IEC 61280-2-2:2008

IEC 61280-2-2
Edition 3.0 2008-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic communication subsystem test procedures –
Part 2-2: Digital systems – Optical eye pattern, waveform and extinction ratio
measurement
Procédures d'essai des sous-systèmes de télécommunications à fibres
optiques –
Partie 2-2: Systèmes numériques – Mesure du diagramme de l’œil optique,
de la forme d’onde et du taux d’extinction

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IEC 61280-2-2
Edition 3.0 2008-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Fibre optic communication subsystem test procedures –
Part 2-2: Digital systems – Optical eye pattern, waveform and extinction ratio
measurement
Procédures d'essai des sous-systèmes de télécommunications à fibres
optiques –
Partie 2-2: Systèmes numériques – Mesure du diagramme de l’œil optique,
de la forme d’onde et du taux d’extinction

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
U
CODE PRIX
ICS 33.180.01 ISBN 2-8318-9637-1

– 2 – 61280-2-2 © IEC:2008
CONTENTS
FOREWORD.4

1 Scope and object .6

2 Normative references.6

3 Apparatus.6

3.1 Time-domain optical detection system.7

3.1.1 Optical-to-electrical (O/E) converter .7

3.1.2 Resistive signal splitter (optional).8
3.1.3 Linear-phase low-pass filter .8
3.1.4 Oscilloscope .10
3.1.5 Overall system response.10
3.2 Oscilloscope synchronization system .10
3.3 Pulse pattern generator .11
3.4 Optical power meter.11
3.5 Optical attenuator .11
3.6 Test cord.11
4 Test sample.11
5 Procedure.12
5.1 Method 1: Basic waveform measurement .12
5.2 Method 2: Extinction measurement method using the histogram function.12
6 Calculation .14
6.1 Basic waveform measurement definitions.14
6.2 Method 2: Extinction measurement method using the histogram function.17
6.3 Eye-diagram analysis using a mask .18
7 Test result .19
7.1 Required information .19
7.2 Available information .19
7.3 Specification information.20

Annex A (informative) Oscilloscope synchronization system.23

Bibliography .27

Figure 1 – Optical eye pattern, waveform, and extinction ratio measurement
configuration .6
Figure 2 – Time-domain optical detection system .7
Figure 3 – Illustrations of NRZ and RZ eye-diagram parameters .17
Figure 4 – Example of eye pattern measured with 0,75/T low-pass filter .18
Figure 5 – Example of eye pattern measured with 3,0/T low-pass filter.19
Figure 6 – Eye diagram with vertical histogram data collected from the central 20 %
window.19
Figure A.1 – Oscilloscope synchronization system .23

Table 1 – Frequency response characteristics .10
Table 2 – Typical parameters for the measurement shown in Figure 4 .21

61280-2-2 © IEC:2008 – 3 –
Table 3 – Typical parameters for the measurement shown in Figure 5 .22

Table A.1 – Example lengths of common RG-58 cable.25

– 4 – 61280-2-2 © IEC:2008
INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
FIBRE OPTIC COMMUNICATION
SUBSYSTEM TEST PROCEDURES –
Part 2-2: Digital systems –
Optical eye pattern, waveform and

extinction ratio measurement
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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by agreement between the two organizations.
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5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61280-2-2 has been prepared by subcommittee 86C: Fibre optic
systems and active devices, of IEC technical committee 86: Fibre optics.
This third edition cancels and replaces the second edition published in 2005 and constitutes a
technical revision. This edition includes the following significant technical changes with respect
to the previous edition:
a) The necessity of DC coupling for extinction ratio measurement is clarified.
b) The definition of extinction ratio has been revised to better harmonize with ITU-T.
c) The definition of OMA has been clarified.

61280-2-2 © IEC:2008 – 5 –
The text of this standard is based on the following documents:

CDV Report on voting
86C/768/CDV 86C/801/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 parts of the IEC 61280 series, published 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 – 61280-2-2 © IEC:2008
FIBRE OPTIC COMMUNICATION
SUBSYSTEM TEST PROCEDURES –
Part 2-2: Digital systems –
Optical eye pattern, waveform and

extinction ratio measurement
1 Scope and object
The purpose of this part of IEC 61280 is to describe a test procedure to measure the eye
pattern and waveform parameters such as rise time, fall time, overshoot, and extinction ratio.
Alternatively, the waveform may be tested for compliance with a predetermined waveform
mask.
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.
ITU-T Recommendation G.957, Optical interfaces for equipments and systems relating to the
synchronous digital hierarchy
3 Apparatus
The primary components of the measurement system are a photodetector, a low-pass filter, an
oscilloscope, and an optical power meter, as shown in Figure 1.

Pulse
Trigger
pattern
generator
Optical
interface point
Time-domain optical detection system
Data (Clock)
Low-pass
Optical O/E
Optical
Oscilloscope
filter
attenuator convertor
transmitter
Test Test cord
cord
Optical
power
meter
IEC  1 519/98
Figure 1 – Optical eye pattern, waveform, and extinction
ratio measurement configuration

61280-2-2 © IEC:2008 – 7 –
3.1 Time-domain optical detection system

The time-domain optical detection system displays the intensity of the optical waveform as a

function of time. The optical detection system is comprised primarily of an optical-to-electrical

(O/E) converter, a linear-phase low-pass filter and an oscilloscope. The detection system is

shown in Figure 2. More complete descriptions of the equipment are listed in the following
subclauses.
Optical Resistive
Oscilloscope
interface signal
vertical
point splitter
(optional)
Low-pass
filter
Trigger
Optical
Optical to
signal
electrical
input
converter
Optional
Oscilloscope
amplifier
synchronization
signal
To
synchronization
system
(optional)
IEC  1 520/98
Figure 2 – Time-domain optical detection system
3.1.1 Optical-to-electrical (O/E) converter
The O/E converter is typically a high-speed photodiode followed by electrical amplification. The
O/E converter is equipped with an appropriate optical connector to allow connection to the
optical interface point, either directly or via an optical test cord.
The O/E converter (including any optional amplification following the O/E converter) shall be

able to reproduce the optical waveform with sufficient fidelity to ensure a meaningful
measurement. Precise specifications are precluded by the large variety of possible
implementations, but general guidelines are as follows:
a) acceptable input wavelength range, adequate to cover the intended application;
b) input optical reflectance, low enough to avoid excessive back-reflection into the transmitter
being measured;
For example, assume that an optical transmitter is specified to tolerate –24 dB reflectance
maximum. If the input reflectance of the O/E converter is –30 dB, the converter can be
directly connected to the transmitter. If, however, the input reflectance of the O/E converter
is –14 dB, a common value, the effective reflectance can be lowered to –24 dB (or less) by
inserting either an optical isolator or a low-reflectance attenuator of 5 dB (or more) between
the transmitter and the O/E converter.
c) responsivity, adequate to produce a readable display on the oscilloscope;

– 8 – 61280-2-2 © IEC:2008
For example, assume that a non-return-to-zero (NRZ) optical data stream with an average

optical power of –15 dBm is to be measured. If the sensitivity of the oscilloscope is 10 mV

per division, a responsivity of 790 V/W is required in order to produce a display of 50 mV

peak-to-peak (that is, five divisions peak-to-peak).

d) optical noise-equivalent power, low enough to result in an accurately measurable display on
the oscilloscope;
For example, assume that a non-return-to-zero (NRZ) optical data stream with an average

optical power of –15 dBm is to be measured. If the effective noise bandwidth of the

measurement system is 470 MHz, and if the displayed root-mean-square noise is to be less
than 5 % of the eye pattern peak-to-peak height, the optical noise-equivalent power shall be

-1/2
145 pW Hz or less.
e) lower cut-off (–3 dB) frequency, 0 Hz;
DC coupling is necessary for two reasons. First, extinction ratio measurements cannot
otherwise be performed. Second, if AC-coupling is used, low-frequency spectral
components of the measured signal (below the lower cut-off frequency of the O/E
converter) may cause significant distortion via amplitude modulation of the detected
waveform.
f) upper cut-off (–3 dB) frequency, greater than the bandwidth of the low-pass filter following
the O/E converter;
In order to ensure repeatability and accuracy, a low-pass filter of known characteristics is
inserted in the signal path before the oscilloscope. This filter alone should primarily
determine the effective system bandwidth. However, the response of the entire
measurement system should conform to the desired frequency response.
g) transient response, overshoot, undershoot and other waveform aberrations minor so as not
to interfere with the measurement;
The low-pass filter following the O/E converter should primarily determine the system
transient response.
h) output electrical return loss, high enough that reflections from the low-pass filter following
the O/E converter are adequately suppressed from 0 Hz to a frequency significantly greater
than the bandwidth of the low-pass filter.
A time-domain measurement may be very inaccurate if significant multiple reflections are
present. Many passive, low-loss, low-pass filters, in addition to being reflective in the stop
band, have frequency responses that are strongly dependent on the termination
impedances at the input and output. A minimum value of 15 dB for the return loss is
recommended when passive low-pass filters are employed following the O/E converter. The
effective output return loss of the O/E converter may be improved with in-line electrical
attenuators, at the expense of reduced signal levels. Finally, the return loss specification
extends to d.c., since otherwise, a d.c. shift in the waveform will occur, causing extinction
ratio measurements to be in error.
3.1.2 Resistive signal splitter (optional)
If the trigger signal for the oscilloscope is to be derived from the optical waveform itself, it is
necessary to tap into the signal path at some point. A resistive signal splitter (power divider) at
the location indicated in Figure 2 provides a branch from which to derive the trigger signal.
3.1.3 Linear-phase low-pass filter
Generally, one of the primary purposes of measuring the optical eye pattern is to verify certain
performance requirements such as rise and fall time, overshoot, etc. If the measurement
system bandwidth is much greater than needed, high frequency (and probably insignificant)
details of the waveform will tend to obscure the desired measurement. Also, since different
measurement setups would have different bandwidths, repeatability between setups would be
almost impossible to achieve.
61280-2-2 © IEC:2008 – 9 –
In order to ensure repeatability and accuracy, a low-pass filter of known characteristics is

inserted in the signal path prior to the oscilloscope. This filter alone should primarily determine

the effective system bandwidth. The type of measurement being performed determines the

bandwidth of the low-pass filter. The bandwidth and transfer function characteristics of the low-

pass filter should be explicitly stated in the detail specifications.

One type of eye pattern measurement effectively simulates the signal that would result at the

output of a bit-rate-specific optical receiver. For NRZ format signals, this type of receiver

typically has a bandwidth that is somewhat less than the clock frequency. For this type of

measurement, a low-pass filter of –3 dB bandwidth of 0,75/T (where T is the bit interval, in

seconds, of the data signal) is often used. The resulting eye pattern is compared to a "mask" to

verify compliance with specification. For RZ format signals, spectral content may be
significantly higher than the NRZ signal at the same optical bit rate. This may require the
reference bandwidth to be in excess of the clock frequency.
A different type of eye pattern measurement involves measuring the rise time, fall time, pulse
width and other time-domain parameters of an optical transmitter unit. For this type of
measurement, the system bandwidth shall be greater than described above. The –3 dB
bandwidth of the low-pass filter in this case needs to be high enough to allow verification of
maximum rise and fall times (for example, one-third of a bit intervals), but low enough to
eliminate unimportant high-frequency waveform details. For NRZ signals, a low-pass filter
bandwidth of 3,0/T is a typical compromise value for this type of measurement. RZ signals can
require a bandwidth of 5,0/T as a typical compromise.
Regardless of the type of eye pattern measurement, the filter should have a linear phase
response at frequencies up to and somewhat beyond the filter –3 dB bandwidth. If the phase
response is linear (implying that the group delay is constant) up to frequencies of high
attenuation, slight variations in filter bandwidths should not significantly affect the waveform
measurements (see Table 1).
Example low-pass filter specifications for a 0,75/T filter are as follows (exact filter
specifications are typically found within the communication standard defining transmitter
performance):
– characteristic impedance: 50 Ω nominal;
– –3 dB bandwidth: 0,75/T, Hz; (RZ ffs);
– filter type: fourth-order Bessel-Thompson (RZ ffs).

– 10 – 61280-2-2 © IEC:2008
Table 1 – Frequency response characteristics

Maximum group
Frequency divided
Nominal attenuation Attenuation tolerance
delay distortion
by bit rate dB dB
s
0,15 0,1 0,3 –
0,30 0,4 0,3 –
0,45 1,0 0,3 –
0,60 1,9 0,3
0,002 T
0,75 3,0 0,3 0,008 T
0,90 4,5 0,3
0,025 T
1,00 5,7 0,3 0,044 T
1,05 6,4 0,39
0,055 T
1,20 8,5 0,64 0,100 T
1,35 10,9 0,90
0,140 T
1,50 13,4 1,15 0,190 T
2,00 21,5 2,0 0,300 T
3.1.4 Oscilloscope
The oscilloscope that displays the optical eye pattern should have a bandwidth well in excess
of the bandwidth of the low-pass filter, so that the oscilloscope is not the bandwidth-limiting
item of the measurement system. The oscilloscope is triggered either from a local clock signal
that is synchronous with the optical eye pattern or from a synchronization signal derived from
the optical waveform itself.
Figures 4 and 5 illustrate oscilloscope bandwidths that are commonly used in eye pattern
measurements.
The oscilloscope shall have a vertical-channel histogram function for extinction ratio
measurement.
3.1.5 Overall system response
The eye pattern measurement is obviously a time-domain measurement and needs to
accurately represent the optical waveform. This should be done without introducing undesirable
overshoot, ringing and other waveform aberrations. While the individual components of the
measurement system are most conveniently specified in the frequency-domain, the final
assembled system may also be required to meet certain time-domain performance limits.
Even an ideal fourth-order Bessel-Thompson filter will have an overshoot of about 1 % and a
rise time (10 % to 90 %) of about 0,35/B, where B is the bandwidth in Hertz. In view of this, the
overall measurement system shall be required to demonstrate performance similar to the
following when stimulated by an ideal step function signal:
– rise time, fall time (10 % – 90 %): 0,43/B maximum, 0,29/B minimum;
– rise time, fall time (20 % – 80 %): 0,35/B maximum, 0,23/B minimum;
– overshoot, undershoot: 5 % maximum.
3.2 Oscilloscope synchronization system
A stable synchronization signal is essential for accurate eye pattern measurement. Ideally, the
optical transmitter unit provides the synchronization signal. However, since this synchronization
signal may not be present at an optical interface point, it may be necessary to derive the signal
from the optical waveform itself.

61280-2-2 © IEC:2008 – 11 –
Some oscilloscopes will allow triggering from transitions in the detected data pattern, either

internally (from the vertical channel) or externally (from a separate trigger input). While

conveniently simple, this method is not recommended for performing accurate measurements.

Optical waveforms are usually fairly "noisy," and by triggering on transitions in the data pattern,

the observed eye pattern will be changed by the precise adjustment of the trigger threshold. An

oscilloscope typically will trigger on either a rising or a falling edge (but not both). For a
Pseudo-random binary sequence (PRBS) data pattern only one of every four bit transitions will
produce a trigger event. Thus 75 % of the data is never measured, even if the pattern repeats.

Also, jitter in the optical waveform itself will be difficult to observe, since the triggering process

may mask jitter. Jitter on the signal being measured is common to the trigger signal and can be

effectively eliminated. Finally, the bandwidth of the transition detector in many oscilloscopes

may be insufficient to allow reliable, repeatable triggering.

When a stable synchronization signal is not available, a more reliable method of using the
optical waveform for oscilloscope synchronization is to use an external oscillator which is
"locked" to the optical signal. This is similar to the method used to recover the clock in an
optical receiver, but is accomplished with commercially available test equipment rather than
with custom circuitry. This method provides a very stable trigger signal, and allows jitter in the
optical waveform to be viewed directly. A detailed description of an oscilloscope
synchronization system and associated equipment is given in Annex A. Care must be taken in
choosing the response bandwidth of the trigger extraction system to achieve an accurate jitter
measurement.
3.3 Pulse pattern generator
The pulse pattern generator shall be capable of providing a pseudo random bit sequence and
programmable word patterns to the system consistent with the signal format (pulse shape,
amplitude, etc.) required at the system input electrical interface of the transmitter device.
3.4 Optical power meter
The optical power meter shall be used which has a resolution of at least 0,1 dB and which has
been calibrated for the wavelength of operation for the equipment to be tested.
3.5 Optical attenuator
The attenuator shall be capable of attenuation in steps less than or equal to 1 dB and should
be able to adjust the input level of the O/E converter.
Care should be taken to avoid back reflection into the transmitter.
3.6 Test cord
Unless otherwise specified, the test cords shall have physical and optical properties normally
equal to those of the cable plant with which the equipment is intended to operate. The test
cords shall be 2 m to 5 m long and shall contain fibres with coatings which remove cladding
light. Appropriate connectors shall be used. Single-mode test cords shall be deployed with two
90 mm diameter loops. If the equipment is intended for multimode operation and the intended
cable plant is unknown, the fibre size shall be 62,5/125 μm.
4 Test sample
The test sample shall be a specified fibre optic transmitter. The system inputs and outputs
shall be those normally seen by the user of the system. The test transmitter shall be installed in
the measurement configuration as shown in Figure 1.

– 12 – 61280-2-2 © IEC:2008
5 Procedure
In measuring an optical eye pattern, a synchronization signal for triggering the oscilloscope

may or may not be available. Many measurement systems include the ability to derive a

synchronization signal from the signal being measured. Measurement considerations, as well

as a scheme for building a synchronization system when one is not integrated into the test

equipment, are discussed in Annex A.

5.1 Method 1: Basic waveform measurement

5.1.1 Unless otherwise specified, standard operating conditions apply. The ambient or

reference point temperature and humidity shall be specified.
5.1.2 Apply appropriate terminal input voltage/power to the system under test. Follow
appropriate operating conditions. Allow sufficient time (30 min, unless otherwise specified by
the manufacturer) for the terminal under test to reach steady-state temperature and
performance conditions.
Determine the data rate of the optical signal to be tested. Select the appropriate low-pass filter
corresponding to the data rate and controlling specification. Connect the test equipment, as
shown in Figure 1, and apply power. If a sampling oscilloscope is used, verify that the
waveform shape is not corrupted through averaging or smoothing and that the sampling loop
gain is properly adjusted, if applicable. Set the horizontal display of the oscilloscope to
approximately 0,2 T per division, where T is as defined in 3.1.3, and the oscilloscope timebase
is displayed in 10 divisions. Allow sufficient warm-up time for the test equipment.
As part of standard operating conditions, all terminal inputs are fully loaded with a signal at the
full data rate and with a pattern that has spectral content representative of actual operation.
This is often achieved with pseudo random data (maximum word length, typically

2 –1).
5.1.5 Use appropriate optical fibre cables; if necessary connect the input of the O/E converter
to the optical interface point being tested.
5.1.6 Adjust the vertical position and sensitivity of the oscilloscope to obtain a centred display
covering about half of the vertical screen dimension.
5.1.7 Oscilloscope synchronization: adjust the trigger level, if necessary, to obtain a stable
waveform display.
5.1.8 Adjust the vertical and horizontal controls of the oscilloscope to produce the desired

eye pattern display.
5.1.9 If desired, photograph, print or store in a memory device the displayed waveform for
later calculations. Otherwise, measurements shall be performed on screen.
5.1.10 Disconnect or otherwise block the optical signal input to the O/E converter by using the
optical attenuator. Observe or record the oscilloscope trace under this condition to determine
the dark level. This information is necessary for extinction ratio calculations.
5.2 Method 2: Extinction measurement method using the histogram function
5.2.1 Unless otherwise specified, standard operating conditions apply. The ambient or
reference point temperature and humidity shall be specified.
5.2.2 Apply appropriate terminal input voltage/power to the system under test. Follow
appropriate operating conditions. Allow sufficient time (30 min, unless otherwise specified by

61280-2-2 © IEC:2008 – 13 –
the manufacturer) for the terminal under test to reach steady-state temperature and

performance conditions.
5.2.3 Determine the data rate of the optical signal to be tested. Select the appropriate low-

pass filter corresponding to the data rate and controlling specification. Connect the test

equipment, as shown in Figure 1, and apply power. If a sampling oscilloscope is used, verify

that the waveform is not corrupted through averaging or smoothing and that the sampling loop

gain is properly adjusted, if applicable. Set the horizontal display of the oscilloscope to

approximately 0,2 T per division, where T is as defined in 3.1.3, and the oscilloscope timebase

is displayed in 10 divisions. Allow sufficient warm-up time for the test equipment.

5.2.4 As part of standard operating conditions, all terminal inputs are fully loaded with a
signal at the full data rate and with a pattern that has spectral content representative of actual
operation. This is often achieved with pseudo random data (maximum word length, typically 2
–1).
5.2.5 Adjust the optical attenuator to set the O/E converter input power at the manufacturer
specified input power level.
5.2.6 Connect the test cord to the input connector of the O/E converter.
5.2.7 Adjust the vertical position and sensitivity of the oscilloscope to obtain a centred display
covering about half of the vertical screen dimension. Synchronization with the data pattern
need not be present at this time.
5.2.8 Oscilloscope synchronization: adjust the trigger level as necessary to obtain a stable
waveform display.
5.2.9 Adjust the vertical and horizontal controls of the oscilloscope to produce the desired
eye pattern display.
5.2.10 If desired, photograph, print, or store in a memory device the displayed waveform for
later calculations. Otherwise, measurements must be performed on-screen.
5.2.11 Disconnect or otherwise block the optical signal input to the O/E converter by using the
optical attenuator.
5.2.12 Adjust the vertical control of the oscilloscope to determine the dark level. Record the
dark level (b ).
dark
5.2.13 Set the optical attenuator to the adjustment of 5.2.5.
5.2.14 Measure the vertical level by histogram function of the oscilloscope, as shown in
Figure 6. The time range of the histogram is mT. Unless otherwise specified, m shall be 0,2 and
centred within the eye diagram 1.
5.2.15 Read the mean voltage corresponding to the optical power of the logic 1 level (b ),
indicated on the histogram.
5.2.16 Read the mean voltage corresponding to the optical power of the logic 0 level (b ),
indicated on the histogram.
– 14 – 61280-2-2 © IEC:2008
6 Calculation
6.1 Basic waveform measurement definitions

Refer to the definitions below and to Figure 3 for sample calculations. Figures 4 and 5 illustrate

typical examples of measured waveforms. Tables 2 and 3 provide typical parameters from the

measurements shown in Figures 4 and 5, respectively.

b : the voltage indicated on the oscilloscope when optical power is removed or blocked
dark
from the O/E converter.
b : the voltage corresponding to the nominal optical power of the logic 0 level.
If available in the oscilloscope, voltage histograms may be useful to determine this
value. The logic 0 level may be estimated from a filtered optical eye pattern (fourth-
order Bessel-Thompson, 0,75/T bandwidth) as the mean of the logic 0 voltage
histogram measured across the central 20 % of the bit period. The histogram mean
over this region provides a good estimate of the energy in the bit as the reference
receiver acts as an integrator. Care shall be taken to assure that the number of
samples taken for the histogram is adequately large to overcome the uncertainty
caused by noise on the waveform.
For the return-to-zero (RZ) format, the logic 0 level may be estimated from a
filtered optical eye pattern (fourth-order Bessel-Thompson, m/T bandwidth, m to be
determined) as the mean of the logic 0 histogram voltage measured across 5 % of
the bit period centred below the logic 1 peak.
b : the voltage corresponding to the nominal optical power of the logic 1 level.
If available in the oscilloscope, voltage histograms may be useful to determine this
value. For non-return-to-zero (NRZ) format, the logic 1 level may be estimated from
a filtered optical eye pattern (fourth-order Bessel-Thompson, 0,75/T bandwidth) as
the mean of the logic 1 histogram voltage measured across the central 20 % of the
bit period. Care shall be taken to assure that the number of samples taken for the
histogram is adequately large to overcome the uncertainty caused by noise on the
waveform. Note that b is assumed to be greater than b .
1 0
For the return-to-zero (RZ) format, the logic 1 level may be estimated from a
filtered eye pattern (fourth-order Bessel-Thompson bandwidth, m/T, m to be
determined), as the mean of the logic 1 histogram voltage measured across 5 % of
the bit period centred at the peak of the logic 1 pulse. The percentage of the bit
period used may vary depending upon the bandwidth of the reference receiver.
(b – b ): the difference in voltage between the nominal logic 1 level and the nominal logic 0
1 0
level. This is similar to the optical modulation amplitude (OMA) parameter defined in
IEEE 802.3. However, OMA is usually measured on a square wave pattern (such as
1111100000) rather than an eye diagram to minimize data dependent effects such as

inter-symbol interference and laser transients. For OMA, b is usually determined in
the centre of the sequence of logic 1’s and b is usually determined in the centre of the
sequence of logic 0’s.
Extinction ratio: the ratio of the average optical energy in the centre of a logic 1
pulse to the average optical energy in the centre of a logic 0.
For non-return-to-zero (NRZ) and return-to-zero (RZ) optical line coding, the extinction ratio
may be determined by measuring the filtered optical eye (fourth-order Bessel-Thompson, m/T
bandwidth where m is 0,75 for NRZ and to be determined for RZ) at full line rate and computing
the ratio:
(b – b ) / (b – b )
1 dark 0 dark
Contrast ratio (RZ format signals): the ratio of the signal amplitude of the logic 1 at its full on
state to the amplitude of the logic 1 at its off state where it returns to 0.

61280-2-2 © IEC:2008 – 15 –
(b – b ) / (b – b )
1on dark 1off dark
The logic 1 off amplitude is composed of data from logic 1 pulses including those preceded or

followed by logic 0’s. Care should be taken to reduce the influence of the logic 0 signal in the

measurement of the logic 1 off amplitude.

Rise time: the time required for the optical pulse to rise from 20 % to 80 %, or from a value of:

b + 0,2 (b – b )
0 1 0
to a value of:
b + 0,8 (b – b )
0 1 0
Because of the generally "noisy" nature of optical waveforms, the 10 % and 90 % levels may be
difficult to resolve with sufficient accuracy. Therefore, 20 % to 80 % rise times are preferred by
this standard. If 10 % to 90 % values are required, those values may either be measured
directly or the 20 % to 80 % values may be measured and a correction factor applied.
Assuming a fourth-order Bessel-Thompson filter response, the 10 % to 90 % rise times are a
factor of 1,25 larger than the 20 % to 80 % rise times. This value is typically measured without
a low-pass Bessel-Thompson filter. If the filter is in place, the rise time measured will be larger
than the actual risetime of the signal
Fall time: the time required for the optical pulse to fall from 80 % to 20 %, or from a value of:
b + 0,8 (b – b )
0 1 0
to a value of:
b + 0,2 (b – b )
0 1 0
See explanation for rise time above.
Pulse width: the width of the logic 1 pulse at the 50 % level, defined as the time duration from
the time the rising edge of the optical pulse first crosses the value of:
b + 0,5 (b – b )
0 1 0
until the time the falling edge first crosses the value of:
b + 0,5 (b – b )
0 1 0
Duty cycle distortion: for non-return-to-zero (NRZ) optical line coding only, the deviation of
pulse width from the ideal, expressed in percent, as follows:
duty cycle distortion (per cent) = ⏐ (T – pulse width) / T ⏐ 100
where T is as defined in 3.1.3.
Jitter: the time variation of the rising or falling edge of the optical waveform as it crosses the
value of:
b + K (b – b )
0 1 0
where K is a constant between 0,2 and 0,8.

– 16 – 61280-2-2 © IEC:2008
Ideally, the value of K is 0,5, so that jitter is measured at the 50 % level. However, the rising

and falling edges of the NRZ eye pattern will often be on top of each other at the 50 % level,

making measurements difficult. The absolute value of jitter should be relatively unaffected by

the choice of threshold. Note that the jitter on rising edges may not be identical to the jitter on

the falling edges.
For the NRZ eye diagram a jitter measuremen
...