If you’re new to fiber or just brushing up before your next project bid, here are some common fiber optic basics.
What is Optical Fiber?
Fiber is made up of a core surrounded by a cladding layer. Both are glass but each has its own index of refraction.
In use today are two general types of optical fiber.
Singlemode (SM) fiber is designed for use with a signal of one wavelength of light, typically at invisible 1310, 1480, 1550 or 1625nm wavelengths. Most often with a core diameter of 250µ (micron), singlemode fiber is commonly used for long distance regional or inter-city transmissions of data.
Multimode (MM) fiber is based on the ability to combine different wavelength signals in the same fiber path, typically at invisible 850 or 1300nm wavelengths. Most often with a core diameter of 900µ (micron), multimode fiber is commonly used for short distance curb to house, or patch cable transmissions of data.
The light signal is applied to the end of the optical fiber and then bounces down the optical path.
Because fiber is glass, you cannot simply tie two optical fiber ends in a knot. There are two methods to properly “splice” two fiber ends together.
Mechanically - Two finely polished fiber ends are mated in a mechanical device with a small amount of index matching gel. The aligning of cores is very important (mismatches increase fiber loss).
Fusion - melting of the 2 fiber ends into one solid glass fiber ensuring core alignment and minimal loss.
1. Two cleaved and cleaned fibers are core aligned between two fusion electrodes.
2. The two fusion electrodes emit a precision arc of electricity to melt and fuse the two fiber ends together.
3. Within seconds the two fiber ends are fused together resulting in a continuous fiber strand.
An ideal core-aligned splice has 0.0 to 0.05 loss.
Used for the greater part of the last century, the “twisted pair” is a twisted thin gauge copper wire pair that only allows a single analog data connection. Today, twisted pairs are used in everything ranging from telephone wires to computer networking cables.
Twisted pairs rely on the use of hardware switching equipment to combine mass amounts of data to be carried over distances, and can be susceptible to interference and/or security concerns.
Revolutionizing the telecommunications industry, optical fiber strands transmit digital (binary) data at the speed of light. This throughput allows each individual fiber to transmit an incredible amount of data, for example tens-of-thousands of telephone calls. As an added bonus, optical fiber strands are very secure and immune to radio frequency interference.
However, unlike the twisted pair, to connect two separate fiber strands you cannot just simply twist them together. A mechanical or fusion splicer must be used to align the fiber cores in order to continue the transfer of data.
First, a Basic Overview:
Fiber optic lines are made up of a core surrounded by a cladding layer. Both are glass but each has its own index of refraction. The light signal is applied to the end of the optical fiber and then bounces down the optical path.
Singlemode fiber is designed for use with a signal of one wavelength of light, typically at an invisible 1310 nm, 1480 nm, 1550 nm or 1625 nm wavelengths.
Multimode fiber is based on the ability to combine different wavelength signals in the same fiber path, typically at an invisible 850 nm or 1300 nm wavelength.
Common signal connection between transmission systems use ST or SC for multimode (generally jacketed in orange protective cabling), ST, SC, FC and LC for singlemode (generally jacketed in yellow protective cabling). Angled connectors are also prevalent in cable video applications: ASC or AFC (generally color coded green for quick identification).
Typical multimode connection losses are 0.2 to 0.5 dB, singlemode connection losses typically 0.5 to 1.0 dB - this is why even today so many inside applications show a preference for multimode connections requesting pigtailing.
There are two ways to join fiber optic cable (working with glass fiber, of course, you can't tie it in a square knot) :
Mechanically - Two finely polished fiber ends are mated in a mechanical device with a small amount of index matching gel.
Arc fusion - Simply cleaving and melting the two fiber ends into one solid glass fiber to ensure minimal loss.
Typical mechanical connection losses are 0.3 dB and fusion are 0.03 dB. These losses, plus the typical loss of the fiber type you are using should fall within the loss budget.
Buffer: Protective coating applied directly on the fiber.
Cladding: The lower refractive index optical coating over the core of the fiber that "traps" light into the core.
Core: Center of an optical fiber which light is transmitted.
dB: A unit of measurement of optical power which indicates relative power.
Index of Refraction: A measure of/allowance for the speed of light in a material at nm wavelengths.
Jacket: The protective outer coating of a cable that contains fiber optic lines.
Jumper Cable: A short fiber cable with connectors on both ends to interconnecting other cables or test devices.
Launch Cable: A reference fiber optic jumper cable of a calibrated length and loss for accurate loss testing.
Loss Budget: Tolerable/acceptable amount of total power lost as light is transmitted through fiber, splices & couplings.
Loss, Connection: The total power lost within a physical connection, affected by cleanliness and alignment.
Loss, Estimated: An onscreen estimate of a completed splice's loss within a fused fiber.
Loss, Insertion: The loss caused by the insertion of a component such as a splice or connector in an optical fiber.
Loss, Microbend: Loss in fiber caused by bent or looped fiber.
Loss, Optical: Actual measured amount of total power lost as light is transmitted through fiber, splices & couplings.
Loss, Typical: Accepted budget loss(es) of cable attenuation inherent to fiber per km by wavelength.
Margin: The calculation of any additional amount of loss that can be tolerated in a tested link.
Multimode: A fiber with a core diameter larger than the wavelength of transmitted light allowing many modes of light to propagate. Used with LED sources for shorter distance links. Typical Loss: 850nm 3.5dB/km, 1300nm 1.5dB/km
Pigtail: A connectorized short length of fiber attached to a fiber for termination.
Singlemode: A fiber with a small core that only allows one mode of light to propagate. Commonly used with laser sources for high speed, long distance links. Typical Loss: 1310nm 0.35dB/km, 1550nm 0.22dB/km
Splice, Arc Fusion: An instrument that splices fibers by fusing or welding them, typically by electrical arc.
Splice, Mechanical: A physical connection between two fibers made with an index matching fluid or adhesive.
Termination: Preparing the end of a fiber to connect to another fiber or an active device, also called connectorization.
Visual fault locator: A visible light source that allows visual tracing and finding jacketed fiber breaks and bends.
Wavelength: "Long wavelength" generally calls for 1310/1550nm singlemode, "Short wavelength" 850/1300nm multimode
OTDR’s: OTDRs come in three basic versions. Full size OTDR’s, the highest performance with a full complement of features like data storage and printers. MiniOTDR’s provide the same type of measurements as a full OTDR, but with fewer features to trim the size and cost. Fault finders use the OTDR technique, but are greatly simplified to just provide the distance to a fault, making the instrument even more affordable and easier to use.
BRT’s or ORL's: Measure the ratio between the optical power into a component or system to its reflected optical power (back reflection), in units of dB. ORL's measure actual insertion loss, so a low number is good. BRT's display return loss so the higher the number the better.
Polishers: Precision cleaning for low loss fiber ends.
Talk Sets: Verify, with a crystal clear connection, communication with servicers at the other end of the fiber.
Light Sources: Chosen for compatibility with the type of fiber in use (singlemode or multimode with the proper core diameter) and the wavelength desired for performing the test. A signal source for an optical loss measurement.
Power Meters: Calibrated to read in linear units (milliwatts, microwatts and nanowatts) and/or dB referenced to one milliwatt or one microwatt optical power. The best meters offer a relative dB scale for laboratory loss measurements.
Attenuators: Precision adjustment of the level of signal in fiber.
Visual Fault Locators: Cable breaks, bending losses caused by kinks in the fiber , bad splices etc. can be quickly detected visually with a visible light source.
Fiberscope: Hand held microscope with a universal adapter to inspect connectors more closely.