The operation system controls and monitors the
performance of submarine repeaters through SV terminals.  The
specifications of the FSA are shown in Table 1.

     Unlike regenerator systems where transmission penalties are
reset, impairments accumulate along the optical amplifier chain
in the FSA.  The most fundamental factor limiting transmission
performance is optical noise (ASE: amplified spontaneous emission
noise) generated from each optical amplifier which degrades the
signal to noise ratio (SNR) of the line signal.  The signal
output power level in the FSA is set at the best value to
maintain the SNR for 1,000 km transmission with a repeater
spacing of 100 km.      In addition to optical noise, such
systems encounter the problems of fiber nonlinearity, which
mainly leads to the
following two effects: Self phase modulation (SPM), the phase
shift induced by the signal intensity, causes waveform distortion
of the signal pulses through interaction with chromatic
dispersion; four wave mixing (FWM), which refers to the
interaction of the signal and the ASE noise, creates excess noise
     To overcome fiber nonlinearity, each fiber is carefully
configured to achieve normal dispersion values at its input end
and anomalous dispersion at its output end to realize dispersion
compensation as shown in Fig. 3. This is because fiber-
nonlinearity impairments become significant as signal output
power increases and they strongly depend on fiber dispersion.
     The configuration of the FSA submarine repeater is shown in
Fig. 4. Each repeater contains 12 EDFAs to form 6 line pairs. 
Each EDFA has a noise figure of about 6 dB and an optical output
power of 6 dBm set by the automatic level control circuit.  An
optical filter of 10 nm bandwidth was located at the output end
of each EDFA to remove unwanted ASE noise and to manage the
amplifier's optical passband.
     A unique in-service supervisory signal transmission scheme
suitable for optical amplifier repeater systems with quite simple
supervisory hardware is developed in the FSA.  The supervisory
signal is transmitted by overmodulation of the line signal as
shown in Fig. 5. The submarine repeater can be mon'ltored even if
the line signal is shut off because the supervisory signal can be
superimposed on the ASE noise.  The oven-nodulation index and
frequency is designed to be properly transmitted and not to
affect line signal transmission.
     In a submarine system, it takes a long time to repair
repeaters or cable breaks because special ships must work on the
sea surface.  To operate the FSA easily, an operation system has
been developed.  The operation system consists of a commercial
personal computer, software, and a public telephone line.  The
personal computer can be used on the ships, in the site of SV
terminals, or anywhere.  The submarine repeaters can receive the
supervisory signal from each SV terminal site since these signals
interconnect the submarine repeaters.
     The FSA has in-service performance monitoring of repeaters
and power feed equipments.  In submarine repeaters, the input
power, output power, electrical current of the pump laser, and
internal temperature can be monitored.  Electrical current and
voltages in the power feed equipment are also monitored.  Any
discrepancy between the initial value and the present one can be
used for forecasting failures.
     In addition to in-service performance monitoring, the
operation system detects alan-ns in SV ten-ninals and the power
feed equipments.  Such as package failures and loss of signal. 
Electrical current and electrical voltages in the power feed
equipment are also monitored.  The operation system can identify
the location of a failure In the FSA.
     Optical Time Domain Reflectometory (OTDR) 'Is an effective
technique for finding faults along a fiber.  However OTDR cannot
be used with transmission lines containing traditional repeaters. 
The FSA repeaters establish the by-pass optical circuit to return
the backscattered light to the OTDR setup.  Then fault location
in the FSA transmission line containing repeaters is carried out.

     This paper introduced very high-speed fiber submarine
transmission systems.  The transmission capacity of this system
is flexible and is available for 600 Mbit/s, 2.4 Gbit/s and 10
Gbit/s.  The FSA has been installed between Kagoshima and Okinawa
in spring 1995 (Fig. 6).  The total length is about 900 km.  
(1) H. Miura : "Construction & Operation of the Transmission Line 
   Network", NTT Review, Vol. 5, No. 2, pp.30-58, Mar. 1993. (2)
K. Nishihata, T. Kuwata and H. Seki : "Introduc-
        tion of New SDH Optical Transmission System",
   NTT Review.  Vol. 6, No. 5, pp.84-89, Sept. 1994.
(3) M. Aiki, S. Saito and T. Ito : "Evolution of Optical
   Amplifier' Submarine Systems", GLOBECOM'92, pp. 1431-1435,   
ORLAND, 1992.

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