What is dB?

dB is an abbreviation for "decibel" and is a unit used to express the relative power of electricity or light.

By "relative," I mean how many times the power has increased compared to a certain standard power? How many times has it decreased? This is expressed on a logarithmic scale.

Expressed as an equation, this is as follows:

Let's take a look at this in more detail.

Here is an example of passing light through an optical fiber: Suppose the input power of the optical fiber is 1 watt and the output power is 0.9 watts.

This can be stated as follows:

- 0.1 watts loss through optical fiber
- Power is reduced to 9/10 through optical fiber
- Fiber optic loss is 0.46dB

The optical fiber loss of 0.46 dB is calculated in dB terms using an input power of 1 watt as the "reference power" and an output power of 0.9 watts as the "target power."

The value is negative, so it represents a loss.

Let us consider the following example.

Here is an example of amplification using an optical amplifier. Let's assume that the power at the input side of the optical amplifier is 0.1 watts and that at the output side is 1 watt.

This can be stated as follows:

- 0.9 watts increase through optical amplifier
- The power is increased by 1/0.1 = 10 times through the optical amplifier
- The gain of the optical amplifier is 10 dB.

The gain of an optical amplifier is 10 dB, calculated in dB terms with the "reference power" being an input power of 0.1 watts and the "target power" being an output power of 1 watt.

Since the value is positive, it represents a gain.

What is dBm?

dBm is an abbreviation for decibel-milliwatt and is a unit that expresses the absolute value of electrical or optical power.

By "absolute," I mean that the reference value is set at 1 milliwatt, and how many times has it increased compared to this 1 milliwatt? What fraction of a milliwatt has it decreased to? This is expressed on a logarithmic scale.

Expressed as an equation, this is as follows:

Let's take a more concrete example using optical fiber.

This can be stated as follows:

- 1 watt of input power can be translated to 30 dBm of input power
- 0.9 watts of power translates to 29.54 dBm of power.
- The difference between the two is that the loss of the optical fiber is 0.46 dB.

Let's convert 1 watt of input to dBm.

Next is the output power of 0.9 watts.

Subtracting the input from the output gives the loss in the optical fiber.

This is the same value as the optical fiber loss calculated above in dB.

What is the difference between dB and dBm?

The main difference between dB and dBm is that dB is a relative measure and dBm is an absolute measure.

dB is referenced to an arbitrary reference value, whereas dBm is referenced to 1 milliwatt.

Thus, dB is a unit of ratio, proportion or change, whereas dBm is a unit of a specific physical quantity.

Understand the differences and use them correctly.

Try the practice questions

Which is the correct expression? Please choose either dB or dBm (in parentheses).

Question 1. The optical power coming out of an optical fiber is -10 (dBm or dB).
Question 2. The gain of an optical amplifier is (5dBm or 5dB).
Question 3.It can also be said that the measurement error of optical power in watts is ±50% → ±3 (dB or dBm).


Answer:
Question 1. The optical power coming out of an optical fiber is -10 (dBm).
Question 2. The gain of an optical amplifier is (5dB).
Question 3.It can also be said that the measurement error of optical power in watts is ±50% → ±3 (dB).

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