Reflection Considerations in Seeded DWDM Transmission LinksSyed Aun Abbas3, Hee-Yeal Rhy1, Changhee Lee2, Bongsoo Kim1, Jun-Young Kim2 1. LG-Ericsson R&D Center, Anyang-shi, South Korea 2. Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Daejeon, South Korea 3. National ICT R&D Fund, Islamabad, Pakistan

Conventional DWDM (G.698.1 ) system Architectures

Seeded DWDM (G.698.3) system architectureRS for signals Rx1 Tx1 MPI-SM Rx2 Tx2 OD /OMfor signals going going

Rx1 Tx1

SS for signalgoing

RS OD /OM SS

Rx2 Tx2

MPI-RMfor signals going

RxN TxN Head-end

RS Seed source SS Black link

RxN TxN Tail-end

Seeded vs. Conventional DWDM Systems Bi directional operation Fiber savings

Color adaptive (less) operations Cost savings and operational ease

G.sdapp is now G.698.3

Back reflections Back reflections in ODN Intrinsic back reflection induced by Rayleigh back scattering Reflections due to imperfect Connectors Splices

Injection seeded WDM-PON uses bi-directional transmission of the same wavelength for upstream. There exist multiple reflection routes Impact of reflections on system performance Contained through specification of three parameters minimum optical return loss, maximum discrete reflectance maximum reflectance of the optical network elements

Various reflection scenarios in a Seeded DWDM systemBlack link ORL R_US Scenario -2 Tx Scenario - 4 Scenario - 3 OM WDM_HEE SS Scenario - 1 Rx Black Link ORL R_DS

Black link

Scenario - 5 Tx WDM_TEE

MPI-SM Scenario - 6

Rx

HEEOD

TEE

BLS

OD/OM

Route 3, Route 4 and Route 6 are suppressed by WDM filters Route 1 is reflection of BLS to OLT > US penalty Route 2 is reflection of DS Tx to OLT > DS penalty Route 5 is reflection of US Tx to TEE > US penalty Route 2 and route 5 are the same type of reflection

Impact of HEE BLS into HEE receiver > US penalty maximum BLS optical power of channel number n of HEE at MPI-SM minus ORL of black link at MPI-SM must be less than the permissible interference optical power. The returned seed signal power is a broadband noise signal into the upstream/HEE receiver. The interferometric (in-band) cross talk level is given by: Crosstalk Level (dB) = Minimum received signal power (Maximum power of the seed signal ORL)

In G.698.3, the typical maximum mean channel seed signal input power at RS will be around -2 dBm. The typical insertion loss of the optical path is between 3-14 dB, the maximum BLS optical power for a channel at MPI-SM can be as much as 1 dBm the minimum received signal power, will thus be at least -16.5 dBm.

As a result, Crosstalk Level = ORL - 17.5

Power penalty vs. interferometric crosstalk simulated power penalty as a function of the interferometric crosstalk @ 10-12 BER for different values of the injection power.5-5 dBm injection -10 dBm injection

Power penalty (dB)

4

-15 dBm injection -18 dBm injection -20 dBm injection

3

2

1

0 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0

Crosstalk (dB)

Impact of HEE BLS into HEE receiver > US penalty For optical return loss of 32 dB, the interfering optical power due to seed signal of channel number n at the MPI-SM is 14.5 dB. For the injection power (greater than -18 dBm) and crosstalk values of interest (14.5 dB and higher), the penalty is somewhere between one quarter to a one third of a dB, where higher penalty corresponds to the lower injection seed signal powers.

Impact of TEE transmitter into HEE receiver In this case, care must be taken to avoid providing high reflection feedback to TEE transmitter. The back reflection can deteriorate the performance due to the reflectively modulating nature.

Signal

Un wanted Signal Reflective Gain : G dB

Reflected signal by ORL : X dB

Worst case scenario of interest The TEE transmit signal will be reflected back at the SS interface by an amount of ORL loss of X dB. It will be reflected back into the optical path from SS to MPI-RM as interferometric crosstalk by an amount equal to the Gain G of the reflective modulator. The unwanted signal power is given as Signal power X dB + G dB . The cross talk level (-X dB + G dB) should be less than the permissible interference level.

Power penalty vs. interferometric crosstalk @ 10-12 BER for different values of the injection power5-5 dBm injection -10 dBm injection -15 dBm injection -18 dBm injection -20 dBm injection

Power penalty (dB)

4

3

2

1

0 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0

Crosstalk (dB)

Crosstalk Level = Signal power (Signal power ORL +Maximum gain of the reflecting modulator) ORL = Crosstalk Level - Maximum gain of the reflecting modulator For the injection power (greater than -18 dBm) and crosstalk values of interest (14-16 dB), the power penalty is less than quarter dB.

CONSIDERATIONS FOR THE MAXIMUM DISCRETE REFLECTANCE OF THE OPTICAL PATH The maximum number of connectors or other discrete reflection points in the optical path must be such that they allow the desired overall optical return loss to be achieved. Maximum discrete reflectance must be less than the maximum optical return loss. In order to avoid unacceptable impairments due to multiple reflections, it may be necessary to limit the number of connectors or use connectors having improved reflectance performance.

CONSIDERATIONS FOR THE MAXIMUM REFLECTANCE OF OPTICAL NETWORK ELEMENTS Maximum Reflectance of the TEE receiver at Rs BLS and HEE optical signal reflection by TEEBlack linkRS WDM_TEE Tx Tx

Scenario -1OM Rx WDM_HEE

Rx

Scenario- 2

R_TEE_Rx

TEE

HEEOD BLS

OD/OM

TEE optical signal reflection by HEEBlack linkTx

R_HEE_Rx

OM WDM_HEE Rx

MPI-RM

Tx WDM_TEE Rx

Scenario - 3

HEEOD BLS

OD/OM

TEE

Conclusions The seed source and reflective transmitters together with single fiber multiplexed architecture create 4 additional reflection scenarios compared to the conventional DWDM systems. The reflectance of the receiving equipment is the same as the conventional DWDM systems, The optical return loss and the maximum discrete reflectance of the optical path needs to be more stringent than the conventional systems.

ConclusionsOptical path from point MPI-SM to RS Minimum optical return loss at MPI-SM Maximum discrete reflectance between MPI-SM and RS Optical path from point SS to MPI-RM Minimum optical return loss at SS Maximum discrete reflectance between SS and MPI-RM Interface at point RS Maximum reflectance of receiver Interface at point MPI-RM Maximum reflectance of optical network element dB -27/-27 dB -27/-27 dB dB 32/24 -35/-27 dB dB 32/24 -35/-27