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Passive WDM channel power equalization with cascaded nonlinear amplifying loop mirrors

We propose a novel cascaded amplifier system for long-haul PSK or FSK transmission that equalizes WDM channel powers completely passively. An unflattened EDFA is asymmetrically placed within an all-fiber Sagnac interferometer. The Kerr nonlinearity of the Sagnac-loop fiber induces a net phase difference between the counterpropagating signals. Individual channels have independent, nonlinear transfer functions upon exiting the loop. This system provides higher gain for weak channels, while strong channels receive reduced gain. Thus, all channels approach and maintain a steady-state power level through successive amplifications. This system requires no active feedback mechanisms to maintain channel power equality. Its performance is not affected by changes in the gain spectrum of the optical amplifier and, unlike all other power equalization or gain-flattening schemes, the degree of equalization improves with increasing number of amplifications. This presentation will discuss the operating principle of this device, theoretical predictions of its properties, and work in progress towards an experimental proof of principle.