Improvements of a spatial frequency analyzer forautomated characterization of holographicrecording materials

Jean J. A. Couture and Donald Tanguay

A spatial frequency analyzer was designed to simplify characterization studies for new holographicrecording materials. Mechanical movements were automated and a complete informational system gaverapid characterization results. A good fringe stabilization unit was improved by adding simple holographicoptical beam combiners. Experimental characterization of two different recording materials shows theversatility of this automated apparatus. Also we present modulation-transfer-function curves ofdichromated gelatin between 500 and 3500 cycles/mm obtained with polarization volume transmissionholograms.

1. IntroductionHolographic characterization studies and four-wavemixing experiments require high mechanical stabilityand versatile experimental arrangements when per-formed with visible cw laser lines. To simplify theseexperiments, a spatial frequency analyzer was de-signed by Maksymik.' This special setup was im-proved by Couture and Lessard 2 and by Tanguay togive rapid characterization results and MTF curvesfor new real-time and classical recording materials.

Our present research involves rapid characeriza-tion of new photopolymer recording materials andtheir applications. In particular we are studyingholographic optical element (HOE) fabrication andreal-time holographic interferometry techniques thatcan be used for industrial quality control. To improvethese techniques, we have considered in our holo-graphic recording materials research the preparationof new dye-polymer systems used in conjunction withwell-known materials such as dichromated gelatinfilms. Their characterizations imply experimentalstudies of parameters such as dye and polymer concen-trations, holographic sensitometry, signal-to-noiseratios, angular selectivity, chromatic selectivity, mod-ulation transfer function curves, effective thickness,and modulation parameter measurements. These

The authors are with the Centre en Optique, Photonique etLaser, D6partement de physique, Universit6 Laval, Cit6 Universi-taire, Quebec GlK 7P4, Canada.

Received 18 June 1991.0003-6935/92/142499-07$05.00/0.6 1992 Optical Society of America.

quantitative evaluations involve experimental re-search that can be simplified if the basic experimentalsetup is versatile.

The first objective of this paper is to introduce aversatile spatial frequency analyzer that has manyautomated parts and a complete informational sys-tem. The second objective is to describe characteriza-tion experiments that have been improved by usingthis spatial frequency analyzer.

II. Improvements of basic setupThe basic experimental setup is illustrated in theupper part of Fig. 1, and the principle of operation isshown in the lower part of Fig. 1. This specialarrangement has been designed by Maksymyk.1 Essen-tially, this is a two-beam arrangement that permitsholographic recording of volume holograms for aninterbeam angle (20) in the range 14°-160° or aspatial frequency in the range 500-4000 cycles/mmat the 488-nm (blue) wavelength; this angular rangecorresponds to major holographic experiments. Alsothe path-length difference between the two recordingbeams is smaller than ± 5 mm. Its basic versatility isevident when the angular mirror positions keep thepathlength difference smaller than the coherencelength of the laser line used. Hologram recording waseasily performed with this special two-beam arrange-ment. Also, in the reconstruction process, the readingbeam (red) is diffracted by the transmission volumegrating recorded in the studied films. Using thetheory of Kogelnik, 4 we find that the diffractionefficiency of the reconstructed image, by a readingbeam with wavelength A', takes the following value at

10 May 1992 / Vol. 31, No. 14 / APPLIED OPTICS 2499

X4

Fig. 1. Maksymyk's basic mechanical setup.

a Bragg angle

(-2aD[ ih2( a1D i ( 'rn 1DILos0) +c sin) X' os O'j

for a spatially modulated grating that has aibance a and a refractive index n, respecti-,scribed by

a = a + a cos(_x),

n = no + ni cos( 2x)T.

In the above equations, a and n represaverage values of the modulated absorbancEthe refractive index; the modulation absorprameter and the modulation phase parametEand ni, respectively.

Returning to the experimental arrangeme1), we observe that a rotating plateholder peristudy of the angular selectivity responserecording samples. Knowing this angular se'bandwidth of a sample, we can evaluate the thickness as described in a previous paper2

quently, the modulation parameter values (n.of recorded gratings can easily be deduced.2

complete characterization study can improvengineering applications if our new hologracording materials are used; these materialsbe used in four-wave mixing experiments.- 7

Automatization of many mechanical movementswas realized. First, we motorized the rotating move-ments of long mirrors. Because such a movement isloose, the two recording beams do not perfectlyintersect on the same section of the sample, so wemounted our long mirrors on adjustable motor-driven holders. Moreover, the plateholder wasmounted on a motor-driven rotational stage to ensurethe best adjustments for recording beam positionsand to permit the study of the angular selectivityresponse of our samples (see Fig. 2). The basicstabilization setup, illustrated in the upper diagramof Fig. 3, has an electronic stabilization system thatuses the two recording beams, which are attenuatedby the recording sample (H); they travel equal pathlengths as shown in the lower diagram of Fig. 3, andthey strike a beam splitter (AB) and give largeinterference fringes on a photosensor (P), whichdrives an electronic circuit and a movable mirror(MM). A linear polarizer sheet is placed between themicroscope objective and the sensor (P) as illustratedin Fig. 4, so that light components of equal linearpolarizations give high-contrast fringes on the sen-

X:485 sor; this is especially interesting for linear-crossedW nm and circular-left and circular-right polarized record-

9 ing beams.At present, our spatial frequency analyzer is cou-

'12 pled with a complete informational system that cangive MTF and angular selectivity curves with real-time holographic sensitometric parameters such aseffective thickness and modulation, n, and a, respec-tively. Figure 5 shows our automated spatial fre-quency analyzer system. Usually a complete character-

(1) ization study takes one year; with our analyzersystem it can be performed within two months. Our

a absor- spatial frequency analyzer works rapidly, but therely, de- major drawbacks in our recording material research

are the mechanical adjustments required for betterlong-term fringe stabilization and the time needed forpreparing new organic recording materials. In Sec-

(2) tion III we discuss our experiments and results that

(3)

ent thee and oftion pa-

er are a,

nt (Fig.mits theof ourlectivity

effective; conse-and al)Also, a

ve manyphic re-.an also Fig. 2. Motorized plateholder that rapidly gives the angular

selectivity response of the studied organic films.

2500 APPLIED OPTICS / Vol. 31, No. 14 / 10 May 1992

Fig. 3. Equal-path-length stabilization system. H, studied sam-ple; P, photosensor; MM, movable mirror; AB, 50/50 beam splitter.

illustrate the usefulness of our automated spatialfrequency analyzer.

III. Experiments

A. Stability Detection

As indicated in Section II, our analyzer has a simpleoptical stabilization system that works in agreementwith interbeam angle changes. Stable fringe forma-tion is easily obtained, and simple adjustments allowthe production of well-contrasted fringes. To verify

Fig. 4. Working stabilization system.

the setup stability, we recorded real-time transmis-sion holograms with recording films2 prepared with amixture containing an azodye (Methyl Orange), whichwas introduced in a polyvinyl alcohol (PVA) aqueoussolution. Solid films such as Methyl Orange-PVArecord real-time holograms; these films can be erasedand reused through thousands of cycles. Figure 6illustrates two real-time hologram recordings ob-tained with two writing beams that have circular-leftand circular-right polarizations. The dashed curverepresents the recording as performed without astabilization system, and the solid curve representsthe real-time hologram recording when our stabiliza-tion unit was working. We can see that the diffractionefficiency value is increased by a factor of 3 when thestabilization unit is used. Whenever we changed theinterbeam angle (20), we made many simple adjust-ments. The large fringes were then imaged upon thephotosensor, and when the fringes were fixed, weverified the stability of the analyzer setup by usingthe real-time recording test as previously explained.Consequently, this test becomes useful for ensuringexcellent recording conditions.

We also studied the grating growth of anotherazodye, Mordant Yellow 3R (Alizarine Yellow GG,sodium salt), in PVA. Thin solid films of MordantYellow 3R-PVA recorded good real-time polarizationholograms when two beams of the same power thathad linear-crossed polarizations lighted these sam-ples. Figure 7 illustrates the real-time recording(0-240-s) and erasing (240-420-s) processes. In thiscase the power of these two polarized beams is thestudied parameter; we also note that an importantmemory effect (t > 300 s) is observed when the totalpower is higher than 2 mW.

B. Holographic Beam Splitter-CombinersWe now study the spatial frequency response of newrecording materials between 500 and 4000 cycles/mm. To rapidly achieve an excellent stability, at anyinterbeam angle, we used one equal path-length setupand a conventional beam splitter (AB) as illustratedin Fig. 3. Also, to obtain high-contrast fringes thatrapidly give a good setup stability, we constructed aHOE that functions as a holographic beam splitter-combiner. The recording films that were used toconstruct these HOE's were dichromated polyvinylalcohol (DC-PVA) films8; under light exposure, theseDC-PVA films were photocrosslinked and gave adiffraction efficiency value of 18% for two plane-wavebeams with parallel-linear polarizations. Also, theseDC-PVA's do not require chemical development, andare instead given a simple thermal treatment at2000C for 60 s, which produces holograms that have adiffraction efficiency value of 36%. Figure 8 illustratesthe principle of the recording and the use of holo-graphic beam splitter-combiners; our combiner wasconstructed with two different recordings that hadthe same power, PR = P0 = 4.25 mW, in the followingthe sequence: (a) A DC-PVA film was placed on theplateholder of our stabilized spatial frequency ana-

10 May 1992 / Vol. 31, No. 14 / APPLIED OPTICS 2501

Fig. 5. Automated spatial frequency analyzer.

lyzer. (b) A first recording was made with two polar-ized blue (488-mm) beams r and 0 as indicated in Fig.8(a) (total exposure was 433 mJ/cm2 and the record-ing time was t = 10 s). (c) A symmetric hologramrecording was performed with two blue beams r and Ras illustrated in Fig. 8(b); the exposure time was

0.1

0.1

0 0I=,,= I= 20 mW/cm2

Methyl orange/PVAd=30 /,.mc=2xl0- M

I, 4

I I v

60 120 180 240t (sec)

Fig. 6. Real-time recording test results that permit verification ofthe stabilization state of our analyzer.

t = tV/2, or 14 s as required for sequential volumemultiplex holograms,9"0 for a total exposure of 612mJ/cm2 . (d) Our light-exposed DC-PVA film wasthermally treated at 200C for 60 s; such a processgave a completely inactive ammonium dichromatethat allows the use of the recorded holograms at thesame wavelength used for the recording process. (e)We placed our HOE's (combiners) in stabilizationsetup; these HOE's function as a conventional beamsplitter (AB) as shown in Fig. 3, but the two diffractedbeams share a collinear path length along the normalaxis to the HOE front surface and give high contrastfringes that can be detected by the photosensor of thestabilization unit, as illustrated in Fig. 8(c).

77M%)

0,35

I5mW

0,30 10mW |t

020 ai = t N

0 60 120 180 240 300 360 420

t(sec)

Fig. 7. Real-time kinetic grating growth in a Mordant Yellow3R-PVA film. The recording beams have linear-crossed polariza-tions.

2502 APPLIED OPTICS / Vol. 31, No. 14 / 10 May 1992

1000 2000 3000 4000

f (cycles/mm)

Spatial frequency responses (MTF) of dichromated gelatin

C) p r oFig. 8. Holographic recording in a DC-PVA film and use of a beamsplitter-combiner.

We prepared nine beam splitter-combiners formany interbeam angles (14°, 20°, 40°, 600, 100° .... ,140°, and 154°). Large fringes were easily obtained forangles up to 100°, but their contrast decreased slowlyfor larger angles; this effect is in agreement with thespatial frequency response8 of DC-PVA films. TheseHOE's are useful whenever a study of the rapidspatial frequency response of new recording films isrequired; also the number of mechanical adjustmentsto give well-stabilized fringes is greatly reduced.

C. MTF of Dichromated Gelatin Films

When our apparatus was stabilized, we consideredthe spatial frequency analysis of dichromated gelatin(DCG) films of 20-jim thickness." 2 (These films wereobtained from a 4.8 wt. % of gelatin solution preparedin demineralized water; gelatin films were treated bya 1% ammonium dichromate aqueous solution for 10min). After the hologram recording process, theseDCG films were developed by using conventionalwater and isopropanol processes" without baking sothat the thickness of DCG films became 30 jm exceptin the exposed sections. Figure 9 shows the spatialfrequency responses (MTF's) of these DCG films. Thefirst response was measured when the two recordingsshared the same linear-parallel polarization ( f ),and the second MTF curve corresponds to recordingsrealized with two beams that had circular-left andcircular-right polarizations. Note that the spatialfrequency responses are nearly the same for thesetwo polarization modes except at 500 cycles/mm;

however, the maximum diffraction efficiency valueswere both 42% for linear-parallel and circular-left andcircular-right polarizations. As explained by Solanoand Lessard" and Solano,"3 one result of chemicaldevelopment of recorded gratings is the loss of result-ant linear polarization information. Consequently,polarization-recorded holograms in DCG films have aspatial frequency response similar to that of conven-tionally recorded holograms obtained with linear-parallel polarization. Present MTF curves show thatthe limiting resolution is higher than 4000 cycles/mm; this is better than earlier published""'6 results.However, Chang 7 attributed an almost uniform spa-tial frequency response over the spatial frequencyrange 100-5000 lines/mm without any graphicalMTF results to the domain 2000-5000 cycles/mm.Our MTF curves show that DCG films display aslowly decreasing uniform response up to 3500 cycles/mm, and they complete the curves presented byChang and Leonard.' 8 After many verifications, thetwo experimental values between 3500 and 4000cycles/mm (Fig. 9) were not good because the twowriting beams were not completely overlapped duringthe recording process at these wide-angle values.However, we did not take into account a correctionfactor for these two experimental values, because weknow that the MTF response of our DCG films isdescribed by a low-gradient, downward-sloped curve,shown in Fig. 9. Moreover, many parameters such asdichromate and gelatin concentrations can optimizethe spatial frequency responses and the diffractionefficiency values; these parameter effects can be easilystudied with our automated spatial frequency ana-lyzer. Similar methods become of primary importancein studying holographic characterization for newlinear and nonlinear dye-polymer systems that would

10 May 1992 / Vol. 31, No. 14 / APPLIED OPTICS 2503

itI1,0

0,80,6

0,4A)

B)

0,2 F

0,1

Fig. 9.films.

I

E (mJ/cm2)Fig. 10. Holographic sensitometry curve for DCG films studied at488 nm.

make feasible a great number of holographic andfour-wave mixing applications.

D. Holographic Sensitometry and Effective ThicknessDCG Films

In this study the interbeam angle was 300. Weinvestigated the dependence of the diffraction effi-ciency level on the total light exposure used forhologram recording in DCG films at 488 nm. Figure10 shows all of the results, which are in good agree-ment with those obtained at 514.5 nm by Lin. 9 Ourexperimental values were readily obtained with anapparatus that has a good fringe stabilization systemthat can be verified at any time by the azo-plate testas indicated in Fig. 6.

In a final experiment, we studied the angularselectivity response of a DCG film. Figure 11 shows atypical angular response. The half-width of 3.80 corre-sponds to an effective thickness of 19.4 m. This

'77

\ DCG

(R -- s 15')

Fig. 11. Angular selectivity response of a DCG fim at 488 nm.

value is less than the original 25 .im determined witha Sloan Dektak II apparatus. We also deduced thatthe holographic grating fringes have been well re-corded inside the depth of the DCG film studied.

IV. ConclusionIn this paper we introduced a new and versatilespatial frequency analyzer that greatly simplifiescharacterization studies of recording films. Presently,many efforts are applied to increase the spatial fre-quency range (wide-angle studies) and diffractionefficiency levels of new recording films. Moreover, ourautomated apparatus permits the study of real-timeand permanent grating applications, such as HOEfabrication. The two major drawbacks are the timerequired to fix the fringes on the photosensor of thestabilization system and the preparation conditionsrequired of new dye-polymer systems. The versatilityof the present analyzer was readily demonstrated bycharacterization experiments with two kinds of record-ing materials.

This research was supported by the Natural Sci-ences and Engineering Research Council of Canadaunder grant CRSNG-A0360 and by the Gouverne-ment du Qu6bec under grants FCAR-89AS-2429 andFCAR-91-AR-0905.

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comme milieu enregistreur en holographie," M. S. thesis(Universite Laval, Sainte-Foy, Quebec, Canada, 1987).

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