1. Module 3 Transmitting Light on a Fibre

2.  An optical fiber is a very thin strand of silica glass in geometry quite like a human hair.  In reality it is a very narrow, very long glass cylinder with special characteristics.  When light enters one end of the fiber it travels (confined within the fiber) until it leaves the fiber at the other end Introduction

3.  When transmitting light, two critical factors need to be taken into account:  1. Very little light is lost in its journey along the fiber.  2. Fiber can bend around corners and the light will stay within it and be guided around the corners. Introduction

4.  Weight and Size: Fiber cable is significantly smaller and lighter than electrical cables to do the same job.  Material Cost : Fiber cable costs significantly less than copper cable for the same transmission capacity.  No Electrical Connection  No Electromagnetic Interference: Because the connection is not electrical, you can neither pick up nor create electrical interference (the major source of noise). This is one reason that optical communication has so few errors Advantages of Fiber

5.  Distances between Regenerators: the repeater spacing is typically +100 kilometers.  Low loss over long transmission  High bandwidth of at least 25 THz and higher  Better Security: It is possible to tap fiber optical cable. But it is very difficult to do and the additional loss caused by the tap is relatively easy to detect  Advantages of Fiber

6.  An optical fiber consists of two parts: 1. The core: a narrow cylindrical strand of glass. 2. The cladding: A tubular jacket surrounding the core.  The core has a (slightly) higher refractive index than the cladding.  This means that the boundary (interface) between the core and the  cladding acts as a perfect mirror.  Light travelling along the core is confined by the mirror to stay within it - even when the fiber bends around a corner. Parts of optical fiber

7. In Figure (right) the interface between two media of refractive index n 1 and n 2 is shown. A light ray from medium 1 is incident on the interface of medium 1 with medium 2. The angle of incidence is the angle between the incident ray and the normal to the interface between the two media and is denoted by θ 1. Part of the energy is reflected into medium 1 as a reflected ray, and the remainder (neglecting absorption) passes into medium 2 as a refracted ray The angle of reflection θ 1r is the angle between the reflected ray and the normal to the interface; similarly, the angle of refraction θ 2 is the angle between the refracted ray and the normal. Parts of optical fiber

8.  As the angle of incidence θ 1 increases, the angle of refraction θ 2 also increases.  If n 1 > n 2, there comes a point when θ 2 = π/2 radians = 90 0.  This happens when θ 1 = sin −1 n 2 / n 1.  For larger values of θ 1, there is no refracted ray, and all the energy from the incident ray is reflected.  This phenomenon is called total internal reflection.  The smallest angle of incidence for which we get total internal reflection is called the critical angle and equals sin −1 n 2 / n 1. Parts of optical fiber Snell's Law n 1 sin θ 1 = n 2 sin θ 2

9.  Thus light is perfectly reflected at an interface between two materials of different refractive index iff: 1. The light is incident on the interface from the side of higher refractive index. 2. The angle θ is greater than the “critical angle”. Parts of optical fiber

10.  Light propagates in optical fiber due to a series of total internal reflections that occur at the core-cladding interface. Only light rays that are incident at an angle at the air-core interface will undergo total internal reflection at the core- cladding interface and will thus propagate. Such rays are called guided rays, and θ 0 max is called the acceptance angle. Parts of optical fiber Snell's Law n 1 sin θ 1 = n 2 sin θ 2

11. The refractive index difference n 1 − n 2 is usually small, and it is convenient to denote the fractional refractive index difference ( n 1 − n 2 )/ n 1 by For small As an example, if = 0.01, a typical value for (multimode) fiber, and n 1 = 1.5, a typical value for silica, assuming we are coupling from air, so that n 0 = 1, we obtain θ max 0 ≈ 12 ◦ Parts of optical fiber Snell's Law n 1 sin θ 1 = n 2 sin θ 2

12. Numerical Aperture (NA) The NA is the measure of the light capturing ability of the fiber. However, it is used for many other purposes. Where n 1 = refractive index of the core and n 2 = refractive index of the cladding. Also: This relates the NA to the RI of the core and the maximum angle at which a bound ray may propagate. Typical NA for single-mode fiber is 0.1. For multimode, NA is between 0.2 and 0.3 (usually closer to 0.2).

13. Fibre Attenuation The lower curve shows the characteristics of a single-mode fibre made from a glass. The upper curve is for modern graded index multimode fibre. Attenuation in multimode fibre is higher than in single-mode because higher levels of dopant are used. The peak at around 1400 nm is due to the effects of traces of water in the glass.

14.  Most of the attenuation in fibre is caused by light being scattered by minute variations (less than 1/10th of the wavelength) in the density or composition of the glass.  This is called “Rayleigh Scattering”. in honour of Lord Rayleigh, who published a paper in 1871 describing this phenomenon.Lord Rayleigh  Rayleigh scattering is also the reason that the sky is blue and that sunsets are red. Fibre Attenuation

15.  Mie Scattering is another form of scattering.  Mie scattering is caused by imperfections in the fibre of a size roughly comparable with the wavelength.  This is not a significant concern with modern fibres as recent improvements in manufacturing techniques have eliminated the problem. Fibre Attenuation

16.  The absorption peak shown in previous Figure is cantered at 1385 nm but it is “broadened” by several factors including the action of ambient heat.  This absorption is caused by the presence of water. Fibre Attenuation

17. Transmission windows

18.  Short Wavelength Band (First Window): 800-900 nm.  Medium Wavelength Band (Second Window): 1310nm.  Long Wavelength Band (Third Window): between 1510 nm and 1600 nm. This band has the lowest attenuation available on current optical fibre (about 0.26 dB/km). Transmission windows

19.  The potential transmission capacity of optical fibre is enormous  The second window is about 100 nm wide (1250 nm - 1350 nm) with a loss of about.4 dB per km.  The third window is around 150 nm wide (1450 nm - 1600 nm with a loss of about.2 dB per km.  The useful (low loss) range of second and third is around 250 nm. Transmission capacity

20.  We have a usable range of about 30 Tera Hertz (3 × 10^13 Hz).  We can expect a digital bandwidth of 3 × 10^13 bits per second.  In 1998 practical fibre systems were limited to 10 Gbps because of the speed of the electronics needed for transmission and reception. Transmission capacity

21.  Almost all light sources used in communications today are made from semiconductors  There are two types of light sources 1. Light Emitting Diodes (LEDs) 2. LASER Light Sources

22.  Light Emitting Diodes are simpler than lasers but have a lot in common with them.  Characteristics of LEDs  Low Cost  Low Power  Relatively Wide Spectrum Produced: Multimode Fibbers  Incoherent Light: Therefore LEDs are not suitable for use with single-mode fibre. Light Emitting Diodes : LED

23.  LASER :Light Amplification by the Stimulated Emission of  Radiation.  Lasers produce the best kind of light for optical communication. LASER

24.  Characteristics of LASERs 1. Ideal laser light is single-wavelength only. 2. Lasers can be modulated (controlled) very precisely. 3. Lasers can produce relatively high power 4. Laser light is produced in parallel beams, therefore a high percentage (50% to 80%) can be transferred into the fibre. LASER

25.  Disadvantages: 1. High cost 2. The wavelength produced by a Laser depends on the characteristic of the material used to build it and of its physical construction. However, tunable lasers exist and are commercially available LASER

26. LASER - Operation Principles of a Laser

27. 1. An electron within an atom (or a molecule or an ion) starts in a low energy stable state often called the “ground” state. 2. Energy is supplied from outside and is absorbed by the atomic structure whereupon the electron enters an excited (higher energy) state. 3. A photon arrives with an energy close to the same amount of energy as the electron needs to give up to reach a stable state. LASER - Operation Principles of a Laser

28. 4. The arriving photon triggers a resonance with the excited atom. As a result the excited electron leaves its excited state and transitions to a more stable state giving up the energy difference in the form of a photon. LASER - Operation Principles of a Laser

29.  The critical characteristic here is that when a new photon is emitted it has identical wavelength, phase and direction characteristics as the exciting photon. LASER - Operation Principles of a Laser

30.  Note: The photon that triggered (stimulated) the emission itself is not absorbed and continues along its original path accompanied by the newly  emitted photon. LASER - Operation Principles of a Laser

31.  Converts light to electrical signal – Voltage – Current  Response is proportional to the power in the beam Optical Detectors