folded optical delay lines - uta › rfmems › teralum_old › a2.pdf · a lissajous pattern of...

Folded Optical Delay Lines

Donald R. Herriott and Harry J. Schulte

A long optical path has been folded between two 7.5-cm diam spherical or aspherical mirrors to provide

an output beam which can be well separated from previous reflections with 1000 or more passes betweenthe mirrors. The 3000-m path provides 10 psec of delay. This system can be used as a dispersionlessoptical delay line for use in filtering or storage of information modulated onto the light beam. The pat-tern of beams between the two mirrors is obtained in one of two ways. A small perturbing mirror may

be inserted to give a series of offset ellipses, or one or both of the mirrors can be made astigmatic to give

a Lissajous pattern of spots on each mirror. The output beam can be separated from others by dis-

criminating in both angle and position. The diffraction losses of the system are much lower than those

for an open beam because of the periodic focusing of the spherical mirrors. The extreme dependence ofthe loss of the delay line upon the absorption and scattering loss of the mirrors makes the system de-

pendent upon very low loss mirrors and also makes the system a suitable method for measuring mirrorloss. Block diagrams are shown for some possible filtering and storage applications.

IntroductionLong optical paths for delay line or storage purposes

can be made compact by reflecting the beam repeatedlybetween two spherical mirrors without interferencebetween adjacent beams. The converging power ofthe spherical mirrors markedly reduces the diffractionlosses.' A previous paper2 has described such off-axispaths between spherical mirrors. These paths in gen-eral lie on hyperboloids having elliptical intersectionswith the spherical mirrors. The light rays are reflectedalternately by the mirrors to successive spots alongthese ellipses.

This paper describes the use of either slightly astig-matic mirrors to cause the spot to follow a Lissajouspattern, or a small perturbing mirror to cause the ellipseon each spherical mirror to shift and follow a set of newellipses each slightly different from the previous one.This allows a much larger number of passes to be in-cluded in a given mirror area with clear separation ofthe input and output spots.

Long optical paths are obtained in a small volume tomake optical delay lines for storage, filtering, or otherlogic and signal processing use. Proposed applicationsof the optical system are discussed at the end of thepaper.

Off-Axis Paths in Spherical Mirrors

In a previous paper,2 a system of off-axis paths hasbeen used to get long optical paths folded between two

The authors are with Bell Telephone Laboratories, Incorpo-rated, Murray Hill, New Jersey.

Received 26 April 1965.

spherical mirrors. The light is injected through onemirror at a proper angle and position, and is reflectedback and forth between the two mirrors in a mannersuch that it traces out elliptical paths on the mirrors asshown in Fig. 1. After many reflections, the beam canbe allowed to leave the cavity. The shape and locationof the ellipses as well as the spacing of the reflectingpoints along the ellipses are determined by the radius ofcurvature of the two mirrors, their spacing, and theangle and position of injection of the ray into the cavityas shown in Fig. 2.

If the injected ray has a wavefront radius and spotsize that corresponds closely to that of resonant modesbetween the mirrors, the diameter of the ray will notgrow even after many reflections and the diffractionlosses will be small.

The maximum delay that is possible by this methodis limited by the spacing of the mirrors and number ofspots of this finite size that can be spaced around anellipse within the diameter of the mirror without over-lapping spots. With present technology, mirror reflec-tance as such is not a limiting factor by an order ofmagnitude or more in the single elliptical path delayline.

Methods of Obtaining Increased Path Length

Pertu rbers

The maximum path length can be increased by allow-ing the beam to be reflected from the spherical mirrorat all but the last spot around one ellipse, then fall ontoa small mirror at the last spot as shown in Fig. 3, tostart the beam on a new elliptical path as shown inFig. 4. The beam will follow this new elliptical path

August 1965 / Vol. 4, No. 8 / APPLIED OPTICS 883

-A

Fig. 1. Pattern of reflections on one mirror of spherical mirrordelay line.

/-,R ) n D-

Y Oy

Y= Cos y n

where optical amplification is used with an integralisolator, modulator, and detector mounted some placealong the path.

Astigmatic Mirror PatternsA second way of obtaining a modified path is to dis-

tort one or both spherical mirrors so that they are astig-matic and have a cylindrical component.

The path of the intersections on a mirror can then befound by independently considering the spot positionin the coordinates transverse to the cavity axis, onechosen parallel to the orientation of the astigmaticmirror. With the help of the previous paper2 we have,for the transverse spot coordinates on the nth mirrorintersection after injection,

x = Asin(nOx + ),

Yn = Bsin(noy + ),

where A, B and a and are constants which includeinjection conditions as well as mirror spacing and focallength. They are unimportant to the present argu-ment. The frequencies O and 0, are given simply by

coso = [1 - (D2fx)],

SPHERICAL MIRRORS

Cos y =Cos Ox=,_ DR

x=Cos (Ox n+(P)

x

Fig. 2. Analysis of pattern in spherical mirror delay line where nis path from injection in units of D.

until it returns to the perturbing mirror which willagain perturb it into yet another ellipse, etc.

It is thus possible to extend the path length as longas the entering spot and exit spot are not overlapped byany other intermediate spots and as long as the apertureof the system is not exceeded. The direction that theperturbing mirror deviates the beam determines theorientation and shape of the new ellipses.

Figure 5 shows a pattern where the perturber deviatesthe beam to a larger ellipse. The beam goes twicearound each ellipse before returning to the perturberand starting into a new ellipse.

Figure 6 shows a pattern where the new ellipses aresmaller than the original. The ellipse actually de-creases to a line and then grows so that the output beamcan be obtained outside of the original ellipse.

Re-entrant Paths

The path can be made re-entrant after a number ofellipses by adjusting the mirrors so that the last dot ofthe last ellipse is superimposed on the entering ray. Asecond small mirror may be placed at that point toreflect the output ray along the path of the input ray.This arrangement may be useful in storage systems

Fig. 3. Cross section of delay line showing perturber mirrorposition.

Fig. 4. Multiple elliptical patterns on mirror with perturbersystem.

884 APPLIED OPTICS / Vol. 4, No. 8 / August 1965

.0 - D

I

J

Fig. 5 (left). Multiple elliptical pattern with perturber tippedfor increasing major axis. Fig. 6 (right). Multiple elliptical

pattern with major axis decreasing through zero.

yk--__Ry

C IETI V

n -p-

Cos &yn

Cos OxnACTIfMTP AAf'

slEaE I 1 I EIITI Iu A,

SPHERICAL MIRRORS

Cos Ox= DRx

n

Cos y 1 DRy

Fig. 7. Analysis of mirror patterns with astigmatic mirrorswhere n is path from injection in units of D.

coso, = [1 - (D/2fy)].

Here D is the mirror spacing and f-, and fy are the focallengths of the mirrors for ray displacements in the x andy directions, respectively. With no cylindrical astig-matism f., = fy, which was the case considered in ref. 2.With astigmatism added, however, the two focal lengthsand hence the two transverse frequencies may be madeto differ by a fraction of a percent. Hence a Lissajouspattern results as shown in Fig. 7. This pattern canbe varied by adjusting the mirror astigmatism to changethe ratio of frequencies and obtain either a simple orcomplex pattern that is either running or re-entrant.Only a small cylindrical component was required toproduce any of the figures shown-a few fringes at most.

The mirrors can have a radius near the confocal caseso that there are only slightly more than two spots percycle of motion in each coordinate, or the mirrors can befar from the confocal case so that there are very closelyspaced spots along the Lissajous path.

The patterns that canbe obtained in the two casesare very similar. Figures 8 through 11 show patternsobtained with 10-m mirrors at 1-m spacing with variousdegrees of astigmatism in which the sampled Lissajouspattern is re-entrant within the number of visible dots.Figure 8 is not a multiple exposure but an ellipse that isrotating and is sampled synchronously with its rotation.

The synchronism can be adjusted by small changes inthe spacing of the mirrors and bending of the mirrors.

Figure 12 was taken with a situation somewhat closerto confocal than was used for the earlier photos. HereR = 10 m, d = 3 m. For this case there are only fourspots per ellipse with a small precession of the ellipseeach time around caused by the bent mirror. Theentrance conditions were chosen to open up the patternand show the later passages which are somewhat fainterin the center region of the rectangular pattern.

Figure 13 is a sketch of the spot sequence in thispattern. The four spots in the first ellipse are darkenedfor emphasis. Note that adjacent spots are separatedby four or fifty-one round trips, respectively, dependingupon whether one considers the tangential or the radialdirection. These numbers are of course merely exem-plary of this particular pattern.

Design Techniques

Patterns

Although many types of patterns are possible withthe two basic methods, a pattern of the general typeshown in Fig. 14 has been found useful in many applica-tions. The entering beam starts near the center of thepattern to give a narrow ellipse that is slowly approach-ing a circle. After passing through the circle, the pathbecomes an ellipse along the other diagonal and thebeam is removed from this narrow ellipse when it is inthe center of the original ellipse.

The beam is therefore injected into the system andremoved from the center region where the spot spacingis a maximum. The beam is also removed from thecenter of the original ellipse where the early beams canbe quite far away. This tends to minimize the cross-

Figs. 8 (left) and 9 (right). Synchronously sampled Lissajouspatterns.

Fig. 10 (left). Synchronously sampled Lissajous pattern. Fig.11 (right). Nonsynchronously sampled Lissajous pattern.


I

Fig. 12. Mirror pattern in optical delay line showing faint outputspots in center of ellipse.

Fig. 13. Order of spot positions in pattern of Fig. 12.

talk between the early intense beams and the weakeroutput beam.

Mode Matching RequirementsIt is necessary to match the injected beam to the

natural modes of the mirror cavity if the spot size is toremain uniform in the system. This requires that thewavefront have a radius such that the minimum diameterof the Gaussian beam is located in the center of thesystem and a Gaussian distribution of energy across thewavefront so that it will generate the same distributionon the other mirror. This can be accomplished byimaging the beam from the laser source into the delayline through a telescope of the proper magnification andfocus setting. The distribution and radius of the wave-front on one mirror will then be correct to give an iden-tical distribution and wavefront radius on the othermirror and in turn at each successive reflection.

If the mode diameter or focus is not correct, the spotsize will vary at twice the frequency that the spot posi-tion does as shown in Fig. 15. Since the spot positionvaries at different frequencies in the two coordinates,the spot will become astigmatic in various parts of thefield.

This oscillation of spot size can be used to improveseparation of the beams. A smaller than normal spot

can be sent into the cavity at the center of the mirrors,allowed to grow in size near the edges of the pattern, andthen removed near the center of the mirror where thespot is again at a part of the cycle when it is smallerthan normal.

Entering and Exit Techniques

The beam can be sent into the system or allowed toescape from the system by drilling a hole in the mirroror removing a small spot of the reflecting film from thesurface as shown in Fig. 16. The actual hole would besuitable when the system is used at a wavelength wherethe substrate is not transparent. A small mirror suchas the perturber mirror can be used also to inject orremove a beam. A transparent fiber can be used toremove a beam from the system but generally cannotbe used to inject a beam because the characteristics ofthe wavefront would be distorted in the fiber.

The small transmittance of the high reflecting dielec-tric films can be used to inject or remove beams espe-cially where more than one beam is to be used at onetime. The scattered light from the front surface of themirror to a detector to one side gives a larger signal thanthe transmitted beam when a ombination of all of thespots is wanted.

Discrimination Between BeamsIt is important that light from other passes through

the system not get mixed with light in the exit beam.Selecting the open center of the pattern for entering andexiting beams is good. Using some oscillation of spotsize so that the beam is small when injected and re-

Fig. 14 (left). Usual pattern used in delay line with centerentrance and exit apertures. Fig. 15 (right). Pattern withentering beam not matched so that spot size is not constant and

some spots are astigmatic.

-L1'- 0- OR - 9 ,

Fig. 16. Entering and exit methods.


Fig. 17. Discrimination of beams by angle.

Fig. 18. Mirrors mounted at close spacing with smoke in air toshow beams.

moved helps. The separate beams are always sepa-rated in either position or angle. Many of the beamsin the region of the output beam have a different anglein the system than the output beam. By placing a lensbehind the exit aperture and imaging the far mirroronto a mask, the output beam can be selected as shownin Fig. 17 according to its angle between the mirrorsas well as its position.

Multiple Delay SystemsMultiple storage or delay systems can be operated in

a single pair of mirrors by injecting the entering beamsthrough the same aperture at different angles therebyobtaining Lissajous patterns of different sizes and out-put beams that can be separated by their differentangles. Multiple systems can also be made by usingmore than one aperture for the beams.

It is probable that the total delay of a multiplicity ofdistinct delay lines in one pair of mirrors can be largerthan the single delay.

Mirror Coatings

The reflectivity of the mirror coatings used in thedelay line is extremely critical because the transmit-tance of the delay line is Rn, where n is the number ofreflections and may be over a thousand. Reflectanceof 99.5% results in loss of half the power in 138 reflec-tions.

Multilayer dielectric mirrors having approximatelythirty alternate layers of high- and low-index evapo-rated X/4 thick films have been used. Perry has de-veloped techniques for evaporating these films withvery low scattering loss as is described in another articlein this journal.' Because of the extreme sensitivity ofthe delay line loss to mirror loss, the delay line is a suit-

able method of measuring this reflectivity as will bedescribed in the applications part of this article.

Astigmatic Mirrors

The astigmatism required in the mirrors is very smalland amounts to only a few wavelengths. The mirrorsthat we have used have been mechanically distorted inthe mount to obtain the aspheric surface. Fused silicamirrors of 7.5-cm diam, 1.25-cm thickness, and 10-mradius have been used for much of this work. At aspacing of 3 m, these mirrors have a reflectance powerloss as low as 10dB for 1000 passes traversing a 3-kmpath of 10-j4sec delay. One of these mirrors is shownon the cover and Fig. 18 shows two of them spaced closetogether with smoke in the path to show the beams.The four mounting points around the edge of the plateare used to distort the mirror by the desired amount.A clear spot in the left mirror is used for the enteringbeam and a small clear spot in the center of the right-hand mirror is used for the exit beam.

Mirrors of 15-cm diam of crown glass 2.5 cm thickhave also been used in similar mounts. K. H. Behrndtdeposited uniform evaporated films onto these mirrorswhich allow larger patterns and greater spot spacing.

Astigmatic mirrors can also be made by distorting ablank during polishing of a spherical surface and thenreleasing the strain to obtain the astigmatic surface.

ApplicationsSeveral applications of the off-axis cavity delay line

can be foreseen. The use most pertinent to the themeof this issue and one which has already produced resultsis the measurement of optical loss. For this applicationone requires long path lengths for measurements ongases or many traversals for interface measurements.For applications in the communication or computationfields one probably requires long time delays (i.e., manymicroseconds) and often multiple output taps as well.These cases are still in a very exploratory state and willbe briefly reported below.

Small Optical Loss MeasurementsAn important application of these off-axis cavity

modes lies in the evaluation of optical properties ofmaterials or interfaces which are nearly perfect, hencenormally difficult to measure quantitatively. Oneexample is the measurement of mirror surface reflec-tance when it is close to unity.

If we let E = 1-R = fraction of power not reflected,x = relative output power after n passes through cavity.

Then obviously x = (1- )n and for small uncer-tainties-6e, 3x we have

e6 x n (e

Hence a measurement of x, conveniently made withphotodetector and oscilloscope to perhaps 10% un-certainty, yields similar accuracy in our knowledge ofe if the number of reflections n is > I/e. For example,if we have mirrors of reflectance R = 0.995, we should


Fig. 19. Multiple exposure oscilloscope picture of successivespot positions in pattern. Horizontal scale: 0.5 jusec per large

division. Mirror separation: 1.52 m.

use n 200 which is easily achieved with this cavityarrangement.

Figure 19 is a multiple exposure oscilloscope pictureillustrating such a measurement of mirror reflectancewhen it is close to unity. A Liassajous pattern wasobtained which consisted of horizontal rows of spots.A small scanning mirror was then moved manuallyacross the cavity, intercepting successive spots in onerow of the pattern and reflecting them to a photomul-tiplier cathode. The cavity input beam was modulatedby a Kerr cell to produce 100-nsec pulses. By actualcount on the visible pattern, motion of the scanningmirror from one exposure to the next removes (oradds) twenty-four spots to the pattern. Using theoscilloscope sweep speed of 2 divisions/psec and themirror spacing of 1.52 m we can independently derivethe same number, confirming the oscilloscope calibra-tion. Thus from any one pulse to the next in Fig. 19the beam made twenty-four round trips, traversing 73 mon the way.

The mirror loss in this case resulted in decay to half-intensity in 168 reflections, which corresponds to afractional reflectance of 1-0.004 = 0.996. An He-Nevisible laser was used, oscillating at a power level of afew milliwatts with no off-axis modes.

Some radiation from the Kerr cell is seen as noise onthe oscilloscope base line, and this in fact distorts theearlier pulse shapes. The delay line itself is relativelydispersionless and contributes a negligible amount ofpulse distortion. (The multilayer mirror reflectance,measured with a conventional spectrophotometer, isconstant to within 1% over a wavelength range of1800 A, centered roughly at X 6328 A.)

For the above example we used two 15-cm diammirrors having 10-m radii of curvature. They wereprepared for us by K. H. Behrndt and D. W. Doughtyof Bell Laboratories. The coatings are thirty-fivelayers of alternating dielectric constant on the polishedblanks which were of /10 quality. Behrndt andDoughty have achieved somewhat better coatingson smaller mirrors for which their evaporation systemwas designed.

The mirrors used in the photograph on the cover ofthis issue and in other figures accompanying this articleare 7.5 cm in diameter and have been kindly prepared

for us by D. L. Perry, also of this laboratory. 3 Wehave measured values of reflectance of 0.998 forthese mirrors.

Rather obvious refinements in the technique ofmeasurement will permit more precise results and willbe reported in the future. We remark in passing thatscattering loss from such many-layer mirrors greatlyexceeds the transmission and hence the usual determina-tion of the latter is not a good measure of reflectance.

Optical Data Storage

Applications of the off-axis cavity which are more inthe future include a serial delay line memory and variousforms of the transversal filter. This form of memory,shown in block diagram form in Fig. 20, was used inmany early synchronous serial computers using acousticdelay lines with electroacoustic transducers at inputand output. The total loop delay must be an integral(and usually constant) number of bit times. Thestorage capacity is the product of total loop delaytimes the serial bit rate. Access time is often a dis-advantage unless special programming care is ex-ercised.

With the advent of optical delay lines of severalmicroseconds duration and, with optical bit rates ofgigacycles per second becoming feasible, a new em-bodiment of an older idea is being studied. As shownin Fig. 20 either or both of the logical OR and the GAINfunctions can be performed either optically or elec-tronically. With electrical circuitry in the loop, ofcourse, optic-to-electronic-to-optic transduction isrequired. Almost any form of modulation is possibleexcept that amplitude modulation in an all-optic loopwould be difficult because of the lack of a fast opticalthreshold device.

The access time problem can be greatly alleviated byinserting a high-quality transparent plate across thecavity paths at nearly Brewster's angle. This wouldmake read-out of any path possible every few nano-seconds. Of course, two more scattering surfaces areadded to the cavity, and a multitude of photodetectorswould be required for this quasi-random access facility.

Optical Signal Filtering

A second application which is still under early studyis the multiply tapped delay line, useful in severalforms in the communications field. In such form,

GAIN

Fig. 20. General diagram of optical storage systems.


OPTICAL EQUIVALENT CIRCUIT

i -4 L f f # f . l

INPUT

IN PUT

ELECTRICAL EQUIVALENT CIRCUIT

_ _ _ _ _ BOUT

SUMMINGCIRCUIT

Fig. 21. Optical system and electrical equivalent transversalfilter.

known generally as a transversal filter4'5 the opticaland electrical equivalent circuits are as shown in Fig.21. We can apply the circuit as a time domain equal-izer to recover distorted pulse shapes.6 Alternatively,we can use it as a filter in the frequency domain. Forthis case we write the transfer function of the electricalcircuit as

H(j) = Ecneiw(n ),n=O

N/2= e(N/ 2)r [ + E a cos(kw)].

k=1

We note the formal similarity to the usual Fourierseries expansion although in the frequency domainrather than the time. Here is the tap spacing and wis the radian frequency of the light if true opticalwavefront addition is performed at the output (i.e.,interferometry.) If the various output tap intensitiesare merely added on a photodetector then is anyFourier component (in the usual sense) of the modula-tion impressed on the light. N+1 is the number oftaps collected from the optical cavity.

As an example the delay line may be used to con-struct a bandpass or band rejection filter. The fre-quency width of the band will be 2/v and the "ripple"frequency and steepness of the skirts will be - N/r.For the present, demonstrated delays of N > 10gsec suggest filter bandwidths of <5 200 kc/sec withouttaps. In this case N = 1, r = 10 gsec. If instead thesame total delay is divided into N sections, we can getbetter relative shaping of the filter response curve(flatter plateaus, steeper skirts) but we must acceptwider frequency bands by a factor N.

We are indebted to G. E. Reitter and H. A. Stein fortechnical assistance, D. L. Perry and K. H. Behrndtfor mirror fabrication, and numerous colleagues forhelpful discussions. The encouragement of R. Kompf-ner and J. A. Young is also acknowledged.

References

1. A. G. Fox and T. Li, Bell Syst. Tech. J. 40, 453 (1961).2. D. R. Herriott, H. Kogelnik, and R. Kompfner, Appl. Opt.

3, 523 (1964).3. D. L. Perry, Appl. Opt. 4, 987 (1965).4. H. E. Kallmann, Proc. Inst. Radio Engrs. 28, 302 (1940).5. L. A. Zadek and C. A. Desoer, Linear System Theory (Mc-

Graw-Hill, New York, 1963), p. 447.6. F. K. Becker, L. N. Holzman, R. W. Lucky, and E. Port,

Proc. IEEE 53, 96 (1965).

Meeti Repo tsSymposium on Inhomogeneity in Glass, Univer-sity of Sheffield, 6-8 April 1965Reported by A. Winter, Laboratoire des Verres de l'Institutd'Optique, Paris, France.

A conference on Inhomogeneity in Glass was held at Sheffieldon 6-8 April under the sponsorship of the Society of Glass Tech-nology. The large attendance indicated the interest rousedby this subject. The meeting was mostly concerned with macro-scopic kinds of inhomogeneities, that is, cords, knots, lines, etc.,defects which alter for the worse the quality of light transmissionof an ordinary glass.

The origin of some of these defects was shown to be related tobatch-mixing problems: the usual form of the hopper induces asegregation in the grain size during their flow into the tank, thusproducing compositions differences. Accessories designed for

homogenizing the inlet and the outlet of the mixture to thehopper were presented.

The importance of a uniform grain size spread in the batchwas emphasized as well as the advantages of bubbling when usedtogether with stirring for homogenizing the melt.

Then an important part of the meeting was devoted to a greatvariety of methods for testing glass inhomogeneities. Most ofthese methods give only a limited aspect of this phenomenon andare designed to be applied under well-determined circumstances.They make use of a wide variety of glass properties and allow arapid control of the glass prepared for a given purpose.

For instance, for sealing of glasses a constant thermal expansionis required, inhomogeneity resulting in a weakness of the seal. Asystematic testing of cords appearing in glass is easily accom-plished by measuring the microhardness. This property issensitive to small composition variation, and its different valuescan be found in the literature. A standard calibration for a givenglass leads to a rapid detection of the origin of the defect.Shelyubskii's method of determining the homogeneity of glass isfrequently used in factory laboratories and is considered con-


Z.

-i-

Thin Films and Their Uses for the Extreme Ultraviolet

W. R. Hunter, D. W. Angel, and R. Tousey

The transmittance characteristics in the extreme ultraviolet of unbacked films are discussed; data arepresented for films of Al, In, Bi, Ge, Si, and certain organic materials. Generally, the metals begin totransmit shortward of their critical wavelengths, determined by the electron eigenloss, and reachmaximum transparency at the nearest x-ray edge; to shorter wavelengths regions of transparency existbetween the other x-ray edges. Their use in order-sorting, and for eliminating long-wavelength straylight, is illustrated with laboratory spectra and solar spectra obtained from rockets.

Introduction

Thin films find many uses in extreme ultraviolet(xuv) research. Their selective transmission prop-erties make them valuable as optical filters for eliminat-ing stray light in spectrographs, or for order-sorting.With their aid, single-dispersion spectrographs can beused where multiple-dispersion would otherwise berequired. With gas filled detectors, they serve aswindows to hold back the gas and transmit the radia-tion.

The first study of the transmitting characteristics ofa thin organic film was made by Laird,' who showedthat celluloid transmits at least to 450 A, and also insoft x rays; quantitative measurements were made byO'Bryan' over the range 1000 A to 300 A, andTomboulian and Bedo3 measured the transmittance ofZapon, a similar material between 260 A and 80 A.

Many years ago Wood4 discovered that thin films ofsodium and potassium, although completely opaque tovisible light, are highly transparent in the ultraviolet.In further investigations' he showed that the positionbelow which transparency sets in moves to shorterwavelengths, the lighter the alkali metal atom:lithium, for example, transmits shortward of 2050 A.This characteristic wavelength is a consequence of theexistence of free charges in metals and has been calcu-lated for different metals by Pines6 on the basis of thenumber of free electrons per atom. Because of theirchemical activity, however, the alkali metals can beused only if sealed as a sandwich between quartz platesas has been accomplished by O'Bryan.7 Thereforethey are of little interest as filters for the xuv.

Gold and silver have long been known to transmit in

The authors are with the E. 0. Hulburt Center for Space Re-search (sponsored jointly by the Office of Naval Research and theNational Science Foundation), U.S. Naval Research Laboratory,Washington, D.C.

Received 26 April 1965.

narrow bands, at 5000 A and 3200 A, respectively.Although silver was reported by Laird' to transmitwavelengths as short as 900 A, recent work by Canfieldand Hass' has shown that the film must be less than300 A thick in order to transmit more than 10% of theincident radiation at 1000 A.

The first measurements of the xuv transmittance of athin metal film were made by Tomboulian and Pell0

who measured aluminum, supported on Zapon, from80 A to 320 X. The LI and L2,3 x-ray edges lie withinthis spectral range, but only the latter was clearlyobserved. Similar measurements, from 130 X to 750 A,have been made by Astoin and Vodar" for aluminumsupported on collodion. The xuv transmittance ofvarious other metals, as well as aluminum has beenmeasured by Walker et al." and Rustgi.3

To obtain complete knowledge of the optical behaviorof metals it is necessary to determine both opticalconstants n and k. This can be done by measuring thereflectance at several angles of incidence, or the reflec-tance and the transmittance. Data for aluminum andindium over most of the spectral range where they aretransparent have been reported by Hunter,'4 and, morerecently, he has obtained similar results for Sn, Ge, Si,and Be." From the optical constants it is possible tocalculate the transmittance of a film of a given thicknessand to include all the interference effects, as well aseffects produced by an oxide layer, if n and k are knownfor the oxide.

Parlodion

Parlodion is a trade name for cellulose nitrate, C12H6(NO3)4. Zapon is a similar material. Celluloid andcollodion are cellulose nitrate containing some camphor.Films of Parlodion were prepared by placing a drop of adilute solution of Parlodion in amyl acetate on a dis-tilled water surface. The films were so thin as to beblack by reflected light and were found to be quiteuniform.


folded optical delay lines - uta › rfmems › teralum_old › a2.pdf · a lissajous pattern of...

Documents