Fibre Optic Basics FIA Summer Seminar 2014 Andrew Cole
Theory Basics Characteristics of Light What is Light? An electromagnetic wave, which like other waves, has a frequency (f), velocity (v) and a wavelength (λ). The velocity v, the speed of light is represented by c. c = f λ. 2
Theory Basics Characteristics of Light 3
Lasers and Eye Safety 4
Lasers and Eye Safety Visible Light Wavelength: 400nm to 700nm Typical Telecom Wavelength: 850nm to 1650nm Light used for communications is not visible light, but it will harm your eye Unless you are 100% sure that the light is switched off Never look into the end of an optical fibre 5
Unless you are 100% sure that the light is switched off Never look into the end of an optical fibre 6
Lasers and Eye Safety 7
Lasers and Eye Safety Video Inspection Probe / Indirect Viewing Aid 8
Unless you are 100% sure that the light is switched off Never look into the end of an optical fibre 9
Theory Basics Characteristics of Light Units of Measure Frequency: Wavelength: THz (x10 12 Hz or x10 6 GHz) (1550nm is approx. 292THz) um or nm (x10-6 m/s) nm (x10-9 m/s) Optical Power: dbm Attenuation: db Pulse Width: us or ns (x10-6 ) or ns (x10-9 ) 10
Theory Basics Characteristics of Light The Speed of Light c = 299,792,458 m/s (3 x 10 8 m/s Approximately) in a vacuum 11
Theory Basics Characteristics of Light But, light travels more slowly in any other material Index of Refraction (IOR), n (in fibre) n = c v (c = speed of light in a vacuum) (v = speed of light in the fibre) IOR is specified by the fibre manufacturer 12
Theory Basics Characteristics of Light The distance that light travels through an optical fibre, in a given time can be calculated d = c t n Where: d = distance c = the speed of light t =duration n = the Index of Refraction (IOR) 13
Theory Basics Characteristics of Light Core (glass) 8-10 microns SM 62.5 microns MM Cladding (glass) 125 microns Buffer / Coating 900 microns In a single mode fibre, not all the light energy is fully contained in the core of the fibre. Some of the light energy travels in the cladding, close to the core. The area of the beam of light, across a section of the fibre, is called the Mode Field or Effective Area and its diameter the Mode Field Diameter. The Core acts as a waveguide. Fibre Type Mode Field Diameter @ 1550nm MFD mid-range Effective Area (nominal) Dispersion compensating fibre 6.0-6.2um 6.1um 30um 2 G.653 7.8-8.5um 8.15um 52um 2 G.655 TW-RS 7.8-9.0um 8.4um 55um 2 G.655 LEAF 9.2-10.0um 9.6um 72um 2 G.652 9.7-10.7um 10.2um 83um 2 G.654.B 9.5-13.0um 11.25um 100um 2 14
How does an OTDR measure? 15
OTDR Basics Rayleigh Scattering Rayleigh Scattering Backscatter Particle Light pulse hits dopant particle Weaker after backscattering 16
OTDR Basics Time of Flight Fibre Core Reflection Light Pulse 17
OTDR Basics Time of Flight Light Pulse Reflection d = t c 2 n Where: d = distance c = speed of light in a vacuum t = elapsed time n = Index of Refraction 18
OTDR Basics OTDR Parameter Settings 19
OTDR Basics Pulse Width and Dynamic Range Too long (good DR but large deadzones) Just right (good DR, short deadzones) Too short (short deadzones but insufficient DR) 20
OTDR Basics Range Setting Range must be at least 25% greater than the fiber under test. If the OTDR stops measuring here... this data won t be acquired! 21
OTDR Basics OTDR Parameter Settings 22
OTDR Basics OTDR Trace 23
OTDR Basics Typical OTDR Trace Reflective Event Return Signal Level Launch Non-Reflective Event End/Fault Distance Noise 24
OTDR Basics - Deadzones Fusion splice Event Deadzone Backscatter Deadzone 25
OTDR Basics Reflectance Reflectance 26
OTDR Basics Loss & Reflectance P3 P1 P2 Insertion Loss P1 P2 (expressed in db) Reflectance P3 P1 (expressed in negative db, i.e. -40dB) Return Loss P1 P3 (expressed in positive db, i.e. +40dB) 27
OTDR Basics Optical Return Loss (ORL) ORL Fresnel Reflections Backreflections from Rayleigh Scattering Transmitted Light 28
Fibre Optic Cleanliness 29
Always clean before you connect!! 30
Always clean before you connect!! 31