In-depth optical technology

1. In-depth optical technology December 19, 2019 Roeland Nuijts Network Services

2. Outline • Introduction • 10Gb/s transmitters and receivers • Optical transmission fiber • Fiber dispersion compensation • EDFAs (Erbium Doped Fiber Amplifiers) and DWDM transmission systems • Advances in optical transmission fibers

3. Introduction – Optical fiber transmission systems Transmission fiber  (dB/km) Tx Rx • 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) PT (dBm) PR (dBm) Transmission distance = (PT-PR) /  (km)

4. 10Gb/s optical transmitter technologies {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 {0,1,1,0,1,1,…,0,1,0} • 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 DFB EA {0,1,1,0,1,1,…,0,1,0} • 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 DFB Mach-Zehnder LiNbO3 modulator

5. decision circuit preamp AGC (A)PD CLK Typical 10Gb/s optical receiver setup data • 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 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 • 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 BER 10-6 10-9 10-12 Psens Prec (dBm)

6. Introduction - System performance - Eye diagrams versus optical pulse shapes • It is common practice in publications, reports and standards, to use eye diagrams as a means to judge system performance without measuring BER • Eye diagrams are generated by superimposing the pulse shape repeatedly on itself, each time shifting it by one bit period, and give a clear picture of eye opening, rise/fall times of bits, overshoot etc 0 1 1 1 01 1 0 1 00 1 001

7. Basic principle of internal reflection known from 19th 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 1979 Optical fiber - Historical perspective

8. n1 n2 INDEX n0 RADIAL DISTANCE Optical fiber – principle of operation jacket • principle of guided propagation is internal reflection, core with refractive index n1 is surrounded by cladding with refractive index n2 • radius core, a, is about 2-4 μm • cladding core, b>>a, is about 50-60 μm • optical energy is confined in fiber cross-section which is slightly larger than the core (electrical field amplitude drops by 1/e at radius of 5.4 μm) cladding a core b FIBER CROSS-SECTION SIDE-VIEW

9. Optical fiber manufacturing • Basic material is silica glass (fusing SiO2) • Refractive index difference is realized by dopants (GeO2 and P2O5 increase refractive index, Boron and Fluorine reduce refractive index) • Fabrication in two steps • Preform fabrication (typically 1m long, 2cm diameter) • MCVD (Modified Chemical Vapor Deposition) • VAD (Vapor-phase Axial Deposition) • OVD (Outside Vapor Deposition) • Fiber pulling • tip of preform is heated beyond melting point and drawn into a fiber while the relative core-cladding dimensions are preserved

10. Preform fabrication using MCVD process FLOW METERS FUSED SILICA TUBE + O2 POCl3 SiCl4 GeCl4 SiF4, SF6, Cl2, BCl4 O2 H2

11. 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 -4 therefore dominant at short wavelengths • Loss at long wavelengths ( > 1625nm) dominated by infra-red absorption • Peak at 1400nm arises from OH impurities, can be removed (AllWave fiber)

12. F r e q u e n c y ( G H z ) - 3 0 - 2 0 - 1 0 0 1 0 2 0 3 0 ) - 5 B d ( r - 1 0 e w o - 1 5 P l a - 2 0 c i t p O - 2 5 - 3 0 Fiber dispersion • Refractive index varies with wavelength which leads to a wavelength dependence of the group delay, g, (delay for different wavelengths) in ps/km • Dispersion coefficient, D, is the derivative of the group delay, g, with respect to wavelength per unit length (ps/nm km) g (ps) Optical pulse shape at Tx output λ (nm) D (ps/nm km) 0 0 λ (nm) Optical spectrum at Tx output • No distortion at zero-dispersion wavelength, 0 • Distortion at other wavelengths

13. Initial (80’s) optical components for transmission through single mode fiber operated at the 1.3m 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 17 Optical fiber – Historical perspective “Standard SMF (G.652)”

14. F r e q u e n c y ( G H z ) - 3 0 - 2 0 - 1 0 0 1 0 2 0 3 0 ) - 5 B d ( - 1 0 r e w o - 1 5 P l a - 2 0 c i t p O - 2 5 - 3 0 Dispersion limited distance, LD B = 10 Gb/s D = 17 ps/nm km LD = ~50km Δλ = 0.112 nm* • Chromatic dispersion places a limit on the maximum transmission distance • LD scales with square of the bitrate • 800km at 2.5Gb/s, no dispersion compensation required in the Netherlands • 50km at 10Gb/s, hence dispersion compensation is needed in case of transmission beyond 50km • Linear effect, can be compensated by fiber with negative dispersion * 3dB bandwidth of 7GHz assumed, doublesided spectrum  14GHz ≡ 0.112nm

15. Fiber nonlinearity • Light transmission through the fiber is affected by a weak dependence of the refractive index on the intensity of the optical pulse • SPM (Self Phase Modulation) • XPM (Cross Phase Modulation) • FWM (Four Wave Mixing)

16. Modeling of pulse propagation in optical transmission fiber • Non-linear Schrödinger equation governs pulse propagation in lossy, dispersive, nonlinear fibers • E is the complex electrical field envelope • a is proportional to fiber absorption  • 2 is proportional to dispersion coefficient • 3 is proportional to dispersion slope with respect to wavelength • n2 is proportional to the nonlinear coefficient • Equation can be numerically solved by split-step Fourier method or beam propagation method

17. - 2 0 2 - 2 0 2 N = 2 . 3 6 1 0 m N = 2 . 6 9 1 0 m • • 2 2 W W 2 2 A = 9 0 . 0 µ m A = 2 1 . 5 µ m e f f e f f D = + 1 7 . 8 p s / n m k m D = - 7 4 . 7 p s / n m k m a a = 0 . 2 7 d B / k m = 0 . 5 5 d B / k m 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 ( a ) A T T 1 1 2 0 k m S M F A T T 2 D C F A T T 3 A A A f T x R x 1 2 3 1 0 G b / s 2 3 P R B S 2 - 1 N R Z l = 1 5 5 7 n m P = + 1 2 . 5 d B m P = 0 d B m S M F D C F ( b ) Example - 120km transmission system setup

18. 1 1 2 2 A T T 1 1 2 0 k m S M F A T T 2 D C F A T T 3 3 3 A A A f T x x R 1 2 3 Measured and calculated optical pulse shape after 120km and after DCF (Dispersion Compensating Fiber) , 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, PDCF =+5dBm After compensation, PDCF = 0dBm

19. ITU standardized dispersion compensation methods for 10Gb/s-80km transmission through standard fiber • Three methods of compensating dispersion as dictated by the ITU-standard ITU-G.691 • Passive (e.g. dispersion compensating fiber) • Inserting a piece of fiber with negative dispersion at transmitter end, receiver end, or both, to compensate for the positive dispersion in the transmission fiber • Negative chirp • EML (Electro-absorption Modulator Laser) • CW-DFB with MZ (Mach-Zehnder) modulator with negative chirp • SPM (Self-Phase Modulation) • Nonlinear Kerr effect, causes pulse compression in the launching end of the fiber where the power is high and which cancels out pulse broadening due to chromatic dispersion g (ps) λ (nm) 17 0 D (ps/nm km) 1310 1550 λ (nm) Red-shift Blue-shift

20. Effect of SPM on transmission performance • Optical eye opening and electrical eye opening increase with transmitted power level • Eye opening after 80km transmission increases with launch power up to +17.5dBm due to SPM • SBS (Stimulated Brillouin Scattering) threshold is around +15dBm for standard fiber (G.652) • SPM requires optical amplifier, large complex, preferred solution uses negative chirp

21. WDM enabling technologies: EDFAs (Erbium Doped Fiber Amplifiers) • Fiber doped with Er3+ 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

22. 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

23. WDM (Wavelength Division Multiplexing) transmission systems – initial systems • 4-8 channels • 2.5Gb/s per channel, total bandwidth of 10-20Gb/s • 1.6nm (200GHz) channel spacing, no wavelength locker necessary on transmitters

24. SURFnet6 – Subnetwork 3 • third generation DWDM • 10Gb/s per channel • 50GHz channel spacing • wavelength locking • optical add/drop • forward error correction • dispersion compensation • C-band and L-band possible • Subnetwork 3 specific • total distance 528km • up to 72 channels x 10Gb/s in C-band • dispersion optimized to allow traffic between any pair of sites • total 5 subnetworks in SURFnet6

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

26. Nonlinear FWM (Four Wave Mixing) • nonlinear FWM effect causes generation of mixing products which coincide with existing wavelengths and deteriorate BER performance of those channels • solutions • use unequally spaced channels in DSF (Dispersion Shifted Fiber) => inefficient use of bandwidth • use fiber with slight amount of dispersion in the 1550nm wavelength window, NZDSF (Non-Zero-Dispersion Shifted fiber)

27. 1 2 Other dispersion compensation methods Total 5000km standard transmission fiber H(f) {0,1,1,0,1,1,…,0,1,0} Rx • Electrical Dispersion Compensation (Postdeadline OFC’05 Nortel)  DFB+MZ OA OA OA h-1 (t) D=+87500ps/nm dispersion compensation off z=5000km z=0km D=-87500ps/nm dispersion compensation on z=5000km z=0km No more need for dispersion compensation via DCFs (Dispersion Compensating Fibers)!!

28. Thanks!roeland.nuijts@surfnet.nl