In-band noise and/or spectral deformation measurement on polarization-multiplexed signals
View Patent ↗There is provided a method to discriminate NLE-induced signal deformation from ASE-noise on polarization multiplexed signals, in order to measure the OSNR under NLE conditions and/or characterize the NLE-induced signal deformation. In accordance with one aspect, the method is based on the acquisition of optical spectrum traces when the (data-carrying) optical communication signal is partially or completely extinguished (ASE-noise only), as well as with a live optical communication signal. Comparing traces acquired with different conditions and/or at different dates allows discrimination of the signal contribution, the ASE-noise contribution and the NLE-induced deformations on the SUT.
1. A method for characterizing an optical signal-under-test (SUT) resulting from the propagation of a polarization-multiplexed optical signal on an optical communication link subject to non-linear effects and optical add/drop multiplexing, the SUT comprising at least a data-carrying signal contribution and an Amplified Spontaneous Emission (ASE) noise contribution N ASE _ SUT within an optical signal bandwidth, the method comprising:
at a test point on said optical communication link and at a first time period:
while the polarization-multiplexed optical signal on said optical communication link is live within said optical signal bandwidth, acquiring, using a test instrument, a first commissioning optical spectrum trace P Rx _ t1 _ L (λ) of said polarization-multiplexed optical signal, said first optical spectrum trace extending over a spectral range encompassing at least a portion of said optical signal bandwidth;
while the polarization-multiplexed optical signal on said optical communication link is extinguished within said optical signal bandwidth, acquiring, using a test instrument, a second commissioning optical spectrum trace P Rx _ t1 _ E (λ) extending over a spectral range encompassing at least said portion of said optical signal bandwidth;
at said test point on said optical communication link and at a second time period:
acquiring, using a test instrument, a test optical spectrum trace P SUT (λ) of said SUT, said test optical spectrum trace extending over a spectral range encompassing at least said portion of said optical signal bandwidth;
using a processing module,
deriving, from said second commissioning optical spectrum trace P Rx _ t1 _ E (λ), a first ASE-noise level N ASE _ Rx _ t1 (λ) on said first commissioning optical spectrum trace P Rx _ t1 _ L (λ);
determining a first spectral shape trace S Rx _ t1 (λ) of said data-carrying signal contribution of said polarization-multiplexed optical signal at said first time period and at said test point using at least said first commissioning optical spectrum trace P Rx _ t1 _ L (λ) and said first ASE-noise level N ASE _ Rx _ t1 (λ) using a relation equivalent to the following equation:
S Rx _ t1 (λ)= P Rx _ t1 _ L (λ)− N ASE _ Rx _ t1 (λ);
determining, within said spectral range, a relative signal deformation k(λ) of the data-carrying signal contribution between said optical signal at said first time period and said SUT at said second time, using a relation equivalent to the following relation:
k
(
λ
)
=
[
dP
SUT
(
λ
)
/
d
λ
d
S
Rx
_
t
1
(
λ
)
/
d
λ
-
1
2
d
d
λ
(
d
P
SUT
(
λ
)
/
d
λ
dS
Rx
_
t
1
(
λ
)
/
d
λ
)
·
S
R
x
_
t
1
(
λ
)
d
S
R
x
_
t
1
(
λ
)
/
d
λ
]
;
wherein d( )/dλ denotes the derivative of corresponding function ( ) as function of wavelength λ, and wherein said relative signal deformation k(λ) is representative of a change in signal deformation induced by non-linear effects between said optical signal at said first time period and said SUT at said second time period; and
determining, within said spectral range, at least said ASE-noise contribution N ASE _ SUT of said signal-under-test at said second time period using a relation equivalent to the following relation:
N ASE _ SUT (λ)= P SUT (λ)− k (λ)· S Rx _ t1 (λ).
2. The method as claimed in claim 1 , further comprising:
at a source point upstream from said test point along said optical communication link and at said first time period:
while the polarization-multiplexed optical signal is live on said optical communication link within said optical signal bandwidth, acquiring, using a test instrument, a third commissioning optical spectrum trace P Tx _ t1 _ L (λ), said third commissioning optical spectrum trace extending over a spectral range encompassing at least said portion of said optical signal bandwidth.
3. The method as claimed in claim 1 , further comprising:
using a processing module,
determining a filtering-induced signal deformation k F (λ) on said SUT using a relation equivalent to the following relation:
k
F
(
λ
)
=
(
P
R
x
_
t
1
_
E
(
λ
)
-
N
ASE
_
res
)
(
P
R
x
_
t
1
_
E
(
λ
p
k
)
-
N
ASE
_
res
)
;
wherein λ pk denotes a central wavelength of said optical SUT and N ASE _ res denotes the first ASE-noise level N ASE _ Rx _ t1 (λ) evaluated at a wavelength within said spectral range and spaced from the central wavelength λ pk .
4. The method as claimed in claim 2 , further comprising:
using a processing module,
determining a filtering-induced signal deformation k F (λ) on said SUT using a relation equivalent to the following relation:
k
F
(
λ
)
=
(
P
R
x
_
t
1
_
E
(
λ
)
-
N
ASE
_
res
)
(
P
R
x
_
t
1
_
E
(
λ
p
k
)
-
N
ASE
_
res
)
;
wherein λ pk denotes a central wavelength of said optical SUT and N ASE _ res denotes the first ASE-noise level N ASE _ Rx _ t1 (λ) evaluated at a wavelength within said spectral range and spaced from the central wavelength λ pk .
5. The method as claimed in claim 4 , further comprising:
using a processing module,
determining a second spectral shape trace S Tx _ t1 (λ) of said data-carrying polarization-multiplexed optical signal at said first time period and at said source point, within said spectral range, using a relation equivalent to the following relation:
S Tx _ t1 (λ)= P Tx _ t1 _ L (λ)− N ASE _ Tx _ t1 (λ),
wherein N ASE _ Tx _ t1 (λ) denotes a second ASE-noise level on said third commissioning optical spectrum trace P Tx _ t1 _ L (λ);
determining a total signal deformation k t1 (λ) on said data-carrying polarization-multiplexed optical signal between said source point and said test point at said first time period, within said spectral range, by comparing said second spectral shape trace S Tx _ t1 (λ) with said first spectral shape trace S Rx _ t1 (λ) using a relation equivalent to the following relation:
k
t
1
(
λ
)
=
S
R
x
_
t
1
(
λ
)
S
T
x
_
t
1
(
λ
)
;
and
determining an NLE-induced signal deformation k NL _ t1 (λ) on said data-carrying polarization-multiplexed optical signal between said source point and said test point at said first time period, within said spectral range, by removing the filtering-induced signal deformation k F (λ) from said total signal deformation k t1 (λ) using a relation equivalent to the following relation:
k
NL
_
t
1
(
λ
)
=
k
t
1
(
λ
)
k
F
(
λ
)
·
k
0
,
wherein k 0 denotes a gain/loss factor.
6. The method as claimed in claim 5 , further comprising:
using a processing module,
determining a spectral deformation k NL _ SUT (λ) induced by Non-Linear Effects (NLE) on said SUT between said source point and said test point at said second time period by combining said NLE-induced signal deformation k NL _ t1 (λ) on said polarization-multiplexed optical signal between said source point and said test point at said first time period, to said relative signal deformation k(λ) to obtain said spectral deformation k NL _ SUT (λ) induced by Non-Linear Effects (NLE) on said SUT between said source point and said test point at said second time period using a relation equivalent to the following relation:
k
NL
_
SUT
(
λ
)
=
k
NL
_
t
1
(
λ
)
·
k
(
λ
)
k
0
.
7. The method as claimed in claim 6 , further comprising:
at a source point upstream from said test point along said optical communication link and at said first time period:
while the polarization-multiplexed optical signal is extinguished on said optical communication link within said optical signal bandwidth, acquiring, using a test instrument, a fourth commissioning optical spectrum trace P Tx _ t1 _ E (λ), said second ASE-noise level N ASE _ Tx _ t1 (λ) being obtained from said fourth commissioning optical spectrum trace P Tx _ t1 _ E (λ), said fourth commissioning optical spectrum trace extending over a spectral range encompassing at least said portion of said optical signal bandwidth.
8. The method as claimed in claim 6 , further comprising determining an NLE-induced signal deformation factor characterizing said SUT from said signal deformation k NL _ SUT (λ) induced by NLE on said data-carrying signal contribution of said SUT.
9. The method as claimed in claim 6 , further comprising determining a NLE-induced signal deformation factor SDF of said SUT at said test point from said spectral deformation k NL _ SUT (λ), said determining a NLE-induced signal deformation factor SDF of said SUT comprising calculating SDF=(<|k NL _ SUT (λ)−k NL _ SUT (λ pk )|>)/k NL _ SUT (λ pk ), where brackets <f(λ)> signify an average of function f(λ) over a given spectral range and the brackets |f(λ)| signify an absolute value operation on f(λ) and λ pk is a central wavelength of the optical SUT.
10. The method as claimed in claim 1 , further comprising determining an ASE-only Optical-Signal-to-Noise Ratio (OSNR ASE ) at least from the discriminated ASE-noise contribution N ASE _ SUT and data-carrying signal contribution S SUT (λ) of said SUT, said determining an ASE-only Optical-Signal-to-Noise Ratio (OSNR ASE ) comprising calculating
OSNR
ASE
=
10
log
10
(
∫
CBW
S
(
λ
)
d
λ
C
NBW
∫
NBW
N
ASE_
SUT
(
λ
)
d
λ
)
,
wherein ∫ x is an integral over range x, CBW is a channel bandwidth, C is a normalized resolution bandwidth and NBW is a bandwidth over which the ASE-noise is to be evaluated.
11. The method as claimed in claim 1 , further comprising:
discriminating a variation ΔN ASE of the ASE-noise contribution between said optical signal at said first time and said signal-under-test at said second time from said relative signal deformation k(λ) of the data-carrying signal contribution on said test optical spectrum trace P SUT (λ) using said first commissioning optical spectrum trace P Rx _ t1 _ L (λ).
12. A non-transitory computer readable storage medium having stored thereon computer-readable instructions that, when executed by a computer, cause the computer to perform a method for characterizing an optical signal-under-test (SUT) resulting from the propagation of a polarization-multiplexed optical signal on an optical communication link subject to non-linear effects and optical add/drop multiplexing, the SUT comprising at least a data-carrying signal contribution and an Amplified Spontaneous Emission (ASE) noise contribution N ASE _ SUT within an optical signal bandwidth, the method comprising:
obtaining a first commissioning optical spectrum trace P Rx _ t1 _ L (λ), acquired using a test instrument at a test point on said optical communication link and at a first time period, wherein said first commissioning optical spectrum trace P Rx _ t1 _ L (λ) is acquired while the polarization-multiplexed optical signal is live on said optical communication link within said optical signal bandwidth, and extends over a spectral range encompassing at least a portion of said optical signal bandwidth;
obtaining a second commissioning optical spectrum trace P Rx _ t1 _ E (λ), acquired using a test instrument at said test point on said optical communication link and at a first time period, wherein said second commissioning optical spectrum trace P Rx _ t1 _ E (λ) is acquired while the polarization-multiplexed optical signal is extinguished on said optical communication link within said optical signal bandwidth and extends over a spectral range encompassing at least a portion of said optical signal bandwidth;
obtaining a test optical spectrum trace P SUT (λ) of said SUT acquired using a test instrument at said test point on said optical communication link and at a second time period, wherein said test optical spectrum trace extends over a spectral range encompassing at least a portion of said optical signal bandwidth;
deriving, from said second commissioning optical spectrum trace P Rx _ t1 _ E (λ), a first ASE-noise level N ASE _ Rx _ t1 (λ) on said first commissioning optical spectrum trace P Rx _ t1 _ L (λ),
determining a first spectral shape trace S Rx _ t1 (λ) of said data-carrying signal contribution of said polarization-multiplexed optical signal at said first time period and at said test point using at least said first commissioning optical spectrum trace P Rx _ t1 _ L (λ) and said first ASE-noise level N ASE _ Rx _ t1 (λ) using a relation equivalent to the following equation:
S Rx _ t1 (λ)= P Rx _ t1 _ L (λ)− N ASE _ Rx _ t1 (λ);
determining, within said spectral range, a relative signal deformation k(λ) of the data-carrying signal contribution between said optical signal at said first time period and said SUT at said second time, using a relation equivalent to the following relation:
k
(
λ
)
=
[
d
P
SUT
(
λ
)
/
d
λ
dS
R
x
_
t
1
(
λ
)
/
d
λ
-
1
2
d
d
λ
(
d
P
SUT
(
λ
)
/
d
λ
d
S
Rx
_
t
1
(
λ
)
/
d
λ
)
·
S
R
x
_
t
1
(
λ
)
d
S
R
x
_
t
1
(
λ
)
/
d
λ
]
;
wherein d( )/dλ is the derivative of corresponding function ( ) as function of wavelength λ, and wherein said relative signal deformation k(λ) is representative of a change in signal deformation induced by non-linear effects between said optical signal at said first time period and said SUT at said second time period; and
determining, within said spectral range, at least said ASE-noise contribution N ASE _ SUT of said SUT at said second time period using a relation equivalent to the following relation:
N ASE (λ)= P SUT (λ)− k (λ)· S Rx _ t1 (λ).
13. The non-transitory computer readable storage medium as claimed in claim 12 , wherein the method further comprises:
obtaining a third commissioning optical spectrum trace P Tx _ t1 _ E (λ) acquired using a test instrument at a source point upstream from said test point along said optical communication link and at said first time period, while the polarization-multiplexed optical signal is extinguished on said optical communication link within said optical signal bandwidth, said third commissioning optical spectrum trace extending over a spectral range encompassing at least a portion of said optical signal bandwidth;
obtaining a fourth commissioning optical spectrum trace P Tx _ t1 _ L (λ) acquired using a test instrument at said source point and at said first time period, while the polarization-multiplexed optical signal is live on said optical communication link within said optical signal bandwidth, said fourth commissioning optical spectrum trace extending over a spectral range encompassing at least a portion of said optical signal bandwidth; and
determining a signal deformation k NL _ SUT (λ) induced by Non-Linear Effects (NLE) on said SUT between said source point and said test point at said second time period, within said spectral range, using a relation equivalent to the following relation:
k
NL
_
SUT
(
λ
)
=
k
(
λ
)
·
P
R
x
_
t
1
_
L
(
λ
)
-
N
ASE
_
R
x
_
t
1
(
λ
)
P
T
x
_
t
1
_
L
(
λ
)
-
N
ASE
_
T
x
_
t
1
(
λ
)
·
(
P
R
x
_
t
1
_
E
(
λ
p
k
)
-
N
ASE
_
res
)
(
P
T
x
_
t
1
_
E
(
λ
)
-
N
ASE
_
res
)
,
wherein N ASE _ Rx _ t1 (λ) denotes a first ASE-noise level on said first commissioning optical spectrum trace P Rx _ t1 _ L (λ), N ASE _ Tx _ t1 (λ) denotes a second ASE-noise level on said fourth commissioning optical spectrum trace P Tx _ t1 _ L (λ), λ pk denotes a central wavelength of said optical SUT, and N ASE _ res denotes the first ASE-noise level N ASE _ Rx _ t1 (λ) evaluated at a wavelength within said spectral range and spaced from the central wavelength λ pk .