Europaisches Patentamt 19 European Patent Office Office europeen des brevets © Publication number: 0 510 919 A1 EUROPEAN PATENT APPLICATION © Application number : 92303570.3 © int. ci.5: G02B 5/28, G02B 1/10 © Date of filing : 22.04.92 © 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 sub- strate (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 quar- ter-wave layers (40,42,44,46,48,64) of high and low refractive index materials, each having an optical thickness substantially equal to a quar- ter-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 sig- nificantly 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).