What Is Optical Power?

Power is the rate at which energy is transferred or used. The standard unit of power is the watt (W).

In optical communications, however, we typically deal with relatively small amounts of optical power. An optical transmitter might launch only a few milliwatts of power into a fiber.

A milliwatt (mW) is one-thousandth of a watt:

• 1 W = 1,000 mW
• 1 mW = 0.001 W
• 10 mW = 0.01 W
• 100 mW = 0.1 W

For this reason, mW is often more convenient than watts when discussing optical power.

What Happens to Optical Power in a Fiber?

An optical transmitter converts electrical energy into light and launches that light into an optical fiber.

As the light travels through the fiber, its power gradually decreases. This reduction in optical power is called attenuation.

Two important mechanisms contribute to attenuation:

Absorption

Impurities and other properties of the glass absorb some of the optical energy and convert it into heat.

Scattering

Microscopic variations in the glass cause some of the light to scatter in different directions, reducing the amount of light that remains guided within the fiber core.

The result is a gradual decrease in optical power as distance increases.

Why Don't We Just Use mW?

This is where things become interesting.

Fiber attenuation is essentially an exponential process. In addition, every fiber segment, splice, connector, and passive component can introduce additional loss.

If we calculate everything directly in mW, we end up multiplying many factors together. For example, a signal might be reduced by one factor in a fiber section, another factor at a splice, another at a connector, and so on.

For a complex optical link, these calculations can quickly become cumbersome.

This is exactly where logarithms come in.

Logarithms transform multiplication into addition and division into subtraction. This makes them extremely useful for describing optical power and loss.

What Is dBm?

To express absolute optical power using a logarithmic scale, we use dBm.

The "m" in dBm refers to 1 milliwatt, which is the reference power.

The relationship is:

Because 1 mW is the reference point:

Power levels above 1 mW have positive dBm values, while power levels below 1 mW have negative dBm values.

For example:

Optical Power	dBm
0.001 mW	−30 dBm
0.01 mW	−20 dBm
0.1 mW	−10 dBm
1 mW	0 dBm
10 mW	+10 dBm
100 mW	+20 dBm

So, dBm tells us how much absolute optical power is present at a particular point in the network.

dBm vs. dB: What's the Difference?

This distinction is extremely important.

dBm is an absolute power level. It tells you how much optical power is present, referenced to 1 mW.

dB is a relative change in power. It describes a ratio between two power levels and is commonly used for fiber loss, attenuation, and amplifier gain.

Think of it this way:

Why Does dBm − dBm = dB?

This often seems strange when first learning optical communications.

Suppose a transmitter launches +10 dBm and a receiver measures −5 dBm.

The change in power is:

Why does this work?

Because dBm is a logarithmic representation of power. Subtracting two logarithmic values is equivalent to taking the logarithm of the ratio of the two corresponding linear power values.

In other words, subtracting two absolute power levels gives us the relative power change, which is expressed in dB.

The Same Principle Applies to Amplifier Gain

The relationship works for gain as well as loss.

For example, if an Erbium-Doped Fiber Amplifier (EDFA) increases an optical signal from −10 dBm to +10 dBm, its gain is:

The amplifier has increased the optical power by 20 dB.

The Key Takeaway

The easiest way to remember the difference is:

Once optical power is expressed in dBm and losses and gains are expressed in dB, optical link calculations become much simpler. Instead of multiplying numerous power ratios, we can simply add and subtract dB values.

This is why dBm and dB are used so extensively in optical networking.

The next step is to apply these concepts to an actual fiber optic link power budget, where transmitter power, fiber loss, splice loss, connector loss, and receiver sensitivity are combined to determine whether a link will operate reliably.