The fiber optic power meter and light source are used together to measure loss in a fiber or fiber optic device. The source launches the light into one end of the fiber, while the power meter is connected to the other end to measure the received optical power. The source can be an optical laser or light emitting diode (LED) designed as part of a test set, or alternately the lightwave communication equipment light source can be used.

 

Because optical fiber loss varies with light wavelength, power meter tests should be performed using the same wavelength as the one used by the lightwave communication equipment. If lightwave equipment operates at the 1310-nm wavelength, the power meter and light source should also be set to 1310-nm testing. If the lightwave equipment operates at 1550 nm, the power meter and source should also beset to 1550 nm. Special consideration should be made when testing through a WDM network.

 

THE DECIBEL (dB)

 

The power meter test is used to determine light power loss in a fiber optic link. The measured unit of light power is the milliwatt (mW). However, a more convenient form of measurement used is called the decibel (dB).

 

The decibel is a common measurement used in the field of electronics to determine loss or gain in a system. It is the ratio, in logarithmic form, of power, voltage, or current levels between two points. One point is located at the beginning, or input, of the system to be measured, and the other point is located at the end, or output, of the system. The power formula for decibel gain is expressed as:

 

G_{(dB) = 10 X log (output power/input power)}

 

When the output power is less than the input power, the value of this equation will always be negative. In most fiber optic applications, light power output from an optical fiber will always be less than the input light power into the optical fiber. Therefore, this value will always be negative. This negative gain can be referred to as a light loss, L_(dB):

 

 

where L(db) = 10 X log (input power/output power).

 

Light loss, L(dB), is a commonly used specification for fiber optic attenuation. For example, to determine the light loss of an optical fiber in a cable, a light source is connected to one end of the fiber cable (input). The light output power of the source is known to be 0.1 mW. When an optical power meter is connected to the opposite end of the fiber optic cable under test (output), the meter measures 0.05 mW. Using the decibel power loss formula, the optical fiber loss can be calculated as follows:

 

 

 

The light power loss of this optical fiber is 3 dB

 

The dB unit is a logarithmic ratio of input and output levels and is therefore not absolute (i.e., has no units). An absolute measure of power in decibels can be made in the dBm form. The dBm unit is a logarithmic ratio of the measured power to 1 mW ^{of reference power.}

 

The formula is as follows:

 

 

The same result in loss can be achieved using the dBm. In the previous example, light power input by the source to the optical fiber is 0.1 mW, which is -10 dBm:

 

 

The light power received by the meter from the optical fiber’s output is 0.05 mW, which is —13 dBm:

 

 

The light power loss in the fiber is equal to the light source power minus the received meter light power

 

 

_{Therefore, the light power lost by the optical fiber is 3 dB.}

 

_{All measurements must be in either decibels or in milliwatts, but not both. Usually, all measurements are made in the decibel scale because it is easier to work with, ft is not necessary to convert between mW and dBm because most equipment specification data also use the decibel scale. The following table shows dBm equivalents for optical power in milliwatts:}

 

Power in dBm	Power in mW
+20	100
+ 10	/0
+3	2
0	1
-3	0.5
-10	0.1
-20	0.01
-30	0.001
-40	0.0001
-50	0.00001

 

_{It is helpful to remember that a loss of 3 dB is equivalent to a 50% loss in power. A loss of 10 dB is equivalent to a power loss of 90%. When you add or subtract a dB to or from a dBm, the result is in dBm. When you add or subtract two dB values, the result is always in dB. Decibel values are never multiplied together—they are always added or subtracted.}

 

_{When measuring the loss in dB of a number of different sections of a fiber optic link, the total loss of all sections is equal to the sum in dB of each individual section.}

 

_{Example: A fiber optic link with a 1-km cable has a loss of 3.4 dB. Patch panel connection loss at each end is 0.8 dB. Pigtail loss is negligible. If a light source with optical power of — 10 dBm is connected to one end of the fiber link, what will the received light power be at the other end?}

 

_{First, the total link loss including patch panel connections is summed:}

 

The optical power loss formula needs to be rearranged to equal the received optical power:

 

_{Therefore, the light power that would be measured by an optical power meter at the end is —15 dBm. It should be noted that two fiber optic connectors contribute to one connection loss}

Related links: Optical power loss (attenuation) in fiber access – types, values and sources