Fiber-optic testing Q-and-A

Webinar produced an abundance of questions for FOA president.

On November 29, Fiber Optic Association (FOA) president Jim Hayes presented a one-hour webcast seminar titled “Five Ways to TestFiber Optics.” The seminar included a brief question-and-answer session at its conclusion; because of time constraints, many questions submitted by attendees could not be answered verbally during the seminar.

Hayes crafted answers to the questions that were left unanswered during the live seminar. The questions-and-answers appear below. Those who did not attend the seminar and wish to put these questions into context can register for and immediately watch the seminar here. It will be available for on-demand viewing until May 2013.

Q: Does a connector adopter/converter at light source and power meter increases loss budget measurement result?

A: Adapters on power meters are designed to accurately locate the fiber end so all light is captured by the detector in the power meter so connectors can be attached/detached/reattached without varying the connection. Consider that connection "lossless." On the source side, most sources have a fixed connector that requires a hybrid launch cable with a connector that will mate with the source on one end and the connector style used on the cable plant being tested on the other end. That end mates with the adapter on the power meter to measure power for the "0 dB" reference. The end attached to the source should not be moved after the reference is set, since the coupling between the source and launch cable may vary with each mating cycle. In fact, to reduce the possibility of the launch cable connection to the source varying, be careful not to stress the cable here as it can change the "0 dB" reference and cause measurement errors.

Q: What are the main tests in fiber optics standardized in Cabling System?

A: Insertion loss testing is considered the primary test since it mimics the way the cable plant will be used. See http://www.thefoa.org/tech/ref/1pstandards/index.html

Q: How do you find the backscatter coefficient of a fiber?

A: Basically, that is what the OTDR is measuring. A good explanation is available from Corning at www.corning.com/WorkArea/downloadasset.aspx?id=7811

Q: I'd like to know what you mean by Stress in the Cable.

A: Stress in a cable is caused by either tension or bending. A fiber optic cable that is too tightly bent will put stress on the fiber and cause light loss at the stress point. Tension on a cable may cause stress on a long length of the fiber and cause higher attenuation in the cable.

Q: In Method 1, why is there no loss from B to C?

A: There is a loss at "B" where the receive cable connects to the cable plant under test so that adds to the measurement you want to make. The receive cable is short (usually 1-2m) so the fiber loss is negligible. The coupling to the meter (see question above) is considered to not have loss since the detector on the meter is supposed to capture all the light coming out of the fiber.

Q: When testing the cable plant, is it required that you test the equipment jumper cords that will be used at each end, or do you just test the permanent link cabling?

A: Generally, we test the "permanent link" - the installed cable plant, including the loss of the connectors on each end. We assume patch cords will be used to connect network electronics to the cable plant, so the loss of the connections on either end are already included and the connections to the transmitter and receiver were included when the loss budget was determined for that link.

Q: What’s the best way to test installed cable plant with terminations in place. I prefer the insertion tests using power meter but the OTDR for troubleshooting.

A: I agree.

Q: In Method #1 testing is the launch cable attached to the power source and the power meter attached at the cable plant?

A: Method 1 - a one cable reference - sets the "0 dB" level at the end of the launch cable using the meter. Then the launch cable is attached to one end of the cable plant, launching test power into the cable plant. The meter is attached to the far end of the cable plant with a receive cable which allows the measurement to include the loss of the connector on the far end. See http://www.thefoa.org/tech/ref/1pstandards/FOA-1.pdf

Q: Does deflection cause loss from power meter?

A: If you mean "deflecting" or stressing the connector at the power meter, yes that can cause stress loss and measurement errors. If you are asking about "reflection" from the power meter, the answer is the detector is reflective but generally that does not vary so it is calibrated out in use.

Q: How common is to use the measurement of 3 cable reference?

A: Fairly common these days. Some international standards call for its use because it is the only "universal" method that will work with any cable plant. And since most prefab cabling systems, terminated in a factory and delivered ready to install, use multifiber connectors like the MTP/MPO, those cable plants will require 3 cable reference testing. However, if those cable plants break out into single fiber connectors like SCs or LCs, the entire cable plant may be tested with a 1- or 2cable reference.

Q: How does the new encircled flux testing method differ?

A: Aha, a topic for a full day seminar! Encircled flux is the latest method intended to control modal distribution in multimode fiber to standardize measurements. ISO documents suggest EF is similar to the "mandrel wrap" method used for the last 20 years. I suggest you read the FOA explanation at http://www.thefoa.org/tech/ref/testing/test/EF.html

Q: These days fiber cable is not simply 62.5um or 50um - we now have 3 different grades of 50um fiber......ie. OM2, OM3, OM4. How important for cables to be matched for this?

A: The difference between OM2/3/4 fiber is the structure of the core used to increase the bandwidth. Since all these fibers are designed to have the same core diameter and NA, doing insertion loss tests with mixed types is not a problem. However, many of these laser-optimized fibers are not made as "bend-insensitive fibers and mixing those fibers with regular fibers can be a problem.

Q: What do you recommend as a test method if, for whatever reason, you have different connector types on either end of the cable under test.....ie. ST in the building Distributor, and MTRJ in the Floor Distributor.

A: That's what the 3 cable reference was designed for. See this page on testing with mixed connector types: http://www.thefoa.org/tech/ref/testing/test/conntypes.html

Q: I try to use the one cable method as much as possible however I have two questions. 1) Our transport fiber is sc/upc and our FTTH plant is sc/apc. I have been told the power meter will accept either however light source is only sc/upc compatible? 2) At times access to the back plane of the patch panel. Should I use the two point method for use of jumpers into the front side patch panel connector and add for a second connector?

A: You should be able to use a hybrid launch cable that is SC/UPC to mate with the source and SC/APC to mate with the cable plant. You can use a SC/APC to SC/APC as a receive cable. It should not introduce more errors in the measurement. It is convenient that APC connectors are color coded (green) so they do not get mis-identified!

Q: How do you determine that the link is meeting the requirements?

A: That comes from either having calculated a loss budget (http://www.thefoa.org/tech/loss-est.htm) or from the specifications for the network (http://www.thefoa.org/tech/Linkspec.htm)

Q: In a GPON network what should be the minimum budget/s to have a successful rollout

A: That depends on the version of GPON and may depend on the manufacturer. A good reference to start with is this ITU manual: http://www.itu.int/dms_pub/itu-t/opb/hdb/T-HDB-OUT.10-2009-1-PDF-E.pdf

Q: What are the tests that should be made in a GPON rollout?

A: Generally the cable plant is tested for insertion loss at all operational wavelengths through the PON coupler (1310 upstream, 1490/1550 downstream). If insertion loss is high, OTDR tests may be done but you must know how to interpret the difference in traces upstream and downstream. See http://www.thefoa.org/tech/ref/appln/FTTH-test.html

Q: Is it possible to have two different cable of different refractive index of SM and what is the implication if this cable spliced together, with their be any mismatch or high loss

A: Yes, you can have small variations in the index of refraction. The biggest difference caused by refractive indices changing is the distance measured on an OTDR as it calibrates distance by the speed of light in the fiber which is a function of index of refraction. But fibers with different indices of refraction may have other differences that also affect measurement or splices, like mode field diameter, backscatter coefficient, etc.

Q: Can you please repeat, when to use 1-cable, 2-cable and 3-cable

A: One cable is used when the connectors on the cable plant match the connectors on the test equipment. Two cables are used for times when the connectors are not compatible to the test equipment but may be mated to each other (e.g. LCs on the cable plant, SCs on the test equipment.) Three cables are used when the cable plant used plug/jack type connectors. See http://www.thefoa.org/tech/ref/1pstandards/index.html for more details.

Q: Are the connections to the equipment counted when doing a loss budget.

A: Not to the source or meter or in a link, the transmitter or receiver, as these are included when setting the standard for the loss of a link.

Q: My question is is there a coupler on the market to hybrid SM and MM to avoid high dB loss in that even?

A: No! Several people have tried to make tapered fibers to mate different fiber sizes but none successfully. You need electronics to do it.

Q: When you are using a receive cable with an OTDR, what length should it be?

A: The receive cable should be long enough to be fully resolved by the OTDR so you can place a marker after the connection between the cable under test and the receive cable.

Q: In the SONET world I am used to a link budget, and design a fiber to accommodate this Xmtr Power-Rcvr Sensitivity loss. When it comes to singlemode GigE circuits I am presented with SFP transceiver budgets in kilometers, not dB. Thus my SFP's are 10 KM, 40 KM and 80 Km, seemingly independent of cable and splice loss. What are the designers basing this on?

A: They usually base this on 1) typical transmitter and receiver specifications 2) assumptions on losses in the cable plant based on standards for fiber attenuation, connector and splice loss and 3)assumptions on installations that include some average number of connectors and splices. When they do this, there is always included a good margin for error.

Q: I found that the loss of connector is varying with different vendor's OTDR. How does that happen and how to solve the problem?

A: Different OTDRs may launch differently and process the data differently. The loss of a connector measured by an OTDR varies according to the test method (2 point or LSA) and placement of markers. For multimode, it varies with the modal distribution on launch.

I do not know how to solve that problem.

Here are two links for more information: http://www.thefoa.org/tech/ref/testing/OTDR/OTDR.html and http://www.thefoa.org/tech/ref/testing/OTDR/OTDR-FAQS.html

Q: Can I use a multimode patch cord on a singlemode media converter and vice-versa?

A: No - the difference in core diameter makes for a ~17-20 dB mismatch.

Q: How do I know if my reference cables are OK?

A: Reference cables should be tested using similar methods. See http://www.thefoa.org/tech/ref/testing/test/FOTP-171.html

Q: Are test methods different for multimode and singlemode fibers?

A: No, not really. The basic processes are the same, but the test conditions (source type and modal conditioning) are slightly different.

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