Limiting factors in fiber optic transmissions Sergiusz Patela, Dr Sc Room I/48, Th. 13:00-16:20, Fri. 9:20-10:50 sergiusz.patela@pwr.wroc.pl eportal.pwr.wroc.pl Copying and processing permitted for noncommercial purposes, on condition that proper reference to the source is given. Sergiusz Patela, 2001-6 www.patela.net
Fiber optics fairy tale Once upon a time, the world assumed that fiber possessed infinite bandwidth and would fulfill mankind s communication needs into the foreseeable future.... Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 2/17
Fiber optics transmission systems limits Fiber optic links are either Attenuation limited Dispersion limited Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 3/17
Limiting factors in fiber optic transmission bandwidth Fiber related factors: 1. Fiber dispersion 2. Fiber nonlinearity 3. Optical noise 4. Fiber attenuation Sources/amplifiers related factors: 1. Laser chirp 2. Electrical bandwidth 3. Amplifiers noise (ASE) Detector related factors: 1. Electrical bandwidth Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 4/17
Center wavelength and FWHM FWHM % of Spectral Emission 100 75 50 25 Center Wavelength 1292 1296 1300 1304 1308 Wavelength Range (nm) Center Wavelength: In a laser, the nominal value central operating wavelength. It is the wavelength defined by a peak mode measurement where the effective optical power resides (see illustration). In an LED, the average of the two wavelengths measured at the half amplitude points of the power spectrum. Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 5/17
Semiconductor laser chirp 5 10 8 4 6 Light output (mw) 3 2 4 2 0-2 -4 Frequency Shift (Ghz) 1-6 -8-10 0 10 20 30 40 50 Drive current (ma) Chirp: In laser diodes, the shift of the laser s center wavelength during single pulse duration. Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 6/17
Dispersion Definition: Any phenomenon in which the velocity of propagation of an electromagnetic wave is wavelength dependent In communication technology dispersion is used to describe any process by which an electromagnetic signal propagating in a physical medium is degraded because the various wave components (of different frequencies and wave vectors) have different propagation velocities In optical fiber communications dispersion relates to a few well defined fiber parameters, namely: multimode, material, waveguide and polarization dispersion. Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 7/17
Mode dispersion. Pulse broadening in step index fiber θ c α n co n cl t T B T B + T Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 8/17
Dispersion of single mode fiber 30 Dispersion [ps/(km-nm)] 20 10 0-10 -20 Dispersion (material) Dispersion (total) λ ZD Dispersion (waveguide) -30 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength [µm] Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 9/17
Refractive index of fused silica Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 10/17
Dispersion shifted fibers (DSF) n 20 1.54 n 1 n 2 n 2 d = ~9 µm 125 µm r Dispersion [ps/(km-nm)] 10 0-10 Standard Dispersion flattened Dispersion shifted -20 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength [µm] Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 11/17
Chromatic dispersion vs. the source quality Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 12/17
Nonlinear optical effects in fiber waveguides (1) Nnonlinear changes of refractive index, resulting in phase modulation: refractive index of fiber glass depends on light intensity; Φ NL = n 2k0L E 2 SPM: (self phase modulation) - caused in the same channel by the pulse power modulation. XPM - (cross-phase modulation) - phase modulation caused by power changes in neighboring channels Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 13/17
Power penalty (normal DFB laser) Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 14/17
Power penalty (hq DFB laser, 0.05 nm) Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 15/17
Nonlinear optical effects in fiber waveguides (2) Stimulated Raman Scattering (SRS): As a result of interaction of photons with medium molecules (optical phonons), new wave of different frequency is created Stimulated Brillouin Scattering (SBS): Scattering of photons on acoustic phonons; Four Wave Mixing (FWM): By frequency mixing channels of frequencies f1 i f2 can generate new frequencies 2f 1 -f 2 i 2f 2 -f 1 ; The new frequencies may interfere with signals in existing channels. Summary: Contemporary technology enables WDM systems of 100 channels separated by 10 GHz, 0,1 mw/channel at λ=1550 nm. Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 16/17
Example - DWDM transmission degradation by stimulated Raman scattering Optical power spectrum Optical power spectrum 20 10 0 20 10 1 5 10 16-10 0-20 -10-30 1 5 10 16 186 188 190 192 194 196 Frequency [THz] -20 186 188 190 192 194 196 Frequency [THz] OSNR [db] 18.0 17.6 17.2 16.8 16.4 16.0 50 100 150 200 250 300 350 400 450 500 Fiber Length [km] ch. 1 ch. 16 BER channel 1 channel 16 1.E-00 1.E-06 1.E-12 1.E-18 1.E-24 1.E-30 1.E-36 50 100 150 200 250 300 350 400 450 500 Fiber Length [km] ch. 16 ch. 1 Sergiusz Patela, 2001-6 Limiting factors in fiber optic transmissions 17/17