Which unit is commonly used to express optical power in small-signal receivers?

Which unit is commonly used to express optical power in small-signal receivers?

• A. dBm
• B. W
• C. dB
• D. mW

Answer: (A)

In small-signal optical receivers, power levels span many orders of magnitude and are tied to how sensitive the receiver is to detect a signal above noise. Using dBm gives a fixed reference (1 milliwatt) and expresses power on a logarithmic scale, which matches how receivers and noise figures are described and how link budgets are calculated. This makes it easy to compare levels, set margins, and understand performance in a single number, even when the actual power is very small. For example, received powers are often in the negative dBm range, like -20 dBm or -40 dBm, which would be awkward to read in watts or milliwatts. The other units—watts or milliwatts—are linear and become unwieldy for such small values, and dB is just a relative ratio without a fixed reference, so it doesn’t convey an absolute power level.

In small-signal optical receivers, power levels span many orders of magnitude and are tied to how sensitive the receiver is to detect a signal above noise. Using dBm gives a fixed reference (1 milliwatt) and expresses power on a logarithmic scale, which matches how receivers and noise figures are described and how link budgets are calculated. This makes it easy to compare levels, set margins, and understand performance in a single number, even when the actual power is very small. For example, received powers are often in the negative dBm range, like -20 dBm or -40 dBm, which would be awkward to read in watts or milliwatts. The other units—watts or milliwatts—are linear and become unwieldy for such small values, and dB is just a relative ratio without a fixed reference, so it doesn’t convey an absolute power level.