Signal Loss in a fiber or any fiber in a cable is measured in:

• A. dBm
• B. Milliwatts
• C. dB
• D. Milliamperes

Answer: (C)

Loss is defined by how much the signal power drops from input to output, and decibels provide a compact, additive way to express that ratio. Using dB lets you quantify attenuation as a logarithmic ratio: loss in dB = 10 × log10(Pin / Pout). This makes large losses easier to compare and, crucially, the total loss of a fiber link is simply the sum of the individual losses in dB.

For example, if you start with 1 mW and end with 0.316 mW, the loss is 10 × log10(1 / 0.316) ≈ 5 dB. This demonstrates why dB is the standard for signal loss.

The other units don’t fit this purpose: milliwatts is an absolute power level, not a loss ratio; dBm is a power level relative to 1 mW, not a loss; milliamperes is current, not power.