Some Basic Concepts of Fiber Optic Loss Testing

Tag : Fiber Optic Adapter
When testing loss in a fiber optic link, some basic principles mustbe kept in mind all the time.

1.The testing wavelength should always be the same as the workingwavelength. Because optical fiber loss varies with lightwavelength, you will get incorrect result if your measuringwavelength is different from the actual working wavelength. Forexample, if a system is designed for 1550nm but you test it with1310nm light source and power meter, the result will not becorrect.
2.The testing light source should be the same as the intendedworking lightwave equipment light source. If the system is designedfor a LED source, you should test it with a LED source. If thesystem is designed for multimode laser light, you should use amultimode laser light source for testing. This is also true forsingle mode laser light source.

Fiber optic equipment used in a loss testing

In a basic loss testing setup, four types of test equipment areneeded. They are the light source, the power meter, the referencepatch cables and the adapter (mating sleeve).

Here are some considerations when choosing your equipment.

The light source should have the same wavelength as the operatingequipment, proper mode (multimode or single mode, should be same asthe operating equipment), type (LED or laser, same as the operatingequipment) and proper connector.

The power meter should have the same wavelength as the lightsource, proper connector and calibrated.

The reference patch cables should be high quality with know loss,proper connectors and be the same type as the fiber plant beingtested.

The adapter (mating sleeve) should be with high quality ceramicsleeves and be proper type (FC, SC, LC, etc).

Understanding dB (decibel) in fiber optic loss testing

As in any power measurement, fiber optic light power measurementunit can be expressed in milliwatt (mW), but a more convenient unitis dB(decibel).

Decibel (dB) is most often used in electronics testing. It is theratio between two levels. One level is the input and the otherlevel is the output. The ratio is calculated in logarithmic asexplained below.

For power measurement, dB is defined as: dB = 10 x log(outputpower/input power)

So for example, after a fiber link, the output light power levelbecomes 50% of its input, the loss of the link will be 10log(0.5)=-3 dB.

Since dB is actually a ratio, it has no absolute units. So fromabove measurement sample, we have no idea of the actual power, mayit be 0.1 mW or 1 mW.

That is why we have another unit dBm. It is the ratio of themeasured power to 1mW of reference power. It is defined as: dBm =10xlog(measured power/1mW)

So for example, a 0.1mW light power expressed in dBm will be10xlog(0.1mW/1mW)=-10 dBm.

From above we know that dBm is a absolute unit, we know exactly howmany mW it is.

For fiber optic loss testing, decibel is the most often used unitsince it is much easier to work with. Why? Because two dB valuescan be simply added or subtracted. For example, a total fiber linkmay have three sections, each has loss of 0.5dB, 5dB and 0.5dB. Thetotal loss can then be easily concluded as 0.5dB + 5dB + 0.5dB = 6dB. You can try to convert it to actual milliwatt and you will seethat I am right!

Colin Yao is an expert on fiber optic communication technologiesand products. Learn more about innerduct coupler , innerduct for fiber , plenum inner duct on Fiber Optics For Sale Co. web site.