Measurement in optical fiber refers to the optical power.

Optical power

Practically, each measurement in optical fiber refers to the optical power. The output of a transmitter or the input to a receiver are "absolute" optical power measurements, that is, the actual value of the power is measured. Loss is a measurement of "relative" power, the difference between the power coupled to a component such as a cable, splice or a connector and the power transmitted through it. This difference in the level of power before and after the component is what we call optical loss and defines the performance of a cable, connector, splice or another component.

Whenever the tests are performed on fiber-optic networks, the results are displayed on the instrument reading screen. Power measurements are expressed in "dB", which is the unit of measure of power and loss in fiber optic measurements. The optical loss is measured in "dB", while the optical power is measured in "dBm". The loss is a negative number (for example, -3.2 dB), as are many power measurements. Measurements in dB can sometimes be confusing.

In the early stages of the optical fiber, the source output power was generally measured in milliwatts, a linear scale, and the loss was measured in dB or decibels, a logarithmic scale. With the passage of time, all measures changed to dB for reasons of convenience, which caused a lot of confusion. Loss measurements were generally measured in dB, since dB is a ratio between two power levels, one of which is considered the reference value. The dB is a logarithmic scale, in which every 10 dB represents a ratio of 10 times the value. The actual equation used to calculate the dB is

dB = 10 log (measured power / reference power).

So, 10 dB is a ratio of 10 times the value (either 10 times or a tenth more), 20 dB is a ratio of 100, 30 dB is a ratio of 1000, etc. When the two optical powers compared are equal, then dB = 0, a convenient value that is easy to remember. If the measured power is higher than the reference power, the dB will be a positive number, but if it is lower than the reference power, it will be a negative number. Therefore, loss measurements are usually expressed as a negative number.

Measurements of optical power, such as the output of a transmitter or input to a receiver, are expressed in units of dBm. The "m" in dBm refers to a reference power of 1 milliwatt. Therefore, a source with a power level of 0 dBm has a power of 1 milliwatt. Also, -10 dBm represents 0.1 milliwatts and +10 dBm represents 10 milliwatts.

In order to measure the loss in a fiber optic system, we make two power measurements, a reference measurement before the light passes through the component we are testing and a loss measurement after the light passes through the component. Since we are measuring the loss, the measured power will be less than the reference power, so that the ratio between the measured power and the reference power is less than 1, and the logarithm is negative, which turns to dB in a negative number. When we set the reference value, the meter marks "0 dB" because the reference value we set and the value that the meter is measuring is the same. Then, when we measure the loss, the measured power is lower, so the meter will mark "- 3.0 dB", for example, if the power being evaluated is half the reference value. 

Although meters measure a negative number for the loss, there is a convention to express the loss as a positive number. Therefore, when the meter marks -3.0 dB, we say that the loss is 3.0 dB. if the power that is evaluated is half the reference value. Although meters measure a negative number for the loss, there is a convention to express the loss as a positive number. Therefore, when the meter marks -3.0 dB, we say that the loss is 3.0 dB. if the power that is evaluated is half the reference value. Although meters measure a negative number for the loss, there is a convention to express the loss as a positive number. Therefore, when the meter marks -3.0 dB, we say that the loss is 3.0 dB.

The instruments that measure in dB can be either optical power meters or optical loss testing equipment (OLTS). The optical power meter generally marks in dBm for power measurements or dB with respect to a reference value set by the user for the loss. While most power meters have ranged from +3 to -50 dBm, most sources are in the range of +10 to -10 dBm for lasers and -10 to -20 dBm for LEDs. Only lasers used in CATV or long-distance telephone systems have sufficient powers to be really dangerous, up to + 20 dBm,

It is important to remember that dB is used to measure loss and dBm is used to measure power, and the more negative the number, the greater the loss. Set the zero reference before measuring the loss and check it occasionally while taking measurements.

Calibration of power measurements

Calibration of fiber optic power measurement equipment requires that a traceable reference standard be set to a national standards laboratory such as the National Institute of Standards and Technology in the United States (NIST) for the purpose of comparison when calibrating each power meter or another instrument. The NIST standard for all power measurements is an electrically calibrated pyroelectric radiometer (ECPR), which measures the optical power by comparing the heat power of the light with the known heat power of a resistor. Calibration is performed at 850, 1300 and 1550 nm. Sometimes, Manufacturers use the laser wavelength at 1310 nm as the wavelength calibrated in a power meter, but the standard for power meter calibration is 1300 nm. To conveniently transfer laboratory standards to the calibration laboratories of fiber optic power meter manufacturers, NIST currently uses a laboratory optical power meter that is sent to laboratories as a transfer standard. But the standard for power meter calibration is 1300 nm. To conveniently transfer laboratory standards to the calibration laboratories of fiber optic power meter manufacturers, NIST currently uses a laboratory optical power meter that is sent to laboratories as a transfer standard. But the standard for power meter calibration is 1300 nm. To conveniently transfer laboratory standards to the calibration laboratories of fiber optic power meter manufacturers, NIST currently uses a laboratory optical power meter that is sent to laboratories as a transfer standard.

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Meters calibrated in this way have a calibration uncertainty of around +/- 5%, compared to the NIST primary standards. Limitations in uncertainty are the inconsistencies inherent in optical couplings, about 1% in each transfer, and slight variations in wavelength calibration. NIST is working continuously with instrument manufacturers and private calibration laboratories to try to reduce the uncertainty of these calibrations.

The recalibration of instruments must be performed annually; However, experience has shown that the accuracy of the meters rarely changes significantly during that period, as long as the meter's electronics do not fail. Calibration of fiber optic power meters requires a considerable investment in capital equipment, so the meters must be returned to the original manufacturer or private calibration laboratories to be calibrated.