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Is my SFP Working, or How to Measure an SFP/SFP+/QSFP

Jim Davis

Recently we have been getting questions about how to determine if an SFP is working. I’m going to use SFP generically here to represent a multitude of the various optical modules that are available. The Fluke Networks fiber testers can be used to measure the light that is being put out by and SFP.

To start, put your CertiFiber™ Pro into “Power Meter” mode. From the home screen, select the TOOLS menu and then the second option, if your CertiFiber module is attached, will be POWER METER. Selecting that, you will go directly to the power meter. It is on and running, you don’t even need to push TEST!

There are some minimal configurations that we need to apply. We are going to focus on the bottom half of the power meter. The top is used for transmitting, but we don’t need that to test the SFP. All that we are looking for is the absolute power that is coming out. I’m partial to reading this in dBm as that reflects the specifications of many SFP modules.

Read out in dBm reflects specifications of many SFP modules.

The one piece of configuration that is important is the λ (Lambda) or wavelength, down there in the bottom left corner. What are you testing? Is it Multimode – use 850, or Single-Mode – Use 1310. Pressing the Lambda key will give you a menu to choose appropriate wavelength:

What are you testing? Multimode – use 850, Single-Mode – Use 1310.

Regarding acceptable power levels, please consult your manufacturer. Be aware that if you are getting a positive number, you may need to put an attenuator on your system.

What about inspection? Inspection does not work well with the SFP modules. The inspection microscopes and the related standards, such as the IEC 61300-3-35, are designed for connectors and not the output port of an SFP. The input port of an SFP should have some type of non-contact large area input port. The fiber doesn’t actually make physical contact with the port, so it should remain fairly clean. Even if dust or other debris has collected on the port, it may be hard to visualize with a microscope.

What if I am using a QSFP to transmit 40 Gig through an MPO connector? How can I test this? Two simple options. One would be to use a fan out cable that goes from an MPO port to multiple legs of LC connectors. This can then be plugged into the CertiFiber Pro.

Another option would be to plug the MPO cable directly into a MultiFiber™ Pro. Have it set to “Power” mode, and you can see the power level for each individual fiber. Here is an example with just one channel receiving light:

Connect MPO cable into MultiFiber™ Pro; set to “Power” mode.

Remember, if you are working with 40 Gig, you are probably only using 4 fibers, so don’t be surprised if you only see a power level on the first or last 4 fibers in the power meter.