  We demonstrate the fabrication of volume holograms using 2-photon
polymerization with dynamic control of light exposure. We refer to our method
as (3+1)D printing. Volume holograms that are recorded by interfering reference
and signal beams have a diffraction efficiency relation that is inversely
proportional with the square of the number of superimposed holograms. By using
(3+1)D printing for fabrication, the refractive index of each voxel is created
independently and thus by, digitally filtering the undesired interference
terms, the diffraction efficiency is now inversely proportional to the number
of multiplexed gratings. We experimentally demonstrated this linear dependence
by recording M=50 volume gratings. To the best of our knowledge, this is the
first experimental demonstration of distributed volume holograms that overcome
the 1/M^2 limit.
