Lecture 3: Fibre Optics

Transcription

1 Lecture 3: Fibre Optics Lecture aims to explain: 1. Fibre applications in telecommunications 2. Principle of operation 3. Single- and multi-mode fibres 4. Light losses in fibres Fibre is a transparent cylinder made of a dielectric. Most common material used in fibres is fused silica (amorphous SiO 2 )

2 Nobel Prize for Fibre Optics Nobel Prize for Physics 2009 Sir Charles K. Kao for "groundbreaking achievements concerning the transmission of light in fibers for optical communication Work was mainly done at Standard Telecommunication Laboratories (STL) in Harlow, England in 1960s, with groundbreaking predictions for use of glass fibres for telecommunications in 1966

3 Applications in telecommunications

4 Information encoded using light Light has very high frequency: f 15 = c / λ 10 Hz About more information can be transmitted than by microwave Very short light pulses can be used to transmit bits of information Information can be encoded using wavelength (or colour) Multiplexing: use of single pathway to transmit simultaneously several signals which nonetheless retain their individuality 1970, Corning Glass Works, first fibre 22 April 1977, first live telephone traffic through fiber optics 6 Mbit/s at 0.8 µm. The second generation: early 1980s, 1.3 µm. By 1987 rates of up to 1.7 Gb/s, repeater spacing up to 50 km. The first transatlantic telephone cable with optical fibre in Third- and fourth-generation in 1990s and 2000s: at 1.55 µm, losses only about 0.2 db/km. Bit rate of 10 Tb/s was reached by Repeaters at 100 km and more.

5 Advantages of fibres 1. Low-loss transmission 2. High information carrying capacity 3. Small size and weight 4. Immunity to electro-magnetic interference (bringing unparalleled signal security), no cross-talk between parallel fibres, can be installed in dense areas 5. Abundant availability of the required raw material (sand) Other major applications: Medical applications (endoscopes etc) Industrial application (e.g. as probes)

6 Principle of operation

7 Total internal reflection in fibres n c n o Fibre: transparent cylinder of refractive index n f imbedded in a material of refractive index n c n f θ p θ i If we consider a ray travelling in the plane containing the optical axis then it will remain constrained as long as: cosθ p n n c f Role of cladding: Cladding provides medium with lower n and protects from frustrated total internal reflection e.g. from fibre touching, dust or moisture on the surface

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9 Single and multi-mode fibres

10 Fibre modes y d k θ p n f z For the wave to propagate in the fibre, electromagnetic wave theory requires waves to interfere constructively Allowed angles of propagation inside the fibre: sinθ = p p λ 2dn f The lowest order mode is p=0 and is along the fibre axis. The highest order is near θ c.

11 Different types of multi-mode fibres Typical sizes 50µm Step-index fibres: abrupt change in material refractive index Disadvantage: Dispersion Graded-index fibres: a gradual decrease of the refractive index towards the cladding. Often by a parabola law Advantage: serpentine modes travel similar time to the central mode, since it is slower (larger n)

12 Single mode fibre If for the mode with p=1 θ 1 is greater than the critical angle for the total internal reflection θ c then it cannot propagate, only the p=0 mode will. This is the case for a single mode fibre The condition for single mode propagation d < 2 λ 2 2 n f nc To generalise a fibre will carry modes 0,1,2 p-1 (that is, p modes) if d < pλ 2 2 / 2 n f nc

13 Light losses in fibres

14 Attenuation in silica fibres Ultraviolet range: electronic absorption Infrared range: lattice vibrations (phonons) Transparent regions at 1.3 and 1.55 µm Losses due to Rayleigh scattering ~1/λ 4 and water-related absorption (max at 1.38µm). Additional losses due to imperfections and sharp bends Minimum loss (attenuation) ~0.1dB/km at 1.55 micron (1550nm): 100km before reamplification (10 times attenuation) db = 10 log10 ( ) P out P in

15 EXAMPLE 3.1: Single mode fibre Calculate the diameter of a single mode fibre with n f =1.62 and n c =1.52 operating at λ=1.55µm EXAMPLE 3.2: Multi-mode fibre A glass fibre with n f = 1.52 and n c = 1.50 and a diameter d = 1.8 micron operates with light of wavelength 1.3 micron. (i) What is the highest order mode that can propagate? (ii)what is the highest order of the mode for wavelength in the visible red region? (iii) What is the external angle of acceptance corresponding to the modes with the highest numbers in (i) and (ii) EXAMPLE 3.3: Fibre design Design a fibre enabling propagation of only two lowest modes at the wavelength of 1.55 micron using glass with low dispersion and n f = 1.45 for the fibre core.

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