  This paper presents a pulse compansion, i.e. compression or expansion,
technique based on co-propagating space-time modulation. An engineered
asymmetric space-time modulated medium, co-propagating with a pulse compands
the pulse continuously and at a constant rate. The space-time medium locally
modifies the velocity of different sections of the pulse in order to shape the
pulse as it propagates. There is no theoretical limit on the compansion factor
with the proposed system. Moreover, it can be designed to transform the pulse
shape and its modulation linearly, without any distortion. Therefore the
proposed technique can be used for up or down-conversion of modulated pulses,
with extreme conversion ratios. The presented compansion technique is linear
with respect to the input wave and therefore can be used to perform compansion
or frequency conversion on multiple pulses simultaneously.
