Patent Publication Number: US-2016246062-A1

Title: Beam splitter apparatus, light source apparatus, and scanning observation apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional application of U.S. Ser. No. 13/461,096, filed May 1, 2012, which is a continuation application of PCT/JP2010/055496, filed on Mar. 23, 2010, the contents of which are incorporated herein by reference. 
     This application is based on Japanese Patent Application No. 2009-251859, filed on Nov. 2, 2009, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to beam splitter apparatuses, light source apparatuses, and scanning observation apparatuses. 
     BACKGROUND ART 
     Beam splitter apparatuses for branching one laser beam emitted from a light source into a plurality of laser beams are well known (refer to, for example, Patent Literature 1). This kind of beam splitter apparatus includes at least two highly reflecting mirrors that are disposed at mutually different distances from a flat semi-transparent mirror interposed therebetween and is provided with a portion formed as a total reflector or an anti-reflection member on the semi-transparent mirror. 
     According to this beam splitter apparatus, a laser beam entering from one side of the semi-transparent mirror is branched by the semi-transparent mirror, reflected by highly reflecting mirrors disposed on either side of the semi-transparent mirror, and returns to the semi-transparent mirror. Through repetition of this step, one laser beam is branched into a plurality of laser beams with different optical path lengths. The plurality of resultant laser beams can be converged on one position by endowing the highly reflecting mirrors with a minute angle. 
     CITATION LIST 
     Patent Literature 
     
         
         {PTL 1} 
         Japanese Patent No. 3927513 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, when the beam splitter apparatus disclosed in Patent Literature 1 is to be applied to a scanning observation apparatus, such as a scanning microscope, it is necessary to not only effectively produce optical responses from the subject but also detect those optical responses by differentiating them for each radiation position. 
     More specifically, when the subject is to be irradiated with a plurality of light beams, as with the beam splitter apparatus described in Patent Literature 1, optical responses produced at different radiation positions spatially overlap one another on the detector due to scattering of light on the surface and in the interior of the subject, and these optical responses cannot be differentiated for each radiation position. The deeper the positions in the subject from which optical responses are to be observed, the more intense the scattering of light and the more noticeable this spatial overlapping. In addition, light beams to be radiated on the subject needs to be adjusted to have appropriate intervals. However, with the beam splitter apparatus disclosed in Patent Literature 1, the point of convergence shifts in the optical-axis direction when the branching laser beams are to be set at different relative angles merely by angle setting of the highly reflecting mirrors. Angle setting alone of the highly reflecting mirrors is not satisfactory to endow the laser beams with different relative angles without shifting the point of convergence in the optical-axis direction, but rather, their positions also need to be shifted. Furthermore, when a laser beam is branched into a plurality of laser beams, fine angle setting of the reflecting mirrors is required for each beam branch. For this reason, the work of setting the highly reflecting mirrors is intricate, and the structure of the apparatus also becomes complicated. 
     The present invention is to provide a beam splitter apparatus and a light source apparatus that can detect the responses in the subject, resulting from irradiation with a plurality of light beams, by separating them on the time axis, even if the responses spatially overlapping one another on the detector, as well as providing a scanning observation apparatus capable of fast scanning using this beam splitter apparatus. Furthermore, the present invention is to provide a beam splitter apparatus and a light source apparatus that can branch one beam into a plurality of beams with different optical path lengths and, at the same time, can converge, with a simple structure, those laser beams on the same position in the optical-axis direction, despite the different relative angles between the beams, as well as providing a scanning observation apparatus capable of fast scanning using this beam splitter apparatus. 
     Solution to Problem 
     A first aspect according to the present invention is a beam splitter apparatus that generates a plurality of pulsed beams to be radiated on a subject from an input pulsed beam, and the beam splitter apparatus includes at least one branching section that branches the input pulsed beam into two optical paths; at least one delaying section that endows pulsed beams passing along the two optical paths branching off via the branching section with a relative time delay to sufficiently separate responses in the subject caused by the pulsed beam; and a beam-angle setting section that endows the plurality of pulsed beams, endowed with the relative time delay by the delaying section, with a relative angle and converges the plurality of pulsed beams on the same position. 
     According to the first aspect of the present invention, the input pulsed beam is branched into the two optical paths by the branching section. The pulsed beam that has branched into each of the optical paths is endowed with the relative time delay by the delaying section while passing along each of the optical paths. Then, the two pulsed beams, endowed with the relative time delay, are endowed with the relative angle by the beam-angle setting section, converged on the same position, and radiated on the subject. 
     Because the pulsed beams are converged on the same position with the relative angle therebetween, all the pulsed beams can be transmitted by arranging the position of convergence of the pulsed beams at a pupil position of an optical system (e.g., an objective optical system) downstream thereof or a position that is optically conjugate to it. Then, the pulsed beams can be focused at a focal position of the optical system and spatially spaced apart in the form of multiple points. 
     In this case, the relative time delay caused by the delaying section is longer than the time of the response such as fluorescence or scattering in the subject. Then, the responses in the subject resulting from the pulsed beams are prevented from being mixed and can be detected by separating them on the time axis. 
     In the above-described aspect, a relay optical system that is disposed in each of the optical paths branching off via the branching section and that relays a pupil in each of the optical paths; and at least one multiplexing section that multiplexes the plurality of pulsed beams relayed by the relay optical systems may be provided. The beam-angle setting section may endow one of the pulsed beams branching off via the branching section with an angle so as to have a relative angle with respect to the other pulsed beam. 
     By doing so, the input pulsed beam is branched by the branching section into the two optical paths with different optical path lengths, and the pulsed beams are relayed by the relay optical systems disposed in the respective optical paths and are multiplexed by the multiplexing section. At this time, one of the pulsed beams branching into the two optical paths via the branching section is endowed with an angle by the beam-angle setting section so as to have a relative angle with respect to the other pulsed beam. By doing so, the pulsed beams in the two optical paths having different optical path lengths and endowed with the relative angle can be converged on one position. 
     In this case, because the pupils of the pulsed beams branching into the two optical paths via the branching section are relayed by the relay optical systems disposed in the respective optical paths, the point of convergence of the pulsed beams can be prevented from being shifted in an optical-axis direction even when the branching pulsed beams are set to different relative angles. In short, according to this aspect, even when the relative angles of the pulsed beams are different, the plurality of pulsed beams can be converged on the same pupil position in the optical-axis direction with a simple structure in the form of the relay optical systems. 
     As a result, even when relative angles of the pulsed beams are changed, the pulsed beams can be made incident on the optical systems disposed downstream thereof under the same incidence conditions. For example, by converging a plurality of pulsed beams endowed with a relative angle on the pupil position of a microscope objective lens, the pulsed beams can be radiated at different positions on the focal plane of the objective lens. The intervals of the radiation positions can be changed by making the relative angles different, and the amount of light can be prevented from fluctuating at this time. 
     In the above-described aspect, the relay optical system may include at least one pair of lenses, and the beam-angle setting section may be disposed between the one pair of lenses or between a plurality of pairs of lenses. 
     By doing so, the pupil is relayed by the one pair of lenses even when the branching pulsed beams are endowed with a relative angle by the beam-angle setting section, and the point of convergence of the pulsed beams can be prevented from being shifted in the optical-axis direction. Furthermore, as a result of a plurality of pairs of such lenses being provided and the pupils in the two optical paths being relayed by the plurality of pairs of theses lenses, the lens diameter can be reduced. 
     In the above-described aspect, the beam-angle setting section may include a first mirror that reflects a pulsed beam branching off via the branching section; a second mirror that reflects the pulsed beam, reflected by the first mirror, towards the multiplexing section; and a rectilinear translation mechanism that rectilinearly translates the first mirror and the second mirror together in the optical-axis direction therebetween. 
     A pulsed beam branching off via the branching section can be endowed with a relative angle by parallel moving the first mirror and the second mirror together by means of the rectilinear translation mechanism in the optical-axis direction between these mirrors. 
     In the above-described aspect, the beam-angle setting section may include a mirror that reflects the pulsed beams branching off via the branching section towards the multiplexing section and a swing mechanism that swings the mirror about an axis orthogonal to optical axes of the pulsed beams. 
     The pulsed beams branching off via the branching section can be endowed with a relative angle by swinging the mirror, with the swing mechanism, about an axis orthogonal to the optical axes of the pulsed beams. 
     In the above-described aspect, the beam-angle setting section may include a swing mechanism that swings at least one of the branching section and the multiplexing section about an axis orthogonal to optical axes of the pulsed beams. 
     The pulsed beams branching off via the branching section can be endowed with a relative angle by swinging at least one of the branching section and the multiplexing section, with the swing mechanism, about an axis orthogonal to optical axes of the pulsed beams. 
     In the above-described aspect, a plurality of units in series that each include the branching section, the multiplexing section, the relay optical systems, and the beam-angle setting section may be provided, and the beam-angle setting sections may be disposed between the respective branching sections and the respective multiplexing sections. 
     The input pulsed beam can be branched into a plurality of optical paths, and each of the branching pulsed beams can be endowed with a relative angle by the beam-angle setting section by providing a plurality of units in series that include the branching section, the multiplexing section, the relay optical systems, and the beam-angle setting section. As a result, pulsed beams in a plurality of optical paths, having different optical path lengths and endowed with a relative angle, can be converged on one position. 
     In the above-described aspect, at least one multiplexing/branching section that multiplexes the pulsed beams in the two optical paths branching off via the branching section and that branches the multiplexed pulsed beams into two optical paths with different optical path lengths may be provided. The relay optical system may be disposed in each of the optical paths branching off via the branching/multiplexing section, and the beam-angle setting section may endow pulsed beams branching off via the multiplexing/branching section with a relative angle. 
     As a result of the at least one multiplexing/branching section being provided, the input pulsed beam can be branched into a plurality of optical paths by the branching section and the multiplexing/branching section, and each of the branching pulsed beams can be endowed with a relative angle by the beam-angle setting section. As a result, pulsed beams in a plurality of optical paths, having different optical path lengths and endowed with a relative angle, can be converged on one position. 
     In the above-described aspect, a polarization modulator that is disposed in one of the optical paths upstream of the multiplexing section and that makes the polarization states of the two optical paths orthogonal to each other may be provided. The multiplexing section may be a polarizing beam splitter. 
     One of the pulsed beams in the two optical paths branching off via the branching section or the multiplexing/branching section can be transmitted, while the other is reflected, by enabling the polarization modulator to make the polarization states of the two optical paths orthogonal to each other and forming the multiplexing section of the polarizing beam splitter. As a result, all the pulsed beams in the two optical paths can be multiplexed by the multiplexing section, thus suppressing the amount of light loss of these pulsed beams, thereby increasing the utilization efficiency of the input pulsed beam. 
     Furthermore, a second aspect according to the present invention is a beam splitter apparatus that generates a plurality of pulsed beams to be radiated on a subject from an input pulsed beam, and the beam splitter apparatus includes at least one branching section that branches the input pulsed beam into two optical paths; at least one delaying section that endows pulsed beams passing along the two optical paths branching off via the branching section with a relative time delay to sufficiently separate responses in the subject caused by the pulsed beams; at least one multiplexing section that multiplexes the two pulsed beams endowed with the time delay by the delaying section; a stationary displacing section that is disposed in each of the optical paths branching off via the branching section, causes pulsed beams multiplexed by the multiplexing section to be incident on different positions of the multiplexing section, and makes principal rays of the pulsed beams parallel to one another after the last multiplexing section; and at least one lens disposed after the last multiplexing section. 
     According to this aspect, the input pulsed beam is branched by the branching section into the two optical paths. The pulsed beam that has branched into each of the optical paths is endowed with the relative time delay by the delaying section while passing along each of the optical paths. Then, the two pulsed beams endowed with the relative time delay are subjected to adjustment of their incident positions on the multiplexing section by the stationary displacing sections provided in the optical paths and are then multiplexed by the multiplexing section. Principal rays of the pulsed beams are adjusted to be parallel to each other by the stationary displacing sections after the last multiplexing section, and the pulsed beams are correctly converged on the same position by the lens disposed downstream thereof. 
     In this case, because the delaying section endows the two pulsed beams with the relative time delay to sufficiently separate the responses in the subject, the responses in the subject resulting from the pulsed beams are prevented from being mixed and can be detected by separating them on the time axis. 
     In the above-described aspect, a relay optical system that is disposed in each of the optical paths branching off via the branching section and that relays a pupil in each of the optical paths may be provided. 
     By doing so, the beam diameters of the pulsed beams branching off via the branching section can be made the same by the relay optical systems. As a result, when a plurality of the generated pulsed beams is applied to a scanning observation apparatus, the resolving power can be prevented from changing. 
     Furthermore, in the above-described aspect, the stationary displacing sections may include at least two mirrors and a rectilinear translation mechanism that rectilinearly translates at least one of the mirrors in a plane parallel to an optical axis of a pulsed beam incident on the mirror so as to change an optical path length between the mirrors. 
     The optical path length between the mirrors can be changed by the operation of the rectilinear translation mechanism, thereby changing the intervals of the incident positions, on the multiplexing section, of the two pulsed beams multiplexed by the multiplexing section. 
     Furthermore, in the above-described aspect, the rectilinear translation mechanism may move the two mirrors in a direction parallel to an optical axis between the mirrors. 
     By doing so, the intervals of the incident positions, on the multiplexing section, of the two pulsed beams multiplexed by the multiplexing section can be changed, and the optical path length can be prevented from changing even in that case. As a result of the optical path length being prevented from changing, it is not necessary to set the optical path length anew. If the pulsed beam is a laser beam, it diverges at a predetermined angle depending on the beam diameter while propagating. Because of this, the beam diameter after propagating changes if the optical path length changes. As a result of the optical path length being prevented from changing, the beam diameter can be prevented from changing, thereby preventing the resolving power from changing when this aspect is applied to a scanning observation apparatus. 
     Furthermore, in the above-described aspect, at least one lens group and a lens-group moving mechanism that moves the lens group in a direction orthogonal to the optical axis by the same amount as an amount of displacement of the optical axis in synchronization with displacement of the optical axis by the stationary displacing section may be provided downstream of the stationary displacing sections. 
     By doing so, even when the optical axis is displaced by the stationary displacing sections, the lens group can be moved by the lens-group moving mechanism in a direction orthogonal to the optical axis by the same amount as the amount of displacement of the optical axis. As a result, even when the relative angle of the pulsed beams is changed by the stationary displacing sections, the principal rays of the pulsed beams after being multiplexed can be kept parallel to one another, thereby preventing the point of convergence from shifting in the optical-axis direction. 
     Furthermore, between downstream of the above-described stationary displacing section and at least one lens disposed after the above-described last multiplexing section, at least one pair of lenses ( 36   b : 104   c  and  37   b : 105   a ) may be disposed such that the focal positions of the lenses coincide with one another, as shown in  FIG. 19  (in short, they serve as a  4   f  optical system). 
     By doing so, even when the optical axis is displaced by the stationary displacing section, because an optical system downstream thereof serves as a  4   f  optical system, the principal rays of pulsed light beams after the last multiplexing section can be kept parallel to one another, thereby preventing the point of convergence from shifting in the optical-axis direction. 
     Furthermore, a third aspect according to the present invention is a beam splitter apparatus that generates a plurality of pulsed beams radiated on a subject from an input pulsed beam, and the beam splitter apparatus includes at least one branching section that branches the input pulsed beam into two; at least two light-guide members with different optical path lengths that propagate the pulsed beams branching off via the branching section; and a beam-angle setting section that endows a plurality of pulsed beams emitted from exit ends of the plurality of light-guide members with a relative angle and that converges the plurality of pulsed beams on the same position. 
     According to the above-described aspect, the input pulsed beam is branched into two by the branching section, and the branching pulsed beams propagate along the at least two light-guide members, are emitted from the exit ends of the light-guide members, are endowed with a relative angle by the beam-angle setting section, and are converged on the same position. Because the at least two light-guide members have optical path lengths different from one another, the pulsed beams emitted from the exit ends are endowed with a relative time delay. As a result, the pulsed beams can be endowed with a sufficient time delay merely by adjusting the length of light-guide members, without increasing the size of the apparatus, and the responses in the subject resulting from the pulsed beams can be prevented from being mixed and can be detected by separating them on the time axis. 
     In this case, the beam-angle setting section may be constructed by setting the directions of the exit ends such that the optical axes of the light-guide members intersect one another at one point. Alternatively, if the light-guide members are set such that the optical axes are parallel, the beam-angle setting section may be in the form of a lens that converges the pulsed beams emitted from these exit ends on the same position. 
     Furthermore, a fourth aspect according to the present invention is a light source apparatus including a pulsed light source that emits a pulsed beam; and one of the above-described beam splitter apparatuses that receives the pulsed beam emitted from the pulsed light source. 
     According to this light source apparatus, a bundle of a plurality of pulsed beams emitted from the pulsed light source, having different optical path lengths and endowed with a relative angle, can be converged on the same position and can all be made to pass through the pupil position of an optical system disposed downstream thereof. 
     In the above-described aspect, a scanning section that spatially scans a plurality of pulsed beams emitted from the beam splitter apparatus may be provided. 
     By doing so, while forming many spots on the subject, a plurality of pulsed beams endowed with a time delay can be scanned over these spots on the subject through the operation of the scanning section. As a result, a wider range of the subject can be irradiated with pulsed beams. 
     Furthermore, a fifth aspect according to the present invention is a light source apparatus including a pulsed light source that emits a pulsed beam; one of the above-described beam splitter apparatuses that receives the pulsed beam emitted from the pulsed light source; and a scanning section that spatially scans a plurality of pulsed beams emitted from the beam splitter apparatus by spatially vibrating the exit ends of the plurality of light-guide members. 
     A sixth aspect according to the present invention is a scanning observation apparatus including one of the above-described beam splitter apparatuses; a scanning section that scans a plurality of pulsed beams from the beam splitter apparatus over the subject; an observation optical system that radiates the pulsed beams scanned by the scanning section on the subject; and a detecting section that detects the signal light from the subject. 
     In the above-described aspect, a processing section that synchronizes the signal light detected by the detecting section with the scanned pulsed beams; a restoring section that reconstructs the signal light synchronized by the processing section as two-dimensional information or three-dimensional information in association with sites on the subject; and a display section that displays the two-dimensional information or three-dimensional information may be provided. 
     According to this scanning observation apparatus, a plurality of pulsed beams having different optical path lengths and endowed with a relative angle can be converged on one position by the beam splitter apparatus and radiated on different positions of the subject. Then, an image of the subject can be generated by scanning radiation positions on the subject two-dimensionally or three-dimensionally with the scanning section and detecting light from the subject with the detecting section. 
     Advantageous Effects of Invention 
     The present invention affords an advantage in that beams can be converged on the same position in the optical-axis direction with a simple structure, even if relative angles between the beams differ. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic structural diagram of a beam splitter apparatus of a first embodiment according to the present invention. 
         FIG. 2  is a diagram depicting temporal multiplexing by the beam splitter apparatus of  FIG. 1 , where (a) shows a time delay produced in a reflection optical system and (b) shows an optical pulse train. 
         FIG. 3  is a schematic structural diagram of a beam splitter apparatus according to a modification of  FIG. 1 . 
         FIG. 4  is a schematic structural diagram of a beam splitter apparatus presented as a reference embodiment according to the present invention. 
         FIG. 5  is a diagram depicting temporal multiplexing by the beam splitter apparatus of  FIG. 4 , where (a) shows a time delay produced by a reflection optical system, (b) shows time delays produced by a reflection optical system, and (c) shows an optical pulse train. 
         FIG. 6  is a schematic structural diagram of a beam splitter apparatus of a second embodiment according to the present invention. 
         FIG. 7  is a diagram depicting a method of deflecting a pulsed beam with the beam splitter apparatus of  FIG. 6 , where (a) shows a case where no deflection is performed and (b) shows a case where deflection is performed. 
         FIG. 8  is a schematic structural diagram of a beam splitter apparatus of a modification of  FIG. 6 . 
         FIG. 9  is a schematic structural diagram of a beam splitter apparatus of a third embodiment according to the present invention. 
         FIG. 10  is a schematic structural diagram of a beam splitter apparatus of a fourth embodiment according to the present invention. 
         FIG. 11  is a schematic structural diagram of a beam splitter apparatus of a modification of  FIG. 10 . 
         FIG. 12  is a schematic structural diagram of a beam splitter apparatus of a fifth embodiment according to the present invention. 
         FIG. 13  is a schematic structural diagram of a beam splitter apparatus of a sixth embodiment according to the present invention. 
         FIG. 14  is a schematic structural diagram of a scanning microscope of a seventh embodiment according to the present invention. 
         FIG. 15  is a diagram depicting temporal multiplexing by the scanning microscope of  FIG. 14 , where (a) shows a pulse train of pulsed beams and (b) shows a pulse train of detected fluorescence. 
         FIG. 16  is a schematic structural diagram depicting a beam splitter apparatus of an eighth embodiment according to the present invention. 
         FIG. 17  is a schematic structural diagram depicting a beam splitter apparatus of a ninth embodiment according to the present invention. 
         FIG. 18  is a schematic structural diagram depicting a beam splitter apparatus of a tenth embodiment according to the present invention. 
         FIG. 19  is a schematic structural diagram depicting a beam splitter apparatus of an eleventh embodiment according to the present invention. 
         FIG. 20  is a magnified view of area AA of  FIG. 19 . 
         FIG. 21  is a magnified view of area AB of  FIG. 19 . 
         FIG. 22  is a schematic structural diagram depicting a beam splitter apparatus of a twelfth embodiment according to the present invention. 
         FIG. 23  is a diagram depicting paths with optical path lengths of the beam splitter apparatus of  FIG. 22 , where (a) shows a path with the smallest optical path length, (b) shows a path with the second smallest optical path length, (c) shows a path with the second largest optical path length, and (d) shows a path with the largest optical path length in a solid line. 
         FIG. 24  is a diagram depicting the time intervals of four pulsed beams generated by the beam splitter apparatus of  FIG. 22 . 
         FIG. 25  is a diagram depicting the relationship between the intervals of the pulsed beams of  FIG. 24  and coherence time. 
         FIG. 26  is a schematic structural diagram depicting a modification of the application example of the beam splitter apparatus in  FIG. 22 . 
         FIG. 27  is an overall structural diagram depicting one example of a fluoroscopy apparatus using the beam splitter apparatus of  FIG. 23 . 
         FIG. 28  is a diagram depicting the relationship between pulsed beams radiated on a subject by the fluoroscopy apparatus of  FIG. 27  and fluorescence emitted from the subject. 
         FIG. 29  is a schematic structural diagram depicting a beam splitter apparatus of a thirteenth embodiment according to the present invention. 
         FIG. 30  is a diagram depicting a cross-sectional view of an optical fiber bundle of four optical fibers of the beam splitter apparatus of  FIG. 29 . 
         FIG. 31  is a cross-sectional view depicting one exemplary morphology of the end of an optical fiber bundle having four cores arranged in a square in a fused and integrated cladding, instead of bundling the four optical fibers of  FIG. 30 . 
         FIG. 32  is a cross-sectional view of a modification of the arrangement of the cores in  FIG. 31 . 
         FIG. 33  is an overall structural diagram depicting one example of a fluoroscopy apparatus provided with the beam splitter apparatus of  FIG. 29 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     A beam splitter apparatus  1  according to a first embodiment of the present invention will now be described with reference to  FIGS. 1 to 3 . 
     As shown in  FIG. 1 , the beam splitter apparatus  1  according to this embodiment includes a reflection optical system (beam-angle setting section)  12 , a beam splitter (branching section)  13 , a beam splitter (multiplexing section)  14 , and relay optical systems (pupil transfer optical systems)  16  and  17 . Furthermore, the beam splitter apparatus  1  of this embodiment and a pulsed light source  11  constitute a light source apparatus  101 . 
     In  FIG. 1 , the intersection points of an optical axis IZ with the reflection surfaces of the beam splitter  13  and the beam splitter  14  are referred to as point A and point C, respectively. Furthermore, the midpoint between point A and point C is referred to as point D, and the intersection point of the optical axis of a pulsed beam from the beam splitter  13  with the reflection optical system  12  is referred to as point B. Here, triangle ABC is an isosceles triangle having point B as the vertex, and side AB and side BC have the same length. 
     The functions of the above-mentioned components will now be described. 
     The pulsed light source  11  oscillates a pulsed beam with a repetition frequency R. 
     The beam splitter  13  is a branching section that branches the pulsed beam into two optical paths with different optical path lengths, more specifically, an optical path A-D-C (hereinafter, referred to as “an optical path  10 ”) and an optical path A-B-C (hereinafter, referred to as “an optical path  20 ”). 
     The reflection optical system  12  includes a mirror that totally reflects the pulsed beam from the beam splitter  13  and a swing mechanism (not shown in the figure) that swings this mirror about an axis orthogonal to the optical axis of the pulsed beam. 
     The reflection optical system  12  swings the mirror about an axis orthogonal to the optical axis of the pulsed beam by means of the swing mechanism, not shown in the figure, to change the angle of the optical axis of the pulsed beam branching off via the beam splitter  13 . 
     As a result, the reflection optical system  12  functions as a stationary deflecting section that endows the pulsed beam passing along the optical path  20  branching off via the beam splitter  13  with a deflection angle of θ through tilting of the reflection surface thereof. Furthermore, the reflection optical system  12  also functions as a delaying section that delays the pulsed beam passing along the optical path  20  so that an optical path length difference L is produced between the optical path  10  and the optical path  20 . 
     The optical path  10  and the optical path  20  include the relay optical systems  16  and  17 , respectively, for relaying pupils of the pulsed beams in their respective optical paths. 
     The relay optical system  16  is composed of one pair of lenses  16   a  and  16   b , and the pupil adjacent to point A is relayed to the vicinity of point C. 
     The relay optical system  17  is composed of two pairs of lenses  17   a  and  17   b , and  17   c  and  17   d , and the reflection optical system  12  is disposed between the lens  17   b  and the lens  17   c . The lenses  17   a ,  17   b ,  17   c , and  17   d  have the same focal length. Because of this, the pupil disposed adjacent to point A is relayed to the vicinity above the reflection optical system  12  by means of the lens  17   a  and the lens  17   b . Furthermore, the pupil that has been relayed to the vicinity above the reflection optical system  12  is further relayed to the vicinity of point C by means of the lens  17   c  and the lens  17   d.    
     The beam splitter  14  is a multiplexing section that multiplexes the pulsed beams that have passed along the optical path  10  and the optical path  20 . 
     Although a beam splitter is used as the branching section and the multiplexing section in this embodiment, a half mirror or a dichroic mirror, for example, may be used instead. This also applies to other embodiments. 
     Temporal multiplexing and spatial multiplexing (spatial modulation) of a pulsed beam that has been oscillated by the pulsed light source  11  in the beam splitter apparatus  1  with the above-described structure will now be described. 
     Temporal multiplexing will be described first. 
     From point A to point Z along which a pulsed beam oscillated by the pulsed light source  11  passes, there are two optical paths: the optical path  10  having the shortest optical path length and the optical path  20  having an optical path length larger than the optical path  10  by an optical-path-length difference L. Here, the pulsed beam passing along the optical path  10  is denoted by P 0 , and the pulsed beam passing along the optical path  20  is denoted by P 1 . 
     Because the optical path  20  is longer than the optical path  10  by the optical-path-length difference L, the pulsed beam P 1  passing along the optical path  20  arrives at point C on the beam splitter  14  with a delay L/c compared with the pulsed beam P 0  passing along the optical path  10 , where c represents the velocity of light. In other words, the time t 1  when the pulsed beam P 1  passing along the optical path  20  arrives at point Z is expressed as t 1 =t 0 +L/c, where t 0  represents the time when the pulsed beam P 0  passing along the optical path  10  arrives at point Z (refer to  FIG. 2( a ) ). Here, as shown in  FIG. 2( b ) , when the optical-path-length difference L is selected so that L=c/2R is satisfied in relation to the repetition frequency R of the pulsed light source  11 , an optical pulse train that is temporally multiplexed with a repetition frequency 2R, i.e., twice the original repetition frequency R of the pulsed light source  11 , is generated. 
     Next, spatial multiplexing in which the pulsed beam temporally multiplexed as described above is spatially deflected will be described. 
     First, the following description assumes as a reference that the relative angle between the pulsed beam P 0  and the pulsed beam P 1  is 0 when they are multiplexed at the beam splitter  14  without spatial multiplexing. 
     An incident angle φ 1  at the beam splitter  13  is given as follows: 
       φ1=(π−cos−1( d/L ))/2
 
     where side AB=side BC=L/2, side AC=d, and side AD=side DC=d/2. 
     At this time, an incident angle φ 2  at the reflection optical system  12  is given as follows: 
       φ2=π/2−cos−1( d/L )
 
     At this time, the pulsed beam P 0  is temporally shifted by L/c but is not spatially shifted relative to the pulsed beam P 1 . 
     Thereafter, when the incident angle φ 2  at the reflection surface of the reflection optical system  12  is converted by θ/2 to an incident angle φ 2 ′, the pulsed beam P 1  passing along the optical path  20  is deflected by θ by the reflection optical system  12 . Because the pupil disposed adjacent to point B on the reflection optical system  12  is relayed to point C by the lenses  17   c  and  17   d , the pulsed beam P 1  passing along the optical path  20  is reflected at point C on the beam splitter  14  while maintaining the deflection angle θ, unlike a case where the reflection optical system  12  is not deflected, and is then propagated towards point Z′. At this time, the difference in deflection angle between side CZ and side CZ′ is θ. In other words, spatial multiplexing with deflection angles of 0 and θ can be accomplished. 
     Furthermore, the pupil of the pulsed beam P 0  passing along the optical path  10  is relayed by the relay optical system  16 . 
     From the description so far, a pulsed beam oscillated by the pulsed light source  11  is not only spatially multiplexed with a deflection angle interval of θ but is also temporally multiplexed being shifted by a time interval of L/c. 
     Because the above-described spatial multiplexing and temporal multiplexing occur at the same time in the beam splitter apparatus  1 , pulsed beams produced by irradiating a space with a plurality of light beams, even if spatially overlapping one another on the detection side, can be separated on the time axis. 
     As described above, according to the beam splitter apparatus  1  of this embodiment, a pulsed beam oscillated from the pulsed light source  11  is branched by the beam splitter  13  into two optical paths  10  and  20  with different optical path lengths, relayed by the relay optical systems  16  and  17  disposed in the respective optical paths, and then multiplexed by the beam splitter  14 . At this time, the pulsed beams P 0  and P 1  in the respective two optical paths  10  and  20  branching off from each other via the beam splitter  13  are endowed with a relative angle by the reflection optical system  12 . By doing so, the pulsed beams P 0  and P 1  in the two optical paths  10  and  20 , having different optical path lengths and also endowed with a relative angle, can be converged on one position. 
     In this case, the pupils of the pulsed beams P 0  and P 1  in the two optical paths  10  and  20  branching off from each other via the beam splitter  13  are relayed by the relay optical systems  16  and  17  disposed in their respective optical paths. Because of this, even when the resultant pulsed beams P 0  and P 1  are set to have different relative angles, the point of convergence can be prevented from shifting in the optical-axis direction. In other words, according to the beam splitter apparatus  1  of this embodiment, even with different relative angles of the pulsed beams P 0  and P 1 , the pulsed beams P 0  and P 1  can be converged on the same pupil position in the optical-axis direction with a simple structure of the relay optical systems  16  and  17 . 
     As a result, even when the relative angles of the pulsed beams P 0  and P 1  are changed, they can be made incident upon an optical system disposed downstream thereof under the same incidence conditions. For example, the pulsed beams P 0  and P 1  can be emitted to different positions on the focal plane of a microscope objective lens by converging the pulsed beams P 0  and P 1  endowed with a relative angle at the pupil position of the objective lens. The spacing between the radiation positions can be made different with different relative angles, and the amount of light can be prevented from fluctuating at that time. 
     Furthermore, because the relay optical system  16  is provided with one pair of lenses  16   a  and  16   b , the relay optical system  17  is provided with two pairs of lenses  17   a  and  17   b , and  17   c  and  17   d , and the reflection optical system  12  is disposed between each of two pairs of relay lenses  17   a  and  17   b , and  17   c  and  17   d , the pupil is relayed by the two pairs of lenses  17   a  and  17   b , and  17   c  and  17   d , even when the pulsed beams P 0  and P 1  branching off from each other are endowed with a relative angle by the reflection optical system  12 . Therefore, the point of convergence can be prevented from shifting in the optical-axis direction. In addition, by providing a plurality of pairs of such lenses and relaying pupils of the pulsed beams P 0  and P 1  in the two optical paths  10  and  20  with the plurality of pairs of lenses, the diameters of the lenses can be reduced. 
     Furthermore, a plurality of units including the beam splitter  13 , the beam splitter  14 , the relay optical systems  16  and  17 , and the reflection optical system  12  may be provided in series, and the reflection optical system  12  may be provided between the beam splitter  13  and the beam splitter  14 . 
     By doing so, a pulsed beam oscillated from the pulsed light source  11  can be branched into a plurality of optical paths, and the resultant pulsed beams can be endowed with a relative angle by the reflection optical system  12 . As a result, pulsed beams in a plurality of optical paths having different optical path lengths and endowed with a relative angle can be converged on one position. 
     In addition, according to the light source apparatus  101  provided with such a beam splitter apparatus  1 , a bundle of a plurality of pulsed beams, oscillated from the pulsed light source  11 , having different optical path lengths, and endowed with a relative angle, can all be made to pass through the pupil position in an optical system disposed downstream thereof. 
     MODIFICATION 
     Alternatively, as a modification of this embodiment, the relay optical system  17  may be constructed with one pair of lenses  17   a  and  17   b , and the pulsed beam P 1  passing along the optical path  20  may be endowed with a deflection angle by at least one of the beam splitter  13  and the beam splitter  14 , instead of the reflection optical system  12 . As shown in  FIG. 3 , this modification will be described assuming that the pulsed beam P 1  passing along the optical path  20  is endowed with a deflection angle by the beam splitter  14 . 
     In a beam splitter apparatus  1 ′ according to this modification, the beam splitter  14  includes a half mirror that transmits the pulsed beam P 0  passing along the optical path  10  and reflects the pulsed beam P 1  passing along the optical path  20  and a swing mechanism (not shown in the figure) that swings this half mirror about an axis orthogonal to the optical axis of the pulsed beam. 
     The beam splitter  14  deflects and reflects the pulsed beam P 1  reflected by the reflection optical system  12  by swinging the half mirror about an axis orthogonal to the optical axis of the pulsed beam P 1  by the swing mechanism, not shown in the figure. 
     In this modification, a collimated beam that is emitted from the pulsed light source  11  and incident upon point A is branched by the beam splitter  13  into a light beam passing along the optical path  10  and a light beam passing along the optical path  20 . The light beam passing along the optical path  10  is converted into a collimated beam by the relay optical system  16  but is not endowed with a deflection angle in this case. On the other hand, the light beam passing along the optical path  20  is reflected at the reflection optical system  12  disposed at point B and converted into a collimated beam by the relay optical system  17 . 
     The beam splitter  14  multiplexes the light beam passing along the optical path  20  and the light beam passing along the optical path  10  at point C. At this time, the beam splitter  14  is endowed with a deflection angle about point C so that the light beam passing along the optical path  20  exhibits a finite angle relative to the light beam passing along the optical path  10 . Because the relay optical systems  16  and  17  propagate the pupil near point A to point C, the two light beams can be made to spatially overlap each other in the vicinity of point C. 
     Although this modification has been described by way of an example where a deflection angle is given by the beam splitter  14 , the pulsed beam P 1  may be endowed with a deflection angle by either the beam splitter  13  or both the beam splitter  13  and the beam splitter  14  instead. 
     A pulsed light source is used in this embodiment. However, any light source is acceptable as long as it emits a pulsed beam. For example, a light source such as an LED or a laser light source that emits a laser beam may be used instead. 
     REFERENCE EMBODIMENT 
     As a reference embodiment of the present invention, a beam splitter apparatus  2  will now be described with reference to  FIGS. 4 and 5 . In the description of this reference embodiment, commonalities with the beam splitter apparatus  1  according to the first embodiment will be omitted, and differences will be mainly described. 
     The beam splitter apparatus  2  according to this reference embodiment differs from the beam splitter apparatus  1  according to the first embodiment in that a beam splitter  24  that multiplexes pulsed beams in two optical paths and branches the multiplexed pulsed beams into two optical paths with different optical path lengths is provided between a beam splitter  23  and a beam splitter  25 . 
     As shown in  FIG. 4 , the beam splitter apparatus  2  according to this reference embodiment includes reflection optical systems  21  and  22 , the beam splitter (branching section)  23 , the beam splitter (multiplexing/branching section)  24 , and the beam splitter (multiplexing section)  25 . Furthermore, the beam splitter apparatus  2  of this reference embodiment and the pulsed light source (laser light source)  11  constitute a light source apparatus  102 . 
     The intersection points of the optical axis IZ of the pulsed beam oscillated from the pulsed light source  11  with the beam splitter  23 , the beam splitter  24 , and the beam splitter  25  are denoted by point A, point C, and point F, respectively. 
     Of the two optical paths branching off from each other by the beam splitter  23  between the beam splitter  23  and the beam splitter  24 , the midpoint in the shorter optical path is denoted by point D, and the midpoint in the longer optical path is denoted by point B. Furthermore, of the two optical paths branching off from each other by the beam splitter  24  between the beam splitter  24  and the beam splitter  25 , the midpoint in the shorter optical path is denoted by point G, and the midpoint in the longer optical path is denoted by point E. 
     The functions of the above-mentioned components will now be described. 
     The pulsed light source  11  oscillates a pulsed beam with a repetition frequency R. 
     The beam splitter  23  is a branching section that branches the pulsed beam into two optical paths with different optical path lengths, more specifically, an optical path A-D-C (hereinafter, referred to as “an optical path  10 ”) and an optical path A-B-C (hereinafter, referred to as “an optical path  20 ”). 
     The reflection optical system  21  is composed of two mirrors  21   a  and  21   b  and endows the pulsed beams passing along the two optical paths  10  and  20  branching off from each other by the beam splitter  23  with a relative angle (deflection angle) of 2θ. In addition, the reflection optical system  21  operates the two mirrors  21   a  and  21   b  to delay the pulsed beam passing along the optical path  20  so that an optical-path-length difference L is generated between the optical path  10  and the optical path  20 . 
     The beam splitter  24  multiplexes the pulsed beams in the two optical paths  10  and  20  branching off from each other by the beam splitter  23  and also branches the multiplexed pulsed beams into two optical paths with different optical path lengths: an optical path C-G-F (hereinafter, referred to as “an optical path  30 ”) and an optical path C-E-F (hereinafter, referred to as “an optical path  40 ”). 
     Like the reflection optical system  21 , the reflection optical system  22  is composed of two mirrors  22   a  and  22   b  and endows the pulsed beams passing along the two optical paths  30  and  40  branching off from each other by the beam splitter  24  with a relative angle (deflection angle) of θ. In addition, the reflection optical system  22  operates the two mirrors  22   a  and  22   b  to delay the pulsed beam passing along the optical path  40  so that an optical-path-length difference 2 L is generated between the optical path  30  and the optical path  40 . 
     The beam splitter  25  multiplexes the pulsed beams passing along the four optical paths  10 ,  20 ,  30 , and  40 . 
     Temporal multiplexing and spatial multiplexing (spatial modulation) of a pulsed beam that has been oscillated by the pulsed light source  11  in the beam splitter apparatus  2  with the above-described structure will be described. 
     Temporal multiplexing will be described first. 
     The pulsed light source  11  oscillates a pulsed beam with a repetition frequency R (Hz). The pulsed beam P 0  oscillated at a certain point in time is branched by the beam splitter  23  disposed at point A into the two pulsed beams P 0  and P 1 , so that the pulsed beam P 0  passes along the optical path  10  and the pulsed beam P 1  passes along the optical path  20 . As shown in  FIG. 4 , because the optical path  20  has a larger optical path length than the optical path  10  by L, the pulsed beams P 0  and P 1  arrive at point C at different points in time. This concept is shown in  FIGS. 5( a ) to 5( c ) . 
       FIG. 5( a )  depicts a time delay produced by the reflection optical system  21 ,  FIG. 5( b )  depicts a time delay produced by the reflection optical system  22 , and  FIG. 5( c )  depicts an optical pulse train. 
     In  FIGS. 5( a ) to 5( c ) , the time when the pulsed beam P 0  arrives at point C is denoted by an arrival time t 0 . Because the difference in optical path length between the optical path  10  and the optical path  20  is L, the pulsed beam P 1  arrives at point C at a time t 1 , with a delay of L/c from the time t 0 , where c represents the velocity of light. 
     Both the pulsed beams P 0  and P 1  are multiplexed by the beam splitter  24  disposed at point C, and the beam splitter  24  also branches the pulsed beams P 0  and P 1 . Because of this, each of the pulsed beams P 0  and P 1  propagates along the two optical paths serving as the optical path  30  and the optical path  40 . As shown in  FIG. 4 , because the optical path  40  has a larger optical path length than the optical path  30  by 2L, the pulsed beams P 0  and P 1  arrive at point F with a time difference of 2L/c between a case where they pass along the optical path  40  and a case where they pass along the optical path  30 . Here, the pulsed beams P 0  and P 1  passing along the optical path  40  are renamed pulsed beams P 2  and P 3 , respectively. 
     Consequently, there are four paths from point A to point Z, and the pulsed beams P 0  to P 3  arrive in the vicinity of point Z via any one of the following optical paths: 
     Optical path  10  (P 0 ): A-D-C-G-F-Z (shortest optical path length) 
     Optical path  20  (P 1 ): A-B-C-G-F-Z 
     Optical path  30  (P 2 ): A-D-C-E-F-Z 
     Optical path  40  (P 3 ): A-B-C-E-F-Z 
     Because the beam splitter  25 , constituting a multiplexing section, is disposed at point F, the four pulsed beams P 0  to P 3  are multiplexed with their optical axes oriented towards point Z. Therefore, as shown in  FIG. 5( b ) , temporal multiplexing in the form of pulsed beams at regular intervals on the time axis is accomplished at the time of arrival at point Z. Here, as shown in  FIG. 5( c ) , when the optical-path-length difference L is selected so that L=c/4R is satisfied in relation to the repetition frequency R of the pulsed light source  11 , an optical pulse train that is temporally multiplexed with a repetition frequency of 4R is generated. 
     Next, spatial multiplexing in which the pulsed beam temporally multiplexed as described above is spatially deflected will be described. 
     In this reference embodiment, the reflection surfaces of the beam splitters  23 ,  24 , and  25  and the two mirrors  21   a  and  21   b  of the reflection optical system  21  are disposed so as to have an angle of 45° relative to the optical axis IZ. Quadrangle ALMC is a rectangle, where L and M represent the centers of the mirrors  21   a  and  21   b , respectively, of the reflection optical system  21 . Therefore, when the pulsed beam P 1  passing along the optical path  20  is multiplexed with the pulsed beam P 0  by the beam splitter  24 , the deflection angle between the pulsed beam P 0  and the pulsed beam P 1  is 0 relative to the completely coaxial state serving as a reference. On the other hand, when at least one mirror of the reflection optical system  21  is rotated by a rotation angle of θ relative to the reference state, as shown in  FIG. 4 , the pulsed beam P 1  arrives at point C with a deflection angle of 2θ.  FIG. 4  shows a case where only  21   b  is rotated. 
     Therefore, when the pulsed beams P 0  and P 1  are multiplexed at the beam splitter  24 , the two pulsed beams exhibit a deflection angle of 2θ immediately after they have entered the optical path  30  and the optical path  40 . In the same manner, when at least one mirror of the reflection optical system  22  is rotated by a rotation angle of θ/2, the pulsed beams P 2  and P 3  having a deflection angle of θ relative to the pulsed beams P 0  and P 1  are multiplexed at the beam splitter  25 .  FIG. 4  shows a case where only  22   b  is rotated. 
     The pulsed beam P 2  is deflected by the reflection optical system  22  so as to have a deflection angle of θ after the pulsed beam P 0  has been branched at point C. On the other hand, the pulsed beam P 3  is produced as a result of the pulsed beam P 1  being endowed with a deflection angle of θ at the reflection optical system  22 . Because the pulsed beam P 3  has been endowed with a deflection angle of 2θ at the reflection optical system  21 , it has a total deflection angle of 3θ. Consequently, as shown in  FIG. 4 , the pulsed beams P 0 , P 1 , P 2 , and P 3  propagate in the directions with deflection angles of 0, 2θ, θ, and 3θ relative to the optical axis IZ, thus accomplishing spatial multiplexing. 
     In this reference embodiment, the deflection angle is 2θ when the amount of delay (the difference in optical path length) is L, and the deflection angle is θ when the amount of delay is 2L. Therefore, when the amounts of delay of the pulsed beams P 0 , P 1 , P 2 , and P 3  are 0, L, 2L, and 3L, the respective deflection angles are 0, 2θ, θ, and 3θ. 
     Because the above-described spatial multiplexing and temporal multiplexing occur at the same time in the beam splitter apparatus  2 , the pulsed beam emitted from the pulsed light source  11  exhibits temporal multiplexing with a time interval of L/c and spatial multiplexing with a deflection angle interval of θ. 
     As described above, according to the beam splitter apparatus  2  of this reference embodiment, the beam splitter  24  that branches and multiplexes pulsed beams is provided so that an input pulsed beam can be branched into a plurality of optical paths by the beam splitter  23  and the beam splitter  24  and so that the resultant pulsed beams can be endowed with a relative angle by the reflection optical systems  21  and  22 . By doing so, pulsed beams in a plurality of optical paths, having different optical path lengths and also endowed with a relative angle, can be produced. 
     In addition, because one pulsed beam can be multiplexed to four in this reference embodiment, the signal acquisition level per unit time is increased. This helps achieve fast image generation processing when it is applied to, for example, a microscope. 
     Although this reference embodiment has been described by way of an example where one beam splitter  24  for branching and multiplexing pulsed beams is provided, two or more beam splitters may be provided. By doing so, a pulsed beam from the pulsed light source  11  can be branched into a larger number of beams, thereby further increasing the speed of image generation processing. 
     Second Embodiment 
     A beam splitter apparatus  3  according to a second embodiment of the present invention will now be described with reference to  FIGS. 6 to 8 . In the description of this embodiment, commonalities with the above-described embodiment will be omitted, and differences will be mainly described. 
     The beam splitter apparatus  3  according to this embodiment differs from the beam splitter apparatus  2  according to the reference embodiment in that relay optical systems (pupil transfer optical systems)  36 ,  37 ,  38 , and  39  serving as means for propagating the pupil position is provided. 
     As shown in  FIG. 6 , the beam splitter apparatus  3  according to this embodiment includes reflection optical systems  31  and  32 ; a beam splitter (branching section)  33 ; a beam splitter (multiplexing/branching section)  34 ; a beam splitter (multiplexing section)  35 ; and the relay optical systems  36 ,  37 ,  38 , and  39  serving as means for propagating the pupil position. Furthermore, the beam splitter apparatus  3  of this embodiment and the pulsed light source  11  constitute a light source apparatus  103 . 
     The relay optical systems  36 ,  37 ,  38 , and  39  each include one pair of lenses and are disposed one each in the branching optical paths. The relay optical systems  36 ,  37 ,  38 , and  39  relay the pupils of pulsed beams in their respective optical paths. 
     More specifically, the relay optical system  36 , for example, is composed of one pair of lenses  36   a  and  36   b  to relay the pupil of the pulsed beam passing along the optical path  20  branching off via the beam splitter  33 . In the same manner, the relay optical systems  37 ,  38 , and  39  include one pair of lenses  37   a  and  37   b , one pair of lenses  38   a  and  38   b , and one pair of lenses  39   a  and  39   b , respectively, to relay the pupils of the pulsed beams passing along the optical paths branching off via the beam splitter  33  or the beam splitter  34 . 
     The reflection optical system  31  includes a mirror (first mirror)  31   a  that reflects the pulsed beam branching off via the beam splitter  33 ; a mirror (second mirror)  31   b  that reflects the pulsed beam reflected at the mirror  31   a  towards the beam splitter  34 ; and a stage (rectilinear translation mechanism)  31   c  that rectilinearly translates these mirrors  31   a  and  31   b  together in the optical-axis direction between these mirrors. 
     The reflection optical system  31  rectilinearly translates the mirrors  31   a  and  31   b  together in the optical-axis direction between these mirrors by means of the stage  31   c  to endow the pulsed beam branching off via the beam splitter  33  with a difference in optical path length, as well as a deflection angle. 
     In the same manner, the reflection optical system  32  includes a mirror (first mirror)  32   a  that reflects the pulsed beam branching off via the beam splitter  34 ; a mirror (second mirror)  32   b  that reflects the pulsed beam reflected at the mirror  32   a  towards the beam splitter  35 ; and a stage (rectilinear translation mechanism)  32   c  that rectilinearly translates these mirrors  32   a  and  32   b  together in the optical-axis direction between these mirrors. 
     The reflection optical system  32  rectilinearly translates the mirrors  32   a  and  32   b  together in the optical-axis direction between these mirrors by means of the stage  32   c  to endow the pulsed beam branching off via the beam splitter  34  with a difference in optical path length, as well as a deflection angle. 
     Temporal multiplexing and spatial multiplexing (spatial modulation) of a pulsed beam that has been oscillated by the pulsed light source  11  in the beam splitter apparatus  3  with the above-described structure will be described. 
     Because temporal multiplexing can be accomplished by following an adjustment procedure similar to that described in the foregoing reference embodiment, a description thereof will be omitted. Thus, spatial multiplexing will be described below. 
     The relay optical systems  36 ,  37 ,  38 , and  39  are each composed of a lens pair including two lenses having the same focal length to form an image of the pupil disposed adjacent to point A of the beam splitter  33  in the vicinity of point C on the beam splitter  34 . Furthermore, they form an image of the pupil disposed adjacent to point C on the beam splitter  34  in the vicinity of point F on the beam splitter  35 . Here, assuming that the optical path A-D-C (hereinafter, referred to as “the optical path  10 ”) and the optical path C-G-F (hereinafter, referred to as “the optical path  30 ”) have the same optical path length L 1 , the focal length f 1  of the lenses of the lens pairs used in the relay optical systems  38  and  39  is selected so as to satisfy f 1 =L 1 /4. 
       FIG. 7( a )  depicts an arrangement where the pulsed beam is not deflected. 
     With reference to  FIG. 7( a ) , the relationship between the amount of delay L in the optical path A-B-C (hereinafter, referred to as “the optical path  20 ”) and the focal length f 1  of the lenses in the relay optical system  36  will be described. It is assumed that the points of incidence of the principal rays upon the two lenses  36   a  and  36   b  provided in the relay optical system  36  are denoted by S and T and that the points of reflection of the principal rays at the two mirrors  31   a  and  31   b  provided in the reflection optical system  31  are respectively denoted by L and M. The quadrangle ALMC formed by connecting these four points is a rectangle with all angles of 90° when no deflection is performed. In this case, because side LM and side AC have the same length, the given amount of delay L is equal to the sum of side AL and side MC and is accordingly equal to 2AL. More specifically, the focal length f 1  of the lenses is f 1 =(L 1 +L)/4, and the mirrors and lenses are arranged so that the two given optical paths satisfy AS=SL+LB=BM+MT=TC=f 1 . 
     The pulsed beam reflected at the beam splitter  33  passes via the lens  36   a , the mirror  31   a , the mirror  31   b , and the lens  36   b  in that order and is then multiplexed by the beam splitter  34  with the pulsed beam passing through the relay optical system  38 . 
       FIG. 7( b )  depicts an arrangement where a pulsed beam is deflected. 
     In the reflection optical system  31 , the mirror  31   a  and the mirror  31   b  face each other such that they are tilted with an angle of 45° relative to the optical axis AZ and are disposed on the stage  31   c  that can be moved in a direction parallel to the optical axis AZ. As shown in  FIG. 7( b ) , when the stage  31   c  is moved in the direction indicated by the arrow, the line segment L′M′ formed by connecting the points of reflection of principal rays at the mirrors  31   a  and  31   b  not only moves towards the lenses relative to the line segment LM assumed when no deflection is performed but also shifts in a direction indicated by the arrow. As a result, the principal ray of the pulsed beam reflected at point M′ of the mirror  31   b  shifts leftwards compared with a case where no deflection is performed and, after having passed through the lens  36   b , is converted into a collimated beam deflected relative to the optical axis MC of the lens. Because the displacement of the optical axis is twice the displacement of the stage (i.e., 2ΔL 1 ), this deflection angle θ satisfies the relation tan θ=2ΔL 1 /f 1 , where ΔL 1  represents the displacement of the stage  31   c.    
     Likewise, a relationship between the amount of delay 2L and the focal length f 2  of the lenses in the relay optical system  37  also holds in the optical path  40 . More specifically, the focal length f 2  of the lenses used in the relay optical system  37  is obtained from f 2 =(L 1 +2L)/4, and the displacement ΔL 2  of the stage  32   c  is set so as to satisfy tan 2θ=2ΔL 2 /f 2 . 
     From the description so far, adjustment is performed so that the deflection angle in the optical path  20  is θ and the deflection angle in the optical path  40  is 2θ. 
     Because the above-described spatial multiplexing and temporal multiplexing occur at the same time in the beam splitter apparatus  3 , the pulsed beam emitted from the pulsed light source  11  exhibits temporal multiplexing with a time interval of L/c and spatial multiplexing with a deflection angle interval of θ. 
     The beam splitter apparatus  3  according to this embodiment differs from the beam splitter apparatus  2  according to the reference embodiment in that relay optical systems are used. When relay optical systems are used as in this embodiment, pulsed beams having four deflection angles can be made to spatially overlap one another in the vicinity of the branching section or the multiplexing section by the effect of propagating the pupil positions. As a result, the size of the optical element used for branching and multiplexing can be reduced. 
     Furthermore, a figure formed by optical paths in which only mirrors are disposed exhibits a trapezoidal shape, which is a deformation of a rectangle. When a deflection angle is changed, the shape of the trapezoid also changes, causing the optical paths to differ from one another. As a result, because the time difference when the pulsed beams P 0  and P 1  are multiplexed differs depending on the deflection angle, changing the interval for spatial multiplexing causes the interval for temporal multiplexing also to change. In contrast, because formation of a pupil image is performed by the lenses of the pupil propagating section in this embodiment, the pupil and the pupil image are optically conjugate. For this reason, the optical path  20  does not change even when the deflection angle is changed. Therefore, the interval for spatial multiplexing alone can be changed by modulating only the deflection angle while keeping the time intervals of a pulse train formed by the pulsed beams P 0 , P 1 , P 2 , and P 3  fixed. 
     Although four relay optical systems are used in this embodiment, one relay optical system may be used. In that case, the one relay optical system is most effectively disposed at the position of the relay optical system  37 . The reason for this will be described below. Normally, a pulsed beam does not propagate in the form of a completely collimated beam but propagates with a slight diverging angle. Therefore, when beams passing along paths with different optical path lengths are multiplexed, as in this embodiment, a wide diversity of beam diameter sizes will result due to divergence of beams passing along the shortest to the longest optical paths. To prevent this, it is a good idea to place the one relay optical system in the longest optical path to correct the spread due to divergence. Therefore, it is most effective that the relay optical system is disposed at the position of the relay optical system  37 . Furthermore, it is desirable that a relay optical system be placed in all optical paths in order to make the beam diameters strictly uniform. 
     Modification 
       FIG. 8  shows a beam splitter apparatus  3 ′ according to a modification of the second embodiment. 
     In comparison with the beam splitter apparatus  3  according to the second embodiment, a polarizing beam splitter  35 ′ is employed instead of the beam splitter  35 , a λ/2 plate  131  is additionally provided as a polarization modulator, and a movable mirror  132  is additionally provided as a variable deflecting section. Furthermore, a relay optical system  133  serving as a pupil transfer section is additionally provided immediately downstream of the polarizing beam splitter  35 ′. 
     The procedures for temporal multiplexing and spatial multiplexing are the same as in the second embodiment. In the second embodiment, while most of the pulsed beams multiplexed at point F on the beam splitter  35  travel towards point Z, some of the same pulsed beams propagate in a direction orthogonal to the optical axis AZ (not shown in the figure). In short, some of the pulsed beams do not proceed in the intended direction. In this modification, the loss of the pulsed beam can be minimized by adjusting the polarization. 
     A pulsed light source  11 ′ oscillates a p-polarized pulsed beam. Thereafter, the p-polarized pulsed beam travels to just before the polarizing beam splitter  35 ′ in the same manner as in the second embodiment. Here, the pulsed beam passing along the optical path  40  is modulated from p-polarized light to s-polarized light by the λ/2 plate  131 . Consequently, the pulsed beams P 0  and P 1  are p-polarized light, whereas P 2  and P 3  are s-polarized light. For this reason, all s-polarized pulsed beams are reflected at the polarizing beam splitter  35 ′, whereas all p-polarized pulsed beams pass through the polarizing beam splitter  35 ′, thus causing all pulsed beams to be guided in the Z direction. 
     Furthermore, the pulsed beams multiplexed at the polarizing beam splitter  35 ′ are relayed to the reflection surface of the movable mirror  132  by the relay optical system  133 . The movable mirror  132  has a rotation axis orthogonal to the drawing, and when it is continuously deflected from angles 0 to θ in the drawing with this movable mirror, scanning can be performed within an angular range from 0 to 4θ in the drawing. 
     As described above, according to the beam splitter apparatus  3 ′ of this modification, the polarization states of the optical paths  30  and  40  can be made orthogonal to each other by the λ/2 plate  131 , and all pulsed beams passing along the two optical paths  30  and  40  are multiplexed by the polarizing beam splitter  35 ′ because the multiplexing section is formed of the polarizing beam splitter  35 ′, thereby enabling the loss of the intensity of these pulsed beams to be suppressed, which increases the utilization efficiency of the input pulsed beams. 
     Third Embodiment 
     A beam splitter apparatus  4  according to a third embodiment of the present invention will now be described with reference to  FIG. 9 . In the description of this embodiment, commonalities with the above-described embodiments will be omitted, and differences will be mainly described. 
     In each of the above-described embodiments, pulsed beams pass along a plurality of optical paths of combined rectangular optical paths and straight optical paths. In this embodiment, on the other hand, a Michelson interferential optical path is used for the optical paths of pulsed beams. 
     As shown in  FIG. 9 , the beam splitter apparatus  4  according to this embodiment includes reflection optical systems (beam-angle setting sections)  41  and  42  composed of one mirror; beam splitters (multiplexing/branching sections)  43  and  44 ; relay optical systems (pupil transfer optical system)  45 ,  46 ,  47 , and  48 ; and stationary mirrors  49  and  50 . Furthermore, the beam splitter apparatus  4  of this embodiment and the pulsed light source  11  constitute a light source apparatus  104 . 
     In  FIG. 9 , there are four optical paths as listed below: 
     Optical path  10 : A-C-A-B-D-B-Z 
     Optical path  20 : A-E-A-B-D-B-Z 
     Optical path  30 : A-C-A-B-F-B-Z 
     Optical path  40 : A-E-A-B-F-B-Z 
     The optical path A-E-A has a larger optical path length than the optical path A-C-A by L, and similarly, the optical path B-F-B has a larger optical path length than the optical path B-D-B by 2L. Therefore, the pulsed beams passing along the optical paths  10  to  40  up to point Z are temporally multiplexed with a time difference of L/c, as in each of the above-described embodiments. Furthermore, the relay optical systems  45 ,  46 ,  47 , and  48  function to establish an optically conjugate relationship between points A and C, points A and E, points B and D, and points B and F, respectively, so that the pupils are propagated. 
     In this embodiment, the reflection optical systems  41  and  42  work as stationary deflecting sections. The tilt angle is changed to θ/2 by the reflection optical system  41  and to θ by the reflection optical system  42  to allow the reflection optical systems to endow pulsed beams with deflection angles of θ and 2θ, respectively. By doing so, four pulsed beams arriving at point E are spatially multiplexed with deflection angles of 0, θ, 2θ, and 3θ. 
     According to the beam splitter apparatus  4  of this embodiment, because optical elements are arranged along a straight line in each of the four optical paths, optical adjustment can be accomplished easily. 
     Fourth Embodiment 
     A beam splitter apparatus  5  according to a fourth embodiment of the present invention will now be described with reference to  FIG. 10 . In the description of this embodiment, commonalities with the above-described embodiments will be omitted, and differences will be mainly described. 
     As shown in  FIG. 10 , the beam splitter apparatus  5  according to this embodiment includes reflection optical systems (beam-angle setting sections)  51  and  52  composed of two mirrors; a beam splitter (multiplexing/branching section)  53 ; relay optical systems (pupil transfer optical systems)  54 ,  55 ,  56 ,  57 , and  153 ; stages  51   c  and  52   c ; a pair of stationary mirrors  58  and  59 ; and movable mirrors  151  and  152 . Furthermore, the beam splitter apparatus  5  of this embodiment and the pulsed light source  11  constitute a light source apparatus  105 . 
     Differences from the above-described third embodiment will be described mainly. 
     In the beam splitter apparatus  5  according to this embodiment, the same beam splitter  53  is used as all means for performing branching and multiplexing. In addition, two-dimensional scanning can be accomplished by using the movable mirrors  151  and  152  in respective light-guide directions of multiplexed pulsed beams. 
     With the above-described structure, because all branching and multiplexing operations are accomplished with just one beam splitter  53 , the number of components can be reduced. 
     Modification 
     Alternatively, like the modification of the second embodiment, the loss of pulsed beams may be minimized through polarization adjustment. In this case, polarizing beam splitters  154  and  155  are arranged as shown in a beam splitter apparatus  5 ′ of  FIG. 11 . In addition, λ/2 plates  156 ,  157 ,  158 , and  159  are disposed in four respective optical paths so as to achieve a polarization of 90° after the end of the branching operation, and furthermore, a λ/2 plate  160  that achieves a polarization of 45° is disposed in the optical path between the polarizing beam splitters  154  and  155 . 
     Fifth Embodiment 
     A beam splitter apparatus  6  according to a fifth embodiment of the present invention will now be described with reference to  FIG. 12 . In the description of this embodiment, commonalities with the above-described embodiments will be omitted, and differences will be mainly described. 
     The beam splitter apparatus  6  according to this embodiment includes reflection optical systems  61  and  62 ; beam splitters  63 ,  64 , and  65 ; and relay optical systems  66  to  69  and  161 . 
     The reflection optical system  61  denotes reflection optical systems (mirrors)  61   a  to  61   f  disposed in the optical path A-B-C-D produced by the beam splitter  63 , and the reflection optical system  62  denotes reflection optical systems (mirrors)  62   a  and  62   b  disposed in the optical path D-E-F produced by the beam splitter  64 . 
     The relay optical system  68  relays the pupil adjacent to point A along the optical path A-G-D produced by the first branching operation. The relay optical systems  66  and  161  relay the pupil adjacent to point A along the optical path A-B-C-D. 
     Likewise, the relay optical systems  69  and  67  relay the pupil adjacent to point D along the optical path D-H-F and the optical path D-E-F, respectively. 
     In this embodiment, two sets of the relay optical systems  66  and  161  are provided in the longer path A-B-C-D of the two delay paths. The reason for this is described below. Assuming that a deflection angle is θ and the focal length of a relay optical system is f, the aperture radius of the relay optical system required to efficiently propagate a collimated beam endowed with the deflection angle needs to be larger than the sum of f tan θ and the beam radius. In other words, when a pupil is to be propagated along a delay path by one set of relay optical systems, as in each of the above-described embodiments, a larger focal length inevitably requires a larger aperture of the relay optical system. For this reason, an optical system having a large aperture needs to be prepared. 
     In this embodiment, the pupil adjacent to point A is relayed by the relay optical system  66  having a very large focal length and the relay optical system  161  having a small focal length. A deflection angle is produced by moving the reflection optical systems  61   d  and  61   e  in the optical axis direction between these reflection optical systems. Because a small focal length is selected for the relay optical system  161 , the aperture sizes of the relay optical systems  66  and  161  can be prevented from becoming large. 
     Sixth Embodiment 
     A beam splitter apparatus  7  according to a sixth embodiment of the present invention will now be described with reference to  FIG. 13 . In the description of this embodiment, commonalities with the above-described embodiments will be omitted, and differences will be mainly described. 
     The beam splitter apparatus  7  according to this embodiment includes reflection optical systems  71  and  72 ; beam splitters  73  and  74 ; and relay optical systems  75  and  76  composed of reflection elements. The relay optical systems  75  and  76  shown here are composed of two reflection optical systems  75   a  formed of two non-flat reflection surfaces to relay the pupils of pulsed beams in their respective optical paths. 
     The reflection optical system  71  rectilinearly translates the mirrors  71   a  and  71   b  together in the optical-axis direction between these mirrors by means of the stage  71   c  to endow the pulsed beam branching off via the beam splitter  73  with a difference in optical path length, as well as a deflection angle. 
     The reflection optical system  72  rectilinearly translates the mirrors  72   a  and  72   b  together in the optical-axis direction between these mirrors by means of the stage  72   c  to endow the pulsed beam branching off via the beam splitter  73  with a difference in optical path length, as well as a deflection angle. 
     The relay optical systems  75  and  76  need not be transmissive (refractive), as shown here, but may be reflective. Furthermore, although two optical systems with positive refractive power are provided as a structure for relaying along the path from point A to point B, positive and negative power may be combined. 
     Seventh Embodiment 
     As a seventh embodiment according to the present invention, an example where the above-described beam splitter apparatus is applied to a scanning microscope will be described with reference to  FIGS. 14 and 15 . 
     As shown in  FIG. 14 , a scanning microscope  8  according to this embodiment includes the beam splitter apparatus  3  with the same structure as in the second embodiment; the pulsed light source  11 ; movable mirrors  81  and  82 ; a relay lens  83 ; a dichroic mirror  84 ; an objective lens  85 ; and a detector  86 . Although not shown in the figure, the scanning microscope  8  further includes a processing section for synchronizing detection timing by the detector  86  with the pulsed light source  11 ; a restoring section; and a display section. 
     The beam splitter apparatus  3 , the pulsed light source  11 , and the movable mirrors  81  and  82  constitute a scanning optical system (scanning section)  87  that scans a subject with a plurality of pulsed beams from the beam splitter apparatus  3 . 
     Furthermore, the relay lens  83 , the dichroic mirror  84 , and the objective lens  85  constitute an observation optical system  88  that irradiates the subject with pulsed beams scanned by the scanning optical system  87  and collects light from the subject. 
     The detector  86  is a detecting section that detects light collected by the observation optical system  88 . 
     As described in the second embodiment, pulsed beams are endowed with respective deflection angles of 0, θ, 2θ, and 3θ by the reflection optical systems  31  and  32  in the beam splitter apparatus  3 . In this manner, a deflection angle is assigned to each pulsed beam by the beam-angle setting section and those pulsed beams are multiplexed to form an optical pulse train (spatial multiplexing). 
     When one pulsed beam is converted into a plurality of (four) spatially multiplexed pulsed beams, a plurality of sites on the subject can be irradiated with those pulsed beams, and therefore, a scanning speed four times as high as when the subject is scanned with a single pulsed beam can be accomplished. 
     Furthermore, while a pulsed beam emitted from the pulsed light source  11  at a repetition frequency R Hz is branched by the branching section, the resultant pulsed beams pass along optical paths with different optical path lengths. As a result, the pulsed beams form an optical pulse train at regular temporal intervals (temporal multiplexing). The optical path lengths are made to differ from one another at the branches, for example, in the beam splitter apparatus  3 , so that the formed overall optical pulse train has a frequency of 4R, as shown in  FIG. 15( a ) . 
     When this optical pulse train is radiated onto sites of the subject, fluorescence is produced for each pulsed beam by multiphoton excitation effect. Because this fluorescence is produced immediately after each pulsed beam of the optical pulse train is radiated, fluorescent signal light with a period of frequency 4R occurs as shown in  FIG. 15( b ) . 
     This fluorescent signal light (one-dimensional time information) with a frequency of 4R is collected by the observation optical system  88  as fluorescent signal light from the subject and is detected by the detector  86 . Thereafter, the detected fluorescent signal light is synchronized with the optical pulse train by the processing section (not shown in the figure), is associated as fluorescent signals for respective sites of the subject, and is reconstructed into two-dimensional information by the restoring section (not shown in the figure). Subsequently, the subject can be imaged when this two-dimensional information is displayed on the display section (not shown in the figure). Although two-dimensional information is obtained in this embodiment because signal light based on two-dimensional scanning is reconstructed, three-dimensional information can be obtained by performing three-dimensional scanning. 
     However, if the interior of a subject is to be examined when the subject is a scatterer, signal light produced from irradiated sites spreads widely, leading to a wide distribution of light on the detector  86 . Therefore, the resolving power will decrease because signal beams from various sites are mixed if temporal multiplexing is not performed. 
     However, according to the scanning microscope  8  of this embodiment, because the optical path lengths are made to differ from one another (temporal multiplexing) for respective pulsed beams in the beam splitter apparatus  3 , as shown in  FIG. 15( b ) , fluorescent signal light beams produced from sites arrive at the detector at different frequencies corresponding to the respective irradiated pulsed beams. 
     Because fluorescent signal light beams from sites correspond to respective pulsed beams, they can be separated easily in the time domain through synchronization by the processing section, and therefore, the correspondence relationship between pulsed beams radiated onto sites and the resultant fluorescent signal light beams is elucidated. Because it is possible to identify which site of the subject is irradiated with a pulsed beam for a particular fluorescent signal light beam originating from that pulsed light beam, the fluorescent signal light can be reconstructed as two-dimensional information by the restoring section. 
     According to the scanning microscope  8  of this embodiment, even when signals at sites are adversely affected by scattering of the subject as a result of increasing the signal frequency, the correspondence relationship between pulsed beams and fluorescent signal light beams can be grasped easily through synchronization. Therefore, imaging can be performed at high speed and with high resolving power. 
     As described above, according to the scanning microscope  8  of this embodiment, temporal multiplexing and spatial multiplexing can be accomplished at the same time by converging, on one position, a plurality of pulsed beams having different optical path lengths and endowed with a relative angle by using the beam splitter apparatus  3 . In this scanning microscope  8 , parallel pulsed beams can be radiated onto different positions on the subject by spatial multiplexing. Furthermore, even when parallel pulsed beams are radiated, fluorescent signal light beams returning from the subject can be synchronized with the parallel pulsed beams through temporal multiplexing and can be separated from one another. For this reason, a decrease in resolving power as a result of radiating a plurality of pulsed beams at one time can be prevented, and therefore, fast scanning can be accomplished. 
     Although this embodiment has been described by way of an example where the beam splitter apparatus  3  according to the second embodiment is applied to a scanning microscope, the same effect can be brought about by applying a beam splitter apparatus according to another embodiment. 
     Eighth Embodiment 
     A beam splitter apparatus  200  according to an eighth embodiment of the present invention will now be described with reference to the drawings. 
     For a description of this embodiment, the structures that are the same as those of the beam splitter apparatus  3  according to the above-described second embodiment are denoted by the same reference numerals, and thus a description thereof is omitted. 
     The beam splitter apparatus  200  according to this embodiment differs from the beam splitter apparatus  3  according to the above-described second embodiment in the incidence direction of pulsed beams from the pulsed light source  11  and the installation angles of the beam splitters  33  and  34 . The other structures are the same as in the beam splitter apparatus  3  according to the second embodiment. 
     More specifically, in the beam splitter apparatus  200  according to this embodiment, as shown in  FIG. 16 , the propagation direction of a pulsed beam B 1  that is emitted from the pulsed light source  11  and is incident upon the beam splitter  33  is deflected in one direction (counterclockwise in the drawing) by an angle of 2θ relative to an extension (indicated by broken lines in the drawing) of a straight line connecting the centers of the beam splitters  33  and  34 . Furthermore, in this embodiment, the installation angle of the beam splitter  33  is rotated in the same direction as above by an angle of θ/2, and the installation angle of the beam splitter  34  is rotated in the opposite direction to that described above (clockwise) by an angle of θ. 
     As a result, the incident angle of the pulsed beam B 1  upon the reflection surface of the beam splitter  33  is increased counterclockwise by an angle of 1.5θ, compared with the case of the beam splitter apparatus  3  according to the second embodiment. Therefore, the propagation direction of a pulsed beam B 12  reflected by the beam splitter  33  is tilted clockwise by an angle of θ relative to the propagation direction (indicated by broken lines in the drawing) of a pulsed beam in the second embodiment. 
     On the other hand, the propagation direction of a pulsed beam B 11  passing through the beam splitter  33  is set on an extension of the incidence pulsed beam B 1 , regardless of the installation angle of the beam splitter  33 . For the pulsed beam B 11  entering the optical path  10 , its tilting direction is inverted by the relay optical system  38  composed of one pair of lenses  38   a  and  38   b , and it is tilted clockwise by an angle of 2θ and is incident upon the beam splitter  34 . 
     For the pulsed beam B 12  entering the optical path  20 , its tilting direction is inverted via the relay optical system  36  composed of one pair of lenses  36   a  and  36   b  and the reflection optical system  31  including one pair of mirrors  31   a  and  31   b . As a result, the pulsed beam B 12  is incident upon the beam splitter  34  at a counterclockwise angle of θ relative to the propagation direction (indicated by broken lines in the drawing) of the pulsed beam in the second embodiment. 
     The pulsed beams B 11  and B 12  are each branched into two at the beam splitter  34 . The pulsed beam B 11  that is incident upon the beam splitter  34  with an angle of 2θ is incident upon the reflection surface of the beam splitter  34 , which is tilted clockwise by an angle of θ, at an incident angle increased by θ clockwise compared with the case of the beam splitter apparatus  3  according to the second embodiment. Therefore, the propagation direction of a pulsed beam B 112  that is reflected at the beam splitter  34  and enters the optical path  40  coincides with the propagation direction of the pulsed beam in the second embodiment. 
     Furthermore, the pulsed beam B 12  that is incident upon the beam splitter  34  with an angle of θ is incident upon the reflection surface of the beam splitter  34 , tilted clockwise by an angle of θ, at an incident angle increased by 2θ clockwise compared with the case of the beam splitter apparatus  3  according to the second embodiment. Therefore, the propagation direction of a pulsed beam B 122  that is reflected at the beam splitter  34  and enters the optical path  30  is tilted clockwise by an angle of 3θ relative to the propagation direction (indicated by broken lines in the drawing) of the pulsed beam in the second embodiment. 
     On the other hand, the propagation directions of pulsed beams B 111  and B 121  passing through the beam splitter  34  are set on extensions of the incident pulsed beams B 11  and B 12 , regardless of the installation angle of the beam splitter  34 . 
     For the pulsed beam B 121  in the optical path  40 , its tilting direction is inverted via the relay optical system  37  composed of one pair of lenses  37   a  and  37   b  and the reflection optical system  32  composed of one pair of mirrors  32   a  and  32   b . Because the pulsed beam B 112  is not tilted, the tilt angle does not change even after it has passed through the relay optical system  37  and the reflection optical system  32 . 
     Furthermore, for pulsed beams B 111  and B 122  entering the optical path  30 , their tilting directions are inverted via the relay optical system  39  composed of one pair of lenses  39   a  and  39   b.    
     More specifically, the pulsed beams B 112  and B 121 , which are tilted clockwise by angles of 0° and θ relative to the incidence axis tilted by 45° relative to the reflection surface, are incident upon the beam splitter  35  and are emitted in a direction tilted counterclockwise by angles of 0° and θ relative to the emission axis which is tilted by 45° relative to the reflection surface. Furthermore, the beam splitter  35  transmits the pulsed beams B 111  and B 122 , which are tilted counterclockwise by angles 2θ and 3θ relative to a straight line connecting the beam splitters  34  and  35 , without changing the tilt angles. 
     As a result, the four pulsed beams B 112 , B 121 , B 111 , and B 122 , endowed with time delays that are different from one another by the two optical paths (delay optical paths)  20  and  40  and spaced apart at the same angular interval of θ, are emitted from the beam splitter  35 . 
     In this case, according to the beam splitter apparatus of this embodiment, for the pulsed beams B 12 , B 121 , and B 112  passing along the delay optical paths  20  and  40  provided with the relay optical systems  36  and  37  and the reflection optical systems  31  and  32 , the tilt angles of their propagation directions can be controlled to an angle of θ or less. Therefore, lenses with a small aperture size can be employed as the lenses  36   a ,  36   b ,  37   a , and  37   b . This is advantageous in preventing an increase in apparatus size. 
     Ninth Embodiment 
     A beam splitter apparatus  201  according to a ninth embodiment of the present invention will now be described with reference to the drawings. 
     For a description of this embodiment, the structures that are the same as those of the beam splitter apparatus  3  according to the above-described second embodiment are denoted by the same reference numerals, and thus a description thereof is omitted. 
     The beam splitter apparatus  201  according to this embodiment differs from the beam splitter apparatus  3  according to the above-described second embodiment in the installation angles of the beam splitters  34  and  35 . The other structures are the same as in the beam splitter apparatus  3  according to the second embodiment. 
     More specifically, in the beam splitter apparatus  201  according to this embodiment, as shown in  FIG. 17 , the installation angles of the beam splitters  34  and  35  are tilted in one direction (counterclockwise in the drawing) by an angle of θ/2 relative to the beam splitters  34  and  35  of the beam splitter apparatus  3  according to the second embodiment. 
     By doing so, the pulsed beams B 11  and B 12  passing along the optical paths up to the beam splitter  34  propagate along an optical axis with a tilt angle of 0°, as in the beam splitter apparatus  3  according to the second embodiment. 
     On the other hand, the pulsed beam B 11  incident upon the beam splitter  34  is branched into the pulsed beam B 111  that passes through it as-is with a tilt angle of 0° and the pulsed beam B 112  that is tilted counterclockwise by an angle of θ relative to a direction orthogonal to its direction. Furthermore, the pulsed beam B 12  incident upon the beam splitter  34  is branched into the pulsed beam B 121  that passes through it as-is with a tilt angle of 0° and the pulsed beam B 122  that is tilted counterclockwise by an angle of θ relative to a direction orthogonal to its direction. 
     The pulsed beam B 112  tilted counterclockwise by an angle of θ is incident upon the beam splitter  35  with its tilting direction inverted clockwise via the relay optical system  37  composed of the pair of lenses  37   a  and  37   b  and the reflection optical system  32  composed of the pair of mirrors  32   a  and  32   b . Furthermore, the pulsed beam B 122  is incident upon the beam splitter  35  with its tilting direction inverted clockwise via the relay optical system  39  composed of the pair of lenses  39   a  and  39   b.    
     Because the beam splitter  35  is tilted counterclockwise by an angle of θ/2, the pulsed beams B 112  and B 121  reflected by the reflection surface of this beam splitter  35  are emitted from the beam splitter  35  in directions tilted counterclockwise by angles of 2θ and θ, respectively. On the other hand, the pulsed beams B 111  and B 122  pass through the beam splitter  35  as-is and are emitted with a tilt angle of 0° in a direction tilted clockwise by an angle of θ. 
     As a result, the four pulsed beams B 112 , B 121 , B 111 , and B 122  endowed with time delays that are different from one another by the two delay optical paths  20  and  40  and spaced apart at the same angular interval of θ are emitted from the beam splitter  35 . 
     In this case, according to the beam splitter apparatus of this embodiment, for the pulsed beams B 11 , B 12 , B 111 , B 112 , B 121 , and B 122  passing along not just the delay optical paths but all optical paths, the tilt angles of their propagation directions can be controlled to an angle of θ. Therefore, lenses with a small aperture size can be employed as the lenses  36   a ,  36   b ,  37   a ,  37   b ,  38   a ,  38   b ,  39   a , and  39   b . This is advantageous in preventing an increase in apparatus size. 
     Tenth Embodiment 
     A beam splitter apparatus  202  according to a tenth embodiment of the present invention will now be described with reference to the drawings. 
     For a description of this embodiment, the structures that are the same as those of the beam splitter apparatus  3  according to the above-described second embodiment are denoted by the same reference numerals, and thus a description thereof is omitted. 
     The beam splitter apparatus  202  according to this embodiment differs from the beam splitter apparatus  3  according to the above-described second embodiment in the incidence direction of the pulsed beam B 1  from the pulsed light source  11  and the installation angles of the beam splitters  33  and  34 . The other structures are the same as in the beam splitter apparatus  3  according to the second embodiment. 
     More specifically, in the beam splitter apparatus  202  according to this embodiment, as shown in  FIG. 18 , the incidence direction of the pulsed beam B 1  from the pulsed light source  11  to the beam splitter  33  is set in a direction orthogonal to a straight line connecting the beam splitters  33  and  34 . 
     Furthermore, the installation angle of the beam splitter  33  is tilted in one direction (counterclockwise in the drawing) by an angle of θ/2 relative to the beam splitter  33  of the beam splitter apparatus  3  according to the second embodiment. Furthermore, the installation angle of the beam splitter  34  is tilted in the opposite direction to the rotation of this beam splitter  33  (clockwise in the drawing) by an angle of θ relative to the beam splitter  34  of the beam splitter apparatus  3  according to the second embodiment. 
     By doing so, the pulsed beam B 12  that enters the delay optical path  20  through the beam splitter  33  propagates along an optical axis with a tilt angle of 0°, as in the beam splitter apparatus  3  according to the second embodiment. 
     On the other hand, the pulsed beam B 11  reflected at the beam splitter  33  is tilted counterclockwise by an angle of θ relative to a straight line connecting the beam splitters  33  and  34 . 
     The pulsed beam B 1  is incident upon the beam splitter  34  after its tilting direction has been inverted via the relay optical system  38  composed of the pair of lenses  38   a  and  38   b . The pulsed beam B 11  incident upon the beam splitter  34  is branched into the pulsed beam B 111  passing through it as-is with a tilt angle of θ and the pulsed beam B 112  tilted clockwise by an angle of θ relative to a direction orthogonal to its direction. 
     Furthermore, the pulsed beam B 12  incident upon the beam splitter  34  is branched into the pulsed beam B 121  passing through it as-is with a tilt angle of 0° and the pulsed beam B 122  tilted clockwise by an angle of 2θ relative to a direction orthogonal to its direction. 
     The pulsed beam B 112  tilted clockwise by an angle of θ is incident upon the beam splitter  35  with its tilting direction inverted counterclockwise via the relay optical system  37  composed of the pair of lenses  37   a  and  37   b  and the reflection optical system  32  composed of the pair of mirrors  32   a  and  32   b . Furthermore, the pulsed beams B 111  and B 122  are incident upon the beam splitter  35  with their tilting directions inverted counterclockwise via the relay optical system  39  composed of the pair of lenses  39   a  and  39   b.    
     The pulsed beams B 112  and B 121  reflected by the reflection surface of the beam splitter  35  are emitted from the beam splitter  35  in directions tilted clockwise by an angle of θ and clockwise by an angle of 0°. On the other hand, the pulsed beams B 111  and B 122  pass through the beam splitter  35  as-is and are emitted in directions tilted counterclockwise by a tilt angle of θ and a tilt angle of 2θ. 
     As a result, the four pulsed beams B 112 , B 121 , B 111 , and B 122  endowed with time delays that are different from one another by the two delay optical paths  20  and  40  and spaced apart at the same angular interval of θ are emitted from the beam splitter  35 . 
     In this case, according to the beam splitter apparatus  202  of this embodiment, for the pulsed beams B 12 , B 121 , and B 112  passing along the delay optical paths  20  and  40  provided with the relay optical systems  36  and  37  and the reflection optical systems  31  and  32 , the tilt angles of their propagation directions can be controlled to an angle of θ or less. Therefore, lenses with a small aperture size can be employed as the lenses  36   a ,  36   b ,  37   a , and  37   b . This is advantageous in preventing an increase in apparatus size. The last branching means in this embodiment may be formed of a polarizing beam splitter. 
     Eleventh Embodiment 
     A beam splitter apparatus  203  according to an eleventh embodiment of the present invention will now be described with reference to the drawings. 
     For a description of this embodiment, the structures that are the same as those of the beam splitter apparatus  3  according to the above-described second embodiment are denoted by the same reference numerals, and thus a description thereof is omitted. 
     The beam splitter apparatus  203  according to this embodiment differs from the beam splitter apparatus  3  according to the above-described second embodiment in the incident positions of pulsed beams from the delay optical paths  20  and  40  upon the beam splitters  34  and  35  and in relay optical systems  104  and  105 . 
     As shown in  FIG. 19 , the beam splitter apparatus  203  according to this embodiment includes a relay optical system  104  composed of lenses  104   a ,  104   b , and  104   c  that relay the pupils of the pulsed beams B 112  and B 121 , entering the optical path  40 , originating from the pulsed beams B 11  and B 12  propagating along the optical paths  10  and  20  branching off from each other via the beam splitter  33 ; and a relay optical system  105  composed of lenses  105   a  and  105   b  that relay the pupils of the pulsed beams B 112  and B 121  from the optical path  40  before and after the beam splitter  35 . 
     Furthermore, the lenses  104   a  and  105   b  constitute a relay optical system that relays the pupils of the pulsed beams B 111  and B 122  passing through the beam splitters  34  and  35 . 
     More specifically, the pulsed beam B 1  incident upon the beam splitter  33  as a collimated beam is branched by the beam splitter  33  into the pulsed beams B 11  and B 12  composed of two collimated beams. 
     The pulsed beam B 11  composed of a collimated beam is collected by the lens  104   a  and is partly reflected by the beam splitter  34 . The reflected portion of the beam B 11  enters the delay optical path  40  as the pulsed beam B 112 . In the delay optical path  40 , the pulsed beam B 112  is converted by the lens  104   b  into the pulsed beam B 112  composed of a collimated beam. 
     Then, it is converted into a collimated beam via the relay optical system  37  and the reflection optical system  32 , is collected by the lens  105   a , is reflected by the beam splitter  35 , and is emitted by the lens  105   b  in the form of a collimated beam again. 
     The pulsed beam B 111  passing through the beam splitters  34  and  35  is emitted by the lens  105   b  in the form of a collimated beam again. 
     On the other hand, the pulsed beam B 12  composed of a collimated beam is introduced into the delay optical path  20 , is converted into a collimated beam via the relay optical system  36  and the reflection optical system  31 , is collected by the lens  104   c , and is incident upon the beam splitter  34 . The pulsed beam B 12  is branched into the pulsed beams B 121  and B 122  at the beam splitter  34 , and the pulsed beam B 121  passing through the beam splitter  34  is emitted from the lens  105   b  in the form of a collimated beam while its pupil is being relayed, like the pulsed beam B 111 . 
     Furthermore, the pulsed beam B 122  reflected at the beam splitter  34  is emitted from the lens  105   b  in the form of a collimated beam while its pupil is being relayed, like the pulsed beam B 111 . 
     In this case, in this embodiment, as shown in  FIG. 20 , the optical axes of the pulsed beams B 11  and B 12  incident upon the beam splitter  34  are shifted so as not to coincide on the reflection surface of the beam splitter  34  by adjusting the positions of the reflection optical system  31  and the relay optical system  36 . Furthermore, the optical axes of the pulsed beams B 111 , B 112 , B 121 , and B 122  incident upon the beam splitter  35  are shifted apart at regular intervals on the reflection surface by adjusting the positions of the reflection optical system  32  and the relay optical system  37 .  FIG. 20  is a magnified view of area AA in  FIG. 19 . 
     Then, the principal rays of the pulsed beams B 111 , B 112 , B 121 , and B 122  multiplexed by the beam splitter  35  are set to become parallel to one another. Furthermore, as shown in  FIG. 21 , the pulsed beams B 111 , B 112 , B 121 , and B 122  multiplexed by the beam splitter  35  are set to be collected on the same flat surface after passing through the beam splitter  35 . By doing so, the lens  105   b  works as a telecentric optical system for the pulsed beams B 111 , B 112 , B 121 , and B 122 , and the pulsed beams B 111 , B 112 , B 121 , and B 122  are made to have different angles by the lens  105   b  and converged on the same position at the back focal position of the lens  105   b .  FIG. 21  is a magnified view of area AB of  FIG. 19 . 
     In other words, the four pulsed beams B 111 , B 112 , B 121 , and B 122 , endowed with time delays different from one another by the two delay optical paths  20  and  40  and made to have different angles, are emitted from the back focal position of the lens  105   b.    
     This brings an advantage in that when the pulsed beams B 111 , B 112 , B 121 , and B 122  are collected by the subsequent objective lens at different sites spaced apart on the subject to generate fluorescence, the generated fluorescence can be prevented from being mixed and observation with high spatial resolving power can be accomplished because the light beams B 111 , B 112 , B 121 , and B 122  are endowed with different time delays from one another. 
     In this embodiment, the optical path length may be adjusted and the intervals between the optical axes of the pulsed beams B 111 , B 112 , B 121 , and B 122  incident upon the lens  105   b  may be adjusted by rectilinearly translating at least one of the mirrors  31   a  and  31   b  disposed in the delay optical path  20  and at least one of the mirrors  32   a  and  32   b  disposed in the delay optical path  40 , for example, the mirrors  31   b  and  32   b , relative to the other mirrors  31   a  and  32   a  on a plane parallel to the optical axis between the mirrors  31   a  and  31   b  or the mirrors  32   a  and  32   b.    
     Furthermore, the reflection optical systems  31  and  32  may be rectilinearly translated in a direction along the optical axis between the mirrors  31   a  and  31   b ;  32   a  and  32   b . By doing so, the intervals between the optical axes of the pulsed beams B 111 , B 112 , B 121 , and B 122  incident upon the lens  105   b  can be adjusted without having to change the optical path length. Therefore, this brings an advantage in that it is not necessary to re-adjust the optical path length. 
     Furthermore, if the optical axes are shifted by moving the mirrors  31   b  and  32   b  of the reflection optical systems  31  and  32 , it is preferable that the lenses  36   b  and  104   c  and  37   b  and  105   a  be moved in a direction orthogonal to the optical axes by the same amounts as the displacement of the optical axes. This brings an advantage in that the principal rays of the pulsed beams B 111 , B 112 , B 121 , and B 122 , after being multiplexed by the beam splitter  35 , can be maintained parallel to prevent the point of convergence from being shifted in the optical-axis direction. 
     Furthermore, in this embodiment, the beam diameters of the pulsed beams B 111 , B 112 , B 121 , and B 122  can be made the same by relaying a pupil with the plurality of relay optical systems  36 ,  37 ,  104 , and  105 . This provides an advantage in that because the beam diameters are not changed, the resolving power can be prevented from changing when this embodiment is applied to a scanning observation apparatus. Furthermore, the lenses  36   a    36   b ,  37   a ,  37   b ,  104   a ,  104   b ,  104   c , and  105   a  disposed in the optical paths  10 ,  20 ,  30 , and  40  may be set to have the same focal length. 
     In addition, a polarizing beam splitter may be employed as the beam splitters  33 ,  34 , and  35 . By doing so, pulsed beams can be used without loss. 
     Furthermore, in this embodiment, because the optical axes of the pulsed beams B 111 , B 112 , B 121 , and B 122  propagating along the optical paths  10 ,  20 ,  30 , and  40  are arranged at regular intervals as a result of the multiplexing operation, the scanning pitches of the pulsed beams B 111 , B 112 , B 121 , and B 122  on the subject can be made uniform to allow images free of nonuniform resolving power to be acquired when this embodiment is applied to a scanning observation apparatus. 
     Furthermore, when this embodiment is to be applied to a scanning observation apparatus, it is preferable that the position of convergence of the pulsed beams B 111 , B 112 , B 121 , and B 122  or a position that is optically conjugate to it be disposed on the swing axis of the scanner. This brings an advantage in that even when the scanner is swung to scan a pulsed beam, the incident position of the pulsed beam upon the scanner does not change and the pupil is maintained intact, allowing the scanning area to be scanned without omission. 
     Furthermore, in the case where the scanner is a raster scanning scanner, it is preferable that the position of convergence of pulsed beams or a position that is optically conjugate to it be disposed on the swing axis of the slower scanner. This brings an advantage in that scanning is completed in a short time without having to increase the scanning frequency of the faster scanner because the scanning area is divided by producing a plurality of pulsed beams. 
     Twelfth Embodiment 
     A beam splitter apparatus  204  according to a twelfth embodiment of the present invention will now be described with reference to the drawings. 
     As shown in  FIG. 22 , the beam splitter apparatus  204  according to this embodiment includes an optical fiber  110  that guides a pulsed beam C 1  emitted from a light source; a fiber coupler  113  that branches the pulsed beam C 1  propagating in the optical fiber  110  into pulsed beams C 11  and C 12  propagating in optical fibers  111  and  112 ; a fiber coupler  116  that branches the pulsed beam C 11  propagating in the optical fiber  111  into optical fibers  114  and  115 ; and a fiber coupler  119  that branches the pulsed beam C 12  propagating in the optical fiber  112  into optical fibers  117  and  118 . Four pulsed beams C 111 , C 112 , C 113 , and C 114  emitted from the ends of the four optical fibers  114 ,  115 ,  117 , and  118  are endowed with relative angles by adjusting the end angles of the optical fibers  114  and  115 ,  117 ,  118  (beam-angle setting section) and are converged on the same position. 
     One set of the optical fibers  111 ,  114 , and  117  branching off via the three fiber couplers  113 ,  116 , and  119 , respectively, is longer than another set of the optical fibers  112 ,  115 , and  118 , so that the lengths of the optical paths along which the four pulsed beams C 111 , C 112 , C 113 , and C 114  propagate until they are emitted from the ends of the optical fibers  114 ,  115 ,  117 , and  118  are made different from each other. In  FIG. 22 , reference numeral  120  denotes a focusing lens that collects the pulsed beams C 111 , C 112 , C 113 , and C 114  converged on the same position by the optical fibers  114 ,  115 ,  117 , and  118  and forms images of the exit ends of the optical fibers  114 ,  115 ,  117 , and  118  on the subject. Reference numeral  121  denotes a scanner that scans the subject with the pulsed beams C 111 , C 112 , C 113 , and C 114 . 
       FIG. 23( a )  shows a path with the shortest optical path length from the optical fiber  110  to the exit port of the optical fiber  118  via the two fiber couplers  113  and  119 .  FIG. 23( b )  shows a path with the second-shortest optical path length from the optical fiber  110  to the exit end of the optical fiber  117  via the two fiber couplers  113  and  119 .  FIG. 23( c )  denotes a path with the second-longest optical path length from the optical fiber  110  to the exit end of the optical fiber  115  via the two fiber coupler  113  and  116 .  FIG. 23( d )  shows a path with the longest optical path length from the optical fiber  110  to the exit end of the optical fiber  114  via the two fiber couplers  113  and  116 . 
     For example, if the difference in length between the optical fibers  111  and  112  is set as 2La and the differences between the optical fibers  114  and  115  and between  117  and  118  are set as La, the differences in path length from the shortest path are La, 2La, and 3La. Consequently, when the pulsed beam C 1  is incident upon the optical fiber  110 , an optical pulse train with a time interval of nLa/c is generated, as shown in  FIG. 24 . Here, n indicates the refractive index of the cores of the optical fibers  110 ,  111 ,  112 ,  114 ,  115 ,  117 , and  118 , and c indicates the velocity of light, assuming that the spatial length converted from the pulse widths of the pulsed beams C 111 , C 112 , C 113 , and C 114  is sufficiently small. 
     Then, with the beam splitter apparatus  204  of this embodiment having the above-described structure, there is an advantage in that when a light beam with small temporal coherence is emitted as the pulsed beam C 1 , deterioration due to illumination interference can be prevented because the four pulsed beams C 111 , C 112 , C 113 , and C 114  emitted with a time interval of nLa/c do not interfere with one another, as shown in  FIG. 25 . 
     In addition, the four pulsed beams C 111 , C 112 , C 113 , and C 114  branching off in this manner are collected by the focusing lens  120  and are scanned by the scanner  121  over the subject, as shown in  FIG. 22 . The focusing lens  120  forms images of the exit ends of the optical fibers  114 ,  115 ,  117 , and  118  on the subject via the scanner  121 . As shown in  FIG. 22 , the scanner  121  is a mirror swung about an axis orthogonal to the drawing and can scan the pulsed beams C 111 , C 112 , C 113 , C 114  in a direction parallel to the drawing while being swung. 
     By doing so, the time required to irradiate an area with pulsed beams can be reduced to one fourth of that when the same area is scanned with a single pulsed beam without spatial multiplexing. There is another advantage in that observed images can be acquired without being adversely affected by interference because delay times are provided among the pulsed beams C 111 , C 112 , C 113 , and C 114  to enable temporal multiplexing. 
     In this embodiment, the following modification can be employed. 
     More specifically, two positive lenses  122  and  123  may be employed, as shown in  FIG. 26 , instead of collecting the four pulsed beams C 111 , C 112 , C 113 , and C 114  with the single focusing lens  120 . In this case, the exit ends of the optical fibers  114 ,  115 ,  117 , and  118  are disposed near the front focal plane of the positive lens  122 , the scanner  121  is disposed near the back focal plane of the positive lens  122 , and furthermore, the scanner  121  is disposed near the front focal plane of the positive lens  123 . By doing so, a telecentric arrangement can be achieved both on the object side and the image side, so that observation without a large change in the magnification can be accomplished even when the subject is moved back and forth on the optical axis. 
     Furthermore, although this embodiment has been described by way of an example where the one pulsed beam C 1  is branched into the four pulsed beams C 111 , C 112 , C 113 , and C 114 , the pulsed beam C 1  may be branched into any other number of pulsed beams. 
     Furthermore, although the above-described embodiment has discussed a member that performs one-dimensional scanning, such as a single galvanometer mirror, as the scanner  121 , two-dimensional scanning may be performed by adding another scanner. 
     An example where this embodiment is applied to a fluoroscopy apparatus  205 , as shown in  FIG. 27 , will be described. This fluoroscopy apparatus  205  includes the beam splitter apparatus  204  according to this embodiment; a pulsed light source  124  that produces the pulsed beam C 1  entering this beam splitter apparatus  204 ; a focusing lens  122  that collects the pulsed beams C 111 , C 112 , C 113 , and C 114  emitted from the beam splitter apparatus  204 ; a scanner  125  provided with two galvanometer mirrors that can swing about axes intersecting each other; an objective lens  126  that focuses on the subject the pulsed beams C 111 , C 112 , C 113 , and C 114  scanned by the scanner  125 ; a dichroic mirror  127  that branches fluorescence (return light) C 2  produced at the subject and collected by the objective lens  126  off from the optical paths of the pulsed beams C 111 , C 112 , C 113 , and C 114 ; and an optical detector  128  that detects the fluorescence C 2  branching off via this dichroic mirror  127 . 
     According to this fluoroscopy apparatus  205 , after a light beam has been emitted from the pulsed light source  124  and branched into four light beams by the beam splitter apparatus  204 , the resultant pulsed beams C 111 , C 112 , C 113 , and C 114  scanned two-dimensionally by the scanner  125  are focused on the subject by the objective lens  126 , so that the fluorescence C 2  can be produced at the subject. Thereafter, the fluorescence C 2  produced in the subject and collected by the objective lens  126  is branched by the dichroic mirror  127  off from the pulsed beams C 111 , C 112 , C 113 , and C 114  so as to be detected by the optical detector  128 . In this case, a two-dimensional fluorescence image can be acquired by storing the scanning position by the scanner  125  and the intensity of the fluorescence C 2  detected by the optical detector  128  in association with each other to perform fluoroscopy of the subject. 
     Because the pulsed beams C 111 , C 112 , C 113 , and C 114  are multiplexed both spatially and temporally by the beam splitter apparatus  204 , the acquired fluorescence C 2  forms a train of pulses that do not interfere with each other, as shown in  FIG. 28 , and if the optical detector  128 , such as a photomultiplier tube having sufficiently high response speed, is used, four pulses of fluorescence C 2  can be detected by separating them in the time domain without having to employ a two-dimensional image pickup element. 
     Because the subject is irradiated with the four pulsed beams C 111 , C 112 , C 113 , and C 114 , processing can be performed at a speed four times as high as that of scanning based on the normal one-point-irradiation and one-point-detection technique. In short, even if the scanning speed of the scanner  125  is changed to one fourth of that of scanning based on the one-point-irradiation and one-point-detection technique, image acquisition with the same frame rate can be accomplished. 
     More specifically, when 1/R=4nLa/c is satisfied, where R is the repetition frequency of pulsed oscillation by the pulsed light source  124  and nLa/c is a pulse interval depending on the lengths of the optical fibers  114  and  115 ,  117 , and  118 , the pulsed beam C 1  oscillated from the pulsed light source  124  is multiplexed into four beams spaced apart at regular intervals, and a fluorescence C 2  pulse train produced by a line of the resultant pulsed beams C 111 , C 112 , C 113 , and C 114  can be acquired with the same repetition period, as shown in  FIG. 28 . 
     Thirteenth Embodiment 
     A beam splitter apparatus  206  according to a thirteenth embodiment of the present invention will now be described with reference to drawings. 
     As shown in  FIG. 29 , in the beam splitter apparatus  206  according to this embodiment, the exit ends of the four optical fibers  114 ,  115 ,  117 , and  118  in the beam splitter apparatus  204  according to the twelfth embodiment are bundled and a scanner  130  that shifts an optical fiber bundle  129  of the bundled fibers in the radial direction is provided. 
     The scanner  130  can resonate the optical fiber bundle  129  one-dimensionally or two-dimensionally in the radial direction and can collect the pulsed beams C 111 , C 112 , C 113 , and C 114  emitted from the exit ends of the optical fibers  114 ,  115 ,  117 , and  118  by the focusing lens  120  disposed at the pupil positions to scan the subject disposed at positions that are optically conjugate to the exit ends. Although only the pulsed beam C 111  is shown in  FIGS. 29 and 33 , actually C 112 , C 113 , and C 114  are scanned near this C 111 . 
     Unlike the twelfth embodiment, in which a mirror  121  is swung to scan the pulsed beams C 111 , C 112 , C 113 , and C 114 , the size can be reduced and adjustment can be simplified. 
     As shown in  FIG. 30 , in this embodiment, the exit ends of the four optical fibers  114 ,  115 ,  117 , and  118  may be bundled so that all the optical fibers  114 ,  115 ,  117 , and  118  are adjacent, or alternatively, the claddings of the four optical fibers  114 ,  115 ,  117 , and  118  may be fused to arrange cores  114   a ,  115   a ,  117   a , and  118   a  so that they are adjacent to one another. In this case, the cores  114   a ,  115   a ,  117   a , and  118   a  may be arranged in a rectangular shape, as shown in  FIG. 31 , or in a line, as shown in  FIG. 32 . 
     The beam splitter apparatus  206  according to this embodiment with the above-described structure is provided in a fluoroscopy apparatus  207 , as shown in  FIG. 33 . This beam splitter apparatus  206  splits the pulsed beam C 1  from the pulsed light source  124  connected to one end of the optical fiber  110  into the four pulsed beams C 111 , C 112 , C 113 , and C 114 , which are emitted from the exit ends and collected by an objective lens  120 . By doing so, images of the exit ends of the optical fibers  114 ,  115 ,  117 , and  118  can be formed on the subject disposed at positions that are optically conjugate to the exit ends of the optical fibers  114 ,  115 ,  117 , and  118  to radiate four pulsed beams C 111 , C 112 , C 113 , and C 114 . 
     In  FIG. 33 , optical fibers  131  and  132  whose end portions are disposed around the objective lens  120  are provided. The fluorescence C 2  generated at the positions irradiated with the pulsed beam C 111 , C 112 , C 113 , and C 114  on the subject is incident upon the end portions of the optical fibers  131  and  132 , is guided in the optical fibers  131  and  132 , and is detected by an optical detector  133  connected to the other ends of the optical fibers  131  and  132 . 
     Although the fluorescence C 2  is guided in the two optical fibers  131  and  132  in  FIG. 33 , a space may be provided around the objective lens  120  to arrange the end portions of three or more optical fibers instead. As a result, fluorescence images with a high SN ratio can be acquired. 
     The present invention is not limited to the above described embodiment of the laser scanning fluorescent microscope, and may be applied to any other type of optical-beam scanning observation apparatus such as a laser scanning endoscope, which can realize a real-time observation of a living biological subject such as cells or a tissue. 
     The present invention enables high speed optical scanning without having detected signals interfere each other even if a plurality of beams illuminate a small region of the subject whereby high-density illuminated points are distributed thereon. Therefore, the present invention is advantageous in the case of detecting an optical signal emitted from the subject with a very low intensity, which would require long time exposure to a detecting section for the detection in a conventional scanning apparatus or method. For example, in the case when a scanning speed is increased four times higher by temporal multiplexing, the exposure time can be four times longer than that without temporal multiplexing. Furthermore, in the present invention, the apparatus needs only a single detecting device such as a photodiode (PD) or a photomultiplier tube (PMT), instead of an image device with a plurality of pixels such as a CCD or a CMOS, in order to detect signals. Furthermore, according to the present invention, the intensity of a pulsed light with temporal multiplexing can be weaker than that without temporal multiplexing in order to detect signals with a desired intensity. Therefore, an apparatus according to the present invention can be preferably used as a microscope or endoscope to image or observe a subject including fragile materials such as a living tissue, nerve cells, and the like. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  2 ,  3 ,  3 ′,  4 ,  5 ,  5 ′,  6 ,  7 ,  200 ,  201 ,  202 ,  203 ,  204 ,  206 : beam splitter apparatus 
           8 : scanning microscope 
           10 ,  20 ,  30 ,  40 : optical path 
           11 ,  124 : pulsed light source 
           12 ,  21 ,  22 ,  31 ,  32 ,  41 ,  42 ,  51 ,  52 ,  61 ,  62 ,  71 ,  72 : reflection optical system (beam-angle setting section) 
           13 ,  23 ,  33 ,  63 : beam splitter (branching section) 
           14 ,  25 ,  35 ,  65 ,  74 : beam splitter (multiplexing section) 
           16 ,  17 ,  36 ,  37 ,  38 ,  39 ,  45 ,  46 ,  47 ,  48 ,  54 ,  55 ,  56 ,  57 ,  66 , 
           67 ,  68 ,  69 ,  104 ,  105 ,  153 ,  161 : relay optical system (pupil transfer optical system) 
           24 ,  34 ,  43 ,  44 ,  53 ,  64 ,  73 ,  154 ,  155 : beam splitter (multiplexing/branching section) 
           31   a ,  32   a : mirror (first mirror) 
           31   b ,  32   b : mirror (second mirror) 
           31   c ,  32   c ,  51   c ,  52   c : stage (rectilinear translation mechanism) 
           35 ′: polarizing beam splitter 
           49 ,  50 : stationary mirror 
           83 : relay lens 
           84 ,  127 : dichroic mirror 
           85 ,  126 : objective lens 
           86 : detector (detecting section) 
           87 : scanning optical system (scanning section) 
           88 : observation optical system 
           101 ,  102 ,  103 ,  103 ′,  104 ,  105 ,  105 ′: light source apparatus 
           205 ,  207 : fluoroscopy apparatus (scanning microscope) 
           110 ,  111 ,  112 ,  114 ,  115 ,  117 ,  118 : optical fiber 
           113 ,  116 ,  119 : fiber coupler 
           120 : focusing lens 
           121 ,  125 ,  130 : scanner 
           122 ,  123 : positive lens 
           128 : optical detector 
           129 : fiber bundle