Raman Amplification for Telecom Optical Networks Dominique Bayart Alcatel Lucent Bell Labs France, Research Center of Villarceaux Training day www.brighter.eu project Cork, June 20th 2008
Outline of the talk Raman interaction of Light with the glass Spectral and power characteristics Noise performance and limitations (Rayleigh, noise transfer) System implementation Lumped Raman amplifiers
Raman amplification : Principle G Pump Signal ( = f pump 13 THz ) db ON / OFF = 10 ln 10 Vibrational excitation O O eff Effect happens for any pump frequency, polarization dependent Instantaneous effect (not the travel time of the pump along the fiber!) Peak Raman shift = 13 THz (glass phonon energy) Top width = 2.5 THz, can be broadened with multi λ pumping Long interaction fibers Confinement Attenuation at the pump wavelength is paramount g A r Si 1 e α α L p p P p Signal O n / o f f G a i n (d B ) 20 15 10 5 0 Mea sured g a in curve : 1 p ump @ 1487 nm DSF fiber 1500 1530 1560 1590 1620 nm A eff is the Raman effective area of the fiber L eff : Effective length of the fiber expressed as : L eff = 1 α ρ
Distributed Raman Preamplification 0 Line fiber WDM Pump Signal (db) 5 10 15 20 with Raman On/off gain 25 Di stan ce (k m ) 0 25 50 75 100 without Raman Preamplification (backward pumping : weak gain saturation) Increase of the signal powers at the EDFA input (improved signal to noise ratio)
Pump wavelength combination On / off Ga in (d B) 25 20 15 10 5 O n / off G a i n (d B) 12,05 12,00 11,95 11,90 11,85 Ca lcul ated ga in cur ve : 20 pum ps Equalization over 60 nm 0 1500 1550 1600 nm 1538 1558 1578 1598 n m 2 or 3 wavelengths enough for a 1 db equalization over the C band 3 to 4 over the C+L band Semiconductor pumps need polarization multiplexing or depolarization using PANDA fiber at 45 deg.
Fiber efficiency per unit length and per W of pump 1486 nm pumping Raman efficiency C R (W 1.km 1 ) 0,8 0,6 0,4 0,2 0 C R = 0.23 G on/off / (P p.l eff_p ) 13.3 THz peak (W 1.km 1 ) NZDSF+ NZDSF TeraLight SMF PSCF C R Max 0.75 0.68 0.51 0.40 0.38 0 3 6 9 12 15 18 Frequency Shift (THz) PSCF : Pure Silica Core Fiber
Normalized Gain Spectra CURVES NORMALISED RELATIVE TO 13.3 THz PEAK 1 1486 n m p um p 1400 n m p um p PSCF 1 0,8 0,8 0,6 0,4 PSCF 0,6 0,4 0,2 0,2 0 0 3 6 9 12 15 18 Fr equency Shift (THz) PSCF 12 15 18 21 24 27 30 Fr equency Shift (THz) 0 Boson peak relative intensity higher with SiO 2 than with GeO 2 13.3 THz peak : Ge dependent, extends up to > 20 THz Sharp 14.5 THz and 18 THz peaks specific to SiO 2
Fiber efficiency is pump wavelength dependent Raman efficiency C R (W 1.km 1 ) 1 0,8 0,6 0,4 0,2 0 1486 nm PUMP 0 3 6 9 12 1 5 1 8 1 0,8 0,6 0,4 0,2 Frequency Shift (THz) 0 1400 nm PUMP NZDSF NZDSF+ TeraLight PSCF SMF + 37 % + 21 % + 46 % + 26 % + 20 % 12 15 1 8 21 24 2 7 3 0 Pump/signal/core overlap
Effect of interchannel Raman depletion Input fiber 10 dbm /channel (linear regime) 0.7 db #1 100 km Tx Rx 5.6 dbm /ch. 2.3 db #32
Small frequency shift energy transfers Raman efficiency (W 1.km 1 ) 0,4 0,3 0,2 0,1 17 C band L band 22 27 0 1486 nm PUMP 0 3 6 9 1529 1561 1569 1602 NZDSF+ NZDSF TeraLight SMF PSCF Frequency shift (THz) Wavelength (nm)
Noise figure for Distributed Raman Preamplification Line Fiber WDM B P u m p Intrinsic noise parameter = 3dB (quantum limit) Equivalent noise figure = the noise figure of an EDFA located in B point and providing with the Raman ON/OFF gain Equivalent noise figures can be equal or lower than 0 db
Double Rayleigh Scattering Ps (z) r.ps(z 0 L P DRS (L) ) Signal (neglecting the DRS term): Simple Rayleigh scattered signal: Double Rayleigh scattered signal: P ( z ) = P ( 0 ). G P P S RS DRS ( y ) = S ( L ) = L y r. P ( z ). G L 0 S r. P net RS ( 0 z ) net ( y ). G ( y z ). dz net ( y L ). dy DRS noise to signal ratio at the end of the fibre: R DRS = P DRS (L)/P S (L)
Double Rayleigh Scattering (DRS) DRS is a delayed copy of the signal => beating noise at detection with the signal by quadratic detection: 2 ν RIN ( f ) R DRS 2 2 π ( f + ( ν ) ) Exists in all transmissions but is more penalizing with distributed Raman amplification: DRS is amplified during its double path Crucial issue: DRS impairment is a limit to high amounts of Raman gain ASE and DRS essentially differ by their spectral distribution: ASE is constant with wavelength in the range of the signal bandwidth DRS is a replica of the signal optical spectrum
Electrical measurement of DRS RIN (db/hz). 95 100 105 110 115 120 125 130 135 140 145 150 155 0 1 2 3 4 5 6 7 8 9 10 Frequency (MHz) 24 db gain 16 db gain 10 db gain 8 db gain 6 db gain 4 db gain 2 db gain 0 db gain source
Signal ASE and signal DRS beat noises Reference case: NRZ, broad optical filtering General case: any format σ 2 sg ASE = 4 η 2 N sg pola r ASE P S B elec σ 2 sg ASE = k ASE. P ASE P S σ 2 sg DRS = 2 η 2 P sg polar DRS P S σ 2 sg DRS = k DRS. P DRS P S depend on: modulation format (signal pattern) optical and electrical filters at reception
Conclusion on Double Raleigh Scattering DRS is a major limitation of the maximum Raman gain that can be obtained in the line fiber Limits Raman advantage in backward pumping Max gain closed to 23 db For all Raman pumping of the line fiber Need for forward pumping See related issues (RIN, ) Or use of Raman pumping into the DCF Increases non linear effects in the DCF
Pump to signal RIN transfer Raman effect is very fast (femtosecond) Locally, the intensity fluctuations of the pump are totally transferred to the signal by gain (db) Effect averaged by counter propagation (backward pumping) only by chromatic dispersion for forward pumping
Transfer functions Assumptions: Distributed Raman amplification: long fiber (50 100km) Moderate Raman gain, moderate pump RIN No pump depletion G ON / OFF f RIN ( ) 20 log 10 log S f = RIN P + 1 + 10 / ln( 10 ) f c α P V S with f c = for backward pumping 4 π α P and f c = for forwa rd pumping 1 1 2 π V S V P 2 Reference: C. R. S. Fludger and al., Electron. Lett., 2001, 37, (1), pp. 15 17
Transfer functions With: RIN P = 120dB/Hz Gonoff =10dB α P =0.25 db λ P =1450nm λ S =1550nm 110 115 + 20log(Gonoff/ 4.34) RIN (d B/ Hz) 120 125 Signal RIN with backward pumping (f c =900 Hz) Pump RIN Signal RIN with forward pumping SMF (f c =6 MHz) Teralight (f c =18 MHz) may happen with TW and LEAF 130 135 50 k Hz 7 GHz 1E+ 0 1E+ 3 1E+ 6 1E+ 9 1E+ 12 Frequency (Hz)
Raman amplification : Implementation Q 40Gb/s 2 15 1 Q Q 40Gb/s 2 23 1 Q #1 #63 #2 #64 #65 #127 #66 #128 1x 32 1x 32 1x 32 1x 32 M Z M Z M Z M Z DCF PBS PBSDCF C L 100 km NZDSF C DCF 4 pumps (distributed Raman ampli.) L DCF C L Rx Use of wavelength multiplexed Raman pumps x 3 Efficiency depends on fiber type and characteristics
Stimulated Raman Scattering Pumps OFF 5 In span 5 0 5 17 Out span 22 27 0 5 17 22 27 P (dbm) Pre tilt 3dB Intraband tilt compensated but C to L energy transfer Pumps ON 8 13 18 18 1427 1439 1450 1485 1525 1565 1605 Wavelength (nm) 23 THz Out span 8 13 Raman gain 12dB C band 10dB L band
All Raman pumping schemes DRA in the link: 31dB on off Raman gain in the link fiber MUX TAP DCF MUX Raman pump Raman pump DRA in the link & DCF: 21.5dB on off Raman gain in the link fiber MUX TAP 3dB net gain in the DCF DCF (11dB on off gain) MUX TAP MUX Raman pump Raman pump Raman pump
Criteria for performance assessment (ULH) Final impairment of the amplifier on the system: Generation of noise Non linear phase 1 5 C noise = N ASE B elec + *. R 2 9 C phase = γ L 0 G Net ( z ) dz DRS P in Achievable distance is proportional to ( C noise C Parameter C = C noise.c phase (the smaller the better) Co pumping issues to be accounted aside phase ) 1 / 2
All Raman transmission of 6 Tbit/s over 6120 km Tx 149x43Gbit/s Rx Lumped Raman amp. C Polarization scrambler L 12 = 6120km Dyn. Gain Equalizer Dyn. Gain Equalizer C L Lumped Raman amp. Link fiber +D +D D GFF x3 GFF GFF GFF All Raman amplification (First+Second order)
Experimental results with all Raman amplification Spectrum after 6,120km with 149 DPSK channels C band L band Power 10dB/div. 1530 1550 1570 1590 1610 Wavelength (nm) In C band: more odd than even channels Gain excursion close to 10 db after 6120km OSNR_0.1nm > 16.9dB in L band OSNR_0.1nm > 14.6 db in C band
Lumped Raman Amplifier Internal Gain (db) 26 24 22 20 18 16 5 db/ div In terna l N oise Fi gure (db) 7 6 5 4 14 10 5 0 5 10 15 20 25 Internal Output Power (dbm) 1580 nm 1590 nm 1600 nm 3 10 5 0 5 10 15 20 25 Internal Output Power (dbm) Use of a specific Raman fiber e.g. Photonic Crystal Fiber On demand gain bandwidth and location First stage Second stage Pump input Output pump Weaker sensitivity to signal gain saturation compared to EDFAs Robust noise figure in high power input signal regime
Bibliography «Erbium doped fiber amplifier, Principles and Applications» Emmanuel Desurvire, Wiley (1994), New York «Erbium doped fiber amplifier, Device and System Developments», Emmanuel Desurvire, Dominique Bayart, Bertrand Desthieux, Sébastien Bigo, Wiley (2002), New York «Undersea Fiber Communications Systems» José Chesnoy Ed., Elsevier, San Diego (2002) Chap. 4, Optical Amplification, Dominique Bayart