Low-Loss Multilayer Dielectric Mirrors

D. L. Perry

The preparation of low-loss multilayer dielectric coatings for laser mirrors is described. Layer thicknessis controlled by a unique monitoring system utilizing a 6328-A gas laser as a light source. Experimentsdesigned to evaluate and reduce losses in the mirror coatings are discussed. It is found that mirror trans-mission losses may be reduced to a negligible value by deposition of a sufficient number of dielectriclayers, without at the same time increasing scattering and absorption losses. Reflectivities in excess of99.8% have been obtained from a single stack of twenty-five quarter-wave layers. Broad-band mirrorswith reflectivities greater than 99.5% from 4300 X to 7400 A have been produced by properly stackingtwo such twenty-five-layer groups.

Introduction

The rapidly expanding use of lasers has been paral-leled by an increasing demand for high-quality multi-layer dielectric-coated mirrors.

For efficient laser operation, particularly with rela-tively low gain transitions such as those occurring inhelium-neon gas discharges, it is imperative that opti-cal losses other than mirror transmission be reduced to aminimum. In general, the unwanted losses in a lasersystem can be attributed to the following:

1. Brewster-angle windows-scattering, absorptionand reflection losses due to poor optical polish, un-relieved strains, wedge, or incorrect mounting angle.Losses encountered here will usually range from 0.1 to0.3% (four surfaces). Careful cleaning and assemblyprocedures are necessary.

2. Mirror substrate-poor optical polish, residueleft from cleaning procedure. Here the highest degreeof optical polish is desirable.

3. Diffraction losses-these depend upon choice ofmirror radius relative to laser cavity geometry.

4. Multilayer dielectric mirrors-losses due toscattering and absorption. The ultimate mirror re-flectivity which may be realized is limited by theselosses.

This paper will be confined to experiments and resultsrelated to Item 4, i.e., the fabrication of multilayerdielectric mirrors.

Experimental Arrangement

The dielectric coatings are obtained by evaporation ina commercial vacuum coating unit. * This unit con-

The author is with Bell Telephone Laboratories, Inc., MurrayHill, New Jersey.

Received 26 April 1965.Paper presented at NEREM Conference, 5 November 1964.* Consolidated Vacuum Corp., Model No. LCI-18B.

tains a 15.2-cm oil diffusion pump and a chevron-typebaffle located between the diffusion pump and workchamber. The baffle is automatically cooled with aFreon refrigeration unit. Two mechanical pumps areused; one pre-exhausts the work chamber and the othercontinually backs the diffusion pump. These areconnected to the station by separate lines. A flexiblebellows is mounted in the backing pump line to mini-mize the transmission of forepump vibrations. Thebell jar diameter is 47 cm. Pressures during evapora-tion range from 1 to 5 X 10-5 torr.

The reflective coating is fabricated by the con-ventional method of evaporating alternate layers ofoptically transparent dielectric materials whose indicesof refraction, n, differ as greatly as possible. ZnS(n 2.3) is commonly used as the high-index materialfor wavelengths longer than -4000 A. ThOF2 (n1.5), NaAlF 6 (cryolite) (n 1.35), and MgF2 (n = 1.4)have been used as low index dielectrics. The thicknessof the layers determines the center of the reflectionband, and the number of layers determines the band-width and transmission characteristic.

Maximum reflection will occur at a specific wave-length when the thickness of each alternate dielectriclayer corresponds to an optical path of one quarterwavelength. Some means of accurately indicatingthese quarter-wave thicknesses is necessary. Themonitoring system for the experiments described hereuses a 6328-A laser whose output beam is chopped at1000 cps and passed through a beam splitter whichdirects a portion of the beam to a monitor slide in thebell jar. The beam reflected from this slide is detectedby a solar cell whose output is compared in a 1000-cpsratiometer to that of a solar cell monitoring the incidentbeam. Maxima (or minima) in the ratiometer outputvery accurately indicate the deposition of one quarterwavelength of the high (or low)-index layers. Themain advantages of this scheme as compared to stand-ard monitoring techniques are the insensitivity of the

August 1965 / Vol. 4, No. 8 / APPLIED OPTICS 987

ratiometer output to variations in the intensity of thelight source and the ability to use highly stable de-tectors. A schematic representation of the workchamber is illustrated in Fig. 1. Also shown is themonitoring arrangement. The monitor slide is a stand-ard 2.5 cm X 7.6 cm microscope slide. After fouralternate high- and low-index layers have been evapo-rated on a given area of the monitor slide, the signalsensitivity is reduced to so low a level that it is desirableto expose a clean portion of the slide. By using aconcave beam splitter, in this instance one having a3-m radius of curvature, the beam is converged to arelatively small area on the monitor slide. This per-mits the monitoring of many groups of four layerseach.

Reflective coatings for the visible or near infraredare made by fixing the monitor slide height relative tothe evaporation source and by adjusting the position ofthe mirror blanks relative to the monitor slide. Forwavelengths shorter than 6328 A, the blanks are locatedfarther from the source than the monitor slide; forwavelengths longer than 6328 A, they are closer.Reflective coatings for different wavelengths can beprepared simultaneously by stacking the mirror blanksat the appropriate levels. For coatings in the nearinfrared this technique has limitations since it is notdesirable to bring the blanks too near to the source.This limitation may be overcome as follows: for coat-ing at an infrared wavelength X, the mirror blanks arepositioned at the X/n level and n quarter-wave layersof one material are evaporated for each layer, therebyincreasing the center wavelength of the reflection bandn times. It is assumed here that both coating materialshave the same optical dispersion. In practice this as-sumption appears valid. When monitoring multiplesof half-wavelength layers, the same slide section may beused repeatedly as the reflectivity after each layer isessentially zero.

The tracings of the maxima and minima from a penrecorder are shown in Fig. 2. The monitoring of fouralternate quarter-wave layers is shown in Fig. 2A andthe monitoring of two multiple quarter-wave layers isshown in Fig. 2B. The horizontal pen rate used here is

Fig. 1. Schematic of station and monitoring equipment.

t5-

C,

La

Fig. 2. Monitor tracings from pen recorder.

approximately 50 see every 2.5 cm. From the multiplequarter-wave tracings it may be seen that the maximaand minima are relatively sharp. With a little ex-perience the operator can terminate the evaporationvery nearly at these points. The approximate timerequired to evaporate the various layers may be deter-mined from these monitor tracings.

The evaporation is stopped by rapidly rotating theevaporation boat to a location beneath a shield. Theother dielectric then is rotated beneath the evaporationfilament, and the procedure is repeated. For singlealternate quarter-wave layers the pen usually is oper-ated continuously for a four-layer group. The monitorslide then is repositioned, exposing a clean area, and theevaporation cycle is continued.

Mirror SubstratesQuartz generally is preferred for a laser mirror sub-

strate because of its thermal stability and low absorp-tion losses. Coatings on this material have beenstripped using aqua regia or nitric acid with no ap-parent ill effect on the substrate polish or on mirrorquality when such blanks are recoated. On the otherhand, the polished surface of certain glass substratesfrequently have a hazy or etched appearance afterstripping. This condition seriously impairs the qualityof the recoated mirror.

The last mechanical step in preparing the quartzsubstrate is the final polishing of the surface to becoated. The cleaning procedure, just prior to coating,consists of washing the blank with a solution of Alconoxand water applied with a soft camel's hair brush. Thissolution is rinsed off with tap water, and the blankthen is rinsed thoroughly with deionized water. Thesurface to be coated then is blown dry with nitrogen. Ifthe surface is clean, the water will disappear rapidlywith no droplets remaining. Such cleaning eliminatesevaporation of the water from the surface which, ifallowed to occur, will usually produce water marks thatare difficult to detect until after coating. The rest ofthe mirror blank then is dried with nitrogen, makingsure that no water reappears on the surface to be coated.The surface to be coated then is inspected by shining abright light (microscope lamp) through the blank ardobserving the amount of scattered light from the sur-

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

FOUR ALTER TWO ALTERNATE -4 LAYERS

-0 5SEC.r.

mA B

MONITOR RECORDINGS

face. Poor polish and grinding defects may be detectedin this manner.

The above cleaning procedure is performed in closeproximity to a dust-free portable hood. In order tominimize further the possibility of dust particles ac-cumulating on the clean surfaces, the hood containingthe clean substrates is located next to the coatingstation. The mirror blanks are placed into the coatingadapters within 5 or 10 min after the cleaning procedure.The back surfaces are kept free of unwanted vapordeposition by being covered during the evaporation.Initial roughing of the bell jar is done slowly to avoidraising a dust of small particles remaining from previousevaporations.

Experimental Results

Improvements in mirror quality have been attainedas the result of a number of experiments and observa-tions. As stated earlier, an upper limit on the numberof dielectric layers is set by absorption and scatteringlosses in the coatings. Before an increase in reflectivitycould be achieved by applying additional layers, it wasnecessary to determine the nature of these coatinglosses and eliminate them wherever possible.

One step in the coating procedure recommended inthe literature' is a glow discharge of the substrate justprior to the evaporation. In our processing the coatingstation is in nearly continuous operation and can becleaned thoroughly only infrequently. Therefore, thebell jar walls and the various fixtures in the coatingchamber unavoidably accumulate a coating of smallparticles, and it appears that a glow discharge sputtersthese particles onto the mirror blanks. The evidencefor this is shown in Fig. 3, which shows the light scat-tered from an initially clean mirror blank after 10min of glow discharge. The surface was illuminatedat an oblique angle with white light and viewed througha microscope. The over-all magnification is approxi-mately 45, and the total area of the photograph is 4mm2 . A marked increase in output power from astandard laser setup resulted when use of the glow dis-charge was discontinued.

A further study of the mirror coatings showed thatthe number density of scattering particles appeared to

'SURFACE CONTAMINATION

BY GLOW DISCHARGE

Fig. 3. Particles sputtered on clean glass during 10 min in aglow discharge.

MIRROR COMPARISON

GOOD

POOR

Fig. 4. Particle density comparison of a good and a poor mirror.

vary from one coating run to another. The dielectricsbeing used were ZnS and ThOF 2 . By testing themirrors in a standardized laser geometry, it becamequite evident that these particles were playing an im-portant role in determining the over-all quality of themirror. A comparison of this particle density on agood and a poor mirror is shown in Fig. 4. Both ofthese mirrors have thirty-one layers and both exhibitvery low transmission; however, a considerably greateroutput from the laser resulted when the mirror with thefewer scattering centers was used, indicating a higherreflectivity. To determine the source of these particles,a thick layer, corresponding to many quarter-wavelayers, of each of the dielectrics was evaporated onseparate slide sections. The results of a microscopicexamination under oblique lighting, shown in Fig. 5,indicate that the ThOF 2 is responsible for nearly all ofthe scattering particles. Further experiments indi-cated that a minimum particle density resulted whenthe ThOF2 was evaporated just below its wetting tem-perature. Mirrors of consistently good quality resultwhen this procedure is followed in the evaporation of theThOF2. Further improvement in mirror quality re-sulted when the ThOF 2 powder was sifted through aU.S. Standard series 200 sieve.

The most pronounced decrease in the particle densityon prepared mirror coatings occurred, however, whenthe ThOF 2 was evaporated from chunks as receivedfrom the supplier.* A possible reason for this may bethat in the powder state trapped gases can form pocketswhich are liberated suddenly when the material isheated, releasing small showers of the surface material.The temperature required to evaporate the ThOF2 inthe chunk state appeared to be more nearly constant fora given number of layers than when the powdered statewas used. A subsequent experiment was performed inwhich a thick layer of ThOF2 at 6328 A was evaporated

* Fish-Schurman, New Rochelle, N.Y.

August 1965 / Vol. 4, No. 8 / APPLIED OPTICS 989

- ZnS

SURFACE SCATTERING COMPARISON

THOF2

Fig. 5. Particle comparison of ZnS and ThOF 2 layers.

CRYOLITE AND ZnS 27 LAYERS

Fig. 6. Particle density of mirror coatings using cryolite.

on a slide section while the material was in a completelyliquid state. Practically no particles were discernible.However, certain mechanical modifications of the evap-oration station are necessary before an attempt canbe made to coat mirrors using liquid ThOF2.

Evaporation of the ZnS has not presented any prob-lems. It appears that the more rapid the evaporationrate the more durable the coating. This is consistentwith statements in the literature. 2 The average evap-oration time per layer for the ZnS was approximately20 see to 25 see during a coating run which producedexcellent mirrors. This rate can be controlled easilywith good reproducibility from one evaporation run toanother.

The ZnS and ThOF 2 coatings are quite durable.After extended usage the mirror surfaces may sufferfrom an accumulation of smoke or dust from the sur-rounding atmosphere. By careful cleaning with ace-tone, distilled water, and dry nitrogen, these defectscan be removed, restoring the mirror surface to itsoriginal quality.

Further experiments have been conducted usingcryolite instead of ThOF 2 as the low-index dielectric.Again the most satisfactory results were obtained when

this material was evaporated from a lump as opposed toa powder. The first set of mirrors fabricated usingcryolite received twenty-seven layers and proved su-perior to any mirrors previously coated using ThOF2.Scattering losses were extremely small, as is illustratedin Fig. 6, which shows the particle density of a twenty-seven-layer mirror made using cryolite and ZnS. Afurther indication of low losses in the coatings was ob-tained by measuring the output power from the stand-ardized laser geometry with a twenty-seven-layer mir-ror, and by then adding twenty-three more layers andobserving that the power output did not change.

Because of the lower refractive index of cryolite(n 1.35 as compared to n 1.5 for ThOF2 ) fewerlayers are required to obtain the same reflectivity andreflection bandwidth as when ThOF 2 is used. Alsocryolite can be evaporated more rapidly than canThOF2 . Generally, the grain size of most substancesreduced as the rate of deposition is increased.3 Thedurability of these coatings with respect to cleaningwith water is not as good as those made with ThOF 2.

The initial cryolite coatings were made from naturalcryolite powder which had been fused into a chunk in ahelium atmosphere. These coatings were destroyedalmost immediately when contacted with water. Sub-sequent coatings were made using natural cryolite in theas-received chunk form. This material was preheatedin a vacuum to remove water vapor and gases whichsometimes cause it to break apart quite violently.Coatings from this material proved to be considerablymore resistant to cleaning by water than those madefrom the fused powder. However, the degree ofresistance varies erratically. Some mirrors were re-peatedly washed with water with no noticeable ill ef-fects; on others, water causes small areas of the coatingto lift. More recent investigations of mirror cleaning

Z0- w

I.-

3.0l

,2.0 -

1.0

4200 5000 6000 . 7000 MLC

WAVELENGTH A

Fig. 7. Broad-band mirror transmission characteristic.

6.0

5.0 ]

4.0

3.0

110

C - -_ I

WAVE LENGTH A

Fig. 8. Twenty-five-layer ZnS cryolite mirror transmissioncharacteristic.

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

00

I

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10LASER TUBE BORE mm

Fig. 9. Output power at 6328 X/cm of laser tube discharge vsbore diameter.

have shown that dust and smoke films could be re-moved successfully from cryolite-ZnS mirrors by usingacetone. The coatings are flushed with this solventand quickly blown dry with nitrogen.

The reduction of scattering losses in the dielectriclayers has enabled the fabrication of high-reflecting(>99.5%) broad-band mirrors spanning the wavelengthrange 4300 A to 7400 A.4 The transmission characteris-tic of such a mirror is shown in Fig. 7. This mirror wasmade by stacking two twenty-five-layer reflection bandswhose band centers are separated by approximately1500 A. For comparison the transmission characteris-tic of a twenty-five-layer ZnS cryolite mirror is shown inFig. 8. Preliminary measurements by the techniquesdescribed by Herriott and Schulte in another paper inthis journal5 indicate that the reflectivity of the twenty-five-layer mirror is 99.8%.

Laser output power as a function of capillary dia-meter and discharge length can be estimated from thecurve shown in Fig. 9. The laser tubes from which thisinformation was obtained were filled with He 3-Ne andoperated in a multimode configuration at optimum dis-charge current in spherical mirror cavities using low-loss mirrors as described above. The quoted powerswere observed at one end.

For the He-Ne system which has relatively low gain,the output power is dependent critically upon mirrorquality. For the high gain transitions in many of thenew ionic lasers, the requirement for extremely high re-flectivity is reduced. However, at continuous dutyoutput levels of several watts, reductions in outputpower arising from thermal effects in the mirror coatinghave been observed in the A II laser.6 This places apremium on mirror coatings which have the lowest pos-sible absorption and highest resistance to heatingeffects.

References1. L. Holland, Vacuum Deposition of Thin Films (Wiley, New

York, 1961), p. 74.2. L. Holland, Vacuum Deposition of Thin Films (Wiley, New

York, 1961), p. 295.3. L. Holland, Vacuum Deposition of Thin Films (Wiley, New

York, 1961), p. 295.4. D. L. Perry, Proc. IEEE 53, 76, 77 (1965).5. D. R. Herriott and H. J. Schulte, Appl. Opt. 4, 883 (1965).6. E. F. Labuda, private communication.

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