Patent Publication Number: US-2011051250-A1

Title: Optical element, and processing apparatus and method for reducing reflection

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical elements, and processing apparatuses and methods for reducing reflection. The invention is suitable for, for example, optical elements for which surface reflection of light needs to be prevented. 
     2. Description of the Related Art 
     Lenses that use translucent substrates such as glass and plastic have been widely used as optical elements. To reduce the surface reflected light and increase transmission characteristics, such lenses often use a multilayer film coating that includes an anti-reflective film formed by vapor deposition of material such as an oxide on surface. 
     In the multilayer film coating, the number of coating layers is increased to reduce incident angle dependence or wavelength dependence. This complicates the designing procedures and increases the number of manufacturing steps. 
     As a countermeasure, an element with a structure called a moth-eye structure has been proposed in which microscopic indentations equal to or shorter than the wavelength of light are formed on a lens surface to continuously vary the refractive index of the lens along the thickness direction (see, for example, JP-A-2003-131390). 
     The moth-eye structure does not depend on the incident angle of external light, and has anti-reflection effects over a relatively wide wavelength range. 
     SUMMARY OF THE INVENTION 
     A problem of the moth-eye structure, however, is that designing of indentations that can produce desirable refractive index changes along the thickness direction is difficult, because the refractive index along the thickness is varied using the microscopic indentations formed on the surface of an optical element. 
     Accordingly, there is a need for an optical element, and a processing apparatus and method for reducing reflection with which surface reflection of light can be relieved with great freedom of design. 
     According to an embodiment of the present invention, there is provided an optical element that includes a pit forming portion of a material that forms a pit in the vicinity of each focal point of a predetermined light beam upon condensation, wherein the pits are formed in such a manner that the volume percentage of the pits with respect to the material at distances from a light incident face becomes smaller away from the incident face. 
     With the optical element, the average refractive index in a predetermined range of an equal distance from the incident face along the normal direction can be gradually varied from the refractive index of air to the refractive index of the material toward inside away from the incident face, and the extent of refractive index change can be set with great freedom. 
     According to another embodiment of the present invention, there is provided a processing apparatus for reducing reflection that includes: a light source that emits a light beam; an objective lens that condenses the light beam to form pits inside an optical element of a predetermined material; a moving unit that moves a focal point position of the light beam; and a control unit that controls the light source and the moving unit to form the pits inside the optical element in such a manner that the volume percentage of the pits with respect to the material at distances from a light incident face of the optical element becomes smaller away from the incident face. 
     With the processing apparatus, the average refractive index in a predetermined range of an equal distance from the incident face of the optical element along the normal direction can be gradually varied from the refractive index of air to the refractive index of the material toward inside away from the incident face, and the extent of refractive index change can be set with great freedom. 
     According to the embodiments of the present invention, the average refractive index in a predetermined range of an equal distance from the incident face of the optical element along the normal direction can be gradually varied from the refractive index of air to the refractive index of the material toward inside away from the incident face, and the extent of refractive index change can be set with great freedom. The present invention can thus realize an optical element, and a processing apparatus and method for reducing reflection with which surface reflection of light can be relieved with great freedom of design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram representing a configuration of lens processing apparatuses of First and Second Embodiments. 
         FIG. 2  is a schematic diagram representing the concept of pit formation. 
         FIGS. 3A to 3E  are schematic diagrams representing a pit forming method of First Embodiment. 
         FIGS. 4A and 4B  are schematic diagrams representing a lens substrate of First Embodiment. 
         FIGS. 5A and 5B  are schematic diagrams representing a lens substrate with no pits. 
         FIGS. 6A and 6B  are schematic diagrams representing a lens substrate of Second Embodiment. 
         FIG. 7  is a schematic diagram representing a configuration of a pit forming apparatus of Third Embodiment. 
         FIGS. 8A to 8D  are schematic diagrams representing a pit forming method of Third Embodiment. 
         FIGS. 9A and 9B  are schematic diagrams illustrating an anti-reflective sheet and a lens according to Third Embodiment. 
         FIGS. 10A to 10D  are schematic diagrams illustrating lens substrates of other embodiments. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe embodiments of the present invention, in the order below. 
     1. First Embodiment (an example in which the distribution density of pits is varied) 
     2. Second Embodiment (an example in which the volume of individual pits is varied) 
     3. Third Embodiment (an example in which an anti-reflective sheet is used) 
     4. Other embodiments 
     1. First Embodiment 
     1-1. Configuration of Lens Processing Apparatus 
     A lens processing apparatus  1  illustrated in  FIG. 1  is configured as a whole to cut a lens substrate  100  (workpiece) into a desired shape, and to form pits by irradiating the lens substrate  100  with a light beam. 
     An integrated control unit  11  is adapted to integrally control the lens processing apparatus  1 . The integrated control unit  11  is configured to include a CPU (Central Processing Unit), a ROM (Read Only Memory) storing various programs and other data, and a RAM (Random Access Memory) used as a work memory for the CPU (all not shown). 
     In actual practice, the integrated control unit  11  executes various programs to drive and rotate a spindle motor  13  about the Z axis via a drive control unit  12 , and to thereby rotate a main shaft  14  at a desired speed. A lens anchoring unit  15  is attached to the main shaft  14 . Accordingly, the lens anchoring unit  15  rotates with the main shaft  14 . 
     The lens substrate  100  (workpiece) is anchored on the lens anchoring unit  15 . Accordingly, the lens substrate  100  rotates with the lens anchoring unit  15 . 
     In this manner, the integrated control unit  11  is adapted to drive and rotate the spindle motor  13  via the drive control unit  12  to rotate the lens substrate  100  at a desired speed. 
     The lens substrate  100  is formed of optical glass. Irradiation of the lens substrate  100  with a light beam of a predetermined light quantity causes a local temperature increase in the vicinity of the focal point, and a pit is formed by the resulting thermochemical reaction. Before cutting, the lens substrate  100  is substantially cylindrical in shape with the bottom face in contact with the lens anchoring unit  15 . 
     The optical glass is a blend of at least 5 or 6 kinds of materials such as silica stone, lanthanum oxide, and boric acid, and melts at temperatures of about 1,200° C. to 1,400° C. The optical glass allows for passage of incident light from one surface to the other with high transmittance. The optical glass has a refractive index of about 1.5. 
     The pits formed in the lens substrate  100  are filled with the gas generated by the heat-decomposition of the optical glass. Since the main component of the lens substrate  100  is oxide material such as silica stone, the gaseous component in the pits is considered to be oxygen. The refractive index of oxygen is about 1.0, substantially the same as that of air but different from that of the optical glass. 
     The integrated control unit  11  is also adapted to execute various programs to control the driving of a support unit  16  via the drive control unit  12  in three directions along the X, Y, and Z axes, and in the rotational direction about the X axis. 
     A tool anchoring unit  17  is attached to the support unit  16 . The tool anchoring unit  17  includes a tool  18  made of, for example, diamond, used to cut the lens substrate  100 . 
     In this manner, the integrated control unit  11  controls the driving of the support unit  16  via the drive control unit  12  in such away that the tool  18  anchored on the tool anchoring unit  17  is controlled at a desired position and at a desired angle with respect to the lens substrate  100 . 
     In addition to the tool anchoring unit  17 , an optical unit  19  is also attached to the support unit  16 . The optical unit  19  is thus movable with the tool anchoring unit  17  under the drive control of the drive control unit  12 . 
     The optical unit  19  has substantially the same configuration as a common optical pickup, and includes a laser driving unit  20 , a laser diode  21 , an actuator  22 , a lens holder  23 , and an objective lens  24 . 
     In forming pits in the lens substrate  100 , the integrated control unit  11  performs predetermined processes by, for example, supplying information such as pit volume to a signal processing unit  25 , and produces laser control signals according to the information and sends the signals to the laser driving unit  20  of the optical unit  19 . 
     The integrated control unit  11  also controls the driving of the actuator  22  of the optical unit  19  via the drive control unit  12 . In this way, the integrated control unit  11  causes the lens holder  23  carrying the objective lens  24  to lightly move in directions toward and away from the lens substrate  100  for the position adjustment of the objective lens  24 . The integrated control unit  11  is thus able to move the focal point of a light beam along the depth direction (Z direction) of the lens substrate  100 . 
     The laser driving unit  20  produces a laser drive signal based on the laser control signal supplied from the signal processing unit  25 , and sends the laser drive signal to the laser diode  21 . In response to the laser drive signal, the laser diode  21  emits a pit-forming light beam according to the laser drive signal to irradiate the lens substrate  100  via the objective lens  24  that has undergone a position adjustment. In this way, the optical unit  19  is able to form pits in the lens substrate  100 . 
     The signal processing unit  25  controls parameters such as the peak value, pulse width, and period of the laser control signal sent to the laser driving unit  20 , under the control of the integrated control unit  11 . In this way, the signal processing unit  25  is able to control parameters such as the peak value of light beam intensity, and the irradiation time and period of the light beam shone on the lens substrate  100 . The pit volume increases as the light intensity and/or the irradiation time of the light beam shone on the lens substrate  100  increase. 
     In the actual forming of the pits that proceeds concurrently with the cutting of the lens substrate  100 , the drive control unit  12  rotates the spindle motor  13  under the control of the integrated control unit  11  to cause the rotation of the main shaft  14  and the lens substrate  100  anchored on the lens anchoring unit  15 . 
     The drive control unit  12  then moves the support unit  16  to contact the tool  18  with the lens substrate  100  undergoing rotation, cutting the lens substrate  100  and forming a lens of a desired shape. 
     Here, the signal processing unit  25  drives the laser diode  21  under the control of the integrated control unit  11 , and causes the laser diode  21  to emit a light beam of a predetermined light intensity. The objective lens  24  at a controlled position focuses the light beam onto the position distant apart from the surface of the lens substrate  100  by a desired distance (depth) along the Z direction. 
       FIG. 2  is a conceptual diagram representing the cutting of the lens substrate  100  and pit formation. In  FIG. 2 , only the lens anchoring unit  15 , the lens substrate  100 , the objective lens  24 , and the tool  18  are shown, and the other components are omitted. Here, the lens substrate  100  is being cut to provide a planoconvex lens that transmits and condenses the parallel rays incident on the Z 1  side, and has a focal point on the Z 2  side. 
     The rotation of the lens anchoring unit  15  in direction R about the Z axis causes the lens substrate  100  to rotate with the lens anchoring unit  15 . The lens substrate  100  is thus cut by the tool  18  in contact with the substrate surface. Pits are then formed in the lens substrate  100  as the light beam through the objective lens  24  irradiates the lens substrate  100 . 
     As illustrated in  FIG. 1 , the optical unit  19  carrying the objective lens  24  is attached to the support unit  16  as is the tool anchoring unit  17  anchoring the tool  18 . As such, the objective lens  24  moves in three directions along the X, Y, and Z axes, and in the rotational direction about the X axis, following the tool  18 . Note, however, that, by the provision of the actuator  22 , the movement of the objective lens  24  is independent from the tool  18  with regard to the movement directions toward and away from the lens substrate  100 . 
     As described above, the lens processing apparatus  1  is adapted to form pits by irradiating the lens substrate  100  with a light beam through the objective lens  24  that is lightly moved in directions toward and away from the lens substrate  100  while undergoing movement following the movement of the tool  18  as it cuts the lens substrate  100 . 
     1-2. Formation of Pits 
     The following describes the procedures of forming the pits in the lens substrate  100 . The pits that prevent reflection of external rays are formed in the surface of the lens substrate  100  on the side of the objective lens  24  (the Z 1  side; hereinafter, also referred to as “incident face  100 N”). 
       FIGS. 3A to 3E  are magnified cross sectional views of a lens substrate portion PT 1  of the lens substrate  100  (a portion on the Z 1  side) illustrated in  FIG. 2 , showing how the pits are formed. 
     The lens processing apparatus  1  first uses the drive control unit  12  to move the objective lens  24  with the support unit  16 , and focuses a light beam at a focal point position within the lens substrate  100  of  FIG. 3A  a predetermined distance away from the incident face  100 N, as illustrated in  FIG. 3B . The lens processing apparatus  1  then uses the signal processing unit  25  to control the laser driving unit  20 , causing the laser diode  21  to emit a light beam of a predetermined light intensity for a predetermined time period. As a result, a pit is formed. In this manner, the lens processing apparatus  1  forms a plurality of pits, all in substantially the same volume, by shining a light beam of the same intensity at different positions for the same duration without changing the distance from the incident face  100 N. 
     As a result, as illustrated in  FIG. 3B , a layer of pits (hereinafter, also referred to as “pit layer L 1 ”) is formed in the lens substrate  100  a certain distance (depth) away from the incident face  100 N. The pits actually formed have substantially a spherical shape, even though they appear circular in  FIGS. 3A to 3E . 
     The lens processing apparatus  1  then uses the drive control unit  12  to control the objective lens  24 , and, as illustrated in  FIG. 3C , forms a plurality of pits having substantially the same volume as the pits of the pit layer L 1  by moving the focal point position of the light beam closer to the incident face  100 N than the pit layer L 1 . As a result, as in the pit layer L 1 , a layer of pits (hereinafter, also referred to as “pit layer L 2 ”) is formed in the lens substrate  100  a certain distance away from the incident face  100 N. 
     Here, the lens processing apparatus  1  forms larger numbers of pits than in the pit layer L 1 . Accordingly, the pit density is higher in the pit layer L 2  than in the pit layer L 1  in the lens substrate  100 . 
     The lens processing apparatus  1  repeats the same procedure, each time forming a layer of pits of substantially the same volume but in greater numbers than in the previous layer farther away from the incident face  100 N, by controlling the objective lens  24  with the drive control unit  12 , and shining a light beam on the lens substrate  100  at a focal point position that is gradually moved toward the incident face  100 N in each procedure. 
     In this manner, the lens processing apparatus  1  controls the support unit  16  and the objective lens  24  using the drive control unit  12  to shine a light beam at a focal point position that is gradually moved toward the incident face  100 N of the lens substrate  100  from a position farther away from the incident face  100 N. 
     As a result, as shown in  FIG. 3D  the pits are formed in the lens substrate  100  in three dimensions in the X, Y, and Z directions, and the pits in the Z direction form layers. 
     The lens processing apparatus  1  forms pits of substantially the same volume in the lens substrate  100  in gradually increasing densities toward the incident face  100 N. 
     If the lens processing apparatus  1  were to reverse the procedure and form pits a direction away from the incident face  100 N of the lens substrate  100 , there is a possibility that the light beam shone on the lens substrate  100  passes through the previously formed pits at nearer positions. 
     The light beam that passes through the previously formed pits is influenced by the difference in the refractive indices of the lens substrate  100  and the pits, and may fail to be focused at a desired focal point position. This may lead to poor product quality. 
     In this case, the lens processing apparatus  1  may fail to form pits at desired positions of the lens substrate  100 , or may fail to form pits of desired volumes. 
     To avoid the influence of the previously formed pits, the lens processing apparatus  1  is adapted to sequentially form pits toward the incident face  100 N of the lens substrate  100  from a position father away from the incident face  100 N. 
     Then, as illustrated in  FIG. 3E , the lens processing apparatus  1  moves the focal point position of the light beam to the incident face  100 N of the lens substrate  100 , and shines a light beam so as to increase the pit density more than in the pit layer LN closest to the incident face  100 N illustrated in  FIG. 3D . 
     However, in  FIG. 3E , because the light beam is shone on the surface of the lens substrate  100 , substantially hemispherical depressions with substantially half the volume of the pits formed inside the lens substrate  100  are formed on the incident face  100 N. As a result, indentations are formed on the incident face  100 N of the lens substrate  100 . 
     In the following, the term “pit forming portion  100 H” is used to refer to a portion of the lens substrate  100  where the pits are formed. A portion beneath the incident face  100 N past the pit forming portion  100 H inside the lens substrate  100  where no pits are formed is referred to as an “optically functional portion  100 L.” 
     The surface indentations of the lens substrate  100  may be formed using a chemical treatment, for example, such as etching. However, irradiation of a light beam is more desirable than chemical treatment, because it can simplify the configuration of the lens processing apparatus  1 , and can reduce the number of working processes. 
     1-3. Varying Refractive Index 
     As illustrated in  FIG. 4A , pits of substantially the same volume are formed inside the lens substrate  100 . 
     Here, a range of a predetermined width in a direction normal to the incident face  100 N (depth direction) equally distant apart from the incident face  100 N is depth range DR. For example, when the depth range DR is a range that contains a single pit layer, the material of the lens substrate  100  and the pits are present in a predetermined volume ratio in the depth range DR. In the following, the depth range DR is described as being a range that includes a single pit layer distant apart from the incident face by a predetermined distance. 
     The average refractive index in the depth range DR (hereinafter, also referred to as “depth range refractive index”) is believed to take a value between the refractive index of the material of the lens substrate  100  and that of the pits according to the volume ratio of the pits with respect to the material of the lens substrate  100 . 
     As noted above, the refractive index of the pits is about 1.0, about the same as the refractive index of air outside the lens substrate  100 . The lens substrate  100  (optical glass) has a refractive index of about 1.5. 
     Thus, in the predetermined depth range DR, the depth range refractive index approaches 1.5 as the pit volume decreases with respect to the material of the lens substrate  100 , and approaches 1.0 as the pit volume increases with respect to the material of the lens substrate  100 . 
     As illustrated in  FIG. 4A , the number of pits in the lens substrate  100  increases toward the incident face  100 N, and gradually decreases away from the incident face  100 N into the substrate.  FIG. 4A  also shows incident light LT 1  shone on the lens substrate  100  on the side of the air outside the lens substrate  100 . 
     Thus, as represented in  FIG. 4B , the depth range refractive index gradually increases from 1.0 to 1.5 toward inside the lens substrate  100  away from the incident face  100 N. 
     In the pit layer LN, the depth range refractive index is about 1.0, because of the high pit density attributed to the very large numbers of pits formed in the layer. In contrast, the depth range refractive index is about 1.5 in the pit layer L 1 , because the number of pits formed in the layer is very small and thus the pit density is low. Thus, there is only a small difference in refractive index at the interface between air and the lens substrate  100 . 
     As a rule, where there is a difference in refractive index between two materials on which light is incident, some of the incident rays are reflected at the interface between the two materials. The percentage of the reflected light with respect to the incident light becomes smaller as the difference in refractive index between two materials decreases. 
     Thus, as illustrated in  FIG. 4A , the reflected light LT 2  that occurs as the incident light LT 1  is reflected at the lens substrate  100  has a much smaller light quantity than the incident light LT 1 . 
     Further, because the lens substrate  100  has indentations on the incident face  100 N, the difference in refractive index between air and the lens substrate  100  can be further reduced to continuously vary the refractive index. Thus, reflection of external light on the lens substrate  100  can be suppressed. 
     1-4. Operation and Effects 
     Configured as above, the lens processing apparatus  1  shines a light beam on the lens substrate  100  formed of an optical glass. 
     The lens substrate  100  irradiated with a light beam of a predetermined light quantity undergoes a thermochemical reaction as a result of a local temperature increase in the vicinity of the focal point, and a pit is formed. The lens processing apparatus  1  forms pits of substantially the same volume in such a manner that the pits gradually decrease in density toward inside the lens substrate  100  away from the incident face  100 N. The lens processing apparatus  1  also shines a light beam onto the incident face  100 N of the lens substrate  100  to form indentations. 
     Thus, in the lens substrate  100 , the volume ratio of pits with respect to the material gradually becomes smaller toward inside, away from the incident face  100 N. 
     Because the refractive index of air is about 1.0 and that of the lens substrate  100  about 1.5, an abrupt change occurs in refractive index at the interface between air and the incident face  100 N of the lens substrate  100 , as represented in  FIG. 5B , when no pits are formed in the lens substrate  100  as illustrated in  FIG. 5A . 
     In this case, the percentage of the reflected light LT 2  on the incident face  100 N that occurs as a result of the reflection of the incident light LT  1  externally incident on the lens substrate  100  increases relative to the incident light LT 1 , as illustrated in  FIG. 5A . 
     In contrast, such an abrupt change does not occur in the lens substrate  100  of the present embodiment ( FIGS. 4A and 4B ), because the refractive index continuously varies toward inside the lens substrate  100  away from the air side. 
     Accordingly, only a small difference in refractive index occurs at the interface between air and the lens substrate  100 , and reflection of externally incident light at the surface of the lens substrate  100  can be suppressed. 
     Further, because the lens processing apparatus  1  uses the integrated control unit  11  to control the light beam shone on the lens substrate  100 , the distribution density of the pits in the lens substrate  100  can be freely set. 
     The lens processing apparatus  1  is thus able to set the extent of refractive index change with great freedom over the range extending from the incident face  100 N of the lens substrate  100  into the substrate. 
     The anti-reflection processes of related art can be used to adjust the extent of refractive index change in a direction normal to the incident face. For example, in the multilayer film coating, adjustments can be made by changing the ways a high refractive index layer and a low refractive index layer are combined. In the moth-eye structure, adjustments are possible by varying the indentation height. However, these are difficult to achieve in terms of design. 
     In contrast, with the lens processing apparatus  1  of the present embodiment, the extent of refractive index change in a direction normal to the incident face  100 N of the lens substrate  100  can be adjusted simply by adjusting the density of the pits formed in the lens substrate  100  at each distance from the incident face  100 N. 
     In contrast to the moth-eye structure of the related art in which indentations are formed only on the surface subjected to the anti-reflection process, the lens substrate  100  of the present embodiment forms pits in a normal direction of the incident face  100 N of the lens substrate  100 . Thus, changes in refractive index can be reduced further by forming large numbers of layers relatively deep down the lens substrate  100  in a normal direction of the incident face  100 N. Further, because the lens processing apparatus  1  can form pits down into the lens substrate  100  simply by shining a light beam on an optical glass having a high light transmissivity, the apparatus configuration can be simplified. 
     The multilayer film coating of the related art requires materials such as oxides, in addition to the material subjected to the anti-reflection process. In contrast, irradiation of a light beam is all that is required for the lens substrate  100  of the present embodiment, and other materials are not required. This simplifies the configuration of the lens processing apparatus  1  for the anti-reflection process, and reduces the material cost. 
     According to the foregoing configuration, the lens processing apparatus  1  shines a light beam on the lens substrate  100  under the control of the integrated control unit  11 , and forms pits of substantially the same volume in such a manner that the pits gradually decrease in density toward inside the lens substrate  100  away from the substrate surface. The refractive index of the lens substrate  100  thus continuously varies toward inside the lens substrate  100  away from the air side. The lens processing apparatus  1  is therefore able to gradually vary the depth range refractive index of the lens substrate  100  from the refractive index of air to the refractive index of the material toward inside the substrate away from the incident face  100 N, and set the extent of refractive index change with great freedom. 
     2. Second Embodiment 
     2-1. Pit Formation 
     A lens processing apparatus  1  ( FIG. 1 ) of Second Embodiment is configured in the same way as the lens processing apparatus  1  of First Embodiment. 
       FIG. 6A  is a magnified cross sectional view illustrating a portion of a lens substrate  200  as with  FIG. 4A . The pits for preventing the reflection of external light are formed in the incident face  200 N on the objective lens  24  side (Z 1  side) of the lens substrate  200 . 
     As in First Embodiment, the lens processing apparatus  1  of Second Embodiment controls the objective lens  24  using the drive control unit  12 , and forms the pits by shining a light beam at a focal point position that is gradually moved toward the incident face  200 N of the lens substrate  200  from a position farther away from the incident face  200 N. 
     In the lens processing apparatus  1 , for example, the irradiation time of a light beam for the lens substrate  200  is gradually extended under the control of a signal processing unit  25  as the focal point position is moved toward the incident face  200 N of the lens substrate  200  from a position farther away from the incident face  200 N. Note, however, that the lens processing apparatus  1  maintains the same irradiation time for the pits formed at the same distance from the incident face  200 N. 
     Thus, pits of gradually increasing volumes are formed in the lens substrate  200  from inside the substrate toward the incident face  200 N. Note, however, that the pit volume is substantially the same for the same layer in the lens substrate  200 . Further, indentations are formed on the incident face  200 N of the lens substrate  200 . 
     In the following, the term “pit forming portion  200 H” is used to refer to a portion of the lens substrate  200  where the pits are formed. A portion beneath the incident face  200 N past the pit forming portion  200 H inside the lens substrate  200  where no pits are formed is referred to as an “optically functional portion  200 L.” 
     2-2. Varying Refractive Index 
     As illustrated in  FIG. 6A , the lens substrate  200  has pits that gradually decrease in volume away from the incident face  200 N. 
     Thus, in the lens substrate  200 , the volume percentage of the pits with respect to the material of the lens substrate  200  in the depth range DR gradually decreases toward inside the substrate from the incident face  200 N. 
     As noted above, the refractive index of the pits is about 1.0, substantially the same as the refractive index of air outside the lens substrate  200 . The lens substrate  200  (optical glass) has a refractive index of about 1.5. 
     Thus, as represented in  FIG. 6B , the depth range refractive index gradually increases from 1.0 to 1.5 toward inside the lens substrate  200  away from the incident face  200 N. 
     In the pit layer LN, the depth range refractive index is about 1.0, because the pits formed in the layer have a large volume and thus a large volume percentage with respect to the material of the lens substrate  200 . In contrast, the depth range refractive index is about 1.5 in the pit layer L 1 , because the pits formed in the layer have a small volume and thus a small volume percentage with respect to the material of the lens substrate  200 . Thus, there is only a small difference in refractive index at the interface between air and the lens substrate  200 . 
     Thus, as illustrated in  FIG. 6A , the reflected light LT 2  that occurs as the incident light LT 1  is reflected at the lens substrate  200  has a much smaller light quantity than the incident light LT 1 . 
     Further, because the lens substrate  200  has indentations on the incident face  200 N, the difference in refractive index between air and the lens substrate  200  can be further reduced to continuously vary the refractive index. Thus, reflection of external light on the lens substrate  200  can be suppressed. 
     2-3. Operation and Effects 
     Configured as above, the lens processing apparatus  1  shines a light beam on the lens substrate  200  formed of an optical glass. 
     The lens substrate  200  irradiated with a light beam of a predetermined light quantity undergoes a thermochemical reaction as a result of a local temperature increase in the vicinity of the focal point, and a pit is formed. The lens processing apparatus  1  forms pits in such a manner that the pits gradually decrease in volume toward inside the lens substrate  200  away from the incident face  200 N. The lens processing apparatus  1  also shines a light beam onto the incident face  200 N of the lens substrate  200  to form indentations. 
     Thus, in the lens substrate  200 , the volume percentage of pits with respect to the material gradually becomes smaller toward inside away from the incident face  200 N. 
     Thus, the refractive index of the lens substrate  200  continuously varies toward inside the lens substrate  200  away from the air side, and does not vary abruptly. Reflection of externally incident light at the surface of the lens substrate  200  can thus be suppressed. 
     Further, because the lens processing apparatus  1  uses the integrated control unit  11  to control the light beam shone on the lens substrate  200 , the volume of individual pits in the lens substrate  200  can be freely set. 
     The lens processing apparatus  1  is thus able to set the extent of refractive index change with great freedom over the range extending from the incident face  200 N of the lens substrate  200  into the substrate. 
     The lens substrate  200  of Second Embodiment also has advantages substantially the same as those described in conjunction with the lens substrate  100  of First Embodiment. 
     According to the foregoing configuration, the lens processing apparatus  1  shines a light beam on the lens substrate  200  under the control of the integrated control unit  11 , and forms pits in such a manner that the pits gradually decrease in volume toward inside the lens substrate  200  away from the substrate surface. Thus, the refractive index of the lens substrate  200  continuously varies toward inside the lens substrate  200  away from the air side. The lens processing apparatus  1  is therefore able to gradually vary the depth range refractive index of the lens substrate  200  from the refractive index of air to the refractive index of the material toward inside the substrate away from the incident face  200 N, and set the extent of refractive index change with great freedom. 
     3. Third Embodiment 
     3-1. Configuration of Pit Forming Apparatus 
     A pit forming apparatus  31  ( FIG. 7 ) of Third Embodiment differs from the lens processing apparatus  1  of First Embodiment in that a light beam is shone on an anti-reflective sheet  300  to form pits. 
     Unlike the lens processing apparatus  1 , the pit forming apparatus  31  does not include the tool anchoring unit  17  and the tool  18 . The other configuration is the same except that a sheet anchoring unit  315  that anchors the anti-reflective sheet  300  is provided instead of the lens anchoring unit  15 . 
     As with the lens substrate  100  of First Embodiment, the anti-reflective sheet  300  is made of a material that forms a pit as a result of a thermochemical reaction following a local temperature increase in the vicinity of the focal point of a light beam of a predetermined light quantity shone on the material. 
     The anti-reflective sheet  300  allows for passage of incident light from one surface to the other with high transmittance. The anti-reflective sheet  300  has a refractive index of about 1.5, as with the lens substrate  100  of First Embodiment. 
     Further, the anti-reflective sheet  300  is a flexible sheet thinner than the lens substrate  100  (along the Z direction). Thus, the anti-reflective sheet  300  can be attached to conform to the surface shape of various objects. 
     In the actual forming of the pits in the anti-reflective sheet  300 , the drive control unit  12  rotates the spindle motor  13  under the control of the integrated control unit  11  to cause the rotation of the main shaft  14  and the anti-reflective sheet  300  anchored on the lens anchoring unit  15 . 
     The drive control unit  12  then moves the support unit  16  to bring the optical unit  19  closer to the anti-reflective sheet  300 . 
     The signal processing unit  25  then drives the laser diode  21  under the control of the integrated control unit  11 , and causes the laser diode  21  to emit a light beam of a predetermined light intensity. The objective lens  24  at a controlled position focuses the light beam to a position distant apart from the surface of the anti-reflective sheet  300  by a desired distance (depth) along the Z direction. 
     In this manner, the pit forming apparatus  31  is adapted to form pits using a light beam that is shone upon moving the optical unit  19  over a wide range with the support unit  16 , and moving the objective lens  24  toward and away from the anti-reflective sheet  300 . 
     3-2. Formation of Pits 
       FIG. 8A  illustrates the anti-reflective sheet  300  of the present embodiment. As in First Embodiment, the pit forming apparatus  31  controls the objective lens  24  with the drive control unit  12 , and forms the pits by shining a light beam at a focal point position that is gradually moved toward the incident face  300 N of the anti-reflective sheet  300  from a position farther away from the incident face  300 N. 
     The pit forming apparatus  31  forms pits of substantially the same volume in gradually increasing densities under the control of the signal processing unit  25  as in First Embodiment, by moving the focal point position toward the incident face  300 N of the anti-reflective sheet  300  from a position farther away from the incident face  300 N. 
       FIG. 8B  is a magnified cross sectional view of an anti-reflective sheet portion PT 2  in part of the anti-reflective sheet  300  illustrated in  FIG. 8A . As illustrated in  FIG. 8B , pits are formed in the anti-reflective sheet  300  in three dimensions in the X, Y, and Z directions, and the pits in the Z direction form layers. 
     In the lens substrate  100  of First Embodiment, the pits are formed over a certain distance from the incident face  100 N of the lens substrate  100  (specifically, over the range of the pit forming portion  100 H). However, in the lens substrate  100 , no pits are formed in the optically functional portion  100 L farther down the lens substrate  100 , and this portion of the lens substrate  100  is solely the material of the lens substrate  100 . In other words, the pits are formed only in the vicinity of the surface of the lens substrate  100 . 
     In contrast, the anti-reflective sheet  300 , thinner than the lens substrate  100 , includes pits over the whole distance from the incident face  300 N (Z 1  side) irradiated with a light beam to the transmission face  300 T (Z 2  side) in contact with a lens  400 . In the following, the portion of the anti-reflective sheet  300  where the pits are formed is also referred to as a pit forming portion  300 H. As in the pit forming portion  100 H of the lens substrate  100  of First Embodiment, the pit forming portion  300 H includes pits of substantially the same volume in gradually decreasing densities toward inside, away from the incident face  300 N. 
     As illustrated in  FIG. 8C , in the present embodiment, reflection of light is prevented with the anti-reflective sheet  300  having pits, by attaching it to the surface of the lens  400  where reflection needs to be prevented. 
     As illustrated in  FIG. 8D , the anti-reflective sheet  300  having pits is attached to the lens  400  in such a manner that the transmission face  300 T conforms to the curved surface of the lens  400 . The lens  400  has a refractive index of about 1.5 throughout. 
     3-3. Varying Refractive Index 
       FIG. 9A  is a magnified cross sectional view of an anti-reflective sheet portion PT 3  in part of the anti-reflective sheet  300  and the lens  400  illustrated in  FIG. 8D . 
     As illustrated in  FIG. 9A , the anti-reflective sheet  300  includes pits of substantially the same volume in gradually decreasing densities away from the incident face  300 N.  FIG. 9A  also shows incident light LT 1  entering from the air side, i.e., outside the anti-reflective sheet  300 , onto the lens  400  attached to the anti-reflective sheet  300 . 
     Thus, in the anti-reflective sheet  300 , the volume percentage of the pits with respect to the material of the anti-reflective sheet  300  in the depth range DR gradually decreases toward the transmission face  300 T away from the incident face  300 N. 
     The refractive index of the pits is about 1.0, substantially the same as the refractive index of air outside the anti-reflective sheet  300 . The anti-reflective sheet  300  has a refractive index of about 1.5. 
     Thus, as represented in  FIG. 9B , the depth range refractive index of the anti-reflective sheet  300  gradually increases from 1.0 to 1.5 as in First Embodiment toward the transmission face  300 T away from the incident face  300 N. 
     In the pit layer LN, the depth range refractive index is about 1.0, because of the high pit density attributed to the very large numbers of pits formed in the layer. Thus, there is only a small difference in refractive index at the interface between air and the anti-reflective sheet  300 . 
     In contrast, the depth range refractive index is about 1.5 in the pit layer L 1 , because the number of pits formed in the layer is very small and thus the pit density is low. Accordingly, the depth range refractive index of the anti-reflective sheet  300  in the vicinity of the transmission face  300 T becomes about 1.5, substantially the same as the refractive index of the lens  400 . Accordingly, there is only a small difference in refractive index at the interface between the anti-reflective sheet  300  and the lens  400 . 
     Thus, as illustrated in  FIG. 9A , the reflected light LT 2  that occurs as the incident light LT 1  is reflected at the anti-reflective sheet  300  has a much smaller light quantity than the incident light LT 1 . 
     Further, the anti-reflective sheet  300  has indentations on the incident face  300 N. Thus, the difference in refractive index between air and the anti-reflective sheet  300  can be further reduced to continuously vary the refractive index. Reflection of external light on the anti-reflective sheet  300  can be suppressed this way. 
     3-4. Operation and Effects 
     Configured as above, the pit forming apparatus  31  shines a light beam on the anti-reflective sheet  300  of a material that forms a pit by a thermochemical reaction that occurs as a result of a local temperature increase in the vicinity of the focal point of an irradiated light beam of a predetermined light quantity. 
     The pit forming apparatus  31  forms pits of substantially the same volume in the flexible and thin, anti-reflective sheet  300  in such a manner that the pit density gradually decreases toward the transmission face  300 T away from the incident face  300 N. 
     Thus, the volume percentage of the pits with respect to the material of the anti-reflective sheet  300  gradually becomes smaller in the anti-reflective sheet  300  toward the transmission face  300 T away from the incident face  300 N. 
     The anti-reflective sheet  300  having pits is attached to the lens  400  in such a manner that the transmission face  300 T opposite the incident face  300 N is in contact with the lens  400 . 
     The anti-reflective sheet  300  is therefore able to continuously vary the refractive index over the range from the air side to the lens  400 , and thus there is no abrupt change in refractive index. Reflection of externally incident light on the anti-reflective sheet  300  can thus be suppressed. 
     Further, the anti-reflective sheet  300  has substantially the same refractive index as the lens  400 . This enables the anti-reflective sheet  300  to relieve the reflection of light that occurs because of the difference in refractive index between the anti-reflective sheet  300  and the lens  400  upon the incidence of external light on the lens  400  through the anti-reflective sheet  300 . 
     Further, because the anti-reflective sheet  300  of the present embodiment has a form of a sheet, the anti-reflective sheet  300  can be used to suppress reflection of external light by being attached to, for example, a lens of a material that does not allow for formation of the pits by irradiation of a light beam. 
     The anti-reflective sheet  300  also can be attached to suppress reflection of external light even when the lens  400  has a complex surface shape that makes the accurate focusing of a light beam difficult for the pit formation. 
     Further, because the pit forming apparatus  31  uses the integrated control unit  11  to control the light beam shone on the anti-reflective sheet  300 , the distribution density of the pits in the anti-reflective sheet  300  can be freely set. 
     The pit forming apparatus  31  is therefore able to set the extent of refractive index change with great freedom over the range from the surface of the anti-reflective sheet  300  to the lens  400  attached to the anti-reflective sheet  300 . 
     The anti-reflective sheet  300  of Third Embodiment also has advantages substantially the same as those described in conjunction with the lens substrate  100  of First Embodiment. 
     According to the foregoing configuration, the pit forming apparatus  31  forms pits of substantially the same volume in the anti-reflective sheet  300  under the control of the integrated control unit  11  in such a manner that the pit density gradually decreases toward the transmission face  300 T away from the incident face  300 N. The anti-reflective sheet  300  having the pits is attached to the lens  400  with the transmission face  300 T in contact with the lens  400 . In this way, the pit forming apparatus  31  can gradually vary the depth range refractive index of the anti-reflective sheet  300  from the refractive index of air to the refractive index of the material in a direction from the incident face  300 N to the transmission face  300 T, and can set the extent of refractive index change with great freedom. 
     4. Other Embodiments 
     The foregoing First Embodiment described varying the distribution density of the pits in the lens substrate  100  according to the distance from the incident face  100 N. In Second Embodiment, the volume of individual pits was varied according to the distance from the incident face  200 N. 
     However, the present invention is not limited to these, and the distribution density of pits and the volume of individual pits may be varied in combination as in, for example, a lens substrate  500  illustrated in  FIG. 10A , in which the distribution density of pits and the volume of individual pits vary layer to layer in the lens substrate  500 . 
     The foregoing First Embodiment described forming pits in such a manner that the pits gradually decrease in density across the layers toward inside the lens substrate  100  away from the incident face  100 N. 
     However, the present invention is not limited to this, and more than one layer with the same pit density may be formed as in, for example, a lens substrate  600  illustrated in  FIG. 10B . Likewise, more than one layer with the same pit volume may be formed in the configuration of Second Embodiment, though not illustrated. Furthermore, only a single pit layer may be formed in the vicinity of an incident face  700 N as in a lens substrate  700  illustrated in  FIG. 10C . This configuration is also possible in Second and Third Embodiments. 
     In short, the pits may be formed in any ways as long as the depth range refractive index of the lens substrate  100  gradually varies from a refractive index substantially the same as that of air, to a refractive index substantially the same as that of the material of the lens substrate  100 , in a direction from outside to inside the lens substrate  100 . 
     The foregoing First Embodiment described shining a light beam on the incident face  100 N of the lens substrate  100  to form indentations on the incident face  100 N of the lens substrate  100 . 
     However, the present invention is not limited to this, and the indentations may not be formed on the lens substrate surface, as in, for example, a lens substrate  800  illustrated in  FIG. 10D . In this way, the lens substrate  800  can suppress reflection of light under no influence of damages caused by external contact, or adhesion of liquid. This configuration is also possible in Second and Third Embodiments. 
     The foregoing Second Embodiment described adjusting the pit volume by varying the irradiation time of a light beam on the lens substrate  200 . 
     However, the present invention is not limited to this, and the pit volume may be adjusted by varying the intensity of the light beam shone on the lens substrate  200 , or by varying the irradiation time and the intensity of the light beam in combination. 
     The foregoing First Embodiment described the lens substrate  100  formed of an optical glass in which pits are formed as a result of a thermochemical reaction following a local temperature increase in the vicinity of the focal point of a light beam of a predetermined light quantity shone on the substrate. 
     However, the present invention is not limited to this, and the lens substrate  100  may be formed of an optical glass in which pits are formed as a result of photo irradiation of a light beam, in addition to a temperature increase in the vicinity of the focal point. 
     Instead of optical glass, optical crystal such as fluorite, quartz, silicon, and germanium, or plastic such as a polycarbonate resin may be used. 
     The pits are not necessarily required. For example, a photopolymerizable photopolymer may be used, and the refractive index in the vicinity of a focal point may be varied by causing a photopolymerization reaction and/or a photocrosslinking reaction in the vicinity of the focal point of a light beam. 
     In short, any material can be used as long as the material can vary its refractive index by undergoing state changes as a result of various types of reaction in the vicinity of a focal point upon irradiation of a light beam. This is also the case for Second and Third Embodiments. 
     The target of anti-reflection process is not necessarily limited to the lens, and may be, for example, a solar panel or a protection panel for displays. In short, the anti-reflection process can be applied to any object for which the surface reflection of light needs to be prevented while allowing for passage of incident light. 
     The foregoing First Embodiment described forming pits of substantially the same volume within a predetermined layer inside the lens substrate  100 . 
     However, the present invention is not limited to this, and the pits within the same layer may have different volumes to certain extent. However, the anti-reflection effect can be more uniformly obtained over a wide range on the XY plane when pits of the same volume are spread over the XY plane in the same layer ( FIGS. 3A to 3E ). 
     Further, in the foregoing First Embodiment, the depth range DR was described as being a range that includes a single pit layer distant apart from the incident face  100 N by a predetermined distance. 
     However, the present invention is not limited to this, and the depth range DR may be a range that includes a plurality of pits in a direction normal to the incident face  100 N. This configuration is also possible in Second and Third Embodiments. 
     Further, in the foregoing First Embodiment, the optically functional portion  100 L of the lens substrate  100  was described as being optically functional to transmit and condense incident parallel rays. 
     However, the present invention is not limited to this, and the optically functional portion  100 L may have various optical functions, for example, including transmitting and diverging incident parallel rays. Further, for example, the optical function may be simply the function to transmit incident light. This is also the case for Second Embodiment. 
     The foregoing embodiments described moving the focal point position of a light beam by lightly moving the objective lens  24 . 
     However, the present invention is not limited to this. For example, the light beam emitted by the laser diode  21  may be condensed by the objective lens  24  through an expander lens movable along the direction of the light path of the light beam, and the focal point position may be moved by varying the divergence angle of the incident light beam on the objective lens  24  by moving the expander lens. 
     In the foregoing Third Embodiment, the anti-reflective sheet  300  and the lens  400  were described as having substantially the same refractive index. 
     However, the present invention is not limited to this, and the anti-reflective sheet  300  and the lens  400  may have different refractive indices to certain extent. However, the amount of reflected light at the interface between the anti-reflective sheet  300  and the lens  400  becomes smaller as the difference in refractive index between the anti-reflective sheet  300  and the lens  400  becomes smaller. 
     The foregoing First Embodiment described controlling the objective lens  24  using the drive control unit  12 , and sequentially forming pits by moving the focal point position of a light beam in a direction normal to the incident face  100 N. 
     However, the present invention is not limited to this, and the objective lens  24  and the support unit  16  may be controlled together using the drive control unit  12  to move the focal point position of a light beam in a direction normal to the incident face  100 N. This configuration is also possible in Second Embodiment. 
     Further, the foregoing embodiments described the lens substrates  100  and  200 , and the anti-reflective sheet  300  provided as optical elements that include the pit forming portions  100 H,  200 H, and  300 H, respectively. 
     However, the present invention is not limited to this, and the optical element may be configured to include various forms of other pit forming portions. 
     The foregoing embodiments described the lens processing apparatus  1  and the pit forming apparatus  31  configured as processing apparatuses for reducing reflection that include the laser diode  21  (light source), the objective lens  24  (objective lens), the drive control unit  12  (moving unit), the integrated control unit  11  (control unit), and the signal processing unit  25  (control unit). 
     However, the present invention is not limited to this, and the lens processing apparatus  1  and the pit forming apparatus  31  may be configured as processing apparatuses for reducing reflection that include a light source, an objective lens, a moving unit, and a control unit of various other circuit configurations. 
     The present invention is usable for optical elements for which surface reflection of light needs to be prevented. 
     The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2009-195688 filed in the Japan Patent Office on Aug. 26, 2009, the entire contents of which is hereby incorporated by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.