Abstract:
A confocal optical system comprising a scanning fiber is provided. The scanning fiber is a single-mode fiber of which a first end is shaped as a curved surface. The scanning fiber transmits illumination light to the first end. The illumination light is emitted toward an observation area. The illumination light emanates from the first end. The illumination light emanates from the first end striking a target area within the observation area. The first end receives at least one of reflected light and fluorescence from the target area. The reflected light is the illumination light reflected from the target area. The fluorescence is induced at the target area by illumination from the illumination light.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a confocal optical system used for a scanning apparatus, such as a confocal endoscope apparatus, that can display a highly enlarged subject image at high resolution. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Publication No. 2005-80769 discloses a confocal endoscope apparatus that can display a highly enlarged image at high resolution compared to a usual endoscope. In the confocal endoscope apparatus, a scanning fiber is moved along a predetermined course while emitting illumination light toward an observation area. Reflected light or autofluorescence from a point illuminated by the illumination light is made incident on the scanning fiber. The scanning fiber transmits the reflected light or autofluorescence to a light receiving unit, which detects the amount of received light. 
     To carry out a confocal observation with a confocal endoscope apparatus having the above structure, it is preferable to condense a beam of light with a thick diameter. 
     In addition, a single mode fiber should be used as the scanning fiber in a confocal observation. However, it is difficult to emit a beam with a sufficiently large diameter from a single mode fiber. Accordingly, a lens unit mounted in the emission end of the scanning fiber needs to include an optical enlargement system. 
     However, the size of the lens unit increases by including the optical enlargement system. In addition, movement of a point illuminated by the illumination light according to the movement of the emission end of a scanning fiber with the optical enlargement system is much less than it is relative to a scanning fiber without the optical enlargement system. In order to capture an image of a subject of considerable size, the emission end needs to be moved substantially. Accordingly, it is difficult to manufacture a thin insertion tube that includes the above lens unit and scanning fiber. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a confocal optical system that enables the entire lens unit to be downsized to fit inside a thin insertion tube. 
     According to the present invention, a confocal optical system, comprising a scanning fiber is provided. The scanning fiber is a single-mode fiber of which a first end is shaped as a curved surface. The scanning fiber transmits illumination light to the first end. The illumination light is emitted toward an observation area. The illumination light emanates from the first end. The illumination light emanates from the first end striking a target area within the observation area. The first end receives at least one of reflected light and fluorescence from the target area. The reflected light is the illumination light reflected from the target area. The fluorescence is induced at the target area by illumination from the illumination light. 
     Further, the first end is shaped so that a numerical aperture of the first end is greater than that of the single-mode fiber. 
     Further, a portion of the single-mode fiber within a mode field diameter at the first end is shaped as a spherical surface. Another portion of the single-mode fiber between the mode field diameter and an outside diameter at the first end is shaped as a conical surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a confocal endoscope apparatus comprising a confocal optical system of the embodiments of the present invention; 
         FIG. 2  is a block diagram schematically showing the internal structure of the confocal endoscope processor; 
         FIG. 3  is a block diagram schematically showing the internal structure of the confocal endoscope; 
         FIG. 4  is an enlarged diagram of the emission end of the scanning fiber; and 
         FIG. 5  is a cutaway view of a schematic structure near the emission end of the scanning fiber in a prior confocal endoscope. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described below with reference to the embodiment shown in the drawings. 
     In  FIG. 1 , the confocal endoscope apparatus  10  comprises a confocal endoscope processor  20 , a confocal endoscope  30 , and a monitor  11 . The confocal endoscope processor  20  is connected to the confocal endoscope  30  and the monitor  11 . 
     The confocal endoscope processor  20  provides excitation light that is shined on an observation area (see “OA” in  FIG. 1 ). The excitation light produced by the confocal endoscope processor  20  is transmitted to the distal end of the insertion tube  31  of the confocal endoscope  30  and emitted towards one point in the observation area. Fluorescence from the point struck by the excitation light is transmitted from the distal end of the insertion tube  31  to the confocal endoscope processor  20 . 
     An emission direction, which is the direction in which the excitation light is emitted from the insertion tube  31 , is changed by an actuator (not depicted in  FIG. 1 ) mounted in the distal end of the insertion tube  31 . By varying the emission direction the observation area is scanned with the excitation light emitted from the illumination fiber. The actuator is controlled by the confocal endoscope processor  20 . 
     The confocal endoscope processor  20  determines the emission direction based on the control status of the actuator. The confocal endoscope processor  20  receives fluorescence corresponding to the emission direction, and generates a pixel signal according to the quantity of fluorescence received. One frame of an image signal is generated from pixel signals corresponding to the illuminated points dispersed throughout the observation area. The generated image signal is transmitted to the monitor  11 , upon which an image corresponding to the received image signal is displayed. 
     As shown in  FIG. 2 , the confocal endoscope processor  20  comprises a laser light source  21 , a supply-fiber  22   s , a connection-fiber  22   c , an image-transmission fiber  22   i , a detection-fiber  22   d , a photocoupler  23 , first and second light-capturing units  24   a  and  24   b , an excitation light cut filter  25 , an image-processing circuit  26 , a scanning driver  27 , a system controller  28 , and other components. 
     Excitation light, which causes certain types of subjects, such as human organs, to fluoresce, is generated by the laser light source  21 . The laser light source  21  is optically connected to the supply fiber  22   s . The excitation light from the laser light source  21  is transmitted to the supply-fiber  22   s.    
     The supply-fiber  22   s  is optically connected to both the connection-fiber  22   c  and the detection-fiber  22   d  by the photocoupler  23 . Light can be transmitted between the supply-fiber  22   s  and the connection-fiber  22   c  and detection-fiber  22   d.    
     In addition, the connection-fiber  22   c  and the detection fiber  22   d  are also optically connected to the image-transmission fiber  22   i  by the photocoupler  23 . Light can be transmitted between the image-transmission fiber  22   i  and the connection-fiber  22   c  and detection-fiber  22   d.    
     The photocoupler  23  is a four-port directive coupler. The excitation light transmitted from the supply-fiber  22   s  is divided and routed into the connection-fiber  22   c  and the detection-fiber  22   d . The reflection light and fluorescence transmitted from the connection fiber  22   c  is divided and routed into the image-transmission fiber  22   i  and the supply-fiber  22   s.    
     The detection fiber  22   d  is optically connected to the second light-capturing unit  24   b . The excitation light routed to the detection-fiber  22   d  is transmitted to the second light-capturing unit  24   b.    
     The second light-capturing unit  24   b  detects the amount of the excitation light. The detected amount of excitation light is communicated to the system controller  28 , which controls the amount of excitation light generated by the laser light source  21  based on the amount of detected light. 
     The connection-fiber  22   c  is optically connected to the proximal end of the scanning fiber  32  mounted in the confocal endoscope  30 . The excitation light routed into the connection-fiber  22   c  is transmitted through the scanning fiber  32  to the distal end of the insertion tube  31 , from which it is emitted. In addition, as described later, reflected light and fluorescence are transmitted from the distal end to the connection-fiber  22   c  by the scanning fiber  32 . 
     As described above, the reflected light and fluorescence transmitted to the connection fiber  22   c  is divided and routed into the image-transmission fiber  22   i  and the supply-fiber  22   s  by the photocoupler  23 . The image-transmission fiber  22   i  is optically connected to the first light-capturing unit  24   a . The excitation light cut filter  25  is mounted between the image-transmission fiber  22   i  and the first light-capturing unit  24   a.    
     The reflected light, which is excitation light that has traveled through the image-transmission fiber  22   i , is attenuated by the excitation light cut filter  25  and prevented from entering the first light-capturing unit  24   a . On the other hand, the fluorescence that travels through the image-transmission fiber  22   i  passes through the excitation light cut filter  25  and enters the first light-capturing unit  24   a.    
     The first light-capturing unit  24  generates a pixel signal according to the amount of fluorescence detected from a point in the observation area that has been illuminated by the excitation light. The pixel signal is transmitted to the image-processing circuit  26 , which stores the pixel signal in an image memory (not depicted). 
     As described later, after pixel signals corresponding to a succession of points dispersed throughout the observation area that have been illuminated by the moving excitation light are generated and stored in the image memory, the image-processing circuit  26  carries out predetermined signal processing on the pixel signals, and then one frame of the image signal is transmitted to the monitor  11 . 
     Next, the structure of the confocal endoscope  30  is explained. As shown in  FIG. 3 , the confocal endoscope  30  comprises the scanning fiber  32 , a lens unit  33 , the actuator  34 , and other components. 
     The scanning fiber  32  is arranged inside the confocal endoscope  30  from the connector  35  to the distal end of the insertion tube  31 . As described above, the excitation light generated by the laser light source  21  is transmitted to the proximal end of the scanning fiber  32  via the supply-fiber  22   s , the photocoupler  23 , and the connection-fiber  22   c . The excitation light made incident on the proximal end is transmitted to the distal end, from which it is emitted toward a subject. 
     A single-mode fiber is used for the scanning fiber  32 . In addition, the emission end of the scanning fiber  32 , which is arranged inside of the distal end of the insertion tube  31 , has been ground so that the portion of the emission end inside the mode field diameter, whose center coincides with a core  32   c , has a spherical surface, as shown in  FIG. 4 . In addition, the spherical surface is shaped so that the numerical aperture at the emission end is greater than that of the single-mode fiber in use. Accordingly, the end of the scanning fiber  32  is a “lens-ed” fiber. 
     Generally, the diameter of a beam emitted from a single-mode fiber is not thick enough to use for confocal observation because the numerical aperture of the single-mode fiber is too small. However, because the emission end of the scanning fiber  32  is shaped as a spherical surface as described above, the beam of emitted excitation light has a diameter that is thick enough to use for in a confocal observation. 
     The actuator  34  is mounted near the emission end of the scanning fiber  32 . The actuator  34  moves the emission end of the scanning fiber  32  based on a fiber driving signal transmitted from the scanning driver  27  so that the emission end of the scanning fiber  32  traces a predetermined course, such as a spiral course. By emitting the excitation light from the moving emission end of the scanning fiber  32 , the observation area is scanned with the excitation light. 
     The lens unit  33  is mounted in the downstream direction of the excitation light from the emission end of the scanning fiber  32 . The lens unit  33  has a condenser optical system. Accordingly, the excitation light emitted from the emission end of the scanning fiber  32  is condensed by the lens unit  33 , and directed towards a point in the observation area. 
     Even though the excitation light is directed towards one fine point, a peripheral area surrounding the point is illuminated with the excitation light due to diffraction limited so that the amount of the excitation light is distributed according to Gaussian distribution. 
     Excitation light is reflected and fluoresced from every point struck by the excitation light, but only the portion of the reflected light and fluorescence that emanates from the targeted points&#39; central pinpoint areas, which are confocal points with respect to the emission end of the scanning fiber  32 , is made incident on the emission end of the scanning fiber  32 . The remaining portion of reflected light and fluorescence that emanates from within the targeted points but outside of the central pinpoint area is not made incident on the emission end. 
     The reflected light and fluorescence made incident on the emission end is transmitted to the proximal end of the scanning fiber  32 . The reflected light and fluorescence is transmitted to the first light-capturing unit  24   a  via the connection-fiber  22   c , the photocoupler  23 , and the image-transmission fiber  22   i . As described above, the reflected light, which is the excitation light, is blocked by the excitation light cut filter  25 , so that only the fluorescence is made incident on the first light-capturing unit  24   a.    
     In the above embodiment, a lens unit  33  can be downsized, the diameter of an insertion tube  31  can be reduced, and the signal-to-noise ratio can be improved. Those effects are explained as follows. 
     In a confocal observation, optical information from only a pinpointed area of each illuminated point needs to be captured. Accordingly, a single-mode fiber is adequate for the scanning fiber  32  used in the confocal endoscope apparatus. In addition, in a confocal observation a light beam with a thick diameter must be condensed by a condenser optical system that has a large numerical aperture. 
     As described above, the numerical aperture of a standard single-mode fiber is low, and the diameter of the beam of excitation light emitted from the single-mode fiber is not thick enough. Accordingly, as shown in  FIG. 5 , an optical enlargement system  33   e  that expands the diameter of the excitation light beam generally needs to be included in a lens unit  33 . 
     On the other hand, in the above embodiment, because the diameter of the beam of excitation light emitted from the emission end of the scanning fiber  32  has adequate thickness, the optical enlargement system  33   e  is unnecessary. Consequently, a lens unit  33  can be downsized as a result of the omitted optical enlargement system  33   e.    
     In addition, a mounted optical enlargement system  33   e  restricts the scope of a view angle. So, to capture an image with sufficient breadth, the emission end of the scanning fiber  32  needs to have a broad range of movement. The diameter of a tube (not depicted) around the scanning fiber  32  needs to be thick enough to enable a broad range of movement for the emission end. As a result, it is difficult to provide an insertion tube  31  with a thin diameter. 
     On the other hand, in the above embodiment, because the optical enlargement system  33   e  can be omitted, the breadth of the view angle is increased. As a result, the emission end has a broader range of motion than it would with an optical enlargement system  33   e . Consequently, a thinner insertion tube  31  can be adopted relative to an apparatus equipped with the optical enlargement system  33   e.    
     In addition, in order to generate an image with a high signal-to-noise ratio, light having bands other than fluorescence must be prevented from entering the first light-capturing unit  24   a . However, not only fluorescence but also excitation light is emitted from the image-transmission fiber  22   i . Although the excitation light is attenuated by the excitation light cut filter  25 , as described above, it is difficult to block the excitation light. So, it is preferable to have only a minimal amount of excitation light transmitted by the image-transmission fiber  22   i.    
     Generally, the excitation light transmitted to the image-transmission fiber  22   i  includes not only the excitation light reflected from the illuminated point in the observation area that is made incident on the emission end of the scanning fiber  32 , but also includes excitation light that is partially reflected by the surface of the emission end of the scanning fiber  32 . 
     In the above embodiment, a portion of the excitation light that is transmitted to the emission end is reflected in a direction that is inclined from the axis of the scanning fiber  32  because the emission end within the mode field diameter is shaped as a spherical surface. The excitation light reflected in the direction inclined from the axis cannot be transmitted to the proximal end because the reflected excitation light cannot meet the requirement of the core propagation mode. As a result, the amount of the excitation light transmitted to the image-transmission fiber  22   i  is mitigated. Finally, the signal-to-noise ratio can be improved. 
     The emission end of the scanning fiber within the mode field diameter is shaped as a spherical surface in the above embodiment. However, the emission end is not limited to the spherical surface. For example, the emission end can be shaped as a curved surface, such as an aspheric surface. The emission end within the field mode diameter can take on any shapes as long as the numerical aperture of the shaped emission end is greater than that of the original single-mode fiber. 
     The emission end is ground so that the emission end is shaped as a spherical surface in the above embodiment. However, the method for shaping the emission end is not limited to grinding. Of course, an end can be finely shaped by grinding, and a “lens-ed” fiber can be manufactured with a numerical aperture that is large enough relative to the lens unit  33  in use. 
     The emission end of the scanning fiber outside of the mode field diameter is shaped as a conical surface in the above embodiment. However, this portion can be shaped as a spherical surface similar to the portion within the mode field diameter. 
     The confocal optical system is adopted for a confocal endoscope apparatus in the above embodiment. However, the same effect can be achieved even if the confocal optical system is adopted for another scanning observation apparatus, such as a confocal probe, a multi-photon fluorescence microscope apparatus, or a second harmonic microscope apparatus. 
     Although the embodiments of the present invention have been described herein with reference to the accompanying drawings, obviously many modifications and changes may be made by those skilled in this art without departing from the scope of the invention. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2009-116408 (filed on May 13, 2009), which is expressly incorporated herein, by reference, in its entirety.