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