Abstract:
An apparatus has a rangefinder portion that includes: a radiation generator which emits radiation having a selected wavelength; a radiation detector which detects radiation having the selected wavelength; and an optical portion which includes a non-reciprocal optical part. The optical portion routes radiation emitted by the radiation generator at the selected wavelength through the non-reciprocal optical part and then through an aperture toward a location remote from the apparatus, and also routes radiation received via the aperture at the selected wavelength through the non-reciprocal optical part and then to the radiation detector.

Description:
This application claims the priority under 35 U.S.C. §119 of provisional application No. 60/552,269 filed Mar. 10, 2004. 

   TECHNICAL FIELD OF THE INVENTION 
   This invention relates in general to rangefinders and, more particularly, to rangefinders that can be integrated into an optical weapon sight. 
   BACKGROUND OF THE INVENTION 
   Optical sights are used for various purposes. One example is that an optical sight can be mounted on a weapon, in order to help a user accurately aim the weapon. The optical sight accepts image information from a distance scene, and presents this image information within a field of view that is visible to the eye of a user. 
   In some applications, it would be desirable to integrate into the sight a rangefinder, such as a laser rangefinder, so that the user will have a tool for making an accurate determination of the distance to an actual scene or target of interest. Although various possible approaches to laser rangefinders have previously been proposed, they have not been satisfactory in all respects. For example, one possible approach would be to use separate optical apertures for the outgoing laser pulse and the incoming reflected pulse, in order to obtain high optical efficiency. In particular, in a configuration that used a single aperture and a beam splitter for the laser energy, optical energy would be lost, due to the beam splitter. Another possible approach would be to time-division multiplex the transmission optics between the laser which generates the outgoing pulse and the detector which receives the incoming reflected pulse. This approach would require a high-speed optical switch, along with high-speed and potentially high-voltage electronics to drive the switch. However, suitable high-speed optical switching technology is not readily available. Moreover, and in any event, the switches and circuits would significantly increase the size, cost and weight of any weapon sight, and would also significantly increase power consumption, so as to seriously degrade the effective battery life of a portable sight. 
   SUMMARY OF THE INVENTION 
   One form of the invention involves effecting range finding by: emitting radiation having a selected wavelength from a radiation generator; detecting radiation having the selected wavelength with a radiation detector; routing radiation emitted by the radiation generator at the selected wavelength through a non-reciprocal optical part and then through an aperture toward a remote location; and routing radiation received via the aperture at the selected wavelength through the non-reciprocal optical part and then to the radiation detector. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be realized form the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic view of an apparatus which is an optical sight for a weapon, and which embodies aspects of the present invention; 
       FIG. 2  is a diagrammatic sectional side view of a portion of the sight of  FIG. 1 , in a significantly enlarged scale; and 
       FIG. 3  is a graph depicting a curve that represents the transmittance in the visible and infrared spectrums of a thin-film filter which is a component of the sight of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a diagrammatic view of an apparatus which is an optical sight  10  for a weapon, and which embodies aspects of the present invention. The sight  10  could be mounted on a rifle, in order to assist a user in aiming the rifle at a target within a remote scene  13 . The scene  13  could be any of a wide variety of different things, and is therefore depicted diagrammatically in  FIG. 1  by a broken line.  FIG. 1  does not depict all of the structure of the sight  10 , but only selected components that facilitate an understanding of the present invention. 
   The sight  10  has a housing, which is represented diagrammatically in  FIG. 1  by a broken line  11 . Another broken line  12  represents a path of travel through the sight  10  of visible radiation which embodies an optical image of the remote scene  13 . This radiation from the scene  13  travels along the path  12  to an eye  14  of a user. 
   The sight  10  has an objective lens doublet  16 , and two removable lenses  17  and  18 . The lens doublet  16  defines an optical aperture for the sight  10 , and the removable lenses  17  and  18  determine the magnification of the sight  10 . The sight  10  also has a prism assembly which includes three prisms  21 – 23 . The prisms  21 – 23  have surfaces  31 – 35 , and each of these surfaces has at least a portion thereof covered by a reflective coating. For clarity, the coatings are not separately shown in  FIG. 1 . The coatings on the surfaces  31 – 34  are each a type of coating which is well known in the art. The coating on the surface  35  is described in more detail later. Radiation from the scene  13  propagates along the path of travel  12 , passes successively through the lens doublet  16  and the lenses  17 – 18 , and then passes successively through the prisms  21 – 23 , while being successively reflected at each of the surfaces  31 – 35 . 
   The sight  10  also has a lens assembly  41 , and a lens  42 . After exiting the prism  23 , radiation that is propagating along the path of travel  12  passes successively through the lens assembly  41  and lens  42 , and then travels to the eye  14  of the user. 
   The sight  10  includes a ferrule  51 , which is fixedly supported near the surface  35  of the prism  23 . The ferrule stationarily supports one end of a single-mode optical fiber  52 , so that the end of the fiber extends at a selected angle with respect to the surface  35  of the prism  23 . The sight  10  also includes an electro-optical section  53 , which is in optical communication with the end of the fiber  52  remote from the ferrule  51 . The electro-optical section  53  is shown in more detail in  FIG. 2 . In particular,  FIG. 2  is a diagrammatic sectional side view of a portion of the sight  10  of  FIG. 1 , in a significantly enlarged scale.  FIG. 2  shows the prism  23 , the ferrule  51 , the optical fiber  52 , and a block diagram of the electro-optical section  53 . 
   As mentioned above, a coating is provided on the surface  35  of the prism  23 , and this coating is shown at  57  in  FIG. 2 . In the disclosed embodiment, and as explained earlier, the coating  57  is a more sophisticated coating than the coatings which are provided on the other prism surfaces  31 – 34  ( FIG. 1 ). In particular, the coatings provided on the surfaces  31 – 34  are coatings of a type known in the art, and are each highly reflective to all radiation within both the visible and infrared spectrums. In contrast, the coating  57  on the surface  35  is more sophisticated than the coatings which are provided on the surfaces  31 – 34 , and is described in detail later. For now, it is sufficient to briefly explain that the coating  57  is a thin-film filter, and that the thin-film filter  57  is designed to have certain selected characteristics for radiation at three specific wavelengths, as follows. 
   First, for infrared radiation with a wavelength of 1550 nm, the filter  57  has a transmittance of at least about 80%. In the disclosed embodiment, the transmittance for infrared radiation with a wavelength of 1550 nm is nearly 100%. Second, for infrared radiation with a wavelength of 850 nm, the filter  57  has a transmittance in the range of approximately 30% to 70%, and a reflectance in the range of approximately 70% to 30%. In the disclosed embodiment, the transmittance and reflectance for the wavelength of 850 nm are each approximately 50%. Third, the filter has a reflectance for all radiation in the visible spectrum which is at least about 80%, and nearly 100% in the disclosed embodiment, except for a narrow band of visible radiation centered at a wavelength of about 630 nm. As to visible radiation with a wavelength of approximately 630 nm, the filter  57  has a transmittance in the range of approximately 30% to 70%, and a reflectance in the range of approximately 70% to 30%. In the disclosed embodiment, the transmittance and reflectance for the wavelength of 630 nm are each approximately 50%. 
   The electro-optical section  53  includes three laser diodes  61 – 63 , which are each a commercially-available device that is well known to persons skilled in the art. The laser diode  61  outputs a laser beam of infrared radiation with a wavelength of 1550 nm, the laser diode  62  outputs a laser beam of infrared radiation with a wavelength of 850 nm, and the laser diode  63  outputs a laser beam of visible radiation with a wavelength of 630 nm. As discussed in more detail later, the radiation from the laser diode  61  is used for laser range finding, the radiation from the laser diode  62  is used as an infrared pointer, and the radiation from the laser diode  63  is used as a visible pointer. 
   The electro-optical section  53  includes two wavelength division multiplexers  67  and  68 , which each have three optical ports. These multiplexers are devices of a type known in the art, and can be purchased commercially, for example from Oplink Communications, Inc. of San Jose, Calif., and/or the ATI Optique division of ATI Electronique of Courcouronnes, France. The multiplexer  67  is transmissive to infrared radiation having a wavelength of 1550 nm, and is reflective to certain radiation having somewhat shorter wavelengths, including infrared radiation having a wavelength of 850 nm, and also visible radiation having a wavelength of 630 nm. The multiplexer  68  is transmissive to infrared radiation having a wavelength of 850 nm, and is reflective to certain radiation having somewhat shorter wavelengths, including visible radiation having a wavelength of 630 nm. The electro-optical section  53  also includes an infrared detector  71 , which is a commercially-available component. The infrared detector  71  is responsive to infrared radiation having a wavelength of 1550 nm. The laser diodes  61 – 63  and the detector  71  are each operably coupled to a control circuit, which is shown diagrammatically at  73 . 
   The electro-optical section  53  includes a non-reciprocal optical element which, in the disclosed embodiment, is a commercially-available three-port circulator  77 . The circulator  77  is optimized for a wavelength of 1550 nm, but also happens to have, for each of the wavelengths of 850 nm and 630 nm, a relatively high transmittance. A circulator suitable for use at  77  can be obtained commercially from Oplink Communications, Inc. of San Jose, Calif. 
   A single-mode optical fiber  81  couples the output of the laser diode  62  to an input port of the multiplexer  68 , and a different single-mode optical fiber  82  couples the output of the laser diode  63  to another input port of the multiplexer  68 . A single-mode optical fiber  83  couples an output port of the multiplexer  68  to an input port of the multiplexer  67 , and a single-mode optical fiber  84  couples the output of the laser diode  61  to an input port of the multiplexer  67 . A single-mode optical fiber  86  couples an output port of the multiplexer  67  to an input port of the circulator  77 , and a single-mode optical fiber  87  couples an output port of the circulator  77  to an input of the detector  71 . The end of the optical fiber  52  remote from the ferrule  51  is coupled to a further port of the circulator  77 . 
   Turning now in more detail to the thin-film filter  57 , the filter  57  has a plurality of thin layers of different materials, which are selected and ordered so that the filter  57  has certain specific properties with respect to radiation impinging at selected angles onto either side of the filter  57 . In more detail, the filter  57  in the disclosed embodiment has  140  layers. As discussed above, the filter  57  is configured to be highly transmissive to infrared radiation with a wavelength of 1550 nm, and to have a transmittance and reflectance of approximately 50% for infrared radiation with a wavelength of 850 nm. Further, the filter  57  is configured to be highly reflective to virtually all visible radiation, except for visible radiation falling within a narrow passband which is centered at a wavelength of 630 nm, and which has a width of approximately 4 nm. As to visible radiation with a wavelength of approximately 630 nm, which falls within this passband, the filter  57  has a transmittance and reflectance of approximately 50%. 
     FIG. 3  is a graph depicting a curve that represents the transmittance of the filter  57 , across a spectrum which includes visible and infrared radiation. As indicated at  91 , the visible spectrum ranges from approximately 400 nm to approximately 700 nm, and it will be noted that the filter  57  is highly reflective to all wavelengths of visible radiation, except for a narrow passband  92  centered at 630 nm, where the filter  57  has a transmittance of approximately 50%, and thus a reflectance of approximately 50%. In the disclosed embodiment, the passband  92  has a width of approximately 4 nm. Reference numeral  93  identifies the wavelength of 850 nm, where the filter  57  has a transmittance of approximately 50% and thus also a reflectance of approximately 50%. Reference numeral  94  identifies the wavelength of 1550 nm, where the filter  57  has a transmittance of nearly 100%. 
   Although the filter  57  in the disclosed embodiment is configured to have certain characteristics at the selected wavelengths of 630 nm, 850 nm, and 1550 nm, it would alternatively be possible to use other wavelengths. Further, even though the filter  57  in the disclosed embodiment has a passband  92  with a width of approximately 4 nm, the passband could alternatively have some other suitable width. For example, the advantages of a relatively narrow passband such as 4 nm may justify the added manufacturing cost in some applications, whereas a wider passband such as 8 nm can be made at a lower cost and may be adequate for other applications. 
   The following is the specific prescription for the exemplary 140-layer thin-film filter  57  in the disclosed embodiment, using a notation form which is well known to those skilled in the art: 
   
     
       
             
             
             
             
             
             
             
             
           
         
             
                 
             
           
           
             
               1 
               .87417D 
               2 
               .93707Q 
               3 
               .59373D 
               4 
               .68729Q 
             
             
               5 
               .84367D 
               6 
               .86474Q 
               7 
               .69571D 
               8 
               .80691Q 
             
             
               9 
               .68922D 
               10 
               .94277Q 
               11 
               .95296D 
               12 
               .96184Q 
             
             
               13 
               .84188D 
               14 
               .90343Q 
               15 
               .88798D 
               16 
               .94453Q 
             
             
               17 
               .91814D 
               18 
               .9222Q 
               19 
               .82828D 
               20 
               .88612Q 
             
             
               21 
               .89998D 
               22 
               .9322Q 
               23 
               .83963D 
               24 
               .62469Q 
             
             
               25 
               .53525D 
               26 
               .83988Q 
               27 
               .80437D 
               28 
               .65889Q 
             
             
               29 
               .66428D 
               30 
               .77243Q 
               31 
               .77065D 
               32 
               .62333Q 
             
             
               33 
               .60079D 
               34 
               .85563Q 
               35 
               .9025D 
               36 
               .54333Q 
             
             
               37 
               .69335D 
               38 
               .8133Q 
               39 
               .69358D 
               40 
               .66139Q 
             
             
               41 
               .61176D 
               42 
               .83722Q 
               43 
               .92406D 
               44 
               .61004Q 
             
             
               45 
               .66767D 
               46 
               .59076Q 
               47 
               .73681D 
               48 
               .7168Q 
             
             
               49 
               .45866D 
               50 
               1.08G 
               51 
               .74794D 
               52 
               .93892Q 
             
             
               53 
               1.08487D 
               54 
               1.0855Q 
               55 
               1.0094D 
               56 
               1.08479Q 
             
             
               57 
               1.03634D 
               58 
               1.08205Q 
               59 
               2.06169D 
               60 
               1.09419Q 
             
             
               61 
               1.04682D 
               62 
               1.09012Q 
               63 
               1.03303D 
               64 
               1.11933Q 
             
             
               65 
               1.06649D 
               66 
               1.03346Q 
               67 
               1.2058D 
               68 
               1.44909Q 
             
             
               69 
               1.06938D 
               70 
               1.09292Q 
               71 
               1.06419D 
               72 
               1.09073Q 
             
             
               73 
               1.03179D 
               74 
               1.08894Q 
               75 
               1.03547D 
               76 
               1.08318Q 
             
             
               77 
               6.21126D 
               78 
               1.08677Q 
               79 
               1.03211D 
               80 
               1.08539Q 
             
             
               81 
               1.04187D 
               82 
               1.08426Q 
               83 
               1.02083D 
               84 
               1.16822Q 
             
             
               85 
               1.20809D 
               86 
               1.1466Q 
               87 
               1.0841D 
               88 
               1.20698Q 
             
             
               89 
               1.08544D 
               90 
               1.1133Q 
               91 
               1.07304D 
               92 
               1.11622Q 
             
             
               93 
               1.0515D 
               94 
               1.10198Q 
               95 
               10.36312D 
               96 
               1.09983Q 
             
             
               97 
               1.05013D 
               98 
               1.10794Q 
               99 
               1.03736D 
               100 
               1.06045Q 
             
             
               101 
               1.02806D 
               102 
               1.03392Q 
               103 
               .76535D 
               104 
               .88209Q 
             
             
               105 
               1.04362D 
               106 
               1.04018Q 
               107 
               1.05205D 
               108 
               1.15664Q 
             
             
               109 
               1.07093D 
               110 
               1.10304Q 
               111 
               1.04894D 
               112 
               1.09883Q 
             
             
               113 
               6.2272D 
               114 
               1.09448Q 
               115 
               1.05137D 
               116 
               1.10979Q 
             
             
               117 
               1.04732D 
               118 
               1.12319Q 
               119 
               1.09733D 
               120 
               .9596Q 
             
             
               121 
               .76689D 
               122 
               1.47182Q 
               123 
               1.23068D 
               124 
               .82947Q 
             
             
               125 
               1.12015D 
               126 
               1.16279Q 
               127 
               1.01184D 
               128 
               1.07944Q 
             
             
               129 
               1.05393D 
               130 
               1.10461Q 
               131 
               2.09452D 
               132 
               1.09515Q 
             
             
               133 
               1.04623D 
               134 
               1.08607Q 
               135 
               .97473D 
               136 
               1.07056Q 
             
             
               137 
               1.19518D 
               138 
               .91173Q 
               139 
               .69818D 
               140 
               1.76913Q 
             
             
                 
             
           
        
       
     
   
   In the foregoing prescription, the layer numbers are not part of the prescription itself, but instead are provided for clarity. Layer  1  is at the side of the filter  57  located adjacent the surface  35  of the glass prism  23 , and layer  140  is at the opposite side of the filter  57 . The prescription assumes that the prism  23  is made from a glass material having a refractive index of 1.52, and assumes that the prism is sufficiently thick so that the opposite side of the prism can be effectively ignored. The prescription is configured for random polarization, with incidence on the filter in glass at an angle of 22.5° from a reference line perpendicular to the filter, and with incidence on the filter in air at an angle of 35.6° (and then exiting into the glass prism). 
   In the prescription, each “D” and each “Q” represents a respective layer with an optical thickness of one-quarter wavelength at normal incidence for the design wavelength of 630 nm. The number preceding each “D” or “Q” is a coefficient that represents a thickness adjustment. The “D” layers have a refractive index of 2.1 and can, for example, be implemented with tantalum pentoxide. The “Q” layers have a refractive index of 1.444 and can, for example, be implemented with silicon dioxide. The exact values may vary slightly in dependence on fabrication considerations, such as the method of deposition, residual gases, and rates of deposition. Alternatively, other high-index coating materials could be used with similar scalable results, including niobium pentoxide, zirconium oxide, and/or titanium dioxide. 
   It is emphasized that the foregoing prescription for the filter  57  is merely one possible way of implementing the filter  57 . The invention encompasses this approach, as well as any other suitable approach. 
   An explanation will now be provided of how the disclosed system operates. With reference to  FIG. 1 , visible radiation which originates from the scene  13  propagates along the path of travel  12 , traveling through the lenses  16 – 18 , the prisms  21 – 23 , and the lenses  41 – 42 , until it reaches the eye  14  of a user. As shown at  91  in  FIG. 3 , the filter  57  is highly reflective to all visible radiation, except that it has a reflectance of approximately 50% for the narrow passband  92  which is centered at the wavelength of 630 nm. Thus, to the extent that radiation within the visible spectrum  91  ( FIG. 3 ) impinges on the filter  57  along the portion  97  ( FIG. 2 ) of the path  12 , almost all of this visible radiation will be reflected by the filter  57  and will then travel along the portion  98  of the path  12  to the eye  14  ( FIG. 1 ) of the user. The exception is that only about 50% of the radiation within the passband  92  will be reflected, and the other 50% will pass through the filter  57  and effectively be lost or ignored. 
   In  FIG. 2 , the laser dioxide  63  outputs visible radiation with a wavelength of 630 nm, which is virtually completely reflected into the optical fiber  83  by the multiplexer  68 . Infrared radiation emitted by the laser diode  62  at a wavelength of 850 nm is passed through the multiplexer  68  with a high level of efficiency, and enters the optical fiber  83 . Thus, the multiplexer  68  efficiently combines or multiplexes the radiation at wavelengths of 630 nm and 850 nm, and transmits this combined radiation through the optical fiber  83 . When this combined radiation reaches the multiplexer  67 , the multiplexer  67  effects almost a complete reflection of this radiation into the optical fiber  86 . The laser diode  61  emits infrared radiation with a wavelength of 1550 nm, which passes through the multiplexer  67  with a high degree of efficiency, and enters the optical fiber  86 . Thus, the multiplexer  67  efficiently combines or multiplexes the radiation at all three wavelengths of 630 nm, 850 nm and 1550 nm. 
   The combined radiation with these three wavelengths propagates through the optical fiber  86  until it reaches the circulator  77 , where it is passed with a high degree of efficiency into the optical fiber  52 , and then travels to the filter  57 . When this combined radiation reaches the filter  57 , each wavelength is treated separately. In particular, nearly 100% of the infrared radiation at the wavelength of 1550 nm will pass through the filter  57  and enter the prism  23 . As to the radiation at each of the wavelengths 850 nm and 630 nm, approximately 50% will be reflected by the filter  57  and will be effectively lost or ignored, and the other 50% will pass through the filter  57  and enter the prism  23 . 
   As to the radiation at each of the three wavelengths which does enter the prism  23 , this radiation will all be propagating through the prism  23  along the portion  97  of the optical path  12 . With reference to  FIG. 1 , this radiation will then travel along the path of travel  12  in  FIG. 1 . It will be successively reflected by the surfaces  34 ,  33 ,  32  and  31  as it travels through the prisms  23 ,  22  and  21 , and will then pass through the lenses  18 ,  17  and  16 , and travel to the scene  13 . 
   A typical scene  13  will reflect some of the energy at each wavelength back along the path  12 . This reflected energy will travel through the lenses  16 - 18  and the prisms  21 – 23 , and will reach the filter  57 . With reference to  FIG. 2 , approximately 50% of the energy at each of the wavelengths 630 nm and 850 nm will pass through the filter filter  57 , and will be effectively ignored or lost. The other 50% of the energy at each of the wavelengths of 630 nm and 850 nm will be reflected by the filter  57  so as to be propagating in the direction  98 , and will travel along the path of travel  12  to the eye  14  of the user. As to the radiation with a wavelength of 1550 nm, virtually none of this radiation will be reflected by the filter  57 . Instead, nearly 100% of this radiation will pass through the filter  57  and will enter the optical fiber  52 . It will then travel through the optical fiber  52  to the circulator  77 , which will route it with a high degree of optical efficiency into the optical fiber  87 , and thus to the detector  71 . 
   It will be helpful to now briefly discuss each wavelength separately. Beginning with the wavelength of 1550 nm, the circuit  73  can use the laser diode  61  to emit a pulse of infrared radiation having the wavelength of 1550 nm. This pulse then travels through the multiplexer  67 , the circulator  77  and the filter  57 , and into the prism  23 . It then travels along the path of travel  12  and out of the sight  10  to the scene  13 , where some of the energy of the pulse is reflected. This reflected energy with the wavelength of 1550 nm then travels back along the path of travel  12  until it is in the prism  23  and reaches the filter  57 . Virtually none of this returning energy is reflected by the filter  57 . Instead, almost 100% of this energy passes through the filter  57  and into the fiber  52 , and then is directed by the circulator  77  to the detector  71 . The circuit  73  can determine the time interval which elapses between transmission of the pulse by the laser diode  61  and reception of the reflected pulse by the detector  71 , and can then use known techniques to calculate the distance or range from the sight  10  to the scene  13 . 
   Turning to the wavelength of 630 nm, the laser diode  63  outputs visible radiation at this wavelength, which passes through the multiplexer  68 , multiplexer  67 , and circulator  77 , and eventually reaches the filter  57 . Approximately 50% of this energy will reflected by the filter  57 , and will be effectively ignored. The other 50% of the energy travels out of the sight  10  along the path of travel  12 , until it reaches the scene  13 . A portion of this energy at the wavelength of 630 nm will be reflected by the scene  13 , and will travel back along the path of travel  12  until it reaches the filter  57 . Approximately 50% of this reflected energy will pass through the filter  57 , and will be effectively ignored. The other 50% will be reflected, and will continue propagating along the path of travel  12  until it reaches the eye  14  of the user. As a result, the user can see a small dot or pointer of visible laser light, which is being projected onto the scene  13 . The user can move the weapon carrying the sight  10 , in order to position this visible dot or pointer on a portion of the scene  13  which represents a target that the user wishes to hit with a bullet or other projectile from the weapon. 
   As to the wavelength of 850 nm, the laser diode  62  emits infrared radiation at this wavelength. The purpose and use this radiation is similar to that of the radiation from the laser diode  63 . The fundamental difference is that one beam is infrared radiation, and the other beam is visible radiation. In more detail, the radiation from the laser diode  62  passes through the multiplexers  68  and  67 , and through the circulator  77 . When it reaches the filter  57 , approximately 50% of the energy is reflected, and is then effectively ignored. The remaining 50% of the energy passes through the filter  57 , and then propagates out of the sight  10  along the path of travel  12 , until it reaches the scene  13 . A small portion of this radiation at wavelength 850 nm is reflected by the scene  13 , and travels back along the path of travel  12  until it reaches the filter  57 . Approximately 50% of this reflected energy passes through the filter  57 , and is effectively ignored. The remaining 50% is reflected by the filter  57  so that it travels in the direction  98 , and then propagates to the eye  14  of the user. Since this is infrared radiation, which is not normally visible to the naked eye  14 , the user can wear special glasses of a known type in order to see this infrared radiation. Alternatively, the eyepiece of the sight  10  can be configured to be a detachable assembly, which can be replaced with a substitute assembly that will make this infrared radiation, and also the visible radiation from the scene  13 , visible to the eye  14  of the user. In either case, what the user will see is a small dot or pointer, which is being projected onto the scene  13 . The user can move the weapon carrying the sight  10  in order to position this dot or pointer on a portion of the scene  13  which represents a target that the user wishes to hit with a bullet or other projectile. 
   The disclosed embodiment includes laser diodes  62  and  63  that respectively produce both infrared and visible radiation, because there are applications where it is desirable to have a sight  10  with both infrared and visible pointers. However, for other applications, it would optionally be possible to omit either one or both of the laser diodes  62  and  63 , so as to provide a sight with only a visible pointer, only an infrared pointer, or no pointer at all. If either of the laser diodes  62  and  63  is omitted, then the multiplexer  68  would also be omitted. In addition, if the laser diodes  62  and  63  are both omitted, then both of the multiplexers  68  and  67  can be omitted. 
   Through use of the non-reciprocal optical element  77 , the outbound and inbound laser pulses of the laser rangefinder can all pass through a single common optical aperture, while achieving a high degree of optical efficiency, and while avoiding the cost of multiple apertures or high-speed optical switches and associated circuitry. This permits the laser rangefinder to be compact and lightweight. Further, by avoiding optical switches and the associated control electronics, battery life is extended for a portable sight, such as the sight  10  of  FIG. 1 . 
   In addition, through the use of wavelength division multiplexers, one or more pointers in either or both of the visible and infrared spectrums can be implemented, and can use the same optical aperture as the laser rangefinder, and with a suitable degree of optical efficiency. A further consideration is that, through the use of fiber optics for inbound and outbound laser beams, excellent flexibility is provided for incorporating the invention into the free space available within existing weapon sights. Consequently, with only minimal redesign, structure providing enhanced functionality can be easily retrofit into previously-manufactured sights, and/or can be easily assembled into new sights at the factory. The invention permits a single compact housing with a single optical aperture to contain each of several distinct functional capabilities, including an optical weapon sight, a laser rangefinder, and one or more laser pointers. 
   Although one embodiment has been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and the scope of the invention, as defined by the following claims.