Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
1999-12-07
2004-03-09
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S081000, C398S029000, C398S034000
Reexamination Certificate
active
06704512
ABSTRACT:
BACKGROUND
1. Technical Field
The present invention relates to a signal maintenance and management technology in a Wavelength Division Multiplexed(WDM) transmission network. Particularly, the present invention relates to an optical channel: dispersion compensating and monitoring apparatus when an optical signal transmits from an optical transmitter to an optical receiver through an optical fiber.
And the present invention relates to a WDM optical amplifier comprising the optical channel dispersion compensating and monitoring apparatus, for a long distance.
2. Background
A WDM method is widely used for maximizing data transmission capacity by multiplexing optical signal having a plurality of wavelengths different from each other, on an optical fiber. In the WDM method where the optical signal having several channels is commonly existing, it is very important to figure out each channel state for an efficient maintenance and management of the transmission network, thereby it is essential to monitor the optical channel.
Additionally, in order to increase the data transmission capacity in the WDM method, the number of the optical channel should be increased or transmission speed of each optical channel should be improved. In case of improving the transmission speed of the optical channel, it may happen a serious signal distortion by the optical fiber dispersion as a frequency bandwidth of each optical signal becomes widen. Accordingly, it has been studied for establishing the optical channel dispersion compensating and monitoring technology in the bulk WDM transmission system.
When the optical signal is transmitted through the optical fiber, the transmission speed becomes different according to the optical wavelength. The dispersion is caused by the different transmission speed in the optical fiber. In the dispersion compensating technology, optical element(for example, a dispersion compensating optical fiber, a dispersion compensating filter, etc.) is used for compensating the optical signal distortion generated by the optical fiber dispersion, and thereby makes overall dispersion value to “0” by having a dispersion value opposite to the transmission optical fiber dispersion value.
FIG. 1
shows a configuration diagram of a common dispersion compensating device. An optical signal outputted from an optical transmitting unit
11
, passes through a Single Mode Fiber(SMF)
12
. When the optical signal passes through the SMF, the signal is distorted. A compensated optical signal is detected in an optical receiving unit
14
after passing through a dispersion compensating optical element
13
. A location of the SMF
12
and the dispersion compensating optical element
13
may be changed.
FIG. 2
shows a configuration diagram of a common optical channel monitoring device. The optical channel monitoring technology is used for obtaining information of each channel optical signal, such as optical signal power in the WDM system where optical signals having several channels are multiplexed. Accordingly, the technology is very important when improving the optical amplifier capacity as well as network operating, managing and control in the WDM system.
A portion of WDM optical signal
21
to be monitored is tapped
22
and then the tapped signal is applied to an optical channel monitoring circuit
23
. Here, the tapped signal is a little amount which does not affect a signal transmission process. The optical channel monitoring circuit
23
may have information
24
of each channel from the tapped optical signal. The information
24
of each channel indicates an optical power of each channel as an electric signal.
Now, referring to
FIGS. 3
to
4
the configuration and the operation of the conventional dispersion compensating technology is explained.
The dispersion compensating optical element
13
of
FIG. 1
can be a Dispersion Compensating Fiber(DCF), a Chirped Fiber Bragg Grating(Chirped FBG), etc.
FIG. 3
shows a schematic diagram of a conventional dispersion compensating method using the DCF. It is composed of an optical transmitting unit
31
, an SMF
34
, a DCF
36
and an optical receiving unit
38
. Wavelength elements(&lgr;
L
~&lgr;
H
) are limited in a given pulse width before being transmitted, referring to reference No.
33
. But the optical signals of the transmitting unit
31
have a widening pulse width, caused by a dispersion effect after the optical signals are transmitted through the SMF
34
(referring to No.
35
). Here, an SMF dispersion coefficient(D
SMF
(ps
m/km)) has a positive value. The DCF
36
compensates the dispersed optical signal
35
with a negative dispersion value(D
DCF
(ps
m/km<0)) and applies the compensated optical signal pulse
37
(same as the pulse
33
) to the optical receiving unit
38
. In other words, L
SMF
*D
SMF
+L
DCF
*D
DCF
≈0). The L
SMF
is a length of the SMF. The L
DCF
is a length of the DCF.
Therefore, it is capable to obtain the optical signal without distortion in the optical receiving unit
38
by setting a sum of the dispersion coefficients of the SMF
34
and the DCF
36
to be about “0”.
FIG. 4
shows a schematic diagram of a conventional dispersion compensating method using a Chirped FBG element.
A dispersion compensating device
41
is composed of a Chirped FBG element
42
for compensating channel dispersion and a circulator
43
. When making gratings on an optical fiber, the chirped FBG element
42
is manufactured to have several different gratings to regulate wavelength path to be long or short. For example, &lgr;
L
is for a long reflecting path, &lgr;
O
is for a middle path, and &lgr;
H
is for a short path. For regulating the dispersion generated in the FBG element by managing the reflective paths according to the wavelengths, a dispersion value of the Chirped FBG element is a value opposite to a dispersion value of the SMF, thereby it is capable of compensating the dispersion distortion. A sum of the dispersion value between the chirped FBG and the SMF is about “0”. Because of using the gratings, the compensated optical signal of the Chirped FBG element is a reflective wave. Accordingly, a directional optical element such as the circulator should be used for transmitting the compensated optical signal to the optical receiver.
A signal incoming to first port of the circulator
43
is a distorted optical signal having a channel(&lgr;
O
) after passed through the SMF. Reference No. 45-1 is a pulse of the optical signal of the channel(&lgr;
O
) in a time zone. And reference No. 45-2 is a spectrum of the WDM optical signal in a frequency zone.
The distorted optical signal is outputted from second port of the circulator
43
, passes through several reflective paths at each wavelength by the Chirped FBG element
42
to compensate the dispersion and then is applied to the optical receiver via third port of the circulator
43
. The optical signals except wavelength(&lgr;
L
~&lgr;
H
) in a dispersion compensating bandwidth having wavelength(&lgr;
−1
~&lgr;
1
) is passed therethrough, regardless of the FBG grating. A reference No.
47
is output signal spectrums.
Nowadays, the DCF is widely used. However, it is expected that the FBG element will be highly positioned in cost, size, and less error views according to fast development. Recently, the dispersion compensating device having an FBG element is provided for commercial usage.
Now, the configuration and the operation of the conventional optical channel monitoring method will be explained, referring to
FIGS. 5
to
6
.
FIG. 5
shows a schematic diagram of a conventional optical channel monitoring technology using a spectral element. A portion of WDM optical signal
51
to be monitored is tapped
52
and then the tapped signal is applied to a spectral element
53
which is different from the spectral element (for example, the FBG element in
FIG. 4
) for compensating dispersion. By using the spectral element
53
, the tapped signal is divided into each channel wavelength(&lgr;
1
~&lgr;
N
).
Each of channel wavelengths(&lgr;
1
~&lgr;
N
) Is converted to electric s
Fleshner & Kim LLP
LG Information & Communications Ltd.
Pascal Leslie
Payne David C
LandOfFree
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