In-band OSNR Monitoring Technique based on Brillouin Fiber Ring Laser

Brillouin Fiber Ring Laser based In-Band OSNR Monitoring Method for

Transparent Optical Networks

David Dahan, Uri Mahlab, Yuval Shachaf

July 2nd, 2012

ECI Telecom Network Division Solutions

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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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Motivations

Delay tap asynchronous sampling

Nonlinear transfer functions using an

optical parametric amplifier

Nonlinear loop mirror

*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), 2010

PMD and PDL sensitive and not

compliant with polarization

multiplexed modulation formats

CD & PMD sensitive

CD & PMD insensitive*

Compliant with Polarization

multiplexing**

Several in band OSNR monitoring techniques have been proposed

such as :

Polarization nulling techniques

Stimulated Brillouin scattering*

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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

Stimulated Brillouin Scattering (SBS)

process

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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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SBS based in band OSNR monitoring

technique : principle & challenges

For a given fixed input power, the back-reflected power is OSNR dependent

5

EDFA

Power Meter

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!

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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

Because of the optical circulator

configurations, only the Stokes

waves undergoes multiple round trip

into the ring

1 2dB OC OC feedbackfiber splitterR IL IL IL IL

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Power equations of the BFRL

sig Bsig sig Stokes B sig Stokes

eff

Stokes BStokes sig Stokes B sig Stokes

eff

Rayleigh

Rayleigh R sig

dP gP P P B g P P

dz A

dP gP P P B g P P

dz A

dPP P

dz

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 :

With feedback loss R, DCF length L, the boundary condition are

00

0

0

sig

Stokes

Stokes

Rayleigh

Rayleigh

P P

PP L

R

PP L

R

Simulation parameters Value

L DCF Length 6.1 km

α DCF loss coefficient 0.75 dB/km

αR Rayleigh backscattering

coefficient

2.7 10-3 dB/km

gB Brillouin gain coefficient 1.65 10-11 m/W

B Spontaneous Brillouin

scattering noise coefficient 8.5 10-3

Aeff DCF effective mode area 16 μm2

R Feedback loss (open loop) ∞

Feedback loss (close loop) 4.4 dB

3W m

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Principle of operation Experimental & numerical results for CW signal

Close loop

RdB=4.4 dB

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

Open loop

RdB=∞

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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

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In Band OSNR monitor Experimental results for 10.7 Gb/s OOK NRZ signal with

frequency dithering

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

Power dynamic range= Pout variations over a given OSNR range variations

Close loop configuration

(RdB=4.4 dB)

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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 :

signal

pilotTone

POSPR

P

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

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

Principle of operation Experimental results for 44.6 Gb/s RZ-DQPSK signal

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In Band OSNR monitor Experimental & numerical results for 44.6 Gb/s RZ- DQPSK signal

OSNR=24 dB, OSPR=13 dB,

offset Δf=-12.3 GHz 44.6 Gb/s RZ-DQPSK signal with OSPR=13 dB,

offset Δf=-12.3 GHz

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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.

OSNR penalty < 0.5 dB at BER=1.5E-2 is

achieved for OSPR =16dB in the case of

transmission over a CD uncompensated

link of 1000km.

With OSPR =16dB , the pilot tone

peak is 6 dB above the signal

spectrum

6dB

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Close loop configuration –

RdB=4.4 dB

120 Gb/s DP-QPSK signal with OSPR=16 dB,

offset Δf=0 GHz

In Band OSNR monitor Numerical results for 120 Gb/s PM-QPSK signal

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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Numerical results :

224 Gb/s PM-OFDM signal

The 224 Gb/s PM-OFDM signal is

composed by 128 subcarriers with cyclic

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

8dB

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Close loop configuration

RdB=4.4 dB

In Band OSNR monitor Numerical results for 224 Gb/s PM-OFDM signal

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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In Band OSNR monitor System calibrations

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

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

Estimated OSNR measurement uncertainty for power monitoring accuracy of +/- 0.1dB

Good

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.

Solution :

Working in the optimized OSNR range of

10 dB -15 dB by adding a known level of ASE

noise before the monitor

Deriving the altered OSNR level

With the knowledge of the ASE added level,

the real OSNR level is estimated *

*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), 2010

EDFA

PS

VOAOTF

PD1 DCF OC1

Stokes

signal

Monitored optical signal

Psig,out

OSATX

(Pin)

(Pout)

ASE source VOA

PSPC

50%

50%

50%

50%

MUX

50%

50%

OC2

PD2

ASE sourceVOA

PD0

PC

OSNR range shifter

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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