GAS LASERS

Spectral-splitter coatings in laser systems for the visible and infrared regions

E. N. Kotlikov,a� E. V. Khonineva, and V. N. Prokashev

St. Petersburg State University of Aerospace Instrumentation, St. Petersburg

A. N. Tropin

Girikond Scientific Research Institute, St. Petersburg�Submitted April 28, 2009�Opticheski� Zhurnal 76, 27–31 �November 2009�

This paper describes examples of the synthesis and fabrication of optical coatings that operate inthe mid-IR region and the visible region simultaneously. In particular, it tells how to separate theradiations of CO2 and He–Ne lasers by means of interference coatings that transmit �or reflect�the radiation of a CO2 laser and simultaneously reflect �or transmit� pilot radiation in the visibleregion, including the radiation of a He–Ne laser. Structures based on doped BaF2 films are con-sidered that reflect the visible range and simultaneously operate as antireflection coatings in theIR region. The use of metal-dielectric coatings makes it possible to implement structures thatreflect radiation in the IR region and simultaneously transmit radiation in the visible and near-IRregions of the spectrum. Examples are given of the synthesis and practical implementation ofthis type of coatings. © 2009 Optical Society of America.

INTRODUCTION

There are two ways to implement interference coatingsthat operate simultaneously in the visible and IR regions.First, dielectric films with different refractive indices andsmall losses in the spectral range from 0.5 to 12 �m may beused. Second, metal-dielectric coatings based on silver orgold films may be synthesized and implemented.

Zinc sulfide and selenide films are promising for theformer approach, since they are transparent in the spectralrange from 0.7 to 15 �m at virtually any thickness. Filmswith low refractive index: barium, calcium, strontium, andlead fluorides, with total optical thicknesses greater than3–5 �m in multilayer coatings are only slightly transparentin the visible range because of losses to scattering.1,2 To ob-tain interference coatings for the visible and IR regions, thispaper presents studies of the refractive index nf and relativedensity Q of doped films of BaF2, CaF2, and PbF2, withvariation of the concentration and composition of the dop-ants. Studies have also been carried out of how dopants af-fect the stress in films and interference coatings for the IRregion. The properties of barium fluoride films were studiedin greatest detail. Using doped fluoride and zinc sulfos-elenide films, dielectric coatings on zinc selenide substratesthat simultaneously operate in the visible and IR regions ofthe spectrum were synthesized, fabricated, and studied.

This paper describes a number of synthesized metal-dielectric coatings on synthetic sapphire substrates, repre-senting interference filters in the visible region that are si-multaneously high-reflectance mirrors in the mid-IR region.

MEASURING THE OPTICAL CONSTANTS OF DOPEDBARIUM FLUORIDE FILMS

To determine the refractive index of doped barium fluo-ride films, the technique was to measure the transmission of

693 J. Opt. Technol. 76 �11�, November 2009 1070-9762/2009/

the film on a transparent substrate in vacuum right after sput-tering. Silicon plane-parallel plates with ns=3.5 were used assubstrates to develop a contrasty interference pattern in thesubstrate-BaF2-film system. It was necessary in the experi-ment to deposit a film with optical thickness �0 /4 to deter-mine the refractive index of the deposited layer, with thereference wavelength �0 being chosen as 1.2 �m.

The transmittance at the maximum, Tmax, correspondingto the deposited quarter-wave film can be used to computethe refractive indices of a single film in the region with theminimum absorption, using the following relationship:

nf = ��− A − �A2 − B2�/2Tmax, �1�

where A=Tmaxns2−2ns

2+Tmax−2, B=2Tmaxns, and Tmax is nor-malized to the transmittance of the substrate with no depos-ited layer. The transmittance of a pure nonabsorbing sub-strate, taking into account multiple reflection from theparallel faces, is computed as

T0 = 2ns/�ns2 + 1� . �2�

Equation �1� also takes into account the multiple reflection oflight in the substrate.

The transmittance Tmax was determined by means of aphotometric monitoring system. The accuracy �T with whichthe transmittance of the substrate with deposited films wasdetermined was less than 0.5%. Calculations show that, if thefilm’s refractive index nf lies in the limits from 1.3 to 1.8, itsmeasurement error �nf equals 0.015 in the absence of ab-sorption in the films. Thus, the accuracy with which the re-fractive index is measured by this method is high and iscomparable with the accuracy with which refractive indicesare measured by other methods—for example, by ellipsom-etry, spectrophotometry, etc.

693110693-04$15.00 © 2009 Optical Society of America

The relative density Q of the film is determined by theratio of the film’s density � f to the density �m of the bulksample. The relative density �packing factor, fill factor� isassociated with the refractive index of the bulk sample nm

and of the film nf by the relationship3

Q = ��nf2 − 1�/�nf

2 + 2����nm2 + 2�/�nm

2 − 1�� . �3�

A VU-2M apparatus and the standard technique4 wereused for depositing the films and coatings. The substrate tem-perature tS was determined during deposition to within�3 °C by a thermocouple attached to the support. The stan-dard heating system of the vacuum apparatus was used toheat the holder and the substrate. The temperature scattercaused by periodically switching on the heating system was�5 °C; i.e., the accuracy with which tS was recorded duringthe deposition was �5 °C. The thickness of the depositedfilms was monitored by a photometric system at a wave-length of 1.2 �m.

In the course of fabricating the films, their accommoda-tion �adhesion� coefficient was estimated from the amount ofevaporated substance and from the film-growth rate at fixedpower, dissipated by the crucible with the substance beingdeposited. The adhesion factor for pure BaF2, CaF2, andPbF2 remained virtually unchanged in the entire temperaturerange used here �50–120 °C�.

The scattering of single BaF2 films is small, even whentheir thicknesses exceed several micrometers. At the sametime, the losses to scattering in coatings that consist ofquarter-wave layers of BaF2 and of a substance with highrefractive index is many times as great as in a single film ofthe same thickness. Because of this, the authors studied notonly single films at �=10.6 �m, but also interference coat-ings of the type of 5–11-layer mirrors.

The authors of this article established earlier that thescattering of BaF2 films is reduced when some amount ofanother fluoride—MgF2—is added to them.1,2 Doping offluoride films with other fluorides �especially magnesiumfluoride� increases the relative density of the films. The re-sults of measuring the refractive index and the relative den-sity of the BaF2 films at the deposition temperature of+75 °C and various concentrations of the MgF2 dopant areshown in Table I. Simultaneously with the increase of therelative density, the scattering in the films and mirrors wasreduced.

The scattering of multilayer coatings with doped filmswas investigated by the following method: Nine-layer mir-rors were fabricated from films of doped BaF2 and ZnS for awavelength of 10.6 �m; the reflectance R and transmittance

TABLE I. Results of the measurement of the refractive index nf and therelative density Q for BaF2 films doped with MgF2 at a deposition tempera-ture of +75 °C.

Concentrationof MgF2,% 0 0.2 0.5 1 2 5

nBaF21.30 1.32 1.36 1.41 1.43 1.36

QBaF20.67 0.71 0.79 0.94 0.95 0.79

694 J. Opt. Technol. 76 �11�, November 2009

T of these mirrors were then determined at 0.63 �m and thereflectance at 10.6 �m. For a wavelength of 0.63 �m, theabsorption in the BaF2 and ZnS films and the ZnS substrateis small,4 and all the losses were ascribed to losses to scat-tering; i.e., Ascat=1−R−T. The breakdown threshold wasalso determined for these mirrors by means of a pulsed CO2

laser.2 The results of the measurements of eight-layer mirrorsfor the CO2 laser composed of ZnS and BaF2 films with aMgF2 impurity of from 0 to 2% on a ZnS substrate are givenin Table II. Mirrors were also fabricated with more than eightfilms. However, they were unstable in time and sometimesbroke up in the process of fabrication.

When more than 1% weight concentration of magnesiumfluoride was added, the scattering was reduced by a factor of5. Reducing the MgF2 concentration increases the scattering.A larger admixture �up to 4–5%� of MgF2, although it re-duces the scattering somewhat, results in the appearance offilm defects in the form of islands on the substrates, onwhich virtually no film grows, as well as reducing the refrac-tive index of the film. The diameter of the foci is0.1–0.3 mm, and their number increases as the magnesiumfluoride concentration increases. The authors explain thelarge scatter in the results of the investigation of the radiationstrength E /S�10.6 �m� by the presence of such defects. Theaccommodation factor is reduced by a factor of 3–5 when a2% MgF2 impurity is used. When the percentage of MgF2 inBaF2 films is increased further, the stress in the films reachescritical values, and, as a result, the coating breaks up. Mirrorswith a barium fluoride concentration greater than 4% some-times break up even in the process of fabrication. Increasingthe MgF2 concentration reduces the packing density of thefilm.

Mirrors made from BaF2 and ZnSe were also fabricatedfor a CO2 laser. The physicomechanical properties of thesemirrors are similar to those of mirrors made from BaF2 andZnS films, except that their transmittance in the visible re-gion is a factor of 2–4 less because of high absorption in theZnSe films.

DIELECTRIC COATINGS FOR SEPARATING THE VISIBLEAND IR RADIATIONS OF He–Ne AND CO2 LASERS

One of the tasks that confront laser instrumentation is toseparate �or combine� laser radiation in the visible and IRregions. The ray of a CO2 laser in the IR region is invisibleand, to aim it at a target, it is combined with pilot radiationof a laser of the visible region. Semiconductor or He–Nelasers are most often used as the source of the pilot radiation.

TABLE II. Results of a study of the transmittance and radiation stability ofCO2-laser mirrors made from BaF2 and ZnS films.

MgF2 concentration, % 0 1 4

T�10.6 �m�, % �8 7–8 7–8R�10.6 �m�, % �90 �90 �90T�0.63 �m�, % �10 �60 �60

E /S�10.6 �m�, J /cm2 6–7 8–9 5–10

694Kotlikov et al.

The problem is formulated in general as follows: Theunpolarized �or polarized� radiation of two lasers is incidenton a spectral-splitter plate at a 45° angle. After it passesthrough the plates, the radiation of the two lasers convergesin space. A necessary condition is the minimum loss of ra-diation for the strong and pilot rays.

Let us consider some ways to solve this problem, basedon the material presented above, using zinc selenide or sul-fide and barium fluoride films. Two solutions are possible:High reflection at 10.6 �m and high transmission at0.63 �m, or high reflection at 0.63 �m and high transmis-sion at 10.6 �m.

There are a number of difficulties in implementing theformer solution on the basis of dielectric films. To obtainreflection of the order of 98–99% at 10.6 �m requires a mir-ror composed of 13–16 films of ZnS and BaF2. Althoughsuch mirrors can be fabricated by using special technologies,large expenditures of both materials and time are required.The authors have fabricated ten-layer mirrors based on ZnSand doped BaF2 films. Their transmittance at 0.63 �m didnot exceed 30%, with a reflectance at 10.6 �m of 90–92%.Mirrors with a larger number of layers are unstable.

The use of ZnSe and BaF2 films reduces the number offilms, but such mirrors are virtually opaque in the visibleregion, mainly because of absorption in the ZnSe films andpartially because of the scattering in the BaF2 films. Ten-layer mirrors based on ZnSe films and doped BaF2 filmshave been fabricated. Their transmittance at a wavelength of0.63 �m did not exceed 10–15% for a reflectance of 96–97% at 10.6 �m.

Implementation of the second solution is preferable,since much thicker zinc sulfide or selenide films can be usedin this case, in combination with films of doped barium fluo-ride.

The most successful solution is to use unequal-thicknessfilms, whose total optical thickness is close to 2.65 �m.

By using the program described in Ref. 5, spectral-splitter coatings were synthesized and fabricated that sepa-rate the radiations of He–Ne and CO2 lasers, on substratesmade from zinc selenide, with the structure S�H3L�4, whereL is a layer of doped BaF2, H is a layer of As2S3, and S is thesubstrate. The optical thicknesses h of the As2S3 layers were�0 /4, while those of the BaF2 layers were 3�0 /4, where �0

=0.7 �m. The resulting coating is a good reflector of theunpolarized radiation of a He–Ne laser and is antireflectivefor the radiation of the CO2 laser with wavelength 10.6 �m.For an angle of incidence of 45°, the reflectance of the coat-ing at a wavelength of 0.63 �m was more than 90%, whileits transmittance for unpolarized radiation at a wavelength of10.6 �m was greater than 98%.

The calculated reflection spectra of this coating for un-polarized radiations incident at an angle of 45° are shown inFig. 1.

The synthesized coatings were fabricated on zinc se-lenide substrates. The back side of the substrate wasantireflection-coated at a wavelength of 10.6 �m. The fabri-cated coatings had a transmittance for unpolarized radiationgreater than 98% at a wavelength of 10.6 �m and reflectance

695 J. Opt. Technol. 76 �11�, November 2009

greater than 90% at a wavelength of 0.63 �m.

METAL-DIELECTRIC COATINGS FOR SEPARATING THEVISIBLE AND IR RADIATIONS OF He–Ne AND CO2LASERS

For metal-dielectric coatings, only the version of maxi-mizing the reflectance at 10.6 �m while maintaining suffi-cient transmittance at the wavelength of the pilot radiation ispossible. In this case, a dielectric separating layer is placed inthe structure of the coating between the metal films; i.e., asystem similar to a Fabry-Perot interferometer is formed.The thickness of the separating layer is chosen in such a waythat the filter was tuned to the wavelength of the pilot radia-tion incident at an angle of 45°. Silver films are traditionallyused as metallic layers; it is also possible to use more stableand chemically inert gold films.

Layers of zinc selenide, arsenic sulfoselenide, and fluo-rides can be used in tandem with gold films. Because ofchemical interaction, silver films are not combined withcompounds of sulfur and selenide, and therefore the separat-ing layer is usually made, for example, from magnesiumfluoride. The calculated spectral characteristics of some

0.6 0.8 1.0

8 10 12

0

96

100

98

20

40

60

Wavelength, µm

Wavelength, µm

Tra

nsm

itta

nce

,%

Ref

lect

ance

,%

FIG. 2. Reflectance and transmittance of a metal-dielectric coating with thestructure Substrate-Au �25 nm�-ZnSe �72 nm�-Au �25 nm�.

0.6 0.8 1.0

8 10 12 14

0

0

1

2

50

100

Wavelength, µm

Wavelength, µmRef

lect

ance

,%

Ref

lect

ance

,%

FIG. 1. Spectral responses of the reflection of dielectric coatings.

695Kotlikov et al.

metal-dielectric coatings on synthetic sapphire substrates areshown in Figs. 2 and 3. The geometrical thicknesses of thelayers are indicated in the coating structures given in thefigure captions.

With time, silver films are subject to aging. To avoidthis, a protective layer of quartz several hundred angstromsthick, which does not qualitatively alter the spectral charac-teristics of the coating but protects the outer layer of silverfrom rapid degradation in time, is deposited from above onthe entire coating.

The calculated reflection curves of metal-dielectric coat-ings synthesized using gold and silver films have a maximumreflectance of 98% at a wavelength of 10.6 �m. The reflec-tance of the coatings fabricated by the authors was no greaterthan 95% in this region. The cause of this degradation of thereflectivity, in all likelihood, was the island structure of therelatively thin films of gold and silver. The thickness of themetallic layers could not be increased in this case because ofthe necessity of maintaining the transmission in the visibleregion. The transmittance of the coatings reached 50% at themaximum.

12

0.6 0.8 1.0

8 10

0

96

100

98

20

40

60

Wavelength, µm

Wavelength, µm

Tra

nsm

itta

nce

,%

Ref

lect

ance

,%

FIG. 3. Reflectance and transmittance of a metal-dielectric coating with thestructure Substrate-Au �18 nm�-MgF2 �200 nm�-Au �18 nm�.

696 J. Opt. Technol. 76 �11�, November 2009

Metal-dielectric coatings thus have sufficient transmit-tance at the given wavelength of the visible region and re-flect virtually all the IR region of the spectrum.

CONCLUSION

It has been shown in this paper that there are severalways to solve the problem of separating the strong radiationof a CO2 laser from pilot radiation by means of interferencecoatings. The use of doped films of barium fluoride in tan-dem with zinc or arsenic sulfoselenides in the synthesis andfabrication of dielectric coatings makes it possible to obtainthin-film systems that simultaneously reflect the visible pilotradiation and antireflective optical elements made from zincselenide at a wavelength of the strong radiation of a CO2

laser. The reverse version is implemented by using metal-dielectric coatings: the pilot radiation of the visible range istransmitted, and the radiation at a wavelength of 10.6 �m isreflected. The possibility of using one solution or the other ineach specific case is determined from the combination oftechnical requirements imposed on such optical systems.

a�Email: ekotlikov@mail.ru, prokashev@aanet.ru, tropal@mail.ru

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2E. N. Kotlikov, E. V. Khonineva, and V. N. Prokashev, “The problem ofreducing optical losses in fluoride films,” Opt. Zh. 71, No. 6, 84 �2004��J. Opt. Technol. 71, 407 �2004��.

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