We propose an improved technique for in-band OSNR monitoring based on a Brillouin fiber ring laser seeded by the optical channel to be monitored. This technique shows a reduction of the required input power into the monitor along with a large and tunable dynamic OSNR monitoring range. It is demonstrated experimentally and numerically for various bit rates and modulation formats

1. July 2nd, 2012
Brillouin Fiber Ring Laser based In-
Band OSNR Monitoring Method for
Transparent Optical Networks
David Dahan, Uri Mahlab, Yuval Shachaf
ECI Telecom
Network Division Solutions

2. Motivations :
Requirement of in-band OSNR monitor
Deployment of high speed transparent and
reconfigurable optical networks requires
effective, flexible and robust Optical Performance
Monitoring techniques
The most common method to monitor the OSNR
derives the OSNR level by estimating the in-band
noise level using the out-of-band noise level
measurement
However out-of band OSNR approaches lead to
very large underestimation of real OSNR level in
ROADM based networks
There is a strong requirement in developing
efficient in-band OSNR monitor techniques
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3. Motivations
Several in band OSNR monitoring techniques have been proposed
such as :
PMD and PDL sensitive and not
Polarization nulling techniques compliant with polarization
multiplexed modulation formats
Delay tap asynchronous sampling
Nonlinear transfer functions using an CD & PMD sensitive
optical parametric amplifier
Nonlinear loop mirror
CD & PMD insensitive*
Stimulated Brillouin scattering* Compliant with Polarization
multiplexing**
*M.D. Pelusi et al., “Multi channel in band OSNR monitoring using Stimulated Brillouin
Scattering” , Opt. Express,18(9), 9435-9446, 2010
**Dahan et al. , “Stimulated Brillouin Scattering based in-band ONSR monitoring technique
for 40 Gbps and 100 Gbps optical transparent networks”, Opt. Express, 18(15), 2010not for distribution
Confidential , 3

4. Stimulated Brillouin Scattering (SBS)
process
Brillouin scattering is the interaction between light and sound waves in the
matter. The propagating light beam in the fiber generates a propagating sound
wave which creates a periodic variation of the fiber refractive index. This
generates a Fiber Bragg Grating that backscatters the light through Bragg
diffraction process . The back scattered wave , called “Stokes wave” is
downshifted by ~10 GHz with regard to the incident wave frequency
When increasing the launched power of the optical beam, the reflected power
increase linearly due to back Rayleigh scattering effect in the fiber.
Above a given threshold, the reflected power increases exponentially ; this is
due to the stimulated Brillouin scattering effect
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5. SBS based in band OSNR monitoring
technique : principle & challenges
EDFA
Power Meter
For a given fixed input power, the back-reflected power is OSNR dependent
For bit rates higher than 10 Gb/s , OSNR requirements at the RX become stronger
and links should be planned to meet OSNR>15dB. Therefore, the in band noise is
not high enough to cause a significant change of the back reflected power in the
OSNR monitor, limiting the accuracy of the OSNR measurement.
Beyond 10 Gb/s, the optical channels present very high SBS threshold due to the
use of carrier-less modulation formats (DQPSK, PM-QPSK,PM-16QAM). This requires
the use of long and expensive nonlinear fiber along with high power optical amplifier to
generate the SBS effect: prohibitive cost of the monitor unit! Confidential , not for distribution 5

6. Brillouin Fiber Ring laser based in
band OSNR monitoring technique
• A novel, relatively low cost technique for SBS based in-band OSNR monitoring,
compliant with very high bit rates and various modulation formats.
• Enabling to increase and tune effectively the OSNR sensitivity monitoring range
• This technique is based on the lasing process of a Brillouin Fiber Ring Laser
(BFRL) where the optical seed is the modulated signal to be monitored
A 6km DCF is used in the fiber ring
to stimulate the SBS process
The feedback section loss R is
defined as
RdB  ILOC  ILOC  IL feedbackfiber  ILsplitter
1 2
Because of the optical circulator
configurations, only the Stokes
waves undergoes multiple round trip
into the ring
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7. Power equations of the BFRL
Assuming parallel SOP of the signal and Stokes waves, steady states differential
equations governing the signal, Stokes and Rayleigh backscattering powers in
the DCF are :
Simulation parameters Value
 dPsig gB
  Psig 
DCF Length 6.1 km
 Psig PStokes  B g B Psig PStokes L
α DCF loss coefficient 0.75 dB/km
 dz Aeff
αR Rayleigh backscattering 2.7 10-3 dB/km
 dPStokes
 gB coefficient
   PStokes  Psig PStokes  B g B Psig PStokes gB Brillouin gain coefficient 1.65 10-11 m/W
 dz Aeff B Spontaneous Brillouin 8.5 10-3 W m3
 dP
scattering noise coefficient
Aeff DCF effective mode area 16 μm2
 Rayleigh   PRayleigh   R Psig Feedback loss (open loop) ∞
 dz
 R Feedback loss (close loop) 4.4 dB
With feedback loss R, DCF length L, the boundary condition are

 Psig  0   P0

 P
PStokes  L   Stokes
 0

 R
 PRayleigh  0 
 PRayleigh  L  
 R
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8. Principle of operation
Experimental & numerical results for CW signal
Close loop Open loop
RdB=4.4 dB RdB=∞
The power threshold is defined as the
input power that leads to
PTH=Pout=PStokes+PRayleigh=2PRayleigh
PTH Pout=PTH+20dB
Pin in close loop 0.3 dBm 0.95 dBm
Pin in open loop 6.5 dBm 9.5 dBm
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9. Principle of operation
Experimental & numerical results for 10.7 Gb/s OOK NRZ
signal
10.7 Gb/s NRZ OOK without frequency dithering 10.7 Gb/s NRZ OOK with 10 kHz frequency
dithering
PTH Pout=PTH+20dB PTH Pout=PTH+20dB
Pin in close loop 4.2 dBm 5.6 dBm 10.2 dBm 14 dBm
Pin in open loop 10.7 dBm 12.9 dBm 17.2 dBm 22.6 dBm
Without frequency dithering With 10 kHz frequency dithering 9
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10. In Band OSNR monitor
Experimental results for 10.7 Gb/s OOK NRZ signal with
frequency dithering
Close loop configuration
(RdB=4.4 dB)
Power dynamic range= Pout variations over a given OSNR range variations
OSNR range Pin=11.5 dB Pin=11.9 dB Pin=12.3 dBm Pin=13.1dBm
10 dB - 15 dB 0 dB 2 dB 8.6 dB 8.2 dB
15 dB - 20 dB 5 dB 8.5 dB 4.5 dB 2.2 dB
20 dB - 30 dB 5.2 dB 3.3 dB 0.7 dB 0.6 dB
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11. Principle of operation
Experimental results for 44.6 Gb/s RZ-DQPSK signal
Carrier-less modulation formats such as DQPSK exhibit a very high SBS
threshold leading to a very high required optical launched power.
In order to reduce the required launched power, a small power fraction of an
optical pilot tone is inserted to a 44.6 Gb/s RZ-DQPSK signal at the output of the
transmitter
We define Optical Signal to Pilot tone Ratio
(OSPR) as :
Psignal
OSPR 
PpilotTone
An optimal frequency detuning, Δf=fsig-fpilotTone
can be found with reduced the pilot tone induced
penalty at the receiver thanks to the transfer
frequency response of the DLI at the receiver and
the balanced detection
For 44.6 Gb/s RZ-DQPSK signal, OSPR level of 13 dB and frequency offset Δf=-12.3 GHz, give
an OSNR penalty of 0.3 dB
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12. In Band OSNR monitor
Experimental & numerical results for 44.6 Gb/s RZ- DQPSK signal
OSNR=24 dB, OSPR=13 dB, 44.6 Gb/s RZ-DQPSK signal with OSPR=13 dB,
offset Δf=-12.3 GHz offset Δf=-12.3 GHz
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13. Numerical results :
120 Gb/s PM-QPSK signal
Since the PM-QPSK modulation format presents carrier-less spectrum
characteristics, an optical tone is added at the signal carrier frequency.
6dB
OSNR penalty < 0.5 dB at BER=1.5E-2 is
With OSPR =16dB , the pilot tone
achieved for OSPR =16dB in the case of
peak is 6 dB above the signal
transmission over a CD uncompensated
spectrum
link of 1000km.
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14. In Band OSNR monitor
Numerical results for 120 Gb/s PM-QPSK signal
Close loop configuration – 120 Gb/s DP-QPSK signal with OSPR=16 dB,
RdB=4.4 dB offset Δf=0 GHz
OSNR range 10 dB -15 dB 15 dB -20 dB 20 dB -30 dB
Optimum Pin 17.5 dBm 16.9 dBm 16.6 dBm
Power dynamic range 10.8 dB 6.2 dB 3.2 dB
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15. Numerical results :
224 Gb/s PM-OFDM signal
The 224 Gb/s PM-OFDM signal is
composed by 128 subcarriers with cyclic
8dB
prefix of 12.5%.
Some subcarriers are used as pilot tones for
equalization purposes at the receiver while
the modulated subcarriers use a 16-QAM
modulation scheme.
The OFDM signal presents an RF pilot tone
at the optical carrier frequency for blind
phase noise compensation purposes at the
receiver :this is the main contributor of the
SBS effect
The RF pilot tone peak is 8 dB above the other subcarrier components
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16. In Band OSNR monitor
Numerical results for 224 Gb/s PM-OFDM signal
Close loop configuration
RdB=4.4 dB
OSNR range 10 dB -15 dB 15 dB - 20 dB 20 dB - 30 dB
Optimum Pin 19 dBm 16 dBm 15 dBm
Power dynamic range 16 dB 14 dB 11.3 dB
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17. In Band OSNR monitor
System calibrations
Estimated OSNR measurement uncertainty for power monitoring accuracy of +/- 0.1dB
44.6 Gb/s 120 Gb/s 224 Gb/s
OSNR [dB] Min Max Min Max Min Max
10 9.6 10.4 9.5 10.5 9.8 10.2
12 11.4 12.5 11.4 12.5 11.8 12.3
Good
15 14 16 14 16.1 14.7 15.3
18 16.3 20.3 16.3 20.3 17.5 18.6
20 17.5 24.4 17.5 24.3 19.3 20.7
25 19.8 30 19.9 30 23.2 27.1 Not good
Such an increase in the OSNR inaccuracy is caused by :
 The power dynamic range decreases at high OSNR
 Optimum input power into the monitor approaches the lasing threshold level where the
Brillouin laser is very sharp and power monitoring inaccuracies might lead to large errors.
MUX PC PS
Solution : TX 50% 50%
OSA
 Working in the optimized OSNR range of 50% 50%
ASE source VOA
10 dB -15 dB by adding a known level of ASE
noise before the monitor Monitored optical signal
 Deriving the altered OSNR level
PD0 PD1
OTF OC1 DCF OC2
PC VOA (Pin)
 With the knowledge of the ASE added level, EDFA
Psig,out
Stokes
the real OSNR level is estimated * signal
50%
*Dahan et al. , “Stimulated Brillouin Scattering based in-band ONSR ASE source OSNR range 50%
VOA PS
monitoring technique for 40 Gbps and 100 Gbps optical transparent shifter
PD2 (Pout)
networks”, Opt. Express, 18(15), 2010 17
Confidential , not for distribution

18. Conclusions
We have proposed a novel and improved approach for in-band
OSNR monitoring based on Brillouin fiber ring laser seeded by the
signal to be monitored
We have demonstrated experimentally and numerically that such a
technique enable to reduce drastically the required input power into
the OSNR monitor and provided a large OSNR dynamic power
variations for acceptable monitoring accuracy
In order to provide acceptable monitoring accuracy, the OSNR
monitor should be operated in the optimized OSNR range of 10-15dB
by adding a known ASE level into the signal if needed
For carrier-less modulation formats, a relative low power pilot tone
can be inserted into the signal at the transmitter to reduce the SBS
threshold to acceptable values while leading to relative low OSNR
penalty Confidential , not for distribution 18