What is the meaning of the y-axis on my OSA?

Traditionally, the OSA Y-Axis is displayed in either Linear or Log mode. And, the units can be set to either dBm or dBm/nm. The dBm/nm meaning: When the spectral distribution of optical power of a source, of a laser for example, is measured w an OSA, then the result is usually given either:

1. As a power spectral density (in units of mW/nm or dBm/nm, with dBm = dB relative to 1 mW) or

2. As a power for a given measurement bandwidth (dBm)

As with an FFT, the distribution of power is sorted into bins, on the x-axis.