  We study electromagnetic field propagation through a planar three-waveguide
coupler with linear gain and loss, in a configuration that is the optical
analog of a quantum $\mathcal{PT}$-symmetric system, and provide its
closed-form analytic propagator. At an specific propagation length, we show
that the device provides all-optical amplitude to phase modulation with a $\pi$
modulation range, if an extra binary phase is allowed in the reference signal,
as well as phase to amplitude modulation, with an amplitude modulation range
that depends linearly on the gain-to-coupling ratio of the system.
