The optical fiber is a light guide which conducts the light from the Transmitter to the Receiver.

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2 The optical fiber The optical fiber is a light guide which conducts the light from the Transmitter to the Receiver. The Transmitter and Receiver are Electro-Optics Converters (EOC). The Transmitter converts an electrical signal into an optical signal. The Receiver converts an optical signal into an electrical signal. When the Transmitter and the Receiver are inside the same component, it is called a Transceiver. More than 90% of fiber optics application are data networks. The optical fiber can also be used as a strain sensor or just as a light guide.

3 The optical fiber: main advantages Mass saving: the optical fibers are much lighter than traditional copper wires (ex electrical quadrax = 40 g/m, optical fiber with 1.8 mm jacket = 4 g/m). Small dimensions: fiber optic solutions are smaller than electrical solutions (ex : ribbon fiber with 18 fibers side to side, 125 µm between fibers). Immunity to electromagnetic interferences: The fiber is a totally passive support and is insensitive to electromagnetic effects. Immunity to electrical sparks: the optical fiber is not conductive (high electrical resistance) and is insensitive to electrical sparks. Network security: Unlike to an electricity network (possible recovery of the information by induction), the signal transmitted in an optical fiber cannot be intercepted from the outside. No phenomenon of "cross-talk between different links. Huge bandwidth: combined with multiplexing methods, the data rate inside a single fiber can reach 4 Terabits/s!! ( bit/s). Very low attenuation: a few db/km only. Repeater-less transmission over long distances is possible.

4 Units and comparison between copper/optical fiber Characteristics Optical losses Attenuation Bandwidth wavelength Units db (decibel) db / km Hz.km nm Characteristics Cu Fiber optics Multimode Singlemode Bandwidth for 1 km 10 MHz 500 MHz > 10 GHz Maximum distance of transmission 100 m 2 km 40 km

5 The optical fiber The core: inner light carrying member (glass or plastic) The cladding: layer which serves to confine light in the core (glass or plastic) Buffer: «shock absorber» layer Core Cladding Buffer

6 Reflection/transmission between two layers i r c n1 i c r n1 t n2 n2 Incidence angle larger than critical angle = reflection + transmission Incidence angle smaller than critical angle = Total reflection c vacuum n = = c glass Speed of light in vacuum Speed of light in glass 1.5

7 Optical fiber basic principle Buffer Cladding Core Propagation in a waveguide with total reflections Basic principle of optical fibers

8 Use of optical fibers Different fibers available on the market: Optical fiber Dimensions mm Wavelength l (nm) Application Advantages Weaknesses Silica/silica 9/ / 1310 Telecom 50/ Datacom 62.5/ Silica/polymer 200/ Industrial Aeronautic Industrial Railway High data rate Performances and low cost Performances And low cost Robustness Performances Limited bandwidth Limited bandwidth Low bandwidth Plastic 980/ Datacom Cars Machines Very lost cost (fiber & termination) Limited Temp. T max =85 C There are a lot of other fibers...

9 Attenuation in the optical waveguides Attenuation Rate Cost Wavelength Opto-electronic components : LED (200 mw max), low cost, 150 MHz max nm) LASER (2000 mw max) Up to 10 GHz, and more VCSEL (2000 mw max) 5 to 10 GHz

10 Numerical aperture of optical fibers Only a small cone of light can enter the fiber. Above the critical angle of incidence, rays of light are trapped in the cladding and can t propagate. The Numerical Aperture (NA) is given by the sine of the critical angle.

11 Attenuation in optical fibers The attenuation characterizes the loss of power in an optical link. There are 2 types of attenuations: Intrinsic attenuation (glass properties) diffusion / dispersion of light absorption of the glass Extrinsic attenuation micro-curves macro-curves Attenuation limits the propagation distance Attenuation depends on wavelength

12 Index of optical fibers The light must be kept into the core, otherwise information is lost. There are two ways of keeping the light confined in the core: Step index Graded index

13 Step index Step index: the fiber is composed of elements of different indexes of refraction (the core and the cladding). The light is reflected at the interfaces. The media are homogeneous. n2 n1 n2 Step index The light propagates in straight lines and is reflected in the core/cladding interfaces.

14 Graded index The core of the fiber is «doped» to trap the light in its center. The doping creates a variable index in the glass. n2 n1 n2 Graded index The doping creates contraints in the core of the fiber and fragilize it. The larger is the core, the larger is the constaint. Graded index fiber can be used for core up to 100 µm (100/140 fiber). The light is constanly refracted in the core of the fiber or can be reflected on the interfaces (core/cladding).

15 Modes & rays Rays and modes theories are two different approaches of the resolution of the propagation equation of light in a wave guide. The rays theory: is an high frequency approximation of the propagation equation. It is easily visualisable (drawn by rays) and is an excellent estimate. The solution is independent of the wavelength. The theory can t be use for small diameter of fiber (like 9/125 fiber). The mode theory: is an «exact» approach of the propagation equation but complex. It takes into account the wavelength. A mode represents a possible solution of the propagation equation. The modes only depends on the physical properties of the medium (core diameter, index, cladding, etc) and the wavelength. There are two types of optical fibers : Singlemode fiber: only 1 mode of propagation is possible. This provides an unique propagation time (no possible dispersions and no echoes). The core diameter is very small (5 to 9 µm for =1300 nm to 1500 nm). Bandwidth is large. Multimodes fiber: many modes of propagation are possible. The modes are super-imposed. Propagation times of each mode are different providing echoes. Bandwidth is limited. Minimum diameter of multimode fibers is 50µm.

16 Dispersion Several optical paths may exist between the source and and the receiver This multi-path phenomenon creates temporal dispersion of transmitter signals Transmitted signal 0 amplitude time Dispersion reduces the bandwidth of the fiber The larger is the propagation distance, the larger is the dispersion Bandwidth is given in MHz.km Received signal

17 Dispersion effect on optical fibers Dispersion in the fiber results in a broadening of the signal pulses At high data rates, dispersion allows overlap between pulses. This limits the bandwidth Step index multimode fiber Index Profile Input Pulse Output Pulse Step index multimode fiber: high number of modes, large dispersion low bandwidth, lower operation speed low cost Graded index multimode fiber Step index singlemode fiber Graded index multimode fiber: lower dispersion higher bandwidth, higher operation speed Step index single mode fiber: minimum dispersion high bandwidth

18 Optical cables 3 families of cables Loose tube cable telecom / datacom applications Tight structured cable space / aeronautic Ribbon cables - telecom / datacom applications Jacket Strength members (Kevlar / Aramid) Buffer tube Jacket Strength members (Kevlar / Aramid) Tight buffer Jacket Strength members (Kevlar / Aramid) Coated fiber Coated fiber Laminated film Coated fiber Loose tube Tight structure cable Ribbon cable

19 Optical technologies Expanded beam: a lens is fixed on each fiber so that light goes from a fiber to the other one. There is no contact between the two fibers. The mechanics must be perfectly aligned to guaranty low IL. Ferrule 1 Lens Ferrule 2 Physical contact (Butt-joint): the two fibers are placed in front of each other. There is a physical contact. The alignment is obtained thanks to a sleeve. Ferrule 1 Ferrule 2 Sleeve «Butt-joint»

20 Optical technologies mapping Lens Butt-Joint Weight - + Size - + Density - + Insertion Loss - + Price - + Cleaning + - Robustness + - Because of the lenses, the expanded beam technology is heavier, bigger and more expensive than the butt-joint. But it is easier to clean: the lenses are stuck with the insert so that it is hermetic; the connector can thus be drawn into water for cleaning.

21 The optical connector The optical connector shall permit the alignment of the fiber cores. In the case of the single mode fiber, the misalignment shall not be over 1 micrometer! In case of a butt-joint contact, the fibers are glued inside 2 ferrules which are aligned through a guiding sleeve. The butt-joint optical connector shall: Allow a perfect guiding of the fibers core through the sleeve whatever are the external constraints. Permit an easy access to the contact end face for inspection and cleaning.

22 Example of fiber optic connector Ferrule Optical fiber Adaptor Sleeve (alignment) Boot Butt-joint technology

23 Insertion loss Insertion Loss (I.L.) = Optical power loss due to an optical component I.L. = -10 log (P_transmitted( / P_emitted) (in db) Typical I.L. Connector: 0.50 db. Fusion splice: 0.05 db. 3dB of insertion loss means that 50% of the power is transmitted. Loss factors Intrinsic factor: connection between different optical fiber. External factor: misalignments of ferrules inside the connector.

24 Optical losses Intrinsic Factor Different NA Extrinsic factor Air gap between fibers Different core diameters Dust / bad polishing Different cladding diameters Axial displacement Different concentricities Angular displacement

25 Return loss (reflection losses) Return Loss (R.L.) = Optical losses due to the reflection of the light at the interface of the optical connectors: R.L. = 10 log (P_reflected( / P_emitted) ) (in db) LASER emitters are very sensible to reflected light. LED and VCSEL are not sensible to back reflections.

26 End face shape Flat Polish (FP) RL < -10dB Flat polish Air Gap High RL Physical Contact (PC) RL < -30dB Ultra Physical Contact (UPC) RL< -45dB Good physical contact Low RL No Air gap Angled Physical Contact (APC, angle = 8 o ) RL < -55dB Minimum RL