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CPC Applications & Suggestions
- LiDAR Doppler shift of atmospheric particles
- Particle Size Measurement
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Rotary Joint Applications & Suggestions
- Robotic Arm Optical Interconnect
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- Stock CPCs are available for evaluation
- Custom CPCs can be made upon request
- Design Wavelength: One specific laser wavelength (e.g. 650nm)
- Diffraction Efficiency: 70-85% depending upon various factors
- Number of Annuli: up to 24 annuli have been made
- Spot Size: ~200 microns
- Spot Configuration: a line of spots is standard. A 2D array and other geometries are also possible
- Focal Lengths: ~F2 to F3
- Active Aperture: 25mm
- Glass: BK7 on both sides seals gelatin that contains the recorded interference patterns.
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By: Richard Rallison
of Ralcon Development Labs & Wasatch Photonics
Doppler
shifts in the optical spectrum may be detected either
by heterodyne methods using coherent sources or by direct
detection of filtered and dispersed return light. The
coherent method requires diffraction limited optical
trains and a local oscillator. The direct detection
method is far more relaxed, requiring only "photon
bucket" collection optics and a series of blocking
filters and etalons to separate the frequency shifted
light into a radial pattern. The direct detection of
a Fabry-Perot pattern can best be done with a PMT or
microchannel plate that has been constructed to have
many equal area electrically isolated detection rings.
Such a detector placed at the image plane can handle
large fields of view and is good at preserving precious
photons.
Circle
to Point Converter
A
less expensive alternative would be a device that could
effectively transform the output of the etalons into
a string of foci spaced correctly to fit into a line
of fibers or onto a linear CCD or photo diode array.
This device could be called a circle-to-point converter
and simply redirects all the rays that enter each of
the circular annuluses into unique off axis focal points.
One way to do this is to cut out annular sections of
the edges of lenses and piece them together with appropriate
offsets in their respective focal positions. An extension
of this method would be to cut up plastic Fresnel lenses
or diffractive lenses.
These
devices can also be thought of as fractured zone plates,
re-assembled for the purpose of separating and detecting
the Doppler shift imparted to a narrow frequency laser
pulse by winds or moving objects. The increments in
frequency occur in equal area annuluses so the coarse
appearance of such an optic is that of a zone plate
as well. The main purpose for this optical element is
the remote detection of regional wind speeds.
Rotary
Optical Joint
The
same peculiar optical element constitutes a kind of
rotary optical joint, useful in coupling wideband signals
from a spinning platform to a stationary platform. In
this alternative application the device is best made
with each annulus being the same width rather than the
same area. That would make it appear to be a coarse
axicon rather than a coarse zone plate. Otherwise, they
are or can be the very same optical element. Each annulus
is simply an off axis focusing DOE or HOE that gets
shifted laterally by some arbitrary increment between
zones. We would change the way they are masked if shooting
them in a step and repeat fashion and nothing else.
Figure 1 shows a likely layout of the circle to point
pattern and figure 2 is the corresponding rotary joint
pattern. The circle to point pattern is used with full
illumination over the aperture which has been made to
have as many as 24 annular regions in a 25 mm diameter.
The rotary joint would be used with individual modulated
lasers or fibers addressing each zone with only a pencil
beam. The number of channels would be limited by the
size of the beams and the allowable crosstalk. The focus
is shifted conveniently off the axis of rotation and
out of the path of any zero order light in both applications.
The
method of fabrication and replication can be the same
for each device. We made units with 24 zones as computer
generated binary patterns recorded directly in photopolymer.
They could also have been patterned into photo-resist
and dry etched into silica. We opted to add a high frequency
carrier to eliminate the possibility of cross talk from
higher orders, alternatively a correct blaze could have
been used at lower frequencies so that the parts could
easily be fabricated by mechanical replication. We also
made the same functional units by starting with a holographically
constructed master which was subsequently stepped between
exposures along with a mask of 7 or 24 rings. The HOE
constructed this way can then be copied optically in
one step into another volume recording material for
higher volume production.
Acknowledgements
The
diffractive design and fabrication of the rotary joints
were carried out independently by Mathias Johansen and
Sverker Hard at the Chalmers University of Technology
in Sweden
and the design of the circle to point converter was
contributed by Matt McGill and others at Goddard Space
Flight Center with fabrication being done at Ralcon
development lab. Patents have been filed for or granted
to both parties and both are deserving but I can't help
being amused by the entirely coincidental invention
of the same complex diffractive optical element for
two widely differentiated applications. It makes me
wonder how often such things occur. More details about
each of these optical elements and the applications
they were designed for can be found in the references
and from the authors. Optical Interconnects is another
field where similar devices may be used and could be
searched for. It would not be a surprise to find the
same device in the literature of that field and possibly
also in holographic memories.
•
Matthew
J. McGill, M. Marzouk, V. S. Scott, J. D. Spinhirne
<Holographic circle- to-point converter with a particular
application for Lidar work> Opt. Eng. Vol 36 pp 2171-2175
Aug 1997.
•
Mathias Johansson, Sverker
Hard <Design, fabrication, and evaluation of a multichannel
diffractive optic rotary joint>, App Opt, vol 38,
no. 8, pp 1302-1310, 10 Mar 1999.
•
R. D Rallison <Fractured
zone plates for spatial separation of frequencies>
SPIE vol 3633, pp 92-102, Diffractive and Holographic
Elements, San Jose, CA, Jan 1999
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