Dr. Monir Hossen ECE, KUET 1
Outlines of the Class Principles of WDM DWDM, CWDM, Bidirectional WDM Components of WDM AWG, filter Problems with WDM Four-wave mixing Stimulated Brillouin scattering WDM Network Ring network Mesh network Optical cross connect 2
Principles of WDM 3
Why WDM? (before WDM & EDFA) ~ 1 st Generation CH 1 O/E E/O O/E E/O O/E E/O O/E E/O O/E E/O RECV CH 2 O/E E/O O/E E/O O/E E/O O/E E/O O/E E/O RECV CH 8 O/E E/O O/E E/O O/E E/O O/E E/O O/E E/O RECV (after WDM & EDFA) ~ 2 nd Generation CH 1 CH 2 (30~40 km) O/E/O Repeater RECV RECV CH 8 EDFA (80~120 km) EDFA RECV MUX DEMUX 4
Network Terminals Multiplexer Demultiplexer Basic Design of WDM NT TX1 1 RX1 NT NT TX2 2 RX2 NT NT NT TXn-1 TXn n Monitor Points RXn-1 RXn NT NT WDM transmitter Receiver Channels: 16 Spacing: 0.8 nm Amplified Spontaneous Emission (ASE) 5
Wavelength Options Various types of WDM upon wavelength spacing CWDM (Coarse WDM) Bidirectional Triple play (1310, 1490, 1550 nm) DWDM (Dense WDM) CWDM Over than 20 nm spacing between adjacent channels ITU G.695 ~ Optical interfaces for CWDM ITU G.694.2 ~ Spectral grid for CWDM (20 nm) No tight control of wavelength needed no temp. control Less channel crosstalk Known as 40% cheaper than DWDM Limited number of wavelengths available 18 CWDM channels can be obtained over 1271~1611 nm 6
Wavelength Options - CWDM ITU G.694.2 - Nominal central wavelengths of CWDM 1270 1290 1310 1330 1350 1370 1390 1410 1430 1450 1470 1490 1510 1530 1550 1570 1590 1610 Why Wavelengths Spacing of 20nm? Uncooled lasers: wavelength change with temperature Wide pass band filters : wavelength variation of ± 6-7 nm with current filter technologies 1/3 of the channel spacing is needed for the guard-band 7
Wavelength Options - CWDM OFC 2004, RBN 8
Wavelength Options - CWDM CWDM example #1 : Triple play service Assign extra downstream wavelength for CATV since the video stream is dominant at present time. 1550 nm for optional video CATV (down) ~ CATV analog + digital channels 1490 nm for voice, data, IP video (down) 1310 nm for voice, data, IP video (upstream) CWDM example #2 : 1400 nm (E-band: 1360~1480) CWDM for premium business service 1490, 1550, 1310 nm for triple play of EPON for normal EPON service 9
Wavelength Options - CWDM CWDM: OFS used E-band for CWDM E-band; 1360-1460 nm 10
Wavelength Options - DWDM DWDM Less spacing than CWDM, but, usually less than 3.2 nm spacing (3.2, 1.6, 0.8, 0.4 nm) Tight control of wavelength needed DWDM LD Careful about the channel crosstalk DWDM Filter Can provides a lot of wavelengths 11
Bidirectional WDM Links Different wavelengths are used in TX & RX What will happen if the same wavelengths are used? 12
Components of WDM 13
EDFAs In DWDM Systems Optical amplifiers in DWDM systems require special considerations because of: Gain flatness (gain tilt) requirements Gain competition Nonlinear effects in fibers 14
Gain Flatness (Gain Tilt) of EDFA Gain versus wavelength The gain of optical amplifiers depends on wavelength Signal-to-noise ratios can degrade below acceptable levels (long links with cascaded amplifiers) G Compensation techniques Signal pre-emphasis Gain flattening filters Additional doping of amplifier with Fluorides 15
Gain Competition of EDFA Total output power of a standard EDFA remains almost constant even if input power fluctuates significantly If one channel fails (or is added) then the remaining ones increase (or decrease) their output power Output power after channel one failed Equal power of all four channels 16
Output Power Limitations of EDFA High power densities in SM fiber can cause Stimulated Brillouin scattering (SBS) Stimulated Raman scattering (SRS) Four wave mixing (FWM) Self-phase and cross-phase modulation (SPM, CPM) Most designs limit total output power to +17 dbm Available channel power: 50/N mw * 17 dbm = 50 mw, N = number of channels 17
Fiber Bragg Grating (FBG) Zones of high refractive index scatter light Selectively reflect one wavelength Transmits other wavelengths ADD/DROP of a single wavelength 2n eff ~ grating period n ~ refractive index m ~ integer 1, 2, 3, 4 2 1, 2 ', 3, 4 2 2 ' 18
Arrayed Waveguide Grating (AWG) Application WDM multiplexing/demultiplexing ADD/DROP of multiple wavelengths Optical switching, etc 19
Arrayed Waveguide Grating (AWG) Based on diffraction principles Optical length difference of each waveguide introduces different phase delays Difference wavelengths having maximal interference at different locations, which correspond to the output. Called Rowland Circle Maximal interference when arrived E-fields thru all paths are in-phase Difference of phase delay 20
Arrayed Waveguide Grating (AWG) The index of refraction changes depending upon temperature, which means that the wavelengths of transmitted light also change. To prevent this, a separate temperature control device had to be used. Some athermal AWG solves this problem by using a special silicon pitch in part of the lightwave circuit that has a different temperature coefficient than quartz glass. This design cuts the temperature dependence of the wavelengths of transmitted light to less than one-tenth of its original value, which makes using a temperature-control device unnecessary. http://www.nel-world.com/products/photonics/ather_awg.html 21
Dispersion Compensation 22
Effects of Dispersion & Compensation 80Gb/s transmission system Transmitter PC # 1 # 2 # 8 AWG 8 x 1 Polarization Control (PC) PBS M - Z 10 Gb/s PRBS 2 31-1 Booster Attenuator SMF 100 km ASEF EDFA DCF EDFA SMF 100 km EDFA DCF EDFA DCM # 2 DCM # 1 ASEF AWG 1 x 8 Receiver Rx # 7 # 8 23
Effects of Dispersion & Compensation back to back after 20-km after 40-km after 60-km 24
Effects of Dispersion & Compensation after 80-km after 100-km after DCM1 (100-km) 25
Problems with WDM 26
Four-Wave Mixing (FWM) Nonlinear crosstalk when Fiber dispersion are small enough to satisfy the phase matching condition With presence of chromatic dispersion, different signals travel with different group velocities, reducing the effect 27
WDM MUX Four-Wave Mixing (FWM) Transmission test using DSF DFB LD Ch 1 Polarization Controller Pulse Pattern Generator 10Gbps NRZ 2 23-1 PRBS Booster Ch 2 Ch 8 Polarizing Beam Splitter LiNbO 3 Modulator DSF 80km Error Detector Att. Optical Sampling Scope Band Pass Filter Optical Spectrum Analyzer before after Eye Diagram 28
Stimulated Brillouin Scattering (SBS) Produces gain in opposite direction when signal power density is very high signal line width is very narrow P th 21A g B L eff eff g B ~ gain, L eff ~ effective length A eff ~ effective mode cross section A?? a. 0 1 0 1 0 1 0 1 0 t?? b. 29