High-efficiency multiwavelength beam expander employing dielectric-enhanced mirrors

Abstract

A high-efficiency, multiwavelength beam-expander optical system that employs dielectric-enhanced mirrors is disclosed. Each mirror includes a reflective multilayer coating formed from alternating layers of HfO.sub.2 and SiO.sub.2 that define, in order from the substrate surface, at least first and second sections, wherein the HfO.sub.2/SiO.sub.2 layer thicknesses are generally constant within a given section and get smaller section by section moving outward from the substrate surface. The first and second sections are respectively configured to optimally reflect different operating wavelengths so that the beam-expander optical system has an optical transmission of greater than 95% at the different operating wavelengths.

Claims

1. A method of forming a high-efficiency beam-expander optical system for use at ultraviolet (UV), visible (VIS) and infrared (IR) operating wavelengths, comprising: diamond-turning and polishing a first mirror substrate and a second mirror substrate to respectively form a first mirror having a convex substrate surface and a second mirror having a concave substrate surface; forming on each of the convex substrate surface and the concave substrate surface a reflective multilayer coating comprising alternating layers of HfO.sub.2 and SiO.sub.2 having respective layer thicknesses τ.sub.H and τ.sub.S, including arranging the HfO.sub.2 and SiO.sub.2 layers in at least three sections S1, S2 and S3 in order outward from the convex substrate surface or the concave substrate surface, with the three sections S1, S2 and S3 respectively configured to optimally reflect the IR, VIS and UV operating wavelengths; and arranging the first mirror and the second mirror in an off-axis, afocal configuration having greater than unity magnification and an optical transmittance T.sub.BE>95% at each of the UV, VIS and IR operating wavelengths.

2. The method according to claim 1, the forming the reflective multilayer coating comprising alternating layers of HfO.sub.2 and SiO.sub.2 comprises using plasma ion assisted deposition (PIAD).

3. The method according to claim 2, wherein the plasma ion assisted deposition (PIAD) comprises rotating the first mirror substrate and the second mirror substrate.

4. The method according to claim 2, wherein the plasma ion assisted deposition (PIAD) comprises employing a reversed mask process.

5. The method according to claim 1, wherein at least one of the convex substrate surface and the concave substrate surface includes one or more surface defects, and further including: coating the at least one of the convex substrate surface and the concave substrate surface with an SiO.sub.2 layer; plasma processing the SiO.sub.2 layer; and forming the reflective multilayer coating comprising alternating layers of HfO.sub.2 and SiO.sub.2 atop the plasma-processed SiO.sub.2 layer.

6. The method according to claim 1, wherein the diamond-turning and polishing is carried out so that the convex substrate surface and the concave substrate surface each has a root-mean-square (RMS) substrate surface roughness MSR.sub.RMS of less than 4.5 nm.

7. The method according to claim 1, wherein the first mirror substrate and the second mirror substrate are each formed from an aluminum alloy, and wherein the layer thicknesses τ.sub.H and τ.sub.S in section S1 are on average greater than those in section S2 and the layer thicknesses τ.sub.H and τ.sub.S in section S2 are on average greater than those in section S3.

8. The method according to claim 1, wherein section S1 is substantially transmissive to a mid-wavelength IR (MWIR) wavelength, section S2 is substantially transmissive to the IR operating wavelength and the MWIR wavelength, and section S3 is substantially transmissive to the VIS and IR operating wavelengths and the MWIR wavelength.

Description

BRIEF DESCRIPTION OF τHE DRAWINGS

(1) The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) and together with the Detailed Description serve to explain the principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which:

(2) FIG. 1 is a plot of the reflectance R (%) versus wavelength λ (nm) for a diamond-turned and optically polished aluminum mirror having a mirror substrate made of an aluminum alloy (Al 6061-T6) and including a conventional quarter-wavelength HfO.sub.2/SiO.sub.2 multilayer reflective coating optimized at λ=1064 nm, wherein the plot illustrates relatively low reflectances at operating wavelengths λ of 355 nm, 532 nm and 1064 nm;

(3) FIG. 2 is a schematic diagram of an example two-mirror, off-axis beam expander according to the disclosure, shown along with a laser source, wherein each mirror includes the HfO.sub.2/SiO.sub.2 reflective multilayer coating as disclosed herein that provides the beam expander with an optical transmittance T.sub.BE>95% for ultraviolet, visible and infrared operating wavelengths;

(4) FIG. 3 is a close-up, cross-sectional view of an example mirror used in the beam expander of FIG. 2 and shows a portion of the mirror substrate and the corresponding reflective multilayer coating formed thereon, illustrating how the coating layers have a thickness that generally decreases with distance (z-direction) from the substrate surface;

(5) FIG. 4 is a plot of the refractive index n (at 355 nm) versus the distance z (nm), illustrating an example of how the HfO.sub.2/SiO.sub.2 layers of the reflective multilayer coating define different sections S1 through S3 having different (average) layer thicknesses;

(6) FIG. 5 is a plot of the reflectance R (%) versus wavelength λ (nm) for an example beam-expander mirror with a reflective multilayer coating having a varying-thickness configuration such as shown in FIG. 4;

(7) FIG. 6 is a cross-sectional diagram of an example beam-expander mirror illustrating how the reflective multilayer coating includes three sections S1, S2 and S3, with the uppermost section S3 reflecting UV light, the middle section S2 reflecting visible light, and the lowermost section S1 reflecting NIR light, while the substrate surface reflects MWIR light;

(8) FIG. 7 is a plot of the reflectance R (%) versus wavelength λ (nm) illustrating an example reflectance spectrum for an example beam-expander mirror having a reflective multilayer stack configured for high-efficiency reflectance at operating wavelengths λ of 355 nm, 532 nm and 1064 nm, as well as at SWIR wavelengths;

(9) FIG. 8 is similar to FIG. 7 and illustrates an example reflectance spectrum for an example beam-expander mirror having a reflective multilayer stack configured for high-efficiency reflectance at operating wavelengths λ of 355 nm, 532 nm, 1064 nm, SWIR wavelengths and MWIR wavelengths;

(10) FIG. 9 is a plot of the scattering loss SL (%) versus the root-mean-square (RMS) substrate surface roughness MSR.sub.RMS(nm) for different operating wavelengths λ of 355 nm, 532 nm and 1064 nm, illustrating the impact of the substrate surface roughness on the scattering loss due to optical scattering of light at the different operating wavelengths;

(11) FIG. 10 is a plot of the optical transmittance T.sub.BE(%) of the beam expander of FIG. 2 as a function of the RMS substrate surface roughness MSR.sub.RMS(nm) for different operating wavelengths λ of 355 nm, 532 nm and 1064 nm, illustrating the impact of the substrate surface roughness on the beam expander optical transmittance and the need to maintain the RMS substrate surface roughness below a threshold value in order to achieve a high-efficiency optical transmittance of T.sub.BE>95%; and

(12) FIGS. 11A through 11C are schematic side views that illustrate an example method of mitigating defects on the substrate surface using an intermediate SiO.sub.2 layer that is processed using plasma-ion etching prior to the formation of the HfO.sub.2/SiO.sub.2 reflective multilayer coating as disclosed herein.

DETAILED DESCRIPTION

(13) Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure.

(14) The claims as set forth below are incorporated into and constitute a part of this Detailed Description.

(15) Cartesian coordinates are shown in some of the Figures for the sake of reference and are not intended to be limiting as to direction or orientation.

(16) The acronym “SWIR” stands for “short-wavelength infrared” and represents an example wavelength range from about 900 nm to about 1700 nm. Likewise, the acronym “MWIR” stands for “mid-wavelength infrared” and represents an example wavelength range from about 1700 nm to about 5000 nm. The acronym “IR” stands for “infrared” and can include NIR, SWIR and MWIR wavelengths unless otherwise noted. The acronym “RMS” stands for “root-mean square.”

(17) Also in the discussion below, the term “operating wavelength” is denoted λ and means a wavelength for which the beam expander and the mirrors therein are designed to be used. In the examples below, the beam expander is designed to work for at least three operating wavelengths λ in the ultraviolet (UV), visible (VIS) and infrared (IR) ranges, respectively. In an example, the operating wavelengths are those that can be generated by a high-power Nd:YAG laser, including by frequency multiplication and/or frequency modification techniques known in the art.

(18) It will be understood by those skilled in the art that the operating wavelength has an attendant “operating waveband” or “operating bandwidth” or “linewidth” Δλ about the operating wavelength, which in an example is defined by the bandwidth of the light source (or light sources) that generates/generate light of the operating wavelengths λ. A typical linewidth Δλ of an Nd:YAG laser is less than 1 nm.

(19) The alternating layers of HfO.sub.2 and SiO.sub.2 are described herein using the shorthand notation “HfO.sub.2/SiO.sub.2” and in like manner the respective thicknesses τ.sub.H and τ.sub.S of the alternating layers are denoted τ.sub.H/τ.sub.S.

(20) The term “high efficiency” as used in connection with the beam expander disclosed herein means that the beam expander has an optical transmittance T.sub.BE>95% at each of the operating wavelengths for which the beam expander was designed.

(21) Also, the term “optimally reflect,” when used in connection with a given operating wavelength and a section SN of the HfO.sub.2/SiO.sub.2 layers, is understood as being measured relative to the other operating wavelengths, so that the given operating wavelength is understood as having a higher reflectance than the other operating wavelengths.

(22) High-Efficiency Beam Expander

(23) FIG. 2 is a schematic diagram of an example beam-expander optical system (“beam expander”) 10 that includes two off-axis mirrors 20 and 30. The mirror 20 includes a mirror substrate 21 with a convex substrate surface 22 that supports a reflective multilayer coating 24 having a top surface 26. The mirror 30 includes a mirror substrate 31 with a concave substrate surface 32 that supports a reflective multilayer coating 34 having a top surface 36. In an example, top surfaces 26 and 36 interface with an ambient environment 60 that includes air or vacuum having a nominal refractive index of n=1. In an example, top surfaces 26 and 36 define respective “mirror surfaces” or “reflective surfaces” for mirrors 20 and 30.

(24) In an example, reflective multilayer coatings 24 and 34 have the same structure, in which case the different reference numbers for the coatings denote on which mirror substrate 21 or 31 the reflective multilayer coating resides. As discussed in greater detail below, reflective multilayer coatings 24 and 34 are made up of layers of the dielectric materials HfO.sub.2 and SiO.sub.2. The HfO.sub.2/SiO.sub.2 layers are configured to enhance the reflectance of mirrors 20 and 30 so that beam expander 10 can have a high-efficiency optical transmittance T.sub.BE of greater than 95% for UV, VIS and IR light.

(25) In an example, beam expander 10 includes a housing H that operably supports mirrors 20 and 30. In an example, housing H is made of diamond-turned lightweight metal, such an aluminum alloy, and mirrors 20 and 30 are formed such that they are integral with the housing to provide mechanical and thermal stability.

(26) In an example, mirror substrates 21 and 31 are made of metal. Example metals include non-ferrous metals that can be diamond turned, and in particular include nickel and nickel alloys, magnesium and magnesium alloys, copper and copper alloys, and aluminum and aluminum alloys. In an example, the metal is a lightweight metal such as aluminum, an aluminum alloy, magnesium or a magnesium alloy, so that the beam expander 10 can be made lightweight.

(27) The beam expander 10 is configured to receive a collimated laser beam 40 of a first diameter D1 and form therefrom an expanded, collimated laser beam of diameter D2. The beam expander 10 thus has an afocal configuration with a magnification M.sub.BE=D2/D1. For beam expansion, M.sub.BE>1, i.e., beam expander 10 has greater than unity magnification. In an example, collimated laser beam 40 originates from a high-power laser 50 that can emit wavelengths over multiple operating wavelengths (and thus wavelength bands) in the UV, VIS and IR ranges, e.g., 355 nm, 532 nm and 1064 nm.

(28) In an example, mirror substrates 21 and 31 are made of a lightweight metal, such as an aluminum alloy, e.g., Al 6061-T6. In an example, substrate surfaces 22 and 32 are diamond turned to define the respective convex and concave curvatures. As noted above, in an example, mirror substrates 21 and 31 can be defined by housing H and are formed integrally therewith. In an example, mirrors 20 and 30 are spherical mirrors, while in other examples they can have different shapes, such as aspheric, cylindrical, anamorphic, etc.

(29) HfO.sub.2/SiO.sub.2 Multilayer Coatings

(30) In an example, reflective multilayer coatings 24 and 34 are each formed from alternating layers of HfO.sub.2 and SiO.sub.2, denoted as HfO.sub.2/SiO.sub.2. FIG. 3 is a close-up, cross-sectional view of either mirror 20 or mirror 30 and shows a portion of mirror substrate 21 or 31 and the corresponding reflective multilayer coating 24 or 34. The +z-direction is perpendicular to substrate surface 22 or 32 as shown. The HfO.sub.2/SiO.sub.2 layers have respective thicknesses τ.sub.H and τ.sub.S, which in analogous fashion are denoted in shorthand as τ.sub.H/τ.sub.S. In the discussion below, the HfO.sub.2/SiO.sub.2 layers that define reflective multilayer coatings 24 and 34 are divided up into N sections SN, e.g., sections S1, S2, . . . , SN, with section S1 being immediately adjacent (i.e., closest to) substrate surface 22 or 32. The thicknesses τ.sub.H/τ.sub.S of the HfO.sub.2/SiO.sub.2 layers in a given section SN are denoted SN (τ.sub.H/τ.sub.S) for ease of discussion. The average thicknesses τ.sub.H/τ.sub.S of the HfO.sub.2/SiO.sub.2 layers in a given section SN are denoted SN(τ.sub.H/τ.sub.S).sub.AVG.

(31) A characteristic of reflective multilayer coatings 24 and 34 is that the thicknesses τ.sub.H/τ.sub.S of the HfO.sub.2/SiO.sub.2 layers generally change with distance in the +z direction, i.e., in the direction away from substrate surface 22 or 32 to top surface 26 or 36 of the reflective multilayer coating. In an example, the changes in thicknesses τ.sub.H/τ.sub.S can occur in a stepped fashion, i.e., wherein within each section SN the layer thicknesses τ.sub.H/τ.sub.S are substantially constant but change from section to section. Also, there can be some variation in the layer thicknesses τ.sub.H/τ.sub.S in a given section SN wherein most but not all of the layer thicknesses τ.sub.H are substantially the same and most but not all of the layer thickness τ.sub.S are substantially the same. In an example, SN(τ.sub.H/τ.sub.S).sub.AVG>SN+1(τ.sub.H/τ.sub.S).sub.AVG, i.e., the average thicknesses τ.sub.H/τ.sub.S within a given section SN are greater than those of the overlying section SN+1. So, for example, if reflective multilayer coatings 24 and 34 are divided into three sections S1, S2 and S3, then in the example, S1(τ.sub.H/τ.sub.S).sub.AVG>S2(τ.sub.H/τ.sub.S).sub.AVG>S3(τ.sub.H/τ.sub.S).sub.AVG.

(32) FIG. 4 is a plot of the refractive index n (at λ=355 nm) versus distance z (nm) that shows an example of the varying thicknesses of the different SiO.sub.2 and HfO.sub.2 layers, as well as the overall thickness of reflective multilayer coating 24 or 34. The SiO.sub.2 layers have a refractive index n of about 1.5 while the HfO.sub.2 layers have a refractive index of about 2.1. In an example, the SiO.sub.2 layers generally decrease in thickness τ.sub.S in stepwise fashion between sections S1 and S3, e.g., from 90 nm≤τ.sub.S≤190 nm in section S1 to 60 nm≤τ.sub.S≤70 nm in section S3. Likewise, the HfO.sub.2 layers generally decrease in thickness τ.sub.H between sections S1 and S3, e.g., from 130τ.sub.H 140 nm in section S1 to 40 nm≤τ.sub.H≤50 nm in section S3.

(33) Table 1 below summarizes example ranges for the thicknesses τ.sub.S and τ.sub.H in each of the sections S1, S2 and S3.

(34) TABLE-US-00001 TABLE 1 Summary of τ.sub.S and τ.sub.H for sections S1, S2 and S3 Thickness S1 (IR) S2 (VIS) S3 (UV) τ.sub.S  90 nm to 140 nm 70 nm to 90 nm 60 nm to 70 nm τ.sub.H 130 nm to 140 nm 50 nm to 70 nm 40 nm to 50 nm

(35) The reflective multilayer coating 24 or 34 of FIG. 4 has an overall thickness TH as measured from substrate surface 22 or 32 to top surface 26 or 36 of about 7250 nm. In an example, a relatively thick SiO.sub.2 layer 68 may be added as an outermost capping layer that defines top surface 26 or 36 and that further increase durability to laser irradiation. Likewise, FIG. 4 shows that the SiO.sub.2 layer immediately adjacent substrate surface 22 or 32 is thinner (e.g., 90 nm) than the next SiO.sub.2 layer (e.g., 180 nm or 190 nm). So, as emphasized above, there can be some variation in the thicknesses τ.sub.H/τ.sub.S of the HfO.sub.2/SiO.sub.2 layers in a given section SN.

(36) FIG. 5 is a plot of the reflectance R (%) versus wavelength λ (nm) for an example “triple band” reflective multilayer coating 24 or 34 designed for use at operating wavelengths λ of 355 nm (UV), 532 nm (VIS) and 1064 nm (IR). The horizontal dashed line shows the R=95% reflectance value, and it can be seen that the reflectance R is greater than 95% for each of the wavelengths λ of interest.

(37) FIG. 6 is a schematic diagram of an example configuration of reflective multilayer coating 24 or 34 that illustrates the operation of the multilayer coating in providing such high reflectance at the three example operating wavelengths of interest (λ=355 nm, 532 nm and 1064 nm), as well as MWIR wavelengths that pass through the reflective multilayer coating and reflect from substrate surface 22 or 32. The reflectance of the MWIR wavelengths from substrate surface 22 or 32 (e.g., R>92% or R>95%) enables these wavelengths to be transmitted through beam expander 10, e.g., in the opposite direction of laser beam 40. In an example, this allows for detecting targets (not shown) at the MWIR wavelengths through beam expander 10, with the signal-to-noise ratio being defined by the beam expansion ratio or magnification M=D2/D1.

(38) The example reflective multilayer coating 24 or 34 of mirror 20 or 30 shown in FIG. 6 is divided into three sections S1, S2 and S3, with section S1 being configured to provide a high reflectance R (i.e., optimally reflect) for the NIR operating wavelength of λ=1064 nm while being substantially transmissive to MWIR wavelengths. The close-up inset shows the HfO.sub.2/SiO.sub.2 layers in section S1. The sections S2 and S3 also include the HfO.sub.2/SiO.sub.2 layers, wherein S1(τ.sub.H/τ.sub.S).sub.AVG>S2(τ.sub.H/τ.sub.S).sub.AVG>S3(τ.sub.H/τ.sub.S).sub.AVG.

(39) Section S1 has an HfO.sub.2/SiO.sub.2 multilayer structure with the thickest HfO.sub.2/SiO.sub.2 layers (on average), e.g., the thickness τ.sub.H is in the range from 130 nm to 140 nm for most if not all of the HfO.sub.2 layers and the thickness τ.sub.S is in the range from 90 nm to 190 nm for most if not all of the SiO.sub.2 layers.

(40) Section S2 is the middle section and is configured to provide a high reflectance R (i.e., to be optimally reflective) in the visible (VIS) operating wavelength of λ=532 nm while being substantially transmissive for the NIR and the MWIR wavelengths. The HfO.sub.2/SiO.sub.2 layers in section S2 have intermediate thicknesses τ.sub.H and τ.sub.S, e.g., a thickness τ.sub.H in the range from 50 nm to 70 nm for most if not all of the HfO.sub.2 layers and a thickness τ.sub.S in the range from 70 nm to 90 nm for most if not all of the SiO.sub.2 layers.

(41) Section S3 is the uppermost section and is configured to provide a high reflectance R (i.e., to be optimally reflective) at the UV operating wavelength of λ=355 nm while being substantially transmissive for the VIS, the NIR and the MWIR wavelengths. Section S3 has the thinnest HfO.sub.2/SiO.sub.2 layers, e.g., a thickness τ.sub.H in the range from 40 nm to 50 nm for most if not all of the HfO.sub.2 layers and a thickness τ.sub.S in the range from 60 nm to 70 nm for most if not all of the SiO.sub.2 layers. To facilitate manufacturability of the mirror, the UV and VIS bands are connected in the design. As noted above, a thick SiO.sub.2 layer 68 may be added as an outmost capping layer atop section S3 to further increase durability to laser irradiation.

(42) The above-described method of forming reflective multilayer coating 24 or 34 in sections SN, with each section configured to have a select reflectance R for a given operating wavelength, can be used to design mirrors 20 and 30 for use in beam expander 10. FIG. 7 is a plot of the reflectance R (%) versus wavelength λ for an example reflective multilayer coating 24 or 34 with a high reflectance at operating wavelengths λ of 355 nm, 532 nm, and 1064 nm as well as at SWIR wavelengths. FIG. 8 is similar to FIG. 7 and plots the reflectance R (%) at operating wavelengths λ of 355 nm, 532 nm, 1064 nm and at SWIR wavelengths while the substrate surface 22 or 32 has high-reflectance at MWIR wavelengths that are substantially transmitted by the different sections SN of the reflective multilayer coating 24 or 34.

(43) Controlling Loss Due to Scattering

(44) In an example embodiment, reflective multilayer coatings 24 and 34 each has a reflectance of 98% or greater at each of the UV, VIS and IR operating wavelengths λ, and beam expander 10 has an optical transmittance T.sub.BE>95%, which makes it a high-efficiency optical system. To achieve this high reflectance for mirrors 20 and 30 and high efficiency for beam expander 10, the amount of loss due to scattering for each mirror needs to be controlled.

(45) In the reflectance plot of FIG. 5, a reflectance R of greater than 99.9% can in theory be achieved at all of the “triple bands,” i.e., at λ=355 nm, 532 nm and 1064 nm, by having a perfect substrate surface and by coating interfaces having zero RMS surface roughness.

(46) In practice, however, substrate surfaces and coating interfaces have some degree of surface roughness that diminishes the reflectance. FIG. 9 is a plot of the scatter loss SL (%) for a single mirror 20 or 30 as a function of the RMS substrate surface roughness MSR.sub.RMS (nm) at the operating (design) wavelengths λ=355 nm, 532 nm and 1064 nm. The plot shows that the scatter loss SL is zero when the RMS substrate surface roughness MSR.sub.RMS is zero, i.e., for a perfect surface. The scatter loss SL increases as the RMS substrate surface roughness MSR.sub.RMS increases. For example, when MSR.sub.RMS=3 nm, the scatter loss SL is 0.22% at 1064 nm, 0.50% at 532 nm and 1.13% at 355 nm. For MSR.sub.RMS=6 nm, the scatter loss SL is 0.62% at 1064 nm, 2% at 532 nm and 4.5% at 355 nm.

(47) FIG. 10 is a plot of the transmittance T.sub.BE(%) of beam expander 10 versus the RMS substrate surface roughness MSR.sub.RMS(nm). The plot shows that an amount of RMS substrate surface roughness MSR.sub.RMS=3 nm results in an optical transmittance T.sub.BE for beam expander 10 of T.sub.BE=99.75% at λ=1064 nm, T.sub.BE=99.00% at λ=532 nm and T.sub.BE=97.76% at λ=355 nm. An RMS substrate surface roughness MSR.sub.RMS=6 nm results in an optical transmittance T.sub.BE for beam expander 10 of T.sub.BE=97% at λ=1064 nm, T.sub.BE=96% at λ=532 nm and T.sub.BE 90% at λ=355 nm (obtained by extrapolating the curve for 355 nm to an MSR.sub.RMS of 6 nm).

(48) The optical transmittance T.sub.BE of beam expander 10 is limited by the scattering loss at the UV operating wavelength of λ=355 nm. In other words, according to the plot of FIG. 10, mirrors 20 and 30 of beam expander 10 each need to have an RMS substrate surface roughness MSR.sub.RMS<4.5 nm for the beam expander to have a high-efficiency optical transmittance of T.sub.BE>95%.

(49) As discussed above, reflective multilayer coating 24 or 34 has multiple HfO.sub.2/SiO.sub.2 stacks or sections SN, such as sections S1 through S3, wherein the HfO.sub.2/SIO.sub.2 layer thicknesses τ.sub.H/τ.sub.S generally decrease section by section, from the first or lowermost section S1 closest to mirror substrate 21 or 31 to the top or uppermost section S3 that defines top surface 26 or 36 of the reflective multilayer coating.

(50) The HfO.sub.2/SiO.sub.2 layers of section S3 are configured to provide high reflectance in the UV at λ=355 nm while being substantially transmissive to the VIS, NIR and MWIR wavelengths. Forming section S3 as the top or uppermost section ensures that there is the lowest amount of scattering loss for the shortest operating wavelength. In other words, the UV operating wavelength has the shortest optical-path length within reflective multilayer coating 24 or 34, thereby providing the lowest amount of scatter loss. Although this approach may increase the amount of scatter loss at the IR operating wavelength due to the IR light having to traverse the longest optical-path length within reflective multilayer coating 24 or 34, the scatter loss is not as sensitive at this longer NIR wavelength as compared to at the shorter UV wavelength.

(51) Thus, the configuration of reflective multilayer coating 24 or 34 such as is shown in FIG. 6, where the top or uppermost section S1 defines the reflectance for the shorter UV operating wavelength and the bottom or lowermost section S3 defines the reflectance of the longer IR operating wavelength, is exploited to optimize the optical transmittance T.sub.BE of beam expander 10 and achieve the high-efficiency optical transmittance of T.sub.BE>95%.

(52) Forming the HfO.sub.2/SiO.sub.2 Layers

(53) The plots of FIGS. 9 and 10 indicate that achieving an optical transmittance T.sub.BE>95% for beam expander 10 requires that the amount of RMS substrate surface roughness MSR.sub.RMS be maintained below a certain threshold value TV, such as TV=4.5 nm in the example discussed above. This requires forming the HfO.sub.2/SiO.sub.2 layers in each of the reflective multilayer coatings 24 and 34 so that they are as smooth as possible.

(54) In an example, the HfO.sub.2/SiO.sub.2 layers are formed using the systems and methods disclosed in the '450 patent. In particular, in an example, the HfO.sub.2/SiO.sub.2 layers are formed on the respective diamond-turned and optically polished substrate surfaces 22 and 32 of mirror substrates 21 and 31 using plasma ion assisted deposition (PIAD) in combination with rotating the respective mirror substrate and employing a “reversed mask” process. This method ensures that reflective multilayer coatings 24 and 34 do not increase the RMS substrate surface roughness MSR.sub.RMS beyond that of the original diamond-turned and optically polished substrate surfaces 22 and 32.

(55) Thus, in an example, the diamond-turned and optically polished substrate surfaces 22 and 32 of mirror substrates 21 and 31 each has a RMS substrate surface roughness MSR.sub.RMS less than a threshold value TV, such as the aforementioned 4.5 nm, which results in an optical transmittance T.sub.BE>95% for all the operating (design) wavelengths λ. Such threshold values TV are readily achievable using diamond-turning and polishing of an Al surface.

(56) Mitigating Surface Defects

(57) In some instances, substrate surface 22 or 32 can have surface imperfections or defects that need to be smoothed out or otherwise mitigated to achieve a high-efficiency optical transmittance T.sub.BE for beam expander 10. For example, metal inclusions are sometimes formed in aluminum (Al) alloys to increase the mechanical strength. The metal inclusions can have a hardness different from that of the bulk Al alloy of mirror substrate 21 or 31 and can appear in the form of small (e.g., submicron-size) particles on the polished substrate surface 22 or 32. Such surface defects can increase scatter loss SL and may also reduce the laser-induced damage threshold, especially at the UV operating wavelength.

(58) In one example, surface defects can be mitigated by the depositing of an SiO.sub.2 layer on substrate surface 22 or 32 to seal the surface defect while eliminating defect lateral growth with an inversed mask, followed by the smoothing of the SiO.sub.2-coated surface via plasma-ion etching. In another example, a pure Al film is deposited on substrate surface 22 or 32 to seal the surface imperfection and homogenize the surface, followed by optical polishing the pure Al film. In another example, a layer of aluminum is deposited on substrate surface 22 or 32 to seal the surface imperfection and homogenize the surface, and then the Al-coated surface is smoothed via plasma-ion etching. In another example, a metal such as nickel or a nickel alloy is deposited on substrate surface 22 or 32 and then the coated substrate is processed to define the require curvature to within a desired surface roughness.

(59) FIGS. 11A through 11C are schematic side views of a portion of an example mirror 20 or 30 that illustrates an example method of performing surface-defect mitigation when the mirrors are formed. In FIG. 11A, substrate surface 22 or 32 of mirror substrate 21 or 31 includes surface defects 64. In FIG. 11B, substrate surface 22 or 32, and the surface defects 64 thereon, are coated with an SiO.sub.2 layer 70 using a reversed mask process. The SiO.sub.2 layer 70 is then plasma-ion etched using a plasma-ion etch process that employs a plasma 80 to form a flat (i.e., substantially defect-free) surface 72. The reflective multilayer coating 24 or 34 consisting of the aforementioned HfO.sub.2/SiO.sub.2 layers is then formed atop surface 72 of processed SiO.sub.2 layer 70, as shown in FIG. 11C. The optional SiO.sub.2 layer 68 is also shown in FIG. 11C as being deposited atop the uppermost section S3 of reflective multilayer coating 24 or 34.

(60) The resulting mirrors 20 and 30 are then employed in beam expander 10 to achieve the multiwavelength performance over the UV, VIS and NIR wavelengths with a high-efficiency optical transmittance T.sub.BE>95%.

(61) An advantage of beam expander 10 disclosed herein is that reflective multilayer coatings 24 and 34 can have a relatively high laser-induced damage threshold at all the operating wavelengths λ. The HfO.sub.2/SiO.sub.2 layers are formed to be dense and smooth to reduce or otherwise minimize scatter loss SL while also being resistant to laser damage and environmental erosion. When necessary, substrate surface defect mitigation is carried out as described above to reduce or eliminate the adverse effects of substrate surface defects on the optical transmittance T.sub.BE of beam expander 10. Further, because reflective multilayer coatings 24 and 34 are formed in sections SN that are designed to reflect a particular operating wavelength λ while substantially transmitting other wavelengths, substrate surfaces 22 and 32 of mirrors 20 and 30 can have a relatively high reflectance at MWIR wavelengths.

(62) It will be apparent to those skilled in the art that various modifications to the preferred embodiments of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, the disclosure covers the modifications and variations provided they come within the scope of the appended claims and the equivalents thereto.