S72 227 Digital Communication Systems Fiberoptic Communications Supplementary

S-72. 227 Digital Communication Systems Fiber-optic Communications - Supplementary

Timo O. Korhonen, HUT Communication Laboratory G. Keiser: Optical Fiber Communications, Mc. Graw-Hill, 2 nd Ed.

EDFA - energy level diagram Fluoride class level (EDFFA) E 4 excited state absorption 980 nm E 3 Er 3+ levels E 2 1530 nm 980 nm 1480 nm E 1 u u u Pump power injected at 980 nm causes spontaneous emission from E 1 to E 3 and there back to E 2 Due to the indicated spontaneous emission lifetimes population inversion (PI) obtained between E 1 and E 2 The higher the PI to lower the amplified spontaneous emission (ASE) Thermalization (distribution of Er 3+ atoms) and Stark splitting cause each level to be splitted in class (not a crystal substance) -> a wide band of amplified wavelengths Practical amplification range 1525 nm - 1570 nm, peak around 1530 nm Timo O. Korhonen, HUT Communication Laboratory

Band Description Wavelength (nm) O-band E-band S-band C-band L-band U-band Original 1260 -1360 Extended 1360 -1460 Short 1460 -1530 Conventional 1530 -1565 Long 1565 -1625 Ultra-long 1625 -1675 50 Water spike 10 Loss (d. B/km) Fundamental limits of silica fibers 100 5 1 0. 5 0. 1 Rayleigh scattering Infrared absorption 0. 8 1. 0 1. 2 1. 4 1. 6 1. 8 Wavelength (mm) u u C-band: supports early EDFA C+L-band: support for EDFA’s of today Raman amplifiers can be used over all bands new (medium loss) bands are now applicable (as S & U bands) New fibers can reduce loss at E & S bands (however, EDFA does not work here & Raman gain small) Timo O. Korhonen, HUT Communication Laboratory u u Inter- and Intra-modal dispersion Attenuation (Loss) Non-linear effects – Four-wave mixing (FWM) – Stimulated Raman & Brillouin scattering (SRS, SBS) – Cross-phase & self-phase modulation (SPM, XPM) Polarization fluctuations

Modulation of lasers Timo O. Korhonen, HUT Communication Laboratory

LD distortion coefficients u Let us assume that an LD transfer curve distortion can be described by u where x(t) is the modulation current and y(t) is the optical power n: the order harmonic distortion is described by the distortion coefficient and For the applied signal we assume Timo O. Korhonen, HUT Communication Laboratory and therefore

Link calculations u u u In order to determine repeater spacing on should calculate – power budget – rise-time budget Optical power loss due to junctions, connectors and fiber One should be able to estimate required margins with respect of temperature, aging and stability For rise-time budget one should take into account all the rise times in the link (tx, fiber, rx) If the link does not fit into specifications – more repeaters – change components – change specifications Often several design iteration turns are required Timo O. Korhonen, HUT Communication Laboratory

Link calculations (cont. ) u u Specifications: transmission distance, data rate (BW), BER Objectives is then to select FIBER: – Multimode or single mode fiber: core size, refractive index profile, bandwidth or dispersion, attenuation, numerical aperture or modefield diameter SOURCE: – LED or laser diode optical source: emission wavelength, spectral line width, output power, effective radiating area, emission pattern, number of emitting modes DETECTOR/RECEIVER: – PIN or avalanche photodiode: responsivity, operating wavelength, rise time, sensitivity Timo O. Korhonen, HUT Communication Laboratory

The bitrate-transmission length grid SI: step index, GI: graded index, MMF: multimode fiber, SMF: single mode fiber Timo O. Korhonen, HUT Communication Laboratory