Characterizing Mode-Dependent Loss and Gain in Multimode Components

Mode-dependent loss and gain (MDL and MDG) of multimode components are fundamental impairments that reduce the capacity of mode-division-multiplexed (MDM) systems. MDL of components is commonly quantified either in terms of the root mean square (rms) or peak-to-peak (P-P) gain and lass variations. It is incorrect to specify only the P-PMDL of components if they are to be used in an MDM system with nonnegligible mode coupling, because the system's overall coupled gains are random variables whose statistics cannot be determined from the P-P MDL values. On the other hand, measurements of the rms MDL of components are sufficient to determine the rms value of the system's overall coupled MDL, regardless of whether the system has weak or strong coupling. We propose novel algorithms based on convex optimization, which can efficiently measure all modal gains of any multimode component using low-cast direct-detection hardware. In particular, we propose an efficient algorithm that produces accurate measurements of all modal gains by estimating a high-dimensional MDL ellipse using a sequence of power measurements.