Regular Papers

Technology for Flexibly Monitoring Optical
Signal Quality in Optical Communications
Ippei Shake†, Hidehiko Takara, and Satoki Kawanishi
Abstract
Optical signal quality monitoring is an important function for optical transport networks and future alloptical networks. To monitor the optical signal-to-noise ratio and/or waveform distortion transparently
with respect to the signal format, data format, and signal bit rate, we propose a new optical signal quality monitoring method that uses asynchronous sampling, which is a sampling technique that does not use
timing extraction. The use of high-speed asynchronous sampling and the adjustment of the sampling rate
enable simple open eye-diagram monitoring and evaluation of a fixed-timing Q-factor (Qt) at the maximum eye opening timing phase. This method was experimentally verified using an optical signal quality monitoring circuit. We obtained a good relationship between the measured Qt and Q (which is a Qfactor calculated from the bit error rate (BER)), indicating that the BER was estimated by the measured
Qt with good accuracy. Moreover, as an easier method, we also introduce an average Q-factor (Qavg) evaluation method, which measures the Qavg value from an asynchronous eye-diagram (timing drifted eyediagram). This method is useful when the sampling rate is low or when adjusting the sampling rate is difficult, although a correction procedure for converting from Qavg to Q is needed. Standardization of our
optical signal quality monitoring technology is in progress in the IEC (International ElectrotechnicalCommission).

1. Introduction
Monitoring the quality of signals is an important
aspect of the design, operation, and maintenance of
optical transport networks [1]-[3]. From the network
operator’s viewpoint, monitoring techniques are
required to establish connections, establish protection
or/and restoration, and perform maintenance. To
achieve these functions, monitoring techniquesshould satisfy some general requirements: theyshould provide in-service measurement, signal deterioration detection, fault isolation, transparency, scalability, and simplicity. Figure 1 shows an example ofan optical network that accommodates a diverse
range of clients and has a flexible optical path configuration. If a fault or signal deterioration occurs, itis necessary to detect the problem to protect/restore† NTT Network Innovation Laboratories
Yokosuka-shi, 239-0847 Japan
E-mail: shake.ippei@lab.ntt.co.jp

Vol. 2 No. 5 May 2004

and localize the impaired section or node. A fundamental performance monitoring parameter of any
digital transmission system is its end-to-end bit error
rate (BER). However, the only way to correctly evaluate the BER is to use out-of-service BER measurement, that is to measure it using a known test bit pattern instead of a real signal while the service is notoperating. In-service measurement can give only
rough estimates by measuring digital parameters(e.g., the error block detection and error count in forward error correction) or analog parameters (e.g.,
optical power, optical signal-to-noise ratio (OSNR),
and Q-factor*1).
What has been greatly desired and studied is a signal quality monitoring method that can provide agood measure of signal quality without complex ter*1 Q-factor: a parameter for indicating quality. In this paper, the Qfactor is the optical signal quality in optical communications. It isdefined by the amplitude distributions of both the mark (1) andspace (0) levels of a binary signal.

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ATM
IP
Monitoring

GbE

SDH
Opticalnetwork
SDH
GbE

IP

IP
ATM
GbE

ATM: asynchronous transfer mode
IP: Internet protocol
GbE: gigabit Ethernet
SDH: synchronous digital hierarchy

Monitoring
Optical link

Fig. 1. Optical network with diverse clients (a monitoring function is needed at nodes and repeaters).

mination. When the system BER is too low to bemeasured within a reasonable amount of time, it isuseful to use Q-factor measurement [3]-[10]. However, sampling-based methods basically require synchronization and then some analysis [4], [5], whichmakes them similar to protocol-aware termination interms of cost and complexity. In fact, synchronous
sampling requires timing extraction using complex
equipment that is specific to each bit error rate andeach format.
This paper proposes a simple Q-factor measurement technique as a cost-effective alternative to BER
measurements. This method is a fixed timing Q-factor (Qt) monitoring method that utilizes the open eyediagrams captured by asynchronous sampling [6].
Asynchronous sampling allows bit-rate independent
Q-factor monitoring, and the same equipment cancover a bit rate range of up to 40 Gbit/s. Average Qfactor (Qavg) measurement through asynchronous
sampling [3], [8] is also introduced and comparedwith Qt measurement. The experimental results arealso summarized.
2. Qt monitoring
Parameter Qt is estimated from the open eye-diagrams captured by asynchronous sampling as shownin Fig. 2. To obtain well opened eye-diagrams, all30

sampling points are plotted in time order and superimposed every k (or multiple of k) samples. The following conditions must be satisfied:
Tstep = 1/ fc – 1/{(n/m) fs} = 1/(k fs),

(1)

where fs is the signal bit rate, fc is the sampling clock
rate, Tstep is the sampling time interval, n/m is a value
pertaining to the ratio fs and fc, n and m are natural
numbers, and k is the number of sampling points pertime slot. From these equations, fc is obtained as
fc = (1/k + m/n)–1 fs.

(2)Here, we assume some knowledge of fs such as thedata format (e.g., SONET/SDH (synchronous optical
network, synchronous digital hierarchy) or Ethernet).
Such information is relatively easy to obtain. Notethat when timing extraction is not used, fs is not accurately known at the signal quality monitoring circuit,so fc must be determined independently. Moreover,the performance of the sampling clock source causes
inaccuracy in the setting of fc. These inaccuracies in
fs and/or fc cause Eqs. (1) and (2) to fail. However,high-speed sampling allows us to obtain well opened
eye-diagrams even under this condition, which meansthat Qt can be evaluated as described in the following
theoretical evaluation.
NTT Technical Review

Regular Papers

Amplitude
Optical Signal Quality Monitor

σ1|µµ1 – µ0 |σ0Histogram

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Eye Pattern

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[%]
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30

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10

0

0

–10

–10

Maximumeye
opening
phase

–20
0.0

–20
Number of times
alpha
0.30

Q factor [dB]
20.04

t
200.P
Relative Time [s]Range [V]Q-factor measurement
500.00mm
average Q-factor
Date Rate [Hz]Q-factor at a fixed timing phase
100.00M

Fig. 2. Open eye-diagram captured by asynchronous sampling and amplitude histograms at fixed timing phase t (Nsamp =8000, k = 100, |df | ~~ 0, fs =10 Gbit/s, fc ~~ 100 MHz, n/m ~~ 1/10).

We assume frequency detuning df due to the inaccuracy in determining fs and/or fc. The time shift ofsampling time interval ∆Tstep due to df is written as∆Tstep = 1/fc – 1/( fc + df ).

(3)

When | Nsamp ∆Tstep | is 1/(2fs) or less, where Nsamp is
the total sampling number, a well opened eye-diagram is obtained. Therefore, the following condition
must be satisfied.
fc ≥ (2fs j k | df |)1/2,(4)

where Nsamp = j k ( j and k are natural numbers). For
example, when Nsamp is 250, fs is approximately 10
Gbit/s, the frequency detuning | df | is 20 ppm (200kHz), and the requirement for fc is 1 GHz or more.
Therefore, a sampling clock rate on the order of 1
GHz allows our measurement circuit to capture theopened eye-diagrams even when the frequency
detuning | df | is 20 ppm. In other words, if the sampling clock rate is on the order of 1 GHz, our measurement circuit tolerates setting inaccuracy in fs
and/or fc of up to ±200 kHz. Therefore, high-speed
asynchronous opto-electrical (OE) sampling *2
enables us to achieve simple Q-factor monitoring
without complicated software calculations, which arerequired when using the periodogram [7].
In addition, we must confirm the influence of signal
Vol. 2 No. 5 May 2004

wander. Signal wander is sometimes estimated fromthe group delay due to a change in the transmission
fiber caused by temperature fluctuations. When the
total sampling number is Nsamp points, the transmission fiber length is L m, the temperature change is dT°C/s, the group delay coefficient of optical fibers is α
ps/m/°C, and the total group delay divided by the
total sampling time, ∆twander, satisfies∆twander = α Nsamp L dT / fc .

(5)

For example, when α is 0.2 ps/m/°C, N is 250, L is
320 × 103 m, dT is 0.5 × 10–3 °C/s (20°C per 12hours), and fc is approximately 1 GHz, the groupdelay divided by the total sampling time is approximately 7.5 × 10–6 ps, which is small enough to measure the open eye-diagrams. The eye-diagrams, theamplitude histograms at fixed timing phase t, and Qt
are shown in Fig. 2. Parameter Qt is defined by
Qt = | µ1 – µ0 | / (σ1 + σ0),(6)where µi and σi are the mean and standard deviation,
respectively, of the mark (i = 1) and space (i = 0) level
distributions of the amplitude histograms. The midpoint of the timing phase between the two white lines
in Fig. 2 is t and the sampling points between the two
white lines are used in the estimation.

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The optical signal quality monitoring circuit consists of an OE sampling module, an internal clock
source, an O/E converter, and a signal processing circuit, as shown in Fig. 3. OE sampling is performed
using optical gating with an electrical clock. An electro-absorption (EA) modulator*3 is used as the OE
sampling module. The repetition rate of the electricalclock is approximately 1 GHz. The EA modulator
and electrical clock source are relatively small andsimple compared with the conventional optical sampling components or electrical high-speed sampling
modules. In the conventional electrical sampling
case, the O/E converter bandwidth should be wider
than the signal bit rate. On the other hand, in the OE
sampling method, the signal is optically sampled at arepetition rate lower than the signal bit rate, so the
O/E converter bandwidth is narrower than the signal
bit rate. The signal processing circuit analyzes the*2 Opto-electrical (OE) sampling: a technique in which an opticalsignal is sampled (gated) with electrical pulses.
*3 Electro-absorption (EA) modulator: an optical device in which theoptical absorption coefficient is changed by inducing an electricfield. The optical signal launched into the device is modulated byusing an electrical signal and changing the optical absorption coefficient.

sampled signal to determine Qt and Qavg.
Using this technique, we constructed a prototype
optical signal quality monitor. A polarization-independent EA modulator with bandwidth of 40 GHz
was used to achieve polarization-independent operation and excellent time resolution (8 ps). The signal
bit rate can range up to 40 Gbit/s. We can also measure the wavelength dependence of the Q-factor. The
bandwidth allowing a 2-dB decrease from the maximum Q-factor value is 40 nm (from 1543 to 1583
nm). This range is limited by the characteristics of the
EA modulator used. By shifting the center wavelength to 1550 nm, we can cover the entire C-band.
The main specifications are summarized in Table 1.
4. Impact of OSNR on Qt
Figure 4 shows the relationship between Qt and Q
for 10-Gbit/s NRZ (non-return to zero) optical sig17
16

Table 1. Measured values of main parameters.
Parameters
Sampling rate
Time resolution<=43 Gbit/s>=40 nm (1543 – 1583 nm)

Available input power–5.0 to +5.0 dBm

Polarization dependence<1.0 dB14
13

Qt = Q

12

Measured values<=1 GHz<8
ps=

Wavelength range

Signal bit rate

10 Gbit/s NRZ

15

Qt (dB)

3. Signal quality monitoring circuit using simple
OE sampling*2

11
10
10

11

12

13
14
Q (dB)

15

16

17

Fig. 4. Relationship between a fixed timing Q-factor (Qt)and a Q-factor calculated from BER (Q) for 10Gbit/s NRZ signals.

Optical signal
Electrical signal

Internal clock

Signal processing

OE sampling

O/E conversion

Electrical clock

Optical signal(SDH, GbE…)(NRZ, RZ)
(2.5, 10 Gbit/s…)

Fig. 3. Block diagram of optical signal quality monitoring circuit.

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nals at different signal OSNR values. The OSNR is
defined as the ratio between the optical signal power
and optical noise. The OSNR value is changed byadding amplified spontaneous emission (ASE) noise
to the optical signal. Parameter Qt is obtained byusing the procedure described in the previous section,and parameter Q is derived from the measured BER
using the Gaussian assumption. We set t to the time
where the eye was most widely opened. Good relationships are recognized in the figure, and the slopeof the relationship equals 1. Note that the values of Qt
basically equal those of Q. This means that it is possible to obtain the BER value directly if we estimate
Qt. For instance, when the measured Qt value is 16.4
dB for a 10-Gbit/s optical signal, the BER of the signal is recognized to be 10–10. The important feature of
the technique is that the slope of the relationshipbetween Qt and Q when the OSNR is changed does
not change because Qt is measured from the open
eye-diagram. Therefore, it is useful to estimate theabsolute value of the Q-factor, which is directly related to the BER.
5. Qavg monitoring and impact of OSNR on Qavg
Here, we propose the average Q-factor (Q avg)
method. This can be used when an open eye-diagram
cannot be obtained because no information regarding
the signal bit rate is available and the sampling rate
Amplitudeµ1,avg – µ0,avg ||µσ0,avgQavg = | µ1,avg – µ0,avg | / (σ1,avg + σ0,avg),(7)

where µ i,avg and σi,avg are the mean and standard deviation, respectively, of the mark (i = 1) and space (i =0) level distributions. The asynchronous eye-diagram
includes unwanted cross-point data, which decreases
the measured value of Qavg. Thus, it is necessary to
remove the cross-point data. In this way, we set two
threshold levels: µ th1 = µ1 – αµ and µ th0 = µ 0 + αµ,where µ = | µ1,avg – µ 0,avg | and coefficient α is defined
to lie between 0 and 0.5 [3]. Figure 6 shows the relationship between Qavg and Q for 10-Gbit/s NRZ optical signals at different signal OSNR values. The relationship is good although the slope is not 1 and it isnecessary to use a correction factor for evaluating the
BER. Since the influence of the cross-point data still

Optical Signal Quality Monitor
Histogram

80
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70

σ1,avgcannot be adjusted, when the inaccuracy of fs and/or
fc is large, or when it is difficult to perform highspeed sampling. In these cases, it is impossible toobtain an open eye-diagram, so we must evaluate an
asynchronous eye-diagram, which is a timing drifted
eye-diagram. We proposed an algorithm for evaluating Qavg from an asynchronous eye-diagram [3]. Figure 5 shows an asynchronous eye-diagram obtainedby asynchronous sampling. Parameter Qavg is evaluated using asynchronous amplitude histograms inwhich unwanted cross-point data is included. It isdefined by

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Number of times
alpha
0.30

Q factor [dB]13.29

Eye Pattern

µ th1
µ th0

200.P
Relative Time [s]Range [V]Q-factor measurement
500.00mm
average Q-factor
Date Rate [Hz]Q-factor at a fixed timing phase
100.00M

Fig. 5. Asynchronous (drifted) eye-diagram captured by asynchronous sampling and asynchronous amplitude histograms(Nsamp = 8000, fs =10 Gbit/s, fc ~~ 100 MHz, n/m ~~ 1/10).

Vol. 2 No. 5 May 200433

Regular Papers

No. 10, pp. 1307-1309, 1999.
S. Norimatsu and M. Maruoka, “Accurate Q-factor estimation of optically amplified systems in the presence of waveform distortion,” J.
Lightwave, Tech., Vol. 20, No. 1, pp. 19-29, 2002.
[6] I. Shake, H. Takara, and S. Kawanishi, “Simple Q factor monitoringfor BER estimation using opened eye diagrams captured by highspeed asynchronous electrooptical sampling,” IEEE Photonics Tech.
Lett., Vol. 15, No. 4, pp. 620-622, 2003.
[7] L. Noirie, F. Cerou, G. Moustakides, O. Audouin, and P. Peloso,“New transparent optical monitoring of the eye and BER using asynchronous under-sampling of the signal,” in Proc. ECOC2002, Copenhagen, Denmark, PD 2.2, Sep. 2002.
[8] I. Shake, H. Takara, S. Kawanishi, and Y. Yamabayashi, “Opticalsignal quality monitoring method based on optical sampling,” Electron. Lett., Vol. 34, No. 22, pp. 2152-2154, 1998.
[9] N. Hanik, A. Gladisch, C. Caspar, and B. Strebel, “Application of
amplitude histograms to monitor performance of optical channels,”
Electron. Lett., Vol. 35, No. 5, pp. 403-404, 1999.
[10] C. M. Weinert, C. Schmidt, and H. G. Weber, “Application of asynchronous amplitude histograms for performance monitoring of RZ
signals,” in Proc. OFC2001, WDD41, 2002.
[5]

Qavg (dB)

14

10 Gbit/s NRZ

12

Qavg = 0.44 Q + 5.4

10
13

14

15

16

17

18

Q (dB)

Fig. 6. Relationship between an average Q-factor (Qavg)
and a Q-factor calculated from BER (Q) for 10Gbit/s NRZ signals.

remains, the slope of this relationship changes withthe optical band-pass filter bandwidth or other parameters in the signal quality monitoring circuit [3]. In
this case, the relationship should be measured inadvance and the correction factor should be determined to obtain the BER value.
6. Conclusion

Ippei Shake
Research Engineer, Advanced Photonic Transmission Research Group, Photonic Transport
Network Laboratory, NTT Network Innovation
Laboratories.
He received the B.S. and M.S. degrees inphysics from Kyoto University, Kyoto in 1994
and 1996, respectively. In 1996 he joined NTT
Optical Network System Laboratories, Yokosuka, Japan. Since then he has been engaged in
R&D of high-speed optical signal processing andhigh-speed optical transmission systems. His
research interests also include optical networks
and optical performance monitoring. He is amember of IEEE and the Institute of Electronics,
Information and Communication Engineers(IEICE) of Japan.

Using a new procedure for evaluating the fixed timing Q-factor (Qt), we have devised a simple methodof monitoring the Q-factor using open eye-diagrams
captured by high-speed asynchronous OE sampling.
We designed a signal quality monitoring circuit andexperimentally confirmed its ability to estimate the
BER for 10-Gbit/s NRZ signals. We also showed thatthe circuit offers two modes: Qt and average Q-factor
Qavg evaluation. This technique and circuit will forma powerful solution for satisfying the performancemonitoring requirements of future optical networks.

Hidehiko Takara

References

Satoki Kawanishi

[1]

[2]

[3]

[4]

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G. Bendelli, C. Cavazzoni, R. Girardi, and R. Lano, “Optical performance monitoring techniques,” in Proc. 26th European Conference on
Optical Communication (ECOC 2000), Munich, Germany, Vol. 4, pp.
113-116, Sep. 2000.
R. Giles, “Monitoring the Optical Network,” in Proc. Symposium on
Optical Fiber Measurement, 2002, Boulder, United States, pp. 19-24,
2002.
I. Shake and H. Takara, “Averaged Q-factor Method using Amplitude
Histogram Evaluation for Transparent Monitoring of Optical Signalto-Noise Ratio Degradation in Optical Transmission System,” J.
Lightwave Technol., Vol. 20, No. 8, pp. 1367-1373, 2002.
S. Ohteru and N. Takachio, “Optical signal quality monitor using
direct Q-factor measurement,” IEEE Photonics Tech. Lett., Vol. 11,

Senior Research Engineer, Advanced Photonic
Transmission Research Group, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received the B.S., M.E., and Ph.D. degrees
in electrical engineering from Keio University,
Kanagawa in 1986, 1988, and 1997, respectively.
In 1988, he joined NTT Transmission Systems
Laboratories, Kanagawa, Japan. Since then, he
has been engaged in research on ultrahighspeed/large-capacity optical transmission systems and optical measurement techniques. He isa member of IEEE and IEICE. He received apaper award from IEICE in 1993 and was awarded the Kenjiro Sakurai Memorial Prize fromOEIDA in 1996 and the Electronics Letters Premium from the IEE in 1997.

Senior Research Engineer, Advanced Photonic
Transmission Research Group, Photonic Transport Network Laboratory, NTT Network Innovation Laboratories.
He received the B.E., M.E., and Ph.D. degrees
in electronic engineering from the University ofTokyo, Tokyo in 1981, 1983, and 1993, respectively. In 1983 he joined the Yokosuka ElectricalCommunication Laboratory, Nippon Telegraphand Telephone Public Corporation (now NTT),
Kanagawa, Japan, where he has been engaged in
R&D of high-speed optical transmission systems
and optical signal processing using photonic
crystal fiber. He is a member of the Optical Society of America, IEEE, IEICE, and the Japan
Society of Applied Physics. He received Paper
Awards from IEICE in 1993 and1995 and anachievement award from IEICE and the Sakurai
memorial award in 1996.

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