Optical Power and Decibel Scales

 
Because light is energy, measurements of the rate of delivery or power are important. Compared with common light bulbs of 40 or more watts, LEDs and laser diodes are very low in power.

Power levels commonly experienced in fiber optics are milliwatts (1/1000) of a watt), microwatts (1 millionth of a watt), and nanowatts (1 billionth of a watt). Relative power levels of typical LEDs and laser diodes are shown on the chart below.

The Decibel Scale

When making calculations, it can be difficult to handle factors of 1,000,000 and 1,000,000,000. The decibel scale allows us to compress these figures and use simple addition and subtraction of decibel figures instead of multiplication and division of lengthy numbers.

Many optical test instruments use the dBm and the dBµ scales below. The "m" indicates the zero dB reference value is 1 mw (0.001 watt = 1 milliwatt). The "pe" indicates the zero reference level is one microwatt.

In this scale, a change of 10 dB changes the value considered by a factor of 10. Plus 10 dB (+ 10 dB) increases the value 10 times. Minus 10 dB (-10 dB) de-creases to 1/10 the value.

Table of Decibel Values related to Zero dBm:

Relative Levels
(Exact levels vary with sources and fibers used)

 Device
+30dBm = 1.0w
+20 dBm = .1w
+ 10 dBm = .01w

Laser Diodes
+30dBµ = 0 dBm = .001w = 1 milliwatt
+ 20 dBµ = -10 dBm = .0001w = 100 microwatts
+10 dBµ = -20 dBm = .00001w = 10 microwatts

LEDs
+ 0 dBµ = -30 dBm= .000001w = 1 microwatt
-10 dBµ = - 40 dBm = .0000001w = 100 nanowatts

Received Power
-20 dBµ = - 50 dBm = .00000001w = 10 nanowatts
Two other conventional decibel values to use are:
3dB = Factor of 2: + 3dB = 2x, -3dB = (1/2)x
6dB = Factor of 4: + 6dB = 4x, -6dB = (1/4)x

A loss or reduction of power, is designated by a minus (-dB) decibel number.

 

Metric Conversion Chart:

(Celsius X 1.8) + 32 = Fahrenheit
Kilometers X .6214 = Miles
Kilometers X 3281 = Feet
Meters X 3.281 = Feet
Centimeters X 0.3937 = Inches
Millimeters X .03937 = Inches
Micrometers X .0000394 = Inches

1 Micron = 1 Micrometer = 1 Millionth of a Meter