Source
r/networking•
SysAtMN

Visual Fault Locator + Single Mode Fiber issues

Hello,

I'm new to fiber and recently started to test my fibers with a Fluke VisiFault Visual Fault Locator. To my surprise nearly every strand of single mode has huge faults near the connectors. Even the brand new stuff that we had on the shelf for a few years fails this test. I don't have this problem on most of my Multimode patch cables.

Is it possible I am using too strong of a VFL for this kind of fiber?

Specs on the device say:
Output < 1.3 mW
Wavelength 630-670 nm
Class II laser product

Posted Oct 16, 2020, 6:29 PM UTC

212
netiron•

This isn't atypical. Remember, the VFL is just for visually spotting an issue that you are looking for, not really for verifying that your fiber has no issues.

If you're just testing the jumpers themselves, you should really only need to scope the ends to make sure they're clean. For your fiber runs that have connectors in the middle (patch panels, etc.) you really should be using a certifier or OTDR to verify them, or at the very least looking at the receive level on the equipment on both sides.

6
SysAtMNOP•

We do now have scopes from Viavi to perform clean testing up to the IEC 61300-3-35 standard. This has been a very helpful tool, although we are quickly learning it is impossible to keep the fiber clean for more than a few moments at a time.

Thanks for the tip on VFL. I figured this was a good test for us to perform some some manufacturers provide this on the little fiber certificates on new jumpers. I am going through my piles of old fibers and trying to see how they compare. It doesnt take long to VFL but I fear I may have overvalued this feedback.

We do have Fluke DTX data testers with fiber modules for Loss Testing. However, the reference cables are now failing the IEC clean standard and some of them are falling apart from wear and tear. Replacing the Test Reference Cables is surprisingly expensive. I was hoping we could gain enough confidence between clean testing and VFL testing to postpone ordering new TRCs.

1
D3adlyR3dNetwork Manglineer•

When I'd check fiber (all singlemode) with a VFL it was common (for me at least) to see a lot of light bleed near the boot/connector, I just attributed this to reflection/things get weird in the construction towards the connector.

If you're really worried you can either OTDR your jumper (good luck finding one that's got a fine enough resolution for that) or you can get a light generator that outputs a known level, and a light meter on the other end.

Also, if these jumpers work and are within spec they aren't "failed". If there was a true break in the fiber you wouldn't be getting any light out of the connector end.

I figured this was a good test for us to perform some some manufacturers provide this on the little fiber certificates on new jumpers.

Those certificates have the measurements for return loss and insertion loss, not something a VFL will do.

4
SysAtMNOP•

thanks for confirming your singlemode jumpers do the same thing. i think i will have to adjust my expectations when working with this kind of media. im only going to be concerned about leaks away from the vfl source. ill have to rely more on loss testing for an accurate measurement of performance.

2
nswizdum•

The number of people in here suggesting that OP should get a Visual Fault Locator that uses an Invisible wavelength of light, is concerning.

3
dhawaii808•

Single mode fiber generally isn’t used at this short wavelength, try testing the single mode in 1530-1625nm I would expect it to pass.

1
SysAtMNOP•

Visible light is between 380 and 700 nm. We wouldn't be able to see a laser at 1530 or higher.

2
SysAtMNOP•

I rewatched this Youtube again and compared my results:
https://youtu.be/H5jGPOqQBJo

At 0:52 the video identifies a "leak" as light somewhere outside of the MM SC connector.

At 1:04 the video shows the same light leaking through a single mode cable but says "no breaks appear, meaning the LC fiber cable is undamaged".

I think this is where I am getting confused. I was under the impression that light should only spill out of the far end connector. Perhaps due to its smaller size that light is expected at the source side of the VFL test. That would explain why nearly every SM fiber I have tested so far lights up like a tree on the VCL source side.

1
packet_whisperer•

Wavelength 630-670 nm

Single mode uses longer wavelengths. Standard 10Km singlemode uses 1310nm. If you are using WDM that can vary by a couple hundred nm in either direction. That might be your problem.

-1
SysAtMNOP•

WDM

I am not able to find any specs to suggest our VFL uses WDM:
https://www.flukenetworks.com/datacom-cabling/fiber-testing/VisiFault-Visual-Fault-Locator

​

Do your Single Mode patch cables light up on the source side when you test with your VFL tool? Its starting to look like this is expected behavior.

1
packet_whisperer•

I didn't mean the VFL. I was referring generally to fiber wavelengths. WDM sends multiple wavelengths down a single strand, so you can multiplex multiple connections, similar to channels on TDM (T1s). But those wavelengths are all typically in the 1300 to 1600nm range.

The takeaway is you are testing singlemode fiber with multimode wavelengths. That's not an expected or supported operational configuration, so the test is not valid.

0
StuntedGorilla•

That’s a disturbing lack of understanding of fibre optics coming from a mod. He’s using a visual fault locator which outputs 630-670nm of light which is red in the visual light spectrum. Single mode or multi mode makes no difference, they will pass light just as well as any tube. Whether that light can be modulated to send 1s and 0s fast and accurately is another matter entirely but irrelevant to being able to send a little red beam.

1