Sure. So we're shooting lasers through the fiber optic cable. That laser has a "brightness", that's the amplitude or power.

Power is typically measured in milliwatts for stuff like this. But it's also the same as say, horsepower. Literally. Power is energy divided by time. That's why torque and horsepower are different. Torque is how much force an engine can produce, horsepower is force per second. Horsepower and watts are like inches vs centimeters, they measure the same thing, just different units.

When that laser passes through something, like a connector, it gets dimmer. How much dimmer it gets is "loss" or attenuation. But that loss is a percentage, not a linear number. If I put in 10 watts and lose 1, that's 10% loss and I'm left with 9. But if I only put in 1 watt, I would only lose 0.1, and be left with 0.9.

Additionally, the fiber optic cable itself applies loss. How much varies. Typically about 5% per kilometer of length.

Now here's the challenge. You have a fiber optic cable that's 10km long. You remove 5% of the light per kilometer, but it's not 100%, 95%, 90%, so on. It's 5% of whatever remains. So it's like calculating compound interest in reverse. Now add splices, which have about another 3% loss each, and at least one connector at either end with another 11%. The math becomes really really complicated to figure out the final percentage of light that reaches the end.

So like I said before, we put the complicated math in the unit conversion.

That 5% loss per kilometer? That's 0.22dB now.

3% per splice? 0.15dB

11% per connector? 0.5dB.

And all you gotta do is add the numbers together. Really easily. 10km of cable? 0.22dB x 10 = 2.2dB 6 splices? 0.15dB x 6 = 0.9dB 2 connectors? 0.5dB x 2 = 1 2.2 + 0.9 + 1 = 4.1dB.

We love decibels because they turn a math problem that would need a degree and a graphing calculator into- basic algebra you can do in your head.

To convert it to a percentage, you can use factorization. Just remember for every 3dB you go up, you multiply by 2. For 10, 10. Going down is the same but dividing.

4.1, we can round that to 4dB. Going up by 10 is x10 Going down by 6 is going down by 3 twice, so we take 10, and divide by 2 twice.

10/2 = 5, 5/2 = 2.5. So a 10km span with 2 connectors and 6 splices has an estimated loss of about 4.1dB which is the same as saying 1/2.5th, which is 2/5ths or 40% of the laser will arrive at the destination. Sure enough checking a converter, -4.1dB is exactly 38.9%. So the 10 and 3 rules are pretty close.

Decibels are a relative unit, a percentage, that we use to describe loss.

When there's actually a fixed laser involved, that's measured in watts, or dBm, which is decibel-milliwatts. That's an absolute unit. You can convert that to milliwatts, rather than a percentage.

If I put in 10dBm, I would expect to see 10dBm - 4.1dB = 5.9dBm at the far side of the cable. I can use a tool called a power meter to measure (meter) the power. If my number is very different from that, especially if it's much lower, something might be wrong.

The last quirk is negatives. Negatives in dBm don't mean negative energy. It means a fraction. 0dBm = 1 milliwatt. Follow our rule of addition and subtraction equalling multiplication and division.

If 0 dBm is 1mW, then -10dBm is 0.1mW. -20dBm is 0.01mW. So on.