Dichroic Optical Filter

Europaisches Patentamt 19 European Patent Office

Office europeen des brevets © Publication number: 0 510 919 A1

EUROPEAN PATENT APPLICATION

© Priority : 23.04.91 US 690068 © Inventor : Trost, David 1201 California Street 701 San Francisco, California 94109 @ Date of publication of application (US) 28.10.92 Bulletin 92/44 Inventor : Fischer, Dennis 14205 Edgehill Lane Auburn, California 95603 (US) @ Designated Contracting States : Inventor : Baumeister, Philip CH DE FR GB IT LI 630 Peach Street Newcastle, California 95658 (US) © Applicant : Coherent, Inc. 3210 Porter Drive © Representative : Jackson, David Spence et al Palo Alto California 94304 (US) REDDIE & GROSE 16, Theobalds Road London, WC1X 8PL (GB)

(54) Dichroic optical filter.

© A dichroic optical filter (30) including a substrate (32) that is substantially transparent to 30 32 visible radiation, and an oxide semiconductor 1 layer (34) on the substrate (32) for reflecting an infrared wavelength. A suitable oxide semicon- SUBSTRATE FIG. 4 ductor for treatment beam from 34 reflecting a a INDIUM TIN OXIDE C02 laser is indium tin oxide. A multilayer coat- 36 { ing (36) on the oxide semiconductor layer (34) to enhance the reflection of infrared and longer 38 wavelength radiation includes alternating quarter-wave layers (40,42,44,46,48,64) of high and low refractive index materials, each having an optical thickness substantially equal to a quarter-wavelength of infrared radiation to be reflec- FIG. 5 ted. A thin multilayer coating (41,43,45,47) is provided between each pair of adjacent quar- 36 ^ ter-wave layers (40,42,44,46,48) to enhance the transmission of visible radiation, while not significantly affecting the reflection of infrared wavelengths. A second multilayer coating (38) m.} 65 reflects a narrow band of visible radiation (such as visible radiation from a HeNe laser aiming < beam), while transmitting most visible wavelengths. o> o In o Q_ LU Jouve, 18, rue Saint-Denis, 75001 PARIS 1 EP 0 510 919 A1 2

Field of the Invention parallax between the viewing light (reflected radiation 16) and the treatment radiation. This parallax makes This invention relates to dichroic optical filters. it difficult or impossible for a surgeon to view and treat More particularly, the invention relates to dichroic opt- at the same time inside a restricted orifice of a pa- ical filters that reflect selected infrared radiation while 5 tient's body (or through a hollow instrument inserted transmitting most wavelengths of the visible spec- into such orifice). trum. In another type of conventional system, combiner 4 is replaced by a small, 100% reflective mirror. By Background of the Invention symmetrically positioning such mirror between lenses 10 6 and 8 and patient 10, parallax is eliminated. How- The dichroic optical filter of the invention is par- ever, the constraints on the size of the mirror in such ticularly useful when embodied in a surgical operating a system render the system unsuitable for many ap- microscope micromanipulator. Such a micromanipu- plications. The mirror must be sufficiently small so as lator is an attachment to a surgical operating micro- not to obstruct unduly the surgeon's view of the pa- scope which allows a surgeon to manipulate a high rs tient. Yet, the mirror must not be so small that diffrac- power laser beam (typically superimposed with a visi- tion effects prevent it from directing the treatment and ble, coherent, aiming beam) while viewing a patient aiming beams to a sufficiently small focal spot on the and the aiming beam through the microscope. patient. Due to diffraction effects, the smallest spot The principal optical components of a surgical op- achievable at the focus of the treatment and aiming erating microscope (with micromanipulator) are 20 beams is inversely proportional to the size of the re- shown schematically in Figure 1. The central element flecting mirror. Extremely small spot size is highly de- in the micromanipulator is beam combining optic 4 sirable for some forms of treatment, and yet cannot be (sometimes denoted herein as "combiner" 4). Coher- achieved with this type of conventional system. ent beam source 2 (which may include one or more In yet another conventional system, combiner4 of lasers) emits high power coherent beam 12 and visi- 25 Figure 1 is implemented as a dichroic filter, which ble, coherent aiming beam 14. Beam 12 is typically an transmits visible wavelengths (i.e., visible radiation 16 infrared beam having wavelength 10.6 micrometers and visible aiming beam 14a) and reflects the treat- (from a C02 laser). Aiming beam 14 is typically a visi- ment beam wavelength. With this . approach it is pos- ble beam from an HeNe laser having wavelength sible to make the dichroic filter large (to achieve suf- 0.6328 micrometers, although in alternative embodi- 30 ficiently small treatment beam spot size) and still keep ments beam 14 can have any of a variety of other visi- the dichroic filter on the optical axis (for parallax con- ble wavelengths (such as 0.543 micrometers). Beam trol). One such conventional dichroic filter, suitable for 12 will sometimes be referred to herein as the "oper- use with a C02 laser treatment beam having 10.6 mi- ating" or "treatment" beam. Operating beam 12 and crometer wavelength, is filter 20 shown in Figure 2. aiming beam 14 are incident on patient 10 after they 35 Filter 20 is an "enhanced transmission" filter con- reflect from combiner 4. sisting of transparent glass substrate 20, thin dielec- Visible radiation 16 reflects from patient 10 and tric layer 24 coated on substrate 20, thin gold layer 26 propagates through combiner 4 to microscope objec- coated on layer 24, and thin dielectric layer 28 coated tive lenses 6 and 8. Two objective lenses 6 and 8 are on gold layer 26. Gold layer 26 efficiently reflects 10.6 shown to indicate that the microscope is binocular. 40 micrometer radiation. Portion 14a of aiming beam 14 also reflects from pa- The tendency of gold layer 26 to reflect visible ra- tient 10 and propagates through combiner 4 to lenses diation is partially overcome by dielectric layers 24 6 and 8. In this way, a surgeon may view the radiation and 28, which produce a standing wave in the visible transmitted through lenses 6 and 8 to determine the band, with an antinode at gold layer 26. The net effect portion of patient 1 0 from which beam 14a has reflect- 45 is to enhance the transmission of visible radiation ed. through filter 20. Portion 12a of operating beam 12 reflects from Because coating layers 24, 26, and 28 would not patient 10, and then reflects from combiner 4 in a di- efficiently reflect visible aiming beam 14tothe patient, rection away from microscope objective lenses 6 and it is conventional to include a small aluminized reflect- 8. In this way, combiner4 prevents damage to the sur- so ing spot 29 (shown in Figure 3) in the center of filter geon's eyes while the surgeon views radiation (14a 20. Typically, filter 20 is mounted symmetrically with and 16) transmitted through combiner 4. respect to the microscope objective lenses (6 and 8), In one conventional variation on the system of so that spot 29 is symmetrically positioned relative to Figure 1 , combiner 4 is replaced by a substantially the objective lenses as shown in Fig. 3. However, if 100% reflective mirror that is mounted in a position 55 spot 29 is small enough not to interfere with the micro- offset from the path of visible radiation 16 from patient scope user's view, it tends to produce an imperfect 10 to lenses 6 and 8. An important disadvantage of aiming spot in the field of view (due to the diffractive this type of conventional system is that it introduces effect discussed above, and to loss of light). Further- 2 3 EP 0 510 919 A1 4 more, a typical spot 29 interferes with the microscope ation, while not significantly affecting the filter's reflec- user's view of the patient, particularly when the micro- tion of infrared wavelengths. scope is operated at low magnifications. Also preferably, the multilayer coating includes a For this reason, aluminized reflecting spot 29 and second multilayer coating for partially reflecting a nar- the aiming beam are sometimes omitted. Instead, a 5 row band of visible radiation (such as visible radiation separate aiming spot is developed and projected into from a HeNe laser aiming beam), whileefficiently the microscope field of view as either a real or virtual transmitting wavelengths of the visible spectrum out- image. However, it is difficult to keep such separate side such narrow band. aiming spot aligned with the treatment beam. Conventional enhanced transmission filter 20 (of 10 Brief Description of the Drawings Figures 2 and 3) has a number of additional serious limitations and disadvantages. For example, coatings Figure 1 is a schematic diagram of a system of the 24, 26, and 28 attenuate a significant fraction of visi- type which may embody the optical filter of the inven- ble radiation incident thereon. Furthermore, gold does tion. not adhere well to the usual dielectric materials em- 15 Figure 2 is a cross-sectional view of a convention- ployed as layers 24 and 28. Thus, coating layers 28 al optical filter, which consists of substrate 22 and and 26 do not stand up well to the rugged environment coating layers 24, 26, and 28. of the operating room, and to subsequent cleaning. Figure 3 is a top view of the filter shown in Figure The invention avoids the described limitations 2, in a preferred position relative to lenses 6 and 8 and disadvantages of conventional micromanipulator 20 (shown in phantom view). beam combining optics, by employing an oxide sem- Figure 4 is a side cross-sectional view of an opt- iconductor coating (such as a layerof indium tin oxide) ical filter embodying the invention, which consists of on a substrate (a substrate transparent to visible ra- substrate 32, coating layer 34, first multilayer coating diation), to reflect the treatment beam wavelength (or 36, and second multilayer coating 38. wavelengths) while efficiently transmitting visible wa- 25 Figure 5 is a side cross-sectional view of a portion velengths. Oxide semiconductor layers, such as lay- of an embodiment of multilayer coatings 36 and 38 of ers of indium tin oxide ("ITO") have been used as Figure 4. transparent electrodes in electro-optical devices such Figure 6 is a side cross-sectional view of a varia- as cathode ray and liquid crystal displays. However, tion on the optical filter of Figure 4. until the present invention it had not been known to 30 Figure 7 is a graph showing the reflectance char- employ a transparent substrate with an oxide semi- acteristics of a first preferred embodiment of the in- conductor coating as a dichroic filter, for such appli- ventive optical filter. Distance above the horizontal cations as use in a micromanipulator beam combining axis represents reflectance (in units of percent). Dis- optic. tance from the vertical axis represents wavelength in 35 units of nanometers. Summary of the Invention Figure 8 is a graph showing the transmittance of the first preferred embodiment of the inventive filter as The inventive optical filter is a dichroic optical fil- a function of wavelength (in nanometers). Distance ter including a substrate that is transparent to visible above the horizontal axis represents transmittance (in radiation, and an oxide semiconductor layer on the 40 units of percent). substrate for reflecting an infrared wavelength (such Figure 9 is a graph showing the reflectance char- as an infrared treatment beam wavelength). An exam- acteristics of the first preferred embodiment of the in- ple of a suitable oxide semiconductor for reflecting a ventive filter. Distance above the horizontal axis rep- treatment beam from a C02 layer is indium tin oxide resents reflectance (in percent). Distance from the ("ITO"). 45 vertical axis represents wavenumber in units of in- In a class of preferred embodiments, the filter of verse centimeters. the invention includes a specially designed multilayer Figure 10 is a graph showing the reflectance and coating on the oxide semiconductor layer to enhance transmittance characteristics of a second preferred the filter's reflection of infrared and longer wavelength embodiment of the inventive filter. Distance from the radiation. This multilayer coating includes alternating 50 vertical axis represents wavelength in micrometers. quarter-wave layers of high and low refractive index Figure 1 1 is a graph showing the transmittances materials (each having an optical thickness substan- of the multilayer stacks identified as the "starting de- tially equal to a quarter-wavelength of infrared radia- sign" and the "final design" in Figure 12. tion to be reflected). Preferably also, the multilayer Figure 12 is a table defining three embodiments coating includes a thin multilayer coating between 55 of substrate 32, layer 34, and coating 36, of the filter each pair of adjacent quarter-wave layers, with these of Figure 4. thin multilayer coatings being designed to enhance the transmittance of the inventive filter to visible radi- 3 5 EP 0 510 919 A1 6

Detailed Description of the Preferred Embodiments In some embodiments of the invention, one or both of coatings 36 and 38 are omitted. However, Figure 4 is a side cross-sectional view of a pre- without coating 36, an indium tin oxide layer 34 will ab- ferred embodiment of the inventive dichroic filter. Sub- sorb approximately 20% of the power of an incident strate 32 is preferably composed of an optical filter 5 10.6 micrometer treatment beam. In the typical case glass (such as fused silica or BK-7 glass). Layer 34 that the treatment beam is a high power laser beam, coated on substrate 30, is composed of an oxide sem- this rate of power absorption is unacceptably high, iconductor that reflects infrared (and longer wave- and will drastically shorten the useful operating life of length) radiation while transmitting visible radiation. the filter. For most applications (including virtually all For reflecting infrared treatment beam radiation hav- 10 high power applications), the invention must include ing wavelength 10.6 micrometers (from a C02 laser), coating 36 to reflect enough infrared treatment beam the oxide semiconductor comprising layer 34 may be radiation to prevent filter damage due to excessive indium tin oxide. In alternative embodiments, layer 34 power absorption. may be composed of other oxide semiconductors, Figure 5 is an enlarged view of a portion of a pre- such as CdO or Sn02, for example. 15 ferred embodiment of multilayer coatings 36 and 38 of Use of an oxide semiconductor for layer 34 is su- Figure 4 (the top and bottom layers of coating 36 are perior to use of gold (as in some conventional dichroic shown in Fig. 5, but the inner layers are omitted to sim- filters) since oxide semiconductors are available that plify the drawing). The Figure 5 embodiment of coat- transmit visible radiation more efficiently and over a ing 36 includes thick, high refractive index layers al- broader visible frequency range than gold (even when 20 ternating with thick, low refractive index layers. In Fig- the gold is used with additional visible radiation trans- ure 5, the thick layers are in the following order: thick mission enhancement layers of the type described high index layer 40, thick low index layer 42, thick high above with reference to Figure 2), are more rugged index layer 44, thick low index layer 46, thick high in- than gold, and adhere better to typical substrates than dex layer 48, several layers not shown, and thick low does gold. Oxide semiconductor layer 34 should be 25 index layer 64. Each of thick layers 40, 42, 44, 46, 48, applied with carefully controlled deposition parame- and 64 has an optical thickness substantially equal to ters in order to achieve the desired optical properties. quarter the wavelength of the principal wavelength of For specificity, oxide semiconductor layer 34 will infrared treatment beam 12. Accordingly, the thick be referred to below as "ITO" layer 34, since layer 34 layers will sometimes be referred to herein as "quar- is composed of indium tin oxide in the preferred em- 30 ter-wave layers". bodiment of the invention mentioned above. The number of quarter-wave layers coated on First multilayer coating 36 is coated on ITO layer substrate 32 will depend on the desired optical prop- 34, and second multilayer coating 38 is coated on erties of the filter. The invention in its broadest scope coating 36. Coating 36 is designed so that the se- is not limited a filter having any specific number of lay- quence of its layers (and the optical thickness of each 35 ers. There may be an even number or odd number of layer) is such that coating 36 reflects infrared radiation layers. The layer (layer 40 in Fig. 5) nearest the sub- while having minimal impact on the transmission of strate may be a member of the subset having high re- visible radiation. Each individual layer of multilayer fractive index or may be a member of the subset hav- coating 36 is preferably composed of material that is ing low refractive index. transparent in both the visible and the infrared. 40 A Fresnel reflection naturally occurs at the index Narrow visible band reflecting multilayer coating discontinuity between the thick layers. Because of the 38 should be made out of materials that are transpar- controlled (substantially quarter-wave) optical thick- ent in the visible and the infrared, and should be de- ness of each layer, the multiple reflections interfere signed to partially reflect incident visible aiming beam constructively. The resulting coherent addition of the radiation (in the typical case, the aiming beam radia- 45 multiple reflections is far greater than the simple sum tion does not have significant frequency components of the reflections from each interface. As a result, mul- outside a narrow visible frequency band). Coating 38 tilayer stack 36 increases the reflectivity of the inven- preferably reflects 40% to 60% of the power of inci- tive filter (to the infrared wavelength of interest) to dent radiation in the narrow visible band. Coating 38 95% or more, whereas the reflectivity of oxide semi- should not totally reflect the radiation in the narrow 50 conductor 34 alone is typically about 80%. visible band since the microscope user needs to re- However, an embodiment of IR reflection en- ceive some of this visible radiation (that has passed hancement stack 36 including thick layers only (i.e., through the inventive filter after reflecting from the pa- only layers 40, 42, 44, 46, etc.), and not thin multilayer tient). It is desirable that coating 38 has minimal im- stacks 41 , 43, 45, etc., tends undesirably to interfere pact on transmission of visible radiation outside the 55 significantly with transmission of visible radiation. narrow visible band of the aiming beam, to avoid un- This effect can be understood as follows. If each thick necessarily distorting the image of the patient ob- layer has about quarter the thickness of a 1 0.6 micro- served by the microscope user. meter infrared treatment beam, the optical thickness 4 7 EP 0 510 919 A1 8 of each such layer is anywhere from 1 5 to 26 wave- has been reduced from eight to as few as two. Curve lengths in the visible band. As one scans the visible "F" of Figure 1 1 shows that the visible transmittance band, there will be a series of eleven (or so) reflectivity of the "Final design" is considerably better than that peaks and valleys, as IR reflection enhancement of the "Starting design." stack 36 alternately acts as a reflection and transmis- 5 If each thin stack has a total optical thickness sion enhancer in the visible. much less than the thickness of each thick layer, the To enhance the transmissivity of the inventive fil- thin stacks will have negligible effect on the IR reflec- ter in the visible, a thin multilayer stack (comprising al- tion enhancing properties of stack 36. ternating high and low refractive index materials) is in- In one illustrative embodiment of the invention, serted at the interface between each pair of adjacent 10 multilayer stack 36 includes N quarter-wave (thick) thick layers. For example, thin stack 41 is inserted be- layers, and N-1 thin multilayer stacks between the tween thick layers 40 and 42. Insertion of such "thin" thick layers. The odd quarter-wave layers 40, 44, 48, stacks serves to enhance the transmittance in the visi- etc., are high refractive index layers composed of zinc ble portion of the spectrum and hence the thin stacks sulfide (ZnS) and the even quarter-wave layers (42, are termed "visible transmittance enhancement 15 46, etc.) are low refractive index layers composed of stacks" in subsequent discussion. The following com- thorium fluoride (ThF4). The thin stacks can also be puter modelling procedure was used to design each composed of alternating layers of zinc sulfide and tho- visible transmittance enhancement stack: (1) a group rium fluoride. It should be recognized that other high of ratherthin stacks (each having alternating high and and low refractive index material may be substituted low refractive indices) was interdispersed between 20 for the mentioned materials, provided the substitute each of the thicker layers of stack 36; and (2) the thick- materials are transparent in both the visible and in- nesses of the thin stack layers were adjustedwith the frared. Zinc sulfide and thorium fluoride are the most goal of optimizing the transmittance in the visible por- commonly used and best understood pair of high and tion of the spectrum. The word "thin" in the foregoing low refractive index materials suitable foruse in the in- sentence means thatthe optical thickness of each thin 25 ventive filter (to reflect 10.6 micrometer infrared radi- stack does not exceed 200 nm -i.e., one eighth of a ation). wave in the visible part of the spectrum. The quarter-wave layers in stack 36 need not all As an example, a design that fulfills step (1) in the have identical refractive index, and their optical thick- foregoing paragraph may be written symbolically as ness need not all equal exactly one quarter wave- a/rSHSLSHSLST substrate where H and 30 length of a single selected electromagnetic wave. L represent "thick" layers of a quarterwave in the in- Figure 6 is a side cross-sectional view of a varia- frared. T represents an indium tin oxide layer that is tion on the optical filter of Figure 4. Filter 30' of Figure thick enough to reflect well in the infrared, yet not so 6 differs from filter 30 of Figure 4 only in that narrow- thick as to reduce substantially the visible transmit- band visible reflective coating 38' of filter 30' is coated tance by its absorption. 35 directly on substrate 32 (and ITO layer 32 is subse- Interdispersed between each of the "thick" layers is quently coated on coating 38'), while narrow-band re- the stack S of thin layers, of the design flective coating 38 of filter 30 is coated on multilayer (H' L')4 coating 36 (after coating 36 has been applied directly where H' and L' represent layers of optical thickness to substrate 32). 50 nm, which is substantially thinner than any wave- 40 When using inventive filter 30 (of Figure 4) or in- length in the visible part of the spectrum. The trans- ventive filter 30' (of Figure 6) in the Figure 2 system mittance of such a stack is shown as curve "S" of Fig- as a substitute for combiner 4, the filter side opposite ure 1 1 . Such design has severe, undesirable, oscilla- substrate 32 should face treatment beam source 2 (so tions in the transmittance in the visible. The design of that coating 38 receives radiation directly from source this coating (containing 41 layers) is listed in Table 1 45 2 in filter 30, and coating 36 receives radiation-directly of Figure 12 as the "Starting design." The layer thick- from source 2 in filter 30'). The principle advantage of nesses are metric in this Table. filter30' overfilter30 is that coating 38 of filter 30 must The thicknesses of the layers were then adjusted. not absorb significant treatment beam radiation, while The procedures for doing this are well documented in coating 38' of Figure 6 may or may not be a good ab- the technical literature (see, for example, the teach- 50 sorber of the treatment beam radiation (since virtually ings of J.A. Dobrowolski in an article entitled "Com- all the treatment beam radiation will have reflected pletely automated synthesis of optical thin film sys- from coatings 34 and 36 before reaching coating 38'). tems" appearing in volume 4, page 937 of the period- For most applications, it is much more practical (sim- ical Applied Optics). The "Final design" identified in pler and less expensive) to implement a suitable filter Table 1 of Figure 12 shown the metric thicknesses of 55 30' than to implement a suitable filter 30. the layers in the stack after this adjustment has occur- For this reason, selection of materials for coating red. Ten layers have been dropped from the stack. 38' is much easier, since they need not be transparent The number of "thin layers" between the "thick layers" at 1 0.6 micrometers. In one illustrative example, coat- 5 g EP 0 510 919 A1 10 ing 38' is composed of alternating layers of aluminum ured transmittance of the same three examples of the oxide (Al203) and silicon dioxide (Si02), which are filter over the wavelength range from 400 nm to 800 good choices because of their ease of deposition, nm, while curve D represents the measured transmit- good transparency in the visible, and durability. tance of a reference transparent substrate (composed The design of each of the preferred embodiments 5 of fused silica) over the same wavelength range. of the invention is determined using an iterative optim- Curves A, B, and C show that the filters are highly ization technique as follows: the optical constants of transmissive in the visible, except that they have low the substrate and coating materials are specified; the transmittance (about 40%) over the narrow band from desired reflectance or transmittance spectrum is spe- about 620 nm to about 636 nm. cified; and then each coating layer thickness is deter- 10 Figure 9 is a graph showing the measured reflec- mined using the iterative technique. To insure the filter tance of the same filters measured to generate reflec- is conveniently and repeatably manufactured, each tance curves A, B, and C of Figure 7. In Figure 9, layer's optical thickness must be within a specified tol- curves A, B, and C represent measurements of filter erance of the optimal optical thickness, so that any reflectance over the wavenumber range from 5000 to small variations in each layer's thickness will not sig- 15 600 inverse centimeters. Curve D represents the nificantly alter the filter's reflectance and transmit- measured reflectance of a reference aluminum mirror, tance curves. Those of ordinary skill in the art will be and curve E represents the measured reflectance of familiar with, and capable of performing such an iter- a reference ZnSe wedge, over the same wavenumber ative optimization operation, as a matter of routine de- range. Curves A, B, and C show that the filters have sign. The operation will typically include the steps of 20 high reflectance to infrared radiation with a wavenum- choosing a merit function, and then minimizing the ber 943.4 inverse centimeters (which corresponds to merit function utilizing an optimization routine, to de- a 10.6 micrometer wavelength). termine the optimal set of design parameters. For ex- Figure 10 is a graph showing the predicted reflec- ample, U.S. Patent No.4,536,063, issued August 20, tance and transmittance characteristics of a second 1 985 to Southwell (which patent is incorporated here- 25 preferred embodiment of the invention. Its design is in by reference), discusses the manner in which an listed as the "modified design" in Table 1 of Figure 12. optical coating design merit function may be chosen, The 633 nm reflector consists of nine alternating lay- and then minimized, to generate a desired optical ers of lead floride and thorium floride. coating design. Curve R1 of Figure 10 represents the p-reflec- Figure 7 shows the reflectance characteristics of 30 tance of the second preferred embodiment, and curve a first preferred embodiment of the invention which R2 represents the s-reflectance of the second prefer- consists of a narrow band multilayer reflective coating red embodiment. 38' (having 14 alternating layers of alumina and silica, The above description is merely illustrative of the each having an optical thickness 3XJ4, where invention. Various changes in the details of the mate- XQ = 633 nm) on a fused silica substrate 32, an indium 35 rials and designs described may be within the scope tin oxide layer 34 having an optical thickness of 120 of the appended claims. nm on coating 38', and a multilayer coating 36 on layer 34. Coating 36 consists of a total of 17 layers, including four alternating "thick" layers of thorium fluoride Claims and zinc sulfide, and has an optical thickness of ap- 40 proximately a quarter wave at 10.6 micrometers. The 1. A dichroic optical filter, including: outermost layer can be MgF2 to increase the mechan- a substrate that is substantially transpar- ical durability of the coating. ent to visible radiation, wherein the substrate has In Figure 7, reflectance curves A, B, and C repre- a front surface; sent measurements of the reflectance of three differ- 45 a coating on the front surface, wherein the ent examples of the filter over the wavelength range coating includes: from 400 nm to 800 nm, while reflectance curve D rep- an oxide semiconductor layer that resents the measured reflectance of a reference alu- is highly reflective to a selected infrared wave- minum mirror over the same wavelength range. length and is substantially transparent to visible Curves A, B, and C show that the filters have low re- 50 radiation; and flectance in the visible, except that they have a sub- a first multilayer coating for enhancing the stantial reflectance (in the range from about 40% to filter's reflectance to the selected infrared wave- about 50%) over the narrow band from about 61 0 nm length, wherein the first multilayer coating in- to about 636 nm. cludes thick layers having a higher refractive in- Figure 8 shows the measured transmittance of 55 dex alternating with thick layers having a lower re- the same filters that were measured to generate refractive index, wherein each of the thick layers flectance curves A, B, and C of Figure 7. In Figure 8, has an optical thickness substantially equal to transmittance curves A, B, and C represent the meas- one quarter of the selected infrared wavelength, 6 11 EP 0 510 919 A1 12

and wherein each of the thick layers is substan- 10. The system of claim 9, wherein the coating in- tially transparent to visible radiation. cludes: a multilayer coating which partially reflects 2. The filter of claim 1, wherein the first multilayer the visible aiming beam wavelength, and is highly coating also includes: 5 transmissive to visible wavelengths outside a nar- a plurality of thin layers between each ad- row wavelength band including the visible aiming jacent pair of thick layers, wherein the thin layers beam wavelength. in each said plurality of thin layers have optical thicknesses and refractive indices selected so 11. The system of claim 8, wherein the coating also that said plurality of thin layers enhances the fil- 10 includes: ter's transmission of visible radiation without sig- a first multilayer coating for enhancing the nificantly affecting the filter's reflection of the se- reflectance of the beam combining optic to the selected infrared wavelength. lected infrared wavelength, wherein the first multilayer coating includes thick layers having a high- 3. The filter of claim 1 , wherein the coating also in- 15 er refractive index alternating with thick layers eludes a second multilayer coating that partially having a lower refractive index, wherein each of reflects a narrow wavelength band of visible radi- the thick layers has an optical thickness substan- ation, and is highly transmissive to visible wave- tially equal to one quarter of the selected infrared lengths outside the narrow wavelength band. wavelength, and wherein each of the thick layers 20 is substantially transparent to visible radiation. 4. The filter of claim 3, wherein the second multilayer coating is contiguous with the substrate. 12. The system of claim 11, wherein the first multilayer coating also includes: 5. The filter of claim 3, wherein the first multilayer a plurality of thin layers between each ad- coating is between the second multilayer coating 25 jacent pair of thick layers, wherein the thin layers and the oxide semiconductor layer. in each said plurality of thin layers have optical thicknesses and-refractive indices selected so 6. The filter of claim 1 , wherein the selected infrared that said plurality of thin layers enhances the fil- wavelength is 1 0.6 micrometers. ter's transmission of visible radiation without sig- 30 nificantly affecting the filter's reflection of the se- 7. The filter of claim 1 , wherein the oxide semicon- lected infrared wavelength. ductor layer is composed of indium tin oxide. 13. The system of claim 12, wherein the coating also 8. A micromanipulator system, including: includes a second multilayer coating that partially a source of coherent radiation; and 35 reflects a narrow wavelength band of visible radi- a beam combining optic positioned to re- ation, and is highly transmissive to visible wave- ceive the coherent radiation, and comprising a lengths outside the narrow wavelength band. substrate having a front surface and a coating on the front surface, wherein the substrate is sub- 14. The system of claim 13, wherein the second mul- stantially transparent to visible radiation, wherein 40 tilayer coating is contiguous with the substrate. the coating includes an oxide semiconductor layer, and wherein the oxide semiconductor layer is 15. The system of claim 13, wherein the first multilay- highly reflective to a selected infrared wavelength er coating is between the second multilayer coat- and substantially transparent to visible radiation. ing and the oxide semiconductor layer. 45 9. The system of claim 8, wherein the source of co- 16. The system of claim 13, wherein the source of coherent radiation emits a treatment beam and a herent radiation includes a means for emitting a visible aiming beam, wherein the beam combin- visible HeNe laser beam, and wherein the second ing optic substantially totally reflects the treat- multilayer coating partially reflects the visible ment beam toward a patient and partially reflects so HeNe laser beam. the visible aiming beam toward the patient, wherein the beam combining optic partially trans- 17. The system of claim 8, wherein the selected in- mits visible aiming beam radiation that has re- frared wavelength is 1 0.6 micrometers. flected from the patient, and wherein the beam combining optic substantially totally reflects treat- 55 18. The system of claim 8, wherein the oxide semi- ment beam radiation that has reflected from the conductor layer is composed of indium tin oxide. patient.

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16 EP 0 510 919 A1

European Patent J EUROPEAN SEARCH REPORT Application Number Office

EP 92 30 3570 DOCUMENTS CONSIDERED TO BE RELEVANT category Citation or document with indication, where appropriate, Relevant CLASSIFICATION OF THE of relevant passages to claim APPLICATION ant. CI. 5 ) r A I LN I Abb I HAL I b OF JAPAN 1,2.7,8. GQ2B5/28 vol. 6, no. 133 (P- 129)20 July 1982 18 G02B1/10 & JP-A-57 058 109 ( TOSHIBA ELECTRIC EQUIP. ) 7 April 1982 * abstract *

US-A-4 229 066 ( J . D. RANCOURT) I. 2.7 * column 1, line 17 - line 49; figures * * column 4, line 4 - line 58 * II, 12

Ui-A-4 oo/ b9Z (H. LQERT5CHER) 8,18 * column 3, line 31 - column 4, Une 41; figure 1 * 3.4,13, 14,16

EP-A-0 080 182 ( K. K. TOYOTA CHUO KENKYUSHO) 1,7,11, 18 * page 2, Hne 22 - page 3, Une 10; claim 5; figure 1 * TECHNICAL FIELDS SEARCHED (Int. CLS ) GB-A-2 121 075 ( K. K. TOYOTA CHUO KENKYUSHO) I, 3.4,7, II. 13, G02B 14.18 A61F " aDstract " C03C

Ine present search report has been drawn up for all claims nut or conpieiioa or tae uvea BERLIN 29 JULY 1992 VON MOERS F. uiiHiunr ut uiuj uumr«u.r\ is T : theory or principle underlying the invention E : earlier patent document, but published on, or X ; particularly relevant if taken alone after the filing date V : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document