Optical Fibres and Telecommunications Lecture 2 Fibre Basics Dr Tom Brown Room 284, x3129 ctab@st-and.ac.uk Last Time Why are we doing this course? What s in the course? How will the course be assessed? What other materials are there? 1
Introduction How does a fibre work? Types of optical fibre. What is attenuation? Where does attenuation come from? How does attenuation affect telecommunications. How does a fibre work? Light Ray Total internal reflection Core n 1 Cladding n 2 Optical fibre is a dielectric waveguide. Light is confined within the fibre by total internal reflection. Core refractive index is higher than cladding index ie. n 1 >n 2 2
Numerical aperture θa θ i n a n 1 n 2 θ r Numerical aperture describes the acceptance angle of the fibre. At entrance to fibre: n a sinθ i =n 1 sinθ a (Snell s Law) sinθ i = n 1 sinθ a =n 1 (1-cos 2 θ a ) 0.5 (1) At core/cladding interface: n 1 sinθ r =n 2 sinθ t For total internal reflection: θ t =90 o, so n 1 sinθ r =n 2 Therefore: cosθ a =n 2 /n 1 So: sinθ i ~ θ i = N.A. = (n 12 -n 22 ) 0.5 (2) Numerical aperture For a typical fibre, n 1 = 1.5, n 2 =1.485 (1% Difference) So NA=0.21 or θ i =12 o This gives the maximum divergence angle of the light that can be focussed into the fibre. For comparison a Gaussian Beam focussed to a 2µm spot size has a divergence half-angle of 7 o so this could be efficiently coupled into the fibre. An alternative formation of the NA is: N.A.=n(2 ) 0.5 (3) ( =(n 1 -n 2 )/n ; n=(n 1 +n 2 )/2) 3
Modes An optical fibre can contain many propagation modes. Each mode travels a different distance along the length of the fibre. This gives rise to a blurring of the pulse (intermodal dispersion.) Number of modes supported is proportional to the V number of the fibre where V=(πd/λ).NA (4) Where V is large N modes =V 2 /2 (5) Graded index fibre n 0 1% 2% r One solution is to have a radial variation in core index. Rays on the edge of the core travel faster than those in the middle. This is known as graded index fibre. 4
Single mode fibre More common these days is to reduce d so that only one mode can propagate. This is single mode fibre. Can cause problems with NA if the NA is high, d must be very small for single mode operation. For single mode operation V<2.405. So assume NA=0.21, λ=1500nm d<5.4µm Attenuation Attenuation is the decrease in light power during light propagation along the length of an optical fibre. Unit is normally db/km. Definition: A=10log 10 (P out /P in ) / l fibre (6) P in = Power input into fibre. P out = Power output. l fibre = Length of fibre (km) Example: 10km long fibre, P in =100mW, P out =50mW A = 10log 10 (0.5) / 10 db/km A = (-)0.3 db/km NB. The ve symbol is often omitted. 5
Origins of loss macrobending loss Loss When straight, fibre bounce angle (α) < α c α c α<α c At the bend α>α c. Total internal reflection condition no longer satisfied some light leaks out into cladding. Eg. For standard fibre, one turn of 32mm diameter spindle can cause a 0.5dB loss. Origins of loss microbending loss α>α c Microscopic errors of core / cladding geometry caused during fabrication process. Intrinsic to the fibre. Very low in modern fibres. 6
Origins of loss - Scattering Scattering losses are caused by imperfections within the core of the optical fibre. Very small perturbations in refractive index can cause scattering loss. Rayleigh scattering loss: l scatt =Aλ -4 where A is a constant depending on the material. Reducing the wavelength by half increases the loss by a factor of 16! Origin of loss - absorption All materials absorb some light. Silica glass (the type optical fibre is made from) absorbs weakly for visible light, but absorbs begins very strongly as the wavelength moves further into the infrared. Impurities also cause absorption loss. OH - losses are particularly important. Arise from water impurities during fabrication. Particular problem is with absorption bands around 1.4µm. Best fibre has losses <0.2dB/km at 1.55µm. This means transmission of >95% of input light after 1km. If the sea had losses this low, it would be easily possible to see to the bottom of the deepest oceans! 7
Fibre attenuation spectrum. Loss db/km 10 OH Absorption Peaks 1.55µm 1.0 Impurity metal absorption 0.2 0.1 Rayleigh Scattering 0.8 1.0 1.2 1.4 1.6 1.8 Wavelength / µm Infrared Absorption of Silica Summary Guiding in optical fibres Modes Numerical aperture Multimode, graded index and single mode fibre Losses in optical fibre Bend losses Scattering Absorption Fibre attenuation spectrum 8