DWs could challenge Bragg gratings in many applications of critical importance,
particularly those related to controlling and shaping high-power beams.
to create DWs wherein, instead of the
Cartesian coordinate x, the optical axis
of the LC is rotating as a function of the
azimuthal angle w: a = q ww, where q w
is the polar analog of the wave vector q.
The polar wavevector qw, often referred
to as the vortex degree, is an integer or a
half-integer used to maintain continuity
of the optical axis orientation.
Axial waveplates (AWs) are DWs
with axially symmetric orientation of
the optical axis, and their optical properties can well be understood by analogy
with cycloidal DWs. The AWs between
polarizers appear as a system of axially
symmetric fringes, as opposed to the
linear grating for cycloidal DWs. The
diffraction orders of AWs are extended
into a ring that creates a doughnut
beam—the polar analog of the first-order diffraction by a cycloidal DW. The
beam is widely used for optical tweezers
and imaging applications.
The materials and technology needed
for producing DWs is applicable to axial
waveplates. Moreover, the technological
regimes that meet the half-wave condition for cycloidal DWs can be applied
to the fabrication of axial DWs that
are fine-tuned to a desired wavelength
in the UV, visible or infrared spectral
regions. Large area, high-quality LC
polymer axial DWs tuned in such a way
are demonstrated with as high a degree
as q w = 64! This means that the optical
axis is rotating 128 times with changing
azimuthal angle w from 0 to 2p. The
technology allows for printing arrays of
high-order AWs.
DWs, Bragg gratings
and beyond
DWs could challenge Bragg gratings in
many applications of critical importance,
particularly those related to controlling
and shaping high-power beams. Due
to their thinness, high transparency and
the freedom they offer to choose sub-
strate material from considerations of
minimizing absorption rather than for
the feasibility of fabricating the diffrac-
tive element itself, DWs could withstand
high-power laser beams. The substrates
can be of all practical sizes, shapes and
materials, including metal mirrors.
Nelson V. Tabiryan ( nelson@beamco.com) and
Sarik R. Nersisyan are with BEAM Engineering
for Advanced Measurements Co., Winter Park,
Florida, U.S.A. Brian R. Kimball and Diane M.
Steeves are with the U.S. Army Natick Soldier
Research, Development & Engineering Center in
Natick, Mass., U.S.A.
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