Optical communications – Transmitter and receiver system – Including synchronization
Reexamination Certificate
2000-05-30
2003-09-16
Pascal, Leslie (Department: 2733)
Optical communications
Transmitter and receiver system
Including synchronization
C398S030000, C398S047000, C398S053000, C398S079000, C398S102000, C398S161000
Reexamination Certificate
active
06619867
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmission system utilizing wavelength multiplexing technology and in particular relates to an optical transmission system having stable transmission characteristics in the overhead portion of bit patterns comprised of a plurality of bits.
2. Description of Related Art
Along with the expanded demand for communications, greater demands are also being made for increased transmission capacity along each optical fiber. Wavelength division multiplexing is being developed to a practical level to provide greater transmission capacity. In order to provide greater transmission capacity by means of wavelength division multiplexing (WDM), problems such as attaining high speed signals, a high density placement of optical signals on a wavelength, an expanded bandwidth for the wavelength region being used, a high power optical signal and suppression of the non-linear optical effect must be dealt with.
The non-linear optical effect is an optical phenomenon caused by the non-linear response of matter and is found only in light that is nearly monochrome and has directivity such as laser light. The following are conditions known up until now under which the non-linear optical effect is prone to occur: (1) Relatively large optical power (2) Transmission in low-dispersion range of a fiber transmission path, (3) Narrow wavelength interval (4) Bit pattern matches with other channel intervals.
When using wavelength multiplexing such as WDM technology, the non-linear optical effect is easily prone to occur when the bit or bit pattern of an optical signal is phase-matched.
FIG. 29
is a block diagram of an experimental optical transmission system showing the non-linear optical effect due to phase-matching of the bit. This optical transmission system is comprised of a standard semiconductor laser
12
to output a standard laser beam
11
of a standard wavelength &lgr;
s
, and a reference semiconductor lasers
14
1
. . .
14
N
to output a reference laser beam
13
1
. . .
13
N
on a wavelengths &lgr;
1
. . . &lgr;
N
.
The standard laser beam
11
output from the semiconductor laser
12
is input to an optical modulator
16
and modulated by a specified pulse pattern output from a pulse pattern (PPG) generator
18
. After modulation, the laser light
19
is input to one input of an optical coupler
21
. The reference laser beam
13
1
. . .
13
N
output from the reference semiconductor lasers
14
1
. . .
14
N
, is input to a wavelength multiplexer
24
and subjected to wavelength multiplexing. The laser light
23
is input to an optical modulator
24
after multiplexing, and modulated by a specified pulse pattern
26
output from a pulse pattern generator
25
.
After combining in the optical coupler
22
, the laser light
28
is input to a bit correlation eliminator (DCL)
29
, then input to an optical amplifier
31
, and amplified and sent to a first transmission fiber
31
,. This laser light passes through the transmission path optical fibers
32
1
, . . .
32
k
,
32
(k+1)
as well as the optical amplifiers
31
2
, . . .
31
k
,
31
(k+1)
and is transmitted to the optical band path filter of the other (remote) party. The wavelength of the standard laser
11
and the reference laser beam
13
1
. . .
13
N
is the wavelength within the gain-bandwidth of the optical amplifiers
31
2
, . . .
31
k
,
31
(k+1)
. After only passing the wavelength &lgr; on the receive side, the laser light
35
is input to a receive circuit
36
. This receive circuit
36
is connected to an error detector
37
.
In this kind of experimental system, modulation is applied simultaneously on and overhead comprised of identical bit patterns for all wavelengths &lgr;
s
, &lgr;
1
. . . &lgr;
N
, phase-matching conditions then provided, and on/off settings made on the bit correlation eliminator (DCL)
29
in order to eliminate bit correlation in this state, and the propagation characteristics measured for respective states. The bit correlation eliminator (DCL) is comprised of dispersion compensating fiber and has a dispersion characteristic of approximately −400 ps (picoseconds) per nanometer on the wavelength bandwidth being used.
In this experiment, the type N for the reference laser beam
13
1
. . .
13
N
wavelengths is set as “11” and the figure K for the optical amplifiers
31
2
, . . .
31
k
,
31
(k+1)
is set as “3”. Also, DSF (dispersion shifted fiber) 8.0 kilometers each were used in the respective transmission path optical fibers
32
1
, . . . -
32
4
. The output levels of the
31
2
, . . .
31
4
were all +5 dBm per 1 channel.
Measurement results for the experimental optical transmission systems are shown in FIG.
30
through FIG.
32
. Of these figures,
FIG. 30
shows the code error rate during receive. The test points with X marks
41
in this figure, indicate the bit correlation eliminator
29
is off and bit correlation is not being canceled. In this state, bit phase matches are present. The test point with the “∘” marks
42
in contrast, indicate that the bit correlation eliminator
29
is on and bit correlation is being canceled. The bit phase matches are greatly reduced at this time.
FIG. 31
shows the receive optical waveform when the bit correlation eliminator
29
is off and bit matching of each wavelength &lgr;
s
, &lgr;
1
. . . &lgr;
N
, is being performed. In
FIG. 32
the receive optical waveform is shown, with the bit correlation eliminator
29
on and bit matching of each wavelength &lgr;
s
, &lgr;
1
. . . &lgr;
N
, is not being performed. In both FIG.
31
and
FIG. 32
, a waveform of 2.5 Gb/s (gigabytes per second) is shown.
The horizontal axis in
FIG. 30
shows the average power of the light level being received. Usually, the bit error rate on the vertical axis decreases when the optical signal receive level is raised. However, with the bit correlation eliminator
29
off, the bit error rate will not fall below 10
−5
, even if the receive level (power) is increased and a higher SIN (signal-to-noise) ratio for the optical signal as shown in FIG.
30
. This state is due to a non-linear optical effect in the receive optical signal itself. If the bit correlation eliminator
29
is on however, the extent of bit phase matching decreases greatly. Therefore, the error rate will be reduced to a sufficiently small value if the receive level (power) is increased and the optical signal given a higher S/N (signal-to-noise) ratio, as clearly shown by the one-bit interval change in the waveform in
FIG. 32
much more clearly than in FIG.
31
.
FIG. 33
shows the configuration of an optical transmission system utilized in the related art to eliminate effects from the non-linear optical effect. This system contains one standard clock supply device
51
. The clock signals
52
are output from the standard clock supply device
51
on an identical frequency and identical phase and are supplied to a plurality of transmit side optical transmission devices
53
1
,
53
2
, . . .
53
N
. These transmit side optical transmission devices
53
1
,
53
2
, . . .
53
N
each contain a clock interface (clock I/F)
55
, a frame processor
56
input with frame pulses
56
from these transmit side optical transmission devices
53
1
,
53
2
, . . .
53
N
, and an electrical-optical converter (E/O)
58
.
The clock interfaces
55
are circuits comprised of frequency dividers to convert the clock signal
52
supplied from the standard clock supply device
51
to a clock frequency signal appropriate for its own transmit side optical transmission devices
53
. The respective transmit side optical transmission devices
53
1
,
53
2
, . . .
53
N
require their own unique clock frequency for occasions when discrepancies exist between the signal processing contents themselves or the manufacturer. The frame processor
57
processes the externally input data signals
59
by utilizing the frame pulses
56
. The electrical signals
61
of the frame format that was generated are input to the e
NEC Corporation
Pascal Leslie
Scully Scott Murphy & Presser
Tran Dzung
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