fiber optic basics

Fiber Optic Basics

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 Eye Not 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 1.458 and the cladding region has an index of refraction of 1.440. 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

Geometry Core Diameter Cladding Diameter SM or MM

9 / 125 9 microns 125 microns Single Mode

50/125 50 microns 125 microns Multimode

62.5/125 62.5 microns 125 microns Multimode

100/140 100 microns 140 microns Multimode

Common Types of Loss for Optical Fiber

1. Intrinsic Lossa) 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 Lossa) 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

TransmissionStandard →

100 MbEthernet

1 GbEthernet

10 GbEthernet

40 GbEthernet

100 GbEthernet

OM1 (62.5/125)

Up to 550 meters

220 meters 33 meters Not Supported

Not Supported

OM2 (50/125)

Up to 550 meters

550 meters 82 meters Not Supported

Not Supported

OM3 (50/125)

Up to 550 meters

550 meters 300 meters 100 meters 100 meters

OM4 (50/125)

Up to 550 meters

550 meters > 400 meters

125 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 DuplexSingle mode or Multimode (50 or 62.5um core)1.6mm, 2.0mm, or 3.0mm cordage diameterLC, 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

CeramicFerrule

Spring Fiber900umBuffer 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 length2. Cable prep

a) Strip outer jacketb) Remove Kevlarc) Strip bufferd) Strip coating (exposing glass)

3. Inject epoxy into connector and thread fiber through connector4. Crimp5. Oven cure epoxy6. Cleave excess fiber 7. Quick hand polish (remove excess fiber and epoxy from end face)8. Machine polish the end face9. Visual inspection of end face10.Loss testing11.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 SM Good MM Good SM

• Connector Insertion Loss Power injected into the fiber (P0) compared to power measured at far end or P1

Ins. Loss = P0 – P1

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 bufferedfibers

900um bufferedfibers

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)

Links to more Fiber Optic Information

• http://www.thefoa.org/tech/ref/basic/nets.html• http://

www.ciscopress.com/articles/article.asp?p=170740• http://www.howstuffworks.com/fiber-optic.htm• http://

catalog2.corning.com/corningcablesystems/catalog/DocumentLibrary.aspx

• http://technet.microsoft.com/en-us/library/bb726936.aspx