IEC 61280-2-9:2009
(Main)Fibre optic communication subsystem test procedures - Part 2-9: Digital systems - Optical signal-to-noise ratio measurement for dense wavelength-division multiplexed systems
Fibre optic communication subsystem test procedures - Part 2-9: Digital systems - Optical signal-to-noise ratio measurement for dense wavelength-division multiplexed systems
IEC 61280-2-9:2009 provides a parameter definition and a test method for obtaining optical signal-to-noise ratio (OSNR) using apparatus that measures the optical spectrum at a multichannel interface. Because noise measurement is made on an optical spectrum analyzer, the measured noise does not include source relative intensity noise (RIN) or receiver noise. Three implementations for an optical spectrum analyser (OSA) are discussed:
- a diffraction-grating-based OSA,
- a Michelson interferometer-based OSA,
- and a Fabry-Perot-based OSA.
This second addition cancels and replaces the first edition published in 2002 and constitutes a technical revision. The main changes from the previous edition are as follows:
- A paragraph has been added to the Scope describing the limitations due to signal spectral width and wavelength filtering;
- and Annex B has been added to further explain error in measuring noise level due to signal spectral width and wavelength filtering.
Procédures d'essai des sous-systèmes de télécommunications à fibres optiques - Partie 2-9: Systèmes numériques - Mesure du rapport signal sur bruit optique pour les systèmes multiplexés à répartition en longueur d'onde dense
La CEI 61280-2-9:2009 fournit une définition de paramètres et une méthode d'essai pour obtenir un rapport signal sur bruit optique (OSNR) en utilisant un appareillage destiné à mesurer le spectre optique au niveau d'une interface multivoie. Du fait qu'une mesure de bruit est effectuée sur un analyseur de spectre optique, le bruit mesuré n'inclut pas le bruit à intensité relatif de la source (RIN) ou le bruit de récepteur. Trois réalisations relatives aux analyseurs de spectre optique (OSA) sont en cours de discussion:
- un OSA réalisé à partir d'un réseau de diffraction,
- un OSA réalisé à partir d'un interféromètre Michelson,
- et un OSA réalisé à partir d'un Fabry-Perot.
Cette deuxième édition annule et remplace la première édition parue en 2002 et constitue une révision technique. Les changements principaux par rapport à la première édition sont les suivants:
- Un paragraphe a été ajouté au Domaine d'application, décrivant les limites dues à la largeur spectrale du signal et au filtrage en longueur d'onde;
- et L'Annexe B a été ajoutée, afin de mieux expliquer les erreurs survenant lors de la mesure du niveau de bruit dû à la largeur spectrale du signal et au filtrage en longueur d'onde.
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IEC 61280-2-9 ®
Edition 2.0 2009-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic communication subsystem test procedures –
Part 2-9: Digital systems – Optical signal-to-noise ratio measurement for dense
wavelength-division multiplexed systems
Procédures d'essai des sous-systèmes de télécommunications
à fibres optiques –
Partie 2-9: Systèmes numériques – Mesure du rapport signal sur bruit optique
pour les systèmes multiplexés à répartition en longueur d'onde dense
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IEC 61280-2-9 ®
Edition 2.0 2009-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic communication subsystem test procedures –
Part 2-9: Digital systems – Optical signal-to-noise ratio measurement for dense
wavelength-division multiplexed systems
Procédures d'essai des sous-systèmes de télécommunications
à fibres optiques –
Partie 2-9: Systèmes numériques – Mesure du rapport signal sur bruit optique
pour les systèmes multiplexés à répartition en longueur d'onde dense
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
S
CODE PRIX
ICS 33.180.20 ISBN 978-2-88910-477-2
– 2 – 61280-2-9 © IEC:2009
CONTENTS
FOREWORD.4
INTRODUCTION.6
1 Scope.7
2 Normative references .8
3 Definition.8
4 Apparatus.9
4.1 General .9
4.2 Diffraction grating-based OSA .9
4.3 Michelson interferometer-based OSA .10
4.4 Fabry-Perot-based OSA .10
4.5 OSA performance requirements .11
4.5.1 General .11
4.5.2 Wavelength range .11
4.5.3 Sensitivity.11
4.5.4 Resolution bandwidth (RBW) .11
4.5.5 Resolution bandwidth accuracy .12
4.5.6 Dynamic range .12
4.5.7 Scale fidelity.13
4.5.8 Polarization dependence .13
4.5.9 Wavelength data points .13
5 Sampling and specimens.13
6 Procedure .13
7 Calculations .14
8 Measurement uncertainty .14
9 Documentation .14
Annex A (informative) Error in measuring signal level due to signal spectral width.16
Annex B (informative) Error in measuring noise level due to signal spectral width and
wavelength filtering.19
Bibliography.21
Figure 1 – A typical optical spectrum at an optical interface in a multichannel
transmission system .8
Figure 2 – The OSNR for each channel as derived from direct measurements of the
optical spectrum .9
Figure 3 – A diffraction grating-based OSA .10
Figure 4 – A Michelson interferometer-based OSA.10
Figure 5 – A Fabry-Perot-based OSA.11
Figure 6 – Illustration of insufficient dynamic range as another source of measurement
uncertainty.
Figure A.1 – The power spectrum of a 10 Gb/s, 2 − 1 PRBS signal showing the
considerable amount of power not captured in a 0,1 nm RBW with 0,64 nm filtering
after the signal.17
Figure A.2 – The spectrum of a 2,5 Gb/s 2 − 1 PRBS with 0,36 nm filtering with
considerably less power outside the 0,1 nm OSA RBW .
Figure A.3 – Signal power error versus RBW for a 10 Gb/s modulated signal.18
61280-2-9 © IEC:2009 – 3 –
Figure A.4 – Signal power error versus RBW for a 2,5 Gb/s modulated signal.18
Figure B.1 – Example for noise filtering between channels for a 200 GHz grid .20
Table A.1 – Filtering used in simulation to determine signal power level error.16
Table A.2 – RBW to achieve less than 0,1 dB error in signal power .18
– 4 – 61280-2-9 © IEC:2009
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC COMMUNICATION SUBSYSTEM TEST PROCEDURES –
Part 2-9: Digital systems –
Optical signal-to-noise ratio measurement
for dense wavelength-division multiplexed systems
FOREWORD
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61280-2-9 has been prepared by subcommittee 86C: Fibre optic
systems and active devices, of IEC technical committee 86: Fibre optics
This second addition cancels and replaces the first edition published in 2002 and constitutes
a technical revision. The main changes from the previous edition are as follows:
• A paragraph has been added to the Scope describing the limitations due to signal spectral
width and wavelength filtering.
• Annex B has been added to further explain error in measuring noise level due to signal
spectral width and wavelength filtering.
61280-2-9 © IEC:2009 – 5 –
The text of this standard is based on the following documents:
CDV Report on voting
86C/823/CDV 86C/864/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all the parts in the IEC 61280 series, under the general title Fibre optic
communication subsystem test procedures, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – 61280-2-9 © IEC:2009
INTRODUCTION
At the optical interfaces within wavelength-division multiplexed (WDM) networks, it is
desirable to measure parameters that provide information about the integrity of the physical
plant. Such parameters are necessary to monitor network performance as an integral part of
network management. They are also necessary to assure proper system operation for
installation and maintenance of the network.
Ideally, such parameters would directly correspond to the bit error ratio (BER) of each
channel of a multichannel carrier at the particular optical interface. Related parameters such
as Q-factor or those calculated from optical eye patterns would provide similar information,
that is, they would correlate to the channel BER. However, it is difficult to obtain access to
these parameters at a multichannel interface point. It is necessary to demultiplex the
potentially large number of channels and make BER, Q-factor, or eye-diagram measurements
on a per-channel basis.
In contrast, useful information about the optical properties of the multichannel carrier is
readily obtained by measuring the optical spectrum. Wavelength-resolved signal and noise
levels provide information on signal level, signal wavelength, and amplified spontaneous
emission (ASE) for each channel. Spectral information, however, does not show signal
degradation due to wave-shape impairments resulting from polarization-mode dispersion
(PMD), and chromatic dispersion. Also, intersymbol interference and time jitter are not
revealed from an optical signal to noise ratio (OSNR) measurement. In spite of these
limitations, OSNR is listed as an interface parameter in ITU-T Rec. G.692 [1] , as an optical
monitoring parameter in ITU-T Rec. G.697 [2] and in ITU-T G Rec. Sup. 39 [3].
___________
Figures in brackets refer to the bibliography.
61280-2-9 © IEC:2009 – 7 –
FIBRE OPTIC COMMUNICATION SUBSYSTEM TEST PROCEDURES –
Part 2-9: Digital systems –
Optical signal-to-noise ratio measurement
for dense wavelength-division multiplexed systems
1 Scope
This part of IEC 61280 provides a parameter definition and a test method for obtaining optical
signal-to-noise ratio (OSNR) using apparatus that measures the optical spectrum at a
multichannel interface. Because noise measurement is made on an optical spectrum analyzer,
the measured noise does not include source relative intensity noise (RIN) or receiver noise.
Three implementations for an optical spectrum analyser (OSA) are discussed: a diffraction-
grating-based OSA, a Michelson interferometer-based OSA, and a Fabry-Perot-based OSA.
Performance characteristics of the OSA that affect OSNR measurement accuracy are
provided.
A typical optical spectrum at a multichannel interface is shown in Figure 1. Important
characteristics are as follows.
• The channels are placed nominally on the grid defined by ITU Recommendation
G.694.1.[4]
• Individual channels may be non-existent because it is a network designed with optical
add/drop demultiplexers or because particular channels are out of service.
• Both channel power and noise power are a function of wavelength.
For calculating the OSNR, the most appropriate noise power value is that at the channel
wavelength. However, with a direct spectral measurement, the noise power at the channel
wavelength is included in the signal power and is difficult to extract. An estimate of the
channel noise power can be made by interpolating the noise power value between channels.
The accuracy of estimating the noise power at the signal wavelength by interpolating the
noise power at an offset wavelength can be significantly reduced when the signal spectrum
extends into the gap between the signals and when components such as add-drop
multiplexers along the transmission span modify the spectral shape of the noise. These
effects are discussed in further detail in Annex B, and can make the method of this document
unusable for some situations. In such cases, where signal and noise cannot be sufficiently
separated spectrally, it is necessary to use more complex separation methods, like
polarization or time-domain extinction, or to determine signal quality with a different
parameter, such as RIN. This is beyond the scope of the current document.
– 8 – 61280-2-9 © IEC:2009
Missing channels
Channels on
the ITU grid
Noise
Wavelength
IEC 2407/02
Figure 1 – Typical optical spectrum at an optical interface
in a multichannel transmission system
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 61290-3-1, Optical amplifiers – Test methods – Part 3-1: Noise figure parameters –
Optical spectrum analyzer method
IEC 62129, Calibration of optical spectrum analyzers
3 Terms and definitions
For the purposes of this document, the following terms and definition apply.
3.1
optical signal-to-noise ratio
OSNR
ratio in decibels, from the optical spectrum, defined by the equation
P B
i m
OSNR = 10Log + 10Log dB, (1)
N B
i r
where
P is the optical signal power, in watts, at the i-th channel,
i
B is the reference optical bandwidth, and
r
Optical power
61280-2-9 © IEC:2009 – 9 –
N is the interpolated value of noise power, in watts, measured in the noise equivalent
i
bandwidth, B , given by
m
N(λ − Δλ) + N(λ + Δλ)
i i
N = (2)
i
at the i-th channel, where
λ is the wavelength of the i-th channel, and
i
Δλ is the interpolation offset equal to or less than one-half of the ITU grid spacing.
(The units for B and B may be in frequency or wavelength but must be consistent.)
m r
Typically, the reference optical bandwidth is 0,1 nm. See Figure 2.
NOTE The noise equivalent bandwidth of a filter is such that it would pass the same total noise power as a
rectangular passband that has the same area as the actual filter, and the height of which is the same as the height
of the actual filter at its centre wavelength.
P + N
i i
N(λ – Δλ)
i
N
i
N(λ + Δλ)
i
IEC 2408/02
Figure 2 – OSNR for each channel as derived
from direct measurements of the optical spectrum
4 Apparatus
4.1 General
The required apparatus is an optical spectrum analyzer (OSA) with the performance
necessary to measure the signal and noise powers required for Equation (1). Three common
ways to implement an OSA are with a diffraction grating, a Michelson interferometer, and a
Fabry-Perot etalon.
4.2 Diffraction grating-based OSA
A simplified diagram of a diffraction grating-based OSA is shown in Figure 3. The expanded
input light is incident on a rotatable diffraction grating. The diffracted light comes off at an
angle proportional to wavelength and passes through an aperture to a photodetector. The size
of the input and output apertures and the size of the beam on the diffraction grating determine
the spectral width of the resulting filter and therefore the resolution of the OSA. A/D
conversion and digital processing provide the familiar OSA display.
– 10 – 61280-2-9 © IEC:2009
Diffraction
grating
Light input
A/D
Digital
converter
processing
Slit
Display
Photodiode IEC 2409/02
Figure 3 – Diffraction grating-based OSA
4.3 Michelson interferometer-based OSA
Another type of OSA is based on the Michelson interferometer as shown in Figure 4. The
input signal is split into two paths. One path is fixed in length and one is variable. The
Michelson interferometer creates an interference pattern between the signal and a delayed
version of itself at the photodetector. The resulting waveform, called an interferogram, is the
autocorrelation of the input signal. A Fourier transform performed on the autocorrelation
provides the optical spectrum. The resolution of this type of OSA is set by the differential path
delay of the interferometer.
Photodiode
Digital A/D
processing converter
Display
Beam splitter
Light input
Fixed mirror
Movable mirror
IEC 2410/02
Figure 4 – Michelson interferometer-based OSA
4.4 Fabry-Perot-based OSA
A third type of OSA is based on a Fabry-Perot etalon as shown in Figure 5. The collimated
beam passes through a Fabry-Perot etalon, the free spectral range (FSR) of which is greater
than the channel plan and the finesse is chosen to give the required resolution bandwidth
(RBW). Piezo-electric actuators control the Fabry-Perot mirror spacing and provide spectral
tuning. Digital signal processing provides any combination of spectral display or tabular data.
61280-2-9 © IEC:2009 – 11 –
Display
A/D
spectrum
DSP
converter
or table
Light input
Fabry-Perot etalon
Photodiode
IEC 2411/02
Figure 5 – Fabry-Perot-based OSA
4.5 OSA performance requirements
4.5.1 General
Refer to IEC 62129 for calibration details.
4.5.2 Wavelength range
The wavelength range shall be sufficient to cover the channel plan plus one-half grid spacing
on each end of the band to measure the noise of the lowest and highest channels.
4.5.3 Sensitivity
The sensitivity of an OSA is defined as the lowest level at which spectral power can be
measured with a specified accuracy. The OSA sensitivity must be sufficient to measure the
lowest expected noise level. In terms of OSNR,
Required sensitivity (dBm) = Minimum channel level (dBm) – OSNR (dB) (3)
For example, the sensitivity required for a minimum channel level of –10 dBm in order to
measure a 35-dB OSNR is
–10 dBm – 35 dBm = –45 dBm
4.5.4 Resolution bandwidth (RBW)
The relationship of the measured peak power to the total signal power depends on the
spectral characteristics of the signal and the resolution bandwidth. The resolution bandwidth
must be sufficiently wide to accurately measure the power level of each modulated channel.
The proper RBW setting depends on the bit rate. For example, the signal power of a laser
modulated at an OC-192 (STM-64) rate with zero chirp will measure 0,8 dB lower with a 0,1-
nm RBW than with a wide RBW. This results from the modulation envelope having a portion of
its spectral power outside of the 0,1-nm RBW. If the RBW is decreased to 0,05 nm, the signal
power will measure 2,5 dB lower. This effect is made worse by the presence of laser chirp and
lessened by additional bandwidth limiting in the transmitter laser’s modulation circuitry. This
subject is treated in more detail in Annex A.
– 12 – 61280-2-9 © IEC:2009
When the signal spreads spectrally into the range between the channels, as due to high
modulation rates, then the resolution must be sufficiently narrow to exclude the signal power
from the noise measurement enough to allow the desired accuracy for the given level of
noise. For example in the above case, if the OC-192 (STM-64) signals are spaced 0,2 nm
apart (25 GHz grid), then the spectral power outside an 0,1-nm-RBW signal measurement
would all be included in the noise measurement with 0,1-nm RBW. This 17 % of the signal
power would result in a best measurable OSNR of only about 7 dB. The topic is also
discussed in Annex B.
4.5.5 Resolution bandwidth accuracy
The accuracy of the noise measurement is directly impacted by the accuracy of the OSA’s
RBW. For best accuracy, the OSA’s noise equivalent bandwidth, B , must be calibrated.
m
RBW, in general, differs from B due to the non-rectangular shape of the optical spectrum
M
analyzer’s filter characteristic. The procedure for calibrating B is given in IEC 61290-3-1,
m
where it is referred to as optical bandwidth.
4.5.6 Dynamic range
The dynamic range of an OSA is a measure of the OSA’s ability to make measurements of
low-level signals and noise that are close in wavelength to large signals. It is important to
note that narrowing the RBW does not necessarily correlate to better dynamic range. RBW is
a measure of the 3-dB bandwidth or noise equivalent bandwidth of its filter characteristic.
Dynamic range, on the other hand, is a measure of the steepness of the filter characteristic
and the OSA noise floor. Dynamic range is defined as the ratio, in dB, of the filter
transmission characteristic at the centre wavelength, λ , and at one-half a grid spacing away,
i
λ ± Δλ.
I
Figure 6 shows two channels of a multichannel spectrum, the OSA filter characteristic, the
OSA sensitivity limit, and the transmission system noise that is to be measured. At the noise
measurement wavelength, the dynamic range must be significantly higher than the OSNR for
accurate measurements. The uncertainty contribution can be predicted from the following
equation:
–D/10
Uncertainty in OSNR = 10 log(1+10 ) dB, (4)
where D is the value in dB by which the OSA dynamic range exceeds the actual OSNR. For
example, for an OSNR of 30 dB, a dynamic range of 40 dB (at ½ the ITU grid spacing) will
cause an error of 0,42 dB.
61280-2-9 © IEC:2009 – 13 –
ITU grid spacing
OSA filter
OSA filter
shape Signal
shape
Signal
OSA dynamic range Transmission system
at 1/2 a grid spacing noise
OSA sensitivity limit
IEC 2412/02
Figure 6 – Illustration of insufficient dynamic range
as another source of measurement uncertainty
In general, either the OSA sensitivity limit or dynamic range will limit the value of OSNR that
can be measured. Typically, a Michelson interferometer-based OSA will be limited by the
sensitivity limit and a diffraction grating-based OSA by the dynamic range.
4.5.7 Scale fidelity
Scale fidelity, also called display linearity, is the relative error in amplitude that occurs over a
range of input power levels. Scale fidelity directly contributes to the OSNR measurement
uncertainty.
4.5.8 Polarization dependence
Typically, the signal, P will be highly polarized while the noise, N is unpolarized. OSA
i i
polarization dependence will directly contribute to uncertainty in signal measurement.
4.5.9 Wavelength data points
The minimum number of data points collected by the OSA shall be at least twice the
wavelength span divided by the noise equivalent bandwidth.
5 Sampling and specimens
The device under test (DUT) is a multichannel fibre-optic transmission system or network. The
measurement apparatus is connected to the network at any point by directly connecting to the
optical fibre or via a broadband monitoring port. Measurement points following wavelength-
selective components such as an add-drop multiplexer may be inappropriate due to the noise
filtering effect described in Annex B.
6 Procedure
a) Connect the OSA to the transmission fibre or a monitor port.
b) Choose RBW values sufficiently wide to accurately measure the signal power and with
sufficient dynamic range to measure the noise at ±Δλ from the peak channel wavelength
– 14 – 61280-2-9 © IEC:2009
where Δλ is half the ITU grid spacing or less if this gives a more accurate OSNR value due
to noise filtering. (See annexes, Table A.2 and Subclause 4.5.6.)
c) Set the wavelength range to accommodate all channels plus at least a half grid spacing
below the lowest channel and above the highest channel.
d) Measure the power level at the signal peak for the i-th of n channels. This value is P +N
i i
(refer to Figure 2).
e) Measure the noise at ±Δλ from the signal peak wavelength. Use a calibrated RBW with
noise equivalent bandwidth, B . The measured values are N(λ -Δλ) and N(λ +Δλ).
m
i i
f) Calculate the interpolated value of noise at each channel wavelength (Equation (2)):
N(λ − Δλ) + N(λ + Δλ)
i i
N = (5)
i
g) Calculate P by subtracting N from the value obtained in step d).
i i
h) Repeat steps d) through g) for all n channels.
NOTE This procedure may be done with two RBW settings: one that is sufficiently wide to measure total signal
λ from the peak channel wavelengths.
power, the second with sufficient dynamic range to measure noise at ±Δ
7 Calculations
• For each of the n channels, calculate the interpolated value of noise power, N, using
i
step f) and P using step g) in Clause 6.
i
• For each of the n channels, calculate OSNR from Equation (1).
P B
i m
OSNR = 10 Log + 10 Log (6)
N B
i r
8 Measurement uncertainty
Measurement uncertainty should be calculated based upon the ISO/IEC Guide to the
expression of uncertainty in measurement. [5]
Uncertainty contributions that must be considered are as follows:
• modulated signal power (4.5.4 and Annex A);
• OSA noise bandwidth (4.5.5);
• OSA dynamic range (4.5.6);
• OSA scale fidelity (4.5.7);
• OSA polarization dependence (4.5.8).
9 Documentation
Report the following information for each test:
• test date
• this standard number
• identification of the transmission system being tested and the test location
• description of the equipment used
• OSNR data
• OSA noise equivalent bandwidth, B
m
• reference bandwidth, B
r
61280-2-9 © IEC:2009 – 15 –
• offset wavelength for noise measurement, Δλ, and ITU grid spacing
• measurement uncertainty
– 16 – 61280-2-9 © IEC:2009
Annex A
(informative)
Error in measuring signal level due to signal spectral width
The spectral width of each channel is broadened from that of the CW laser due to several
causes:
• laser chirp
• intensity modulation for signal transmission
• modulation to suppress stimulated Brillouin scattering (SBS)
• self-phase modulation (SPM)
• cross-phase modulation
For dense WDM systems in which external modulation is generally used, laser chirp is not a
factor. Broadening due to SBS suppression and SPM are typically small compared to the
broadening due to the signal modulation at 2,5 Gb/s and higher rates.
Figures A.1 and A.2 show the calculated spectra of an intensity modulated laser for 10 Gb/s
and 2,5 Gb/s line rates respectively. The modulation is an NRZ PRBS with a word length of
2 -1. Optical and electrical filtering values are indicated in Table A.1. For reference, a typical
OSA filter characteristic for a 0,1-nm RBW is also shown.
Because a portion of the signal power is not captured by the OSA, an error in the measured
signal power occurs. Figures A.3 and A.4 show the magnitude of the error for 10 Gb/s and
2,5 Gb/s data rates respectively.
Table A.1 – Filtering used in simulation to determine signal power level error
Modulation rate 10 Gb/s 2,5 Gb/s
Electrical filter bandwidth 30 GHz 7,5 GHz
Optical filter bandwidth 0,64 nm 0,36 nm
61280-2-9 © IEC:2009 – 17 –
OSA’s 0,1 nm RBW
Power spectrum
–10
of 10 Gb/s
modulated signal
–20
–30
dB
–40
–50
–60
–70
–80
–90
1 549,5 1 549,6 1 549,7 1 549,8 1 549,9 1 550 1 550,1 1 550,2 1 550,3 1 550,4 1 550,5
Wavelength nm
IEC 2413/02
Figure A.1 – Power spectrum of a 10 Gb/s, 2 −
1 PRBS signal showing the considerable amount of power not captured
in a 0,1 nm RBW with 0,64 nm filtering after the signal
OSA’s 0,1 nm RBW
–10
–20
Power spectrum
–30
of 2,5 Gb/s
–40
modulated signal
dB
–50
–60
–70
–80
–90
1 549,5 1 549,6 1 549,7 1 549,8 1 549,9 1 550 1 550,1 1 550,2 1 550,3 1 550,4 1 550,5
Wavelength nm
IEC 2414/02
Figure A.2 – Spectrum of a 2,5 Gb/s 2 −
1 PRBS with 0,36 nm filtering with considerably less
power outside the 0,1 nm OSA RBW
– 18 – 61280-2-9 © IEC:2009
–1
–2
–3
–4
–5
–6
0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 0,18 0,2
RBW nm
IEC 2415/02
Figure A.3 – Signal power error versus RBW for a 10 Gb/s modulated signal
–0,2
–0,4
–0,6
–0,8
–1,0
–1,2
–1,4
0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 0,18 0,2
RBW nm
IEC 2416/02
Figure A.4 – Signal power error versus RBW for a 2,5 Gb/s modulated signal
To minimize the error in the signal power measurement, a resolution bandwidth of sufficient
width should be chosen. Table A.2 shows the RBW values that cause less than 0,1 dB error.
Table A.2 – RBW to achieve less than 0,1 dB error in signal power
Modulation rate 10 Gb/s 2,5 Gb/s or lower
RBW
≥0,2 nm ≥0,09 nm
Signal power error dB
Signal power error dB
61280-2-9 © IEC:2009 – 19 –
Annex B
(informative)
Error in measuring noise level due to signal
spectral width and wavelength filtering
The same signal spectral width discussed in Annex A can also influence the uncertainty of
measuring the noise level. When significant power from the signals is present at the mid-point
between the channels, then the OSA is unable to distinguish this from the noise power levels
using measurements between the channels. This limitation becomes significant when
combining higher modulation rates with close channel spacing, like 40 Gb/s signals on a
100 GHz grid. In this case, the noise must be measured with 0,4 nm of the signal, where the
signal strength can be comparable to that at 0,1 nm from the 10 GHz signal, as discussed
in 4.5.4. The optical filtering from multiplexing would however reduce this, similar to the signal
shown in Figure A.1. The degree of this filtering will generally determine whether this OSNR
method can measure such signals to the necessary uncertainty.
A second influence of advanced optical networks is the effect of using optical add-drop
multiplexers (OADM) and other components that have strong wavelength dependence.
Especially the use of reconfigurable OADMs (ROADM) results in channels being separated,
and then recombining channels that have been transmitted along different spans. Especially
the demultiplexing and “remultiplexing” of channels generally reduces the power level
between channels. When this happens, the reduced part of the spectrum cannot be used to
estimate the noise level at the signal wavelength. Other complications can also arise, such as
adjacent channels originating from different spans with differing contributions to OSNR, so
that measuring the noise power between the channels cannot be used for interpolation.
An example for this in Figure B.1 shows four amplified channels with 200 GHz spacing passed
through a multiplexer that has the displayed combined loss curve. The resulting modified
spectrum at the output shows how the noise between the channels has been filtered and can
no longer be interpolated to provide the noise level at the signal wavelength. In this case, the
unmodulated signals are narrower than the passbands, so by using sufficient resolution, noise
plateaus are revealed that are not significantly filtered. When this can be done, OSNR
measurements are possible by reducing the offset of the interpolation to measure the noise
on these plateaus. However when narrower channels are used together with high modulation
rates, good OSNR values will not be measurable.
– 20 – 61280-2-9 © IEC:2009
–10
–20
–30
–40
Mux loss
Noise
–50
Modified signal
–60
1 546 1 547 1 548 1 549 1 550 1 551 1 552 1 553 1 554
Wavelength (nm)
IEC 252/09
Figure B.1 – Example for noise filtering between channels for a 200 GHz grid
The effects discussed in this annex can result in the method of this standard being
inappropriate, independent of the instrumentation used. More complex methods such as the
use of polarization extinction may then be considered for obtaining OSNR. However, when the
time and equipment required to measure the OSNR of multiple channels is higher, the
advantage of using spectral OSNR evaluation with respect to channel characterizations like
RIN and eye diagrams may be reduced.
dBm
61280-2-9 © IEC:2009 – 21 –
Bibliography
[1] ITU-T Recommendation G.692 (1998), Optical interfaces for multichannel systems with
optical amplifiers
[2] ITU-T Recommendation G.697 (2004), Optical monitoring for DWDM systems
[3] ITU-T Supplement 39 to G-series Recommendations (2006): Optical system design and
engineering considerations.
[4] ITU-T Recommendation G.694.1, Spectral grids for WDM applications: DWDM
frequency Grid
[5] ISO/IEC MISC UNCERT: 1995, Guide to the expression of uncertainty in measurement
___________
– 22 – 61280-2-9 © CEI:2009
SOMMAIRE
AVANT-PROPOS.24
INTRODUCTION.26
1 Domaine d’application .27
2 Références normatives.28
3 Définition.28
4 Appareil.29
4.1 Généralités.29
4.2 OSA réalisé à partir d'un réseau de diffraction.29
4.3 OSA réalisé à partir d'un interféromètre Michelson .30
4.4 OSA réalisé à partir du Fabry-Perot.30
4.5 Exigences de performance OSA .31
4.5.1 Généralités.31
4.5.2 Plage de longueurs d'onde .31
4.5.3 Sensibilité .31
4.5.4 Largeur de bande de résolution (RBW).31
4.5.5 Précision de la largeur de bande de résolution .32
4.5.6 Plage dynamique.32
4.5.7 Fidélité d'échelle .33
4.5.8 Dépendance de la polarisation.33
4.5.9 Points de données de longueurs d'onde .33
5 Echantillonnage et éprouvettes.33
6 Procédure .33
7 Calculs .34
8 Incertitude de mesure.34
9 Documentation .34
Annexe A (informative) Erreur de mesure du niveau de signal du fait de la largeur
spectrale du signal.36
Annexe B (informative) Erreur de mesure du niveau de bruit du fait de la largeur
spectrale du signal et du filtrage de la longueur d’onde.40
Bibliographie.
...
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