Reflectance, The Hidden Danger That Increases Bit Error Rates in Your Fiber Networks Denver, May 22nd, 2010 Adrian Young Sr. Customer Support Engineer
Foreword Reflectance is measured using an Optical Time Domain Reflectometer (OTDR). The following presentation contains screen shots from Fluke Networks test equipment to aid the users in understanding the principles of OTDR testing. However, these principles apply to ALL OTDRs and it is hoped that this presentation will be of benefit to ALL OTDR users. OTDR testing is specified in ANSI/TIA-568-C.0 and IEC 14763-3. If you are not OTDR testing today, it is very likely that you will be in the near future and now is a good time to get ready and be prepared. The traces you are about to see have come from real installations.
What is reflectance? Technical explanation warning: When light moves from a medium of a given refractive index n1 into a second medium with refractive index n2, both reflection and refraction of the light may occur. As soon as there is an air gap between the end faces of the fiber then Fresnel reflections will occur. It is these Fresnel reflections that you see when looking at a window. These are caused by the refractive index difference between air and glass. If not too bad, you can still see through the glass.
Telco providers are concerned about reflectance Video does not tolerate reflections.
Is reflectance a requirement? ANSI/TIA-568-C.0 Not for link testing. ANSI/TIA-568-C.3 Connector specification only (20 years old). Multimode -20 db Singlemode-26 db ISO/IEC 11801:2002 Connector specification only (20 years old). Multimode -20 db Singlemode -26 db ISO/IEC 11801 Amd 2 Yes, but no limit given. IEEE 802.3ae The Max. discrete reflectance for 10GBASE-S shall be less than 20 db. The Max. discrete reflectance for 10GBASE-L and 10GBASE-E shall be less than 26 db.
Is reflectance a requirement? IEEE 802.3ba The Max. discrete reflectance for 40GBASE SR4 or 100GBASE SR10 shall be less than 20 db. The Max. discrete reflectance for 40GBASE-LR4 and 100GBASE-LR10 shall be less than 26 db.
Explain those IEEE numbers a little more 100GBASE-SR10 Channel loss permitted 1.9 db One connector at each end of 0.75 db 100 m of OM3 0.35 db (3.5 db/km @ 850 nm) 125 m of OM4 0.31 db (2.5 db/km @ 850 nm) The model is not expecting multiple connections, so reflectance has not been a real consideration.. Fluke Networks has seen issues at 10GBASE-S already caused by poor reflectance. Let s look at the practicality of reducing reflectance in fiber connectors.
Reflectance in connectors When the light travels down the fiber, if it sees a change in refractive index, their will be a reflection (reflectance) The most common causes are: Air gap between the connectors Dirt or residue left behind by the cleaning solution In a perfect world, their would be no air gap between the mated connectors:
Reflectance in connectors In reality, there is always a small air gap, also known an undercut : There is no ANSI/TIA or IEC standard for undercut. However, the very best factory terminated connectors will have an undercut better than 50 nm (that s 0.05 µm). The amount of undercut you see will depend on your polishing technique.
Putting 50 nm into perspective You cannot inspect Undercut using a FiberScope or similar device, these devices are two dimensional. You would need an interferometer to do this, very expensive and requires a skilled user, typically found in laboratories.
Interferometer Not found in the field typically. The shading indicates the amount of undercut on the end face.
Field polishing not the best way When field polishing, each vendor has their own recipe for doing this. It typically starts with an air polish using a 5 to 12 µm paper. Then a puck and 1 or 2 µm paper pad are used to polish the end face of the connector. Then a final polish of either 0.3 µm or 0.05 µm is used.
Field polishing not the best way Problems doing this include: Depends on operator skill, they must have a good day EVERY day. You can over polish the connector. (Big air gap = poor reflectance) You can get away with some sloppy practices and still pass Loss/Length testing but OTDR testing will show up the issues when Loss/Length testing will not. Installers try to save money and not replace the final polishing paper on a regular basis, that paper is only good for 5 polishes. Some will even skip the final paper because they know that the Loss/Length testing will probably pass with a connector loss allowance of 0.75 db.
Better way Use a pre-polished factory terminated connector, examples include: Thread-Lock Corning UniCam CommScope OptiCam There are many others They typically guarantee a minimum reflectance of -35 db for multimode -40 db for singlemode Often better than this But still requires user skill in cleaving the fiber and a Precision Cleaver is a MUST and is often where the problem lies.
Best way Factory terminated pigtails. The connector is polished in the factory on automated machines, has very low loss and excellent reflectance. It is then fusion spliced onto the installed fiber where the splice is typically 0.05 db and much better than the 0.3 db requirement in the standards. Less prone to installer error but the component and equipment cost is more initially.
An Optical Time Domain Reflectometer (OTDR)
The OTDR Port A dirty OTDR port will impact your OTDR measurement. It needs to be inspected and cleaned before you connect the launch fiber. This is true for all fiber connections. The OTDR may have a grading scale for the OTDR Port. It is based on how good the reflectance is.
Test Reference/Launch Cords
Test Reference/Launch Cords
Cleaning an inspecting You must use as a minimum 98% Isopropyl Alcohol (IPA). Anything less will leave a thin film behind, causing a change in reflectance. IPA has issues, it does not evaporate so quickly and you can end up with the halo effect: There is a preference amongst professionals to move to hybrid cleaners.
Hybrid cleaner These new cleaners: Evaporate much quicker Are more aggressive at cleaning Have antistatic properties According to the EPA, the typically dust particle in the office is between 2 and 10 µm. Having a cleaner with an antistatic properties reduces the chance of dust being attracted to the end face of the connector.
Example of a cleaning issue using 93% IPA Poor reflectance causes tailing. If it is really bad, as in this example, the OTDR will not be able to measure the loss (note the loss at 1300 nm is missing). The reflectance is below the desired -35 db (3 db is a factor of two).
Same link, this time using a hybrid cleaner The reflectance is above the desired -35 db (3 db is a factor of two). OTDR is able to make a measurement (-0.01 db), perfect.
Launch lead You must use a launch lead on the OTDR. 105 m for multimode 130 m singlemode (longer for links greater than 15 km) Setting launch fiber compensation ensures the reported length excludes your launch fiber. Launch compensation typically gives the user two options: Launch fiber only Launch + receive fiber (better opt) Using a receive fiber allows you to measure the loss of the connector at the far end and if measured in both directions, you can do bidirectional averaging as described in ISO/IEC 14763-3.
The measurement Good example. Spikes are small (good reflectance).
Example of a bad installation Poor reflectance causes tailing. If it is really bad, as in this example, the OTDR will not be able to measure the loss (note the loss at 1300 nm is missing). The reflectance is below the desired -35 db (3 db is a factor of two).
Example of a good installation This is an exceptional example (not typical). It is done to show how reflectance has an impact on your OTDR measurement. Do not expect to see this good for multimode (better than -35 db is the goal).
Unreliable 10GBASE-LR link (Fiber 01) The height of the event is relative to the amount of reflectance.
Unreliable 10GBASE-LR link (Fiber 01) Excellent reflectance.
Unreliable 10GBASE-LR link (Fiber 02) Reflectance looks to be poor here. Event is wider. Poor reflectance forces the use of a wider pulse to test with.
Unreliable 10GBASE-LR link (Fiber 02) Poor reflectance.
How many connections are there in this link? Looks like there are 6 connections. 1 2 3 4 5 6 If there are cross connects with patch cords, it may look like one connection.
How many connections are there in this link? Let s zoom in on event number 4. 1 2 3 4 5 6 The bump suggests two connections here.
How many connections are there in this link? Changing the pulse width to 20 ns reveals the real number of connections. 1 23 4 6 5 So there is a cross connect here two connections. But, this can give an inaccurate reflectance reading, which is why you would not use 20 ns on a link this length.
How many connections are there in this link? There are actually 13 fiber connections in this 10GBASE-LR link. Reflectance 35 db Not everyone knows how many connections are in the link.
What can I do to prevent reflectance issues? Look at your field polishing practices. If they are not consistent, move to a factory polished connector. See what cleaning and inspecting equipment you have. Consider moving away from IPA and to a hybrid cleaner. Use a launch and receive fiber with excellent end face geometry (under cut of 50 nm or less), do not try and make your own. Retrain the installation crew, but this time do it with an OTDR as well as a loss/length tester so they can see for themselves what it takes to get good reflectance. We know from experience this works better than a PowerPoint presentation. Consider creating a custom test limit with reflectance set at -35 db, so if there is an issue, the OTDR will show FAIL at the time of the test allowing you to fix as you go.
Example of a custom test limit for reflectance This customer was having problems with extreme tailing. Could not get a sensible measurement with the OTDR. After re-training his staff on how to polish fiber correctly, including replacing the polishing paper after five uses:
It fails only at 1550 nm? Two possible causes: Bend in the fiber Cracked fiber Here is an example of a cracked fiber: 1550 nm is worse One reason to test at both wavelengths Never accept single wavelength testing This can also happen when connectors are removed to be inspected.
Summary Review your cleaning practices: You need a visual inspection device, so many technicians who call in have no inspection devices at all. Continue to do your Tier 1 Optical Loss testing: Design the link with 10GBASE-S, 40GBASE-SR4 and 100GBASE-SR10 in mind. Supplement it with Tier 2 OTDR testing to measure reflectance: Define a minimum requirement of at least -35 db. Understand that -35 db may be difficult to achieve with field polishing. Retrain your technicians on field polishing: If they cannot get -35 db or better consistently, consider moving to factory polish connectors.
Thank you