Evaluation of dielectric film beam splitters atcryogenic temperature

Harold P. Larson

The stability of self-supporting dielectric film beam splitters at cryogenic temperature was studied with a test

interferometer that allowed continuous visual monitoring of the surface figure. Test beam splitters using

Mylar film remained flat to within a wavelength of visible light in a 3-cm diam central area during multiple cy-

cles between room temperature and -4 K. Factors that could affect the surface figure, such as assembly

procedures and the mechanical design of the film holder, were studied to determine some of the conditions foremploying Mylar beam splitters in cryogenic space instrumentation.

1. Introduction

Beam splitters fabricated from self-supporting di-electric films are suitable for use in Fourier spectrom-eters at far-IR wavelengths.' One commonly usedmaterial is polyethylene terephthalate, a polyestermarketed under the trade name Mylar in the U.S.A.Its efficiency as a beam splitter is proportional to RT,where R is the reflectance and T is the transmission ofthe film. This efficiency alternates between 0% and amaximum value because of interference between thetwo principal reflected components. Peak efficiencyoccurs at wavelengths X = 2d(n 2 - 1/2)'1 2/m, where d isthe film thickness, n is the index of refraction, m is (1/2, 3/2, 5/2, ... ), and the angle of incidence is 450.Efficiency minima occur at wavelengths correspond-ing to integer values of m (0,1,2.... ). Normally, thefilm thickness is chosen to place the spectrometer'sbandpass in the first order. Thus, for d = 6 ,um, thebeam-splitter efficiency peaks at 245 cm-' (41 Am) andfalls to 0% at 0 and 490 cm-' (20.5 ,gm). The same filmcould also be used in higher orders for shorter-wave-length operation, but absorption in the film and sur-face figure limitations usually restrict this type ofbeam splitter to far-IR applications.

Although the optical properties of Mylar are wellunderstood, questions persist regarding its limits as abeam splitter in cryogenic astronomical instrumenta-tion where low light levels require extra attention to

The author is with University of Arizona, Lunar & PlanetaryLaboratory, Tucson, Arizona 85721.

Received 27 January 1986.0003-6935/86/121917-05$02.00/0.© 1986 Optical Society of America.

optical efficiency. For example, the mechanical sta-bility of the stretched sheet at cryogenic temperature,not the bulk optical properties of the material itself,can set the performance limits of real instruments.Knowledge of these limits is, therefore, important tothe design of IR spectrometers being considered foruse at high altitude or earth-orbiting telescope facili-ties. Their remote operation and high cost requirepredictable instrument performance that remainsclose to theoretical limits. This paper concerns theevaluation of a Mylar beam splitter design intendedfor use in a Shuttle-borne IR spectrometer cooled toliquid helium temperature. Some of the requirementsexceed the traditional applications of Mylar beamsplitters: operation at very short wavelengths (5 Am);use of a noncircular film holder; and the ability tosurvive temperature cycling without major realign-ment or disassembly of the spectrometer. The resultsof these tests should be of general interest to othercryogenic applications of free-standing dielectricbeam splitters.

II. Description of the Project

A. GIRL Spectrometer

A helium-cooled (1.8 K) Fourier spectrometer wasselected as part of the instrument complement for theGerman Infrared Laboratory (GIRL), 2 a 50-cm cryo-genic telescope to be flown on Spacelab in the late1980s. The spectrometer would provide high spectralresolution (0.015 cm-1) and broad spectral bandwidth(20-200 rm) using a Mylar beam splitter.3 A filmthickness of -6 ,um readily met the spectral bandpassrequirement, but the beam splitter also had to satisfysome unusual demands. First, the spectrometer's ref-erence channel, which shared the Mylar beam splitter,used a diode laser at 5 tm. The reference wavelengthcoincides with one of the higher-order efficiency maxi-ma of the 6-jrm film, thus allowing simultaneous opera-

15 June 1986 / Vol. 25, No. 12 / APPLIED OPTICS 1917

tion in a broadband far IR channel and in a narrow-band channel at 5 Aim. The latter mode represents avery short wavelength for the interferometric use ofMylar under any circumstances, and a cryogenic spaceenvironment presents special problems. To meet thisrequirement the Mylar film must remain flat to abouta wavelength of visible light, at least over the smallarea used by the laser beam, to satisfy a X/10 surfacefigure tolerance at 5 jim. Second, the compact spec-trometer design forced the Mylar film holder to benoncircular; its shape was more like a stretched letter0. This geometry meant that effects of differentialcontraction on film flatness during temperature cy-cling could adversely affect the spectrometer's opticalperformance. The GIRL beam splitter design was,therefore, made the subject of this study to determinesome of the limits to which self-supporting Mylar filmcould be used for beam splitters in the near-IR at 2 K.

B. Experimental Procedure

One way to evaluate a beam splitter design is toincorporate it into a working spectrometer and thenuse actual spectra to diagnose performance. This ap-proach has the advantage of defining overall systemresponse, but if more than one problem occurs, it maynot be possible to isolate beam-splitter deficienciesfrom other effects. To avoid such confusion the GIRLbeam splitter was evaluated as a discrete component ina special test interferometer. The experimental ar-rangement is illustrated in Fig. 1. The beam splitterto be evaluated was mounted inside a cryostat where itconstituted one plane mirror of a conventional Michel-son interferometer. Other components of the test in-terferometer were located outside the cryostat. Thesurface figure of the test beam splitter was continuous-ly monitored during temperature cycling by observingthe two-beam interference pattern in visible laser light(Xref = 0.633 Am). The test interferometer was kept inoptimum alignment at all times by adjustment of theroom temperature components. The wave-front dis-tortion in the test interferometer was measured bysubstituting an optically flat mirror for the test beamsplitter; the field was flat to -Xref/10 over the 3-cmdiam test aperture. Thus any changes larger than-Xref/10 in the fringe pattern during testing could bereliably attributed to the surface figure of the Mylarfilm alone.

C. Assembly ProcedureThe samples of Mylar used in this study were from

1.5 to 25 Am thick. In general, the thicker sampleswere easier to work with and gave somewhat betterresults. The tests reported here used 6-jim film, thethickness specified for the GIRL spectrometer.

Part of the evaluation included the investigation ofassembly procedures and properties of the metal filmholder that could influence the optical quality of thebeam splitter. The holder is illustrated in Fig. 2. TheMylar sheet was first clamped between two rings A andB. The metal surfaces in contact with the film wereground flat on a polishing stone to provide a good grip.Consistently better beam splitters resulted when the

Fig. 1. Schematic illustration of the test interferometer used tomonitor the surface figure of Mylar beam splitters at low tempera-ture. The observed interference pattern maps the surface of the testbeam splitter which serves as one of the mirrors in the two-beam

interferometer.

Fig. 2. Cross section of the test beam splitter. The Mylar film isstretched over the aperture D by clamping rings. Tension is applied

to the film by retaining screws.

Mylar was gently prestretched during installation be-tween the retaining rings. The criterion adopted wasthat the Mylar surface should show no ripple afterrings A and B were clamped together. This was ac-complished by gentle radial stretching of the Mylarsheet during installation. The ring assembly with theprestretched film was then attached to the beam-split-ter holder, C in Fig. 2. The film bearing surface on Cwas ground flat to less than a micron, but it was notgiven an optical polish. Final tension was applied tothe film by the retaining screws that held the ringassembly to C. The surface figure was surprisinglyindependent of the applied tension, so no effort wasmade to calibrate this operation. However, if the filmwas stretched too loosely, it became very susceptible toacoustic pickup and other mechanical perturbations.

The film was typically flat after assembly to 3 - 4 Xrefin a 3-cm diam central zone [see Fig. 3(b)]. A signifi-cant improvement in its surface figure resulted fromheat treatment. The beam-splitter assembly wasplaced in an oven at 140-150OC for 1.5 h. It wassometimes necessary to increase the tension on thefilm slightly while the beam splitter was cooling. Oncooling to room temperature, the surface figure usuallyimproved by at least a factor of 2 [see Fig. 3(a)]. Thisimprovement was preserved during subsequent cryo-genic cycling of the beam splitter.

1918 APPLIED OPTICS / Vol. 25, No. 12 / 15 June 1986

Fig. 3. Effect of heat treatment on the surface figure of Mylar beam

splitters: (a) surface figure after heating at '150'C for 1.5 h; (b)

typical flatness after initial assembly of beam splitter. Each brightinterference fringe represents a change by ref = 0.633 m in optical

path or Xref/2 in surface figure. The stressed film in (b) has relaxed

substantially in (a) following the heat treatment.

Ill. Test Results

A. Cryogenic Performance

A very useful feature of the experiment was continu-ous visual monitoring of the test beam splitter duringtemperature cycling. In this way problems could beassociated with a specific time and temperature duringthe test. All beam-splitter failures occurred relativelysoon after cooldown started. The symptom was moreor less sudden loss of interference fringes. Direct visu-al examination usually revealed obvious ripple in thefilm, presumably due to the effects of differential con-traction. Usually, the interference fringes would re-appear on warming up to room temperature, an excel-lent demonstration of the inadequacy of roomtemperature tests for diagnosing cryogenic operation.

Excerpts from one test sequence are presented inFig. 4. The interference patterns record the surfacefigure of the Mylar film during temperature cyclingbetween room temperature, 77 and 4.5 K. A circularholder was used for this sequence to avoid introducingsurface figure distortions by geometric factors. Notethat the clear aperture of the test holder was muchlarger (7-cm diameter) than the central test area (3-cmdiameter).

The sequence begins with a fresh heat-treated filmat room temperature [Fig. 4(a)]. The surface is flat tobetter than Xref over the 3-cm diam test aperture. InFig. 4(b) the surface figure of the test beam splitter isshown at 77 K. Some changes have occurred, but itremains flat to about Xref over most of the test aperture.In general, if the film survived the first phase of thecooldown cycle to 77 K, its figure changed very little at4 K. At liquid helium temperature [Fig. 4(c)] thesurface figure is flat to about Xref over at least a 2-cmdiam area. This fringe pattern convincingly demon-strates that Mylar film at liquid helium temperaturewould easily meet the GIRL spectrometer's require-ment for monochromatic operation at 5 jim.

After being warmed to room temperature [Fig. 4(d)],the surface figure closely resembles the original in Fig.

Fig. 4. Behavior of a Mylar test beam splitter during temperaturecycling: (a) initial heat-treated film at room temperature; (b) 77 K;

(c) 4.5 K; (d) room temperature at the end of the first cycle; (e) room

temperature at the end of the second cycle; (f) room temperature atthe end of the third cycle. Small changes occur to the surface figure,

but a central area flat to Xref or better over at least a 2-cm diameter

was preserved at all times during the tests including periods of rapidtemperature change. The surface returned to a predictable figure atroom temperature after each cycle, demonstrating that Mylar beamsplitters could be used reliably in remote cyrogenic instrumentation.

All temperatures were measured on the film holder.

4(a). This meant that the film had not been stretchedbeyond its elastic limits. This test beam splitter wasthen temperature cycled two more times. The surfacefigures on return to room temperature are presented inFigs. 4(e) and (f). Their identical appearance withthat in Fig. 4(d) demonstrates that no cumulative dete-rioration occurred. These three cycles required morethan two weeks to complete, and there was no evidenceto suggest that the cycles could not have been contin-ued many more times.

B. Other Results

Several useful insights concerning beam-splitter de-sign and assembly were acquired during these tests.Mention was already made of the dependence of sur-face figure on film tension: prestretching in the re-taining ring was important, but the surface figure ofthe assembled beam splitter was not very sensitive to

15 June 1986 / Vol. 25, No. 12 / APPLIED OPTICS 1919

the applied tension. The coefficient of linear expan-sion of Mylar is larger than that of most metals, so theapplied tension would increase at low temperature.Both steel and aluminum test holders were used withno significant differences in their optical properties.This suggested that as long as the Mylar was notstretched beyond its elastic limits, considerable con-traction or expansion in the holder could be accommo-dated by the film without severe degradation of thesurface figure.

The width of the Mylar contact surface (dimension din Fig. 2) might affect the surface figure. Certainly,grinding this surface to interferometric tolerances isimportant for it defines the plane in which the film isstretched. However, beam splitters of comparable op-tical quality were produced with both small (1-mm)and large (5-mm) values of d. This result was impor-tant to the GIRL beam-splitter design, since its dimen-sion d varied from 1.5 to 10 mm because of externaldimensional constraints.

It was noted during this evaluation that, within thecentral 3-cm diam test area, holders with differentclear apertures (dimension D in Fig. 2) yielded beamsplitters of significantly different optical quality inspite of identical assembly techniques. In general,better surfaces were achieved with larger holders. Nodifferences between the various test holders other thandimension D could be implicated. It, therefore,seemed that the best beam splitters were simply over-sized, a condition which apparently allowed relaxationto occur in the film. If the small number of holdersused in this study constitutes a reliable guide, thediameter of the central zone in which the surface is flatto Xref or better is less than half of the clear aperture D.However, this relationship needs to be studied moresystematically.

C. GIRL Design

Temperature cycling tests were also conducted withthe GIRL prototype beam splitter to study the effecton surface figure of its one unusual feature: a clearaperture (dimension D in Fig. 2) that varied from 7.0 to9.7 cm. Its other characteristics were similar to thosestudied independently with circular test holders. Ingeneral, the GIRL beam splitter was trouble prone. Itfailed occasionally on cycling to 77 K, and when it didsurvive the surface figure was usually inferior to thatachieved with the other test holders. Excerpts fromthe best test sequence are presented in Fig. 5. Theinitial heat-treated film [Fig. 5(a)] was flat to Xref overan -1.5-cm diam central area. At 77 K [Fig. 5(b)] theinterference pattern changed, although the centralarea was still sufficiently flat to satisfy the require-ment for monochromatic operation of the referencechannel at 5 jim. After warming to room temperature[Fig. 5(c)] the central area remaining flat to Xref orbetter was definitely smaller than at the beginning.These changes were attributed to differential contrac-tion that introduced radially asymmetric forces in thefilm. This tendency could have led to marginal opticalperformance of the GIRL spectrometer. To avoid this

Fig. 5. Behavior of the prototype Mylar beam splitter for the GIRLspectrometer: (a) initial heat-treated surface at room temperature;(b) 77 K; (c) room temperature at the end of the cycle. This beamsplitter exhibited greater variation in surface figure with tempera-ture change, including complete loss of interference fringes, than didany of the other test beam splitters. This difference was attributedto effects of differential contraction in the noncircular film holder

for the GIRL spectrometer.

possibility, an oversized circular holder could havebeen adopted for the GIRL application. However, apotentially even more satisfactory approach would re-quire only small changes to the prototype design. Themodification is based on test results indicating that theforce used to stretch the film can vary over a broadrange, but it must remain radially symmetric to pre-serve a good surface. Fixed retaining screws and anoncircular holder do not guarantee this conditionduring temperature cycling. However, if the retainingscrews were replaced with spring-loaded fasteners, it islikely that the film would remain more uniformlystretched during cryogenic operation in spite of differ-ential contraction, thus improving the short-wave-length performance limits of the GIRL spectrometer.Implementation and evaluation of this modificationwere beyond the scope of this study, but one possibleapproach is illustrated in Ref. 4.

IV. Conclusions

The stability of self-supporting dielectric film beamsplitters at low temperature was evaluated for possibleuse in cryogenic space instrumentation. The testsdemonstrated that Mylar beam splitters provide inter-ferometrically flat surfaces at short wavelengths,which are stable over multiple cycles between roomtemperature and 4 K. Some important design andassembly considerations include use of oversize circu-lar holders, prestretching of the film during assembly,and heat treatment of the beam splitter after assem-bly. The use of noncircular holders could adversely

1920 APPLIED OPTICS / Vol. 25, No. 12 / 15 June 1986

affect spectrometer performance at any wavelengthunless special attention is given to the film-stretchingmechanism of the holder.

This study was conducted at the Max Planck Insti-tut fur extraterrestrische Physik (Garching) while theauthor was on sabbatical leave. He thanks the Alex-ander von Humboldt Foundation (Bonn) for a SeniorU.S. Scientist Award that supported his participationin this project. Also he thanks his host, SiegfriedDrapatz, and other members of the MPI IR group fortheir cordial welcome and generous assistancethroughout his stay.

References

1. R. J. Bell, Introductory Fourier Transform Spectroscopy (Aca-demic, New York, 1972).

2. D. Lemke, M. Grewing, P. Preussner, W. Martin, D. Offerman, G.Lange, S. Drapatz, R. Katterloher, H. Denner, G. Klipping, F.Dahl, and K. Proetel, "The German Infrared Laboratory (GIRL)-A Progress Report," Adv. Space Res. 5, 11 (1985).

3. S. Drapatz, R. Hofmann, and R. Katterloher, "A Helium CooledMichelson Interferometer for Far Infrared Astronomy on Space-lab," in Proceedings, Eighth International Conference on Infra-red and Millimeter Waves, Miami (1983), paper TH5.2.

4. A. Rudman, "Cooled Beamsplitters Require Special Mounting,"Lasers Appl. IV, No. 12, 81 (1985).

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