This is the instantaneous, most-recently-sampled value of the actual optical carrier frequency measured at this port's output during the current performance-monitoring interval, expressed in MHz. A live frequency reading confirms right now that this transmitter's laser is putting out the exact wavelength the network design assigned to this port, which matters on a densely packed DWDM system where adjacent channels sit close together. For example, right after swapping a transponder, an admin would check outOpticalFrequencyInstant to confirm the new unit is transmitting on the intended grid frequency before enabling traffic.