Fiber Optic Frequently Asked Questions
What is Fiber Optic?
Fiber optic (or "optical fiber") refers to the medium and the technology associated with the transmission of information as light impulses along a glass or plastic wire or fiber. Fiber optic wire carries much more information than conventional copper wire and is far less subject to electromagnetic interference. Most telephone company long-distance lines are now fiber optic.
Transmission on fiber optic wire requires repeating at distance intervals. The glass fiber requires more protection within an outer cable than copper. For these reasons and because the installation of any new wiring is labor-intensive, few communities yet have fiber optic wires or cables from the phone company's branch office to local customers (known as local loop).
Do signals really travel faster in fiber optics? You know that "sending communications at the speed of light" means the speed of light in glass (about 2/3 C), but you might be surprised to know that signals in UTP (unshielded twisted pair) cables like Cat 5e travel at about the same speed (2/3 C). Coax, meanwhile, has a faster NVP (nominal velocity of propagation), about 0.9C, due to it's design. Fiber's "speed" is not referring to the speed of the signal in the fiber, but the bandwidth potential of the fiber.
Can you give me a definition of structured cabling? "Structured Cabling" refers to a standardized cabling architecture, specified by EIA/TIA 568 in the US and ISO 11801 internationally. It uses twisted pair and fiber optic cables to create a standardized cabling system designed for telephones and LANs built by many manufacturers. The nomenclature here is even less precise. Vendors also refer to this as "structured cabling", data-voice cabling, low-voltage cabling and limited-energy cabling.
What is a better way of communication,wire or fiber optics? The telcos and CATV companies use fiber optics because of economics. It's greater bandwidth and lower attenuation allow longer distances and more channels (voice or video) per fiber pair. Typical fiber specs are more than 100 times farther and 1000+ times faster!
What are some of the uses of fiber optic cabling in the business world?
The biggest use is telephony, followed by CATV, then LAN backbones, connecting to switched equipment. Next is connecting remote video cameras for security systems. The building management and security systems are switching to fiber in many buildings due to distance and EMI requirements. Fiber is not often used to the desk because it is perceived to be too expensive, but it allows a system without wiring closets, making the cost less in most instances. Gigabit Ethernet will drive even more fiber into networks, since UTP applications will be too difficult to install.
Will "intelligent buildings" use fiber optics or copper wiring to carry voice/data/video throughout the structure?
Both. Fiber will be used when the distances are longer than 90 meters or data rates are higher (e.g. Gigabit Ethernet). Most backbones will be fiber. Desktop connections to telecom closets will be copper for the near future, until network managers find out what a telecom closet really costs! Phones will continue to use copper until we all go to voice over IP. Video (CCTV) uses fiber for distances over about 150-250 meters.
Can you please tell me what the difference between, dB and dBm when you are trying to test fiber optic cable?
Fiber optic power measurements are generally made in a log scale of "decibels" or "dB" (actually named after Alexander Graham Bell) that has a scale of 10 dB for every factor of 10 in power. The equation is actually: dB=10 log (power 1/power 2) dB is therefore a ratio measurement - 10 times more power is +10 dB and 100 times less is -20 dB, etc. For ABSOLUTE measurements, you must have a reference point. If we use 1 milliwatt of power as our reference, our equation becomes dB= 10 log (power/1 mW) So now 1 mW is 0 dB, 10 mW is 10 dB, 0.1 mW is -10 dB, etc. Here's more information on testing including dB too.
I need to be able to measure the "true" or "useful" power of a VCSEL Laser Diode. To do this, I believe I need to measure the Peak-to-Peak power (the extinction ratio?).
All Fiber Optic power meters measure average power. This is simply peak power diluted by duty cycle. If you know the duty cycle of the signal and the average power, you can calculate peak power as (Avg pwr/duty cycle). If you are measuring a signal with a clock of 50% duty cycle (1-0-1-0, etc), the meter will read half the peak power. Most high speed networks are sending random data, so the duty cycle can probably be assumed to be 50%. Many systems have a test mode that transmits 50% duty cycle just for optical testing. I am confused by the resolution choices in fiber optic power meters. If accuracy is +/-0.2dB, then it doesn't seem worthwhile to have resolution out to the hundredths & thousandths of a dB, when accuracy is only in the tenths (one decimal place). Resolution for power meters is an interesting subject. If you measure power, and the measurement is accurate as (or as NIST prefers - has a measurement uncertainty of ) 0.2 dB, a measurement of 0.00 dB +/- 0.2 dB is confusing. The 1/100 th dB resolution is in fact meaningless. If the uncertainty was 0.02 dB, a hundredth resolution would make sense. Now remember we are talking "absolute power" measurements, calibrated relative to NIST standards.
If we are looking at loss measurements, things changed considerably. The loss of a LC connector, about 0.1 dB, is measured relatively, eg. -15.00 dBm to -15.10, and the measurement uncertainty now has nothing to do with the absolute power levels, but the RELATIVE difference between the two readings. That difference is as precise as the linearity of the power meter ( better than 0.01 dB) and the uncertainty of the mating of the connectors ( a few hundredths too?). So you certainly want a meter with 0.01 dB resolution to test connectors!
If you are testing an installed cable plant with say 3-10 dB loss, the uncertainty is probably 0.5 dB, so 0.1 dB is adequate. If it's a long haul network with 30 dB loss, the uncertainty can be over 1 dB, so 0.1 dB is much more than adequate.
How do you classify fiber optic cable?
Broad question: By NEC - UL - flame retardancy
By cable types (tight buffer/distribution/breakout/loose tube)
By fiber types (multimode/single mode/hybrid)
What type of fiber is required to run at gigabit speed?
Depends on how far you want to go. Plain old FDDI fiber (160 MHz-km bandwidth @ 850 nm and 500 MHz-km @ 1300 nm ) will go ~240 m with a 850 VCSEL or 500 m with a 1300 laser. Practically every fiber manufacturer has 50/125 laser-optimized premium fiber (OM2/OM3/OM4) that will go a lot further -as far as 2 km - and while it's more expensive, we recommend it for any backbone applications.