Optical Power in dB

What is Optical Power? 

• The brightness (or “intensity”) of the light. 

• As light travels through fiber, some of the energy is lost. 

• Either absorbed by the glass particles, and converted to heat;

• Or scattered by microscopic imperfections in the fiber. 

• This loss of intensity is called “attenuation”. 

• We typically measure optical power in “Decibels” 

• A decibel (dB, 1/10th of a Bel) is a logarithmic-scale unit expressing the relationship between two values.
• The decibel is a “dimensionless-unit”, meaning it does NOT express an actual physical measurement on its own. 

Optical Power and the Decibel

A decibel itself is simply a ratio between values! 

• 0 dB is no change, +3 dB is double, -3 dB is half, etc.
• To express an absolute value (i.e. an actual light level), it must be compared to a known reference value. 

In optical networking, this is typically a “dBm”.

• That is, a decibel relative to 1 milliwatt (mW) of power. 
• 0 dBm is 1 mW, 3 dBm is 2 mW, -3 dBm is 0.5mW, etc.
• So what does this make 0mW? Negative Infinity dBm.
• Confusion between dB and dBm is one of the most common mistakes when working with optical networks! 19 Optical Power and the Decibel 

So why do we measure light with Decibels? • Light, like sound, follows the “inverse square” law.

• The signal is inversely proportional to the distance squared.
• After a signal travels distance X, and loses half of its intensity.
• After it travels another distance X, it loses half again.
• Thus after 2X only 25% remains, after 3X only 12.5% remains, etc.

Using a logarithmic scale simplifies the calculations.

• A 3dB change is approximately half/double the original signal. 
• In the example above, there is a 3dB loss per distance X. 
• At distance 2X there is 6dB of loss, at distance 3X it is 9dB, etc.
• Using a logarithmic scale “cancels out” the exponential loss, giving us a linear system that lets us use simple math (like addition and subtraction) when calculating losses. 20 Decibel to Power Conversion Table