Optical Performance Monitoring using Modulated Power Detection
K Balasubramanian*, P Palai
Tejas Networks India Ltd., Khanija Bhavan, 49 Race Course Road, Bangalore 560 001, India.
ABSTRACT
With DWDM networks becoming more and more complex, optical performance monitoring has become a crucial tool
for providing fault management, user quality-of-service (QoS) guarantee and to ensure user compliance to service
agreements. In this paper, we propose a device, which can detect the modulated power in an optical signal. The device
uses a high frequency photodiode which converts the optical signal to the radio frequency (RF) domain. It also blocks the
DC component (0 Hz frequency) and gives a voltage equal to the root mean square value of the AC ( > 0 Hz frequency)
RF signal. Simulation has been done to characterize the modulated power detection circuitry using
VPItransmissionMakerTM tool. Some of the key applications like measurement of extinction ratio, sensitivity of the
device to varying fiber span and in-band optical noise have also been simulated.
Keywords: Optical performance monitoring, modulated power, extinction ratio
1. INTRODUCTION
DWDM networks can accommodate more than 100 channels to be simultaneously transmitted on a single fiber, at data
rates of up to 10 Gbps, per channel over several thousand kilometers. Various degrading effects like noise, waveform
distortion and timing jitter can severely affect the functioning of these complex DWDM networks. These factors have
made performance monitoring of optical signals very crucial. The main aim of performance monitoring it to enable
network operators to provide guaranteed quality of service to the users. It is also necessary to ensure that users of the
networks comply with the service agreement between them and the network operator.
Optical Performance Monitoring can be classified into 3 layers 1 . They are:
1) WDM Channel Management Layer Monitoring. E.g. Optical signal to noise ratio (OSNR), Channel Presence.
2) Channel quality Layer monitoring. For example, per channel eye statistics, electronic SNR, Q factor.
3) Protocol Performance Monitoring. A typical example is monitoring of BER.
Some of the parameters that need to be monitored in a WDM network are average channel power, spectral OSNR, inband OSNR, BER, channel spacing, ?, line-width and drift monitoring. Other advanced signal parameters, which can be
monitored, are accumulated chromatic dispersion, jitter, Polarization Mode Dispersion (PMD) and Q factor.
In this paper, we propose an optical modulated power monitoring device that can be used to determine Extinction Ratio
(ER) of an optical signal. It can also be used to measure in-band OSNR in WDM based systems which is normally very
difficult to measure with a conventional spectrum analyzer type of device.
The Ext inction Ratio (ER)R re of an optical signal can be defined as the ratio of the optical power in Logic 1 (P1 ) to the
power when logic 0 (P0 ) is transmitted. The ER can also be calculated by measuring the average power Pavg (or DC
power ) and modulated power Pmod ( AC power). The average power of the input optical signal ( DC power) is
defined as: (P1 + P0 )/2 and modulated power or AC power is defined as (P1 - P0 )/2. Figure 1 illustrates a typical eye
diagram in an optical communication system. From the diagram, the extinction ration can also be calculated as 2 :
Extinction Ratio re = (Pavg + Pmod) /( Pavg - Pmod ). The power penalty is the ratio of the average power required for a given
value of re to the average power required for the ideal case of re = ∞. Power Penalty3 can be evaluated as δe = Pavg (re) /
Pavg ( re = ∞) = (re +1 )/(re-1).
karthikb@india.tejasnetworks.com, phone: 91-80-22267495; fax 91-80-22267494
Figure 1 Eye Diagram showing Average and Modulated Power
2. DESCRIPTION OF THE DEVICE
Figure 2 shows a schematic of the extinction ration calculation device. The device is traffic non-intrusive. A 5% optical
tap is placed at the output of the signal carrying fiber. The 5% optical power is fed into the modulated power monitor,
which measures both the Pmod and Pavg. The output value of the device are digitized using a ADC and is finally
processed by a microprocessor. Figure 3 gives more details of the components that go into the modulated power monitor.
Input Fiber
95 % Output
Fiber
95:5 Coupler
5 % Output
Fiber
DAC
Modulated
Power
Monitor
ADC
Micro
Processor
Figure 2: Schematic Diagram of Modulated Power Monitor. DAC and ADC are digital to analog and analog to digital
converters, respectively.
Pavg
G
DC
Component
Figure 3: Schematic
details of the modulated power measurement.
PD
RF TRANSFORMER
Pmod
AC
Component
G
RMS to DC
Conversion
Figure 3: Schematic details of the modulated power monitor
A high frequency photodiode (PD) converts the optical signal into the electrical output. The RF transformer filters the
input power into its DC ( 0 Hz) and AC ( > 0 Hz ) components. The DC component gets amplified to yield average
power (Pavg). The AC component goes through a couple of stages of RF amplification and imp edance matching and
subsequently gets fed to a RMS-to-DC converter to yield modulated power (Pmod). The RMS value of a signal, in this
case, can be defined as:
The above mentioned RMS-to-DC converter gives a DC output voltage exactly equal to the RMS value of the input
signal. The energy in the entire RF spectrum, except the DC energy which was diverted already, is integrated and this
energy is expressed as a DC voltage output. There are special RMS Integrated Chips (ICs) that can perform the abovementioned function4 . The operational details of the RMS-to-DC are beyond the scope of the paper, but is welldocumented eleswehere 5 .
The voltages generated proportional to Pavg (Vavg) and Pmod (Vmod) can be digitized by an analog to digital converter
and can be used by a microprocessor to calculate the extinction ratio of the input optical signal. The power penalty in the
system owing to the finite extinction ration can also be calculated. Also, the device can be used to determine
performance degradation due to loss in a system as well as the in-band OSNR in a system. All these applications are
simulated in the next section.
3. SIMULATIONS AND RESULTS
In order to evaluate performance of such an device following simulations were performed using the
VPItransmissionMaker, an optical system level simulation tool:
i) Modulated power output of the device for an optical signal with varying extinction ratio
ii) Modulated power output of the device for an optical signal attenuated by different fiber spans
iii) Measurement of in-band OSNR in WDM based systems.
The details of the three simulations are given below.
Modulated
Transmitte
r with
Variable
Extinction
Ratio 1 dB
to 35 dB
Modulate
d Power
Detection
Circuit
Vmod
RF
Spectrum
Analyzer
Figure 4: Modulated Power Detection Setup
Relative RMS Value (dB)
i) The system shown in Figure 4 was simulated using the VPI Transmission Maker. The source consists of a modulated
laser transmitter whose extinction ratio is varied widely from 1 dB to 35 dB. The RMS power is detected by the
proposed device and corresponding RF spectrum is measured by the RF spectrum analyzer.
0
-5
-10
-15
-20
0
10
20
30
Extinction Ration (dB)
Figure 5: Vmod output of the device for various input
extinction ratios
Figure 5 shows the Pmod (Vmod) value output of the device against the input ER variation (1 to 35 dB) of the optical
transmitter having a constant average power and was modulated at 10Gbps. It can be clearly seen that the RMS value is
almost linear up to 10dB and then it saturates at higher value of Extinction ratio. As most of the time one requires
extinction ratio around 30dB or so for a transmitter for good system performance, any degradation of the extinction ratio
can be monitored by deploying the proposed device with a threshold setting.
Figure 6 shows the variation of the RF spectrum seen
by the device with input ER variation for the 10Gbps
optical transmitter. The amount of the RF power
increases with extinction ratio and eventually saturates
beyond 10dB. It can be seen from the result that the
device gives a voltage proportional to extinction ratio of
the input signal.
There are two major challenges to measuring ER using
this method. Firstly, measuring the modulated power
with minimum signal degradation of the input light
stream. For example, a common source of error is the
offset due to the "dark current" of the photodiode.
Secondly, processing the extracted information. This
includes the statistical processing of the acquired data
and maximum scaling of the signal to the ADC.
Figure 6: RF Spectrum as seen by the device for various
extinction ratios
ii) The variation of the Vmod for various optical attenuation ( variable cable lengths) have also been simulated. The
schematic of simulated system is shown in Figure 7.
Variable
Fiber
Length
Modulated
Power
Detection
Circuit
BER Tester
1.E-08
-65
1.E-23
1.E-38
-70
-75
1.E-53
1.E-68
-80
-85
1.E-83
1.E-98
-90
-95
BER
Modulated
Transmitter
Vmod
RMS Value (dBW)
-60
1.E-113
1.E-128
-100
-105
1.E-143
1.E-158
-110
0
20
40
60
80
100
Fiber Length (km)
Figure 7 Modulated power output as a function of variation
in attenuation
Figure 8: Variation of RMS power and BER with fiber length
Figure 8 depicts the variation of the Vmod which is the RMS power of the modulated signal with various fiber lengths. It
also depicts the corresponding bit error ratio (BER) variation with the fiber lengths. As the fiber length increases it
introduces more attenuations between the modulated transmitter and the measuring devices such as the modulated power
detection circuit and also the BER tester. It can be seen that the proposed device can measure the RMS value which is
indicative of the variation of the BER and can be used as monitoring device for the above mentioned configuration very
easily.
iii) OSNR is a key optical parameter that has been widely used for troubleshooting network faults. OSNR is usually
calculated as the ratio of the average power in the carrier frequency to that of the optical noise power extrapolated from
the power level adjacent to the channel. The assumption here is that the optical signal noise level in-band is nearly same
as that of the power level in the adjacent frequency. This assumption may not be valid for many important noises like
multi-path interference effects, amplifier pump laser RIN transfer noise and four wave mixing.
Variable Inband optical
noise
Modulated
Power
Detection
Circuit
Vmod
-61
1.E-04
-62
1.E-24
-63
1.E-44
-64
1.E-64
-65
1.E-84
-66
1.E-104
-67
BER Tester
Figure 9 Modulated power output as a function of in-band
noise power levels
BER
Modulated
Transmitter
RMS Value (dBW)
.
1.E-124
0
2
4
6
8
In-band Noise Density (10^-15
W/Hz)
Figure10: Variation of RMS value and BER with inband noise in the units of ( 1µW/0.1nm)
The other option is to measure the in-band OSNR that calls for a clear discrimination between the noise and signal inside
the band. Since the device detects both the average power as well as the modulated power in the spectrum, it has
significant advantages over the other spectral optical channel measurement systems
Such an set up is considered in Figure 9. The modulated laser transmitter is subjected to varying in-band noise. Whereas
a typical OSNR measurement equipment based on filtering or diffraction would not take this noise into consideration, the
modulation power detection device clearly gives a different RMS voltage Vmod for different in-band noise power levels.
The results of the simulation are shown in Figure 10. Thus it can be seen that for a variation of 20 dB of the in-band
noise there is a linear variation of modulated power voltage Vmod (measured in dBW).
4. Conclusion
With increasing complexity of DWDM networks, optical performance monitoring has a great role to play in its
management. The modulated optical power meter device can play a significant role in that it measures not only the
average power but also the modulated power, which is the measure of signal quality. Also, the response of the device to
various in-band optical noise, signal extinction ratio and signal attenuation has be simulated. Proper characterization and
optimization of extinction ratio can bring significant benefits in terms of system cost and performance. Cost reduction is
realized when a fiber-optic-based system can reliably transmit and receive a signal over a greater distance. This directly
impacts the number of regenerators or optical amplifiers that are required in the system. This, in turn, can influence
performance, because system-timing jitter will accumulate as a function of the number of active components in the
transmission path. Thus the modulation power detection can greatly add to the management and cost reduction in
network operation.
REFERENCES
1. D.C.Kilper, et. al. , “Optical Performance Monitoring”, Journal of Lightwave Technology, Volume 22, No.1, January
2004.
2. Chapter 8: Dennis Derickson, Analysis of Digital Modulation on Optical Carriers in Fiberoptic Test and
Measurement, Prentice Hall, USA, 2002.
3. Chapter 5: K Sivarajan, Transmission System Engineering in Optical Networks: A practical Perspective, Morgan
Kaufmann, India, 1998.
4. Charles Kitchin and Lew Counts, “RMS to DC Conversion Application Guide”, Analog Devices, www.analog.com ,
1988.
5.“Make a truly linear RF-power detector”, Victor Chang and Eamon Nash, Analog Devices, Page 106, EDN,
www.edn.com, September 19, 2002.