Introduction to Computers Fiber Optic Cables

Cable Construction Core usually made of high quality ultra-pure glass or clear plastic Surrounding the core is a reflective cladding which will keep the light inside the core and keep it traveling through the cable Surrounding the core and cladding are layers of strengthening materials and protective materials This keeps the core from breaking or or getting dirty

Types of Fiber Optic Cables Single Mode Allows one ray of light (Called a mode) Very thin core. 8 - 10.5 microns (millionth of a meter) Can transmit large amounts of data over long distances. It has been tested to transfer 10 gigabits per second over 80 kilometers Multimode Uses multiple rays of light Each mode is slightly different frequencies, allowing the transmission of multiple signals at the same time Much thicker; around 50 - 100 microns Cladding is designed to reflect each mode at a different angle to keep frequencies separate Can’t be nearly as long. Around 2 km for 100 Mbps, 1 km for 1000 Mbps, and 500 m for 10 Gbps

Fiber Optic Cable Advantages and Disadvantages Immune to electromagnetic interference Strongly resists eavesdropping since there is no outside electromagnetic signal. Only way to capture data is to patch into the cable Extremely high bandwidth Allows for extremely long cables Less flexible and harder to install Much more expensive, about 10 times as much as UTP Quite fragile and inflexible Attaching connectors is not as easy and requires more specialized equipment

Fiber Optic Connectors There are many different connectors we can use for fiber optic cables. Although some are only used for multimode or single mode cables, the ones we will look at can all be used for either ST connector - Uses a ceramic ferrule to ensure proper core alignment and prevent light ray deflection Have a keyed bayonet-type connector, sometimes called push and twist As part of the assembly process, it is necessary to polish the exposed fiber tip to make sure light is passed from one cable to the next without dispersion SC connector - Characterized by its square connection Also uses a ceramic ferrule and the core needs to be polished However, it uses a spring loaded ceramic ferrule instead of a keyed bayonet

More Fiber Optic Connectors LC Connector Uses a locking tab similar to an RJ-45 connector About half the size of other fiber optic connectors, making it popular for uses in cramped spaces Like the other connections, uses a ceramic ferrule and needs to have its exposed core polished during manufacturing Separates the transmitting and receiving signal into two wires on one connector MT-RJ Connector Uses a locking tab as well as metal guide pins to ensure proper allignment

Fiber Optic Issues Fiber optic cable is much less forgiving than copper cable. For the signal to pass through the fiber on one side of the connector or splice must line up exactly with the other side Even if the connector is successfully locked into the jack, if it is the wrong connector or misaligned you could lose up to 90% of your bandwidth Dirty connectors can also severely hamper a cable’s effectiveness, so cleaning them should be a priority For connectors where the ferrule is accessible a lint-free cloth soaked in denatured alcohol works well. If the ferrule is recessed you will need a specialized cleaning tool. You will also need a specialized cleaning tool for the fiber optic jack

Insertion loss and ORL Whenever a connector is installed on the end of a fiber optic cable there will be some signal loss. This is called insertion loss Also, problematically some light that is loss will be reflected back down the cable to the source, which can damage the transmitter or corrupt the data being sent. This is called back-reflection or optical return loss (ORL) Manufacturers must polish the connector to ensure minimum loss

Polishing grades The better the polish, the more light will pass through without reflection Physical Contact (PC) polishing is usually used for single mode fiber. It’s end is curved so on the cores of the fiber touch Super Physical Contact (SPC) and Ultra Physical Contact (UPC) use a higher grade of polishing and have a higher curvature than PC Angled Physical Contact (APC) has an 8 degree angle cut into the ferrule. This provides great protection from ORL if used with another APC connector

Fiber Optic Cabling Issues The fiber optic core is very fragile and can easily be damaged. Even bending a fiber optic cable too far can cause it to break Wavelength mismatch will cause almost total signal loss. Wavelength mismatch would occur if connecting single mode fiber to multimode fiber or connecting cables of the same type but different core diameters

Attenuation and Loss Budgeting Fiber optic cable suffers attenuation in a very predictable and calculable way. Cable length, connectors and splices all contribute to the signal loss If you add the average signal loss of all the factors together you get your loss budget You then calculate your link loss margin by subtracting the calculated attenuation from the total power of the transmission source. Your link loss margin should be at least 3 decibels This ensures that the cable will continue to function as its components age

Fiber Optic Loss Budget Calculation Connectors or splices 0.3 dB loss each Multimode cabling 1-3 dB per kilometer, depending on thickness and quality Single mode cabling 0.4-0.5 dB per kilometer, depending on thickness and quality

Video About the Science Behind Fiber Optic and the Transatlantic Cable https://www.youtube.com/watch?v=0MwMkBET_5I