  We present a novel optical device to encode and decode two bits of
information into different Orbital Angular Momentum (OAM) states of a paraxial
optical beam. Our device generates the four angular momentum states of order
$\pm 2$ and $\pm4$ by Spin-To-Orbital angular momentum Conversion (STOC) in a
triangular optical loop arrangement. The switching among the four OAM states is
obtained by changing the polarization state of the circulating beam by two
quarter wave plates and the two-bit information is transferred to the beam OAM
exploiting a single $q$-plate. The polarization of the exit beam is left free
for additional one bit of information. The transmission bandwidth of the device
may be as large as several megahertz if electro-optical switches are used to
change the beam polarization. This may be particularly useful in communication
system based on light OAM.
