Optical Modulation Amplitude vs Average Optical Power Explained

What is Average Optical Power

Average Optical Power (AVG) is the average light level coming out of the transmitter. A transceiver sends data by switching between a bright “1” level (P1) and a dim “0” level (P0). The laser never switches fully off, so P0 is not zero. AVG is simply the middle of the two:

AVG = (P₁ + P₀) / 2

AVG is easy to measure, but it says nothing about how far apart the “1” and “0” levels are.

What is OMA

OMA (Optical Modulation Amplitude) is the gap between the “1” and “0” light levels. This gap is what the receiver actually reads to tell a “1” from a “0”, so it shows the real signal strength.

OMA = P₁ - P₀

PAM4 uses four light levels instead of two. There OMA is called OMAouter: the gap between the highest and the lowest of the four levels.

How to calculate the AVG budget

Take the weakest the transmitter can send and subtract the weakest the receiver can read, both in average power:

AVG budget (dB) = Tx average power (min) - Rx sensitivity (average)

How to calculate the OMA budget

Same idea, but in OMA, and the transmitter’s eye-closure penalty is taken off first:

OMA budget (dB) = (Tx OMA - TDECQ)min - Rx sensitivity (OMA)

TDECQ (TDP for NRZ) is how many dB of signal a real transmitter loses to a partly closed eye and fiber dispersion.

Why the AVG budget is not an accurate value

The AVG budget is an estimate, not a guarantee. It measures how much light there is, not how readable the signal is.

It ignores eye closure. A real transmitter’s eye is partly closed by jitter, noise, and dispersion. TDECQ/TDP measure this, and AVG does not include it.

It does not work for PAM4. With four levels, the receiver must read three small eyes, one in each gap between neighbouring levels (top, middle and bottom). Whether they are open depends on how evenly the levels are spaced, and AVG cannot show that.

For NRZ, the AVG budget is usually close enough. For PAM4, always use the OMA budget.