Service d’Électromagnétisme et de Télécommunications 1 1 Attenuation in optical fibres 5 ème Electricité - Télécommunications II Marc Wuilpart Réseaux de transmission photoniques

Service d’Électromagnétisme et de Télécommunications 2 There are four main causes of attenuation Material absorption –Intrinsic –Extrinsic Linear Scattering –Rayleigh Nonlinear scattering –Brillouin Scattering –Raman Scattering Fibre bend loss

Service d’Électromagnétisme et de Télécommunications 3 The attenuation parameter is expressed in dB/km In uniform fibres, the optical power exponentially decays with distance Where  p is the attenuation coefficient expresses in km -1. When using the dB/km unit, it becomes : Remark :

Service d’Électromagnétisme et de Télécommunications 4 Material absorption is due to the fibre composition Material absorption = loss mechanism related to the material composition and the fabrication process of the fibre : Intrinsic absorption : interaction with one or more of the major components of the glass. Extrinsic absorption : interaction with impurities within the glass.

Service d’Électromagnétisme et de Télécommunications 5 Intrinsic absorption is caused by electronic and vibrational transitions Absorption due to atomic defaults  negligible. Absorption due to electronic transitions in the glass  ultraviolet absorption. Absorption due to molecular vibrations of : –Si-O (9.2µm) –Ge-O (11.0µm) –P-O (8.1µm) –B-O (7.2µm)  infrared absorption. Absorption due to atomic defaults  negligible. Absorption due to electronic transitions in the glass  ultraviolet absorption. Absorption due to molecular vibrations of : –Si-O (9.2µm) –Ge-O (11.0µm) –P-O (8.1µm) –B-O (7.2µm)  infrared absorption. from Senior, "Optical Fiber Communications", Prentice Hall, 1992

Service d’Électromagnétisme et de Télécommunications 6 Extrinsic absorption is caused by metallic impurities  Arises from absorption by transition metal element impurities.

Service d’Électromagnétisme et de Télécommunications 7 Extrinsic absorption is also due to water into the glass  Absorption through molecular vibrations by water dissolved in the glass (hydroxil or OH - ). Absorption band centered on 1.38, 0.95 and 0.72µm Water contamination should be kept as small as possible during the fabrication process and during the fibre life  cable design.

Service d’Électromagnétisme et de Télécommunications 8 Extrinsic absorption due to water creates peaks in the spectrum from Senior, "Optical Fiber Communications", Prentice Hall, 1992 First overtone

Service d’Électromagnétisme et de Télécommunications 9 Linear scattering is mainly due to Rayleigh scattering Rayleigh scattering is the dominant intrinsic loss mechanism in the low absorption window between the ultraviolet and infrared absorption from Senior, "Optical Fiber Communications", Prentice Hall, 1992 Linear scattering = transfer of some or all the optical power contained within one propagating mode to another mode (guided or radiation modes). Linear means that there is no change of frequency or wavelength during the transfer.

Service d’Électromagnétisme et de Télécommunications 10 Rayleigh scattering is the dominant loss mechanism in the short wavelength range It results from inhomogeneities of a random nature occurring on a small scale compared with the wavelength of the light. Inhomogeneities arise from density and compositional variations into the glass material  refractive index fluctuations. Rayleigh attenuation has the form: The light is scattered in every direction  light is thus also backscattered which is used by OTDR (Optical Time Domain Reflectometry).

Service d’Électromagnétisme et de Télécommunications 11 Total attenuation is the sum of all mechanisms from Senior, "Optical Fiber Communications", Prentice Hall, 1992

Service d’Électromagnétisme et de Télécommunications 12 Attenuation is larger in multimode fibres Multimode fibres Singlemode fibres Loss are larger for multimode fibres because of higher dopant concentration.

Service d’Électromagnétisme et de Télécommunications 13 from Keiser, "Optical Fiber Communications" 3 telecommunication windows 1310 nm window minimizes the chromatic dispersion 1550 nm window minimizes the attenuation DSF fibres allows the use the third window with minimization of chromatic dispersion

Service d’Électromagnétisme et de Télécommunications 14 Nonlinear scattering is important at high powers This effect causes the optical power from one mode to be transferred in either the forward or the backward direction to the same or other modes at a different frequency. It critically depends on the optical power density within the fibre and hence becomes only significant above threshold power levels. Brillouin Scattering. Raman Scattering.

Service d’Électromagnétisme et de Télécommunications 15 Brillouin Scattering has the lowest threshold Interaction between acoustic waves and the optical signal. The scattered light appears as upper and lower sidebands which are separated from the incident light by the frequency of the acoustic wave. The incident photon produces by scattering a phonon of acoustic frequency as well as a scattered photon. The BS can only occur in backward direction.

Service d’Électromagnétisme et de Télécommunications 16 Raman Scattering can occur in WDM systems Interaction between vibrational modes of molecules and the optical signal. The scattered light appears as upper and lower sidebands which are separated from the incident light by the frequency depending on the energy levels of vibration modes. The RS can occur in both forward and backward directions.

Service d’Électromagnétisme et de Télécommunications 17 Nonlinear scattering create new optical frequencies in the spectrum from Senior, "Optical Fiber Communications", Prentice Hall, 1992

Service d’Électromagnétisme et de Télécommunications 18 Fibre bend loss is an important practical parameter Optical fibres suffer from radiation losses at bends or curves on their paths. Due to the energy in the evanescent field at the bend exceeding the velocity of the light in the cladding  radiation out of the fiber. from Keiser, "Optical Fiber Communications"

Service d’Électromagnétisme et de Télécommunications 19 Microbending can couple guided modes into radiation modes Microscopic random fluctuation  coupling with radiation modes from Keiser, "Optical Fiber Communications"