Tutorial on optical fibres F. Reynaud IRCOM Limoges Équipe optique F. Reynaud IRCOM Limoges Équipe optique Cargèse sept 2002

optical fibre structure 1) Generalities Cargèse sept 2002 Silica fibres typical refractive index : 1,45 – 1,50 Refractive index difference Core diameter : 5 à 50 µmCladding diameter : 125 à 500 µm Refractive index profil

optical fibre manufacturing Cargèse sept ) preform manufacturing 2) Drawing process (Modified Chemical Vapour Deposition). PCVD (Plasma Chemical Vapour Deposition) OVPO (Outside Vapour Phase Oxydation 1) Generalities

2) Propagation in optical fibres Cargèse sept 2002 Geometrical optics Snell Decartes law : i 1 > i lim Possibility to trap light beams in an high refractive index area surrounded by a low refractive index area

Cargèse sept 2002 Wave theory 2) Propagation in optical fibres First example planar mirror guide Propagation without losses: Intensity = 0 on mirrors a = n i with n = integer Two directions interference between two plane waves For each n one propagation mode

Cargèse sept ) Propagation in optical fibres Propagation without losses: Intensity =0 close to the core/cladding interface a+ 2  = n i with n = integer Two directions interference between two plane waves Second example Planar dielectric waveguide Wave theory One mode n One angle  n solution of the equation

Cargèse sept ) Propagation in optical fibres Second example Planar dielectric waveguide Wave theory Limited number of mode If only one>>>monomode  0    solutions

Cargèse sept ) Propagation in optical fibres Wave theory 3 D interference

Cargèse sept ) Propagation in optical fibres Properties of the modal structure Decomposition of any optical field on the mode basis Wave theory Same transverse field distribution at the input and output Propagation = phase shift  n is the propagation constant Propagation = phase shift

Cargèse sept ) Propagation in optical fibres  depends upon Dispersion Mode in a multimode fibre Modal or intermodal dispersion

Cargèse sept ) Propagation in optical fibres Dispersion Wavelength dependent  Chromatic or intramodal dispersion Wavelength (nm) Chromatic dispersion (ps/nm.km) G.652 (0.08 ps/nm 2.km) G.653 EDFA bandwidth G.655

3) Determination of the mode number Diffraction properties Basic rules General case Multimode beam Monomode beam Cargèse sept 2002

3) Determination of the mode number Core diameter /a Numerical aperture NA Number of spots or speckles Number of modes Case of an optical fibre

Cargèse sept ) Determination of the mode number One speckle diameter surface Fibre core Number of degrees of freedom diameter surface a Warning: N is wavelength dependent

Cargèse sept ) Determination of the mode number Two examples n 2 =1.450 a=8µm Monomode fibre n 1 =1.455 a=50µm n 2 =1.450 n 1 =1.462 Multimode fibre Warning: N is wavelength  = 1.3µm

4) Characterisation of optical fibres Cargèse sept 2002 Numerical aperture Refractive index distribution n( radius ) radius

Cargèse sept ) Characterisation of optical fibres Fibre losses Loss (dB/km) Wavelength (nm) Transmission fibre loss (silica) I2I2 I1I1 Second step Detector Fibre length d Launching assembly =0

5) Optical fibre implementation Cargèse sept 2002 Connectors Plug with a ceramic ferule FCPC E2000 body Loss as function of The transverse position error

Cargèse sept ) Optical fibre implementation couplers Fusion splicing polishing Glued From 2 to 2 From 2 to 8

Cargèse sept 2002 principal use>> optical fibre telecommunications 6) Application of optical fibers Very high bit rate 1 Tbit/sec Very low losses The solution for long distance signal propagation

Cargèse sept 2002 Optical fibre sensors Temperature Pressure Rotation Chemical concentration 6) Application of optical fibers

Possibility to built interferometers Mach Zehnder configuration Cargèse sept ) Application of optical fibers See next lecture

Cargèse sept ) Material and new optical fibers UV 0.3µm Visible Near IR 2µm Far IR 10µm

Cargèse sept 2002 DGD Slow PSP Fast PSP 7) Material and new optical fibers Polarisation preserving fibers Highly birefringent fibres core cladding Stress area Propagation

Cargèse sept ) Material and new optical fibres structure Photonic crystal fibres Monomode over a very large spectral domain