Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
2000-07-05
2003-04-15
Cuneo, Kamand (Department: 2829)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
C324S1540PB
Reexamination Certificate
active
06549018
ABSTRACT:
The invention is directed to a wavelength-division multiplex system and to a method for measuring the individual crosstalk of a specific, of a plurality or of all channels on a payload channel in optical transmission systems, particularly in wavelength-division multiplex systems, comprising n WDM channels, an input-side multiplexer that combines the n WDM channels supplied to it with different frequencies, a following transmission link and an output-side demultiplexer that respectively divides an optical signal incoming on a fiber onto n WDM channels.
In optical transmission systems, the technique of wavelength-division multiplex (WDM) is a frequently utilized method for utilizing the greatest bandwidth of the optical fiber. Within the framework of network monitoring of such systems, it is often desirable to know the crosstalk behavior of all channels on a payload channel being observed. The knowledge of the crosstalk of only one defined channel on the payload channel being observed or the statement about the proportion with which the individual channels participate in the overall crosstalk can also be significant.
For work in the laboratory, there is the solution that the optical spectrum is registered with an extremely great precision. Lines appear in the spectrum at the locations of the crosstalking channels, but these can only be resolved with great outlay. The crosstalk can be identified by measuring these lines and the signal power of the payload channel. This solution is so involved that it can only be employed in the laboratory. It is too complicated and too expensive or, respectively, not economically feasible for an employment in general operations.
It is therefore an object of the invention to develop a method for measuring the individual crosstalk of a specific, or a plurality or of all channels on a payload channel in wavelength-division multiplex systems that is significantly simplified.
Moreover, a wavelength-division multiplex system for application of the inventive method is presented.
The object of developing a method is achieved by the features of the first method claim; the object of developing a wavelength-division multiplex system is achieved by the features of the first apparatus claim.
The inventors have recognized that, given a frequency impressed onto an optical payload signal, the part of the photocurrent of the payload signal this is picked up by a photodiode and is synchronous with the impressed frequency derives as
I
Ph
—
synch,channel
=m·R
Ph
·T
DEMUX
·P
Signal
—
opt,channel
(1)
with the transmission of the demultiplexer T
DEMUX
, the responsitivity of the photodiode R
Ph
, the modulation factor m and the optical signal power of the WDM channel P
Signal
—
opt,channel
.
The synchronous parts of each individual crosstalking signal (n) are calculated from
I
Ph
—
synch,frequency
(n)
=m
(n)
·R
Ph
·T
DEMUX
·P
Signal
—
opt,frequency
(n)
(2)
with the corresponding modulation factor m
(n) and the optical signal power of the corresponding WDM channel P
Signal
—
opt,frequency
(n)
.
The non-synchronous part of the photocurrent is composed of the non-modulated signal levels of the payload channel (1-m
(n)
) P
Signal
—
opt,frequency
, the optical noise P
noise
—
opt
and the photodiode dark current I
D
—
Ph
. The non-synchronous part derives as
I
Ph_nonsynch
=
R
Ph
·
T
DEMUX
·
[
(
∑
n
=
1
M
-
1
(
1
-
m
)
(
n
)
)
·
P
Signal_opt
,
frequency
(
n
)
)
+
(
1
-
m
)
P
Signal_opt
,
channel
+
NB
DEMUX
·
P
Noise_opt
]
+
I
D_ph
(
3
)
with the effective noise bandwidth of the optical demultiplexer NB
DEMUX
.
This non-synchronous part can also be utilized for determining the optical signal-to-noise ratio (OSNR).
The optical signal power of the WDM payload channel derives from (1) as
P
Signal_opt
,
channel
=
I
Ph_synch
,
channel
m
·
R
Ph
·
T
DEMUX
(
4
)
The optical signal power of the individual crosstalking channels derives from (2) as
P
Signal_opt
,
frequency
(
n
)
=
I
Ph_synch
,
frequency
(
n
)
m
(
n
)
·
R
Ph
·
T
DEMUX
(
5
)
The crosstalk (CT) of an arbitrary channel on the payload channel is defined from
CT
Frequency
(
n
)
↔
channel
=
10
·
log
(
P
Signal_opt
,
frequency
(
n
)
P
Signal_opt
,
channel
)
(
6
)
Consequently following with (4) and (5) is
CT
Frequency
(
n
)
↔
Channel
=
10
·
log
(
I
Ph_synch
,
frequency
(
n
)
I
Ph_synch
,
channel
·
m
m
(
n
)
)
(
7
)
It is meaningful, although not necessary, to employ the same modulation factor for all channels. Equation (7) is then simplified into
CT
Frequency
(
n
)
↔
Channel
=
10
·
log
(
I
Ph_synch
,
frequency
(
n
)
I
Ph_synch
,
channel
)
(
8
)
Only the ratio of the synchronous currents thus need be considered for the calculation of the crosstalk. The crosstalk calculated by Equation (8) thus reproduces the influence of a defined channel on the payload channel.
When all influencing channels are to be taken into consideration, then the following is valid for different modulation factors and M crosstalking channels
CT
Frequency
↔
Channel
=
10
·
log
(
(
∑
n
=
1
M
-
1
I
Ph_synch
,
frequency
(
n
)
m
(
n
)
·
m
)
I
Ph_synch
,
channel
)
(
9
)
The following is valid for identical modulation factors
CT
Frequency
↔
Channel
=
10
·
log
(
∑
n
=
1
M
-
1
I
Ph_synch
,
frequency
(
n
)
I
Ph_synch
,
channel
)
(
10
)
Since no component-dependent quantities are utilized for the calculation, the determination of the crosstalk is independent of the generation of the identification frequency as well as of the components needed for the evaluation.
In conformity with the above perceptions, the inventors, according to claim 1, propose that a method for measuring the individual crosstalk of a specific, of a plurality or of all channels on a payload channel in optical transmission systems, particularly in wavelength-division multiplex systems, comprising n WDM channels, an input-side multiplexer that combines the n WDM channels supplied to it with different frequencies, a following transmission link and an output-side demultiplexer that respectively divides an optical signal incoming on a fiber onto n WDM channels be improved to the effect that n identification frequencies f
(n)
are modulated onto a payload signal with a modulation factor m
(n)
and the current crosstalk is calculated.
The current crosstalk can be calculated by coupling a part of the optical power out of the transmission path.
The current crosstalk can also be calculated at the end of the transmission path.
For determining the crosstalk, a low-frequency identification frequency f
(n)
with the modulation index m
(n)
can be additionally modulated onto the high-frequency payload signal. This identification frequency is unambiguously measurably lower in frequency than the payload signal, namely such that it does not influence the behavior of other components in the transmission path such as, for example, fiber amplifiers (EDFAs—erbium doped fiber amplifiers).
The identification frequency differs for each WDM channel in the system.
These n different identification frequencies can, for example, be generated by variable optical attenuator.
Different identification frequencies can also be generated by low-frequency, direct modulation of the lasers or, respectively, of the transmission diode. Further, Mach-Zehnder or some other modulators can also be employed for impressing the identification frequencies.
It can also be advantageous to derive the identification frequency from the high-precision system clock of the transmission system that is also available to other network elements. Further, the identification frequency can be derived from the data rate, for example given SDH signals (SDH=simultaneous data hierarchy), so that a phase-locked reference can be made available for the lock-in amplifiers.
The various optical signals are superimposed on a fibe
Bleck Oliver
Fricke Andres
Wiesmann Rainer
Bell Boyd & Lloyd LLC
Cuneo Kamand
Nguyen Trung
Siemens Aktiengesellschaft
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