  Orbital angular momentum (OAM), due to its theoretically orthogonal and
unbounded helical phase index, has been utilized as an independent physical
degree of freedom for ultrahigh-capacity information encryption. However, the
imaging distance of an OAM hologram is typically inflexible and determined by
the focal length of the Fourier transform lens placed behind the hologram.
Here, 3D orbital angular momentum holography is proposed and implemented. The
Fourier transform between the holographic plane and imaging plane is performed
by superimposing Fresnel zone plates (FZP) onto the computer-generated
holograms (CGH). The CGH is binarized and fabricated on the birefringence
lithium niobate crystal by femtosecond laser micromachining. Experimental
verification demonstrates the feasibility of the encoding method. Moreover, by
superimposing FZPs with different focal lengths into various OAM channels,
OAM-multiplexing holograms are constructed. Target images are separately
projected to different planes, thereby enabling 3D multi-plane holographic
imaging with low crosstalk. The interval between adjacent imaging planes can be
uniform and minimal, free from depth of field constraints, thus achieving high
longitudinal resolution. This work achieves OAM holography in a more compact
manner and further expands its applicability.
