Abstract
Semiconductor optical amplifiers (SOAs) are attractive for integrated
photonic signal processing, but because their response is so fast, delays
in a controller feedback path can jeopardize performance and stability. Using
state-space methods, we quantify the constraints imposed on feedback controllers
by closed-loop delay. We first derive a complete nonlinear state-space control
model of a SOA with an equivalent circuit containing parasitics and dynamic
impedance; the analytical state-space model agrees well with a validated photonic-only
control model. Using a linearized version of the model we demonstrate that
time delay in the feedback path can destabilize the SOA through phase accumulation.
We then apply linear system theory to calculate the best-case stable delay
margin for a given controller norm, and find a potentially severe inverse
relationship between delay margin and controller norm. Finally, guided by
the delay–controller relationship we design a hybrid feedforward–feedback
controller to illustrate that good transient and steady-state regulation is
obtained by carefully balancing the feedforward and feedback components. Our
state-space modeling and design methods
are general and are easily adapted to the design and analysis of more complex
photonic circuits.
© 2009 IEEE
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