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. solution:reduced bend sensitive fibres. There are 2 polarization states in a SM fibre each of which sees a different index leading to pulse broadening. Chromatic dispersion(CD), group velocity dispersion(GVD)and Polarization mode dispersion(PMD). CD is an intramodal dispersion. Units of dispersion is psec/(nm*km). pulse spreading (in psec) = Dispersion*(source spectral width(in nm)*Distance(in km)). FTTx - fibre to the (x:H-home, P-premise). Different types of fibres: 1)UV sensitive(Ge doped) fibre bragg grating. 2)Er doped fibres 3)Attenuating fibre 4)Photonic crystal fibre(used in switching, dispersion slope compensation, wavelength conversion) 4)Polarization preserving fibres 5)High index fibres 6) Bend-insensitive fibres. PMD is a statistical concept. It depends on stress and temperature. units are psec/sqrt(km). typical spectral width of a LED is 75-125nm. Types of laser diodes: 1)Fabry-perot (MM:spectral width ~ 5.26nm) 2)DFB (SM:spectral width ~0.2nm) 3)VCSEL 4)tunable lasers 5)pump lasers. SFF(P)- standard small form factor (pluggable)- a commercial transceiver. Two sections of the L-I curve (Laser optical output vs laser drive current): before threshold(first section) it is spontaneous emission(LED type). Above threshold it is stimulated emission(lasing). Extinction ratio = P(on)/P(off). DCF- Dispersion compensation fibres (with negative dispersion). WDM couplers: 1)Fused fibre couplers. two input two output. if the power at the two outputs is half of the input power we have a 3dB coupler. 2)Mach-Zehnder interferometer(MZI) 3)Dielectric thin-film filter TFF(dielectric reflector stack) 4)Fibre based bragg grating(2 UV rays form an interference pattern to form an index gradient) 5)AWG-Arrayed waveguide grating FSR(free spectral range)-spectral periodicity of the device. Application of optical amplifiers: 1)In-line 2)Pre-amplifier 3)Power amplifier 4)Booster amplifier. EDFA gain vs length of the fibre has a peak. ASE-amplified spontaneous emission. PRBS:Pseudo random bit sequence, typical length of the sequence = 2^(N-1),where N=7,10,15,23. Receiver sensitivity is the minimum average optical power defined at a specific BER(bit-error rate) at a particular data rate. Typical BER desired 10^-9 ->10^-15.