Afii

Optical Power - Power or Loss? ("Absolute" vs. "Relative" Measurements)
Practically every measurement in fiber optics refers to optical power. The output of a transmitter or the input to receiver are "absolute" optical power measurements, that is, you measure the actual value of the power. Loss is a "relative" power measurement, the difference between the power coupled into a component like acable, splice or a connector and the power that is transmitted through it. This difference in power level before and after the component is what we call optical loss and defines the performance of a cable, connector, splice, or other component.
Whenever tests are performed on fiber optic networks, the results are displayed on an instrument readout. Power measurements are expressed in "dB," themeasurement unit of power and loss in optical fiber measurements. Optical loss is measured in “dB” while optical power is measured in “dBm.” Loss is a negative number (like -3.2 dB) as are many power measurements. Measurements in dB can sometimes be confusing.
In the early days of fiber optics, source output power was usually measured in milliwatts, a linear scale, and loss was measured in dB ordecibels, a logarithmic scale. Over the years, all measurements migrated to dB for convenience causing much confusion. Loss measurements were generally measured in dB since dB is a ratio of two power levels, one of which is considered the reference value. The dB is a logarithmic scale where each 10 dB represents a ratio of 10 times. The actual equation used to calculate dB is

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

Thus 10 dB is a ratio of 10 times (either 10 times as much or one-tenth as much), 20 dB is a ratio of 100, 30 dB is a ratio of 1000, etc. When the two optical powers compared are equal, dB = 0, a convenient value that is easily remembered. If the measured power is higher than the reference power, dB will be a positive number, but if it is lower than the referencepower, it will be negative. Thus measurements of loss are typically 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. Thus a source with a power level of 0 dBm has a power of 1 milliwatt. Likewise, -10 dBm is 0.1 milliwatt and +10 dBm is10 milliwatts.
To measure loss in a fiber optic system, we make two measurements of power, a reference measurement before the component we are testing and a loss measurement after the light passes through the component. Since we are measuring loss, the measured power will be less than the reference power, so the ratio of measured power to reference power is less than 1 and the log is negative,making dB a negative number. When we set the reference value, the meter reads “0 dB” because the reference value we set and the value the meter is measuring is the same. Then when we measure loss, the power measured is less, so the meter will read “- 3.0 dB” for example, if the tested power is half the reference value. Although meters measure a negative number for loss, convention is the loss isexpressed as a positive number, so we say the loss is 3.0 dB when the meter reads - 3.0 dB.
Instruments that measure in dB can be either optical power meters or optical loss test sets (OLTS). The optical power meter usually reads in dBm for power measurements or dB with respect to a user-set reference value for loss. While most power meters have ranges of +3 to -50 dBm, most sources are in the rangeof +10 to -10 dBm for lasers and -10 to -20 dBm for LEDs. Only lasers used in CATV or long-haul telephone systems have powers high enough to be really dangerous, up to +20 dBm; that’s 100 milliwatts or a tenth of a watt.
It is important to remember that dB is for measuring loss, dBm is for measuring power and the more negative a number is, the higher the loss. Set your zero reference before...