Fotonica in SURFnet6

Transcriptie

1 Fotonica in SURFnet6 Roeland Nuijts Netwerkdiensten SURFnet Utrecht, 5 april, 2006 Hoogwaardig internet voor hoger onderwijs en onderzoek

2 Outline Introduction Optical transmitters and receivers EDFAs (Erbium Doped Fiber Amplifiers) Optical transmission fiber Dispersion compensation 2 Hoogwaardig internet voor hoger onderwijs en onderzoek

3 Introduction Traditional digital point-to-point optical fiber transmission systems Transmission fiber a (db/km) P T (dbm) P R (dbm) Transmission distance = (P T -P R ) / 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 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 Development of optical fiber amplifiers has enabled DWDM (Dense Wavelength Division Multiplexing Systems) 3 Hoogwaardig internet voor hoger onderwijs en onderzoek

4 {0,1,1,0,1,1,,0,1,0} 10Gb/s optical transmitter technologies DFB {0,1,1,0,1,1,,0,1,0} 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 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 transmission {0,1,1,0,1,1,,0,1,0} DFB Mach-Zehnder LiNbO3 modulator CW-DFB (Continuous Wave DFB laser) and MZ (Mach-Zehnder) combination External modulator Expensive, relatively large, high-power drivers (high power consumption) Low (or deterministic) chirp, excellent performance Used for long reach and DWDM (Dense Wavelength Division Multiplexing) transmission 4 Hoogwaardig internet voor hoger onderwijs en onderzoek

5 Typical 10Gb/s optical receiver setup preamp AGC decision circuit data (A)PD CLK Photodetector converts optical signal to electrical signal. PIN or APD (Avalanche Photo Detector) for improved receiver sensitivity Preamp provides high gain, low noise AGC (Automatic Gain Control) amplifies signal at output of preamp to railto-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 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 BER P sens P rec (dbm) 5 Hoogwaardig internet voor hoger onderwijs en onderzoek

6 WDM enabling technologies: EDFAs (Erbium Doped Fiber Amplifiers) Fiber doped with Er 3+ ions be excited by 980nm or 1480nm photons spontaneous emission generates noise Excited state Erbium ions can be stimulated to decay to ground state via stimulated emission by a 1550nm signal 6 Hoogwaardig internet voor hoger onderwijs en onderzoek

7 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 Fiber loss no longer limiting factor 7 Hoogwaardig internet voor hoger onderwijs en onderzoek

8 λ1 WDM system configuration (one-way) CMD CMD CMD GMD Add/drop site GMD OA OA OA OA OA OA CMD CMD GMD GMD CMD CMD CMD λ36 = 10Gb/s Optical transmitter(om5200/ome6500) = 10Gb/s Optical Receiver (OM5200/OME6500) GMD = Group Multiplexer/Demultiplexer CMD = Channel Multiplexer/Demultiplexer OA = Optical amplifier Scalable and flexible design, easy to add wavelengths Initial release uses 36 channels spaced at 100GHz Can be upgraded to 72 channels, each operating at 10Gb/s Optical add and drop for point-to-point optical lightpaths 8 Hoogwaardig internet voor hoger onderwijs en onderzoek

9 SURFnet6 Subnetwork 3 Optical spectrum at output of transmit OA in Amsterdam1 (RB=0.5nm) λ1 CMD CMD CMD GMD Add/drop site GMD OA OA OA OA OA OA CMD GMD CMD GMD CMD CMD CMD Power (dbm) Wavelength (nm) OSNR (0.1nm) > 40dB λ36 Optical spectrum at output of receive OA in Amsterdam1 (RB=0.5nm) Power (dbm) Wavelength (nm) Subnetwork 3 specific eight spans OSNR (0.1nm) > 28dB total distance 528km dispersion optimized to allow traffic between any pair of sites Hoogwaardig internet voor hoger onderwijs en onderzoek

10 OSNR (Optical Signal-to-Noise Ratio) - Simple formula SMF repeater DCF SMF SMF DCF OA OA OA OA OA ƒ NF 1 NF 2 NF 3 NF N-1 NF N P in,1 P in,2 P in,3 P in,n-1 P in,n OSNR N 1 2 h ν R = OSNR j = 1 Pin, j N sp, j Parameter Description h Planck's constant (J s) c speed of light (m/s) Calculated OSNR (db) Resolution bandwidth = 0.1nm Distance (km) R Pin Nsp OSA resolution BW (Hz) Input power (W) Noise Figure (linear) Simple formula, accurate to within a few tenths of a db 10 Hoogwaardig internet voor hoger onderwijs en onderzoek

11 SURFnet6 third generation DWDM 10Gb/s per channel 50GHz channel spacing wavelength locking optical add/drop forward error correction dispersion compensation C-band, can be upgraded to add L-band 11 Hoogwaardig internet voor hoger onderwijs en onderzoek

12 Optical fiber - Historical perspective Basic principle of internal reflection known from 19 th century (John Tyndall, 1870) 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 Hoogwaardig internet voor hoger onderwijs en onderzoek

13 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 Peak at 1400nm arises from OH impurities, can be removed (AllWave fiber) 13 Hoogwaardig internet voor hoger onderwijs en onderzoek

14 t g (ps) Fiber dispersion Refractive index varies with wavelength which leads to a wavelength dependence of the group delay, t g, (delay for different wavelengths) in ps/km Dispersion coefficient, D, is the derivative of the group delay, t g, with respect to wavelength per unit length (ps/nm km) λ (nm) Optical Power (A.U.) 1,4 1,2 1,0 0,8 0,6 0,4 0,2 Optical pulse shape at output 0, D (ps/nm km) 0 λ (nm) No distortion at zero-dispersion wavelength, l 0 Distortion at other wavelengths l 0 Optical Power (db) Time (ps) Frequency (G Hz) Optical spectrum at output 14 Hoogwaardig internet voor hoger onderwijs en onderzoek -30

15 Optical fiber Historical perspective Standard SMF (G.652) Initial (80 s) optical components for transmission through single mode fiber operated at the 1.3 mm 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 (1.3µm 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 80 s Dispersion-Shifted Fiber (zero-dispersion at 1550nm) later developed and deployed, predominantly in Japan Hoogwaardig internet voor hoger onderwijs en onderzoek

16 Example of impact of dispersion on transmission system performance - optical pulse shape of 10Gb/s signal after 120km 120km standard SMF D=17.8 ps/nm km OA OA ƒ r b = 10Gb/s l c = 1557nm 1 120km-10Gb/s system configuration 2 Optical Power (A.U.) 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0, Time (ps) 1 Optical pulse shape at transmitter output Pulse shape after 120km transmission completely distorted due to pulse broadening, error-free transmission not possible 2 After 120km transmission 16 Hoogwaardig internet voor hoger onderwijs en onderzoek

17 Dispersion Compensating Fiber D (ps/nm km) 0 Standard fiber l 0, SMF l T l 0, DCF Dispersion compensating fiber λ (nm) Advantages Wide band compensation (one DCF compensates for all channels in C-band) Feasible for DWDM systems Easy to use, reliable Disadvantages bulky DCF loss, requires additional optical amplifier smaller core area, hence nonlinear effects 17 Hoogwaardig internet voor hoger onderwijs en onderzoek

18 Measured and calculated optical pulse shape after 120km and after DCF (Dispersion Compensating Fiber) ATT1 120k m SMF ATT2 DCF ATT3 A 1 A 2 A 3 f 1 2 3, 1 After 120km transmission Pulse shape after 120km transmission completely distorted due to pulse broadening, error-free transmission not possible Optical pulse shape recovered after passing through DCF with negative dispersion More residual dispersion if optical power level at the input of the DCF is high due to the (undesirable) nonlinear effect in the DCF Excellent agreement between theory and experiment After compensation, P DCF =+5dBm 2 3 After compensation, P DCF = 0dBm 18 Hoogwaardig internet voor hoger onderwijs en onderzoek

19 NZDSF (Non-Zero Dispersion Shifted Fiber) optimizes dispersion in the EDFA region 19 Hoogwaardig internet voor hoger onderwijs en onderzoek

20 Measured DF data for DWDM spans Fiber type Average measured loss (db/km) Total length (km) G (63.1%) TWRS (29.6%) LEAF (7.2%) Total (100%) 20 Hoogwaardig internet voor hoger onderwijs en onderzoek

21 Other dispersion compensation methods {0,1,1,0,1,1,,0,1,0} Total 5000km standard transmission fiber H(f) h -1 (t) DFB+MZ OA OA OA Optical Power HArb. Units L Optical Power HArb. Units L 2.5 dispersion compensation off D=+87500ps/nm Optical Power HArb. Units L 2.5 z=0km time Hps L Optical Power HArb. Units L z=5000km time Hps L dispersion compensation on D=-87500ps/nm z=0km time Hps L z=5000km time Hps L No more need for dispersion compensation via DCFs (Dispersion Compensating Fibers)!! 21 Hoogwaardig internet voor hoger onderwijs en onderzoek

22 Thanks! 22 Hoogwaardig internet voor hoger onderwijs en onderzoek