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VPH gratings for Telescopes, design and testing
R. D. Rallison, R. W. Rallison, L. D. DicksonRalcon Development Lab, box 142, Paradise UT 84328
ABSTRACTLarge area volume phase holographic (VPH) gratings have been made for use in spectrographs attached to large
telescopes and for scanning LIDAR systems. Examples of the transmitted wavefronts, the spectral efficiency
measurements and other parameters such as uniformity, scatter, absorption and Q have been gathered and presented. Two
exposure layouts have been used and are described along with some discussion of modulation and bulk index of
processed DCG. A discussion of thickness regimes is given. A special case (Dickson) design is presented with examples
of performance and some intrinsic properties.
Keywords: volume phase gratings, dichromated gelatin, holographic optical elements, telescope spectrographs
1. GRATING DESIGN AND CONSTRUCTION CONSIDERATIONS
This paper contains a d iscussion of the elements involved in VPH grating design and fabrication as it has been practiced
at Ralcon for and in behalf of the Telescope community in particular and also for LIDAR, general spectroscopy and
telecom applications. We have delivered a few dozen prototypes to users at ESO, AAO, UNC, ODU, UT@A, GSFC,
OAP, NOAO and a few other organizations. W e have used left over gratings from some of these deliveries to make
representative measurements and to report the results along with explanations of what, why and how we did it. Then we
will propose some possible future improvements in size and performance.
1.1. Regimes
We have often had to begin a design with a discussion about the properties of VPH gratings and which Regime they will
be required to work in. A genuinely thick VPH exhibits high angular and spectral selectivity and operates in the Bragg
regime where most of the diffracted power is in one diffracted order. A thin grating operates in the Raman-Nath regime
and will have broad angular and spectral bands and if it is possib le for higher order modes to be diffracted then they will
be. Gaylord and Moharam1 pointed out that the regime parameter rho (D) has more to do with defining a thin and thick
hologram than does the quality parameter (Q). Rho does not depend on thickness but does depend on modulation. For
operation in the Bragg regime the modulation should be as uniform as possible through the volume and only high enough
to approach coupling all the light out of the zero order. Q is defined by thickness and period but assumes appropriate
modulation. Both parameters depend on wavelength, bulk index and fringe spacing in a similar way. The D or Q of a
thick grating is by definition much greater than 1 while that of a thin grating is less than or equal to 1. The most painful
VPH gratings to make are those with geometries that have a ratio of wavelength (8) to grating spacing (d) much smaller
than one. T hese are the problematic gratings with more than one possible non evanescent order. Larger ratio gratings
benefit little from being overly thick except when used for angular multiplexing of multiple gratings in the same
emulsion, something we have done for a few customers. The relationships for Q and D to quantify thick or thin volume
gratings are shown below.
Figure 1. Equations for Q and D and an approximation for power lost to higher orders
Where 8 is the wavelength of playback light, d is the grating spacing, T is the effective thickness of the grating, n is the
average bulk index of refraction and delta n is the peak index modulation. The lost power estimate is the maximum power
coupled into all higher and negative orders and the relationship applies when D is much greater than 1. When D is 10 then
1% or less escapes. A D of 10 or more is considered ideal but we rarely get that high.
1.2. Effective Thickness
A quick way to determine the effective or optical thickness of a grating is to measure its angular bandwidth. The full
angular bandwidth of a thick uniform grating, from null to null, is approximately 2d/T radians. This approximation is
very close to the thickness predicted by the Kogelnik2 model. Measurements to match thick models of d/T can have large
errors unless surface deformations are not first canceled by index matching with a cover glass. The model and the
approximation are both inaccurate when a strong gradient is present in the modulation as a function of depth through the
film.
Rigorous Coupled Wave Analysis (RCW A) is required to model effectively thin gratings with spatial frequencies that
are low enough to allow higher non evanescent orders. We consider all gratings operating at half angles less than 10
degrees to be problematic in that we have to make the film physically thicker than 17 microns to contro l the losses to
unwanted orders. Thicknesses up to 30 microns are useable but difficult to work with and keep uniform3. For gratings
with half angles of 30 degrees or higher, all orders except the first order are either trapped internally or are evanescent
and it becomes easy to get 95% of the incident light diffracted into a single order with films of even 5 micron thickness.
Spectral bandwidths can be as large as surface phase gratings but with no anomalies and tilting is not required to cover
wide spectrums, although it still moves a less sharp peak around. A special case exists for half angles near 46 or 47
degrees when DCG is used. At these angles it is possible to adjust the modulation upward so that all of both polarizations
are diffracted and will roll off in the same directions. This special grating has the name “Dickson” gratings4 after the
original designer Lee Dickson, an early pioneer in Holographic gratings used in bar code scanning systems at IBM.
1.3. Measurements
The spectral power distribution measurements were made by looking at the zero order component with an Ocean Optics
model 2000 spectrometer configured with fiber fed white light and a fiber pick-up so that gratings of any size could be
easily and quickly measured. The grating under test is simply inserted in the calibrated light path and tilted either for
maximum extinction or for a specified test angle and the missing light equals all the diffracted, reflected, absorbed and
scattered light that missed the input fiber. We then invert the curve and may subtract 8 percent for surface reflection if
small angles are used (this gets really tricky for high angles). W e typically subtract 5% in our models for scatter and
absorption as well. A laser line measurement of actual first order power is made if we have an appropriate laser line to
work with. The real data point helps register the spectrometer plot. Admittedly this method is not the best, but it is the
easiest and allows us to determine rather quickly such things as correct modulation and full bandwidth. A good grating
casts a dark shadow. The biggest shortcoming of this method is that gratings with power in other orders look better than
they are because it measures all power going into all orders at once. Dickson gratings and other high angle gratings
work well only if the surfaces have good AR coatings because the reflection losses in S polarization are far greater than
P so S appears both wider and more efficient than it really is, compared to P.
Wavefronts were measured initially with a 6 inch diameter Continental Optics shearing interferometer, also known as
a collimation checker. This device can easily detect power in a wave but is less useful for higher order aberrations. We
always used it to set up the recording beams and still managed to leave in a focal length of about 2.5 kilometers in most
gratings. The wavefronts transmitted by the gratings in this paper were measured with a ZYGO PTI 4 inch interferometer
equipped with a Diffraction International phase shifter and unwrapped and analyzed with Durango software. In each case
we show the original pattern followed by the unwrapped phase surface with power and tilt removed. The gratings made
for the near IR were measured in the second order to make the paths close to the same angles. The reference flats were
all 20th-wave and measurements were made with index matched 1/8 wave flats.
Scatter measurements were done with an argon laser at 488 nm passing through the gratings then onto a lens with an
obstruction in the center so that only scattered light would pass through. The power in a f/10 cone was collected and
measured and never exceeded 1%. “Holographic” scatter and some cosmetic defects are seen in p ictures.
1.4. Recording Setups
Two recording geometries were used for most of these gratings and a third is being introduced for high frequency very
large format gratings. The first and most general arrangement for recording gratings was implemented to make a 200
mm recording for AAO at 1516 lp/mm. It was the largest precision grating we had ever attempted, in spite of the fact
that we have made many 16 and even 20 inch gratings of a lesser quality for at least 15 years running. This set up is now
used routinely for all gratings smaller than 8 inches in the long direction. We had to buy 10 inch flats for aiming mirrors
but already had several good telescope primaries on hand. The Parabolas are 17 inches in diameter but we can only use
a little less than half of each one in an unobstructed recording configuration. The second recording configuration
derived from the need to make 250 mm diameter precision recordings for the Subaru telescope. This set up does not use
fold mirrors but does require some large parabolas. Both set ups have a pair o f mirrors to be used to adjust the path
lengths to be equal length. W hile not required, it is always a good idea because almost all lasers drift in frequency over
the time required to make a large area exposure. The drift can easily reduce fringe contrast but is less likely if the paths
are equal at the center of the plate or multiples of two cavity lengths.
Figure 2. Two recording set ups used to make high quality plane VPH gratings.
2. GRATING EFFICIENCY MODELS AND PERFORMANCE
2.1. Dickson Grating
The Dickson grating is rather unusual, it works in the range of angles where ordinary gratings either deliver high S or
high P polarizations but both together cannot normally exceed about 50%. The half angles in gelatin turn out to be 46
to 48 degrees and depend on the bulk index after processing, which is low because modulation is typically very high.
Both polarizations are diffracted with a practical potential of 95% efficiency and both roll off as a function of angle and
wavelength in the same direction. We have made a variety of near IR wavelength Dickson Gratings and a few visible
gratings. The model
and measured results
to the left are for a
center frequency of
about 650 nm. The
measured S and P
curves are much more
symmetrical than the
Kogelnik pred icted
curves and may be the
result of gradients in
the modulation that are
n o t p r e s e n t l y
accounted for in the
model. Otherwise they
are in good agreement.
The film thickness in
the model is 5 microns.
The actual thickness in
the product is also 5
m i c r o n s . T h e
modulation is about
.25 in both but no
higher orders can exist
so RCWA would not
be more useful to use
in the model. Literally
a l l t h e l i g h t i s
diffracted into one
order with very high
d i s p e r s io n i n a
Dickson grating and
the bandwidth is much
wider than a normal
grating of the same
frequency. This is a
particularly difficult
grating to fabricate but
the performance is
a l w a y s s o m e w h a t
startling.
Figure 3. Dickson grating 2280 lp/mm model and performance at 650 nm
2.2. Non Dickson high frequency grating model and Performance The 2456 lp/mm grating modeled and measured here illustrates one of the modulation options available in a non Dickson
VPH grating. T he model is modulated to produce near peak efficiency in the S polarization and very little is left for the
P state. The half angle in air
is 59 degrees at 650 nm for
this grating which is
dangerously close to the
a n g l e w h e r e n o P
polarization is allowed to
be diffrac ted a t any
modulation level. The angle
for zero power in P is about
64 degrees5 externally and
of course 45 degrees
internally. For external
angles near 64 degrees, the
P polarization diffraction
efficiency will be nearly
zero. In this case, the
preferred option is to peak
the S polarization at the
expense of P and that is
what has happened in the
measured grating of the
same f requenc y. T his
grating is modulated just to
the value where S is almost
maximum at 650 nm. The
difference in base lines
derives from the difference
in reflectivity of S and P at
high angles and because we
only measured the losses in
the zero order to get these
curves. A lase r l ine
measurement confirms that
this grating diffracts 90%
of 633 nm S light into the
first order. The S curve can
be assumed to actually
return to zero at its low
points and is drawn as seen
by the spectrometer.
Figure 4. Conventional high frequency 2450 lp/mm VPH grating model and performance at 650 nm
This is probably a good place to mention that we follow the current and modern definition of S and P where S is the Te
wave and P is the Tm wave or S is polarized perpendicular to the plane of incidence and P is parallel to that plane. This
puts the E vector parallel to the fringes for S type polarization and is opposite of the convention used by RGL which is
based on a historical use of S and P by RGL founders.
2.3. AAO Atlas Grating
This grating was 200 mm by 160 mm and
caused us to upgrade our aiming mirror
selection to accommodate the size. It was a
r e l a t i v e l y e a s y g r a t i n g t o m a k e
holographically because it worked in the
visible and was a modest 1516 lines per mm.
The exposures went well and several plates
were capped that had a nice appearance, low
scatters, virtually no extra recorded glints and
fairly good uniformity but were mostly under
modulated. They tended to work better in the
green region that in the red although some
regions of each were quite close to optimum
in the red. We modeled it as shown in figure 5
along with the actual measured efficiency in
two locations of a representative plate. It
shows that the center of the plate is very far
under modulated while the lower corners and
edge were nearly where they should have
been. Similar results were had in the first
gratings shipped out to ESO and us later
replaced them with better and more uniform
exposures. We will likely ship replacements
for these 1516 line gratings as well. These
initial orders were filled by R W Rallison and
were his first big gratings, he has steadily
improved since then and some of the
equipment he has to use has also been
upgraded. Wavefront quality was measured
with only unpolished Starphire surfaces
exposed and then with flats matched to both
sides.
The irregularity was .57 waves rms without
flats and .36 with flats over a 4-inch aperture.
Coma and astigmatism were both high at 3.6
and 2.5 waves peak to valley. It is likely that
we could improve on those figures as well as
the efficiency.
Flats with AR coatings applied can be added
to our gratings at any time to improve the
optical performance. This one gets somewhat
better with flats and others may get a lot
better. The errors in the glass are random and
could add to or subtract from the
holographically recorded errors.
Figure 5. The first AAO (Atlas) grating efficiency model and actual performance of one of the gratings.
2.4. ESO 574 l/mm grating
An example of a grating that was low
enough in spatial frequency to almost
require RCWA is the ESO 574 lp/mm
grating for use in the green region, where
the half angle is just 8 degrees. It is
particularly difficult to get both a wide
angular and spectral response and still get
the bulk of the power into the first order at
half angles under 12 degrees . The top plot
was calculated and provided by one of us
and the measured plot below it was
provided by Guy Monnet of ESO. The
power spectrum follows the Kogelnik curve
fairly well only because the sum of all the
power in all the orders does match fairly
well. Data taken here at 514 nm showed
power lost to the -1 order of 2- 4% and the
second order was barely visible. The
gelatin measured 17 microns thick after
processing but the effective thickness is
about 12 microns, due to gradients in the
modulation. The period was 1.74 microns
yielding a Q of about 9 if the modulation
was low and uniform. The value of rho
should be 4 or 5 with a modulation of
approximately .02 so the lost power should
not exceed 4 or 5% , which it did not. The
bandwidth mismatch with the model may
indicate that the thickness was effectively
larger than modeled for the deep blue or
that absorption was higher than anticipated
in that region.
The wavefront quality was good with
residual 0 order power at 1.8 waves,
astigmatism at .6 waves and rms deviation
from a plane wave of .2 waves. In the first
order after removing 1.5 waves of power
and matching flats to it we are left with 1
wave of astigmatism, .9 waves of coma and
a rms deviation of just .27 waves over a 4-
inch aperture. The interferogram made with
flats matched to the grating is shown at the
bottom. The one above is without flats.
The 3 or 4 waves of power amounts to a
focal length of two kilometers and does
not affect image quality.
Figure 6 ESO 574 lp/mm grating efficiency
model and performance.
2.5. ESO 720 lp/mm grating centered at 1120 nm
The Kogelnik plot at the top of the page is created for a 1120 nm center wavelength, a 720 lp/mm grating and an effective
thickness of 5 microns. The actual measured efficiencies at angles from 18.8 to 23.8 degrees match up very well but they
center about 1085 nm and roll off a little too fast in the longer wavelengths. These measurements were again supplied
by Guy Monnet at ESO. A request was made after fabrication was complete that we optimize for 1240 nm. The 1240
region is only down about 2% even though optimization was done at 1085. W e have a 1310 nm laser to measure with
while doing the tuning so we feel very good about the performance as it is.
T h e w a v e f r o n t w a s
i n a d v e r t e n t l y n o t
m e a s u r e d p r i o r t o
shipping. We can only
assume that it was about
the same as the 574 lp/mm
grating since it was made
with the same optics and
substrates. Occasionally
we see a departure from
the wave or so of error
that is typical over 4
inches so we probably
should have done the
measurements. We still do
not have a good handle on
where all the aberrations
come from. The process of
laminating a cover glass
o v e r t h e g r a t i n g
occasiona lly produces
more aberrations than
would be expected. It also
affects the final index
modulation and thus the
efficiency of the grating
and must be compensated
for in the thicker gratings.
Figure 7. The model of a 720 l/mm grating centered at 1120 nm and the under it the measured performance.
2.6. Subaru 10 inch diameter 385 lp/mm near IR Grating
This grating was too large to be made with our 8-inch set up so we used one 17-inch off axis parabola and one half of
a 22-inch diameter parabola in an odd
configuration to record the 385 lp/mm
grating. The layout we used had no fold
mirrors after the collimators which is a
difficult arrangement to initially align
but it produces gratings with a little
less scatter and no random error from
imperfect flats. This was a very low
spatial frequency but the center
wavelength was also very long so
angles were reasonable. We were not
able to test this grating in a
spectrometer because it was designed
to work at 1.3 microns and our
spectrometer cuts off at 1.1 microns so
we have to be content to measure it at
laser wavelengths of 1545 1310 and
1064 nm. All five plates measured
greater than 90% efficiency at 1310
nm. It is seen here with a fluorescent
light behind it. The full spectral range
extended from 900 to 1800 nm and to
see the wavefront quality we did a
double pass through the grating using
the second order at 633 nm which
approximates the correct angles very
closely. The residual optical power
without flats matched to it was almost
3 waves and with flats it was 2 waves
so most of the power was holographic.
A little more than a half a wave of
astigmatism was also present. The zero
orders were all .25 waves or less with
f l a t s . A s a m p l e 2 n d o r d e r
interferrogram with and without flats is
shown here, the penalty for not using
flats is one more wave of power and
another wave of random error over any
four inch aperture. The performance at
the working wavelength will of course
be 2 to 3 times better than at the test
wavelength of 633 nm. W e did this
complete order twice but never capped
the first batch because it lacked
uniformity.
Figure 8. Subaru 385 lp/mm 10 inch grating back illuminated and the wavefront test results.
3. SCATTER AND HOLOGRAPHIC NOISE
Scattered light from the grating itself or reconstructed scattered
light from the recording set up decreases contrast in images
made through the gratings and can also blur focal planes and
add unwanted imagery. It is a continual battle to eliminate both
of these major sources of image degradation. The pictures at
the left were made looking at a screen about 1 meter down
stream from the grating. The grating had a single spot
illuminated with 488 nm laser light and a lens was positioned
to catch scattered light and concentrate it for a quantitative
measure of the scattered light in a fixed cone but these pictures
show some light with definite patterns scattered around the lens
in a larger cone and these are the holographic reconstructions
of objects inadvertently recorded along with the grating fringes.
The scatter was the greatest from the AAO grating pictured at
the top left but it was still around 1% in a f#10 cone. The large
square that appears to be reconstructing is an inline hologram
of the surface of a mirror that had a fairly bad roughness figure
which happens during the coating of aluminum when conditions
are not op timum in the coating chamber. W e had replaced this
mirror with a better one before recording the ESO grating
which is the one in the middle picture where just a faint image
of a mirror appears. We re-coated the same mirror ourselves
and were careful to wait till the pressure was low enough before
starting the evaporation of the aluminum, which always seems
to help to make the coating smoother. No steering mirrors were
used in the Subaru recording set up so the scatter picture at the
bottom has the lowest of all scatter figures. It also had the
lowest scatter in the f#10 cone and was well under 1%.
We have always used two rules to eliminate spurious
recordings of our setups and they always work when strictly
adhered to. We first look back through the plate holder position
and identify any and all glints seen from hard or stable objects
and we place floppy plastic masks in strategic positions to
block all the glints. The masks continue in motion just
enough to avoid being recorded themselves during an exposure.
The things that can never be masked are the mirrors and lens
surfaces used to collimate or steer the beams so the absolute
best recording configuration would be two coherent beams
from point sources perhaps 50 meters away from the recording
plane. That is usually not an option for many reasons but it
could be done if reduction of the scatter were to be the highest
priority. As near as we can tell, all real optical surfaces scatter
some light, it is unavoidable.
Figure 9. Scattered light patterns and the energy gathered in a f#10
cone for all three astronomy gratings.
4. NASA MULTIPLEXED GRATINGS
Most of our work for NASA GSFC since 1990 has been gratings with optical power and most have been slanted
gratings. The sizes of the realized gratings run from 200 to 400 mm in diameter but NASA would like 1 meter in diameter
if they could get it. The wavelength range runs from 2.2 microns to 355 nm and we have made them all. The biggest
difference between what we have done for NASA and what we have been asked to do for the Astronomy community is
the quality of the wavefront. Lidar applications can require diffraction limited performance if it is coherent Lidar but if
it is not coherent then even 1 milliradian of error is useful and none of the requirements have exceeded 50 micro-radians,
which are several waves of error. The atmospheric turbulence is sometimes the limiting parameter and other times the
signal to noise ratio is being boosted by looking at a narrower piece of the sky.
We have had to multiplex some of the plane gratings made for NASA because we have no way to record an accurate
pattern larger than 250 mm. The 400 mm gratings were made by first making a 200-mm master grating and then fixturing
the master so that it could be aligned four times over a single larger plate. The alignment was done by mounting a mirror
on one edge of the master grating and then positioning an auto collimator with 5 arc second calibration marks in the
reticle on a plate holder. The master was moved to each location on the sensitized plate and oriented with a micro mover
till the autocollimator registered in visual alignment, then a shot was made and it was moved to the next location. The
multiplex played back as one grating and was tested by illuminating the core drilled center section with a single
collimated beam and then viewing the diffracted light out some 100 feet to a screen where displacements of the
quadrants could be observed and measured. The spacings at these corners changed by 1 to 2 mm indicating errors on the
order of 50 micro-radians or about 10 seconds of arc and were acceptable for LIDAR. Improvements in alignment and
testing are contemplated for astronomy applications.
Figure 10. A 400-mm dia multiplexed and slanted grating showing the four separate exposure lines and differences in efficiency
5. POSSIBLE METER SIZED GRATINGSWe have some experience with a meter sized grating fabrication
but have not as yet made one. We have coated six 36 inch
diameter plates with DCG and we have an obstructed 36 inch
collimator and an obstructed 36 inch plate holder as well. It
would be possible to make a grating of this size with a hole in
the middle if we simply had a second 36 inch parabola from
which to derive a second collimated wave. Multiplexing can
also be done. The angular match between exposures can be
made based on a purely mechanical measurement but the phase
can only be matched if the previous recording can be
reconstructed and interfered with the current exposure and this
cannot be done easily. One way to do it would be to record an
overlapping grating on the back side of the glass and to
subsequently use that initial grating to feed a signal to a fringe
locker that controlled a micro-mover attached to the master.
Figure 11. Author holding 36 inch DCG coated plate
In the visible region a Dickson grating can be made if the angles
can approach 47 degrees. A model of a 3000 lp /mm grating is
shown where it is seen that both polarizations peak at the same
wavelength. The non Dickson equivalent can only diffract half as
much polarized incident light. This frequency can be recorded in
a number of ways but those options get scarce as the size goes up.
We have a single large collimator that can be used to derive bo th
collimated waves but it has to be used in a certain way to get the
best possible wavefront. A method for getting straight fringes
from symmetrically aberrated wavefronts is shown below.
Figure 12. Efficiency plot for a 3000 lp/mm Dickson grating.
A single shot 400 mm
diameter record ing
g e o met ry tha t i s
currently possible in
our lab is shown in
Figure 13. It shows an
exposure layout using
only one fold mirror to
fold the errors on one
side of a circularly
symmetric parabola
onto the other side, so
that small symmetric
errors cancel and form
straight fringes.
Figure 13. A 400 mm diameter recording layout using only two mirrors, most useful for high frequencies.
ACKNOWLEDGMENTS
We want to thank the following people and organizations for contributing data and for starting us down the path of large
area precision VP H gratings with their trial orders and great patience.
1. Sam Barden of AURA for inviting us to make trial gratings and pointing potential customers our way.
2. Christopher Clemens of the University of North Carolina for assorted orders, published evaluating a UV grating and
being patient as we made some very low frequency mediocre gratings on his expensive substrates.
3. Ivan Baldry, Will Saunders, Karl Glazebrook and o thers associated with AAO and Johns Hopkins U niversity for their
many requests and excellent feedback on good and mediocre gratings.
4. Guy Monnet of ESO for near infinite patience waiting for flat glass and a second chance on a botched order to get it
right and for excellent experimental data for this paper.
5. Marsha Wolfe and Gary Hill with the University of Texas at Austin for excellent models and orders of a variety of
gratings, a visit and some feedback.
6. Emilio Molinari and Claudio Pernechele of the Osservatorio Astronomico di Brera and di Padova for tolerating a
completely lost shipment and waiting a month for a replacement and for some published feedback.
7. Robert Rallison of Ralcon for fabricating each grating measuring over 100 mm on a side or diameter and for making
steady improvement in quality with each new batch.
REFERENCES
1. T. K. Gaylord and M. G. Moharam, "Analysis and applications of Optical Diffraction by Gratings" Proc. of IEEE,
Vol. 73, No. 5, M ay 1985.
2. H. Kogelnik, "Coupled wave theory for thick hologram gratings" Bell Syst Tech J. vol. 48, p2909-47 (1969)
3. R. D . Rallison "Using Thick DCG, 30 to 100 microns" SPIE vol. 1914, Practical Holography VII, (1993).
4. L. D. Dickson, R. D. Rallison, "Holographic Polarization-Separation Elements" Appl. Opt. vol. 33, No 23, p. 5378-
5385, 10 Aug 1994
5. R D Rallison "Polarization properties of gelatin holograms" SPIE vo l. 1667, Practical Holography VI, (1992)
Notice of Reorganization:
R. D. Rallison started making gratings by interference techniques in 1971, Ralcon Development Lab has specialized
in prototype Holographic optics for 20 years. This is the last year of Ralcon prototype construction, Ralcon is being
folded into Wasatch Photonics as this paper is being written and will cease doing business as Ralcon at the end of 2002.
Wasatch Photonics will carry on with production of commercial quality gratings as long as a demand can be found. The
Wasatch team of owners has the proper orientation and skills to excel at grating fabrication well beyond the sometimes
crude prototypes fabricated in the makeshift labs at Ralcon.
Some Recent improvements and new Processes:The results of a applying a post polish to the zero order only of a well made grating is impressive. It is simply
making the whole sandwich into a near perfect widow by cancelling all the phase errors in the volume. The
result is that the first order is vastly improved as w ell because the paths are close when thin glass is used .
The method of correction is clever and simple. A wet etch fountain of weak HF acid is scanned under one
grating surface while an interferometer measures path lengths in real time so that when all path lengths are
equal over the whole window then the window adds no aberrations to the grating. This is a technique worked
out at Lawrence Livermore National Lab and is available for licensing and is practiced on meter size windows
as well as centimeter size with equal performance. We plan to use it for precision gratings in the future,
assuming there is a demand for the same. See http://www.llnl.gov/nif/lst /diffractive-optics/newtechwet.html
Cryogenic operation:
Recent testing by Italian astronomers and others has shown that DCG can withstand a few -40 to -60 C
freezing cycles without coming apart or suffering much degradation. The modulation drops, probably in a
predictable repeatable fashion and so if operation at these low temperatures is required then the grating needs
to be over m odulated at room temp. This is experimental at the moment but seem s completely reasonable.
UV operation:Operation down to 350 nm is demonstrated in 5 micron gel at spatial frequencies above 1000 lp/mm with
90%T, see http://www.noao.edu/ets/vpgratings/papers/clemensreport.pdf
Data taken on part #940 FS 05 at Ralcon and at
LLNL before and after WEF (wet etch figuring)
1st order before Wef
Power: 0.3638 waves
Astigmatism: 0.4189 waves @ -8.97°
Coma: 0.1543 waves @ 26.32°
Spherical: 0.0147 waves
P-V: 0.6166 waves
RMS: 0.0945 waves
Transmitted Wavefront Statistics
0 order data after WEF
Pow er: 0.0497 waves
Astigmatism: 0.0825 waves @ -15.32/
Com a: 0.0304 waves @ -96.60/
Spherical: 0.0076 waves
P-V: 0.0940 waves (size 11 mm dia)
RM S: 0.0182 waves
1st order data after Wef
Pow er: 0.0122 waves
Astigmatism: 0.1237 waves @ 26.49/
Com a: 0.0401 waves @ -103.04/
Spherical: -0.0104 waves
P-V: 0.1437 waves
RM S: 0.0203 waves
Scissorjack / Spring model of DCG
modulation1. The jacks and springs are assembled in the first panel by chromate ions(stick figures) and blue photons
2. Modulation is small before any processing but is measurable, density ishigh everywhere, low modulation
3. Water has been displaced by alcohol while the gelatin was in a swollenstate, lower density, increased modulation
4. The dry layer grows less dense between fringes as thickness increases,springs may break, maximum modulation
Recommended