Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-10-22
2003-07-01
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06587242
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmission system that transmits an OTDM signal (optical time division multiplexed signal) via an optical transmission fiber and one or both of an optical linear repeater and an optical regenerator repeater, and wherein a control signal carrying transmission quality monitoring information, frame information, multiplexed signal channel information, etc., is transmitted by being multiplexed with an OTDM signal.
In addition, the present invention relates to an optical transmission system that transmits an optical signal via an optical transmission fiber and optical linear repeaters, and wherein monitor light used in wavelength dispersion compensation of the transmission path is transmitted by being multiplexed with signal light.
2. Background Art
In an optical transmission system, transmission quality monitoring, frame synchronization, and extracting multiplex signal channels are very important. In conventional electrical time division multiplexing (ETDM) that multiplexes channels with a plurality of lines at the electrical stage, the transmission quality monitoring information, frame information, and multiplexed signal channel information corresponding to each of these functions are accommodated in the overhead of the SDH frame, and by electrical signal processing after conversion of the signal light into an electrical signal by a light receiver, transmission quality monitoring, frame synchronization, and channel extraction are carried out. In addition, a method of improving transmission characteristics consists in lowering the error rate by adding a forward error correction code (see M. Tomizawa et al., “STM-64 linearly repeating optical transmission experiment using forward error correcting codes”, in Electron. Lett., Vol. 31, No. 12, pp. 1001-1003, 1996.)
In contrast, in optical transmission systems, one method of improving the transmission speed is optical time division multiplexing (OTDM), which multiplexes a plurality of an optical short pulses while offsetting the timing along the time axis. Furthermore, for optical time division multiplexing, there is the parallel form shown in
FIG. 18
(see Japanese Unexamined Patent Application, First Publication, No. Hei 10-229364, “Optical Pulse Multiplexing Apparatus”) and the serial form shown in
FIG. 19
(see S. Kawanishi et al., “All-optical time-division-multiplexing of 100 Gbit/s signal based on four-wave mixing in a travelling-wave semiconductor laser amplifier”, Electron. Lett., Vol. 33, No. 11, pp. 976-977, 1997).
In
FIG. 18
, the optical pulse train of the repetition rate f
0
is split into N parts by an optical splitter
61
, and input into respective optical modulators
62
-
1
~
62
-N. The optical signals modulated by each optical modulator are respectively amplified by optical amplifiers
63
-
1
~
63
-N, have a different delay imparted by optical delay devices
64
-
1
~
64
-N, and are coupled by an optical coupler
65
. Thereby, an OTDM signal having a bit rate of Nf
0
is generated. When the bit rates of all the lines are equal, this structure can generate an OTDM signal that time division multiplexes N lines of optical signals having arbitrary bit rates by respectively multiplying the optical pulse train having a fundamental frequency f
0
split into N parts, in the case that the N lines of a modulated signal having has a bit rate m
i
f
0
(i=1, 2, . . . , N, and m
i
is an integer equal to or greater than 1).
In
FIG. 19
, an optical pulse train having a repetition rate &Sgr;m
i
f
0
is generated by a high speed optical pulse train generating means
66
, and modulated by an optical modulator
67
-
1
using a modulation signal having a bit rate of m
1
f
0
, and amplified by optical amplifier
68
-
1
. In the following manner, it is possible to generate an OTDM signal that has a time division multiplexed signal light of N lines by sequential modulation using modulation signals respectively having bit rates m
i
f
0
with each modulator.
In addition, in optical transmission systems, one of the main factors causing deterioration of transmission characteristics is the wavelength dispersion of the optical transmission path. When this wavelength dispersion is large, because the waveform of the signal light is distorted, inter-symbol interference causes bit errors. The influence of this increases as the transmission speed increases. Therefore, when constructing an optical transmission system, it is necessary to understand the wavelength dispersion characteristics of the optical transmission path and carry out dispersion compensation.
A conventional means for measuring wavelength dispersion when implementing a system, as shown in
FIG. 20
, is measuring the zero-dispersion wavelength of the optical transmission path using a PM-AM converter, and using this to determine the amount of wavelength dispersion (see M. Tomizawa, et al., “Nonlinear influence on PM-AM conversion measurement of group velocity dispersion in optical fibers”, Electron. Lett, Vol.30, No.17, pp. 1434-1435,1994).
In
FIG. 20
, CW light having a wavelength &lgr;
1
output from the monitor light generation means
71
of the optical transmitter is input into an optical phase modulation means
72
, and monitor light having applied a phase modulation of frequency &ohgr;
1
is sent to the optical transmission fiber
73
. The monitor light is sent via an optical transmission fiber
73
and an optical linear repeater
74
, amplified by the optical amplifier
75
of the optical receiver, and received by an optoelectric conversion means
76
. At this time, due to the wavelength dispersion that the monitor light undergoes in the optical transmission fiber
73
, an intensity modulation component of frequency &ohgr;
1
depending on phase modulation appears.
Because this intensity amplitude depends on the amount of wavelength dispersion the light of frequency &lgr;
1
receives over the entire optical transmission fiber, if intensity amplitude information from an electric signal output from the optoelectric conversion means
76
of the monitor clock detection means
77
is extracted, the average amount of the wavelength dispersion over the entire optical transmission fiber can be known. This information is fed back to the monitor light generation means
71
using the wavelength dispersion compensation amount control means
78
, and by carrying out the same measurement a number or times by changing the wavelength of input light, it is possible to set an arbitrary wavelength that is suited for the optical transmission fiber that has been introduced into the system. Normally, by matching the average zero dispersion wavelength of the optical transmission fiber as a whole, it is possible to minimize the amount of dispersion of the optical transmission fiber.
However, in high speed transmission systems of 40 Gbit/s or greater, adjusting dispersion equalization that optimally compensates the wavelength dispersion in real time is necessary as a measure against time dependent fluctuation of the wavelength dispersion due to temperature fluctuation. As a conventional applied dispersion equalization method used while a system is in operation, a method, as shown in
FIG. 21
, has been proposed in which monitor light having a wavelength differing from that of the signal light is wavelength multiplexed with the signal sight and transmitted, the monitor light only is separated by a wavelength filter from the wavelength multiplexed light optically split at the receiver, and the amount of wavelength dispersion is measured (see Kuwahara et al., “Study of adjusting dispersion equalization by dispersion fluctuation detection using the PM-AM conversion effect”, Electronic Information Communication Association Technical Research Report OCS 98-5 [in Japanese]).
In
FIG. 21
, the signal light generation means
81
outputs a signal light having a wavelength &lgr;
0
. The monitor light generation means
82
outputs a CW light having a wavelength of &lgr;
1
(≠&lgr;
0
), and the op
Kamatani Osamu
Nonaka Koji
Shake Ippei
Takara Hidehiko
Uchiyama Kentaro
Burns Doane Swecker & Mathis L.L.P.
Chan Jason
Nippon Telegraph and Telephone Corporation
Payne David C.
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