This is the instantaneous, most-recently-sampled value of the received optical power measured on the lower-numbered optical lane of this multi-lane coherent port during the current performance-monitoring interval. A live per-lane reading matters because it lets an admin check right now whether this specific lane, rather than the whole port, is the source of a reported problem. For example, if inOpticalPowerLaneLowInstant reads several dB below inOpticalPowerLaneHighInstant at the same moment, an admin would suspect a defect specific to the low lane's receive path rather than a fiber-wide power loss.