Hi,
The difference between power levels of two i.e Any optical transmission system requires a defined range of optical receiver input power for proper operation. In practice, the received power must be higher than the minimum level and lower than the maximum level is called is the optical margin.
A design margin of 6dB is common in telecom systems, although the installed system may achieve a better value, since “worst case” conditions are of course infrequent. The design margin of some high speed LAN systems may be much smaller, since these systems are typically limited by chromatic dispersion rather than loss.

To measure optical margin, the essential apparatus is:

A complete and operational transmission system.
An instrument to measure transmission quality. For instance an instrument to measure Bit Error Rate or BER, or a signal / noise meter depending on the nature of the transmission.
Some fiber optic measuring instruments:
A Variable Optical Attenuator (VOA) to adjust the attenuation. This must have sufficient power handling capacity, it’s optical characteristics must not degrade the transmission, and it should be accurate at the operational wavlength(s).
An Optical Power Meter to measure the absolute light level, which must be accurate at expected power levels and wavelengths.

Thanks