Fiber Optic Basics

Why fiber? – Low loss & low signal spreading means greater distances between expensive repeater stations. – Less weight means easier & less costly installation – Narrower pulses mean more information per second (or bandwidth). More bandwidth per fiber means more money per fiber for the provider. Initial Pulses Pulses start to spread Pulses start to overlap Pulses no longer recognizable

Visible to Naked EyeNot Visible

TIR – When all incident light is reflected at the boundary. Critical property for all fiber optic transmission.

Light reflects away from the lower index of refraction material (air) and continues to bounce down the object. The key here is the difference in the index of refraction between the two materials. For fiber optics to work, you only need this difference to exist between two glass materials.

Fiber optic fiber is constructed from two glass materials, a core region and a cladding region. Each material has its own index of refraction. In this case, the core region has an index of refraction of and the cladding region has an index of refraction of This difference is enough to achieve TIR as long as the light enters the fiber at a sharp enough angle (critical angle).

Multimode fiber will capture many of these light modes. Since light arrives at the other end at different times, the signal will suffer spreading (dispersion). Single mode fiber has such a small core that it will only capture the ”zero order mode” resulting in very little spreading due to modal dispersion.

Common Optical Fiber GeometryCore DiameterCladding DiameterSM or MM 9 / 1259 microns125 micronsSingle Mode 50/12550 microns125 micronsMultimode 62.5/ microns125 micronsMultimode 100/ microns140 micronsMultimode

Common Types of Loss for Optical Fiber 1.Intrinsic Loss a)Material Absorption - glass impurities absorbing light when hit. b)Scattering – glass impurities and atomic structure variation scatter light as it travels down fiber. 2.Extrinsic Loss a)Micro Bending – manufacturing defects in the core. b)Macro Bending – results from bending or crushing fiber.

Loss versus Wavelength Standard Single Mode Fiber Low or Zero Water Peak SM Fiber (better for Dense WDM systems) 1310nm 1550nm Single wavelength SM transmission systems are designed to operate at 1310 and 1550nm (low loss areas). Water Peak WDM – Wavelength Division Multiplexing (transmission system using several lasers at different wavelengths)

Multimode fiber bandwidth (typical) – 1 gigabit/sec for 550 meters (850nm) Single mode fiber bandwidth (typical) – 10 gigabits/sec for 80,000 meters (1550nm)

Advances in Multimode Fiber Transmission Standard → 100 Mb Ethernet 1 Gb Ethernet 10 Gb Ethernet 40 Gb Ethernet 100 Gb Ethernet OM1 (62.5/125) Up to 550 meters 220 meters33 metersNot Supported OM2 (50/125) Up to 550 meters 550 meters82 metersNot Supported OM3 (50/125) Up to 550 meters 550 meters300 meters100 meters OM4 (50/125) Up to 550 meters 550 meters> 400 meters 125 meters 10 Gb systems typically use OM3 fiber and low cost VCSEL lasers instead of LEDs.

New Bend-Insensitive Fibers Created to limit the affect of macro bending loss due to sharp bends in the fiber during installation. Almost all fiber cable being purchased today are “bend insensitive”.

Common Patchcord Offering Simplex or Duplex Single mode or Multimode (50 or 62.5um core) 1.6mm, 2.0mm, or 3.0mm cordage diameter LC, SC, FC, ST connectors

Multifiber Cable Assemblies

Most Popular Connector Offerings (PC Polish) LC/PC SC/PC FC/PC ST/PC PC = Physical Contact

Most Popular Connector Offerings (Angled Polish – for systems needing low return loss performance) LC/APC SC/APC FC/APC High return loss affects laser performance causing “ghosting” for CATV fiber systems and high bit error rates for very high speed digital systems.

Cross Section of Fiber Optic Connector Ceramic Ferrule Spring Fiber 900um Buffer Epoxy NOTE: All connectors are designed to accept a 900um buffered fiber. Cables designed with un-buffered fibers will need to have fibers up jacketed before connectorization.

Process Steps of Making Patch Cords 1.Cut cable to length 2.Cable prep a)Strip outer jacket b)Remove Kevlar c)Strip buffer d)Strip coating (exposing glass) 3.Inject epoxy into connector and thread fiber through connector 4.Crimp 5.Oven cure epoxy 6.Cleave excess fiber 7.Quick hand polish (remove excess fiber and epoxy from end face) 8.Machine polish the end face 9.Visual inspection of end face 10.Loss testing 11.Pack and ship

Physical Requirements of End Faces (PC Polish) -Need to control for good return loss performance Radius of Curvature (7-25mm) Offset (< 50um) Undercut (+/- 50nm)

Visual Requirements of End Faces - Need to control for good insertion and return loss performance Bad SMGood MM Good SM

Connector Insertion Loss  Power injected into the fiber (P 0 ) compared to power measured at far end or P 1  Ins. Loss = P 0 – P 1  Typical SM value = 0.12dB  Typical MM value = 0.08dB Connector Return Loss  Amount of light reflected back to source (critical parameter for very high speed systems) Patchcord with 2 connectors SM return loss spec is typically < -55dB

Distribution Cable Multimode and Single mode Indoor or Outdoor 900micron sub units Fiber counts of 2, 4, 6, 8, 12, 24, 48, 72 or more Outer jacket can be of various materials Can be terminated with any connector style Unitized (typically for 24 fibers & up) Non-Unitized (typ. for 24 fibers or less) 12 fiber units (with Kevlar) 900um buffered fibers 900um buffered fibers

Breakout Cable Multimode and Single mode Indoor or Outdoor 1.6mm, 2.0mm, 3.0mm sub units Fiber counts of 2, 4, 6, 8, 12, 24, 48, 72 or more Outer jacket can be of various materials Can be terminated with any connector style Individual 1.6mm, 2.0mm, or 3.0mm jacketed fibers (each fiber has its own Kevlar strength members)

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