Fiber Optics Interview Questions & Answers

  1. Question 1. Do Signals Really Travel Faster In Fiber Optics?

    Answer :

    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. Fibers “speed” is not referring to the speed of the signal in the fiber, but the bandwidth potential of the fiber.

  2. Question 2. Can You Give Me A Definition Of Structured Cabling?

    Answer :

    “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.

  3. Networking Interview Questions

  4. Question 3. What Are Some Of The Uses Of Fiber Optic Cabling In The Business World?

    Answer :

    The biggest use is telephony, followed by CATV, then LAN backbones, connecting hubs. 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. 

  5. Question 4. Will “intelligent Buildings” Use Fiber Optics Or Copper Wiring To Carry Voice/data/video Throughout The Structure?

    Answer :

    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.

  6. Networking Tutorial

  7. Question 5. 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?).

    Answer :

    All FO 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.

  8. Telecommunication Project Management Interview Questions

  9. Question 6. How Do You Classify Fiber Optic Cable?

    Answer :

    By NEC – UL – flame retardancy

    By cable types (tight buffer/distribution/breakout/loose tube)

    By fiber types (multimode/singlemode/hybrid)

    Whether it has fiber and wire (composite cable)

  10. Question 7. What Type Of Fiber Is Required To Run At Gigabit Speed?

    Answer :

    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.

  11. Telecom Billing Tutorial
    Network Administrator Interview Questions

  12. Question 8. What Is Modal Bandwidth, And How Does It Effect What Distances Gigabit Ethernet Can Travel Over Fiber?

    Answer :

    Modal bandwidth is caused by the fact that light in multimode fiber travels in rays or “modes” that take different times to get to through the fiber, causing dispersion. The longer the fiber, the greater the effect. This is a major factor in the distance limitation of GBE and the incentive for fiber manufacturers to develop better multimode fiber. While the worst case distance for 62.5/125 FDDI-spec fiber using a 850 nm VCSEL source is only 220 m, laser-optimized 50/125 fiber capable of 1 km is now available. 

  13. Question 9. What Is The Difference In Connectorization In Tight Buffered And Loose Tube Type Of Cables?

    Answer :

    A tight buffer cable can be terminated directly. THe 900 micron coating on the fiber is rugged enough to allow the connection to be connected directly and if there is a 3 mm jacket, it is crimped to the connector for strength. 

    A loose tube cable has 250 micron buffer on the fiber in it and is too fragile to attach a connector directly. It has be be used with a breakout kit that sleeves the fiber in a protective tube before termination.

  14. Telephone Etiquette Interview Questions

  15. Question 10. Can I Splice 62.5/125 Fiber To 50/125 Fiber? If So What Type Of Nominal Loss Would I Be Looking At At For My Loss Budget?

    Answer :

    If you splice it, you will get directional losses. Transmitting from 50 to 62.5 fiber, you’ll get virtually no losses but from 62.5 to 50, you will get a minimum of 1.6-1.9 dB loss due to the size and NA mismatch. (50 micron fiber has a lower numerical aperture (NA) than 62.5)  for a table of interconnection losses with different size fibers. 

  16. Question 11. Does This Still Hold True With Six Fiber Or 12 Fiber Premise Style Cable?

    Answer :

    Absolutely. Remove the jacket, cut off the fibers and central strength member and tie a swivel onto the kevlar. You’ll need about 6 inches of kevlar, tie with a double knot on the swivel then tape the kevlar back along the cable, esp. covering the end of the jacket, to prevent the kevlar from pulling loose or the cable snagging while pulling.

  17. Ethernet Interview Questions

  18. Question 12. Is There Any Independent Testing Procedures On “rodent Proof” Optical Cables And What Sort Of Qualification/s Being Used To Justify?

    Answer :

    There is a test referenced in GR-20 issue 2. I have heard that there are facilities which still perform the test, but I am not familiar with any of them. I know of no-one who is trying to justify the test.

  19. Networking Interview Questions

  20. Question 13. What Measurements Are Manufacturers Using When They Claimed That Their Products Are Of “rodent Resistance”, “rodent Proof” And “rodent Protection”.

    Answer :

    The test referenced above did not use any of these terms, but just gave a damage rating. There is anecdotal information that some cable / duct types have less rodent damage in the field than other cable / duct types.

  21. Question 14. How Are These Three Terms Differentiate In Terms Of Classification With Respect To The Rodent Issue”?

    Answer :

    There is no organization, of which I am aware, which gives any formal definition to the terms. Rodent resistance” and “rodent protection” are currently used by some organizations, for cable designs which they feel have experienced less damage in the field. “Rodent proof” , to the best of my knowledge, has only been used by a company which makes ductwork .

  22. Question 15. Can You Give Me A Definition Of Fiber Optics?

    Answer :

    What we call “fiber optics” is communications by modulated light guided through a transparent optical fiber. As a relatively young technology, the nomenclature can be quite varied among users. In the UK, it’s fiber optics, sometimes its fiber optics or fiber optics (as one word). Within the business, we generally say “fiber” when we refer to the optical fiber itself, although some use it to mean a cable of optical fiber. Lennie Lightwave has a fiber optic glossary on the web.

  23. Telecom Billing Interview Questions

  24. Question 16. What Do I Need For Connecting Optic Fibre Cable To A Cat 5 Cable?

    Answer :

    You need a device called a “media converter” available from a number of companies for $100-200.

  25. Question 17. Will A Single Mode Connector Work On Multi-mode Cable?

    Answer :

    The answer is maybe you can use SM connectors on MM but NOT the reverse. SM connectors are made to tighter tolerances – as is SM fiber – so the ferrule hole may be too small for some MM fibers. MM connectors have bigger holes for the fiber and will have high loss (>1dB) with SM. Also MM connectors may not be PC (physical contact) polish – terrible for return loss. MM fiber may not fit the smaller hole in SM connectors.

  26. Question 18. If You Have A 50 Micron Fiber Backbone, Can You Use 62.5 Fiber Jumpers On Each End?

    Answer :

    NO! On the receiver end it is OK, but on the transmitter end, the larger core of 62.5 into smaller 50 micron fiber will have losses of 2-4 dB.

  27. Telecommunication Project Management Interview Questions

  28. Question 19. Why Do Some Power Meters Have Calibration At 1300 Nm While Others Are 1310 Nm?

    Answer :

    Convention. The “official” laser center wavelength is 1310 nm, but vary between 1290 and 1330 nm. LEDs are broad spectral output devices that have outputs over a broad range of wavelengths, roughly centered around 1300 nm. We prefer to just say 1300 nm, and so does NIST, who calibrate at this wavelength with a 1300 nm YAG laser.

  29. Question 20. Where Do I Find The Best Information On Fiber Optics For Lighting?

    Answer :

    We’re not into fiber optic lighting but we have a tutorial on lighting on our website.