When testing loss within a fiber optic link, some basic principles needs to be taken into account on a regular basis.
1.The testing wavelength should invariably be similar to the working wavelength. Because optical fiber loss varies with light wavelength, you will definitely get incorrect result when your measuring wavelength is different from the exact working wavelength. As an example, in case a system is designed for 1550nm however you test it with 1310nm light source and Fiber Power Meter, the result will never be correct.
2.The testing source of light ought to be the identical to the intended working lightwave equipment source of light. In the event the system is designed for a LED source, you ought to test it by using a LED source. If the system is made for multimode laser light, you need to use a multimode laser source of light for testing. This is especially true for single mode laser light source.
Fiber optic equipment employed in a loss testing
Inside a basic loss testing setup, four forms of test equipment are essential. These are the source of light, the ability meter, the reference patch cables and the adapter (mating sleeve).
Here are some considerations when choosing your equipment.
Light source should have a similar wavelength because the operating equipment, proper mode (multimode or single mode, ought to be identical to the operating equipment), type (LED or laser, just like the operating equipment) and proper connector.
The Fiber Identifier should have similar wavelength since the source of light, proper connector and calibrated.
The reference patch cables needs to be high quality with know loss, proper connectors and stay a similar type as being the fiber plant being tested.
The adapter (mating sleeve) ought to be with high quality ceramic sleeves and become proper type (FC, SC, LC, etc).
Like in any power measurement, fiber optic light power measurement unit may be expressed in milliwatt (mW), but a more convenient unit is dB(decibel).
Decibel (dB) is frequently utilized in electronics testing. It will be the ratio between two levels. One level will be the input along with the other level may be the output. The ratio is calculated in logarithmic as explained below.
For power measurement, dB is described as: dB = 10 x log(output power/input power)
So for example, after a fiber link, the output light power level becomes 50% of their input, losing the link will likely be 10log(.5)= -3 dB.
Since dB is truly a ratio, it offers no absolute units. So from above measurement sample, we do not know of the actual power, may it be .1 mW or 1 mW.
That is why we have another unit dBm. This is basically the ratio of your measured capability to 1mW of reference power. It is described as: dBm = 10xlog(measured power/1mW)
So as an example, a .1mW light power expressed in dBm is going to be 10xlog(.1mW/1mW)=-10 dBm.
From above we all know that dBm is actually a absolute unit, we understand just how many mW it really is.
For Optical Light Source, decibel is considered the most often used unit because it is much better to work together with. Why? Because two dB values can be simply added or subtracted. As an example, an overall total fiber 56devhpky might have three sections, each one has loss in .5dB, 5dB and .5dB. The whole loss could then easily be concluded as .5dB 5dB .5dB = 6 dB. You can attempt to transform it to actual milliwatt and you will recognize that I am right!