Optische netwerken - SNE/OS3 Homepage [OS3 Website] netwerken SNE opleiding - 19 maart 2009 Roeland Nuijts, ... optical fiber transmission systems were “loss-limitedâ€‌,

Optische netwerkenSNE opleiding - 19 maart 2009

Roeland Nuijts, SURFnet, The Netherlands

roeland.nuijts@surfnet.nl

Outline

- Introduction

- Optical transmission fiber

- Optical transmitters and receivers

- DWDM enabling technologies

EDFA (E bi D d Fib A lifi )- EDFAs (Erbium Doped Fiber Amplifiers)

- Optical multiplex and demultiplex filters

- 10Gb/s transmission and Dispersion compensationp p

- High-speed transmission (40Gb/s and 100Gb/s)

- All optical switching WSS (Wavelength Selective Switches)

2

Optical fiber - Historical perspective

- Basic principle of internal reflection

known from 19th century (John

Tyndall, 1870)

E l fib ith l ddi t l - Early fibers with cladding extremely

lossy ~1000dB/km (1960)

- Progress in fabrication (MCVD)

leads to low loss fibers (0.2dB/km

at 1550nm wavelength, limited by

fundamental limit of Rayleigh

scattering) around 1979

3

Fiber absorption

- Fiber loss is wavelength dependent, minimum is around 1550nm- Current fiber loss is close to fundamental limit determined by Rayleigh

scattering, proportional to l-4 therefore dominant at short wavelengths- Loss at long wavelengths (l > 1625nm) dominated by infra-red absorption

4

- Peak at 1400nm arises from OH impurities, can be removed (AllWave fiber)

Fiber dispersion - Refractive index varies with wavelength which leads to a wavelength dependence of the group delay, tg, (delay for different gwavelengths) in ps/km

- Dispersion coefficient, D, is the derivative of the group delay, tg , with respect to wavelength per unit length (ps/nm km)tg

(ps)

1 2

1,4 Optical pulse shape at Tx output

(nm)0,0

0,2

0,4

0,6

0,8

1,0

1,2

4100 4300 4500 4700 4900 5100 5300 5500

Opt

ical

Pow

er (A

.U.)

D(ps/nm km)

0-30 -20 -10 0 10 20 30

Frequency (G Hz)

4100 4300 4500 4700 4900 5100 5300 5500

Time (ps)

0 (nm) -5

-10

-15Pow

er(d

B)

Optical spectrumat Tx output

l0

5

-30

-25

-20

15

Optic

alP

No distortion at zero-dispersion wavelength, l0 Distortion at other wavelengths

Optical fiber Historical perspective Standard SMF Optical fiber Historical perspective Standard SMF (G.652)

- Initial (80s) optical components for transmission through single mode fiber operated at the 1.3 mmwavelength, therefore fiber was developed which had zero-dispersion at this wavelength. For this wavelength, therefore fiber was developed which had zero dispersion at this wavelength. For this reason, this type of fiber is often referred to as standard fiber, conventional fiber or ITU G.652 fiber.

- Installed fiber base in the world is mainly comprised of this standard (1.3m zero-dispersion wavelength) SMF (Single Mode Fiber)

- This embedded base represents an enormous investment, strong incentive to use it

- Development and commercialization of sources and detectors operating in the 1550nm wavelength region, where the minimum fiber loss is achieved, were developed later, more specifically in the 80s

- Dispersion-Shifted Fiber (zero-dispersion at 1550nm) later developed and deployed, predominantly in Japan

17

6

NZDSF (Non-Zero Dispersion Shifted Fiber) optimizes dispersion in the EDFA region

7

Introduction Traditional digital point-to-point Introduction Traditional digital point to point optical fiber transmission systems

Transmission fiber

Tx Rx

a (dB/km)

Tx Rx

PT (dBm) PR (dBm)

T itt d l i l d b t i li ht d ff i

Transmission distance = (PT-PR) / a (km)

- Transmitter sends logical ones and zeros by turning light on and off, receiver converts received optical power to electrical signal, retrieves clock signal and determines on decision moment whether one or zero was sent

- Initial, low-speed, optical fiber transmission systems were loss-limited, transmission distance was limited by the thermal noise in the optical receiverdistance was limited by the thermal noise in the optical receiver

- Increase in transmission bit rate to high speeds (bit rate 2.5Gb/s) has made fiber dispersion, D, an important system parameter which limits the achievable transmission distance

8

Decibel scale versus li llinear scale

Power levels and loss scales in optical systems cover a hugh dynamic range Power levels and loss scales in optical systems cover a hugh dynamic range

Losses in fibers and filters are multiplication factors in linear domain, additions and subtractions in dB

Typically power levels and losses on logarithmic scale in decibels, more practical

Power is mW in linear domain -> dBm in logarithmic domain

Loss dimensionless in linear domain -> dB in logarithmic domain

P (mw) P (dBm) P (mw) P (dBm)

(mW)] [P Log 10=(dBm) P 10

[L]Log10=(dB)L 10

1000 30 1 0800 29 0.8 -1500 27 0.5 -3400 26 0.4 -4250 24 0.25 -6200 23 0 2 7[L]Log 10(dB)L 10 200 23 0.2 -7100 20 0.1 -1080 19 0.08 -1150 17 0.05 -1340 16 0.04 -1425 14 0 025 -1625 14 0.025 1620 13 0.02 -1710 10 0.01 -208 9 0.008 -215 7 0.005 -234 6 0.004 -24

x 2 3dBx 5 7dBx10 10dB

9

2 3 0.002 -271 0 0.001 -30

0 0d

10Gb/s optical transmitter technologies

{0,1,1,0,1,1,,0,1,0}

DFB

DM-DFB (Directly Modulated Distributed Feed Back laser) Cheap, small, low power consumption Chirped, i.e. different wavelength during ones and zeros which leads to a wide optical spectrum and associated transmission impairments Used for short reach transmission

EML (El t Ab ti M d l t L )

{0,1,1,0,1,1,,0,1,0}

DFB EA

EML (Electro-Absorption Modulator Laser) Monolithically integrated laser and modulator combination Potentially cheap, small, medium power consumption Chirped, i.e. different wavelength during ones and zeros Used for intermediate and long reach transmissiong

{0,1,1,0,1,1,,0,1,0}CW-DFB (Continuous Wave DFB laser) and MZ (Mach-Zehnder) combination

DFB

(Mach Zehnder) combination External modulator Expensive, relatively large, high-power drivers (high power consumption) Low (or deterministic) chirp, excellent

f

10

Mach-ZehnderLiNbO3 modulator

performance Used for long reach and DWDM (Dense Wavelength Division Multiplexing) transmission

Typical 10Gb/s optical receiver setup

preamp AGCdecisioncircuit

data

CLK

(A)PD

Photodetector converts optical signal to electrical signal. PIN or APD (Avalanche Photo Detector) for improved receiver sensitivity

P id hi h i l i

BER

Preamp provides high gain, low noise

AGC (Automatic Gain Control) amplifies signal at output of preamp to rail-to-rail voltage of decision circuit

Decision circuit, usually D-flip-flop, signal at input is clocked to the output on rising edge of clocksignal distortion is removed

10-9

10-6

rising edge of clocksignal, distortion is removed

BER (Bit-Error Rate) performance limited by thermal noise in receiver, receiver performance is usually specified in terms of receiver sensitivity, i.e. the amount of optical power needed to achieve a BER of 10-12

10-12

Psens

11

Prec (dBm)

WDM enabling technologies I: WDM enabling technologies I: EDFAs (Erbium Doped Fiber Amplifiers)

Fiber doped with Er3+ ions be excited by 980nm or 1480nm photonsp y p spontaneous emission generates noise Excited state Erbium ions can be stimulated to decay to ground state via stimulated emission by a 1550nm signal

12

Erbium Doped Fiber Amplifier

Erbium DopedFiber

isolator isolator

1480nmoror

980nm

13

ASE (Amplified Spontaneous Emission)

() = 2 h n sp (G() 1)

- Amplifiers are used to overcome fiber losses.- Optical Noise is added by each amplifier.

14

- Engineering rules usually defined for equal spans (e.g. 20 x 20dB) which is not the case in the real fiber networks

Slide courtesy of Kim Roberts, Nortel

Initial two-stage EDFA configuration, Initial two stage EDFA configuration, example

High-gain, low-noise first stage followed by high-power second stage

Current designs state of the art designs are wideband, 1520nm-1560nm (C-band) or 1565nm-1605nm (L-band), can be used for simultaneous amplification of multiple channels at different wavelengths

Bitrate transparent

15

Fiber loss no longer limiting factor

WDM enabling technologies II: WDM enabling technologies II: Multiplex/Demultiplex filters

somewhat analogous to prism input white beam, seperates it spatially onto output fibers works both ways, demux and mux other technologies possible (e.g. thin film filter)

16

g p ( g )

WDM system configuration (one-way)C

MD 1

CM

D

1TxTxTxTx

1

RxRxRxRx

Add/drop siteG

MD

CM

D 2

OA OA OA OA

GM

D

CM

D2

Tx Rx

OA

GM

DG

MD

OA

G

CM

D 9

D

CM

D

9TxTxTx

RxRxRx

DG

CMD CMD

Tx = 10Gb/s Optical transmitter(OM5200/OME6500)

Rx = 10Gb/s Optical Receiver (OM5200/OME6500)

36

C

DTxTx

RxRxT

xTx

Tx

Tx

Rx

Rx

Rx

Rx

GMD = Group Multiplexer/Demultiplexer

CMD = Channel Multiplexer/Demultiplexer

OA = Optical amplifier

S