Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/20—Filters
  • G02B5/28—Interference filters
  • G02B5/285—Interference filters comprising deposited thin solid films
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/08—Mirrors
  • G02B5/0816—Multilayer mirrors, i.e. having two or more reflecting layers
  • G02B5/0825—Multilayer mirrors, i.e. having two or more reflecting layers the reflecting layers comprising dielectric materials only
  • G02B5/0833—Multilayer mirrors, i.e. having two or more reflecting layers the reflecting layers comprising dielectric materials only comprising inorganic materials only
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/08—Mirrors
  • G02B5/0883—Mirrors with a refractive index gradient
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/20—Filters
  • G02B5/28—Interference filters
  • G02B5/289—Rugate filters

Definitions

• This invention relates to dielectric coatings for optical elements, and more particu ⁇ larly, to coatings which have improved resistance to damage by incident radiation of high energy.
• Dielectric coatings for optical applica ⁇ tions are generally formed by vacuum evaporation, sputter ⁇ ing, or low-temperature solution deposition over suitable glass, ceramic, or metal substrate material.
• the particu- lar optical function and the wavelength or wavelengths of use for the optical coating dictates the coating design.
• coating design refers to the number of discrete layers of material to be deposited, the thickness of these layers and materials from which the layers are to be fabricated.
• the difference in refractive index between the materials that form the discrete layers is the physical property that, in combination with the coating design, gives the coating its unique function.
• coatings can be designed to function as reflectors, anti- reflectors, polarizers, and other optical elements.
• Bulk fused silica can be produced by a chemical vapor deposition (CVD) process. Pure silica produced by such a process has very good damage resistance to high-energy laser pulses.
• a CVD process continuously deposits silica in layers each a few angstroms thick. Thousands of layers are deposited to form a monolithic fused silica blank.
• optical waveguide preforms using silica layers with different indices of refraction.
• U.S. Patent No. 3,737,292 issued to D.B. Keck et al. describes an optical waveguide which has a silica core doped to increase its index of refraction and which has another cladding of undoped silica.
• optical coatings such as those used for interference mirrors, interference filters, polariza ⁇ tion filters, antireflecting coatings, beam splitters, etc., are mostly produced by PVD processes (PVD stands for physical vapor deposition), or by immersion methods.
• PVD methods include, for example, high vacuum vapor deposition, cathode sputtering, electron beam sputtering, etc.
• a survey of these methods is given in the article "Systematic Design Approach Leads to Better Optical coatings" by W.T. Beauchamp, B.P. Hichwa and M.H. Imus, Laser Focus/Electro- Optics, May 1988, page 109 ff.
• these techniques still require improvement with regard to the optical quality of the coatings such as that required especially for interference mirrors for laser systems.
• the coating in these processes is built up with individual layers which differ from each other in their properties such as density and thermal expansion coefficient and also differ from the solid substrate material.
• the layers are not amorphous solids but instead they usually have a definite column structure which can in turn lead to depolarization. Improvements can be achieved by ion plating or ion beam sputtering, but defects and voids cannot be avoided. Both of these lead to losses due to absorption and scattering in the passage of light.
• the marked boundaries between the layers where defects generally accumulate are also loca ⁇ tions where losses of transmitted light are increased. At high light power levels (power lasers) this leads to destruction of the layer.
• the layer systems often have great mechanical instability and require use of certain substrates which in turn cause problems with certain applications, e.g., for beam splitters.
• Another disadvantage of the prior art methods of forming coating is that impurities (e.g., due to contamina ⁇ tion by the crucible material) cannot be avoided.
• impurities e.g., due to contamina ⁇ tion by the crucible material
• optical coatings with the smallest possible number of layers are produced, but the layers must differ substantially from each other in their refractive indexes.
• the materials that can be used and the areas of use of layered systems are very limited. Examples of such layer systems include Si0 2 -Si 3 N 4 or Si0 2 -Ti ⁇ 2.
• t is another objective of this invention to provide a multilayer dielectric optical coating in which the index of refraction profile within a single quarterwave layer can be tailored in a stepwise fashion.
• an optical coating is provided which is formed from a number (typically greater than 100) of alternating layers of doped and undoped material.
• the dopant concentra ⁇ tions are held to values such that the variation in the indices of refraction between the doped and undoped layers is small, typically between 0.1 and 5% but can be as high as 15% with special dopants. This permits the doped and undoped layers to be thermomechanically and chemically com ⁇ patible.
• such multi-layer coatings are formed in a controlled atmosphere, preferably an oxidizing atmos- phere containing oxygen, chlorine, fluorine, water or combinations of these or other oxidizing gases, at a temperature high enough to ensure that the coating and any foreign particulates contained in the coating are fully oxidized to thereby prevent either bulk or localized energy absorption when the coating is subject to high energy flux densities such as from a high-energy laser pulse.
• Coatings prepared by the current art have high absorption, typically 10 to 100 times greater than the material deposited by the present invention because of the inability of the current art to fully oxidize the coating material.
• Coatings formed using chemical vapor deposition (CVD) or plasma-assisted CVD permit the various layers of the coatings to be incre ⁇ mentally built up.
• CVD chemical vapor deposition
• plasma-assisted CVD for example, deposition increments as small as 5-10 Angstroms can be achieved.
• Undoped or doped layers of suitable thickness can be formed over such coatings to serve as protective overcoating or even as the support substrates for such coatings.
• the profile of the refractive index within a given coating layer can be tailored with a resolu ⁇ tion of the increment size of between 5-10 Angstroms.
• a given coating layer or series of layers having an index profile approximating a sinusoidal wave or a portion of a sinusoidal wave can be fabricated.
• Reflective optical coatings are produced by having alternating doped and undoped layers or alternating layers of different dopants or doping concentrations, where each of the layers has an optical thickness of one-quarter wavelength for the particular wavelength to be reflected.
• the reflectance and bandwidth of such reflective layers is controlled by the number of layer-pairs of doped and undoped layers or number of layer-pairs of differently doped layers used as well as by the difference in the indices of refrac- tion between the layers.
• Optical reflectors having greater bandwidths are formed by fabricating two or more reflective coatings adjacent to each other in a composite coating. Similarly, optical reflectors that are fully or partially reflecting across more than one spectral region are fabri- cated by controlling the number of layer pairs and quarter- wave optical thickness of the deposited layers.
• a composite stack of reflective coatings is deposited on either the inside or outside (or both) of a quartz envelope surrounding the light source.
• the composite stack consists of sets of reflective coatings of alternating doped and undoped layers or differently doped layers, with each layer of a particular set of layers having a thickness equal to one-quarter of a wavelength to be reflected back into the light source media.
• the composite stack of reflective coatings for a light source is designed to reflect the undesired wavelengths back into the light source medium while transmitting the desired wavelengths.
• a flashlamp is one example of a light source whose broadband spectral output can be tailored using the deposit ⁇ ed reflective coating.
• a flashlamp is a device that converts stored electrical energy into light by means of a sudden electrical discharge.
• Selectively reflective coatings for flashlamps are formed by coating the inside or outside of the fused silica envelope of a flashlamp with a composite stack of reflective coatings.
• the composite reflective coating is comprised of sets of alternating doped and undoped layers or differently doped layers, where each set of layers is designed to reflect a specific wavelength back into the flashlamp media.
• the composite of all sets of coating layers for a flashlamp light source is designed to reflect the undesired wavelengths back into the light source media while transmitting the desired wave ⁇ lengths.
• such flashlamp reflective coatings are easily damaged by the high flux of broadband radiation produced by the flashlamp plasma and thus these current art coatings are of limited utility.
• a flashlamp used to pump solid state laser gain media is one example of a flashlamp light source whose broadband spectral output can be tailored using the inventive deposit ⁇ ed reflective coating. Flashlamps made using the prior art have no reflective coatings and as a consequence only that part of the broadband spatial output that overlaps the narrow absorption bands of the solid state media is used in the pumping process. That portion of the spectral output from the flashlamp that does not overlap the absorption band is unused.
• selectively reflective coatings for laser flashlamps are designed to reflect the unused wavelengths back into the flashlamp media while passing wavelengths that excite the pump bands of the laser. The reflected energy is absorbed and then re-emitted by the flashlamp medium to thereby improve the electrical-to- optical energy conversion efficiency for the flashlamp, over the desired range of wavelengths.
• a flashlamp used to pump Neodymium-doped solid state laser medium is one specific example of a laser flashlamp light source whose broadband spectral output can be tailored to match the pump bands of the dopant Neodymium ion. Neodymium ion pump bands occur over a broad spectral region between 400 and 940 nm.
• a prior art flashlamp has a broad ⁇ band output such that the wavelengths less than 400 nm and greater than 940 nm are unused by the solid state laser medium.
• Selectively reflective coatings for flashlamps used to pump Neodymium-containing solid state laser material are designed to reflect wavelengths between 250 to 400 nm and between 940 to 1200 nm back into flashlamp media while passing wavelengths between 400 and 940 that excite the Neodymium ion pump bands. The reflected energy is absorbed and then re-emitted by the flashlamp medium.
• a typical undoped material for coatings of the type contemplated by the present invention is fused silica.
• Typical dopants are Ti0 2 , Ge0 2 , *P2°5' F ' B 2°3' A1 2°3' 1 > Ce 2 0 3 , Sb 2 0 3 , Ta 2 ⁇ 5 and N.
• the list of typical dopant materials is not meant to be exhaustive nor restrictive; many other elements or materials will produce refractive index changes when used as dopants.
• the important criteria is that the doped and undoped layers maintain thermome ⁇ hani- cal and chemical compatibility.
• This invention is based on the problem of creating an optical coating with improved material properties.
• An optical coating should be largely free of defects.
• Another function of this invention is to develop a process for producing such optical coatings.
• the defect problem is solved according to this invention with regard to the optical coating by means of an optical coating that consists of a glass body with laminar dopings.
• the problem on which this invention is based is solved according to this invention with a process for producing an optical coating whereby a glass body is produced by means of a gas phase process (CVD) by reactive deposition from a reaction gas, and another reaction gas from which the elements doping the glass body are deposited is added to the first reaction gas.
• CVD gas phase process
• the optical coating has a vitreous matrix which is doped differently in a plane in the space coordinate at right angles to the substrate, i.e., it is provided with laminar doping.
• the optical coating is them prepared from a glass body instead of individual layers. To change the physical properties within the glass body, the glass body is doped during production. To achieve different refractive indexes in different planes in the glass body, it is doped with different concentrations - and/or dopants in a laminar pattern.
• a glass body is well known to be a fully dense, defect-free amorphous material, as for example fused silica or other optical glass.
• the reactive layers can b ' e deposited on solid sub ⁇ strates such as glass, glass ceramics, and ceramics.
• the substrate temperature needed for this deposition depends on the reaction gases used is typically about 1000 ⁇ C and may be as high as 1600 ⁇ C.
• the glass matrix is doped periodi ⁇ cally and periods of different doping can also follow each other.
• Doping is understood to refer to the admixture of one or more substances to the chemically pure starting mate ⁇ rials of the glass matrix, and this admixture may also take place in an exchange with one or more starting materials of the glass matrix. Doping is also equated with an increase or decrease in the concentration of one or more starting materials of the glass matrix.
• Suitable dopants include all substances and substance mixtures that can be incorporated into the glass matrix.
• Si0 2 as the glass matrix
• alkali metals, alkaline earth metals, B, Al, Ge, Sn, Pb, Zn, metals of the fourth and fifth secondary groups of the periodic system, W, Y, La, Ce, Nd, or F can be incorporated.
• the amount of dopant(s) depends on the desired change in physical properties of the glass such as refractive index, thermal expansion coefficient, absorption and the maximum compatibility of the dopant in the glass matrix.
• the dopant may be used only in amounts such that the usability of the coating is not impaired, e.g., due to defects.
• the glass matrix can be adapted in composition and/or physical properties to a substrate (lens) that is to be coated. For example, stresses between a glass matrix and a substrate or within the glass matrix itself which develop on cooling due to differences in thermal expansion coefficient can be reduced by adding fluorine and/or titanium dopants.
• optical coatings according to this invention are glass bodies and thus are structures with mechanical stability. They can be produced with relatively great wall thicknesses, so the substrate, which is often useless and interfering for optical purposes can be removed, e.g., by grinding or polishing. In this way, practically absorp- tion-free beam splitters with very little beam misalignment can be produced.
• FIG. 1 is a schematic cross-sectional diagrammatic representation of a conventional dielectric coating.
• FIG. 2 is a schematic cross-sectional representation of a damage-resistant dielectric coating according to the present invention.
• FIG. 3 is a plot of percent reflectance versus number of layer pairs, with small differences in index of refrac ⁇ tion as a parameter, for a multilayer, thin-film dielectric coating comprised, for example, of alternating layers of silica and doped silica.
• FIG. 4 is a plot of optical bandwidth of a reflective coating versus small differences in index of refraction for a multilayer thin film coating.
• FIG. 5 is a schematic representation of a plas a- assisted CVD system for fabricating protective coatings according to the invention.
• FIG. 6 is a plot of measured transmission versus wavelength for a sample coating fabricated using a plasma CVD process according to the invention.
• FIG. 7 is a schematic diagrammatic representation of a multilayer coating according to the invention with a protective over-coating and a substrate formed according to the present invention.
• FIG. 8 is a plot of calculated reflectance versus wavelength for 150 layer-pairs of alternating silica and silica doped with Ti0 2 for a difference in index of refrac ⁇ tion of 0.025.
• FIG. 9 is a plot of calculated reflectance versus wavelength for 250 layer-pairs of alternating silica and silica doped with Ge0 2 (or P 2 0 5 ) for a difference in index of refraction of 0.015.
• FIG. 10 is a plot of calculated reflectance versus wavelength for 400 layer-pairs of alternating silica and silica doped with F for a difference in index of refraction of 0.01.
• FIG. 11 is a plot of calculated reflectance versus wavelength for 1200 layer-pairs of alternating silica and silica doped with B 2 0 3 for a difference in index of refrac ⁇ tion of 0.004.
• FIG. 12 is a diagrammatic representation of a hot plasma broadband ( ⁇ ) light source for which the fused silica envelope used to contain the plasma is coated on the inside with a composite stack of reflective multilayer coatings that are designed to reflect wavelengths __ X 2 and ⁇ 3 and transmit wavelengths ⁇ - ⁇ Q - ⁇ 3 .
• ⁇ hot plasma broadband
• FIG. 13 is a diagrammatic representation of a flash- lamp broadband light source for which the fused silica envelope used to contain the plasma is coated on the inside with a composite stack of reflective multilayer coatings designed to transmit wavelength regions ⁇ 2 and ⁇ 4 and reflect wavelength regions ⁇ 3 and ⁇ i back into the flash- lamp plasma and further schematically illustrating the flow of electrical and optical energy into and out of the flash- lamp plasma.
• FIG. 14 is a schematic cross-sectional representation of the wall of a flashlamp envelope used to pump a Neody- mium-doped solid state laser media and having a dielectric coating for reflecting ultraviolet and infrared energy and a protective coating over the optical coating according to the present invention.
• FIG. 15 is a plot of calculated percent reflectance versus wavelength for the ultraviolet range of the flash- lamp coating of FIG. 14.
• FIG. 16 shows the structure of an interference mirror.
• FIG. 17 shows the structure of an interference filter.
• FIG. 18 shows the structure of an antireflective coating.
• FIG. 1 shows a cross-sectional schematic represen ⁇ tation of a conventional dielectric coating 10 formed on a substrate 12 of, for example, inorganic glass, ceramic, or metal.
• the conventional coating 10 is fabricated by vacuum evaporation, sputtering, or low temperature solution deposi ⁇ tion.
• the coating 10 is formed from a number of layer-pairs of a material A and of a material B, where material A has an index of refraction n a and material B has an index of refraction n b .
• material A and material B are different materials with different physical characteristics.
• a reflective coating for a particular wavelength the individual layers of material A and of material B each have a thickness of one-quarter wavelength within that material.
• the differences in the materials used for each layer and the processes used to produce such coatings cause such coatings to have energy absorbing sites such as voids, local defects, and contamination sites as well as high internal thermomechanical stresses that result in low thresholds for damage when subjected to- high-energy radiation as for example in a laser pulse. This is particu- larly true at the abrupt interfaces between the different materials, typically shown as 14,16.
• the number of layer- pairs of dissimilar materials required to form a reflective coating depends on the magnitude of the refractive index difference between those materials. In general the higher the refractive index difference the fewer the number of layer-pairs required. Typically the index difference may range between 20 to 60% of the value of the low index material and the number of layer-pairs is less than 30-40.
• the chemical stoichiometries of the materials comprising the conventional coating typically are not of the fully oxidized form, leading to bulk optical absorption loss 10 to 100 times greater than that of the desired fully oxidized form.
• FIG. 2 shows a cross-sectional schematic represen ⁇ tation of a damage-resistant dielectric coating 20 accord- ing to the invention.
• the damage-resistant dielectric coating 20 is typically fabricated over a substrate 22 of, for example, fused silica using chemical vapor deposition CVD techniques. When a pulsed plasma assisted process is used, deposition thicknesses as small as 5-10 Angstroms are achieved. Individual coating layers as shown by 24,26 are formed by successively depositing 5-10 Angstrom increments until they build up to the desired layer thickness. Layer- pairs comprised of either pairs of doped and undoped layers or pairs of differently doped layers are formed by continu- ing the CVD gas process and varying the concentration of dopant added to the reactive CVD mixture.
• the index profile of any given layer pair can be a simple square wave 27 varying from a maximum n 2 to n ⁇ or a specific profile such as, for example, a sinusoidal wave 29 having refractive index maxima and minima of n and n l7 respectively.
• the minimum index spatial resolution is 5-10 Angstroms and is determined by the minimum deposition thickness of 5-10 Angstroms. If the doping levels are low enough so that there are distinct interfaces or abrupt changes between layer, the coating as a whole can have properties nearly identical to the undoped substrate material and is substantially free of thermomechanical stress.
• a coating is fabricated at a sufficiently high temperature, for example, greater than 1,000°C under a controlled oxidizing atmosphere condition such as pure 0 2 , this serves to eliminate light absorbing inclusions, elimi ⁇ nate damage-causing structural defects, reduce absorption, and maintain proper chemical stoichiometry.
• the optical absorption losses for the reflective coating by the present invention are measured to be 0.0001% to 0.0002% of the incident light energy at 500 nm compared to measured values of typically 0.05% and at best 0.002% for a conventional coating. This represents an improvement of about 10 to 100 fold for the inventive coating over the prior art.
• CVD CVD
• the difference in the indices of refraction between two adjacent layers can be kept very low and a desired reflectivity characteristic for a coating 2 0 is obtained by use of many alternating layers 24,26 of doped and undoped material or of differently doped material which have very similar indices of refraction.
• the difference in indices of refraction for the coating layers according to the invention would vary between 0.1 and 5%. Because the materials of the differently doped layers 24 and 26 are so similar in thermomechanical properties and because a high temperature CVD produces very dense, defect- free, low optical absorption amorphous films with correct chemical stoichiometry as the films are being formed, it is possible to fabricate a coating 20 having superior damage- resistance to incident optical radiation, approaching the damage resistance of pure fused silica. This means that in high-power laser applications optical coatings produced according to the invention can withstand much higher energy flux densities approaching, for example, that of pure fused silica. Prior art thin film dielectric optical coatings have previously limited laser flux densities to lower levels.
• use of the present invention will permit much higher flux densities to be used.
• Use of the inventive coating will also allow much smaller reflector areas to be able to handle the same energy levels which were previously handled with larger reflectors having the prior art coatings.
• FIG. 3 and FIG. 4 will aid in understanding the significance of the present invention shown by FIG. 2 in comparison to the prior art coatings depicted in FIG. 1.
• FIG 3. is a plot of reflectance R versus number of optical coating layer-pairs for various values for the difference in index of refraction, n -n l between adjacent layers.
• the reflectance R for a multilayer thin-film dielectric coating where each layer thickness is equal to one-quarter wavelength is given by the following equation: where n n ⁇ and ng are the indices for. the high index layer, low index layer and substrate, respectively and N is the number of layer pairs.
• FIG. 4 is a plot of optical coating spectral bandwidth versus the difference in index of refraction, n 2 -n ⁇ , between adjacent layers, and assuming n* j _ « 1.45.
• the band width (4 / for a multilayer, thin-film, dielectric reflector where each layer thickness is equal to one-quarter wavelength is given by
• ⁇ is the bandwidth (half-height, full width) of the
• FIG. 4 shows that using smaller differences in indices of refraction produces smaller bandwidths.
• the present invention permits formation of a large number of layers, in the thousands, a number of groups of different reflector layer-pairs can be formed with each group having a different layer thickness. Each of these different reflector layer-pairs would therefore reflect a different wavelength.
• a wider band reflector is thus formed using a series of layer-pairs, each having a thickness correspond ⁇ ing to a particular portion of a broad band of wavelength to be reflected.
• optical reflectors that are fully or partially reflecting across more than one spectral region are fabricated by the present invention by controlling the number of and quarterwave optical thickness of the deposit ⁇ ed layers.
• FIG. 5 is a schematic representation of a CVD system 30 for producing a series of layers according to the present invention.
• This system is described in an article by H. Bauch, V. Pacquet, and W. Siefert, "Preparation of Optical Fiber Preforms by Plasma-Impulse-CVD", SPIE Vol. 584 Optical Fiber Characterization and Standards (1985) p. 33-37.
• This process is used to make dielectric coatings on the inside of a silica, or quartz, tube 32, which is coaxially positioned in a microwave cavity 34.
• a surrounding tubular furnace 36 is heated to a high temperature of 1000-1200 degrees Centi ⁇ grade which permits deposition of films on a heated sub ⁇ strate to avoid inclusion of undesired elements, defects, or discontinuities in the film and to avoid thermomechanical stresses in the films.
• a gas supply system 38 supplies •SiCl , oxygen, and one or more dopant sources as required to one end of the quartz tube 32.
• the gases are drawn through the quartz tube 32 by means of a vacuum pump 40 at the other end of the quartz tube 32. After the gases have filled the quartz tube, microwave energy from a magnetron 42 triggered by a pulse generator 44 propagates along the axis of the quartz tube 31 and builds up a plasma 46.
• the plasma 46 initiates a reaction between the SiCl 4 and the oxygen to form Si0 2 .
• the pulse duration of about 1.5 ms and the repetition frequency of about -100 cycles per second is adjusted to a continuous flow of gases so that, at the end of the plasma pulse the residual gases flow out and are replaced with a new charge of gases before another microwave pulse is repeated.
• Each microwave pulse produces a deposit thickness of about 5-10 Angstroms which at a rate of 100 microwave pulses per second corresponds to a deposition rate of 500-100 Angstroms per second. This means that in a few seconds a single layer can be formed having a thickness of 1800 Angstroms, which is an optical quarter-wave thickness for a wavelength of 1060 nm in a material with an index of refraction of 1.45. By alternating quarter-wave layers of different doping concentrations, a reflective coating is thus generated for a particular wavelength.
• FIG. 6 is a plot of measured transmission percentage versus wavelength for a sample coating comprised of approxi ⁇ mately 1100 layer-pairs on the interior wall of a quartz tube as described in connection with FIG. 5.
• the sample coating was designed to reflect at 450 nm.
• the difference between the indices of refraction was 0.016 and the layers were alternately doped with and without F and Ge to obtain the difference in indices of refraction.
• Laser damage tests were completed on the optical coating in Fig. 6 by irradiating it with 1.064 ⁇ m wave- length laser pulses having a pulse width of 1.0 x 10-8 sec.
• the measured energy fluence threshold for laser damage was determined to be 36 J/cm 2 .
• Damage thresholds for the best prior art coatings typically range from 5 to 20 J/cm 2 when irradiated with 1.06 m laser pulses having a pulse width of 110 x 10 ⁇ 8 sec.
• FIG. 7 shows a cross-sectional diagrammatic represen ⁇ tation of a multilayer reflective coating 50 of doped and undoped layers according to the invention.
• a protective coating 52 of undoped material such as fused silica, Si0 2 formed to a desired thickness using, for example, a high temperature CVD process as described.
• This protective coating can be formed over reflectors, polarizers, and other optical elements to protect the element from abrasion and wear. In the event of surface damage, such a protective coating could be refinished without disturbance to the underlying reflective coating.
• FIG. 7 also shows a substrate 54 which can be formed as thin layers using a CVD process.
• This substrate 54 which can be formed subsequently or prior to formation of the coating 52, serves as a support substrate for the coating 50 and is made to whatever thickness is required.
• FIGs. 8-11 show calculated reflectance plots for coating designs having layer-pairs ranging in number from 150 to 1200.
• the layer-pairs comprise alternating layers of undoped Si0 2 and doped Si0 with various dopants being used as indicated. These plots were calculated using standard thin film calculation methods and using a thin film software program for IBM compatible personal computers called "FILM*CALC" available from FTG Software Associates, Post Office Box 358, Chatham, New Jersey, 07928.
• FIG. 8 shows a plot of reflectance versus wavelength for 150 layer-pairs of Si0 with alternate layers doped with 6 mol% Ti0 2 which produces a difference in index of refraction of 0.025.
• FIG. 9 shows a plot of reflectance versus wavelength for 250 layer-pairs of Si0 2 with alternate layers doped with 10 mol% GE0 2 (or P 0s) which produces a difference in index of refraction of 0.015.
• FIG. 10 shows a plot of reflectance versus wavelength for 400 layer-pairs of Si0 2 with alternate layers doped with 2% F which produces a difference in index of refrac ⁇ tion of 0.01.
• FIG. 11 shows a plot of reflectance versus wavelength for 1200 layer-pairs of Si0 with alternate layers doped with 5% B 2 0 3 which produces a difference in index of refraction of 0.004.
• FIGS. 8-11 show that the number of layer pairs required to achieve high reflectance increases and bandwidth de ⁇ creases as the magnitude of the index difference decreases. Although these designs contain a large number of layer- pairs, since the alternating doped layers are so lightly doped, the damage thresholds for these coatings approach that of undoped Si0 2 .
• FIG. 12 shows a cross-sectional diagrammatic represen ⁇ tation of a composite stack of selectively reflective coatings 55 deposited on a fused silica envelope 57 used to tailor the spectral output from a broadband light source such as a hot plasma 58 produced by an electrical dis ⁇ charge. The electrical discharge occurs between two conductors 59 and through the gas media contained inside the envelope 57.
• a flashlamp is one example of the type of light source shown in Fig. 12.
• a composite stack of damage resistant reflective coatings 55 is deposited on either the inside (as shown in Fig. 12) or outside or both of the fused silica envelope surrounding the light source.
• the composite stack consists of sets of reflective coatings of either alternating doped and undoped layers or differently doped layers with each layer of a particular set of layers having a thickness equal to one quarter of a particular wavelength to be reflected back into the hot plasma 58.
• the composite stack of reflective coatings 55 is designed to reflect the undesired wavelengths 61 back into the hot plasma 58 while transmitting the desired wavelengths 62.
• a thick, undoped coating layer 63 is deposited over the composite coating stack 55 to protect it from the hot plasma 58. The protect-
• the coating has a high resistance to optical damage by the intense flux of broadband optical radiation produced by the light source.
• FIG. 13 is a diagrammatic representation of a flash- lamp 70 illustrating the flow of electrical energy 80 and optical energy 71 into and out from the hot plasma 74 contained in the flashlamp.
• the flashlamp is an intense, pulsed broadband light source schematically shown as having a significant optical energy output over the spectral region ⁇ 71.
• the fused silica envelope 76 that contains the hot plasma is coated on the inside with a composite sta ⁇ k of the inventive reflective coatings 72 and designed to reflect the spectral regions ⁇ ]_ 78 and ⁇ 3 77 and transient light in the spectral regions ⁇ 2 73, and ⁇ 75.
• the reflected light energy 77,78 is reabsorbed by the plasma 74 and then remitted as a broadband emission 71 of ⁇ ⁇ bandwidth.
• the intense broadband output energy of the flash- lamp can be tailored to give one or more outputs over specific spectral bands. Further, the inventive reflective coating remains undamaged by the intense light incident on it.
• FIG. 14 shows a cross-sectional diagrammatic represen ⁇ tation of a portion of a flashlamp envelope wall that is specifically designed to pump a Neodymium-containing solid state laser gain medium.
• the envelope in operation contains a hot plasma giving an intense, pulsed optical energy output which passes through the envelope and is subsequently used to pump the Neodymium energy bands of the solid state laser gain medium.
• the optical energy produced by a flashlamp is broadband.
• the energy necessary to pump the host laser falls within a narrower band so that a great deal of the flashlamp energy output is wasted because it cannot pump the host laser medium.
• reflectors are added to the flashlamp to return undesired energy back into the flashlamp medium.
• the broadband output energy of the flashlamp output can, in effect, be tailored to match the bandwidth of the pump bands of the host laser. Overall, as described in Fig. 13, this increases the efficiency of converting the electrical energy exciting the flashlamp into stored optical energy in the laser medium. In this specific case the efficiency of a flashlamp can be greatly increased by reflecting energy in the infrared and ultra ⁇ violet wavelengths that is not used to pump the Neodymium- doped solid state laser host back into the flashlamp medium. It is desirable to let wavelengths between 400 and 940 nm pump the Neodymium-doped laser host while reflecting ultraviolet wavelengths between 250 and 400 nm and infrared wavelengths between 940 and 1200 nm.
• FIG. 14 shows an example of a reflector design for a flashlamp used to pump a Neodymium-doped solid state laser host.
• the outer wall 90 of the flashlamp is a fused silica tube.
• An infrared reflective coating 92 designed to reflect between 940 and 1200 nm is formed on the inner surface of the wall 90.
• the infrared coating is formed of a composite stack of groups of multilayer-pairs with each of said multilayer-pairs forming a narrowband reflector for predetermined wavelengths within the range of 940 to 1200 nm.
• an ultraviolet coating 94 is formed adjacent to the infrared reflective coating 92.
• the ultraviolet coating is formed of a composite stack of groups of multilayer-pairs with each of said pairs forming a narrowband reflector for predetermined wavelengths within the range of 250 to 400 nm.
• a protective coating 96 of pure Si0 2 is formed as the inner surface of the flashlamp.
• the protective coating is designed to protect the groups of reflective layer-pairs from corrosion by the hot flashlamp plasma.
• the protective coating overlies the ultraviolet reflective coating 94.
• FIG. 15 shows a plot of calculated reflectance versus wavelength for the ultraviolet coating 94 of the flashlamp coating of FIG. 14.
• the ultra ⁇ violet coating is formed of a stack of 32 groups of 100 layer pairs with each of said layer-pairs forming a " narrow- band reflector for predetermined wavelengths within the range of 250-450 nm.
• the difference in indices of refrac ⁇ tion for alternate layers is 0.05 with the thickness of each layer being the optical quarter-wave thickness.
• Table 1 is a chart indicating the nominal wavelength, bandwidth, and number of layer pairs for each of the narrow ⁇ band reflectors forming the composite broadband ultraviolet reflective coating for the flashlamp.
• FIG. 14 shows that broadband reflectors of the type described immediately above can be obtained by, in effect, cascading a series of narrow-band reflective coatings of the type described in connection with FIGS. 1 and 8-11.
• the principle of the coating technique used to produce optical waveguides is used i.e., CVD or plasma CVD methods with starting materials of extremely high purity.
• the glass body is produced by reactive deposition of the reaction gases by means of a heterogeneous reaction on the substrate.
• these methods include CVD with reactive deposi ⁇ tion on the substrate, plasma impulse CVD, plasma activated CVD, ECR microwave CVD.
• Production of the optical coating can take place at atmospheric pressure (CVD process, plasma burner) , in the mbar range (plasma activated CVD) or in the Pa range (ECR microwave CVD) .
• the plasma impulse CVD method is especially preferred.
• reaction gases examples here include metal chlorides such as metal hydrides, organometal compounds, oxygen, nitrogen oxides, and ammonia.
• flat or curved substrates can be coated.
• planer interference mirrors, hollow interference mirrors, antireflective lenses, polarization mirrors, etc. can be produced.
• the substrate must be heated during coating in accor ⁇ dance with the coating method and reaction gas used in each case.
• the sub ⁇ strate temperature is up to 1600 ⁇ C.
• the glass body can be doped with fluorine and/or titanium during the production process.
• relatively thick optical coatings that may contain more than 1000 periods can be produced.
• Glass processing technology can be used for the glass bodies of the optical coatings according to this invention, especially drawing to yield thinner layers, rolling and/or fusing to another glass or another glass body with an optical coating according to this invention. In this way two glass bodies can be heated to the required temperature as a whole, e.g., in a kiln, and then fused together.
• the thickness of the laminar doping and of the undoped areas is adjusted according to ⁇ the wavelength or wavelength range of interest (e.g., in interference mirrors for lasers) .
• ⁇ the wavelength or wavelength range of interest
• any given refractive index profile can be established easily and reproducibly in the optical coatings according to this invention as long as typical material values for the refractive index are not exceeded. This is accomplished through appropriate mass flow regulation of the doping gas that alters the refractive index of the untreated glass. A given spectral transmission performance of the glass body can easily be achieved on the basis of this possibility.
• Interference filters or interference mirrors can be produced with such a refractive index pattern without any interfering secondary bands.
• Optical coatings produced in this way (having a high reflection for a given wavelength) have a high radiation stability and a very high destruction threshold under bombardment with high energy laser pulses.
• the destruction threshold is especially high when a glass body is produced from oxide glass with a plasma impulse CVD method with microwave excitation.
• FIGURE 16 shows the structure of an interference mirror.
• FIGURE 17 shows the structure of an interference filter.
• FIGURE 18 shows the structure of an antireflective coating.
• FIGURE 16 shows schematically a detail from an inter ⁇ ference mirror 101 consisting of a substrate 102 and an optical coating 103.
• the optical coating contains 1.5 periods of laminar doping 104 whose concentration varia ⁇ tions are shown in a diagram 106 next to the interference mirror 101.
• d indicates the thickness of the optical coating 103
• c indicates the concentration of the dopant(s) and thus also the relative change in refractive index
• P indicates the thickness of a period
• ⁇ /4 is a refraction range for perpendicular incident radiation (arrows) of the wavelength .
• FIGURE 17 shows schematically a detail of an inter ⁇ ference filter 111 which consists only of an optical coating 113.
• the doping area 118 may contain more than 1000 periods of the thickness P A
• the doping area 117 which begins seamlessly- at point D next to doping area 118 may contain more than 1000 periods of a thickness P B .
• Concentration curve 115 of the 2.5 periods of laminar doping 114 is shown in a diagram 116 next to interference filter 111 and can be described by two sine functions linked together at point d* ⁇ which are in turn assigned to different doping areas 117 and 118.
• diagram 116 d indicates the respective thickness of the optical coating (13)
• c is the concentration of the dopant(s) and thus also the relative change in refractive index
• P A is the thickness of a period in the doping area (17)
• P B is the thickness of a period in the doping area (18)
• I s a/4 is a reflection range for perpendicularly incident radiation of the wavelength of Y A
• ⁇ B 4 i* 3 a reflection range for perpendicularly incident radiation of the wavelength r 3 .
• a spectrum 119 is shown above interference filter 111 illustrating the operation of interference filter 111.
• T indicates the transmission of 0-100%, and is the corresponding wavelength.
• the arrows 120 indicate "white" light (or electromagnetic radiation of a different wavelength) which strikes the interference filter at right angles.
• the inter ⁇ ference filter functions like an interference mirror (see FIGURE 1) and reflects the incident light of this wave ⁇ length, symbolized by arrows 121.
• the nonreflected light, symbolized by arrow 122 penetrates practically unhindered through interference filter 111, so the gap 123 in the arrows 122 symbolizes the impermeability of interference filter 111 for light (or the electromagnetic radiation) of the wavelength range around r B and r ⁇ .
• the impermeability of the interference filter 111 for a spectral range is achieved, as indicated in FIGURE 17, by varying the thickness of a period in a doping area. Instead of this, it can also be achieved by varying the concentra ⁇ tion c of the dopant or by means of other or additional dopants or by varying the refractive index of the glass matrix (by adding or reducing the substances that influence the refractive index) or through a combination of these possibilities.
• FIGURE 18 shows schematically a detail of lens 131 with multiple antireflective coatings, consisting of a lens 132 and an optical coating 133.
• the optical coating 133 contains four doping areas 137, 138, 139 and 140) with laminar dopings 134 in two different concentrations.
• the thicknesses of the individual doping areas 137 to 140 are different and can be seen from the concentration curve 135 (of the dopant) in diagram 136.
• d and c have the same meanings as in FIGURES 16 and 17.
• the concentration curve 135 of doping areas 137 to 140 is essentially a stepwise curve and can easily be implemented in a high quality, e.g., by adding to and/or decreasing dopants during production of the optical coating 133.
• the optical coating 133 consists only of a glass matrix, is essentially amorphous and is practically free of defects.
• optical coating consisting of F- and Ge-doped Si0 2 is applied to the inside surface of a quartz glass tube with an inside diameter of 17.2 mm and a length of 1.25 m over a coating area of 0.5 m by means of a PICVD method (see European Patent 36,191).
• the glass body of the optical coating is produced on an atomic scale with a very high "repeat frequency", where the concentration of dopant is varied continuously, i.e., the change in concentration is on a monomolecular layer of the glass body.
• a gas generator supplies the reaction gas mixture of 50 ml (SiCl 4 + GeCl 4 ), 200 ml 0 2 and 2 ml CC1 2 F 2 (all quantities given are based on 20°C and 1 bar) into the quartz glass tube.
• the GeCl 4 mass flow is controlled according to a sine function with a period of 8.1 sec between a mass flow of 1 and 7 ml/min.
• the SiCl 4 mass flow is reduced accordingly so the total mass flow of chlorides is 50 ml/min.
• the plasma is ignited by microwave pulses of a magne ⁇ ton, model XT 1600, pulse power 5kW, frequency 2.45 GH Z .
• the 2 ml CC1 2 F 2 content in the reaction gas remains constant, so there is constant F doping which reduces the mechanical stresses so there are no internal stresses that could lead to destruction of the optical coating during cooling of the tube.
• An optical coating with 1000 periods is produced.
• the result is a mirrorized coating on the quartz glass tube for a length of about 40 cm.
• the quartz glass tube is then sawed in pieces on which a reflection coefficient of 0.99 for a wavelength of 1060 nm is determined.
• This mirror will reflect 10 ns laser pulses without being damaged (laser wavelength 1060 nm) with an energy density of 35 J/cm 2 .
• optical coatings have previous ⁇ ly been produced by applying individual layers usually be means of PVD methods (physical vapor deposition) . These layers have a high number of defects especially at inter ⁇ faces between the layers, and they also differ substan ⁇ tially from each other and the substrate in their physical properties.
• the new optical coating is largely free of defects and has superior physical properties.
• the optical coating is prepared from a glass body instead of individual layers. To change the physical properties within the glass body, the glass body is doped during production. To achieve differences in refractive indexes over areas of the glass body, it is doped with different concentrations and/or different dopants in a laminar fashion. Interference mirrors, interference filters, antireflective coatings, polarization filters.

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• 1989
  • 1989-09-05 EP EP19890910726 patent/EP0402429A4/en not_active Withdrawn
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