Basics of Optical Communication Systems and WDM

Plastic fibres exist. capacity vs distance requirement drives us to move from copper cables to optical fibres. attenuation vs wavelength has minimas at telecom wavelength (1310nm and 1550nm). SONET - synchronous optical network. SDH - synchronous digital heirarchy. notation - OC-N => N x 51 Mbps STM-X => X x 155Mbps some standards OC-48 : 2.5Gbps OC-192 : 10Gbps OC-768 : 40 Gbps Bands O(original) band - 1260 <-> 1360nm E(Extended) band - 1360 <-> 1460nm S(Short) band - 1460 <-> 1530nm C(Conventional) band - 1530 <-> 1565nm L(Long) band - 1565 <-> 1625nm U(Ultra-long) band - 1625 <-> 1675nm At 1310nm (although not an exact minimum) there is zero pulse broadening. CWDM-coarse wavelength division multiplexing,DWDM-dense wavelength division multiplexing. coarse: span:1270->1610nm , spacing:20nm (ex. 1310,1490,1550). Dense: spacing:0.1->0.8nm. refractive index varies with wavelength and doping of the fibre. SM(single mode) core diameter: 8-10um, cladding: 125um. MM(multi mode) core diameter: 50-62.5um, cladding: 125um. step and graded index fibres. Index is graded to minimize pulse broadening. n_cladding = n_core*(1-delta) typical delta for SM: 0.2->1% MM: 1->3%. numerical aperture = sqrt((n1^2)-(n2^2)) ~ n1*sqrt(2*delta) small wavelength limit(or ray picture): if fibre core radius >> wavelength. Power loss occurs when there is coupling of input power with higher order modes and cladding modes. Bend loss: N loops of radius R, example: 9um SM fibre N=3, R=1.15cm has a bend loss of 2.6dB at 1310nm and 23.6dB at 1550nm. N=3, R=1.8cm has a bend loss of 0.1dB at 1310nm and 2.6dB at 1550nm.