Patent Publication Number: US-2017351156-A1

Title: Optical Device and Optical Device Manufacturing Method

Description:
TECHNICAL FIELD 
     The present invention relates to an optical device. 
     BACKGROUND ART 
     When configuring a system with an optical function, in many cases, a so-called spatial optical system in which optical devices having various optical functions are disposed, and in which light is controlled to be propagated through a space by these optical devices, is adopted as a form. On the other hand, in recent years, a technique for realizing a system with an optical function by forming various optical devices at the inside of a transparent board is studied. 
     As a method for forming an optical device at the inside of a transparent board, a change in the refractive index of a transparent material due to a nonlinear optical effect can be used. When a transparent board is irradiated with a short pulse laser, the chemical/physical structure of the transparent board is changed at a focal point of the laser beam, and the refractive index of the material is changed. This phenomenon is caused by the nonlinear optical effect, and the refractive index of the board is changed only at the focal point. Therefore, since the optical device can be disposed at an arbitrary position at the inside of the board and a three-dimensional optical system can be formed, and thus the size of the optical system can be reduced. Further, since the devices are integrated into the inside of one board, there is also an advantage in that the optical system is stable against disturbance such as vibration and contamination. 
     As techniques for realizing an optical function by forming a cavity at the inside of a transparent medium, there are the following PTL 1 to PTL 3. 
     CITATION LIST 
     Patent Literature 
     PTL 1: US 2009122407 A1 
     PTL 2: JP-A-2007-034004 
     PTL 3: JP-A-2003-131053 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the case of forming an optical device using a change in the refractive index by irradiation of a short pulse laser, there is a problem in that an amount of change in the refractive index is small. For example, in the case of a quartz glass which is often used as a material of an optical device, an amount of change in the refractive index by irradiation of a short pulse laser largely depends on a light irradiation condition, but is less than approximately 1%. For this reason, it is difficult to manufacture devices such as some optical devices used in a spatial optical system, specifically, lenses, which cause an optical function by a light refraction effect at an interface. 
     On the other hand, as a method for realizing an optical function using a small amount of change in the refractive index, there is a method using diffraction by a periodic structure. For example, the function of a lens can be realized by forming a concentric circular structure. However, in this method, it takes some time to form a concentric circular structure by laser processing. In addition, in order to obtain diffraction efficiency suitable for a practical use, a measure such as formation of a multilayer structure is required, and thus the time required for forming a device becomes further longer. Further, when a simple concentric circular structure is adopted, chromatic aberration increases because the focal length is inversely proportional to the wavelength. 
     It is also considered that an optical device is manufactured by forming an interface using etching of a transparent board such as a glass. In this case, the problem in that a difference in the refractive index is small in laser processing is solved. However, there is a restriction in which the interface should be formed from the outer surface of the board. In addition, processing for smoothing the etched surface is necessary. 
     In the technique described in PTL 1, an optical function is realized by sequentially disposing fine cavity structures having a substantially spherical shape. Thus, since the technique assumes that a plurality of cavity structures are formed at the inside of a transparent medium, the corresponding processing time is required according to the number of the cavity structures. 
     In the technique described in PTL 2, an optical function is realized by irregularly forming a plurality of flat cavities  5  at the inside of a denaturation region  4  (refer to abstract). Thus, the optical function depends on disposition of the denaturation region  4  and the number of the cavities  5 , and it is considered that the processing process becomes complicated or a corresponding processing time is necessary. 
     In PTL 3, a bubble (cavity) is formed at an inflection point of a waveguide, and the bubble is used as a reflection mirror. In this configuration, since a contact point between the bubble and the waveguide is important, the shape of the bubble is assumed to be a flat plate shape. Thus, it is not clearly mentioned that various optical functions are imparted by controlling the shape of the cavity. 
     The present invention has been made in consideration of the problems, and an object of the present invention is to provide a technique capable of easily manufacturing a desired optical device at the inside of a transparent board. 
     Solution to Problem 
     An optical device according to the present invention is manufactured by denaturing the vicinity of a hollow structure at the inside of a transparent board and deforming the shape of the hollow structure. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to easily manufacture an optical device at the inside of a transparent board in a short time. The objects, configurations, and effects other than those described above will be clarified from the description of the following embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram for explaining a difference in the types of optical systems. 
         FIG. 2  is a conceptual diagram for explaining an optical device according to a first embodiment and a manufacturing method of the optical device. 
         FIG. 3  is a diagram illustrating a configuration example of an optical device manufacturing apparatus  100  according to the first embodiment. 
         FIG. 4  is a timing chart for explaining the operation of the optical device manufacturing apparatus  100 . 
         FIG. 5  illustrates microscope photographs of a hollow structure  21 , and a titanium sapphire laser is used as a short pulse laser. 
         FIG. 6  is a microscope photograph in a case where the irradiation position of LASER 2  is closer to the irradiation position of LASER 1  compared to the case of  FIG. 5 . 
         FIG. 7  is a diagram illustrating another configuration example of an optical device manufacturing apparatus  100  according to a second embodiment. 
         FIG. 8  is a timing chart for explaining the operation of the optical device manufacturing apparatus  100  according to the second embodiment. 
         FIG. 9  is a flow chart explaining a procedure for manufacturing an optical device according to the second embodiment. 
         FIG. 10  is a diagram illustrating an example of the shape of the hollow structure  21  formed by the optical device manufacturing method according to the first and second embodiments. 
         FIG. 11  is a diagram illustrating the optical response of the hollow structure  21  illustrated in  FIG. 10( a ) . 
         FIG. 12  is a diagram illustrating the optical response of the hollow structure  21  illustrated in  FIG. 10( b ) . 
         FIG. 13  is a diagram illustrating an example in which a concave minor is formed by the hollow structure  21 . 
         FIG. 14  is a diagram illustrating a configuration example of an optical system using the optical device illustrated in  FIG. 10( b ) . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Optical Device In Related Art 
     In the following, in order to facilitate understanding of the present invention, first, an optical device in the related art and a manufacturing method thereof will be described, and thereafter an optical device according to the present invention and a manufacturing method thereof will be described. 
       FIG. 1  is a diagram for explaining a difference in the types of optical systems.  FIG. 1( a )  illustrates a configuration example of a spatial optical system, and  FIG. 1( b )  illustrates an example of an optical system formed at the inside of a transparent board. In a spatial optical system, in many cases, an optical device is implemented by fixing the optical device to a base using a fixing tool. In a process in which light emitted from a light source  12  is propagated through air and is reached to a measurement object  11 , the light is controlled by a beam splitter  13 , a mirror  14 , a lens  15 , and the like, and is detected by a detector  16 . On the other hand, in an optical system formed at the inside of a transparent board, an optical device is integrated into the inside of the transparent board, and light is propagated through a waveguide  17  formed at the inside of the board. 
     Specific examples of forming an optical device at the inside of a transparent board include the following examples: (a) an example for manufacturing a waveguide by providing a denaturation region in a linear shape; (b) an example of forming a diffraction type lens by forming a concentric circular structure; (c) an example of forming, at the inside of a transparent board, an interface between the transparent board and air, for example, forming a mirror by etching photosensitive glass, and using the refraction/reflection of light at the interface; and (d) an example of manufacturing a device for measuring the refractive index of a liquid by combining a bragg grating and a micro flow path. 
     In a method of forming a plurality of cavities at the inside of a transparent board and a method of forming an interface by etching, there are problems as described above. Thus, the present invention imparts a desired optical function to a cavity formed at the inside of the transparent board by changing the shape of the cavity. 
     First Embodiment 
     In an optical device manufacturing method according to a first embodiment of the present invention, (a) a hollow structure is formed at the inside of a transparent board by a short pulse laser with a pulse width of 1 ns or less, and (b) the interface shape of the hollow structure is deformed according to a spatial pattern by a separate laser for controlling the interface shape of the hollow structure. Thus, an optical device with a hollow structure having an arbitrary shape is manufactured. It is known that a hollow structure is produced by irradiating a transparent material such as a quartz glass with a high repetition-rate pulse laser having a repetition-rate frequency exceeding 1 MHz. The hollow structure formed by the above-mentioned method has a spherical shape when an irradiation condition is adjusted. 
       FIG. 2  is a conceptual diagram for explaining an optical device according to the first embodiment and a manufacturing method thereof. LASER 1  is a laser beam for forming a hollow structure at the inside of a transparent board  20 . LASER 2  is a laser beam for denaturing the physical properties of the inside of the transparent board  20 . An object lens LENS is disposed so as to condense the laser beams into the inside of the transparent board  20 . 
     First, a hollow structure  21  is formed at the inside of the transparent board  20  by the LASER 1 . Next, a denaturation region  22  is formed at a position at the inside of the transparent board  20  that is different from the position of the hollow structure  21 , by the LASER 2 . The hollow structure  21  is pressed by the denaturation region  22 , and is deformed. Here, the denaturation region means a region where the chemical/physical properties of the transparent board  20  are changed by irradiation of the LASER 2 . Although the type of the change depends on the type of the material of the transparent board  20  and the irradiation condition of the laser beam, for example, the denaturation region is a region where the material is once dissolved. Preferably, the denaturation region does not remain after the laser irradiation. However, in a case where the optical influence of the denaturation region is small, the denaturation region may remain. The hollow structure, which has initially a spherical shape, is deformed by irradiation of the LASER 2 , and is shaped into a desired shape. 
     In  FIG. 2 , although the LASER 1  and the LASER 2  are illustrated to be separated, the LASER 2  may be a laser beam branched from the LASER 1 . In addition, the LASER 2  is not necessarily separated from the LASER 1 , and in a case where irradiation of the same laser beam is performed at different timings, a laser beam in one initial irradiation may be used as the LASER 1  and a laser beam in the rest irradiation may be used as the LASER 2 . The irradiation of the LASER 2  may be performed at approximately the same time as the irradiation of the LASER 1 , or the irradiation of the LASER 2  may be performed after the irradiation of the LASER 1 . it is not necessary to control the shape of the hollow structure by one laser irradiation, and the shape of the hollow structure may be controlled in stages by irradiation of the LASER 2  in multiple times. The shape of the hollow structure may be controlled by changing the spatial pattern of the LASER 2  by a spatial light modulator or the like, for example, by a plurality of light spots. 
     In a manufacturing method of the optical device according to the first embodiment, three-dimensional processing by a nonlinear optical effect is used. Therefore, linear absorption of the laser beam by the transparent board  20  should be sufficiently small. At the wavelength of the laser beam forming the hollow structure, the absorption coefficient of the material of the transparent board  20  is equal to or less than 1 cm −1 . 
     The optical device using the hollow structure  21  according to the first embodiment functions basically by reflection or refraction of the light at the interface between the transparent board  20  and the hollow portion. In particular, the optical device functions as an optical device that changes a spatial pattern such as a light propagation direction and a light intensity distribution by using the phenomenon. Therefore, the function of the device is determined by the interface shape of the hollow structure  21 . As particularly important shapes, there are a shape having a spherical surface and a shape having a substantially flat surface (realized as a spherical surface having a very large radius of curvature). The spherical shape functions as a lens for the refracted/reflected light. The shape of the optical device is not limited to a shape having only one spherical surface or one substantially flat surface. For example, the shape of the optical device may be a shape in which one or more spherical surfaces are combined with one or more substantially flat surfaces, or may be an arbitrary shape realized as a set of substantially flat surfaces. 
       FIG. 3  is a diagram illustrating a configuration example of an optical device manufacturing apparatus  100  according to the first embodiment. Here, an example configuration in which the LASER 1  and the LASER 2  are the same beam is illustrated. The optical device manufacturing apparatus  100  includes an optical processing system ( 102  to  106 ) and a control device  101 . A short pulse laser  102  emits a laser beam  103 . An optical shutter  104  adjusts the irradiation time of the laser beam  103 . An attenuator  105  adjusts the power of the laser beam  103 . An objective lens  106  condenses the laser beam  103  into the inside of the transparent board  20 . An automatic stage  107  controls the position of the transparent board  20 . 
       FIG. 4  is a timing chart for explaining the operation of the optical device manufacturing apparatus  100 . The automatic stage  107  moves the transparent board  20  such that a position where the hollow structure  21  will be formed is irradiated with the LASER 1 . Next, the optical shutter  104  is opened, and irradiation of the LASER 1  is performed. Thus, the hollow structure  21  is formed. Next, the automatic stage  107  moves the position of the transparent board  20 . At this time, the attenuation rate of the optical power of the attenuator  105  may be simultaneously changed. After the position of the transparent board  20  is moved, the optical shutter  104  is opened again, and irradiation of the LASER 2  is performed. The denaturation region  22  is formed and the shape of the hollow structure  21  is changed, by the LASER 2 . 
       FIG. 5  illustrates microscope photographs of the hollow structure  21 . As a short pulse laser, a titanium sapphire laser is used. The pulse energy of the emitted laser beam is 24 nJ, and the repetition-rate frequency of the pulse is 76 MHz. As the transparent board  20 , a quartz glass is used. In the example illustrated in  FIG. 5 , a hollow structure  21   a  is formed by keeping the attenuation rate of the attenuator  105  constant, and by irradiating the transparent board  20  twice with laser beams having the same power, as the LASER 1  and the LASER 2 , respectively. Then, the shape of the hollow structure  21   a  is controlled by the denaturation region  22 . The irradiation time for both the LASER 1  and the LASER 2  is 100 ms. 
       FIG. 5( a )  illustrates a state where the hollow structure  21   a  is formed by irradiation of the LASER 1 .  FIG. 5( b )  illustrates a state where the shape of the hollow structure  21   a  is controlled by the LASER 2  after the hollow structure  21   a  is formed. As apparent from  FIG. 5 , the hollow structure  21   a,  which has a spherical shape in a state where the shape thereof is not controlled, is shaped into a hemispherical shape by the denaturation region  22  formed by irradiation of the LASER 2 . In the case of the irradiation conditions as described above, it is said that the denaturation region  22  is a region where heat is accumulated at the inside of the transparent board  20  by the laser irradiation and thus the board medium material is dissolved. 
     In the example illustrated in  FIG. 5 , since the irradiation condition of the LASER 1  is the same as that of the LASER 2 , a hollow structure  21   b  is formed by irradiation of the LASER 2 . When the hollow structure  21   a  is used as an optical device, the hollow structure  21   b  may interfere with the hollow structure  21   a  in some cases. In such a case, it is possible to eliminate the hollow structure  21   b  by an extension of the present invention. 
       FIG. 6  is a microscope photograph in a case where the irradiation position of the LASER 2  is closer to the irradiation position of the LASER 1  compared to the case of  FIG. 5 . In  FIG. 6 , it is understood that the hollow structure  21   a  formed by the LASER 1  is completely disappeared. According to this method, the hollow structure is transferred to a different position by sequentially eliminating unnecessary hollow structures, and thus it is possible to dispose the hollow structure at a position which does not hinder other optical functions. 
     In the example illustrated in  FIG. 5 , it is understood that the denaturation region  22  which is produced by dissolution of the board material due to the LASER 2  remains. The refractive index of the denaturation region  22  is slightly changed compared to the refractive index of a non-processing region, but the amount of the change is small. Thus, the influence of the refractive index of the denaturation region  22  on the optical response is small. In a case where, even though the change of the refractive index of the denaturation region  22  is small, the influence of the refractive index of the denaturation region  22  on the optical response becomes a problem, the influence due to the interface of the denaturation region  22  on the optical response can be relaxed, by guiding the light to the inside of the denaturation region  22  using the waveguide. 
     In the examples described in  FIGS. 5 and 6 , although the hollow structure  21   b  is produced when the denaturation region  22  is produced, only the denaturation region  22  may be produced by adjusting parameters of the LASER 2  such that the hollow structure  21   b  is not produced and performing irradiation of the LASER 2 . 
     Hereinafter, an advantage of the optical device manufacturing method described in the first embodiment will be described. In the case of forming a three-dimensional optical system at the inside of the transparent board  20 , preferably, the periphery of the formed optical device remains unchanged so as not to influence other optical devices. When forming a cavity by a femtosecond laser, since a nonlinear absorption effect of light is used, in a place other than the vicinity of the focus of light, there is no change. Therefore, compared to a method of cutting the transparent board  20  from the outside by using etching or the like, the other places of the transparent board  20  are unlikely to be influenced. Further, since the nonlinear absorption effect is used, an optical device can be formed at an arbitrary position at the inside of the transparent board  20 . 
     An example of forming a lens as an optical device is considered. In order to form a Fresnel lens using a change in the refractive index by laser irradiation, it is necessary to scan the laser spot many times in a concentric circular shape. On the other hand, in the optical device manufacturing method according to the first embodiment, an optical device is completely formed by several times of laser irradiation for forming the hollow structure  21  and controlling the shape of the hollow structure  21 . Therefore, it is possible to manufacture an optical device in a short time. 
     Second Embodiment 
       FIG. 7  is a diagram illustrating another configuration example of an optical device manufacturing apparatus  100  according to a second embodiment of the present invention. in the present configuration example, the degree of freedom in control is increased as compared with the configuration example illustrated in  FIG. 3 , by individually controlling the LASER 1  and the LASER 2 . 
     A short pulse laser  102  emits a laser beam  103 . An optical branch device  108  branches the laser beam  103  into a laser beam (LASER 1 ) indicated by a solid line and a laser beam (LASER 2 ) indicated by a broken line. Here, although the LASER 1  and the LASER 2  are generated by branching a single laser beam, laser beams emitted from two different lasers may be used. After the laser beam  103  is branched by the optical branch device  108 , an optical shutter  104  adjusts the irradiation time of the LASER 1 . An attenuator  105  adjusts the power of the LASER 1 . A mirror  109  reflects the LASER 2 . An optical shutter  110  adjusts the irradiation time of the An attenuator  111  adjusts the power of the LASER 2 . An irradiation timing control device  112  controls the irradiation timing compared with the pulse of the LASER 2 . The adjustment of the irradiation timing of the LASER 2  may be performed by the optical shutter  110 . However, when there is a small difference between the irradiation timing of the LASER 1  and the irradiation timing of the LASER 2 , preferably, for example, a delay line for adjusting the optical distance is used as the irradiation timing control device  112 . A spatial pattern control device  113  modulates the LASER 2  such that a desired optical pattern is formed on the transparent board  20 . As the spatial pattern control device  113 , for example, a spatial light modulator may be used. A mirror  114  reflects the LASER 2 . A multiplexer  115  multiplexes the LASER 1  and the LASER 2  (adjusts the irradiation position on the same axis) such that the LASER 1  and the LASER 2  travel in the same direction. An objective lens  106  condenses the multiplexed laser beams into the inside of the transparent board  20 . 
       FIG. 8  is a timing chart for explaining the operation of the optical device manufacturing apparatus  100  according to the second embodiment. An automatic stage  107  disposes the transparent board  20  such that a position where the hollow structure  21  will be formed is irradiated with the LASER 1 . Next, the optical shutter  104  is opened, and irradiation of the LASER 1  is performed. Thus, the hollow structure  21  is formed. Next, the optical shutter  110  is opened, and irradiation of the LASER 2  is performed. In  FIG. 8 , although irradiation of the LASER 2  is performed after irradiation of the LASER 1  is performed, irradiation of these two laser beams may be performed at the same time (at approximately the same time). The beam shape of the LASER 2  is shaped by the spatial pattern control device  113 , and a denaturation region  22  according to the shape is formed. The shape of the hollow structure  21  is changed by the denaturation region  22 . In  FIG. 8 , although irradiation of the LASER 2  is performed only once, a plurality of denaturation regions  22  may be formed by performing irradiation of the LASER 2  a plurality of times while changing the spatial pattern of the LASER  2 . 
       FIG. 9  is a flow chart explaining a procedure for manufacturing an optical device according to the second embodiment. First, the transparent board  20  is moved and disposed such that the laser beam is focused on a position where the optical device will be formed (S 11 ). Next, the hollow structure  21  is formed at the inside of the transparent board  20  by performing irradiation of the LASER 1  (S 12 ). Next, a spatial pattern of the LASER 2  is determined according to the shape of the hollow structure  21  to be finally formed (S 13 ). In a case where irradiation of the LASER 2  is performed a plurality of times, the order of irradiation is also determined in S 13 . Next, the spatial pattern for irradiation of the LASER 2  is input to the spatial pattern control device  113  (S 14 ). Thereafter, the optical shutter  110  is opened, and irradiation of the LASER 2  is performed (S 15 ). In a case where the shape of the hollow structure  21  becomes the target shape by the irradiation, the procedure is terminated, and in a case where the shape of the hollow structure  21  does not reach the target shape, the steps S 14  to S 15  are repeated (S 16 ). 
     Third Embodiment 
       FIG. 10  is a diagram illustrating an example of the shape of the hollow structure  21  formed by the optical device manufacturing method according to the first and second embodiments.  FIG. 10( a )  illustrates an example of the hollow structure  21  which is configured with a convex spherical surface and a substantially flat surface. The hollow structure  21  illustrated in  FIG. 10( a )  is further shaped, and thus, as illustrated in  FIG. 10( b ) , a hollow structure  21  which is surrounded by a substantially flat surface, a concave spherical surface, and convex spherical surfaces is formed. Alternatively, as illustrated in  FIG. 10( c ) , a hollow structure  21  surrounded by concavo-convex spherical surfaces can be formed.  FIG. 10( d )  illustrates an example in which a plurality (two in  FIG. 10 ) of concave spherical surfaces are formed by further processing the right surface of the hollow structure  21  illustrated in  FIG. 10( a ) . The hollow structure  21  and each spherical portion may be formed in a direction different from the direction illustrated in  FIG. 10 . The shape of the hollow structure  21  is not limited to the shape illustrated in  FIG. 10 , and other shapes may be adopted depending on the use. 
       FIG. 11  is a diagram illustrating the optical response of the hollow structure  21  illustrated in  FIG. 10( a ) .  FIG. 11( a )  illustrates the optical response of a general lens for comparison. A general lens that is formed of a transparent material and has a convex surface and a substantially flat surface functions as a convex lens, and has a function of condensing parallel light (light), for example. On the other hand, the optical device using the hollow structure  21  according to the present invention has a reversed refractive index as compared with a general optical device that is formed of a transparent material  30  and has the same shape. Thus, the optical device according to the present invention has a function different from that of the general optical device. For example, in the example illustrated in  FIG. 11 , the optical device according to the present invention functions as a so-called concave lens for diffusing parallel light. 
       FIG. 12  is a diagram illustrating the optical response of the hollow structure  21  illustrated in  FIG. 10( b ) . As illustrated in  FIG. 12 , the hollow structure  21  functions as a so-called convex lens for condensing parallel light (light). 
     The lens using the hollow structure  21  illustrated in  FIGS. 11 and 12  exhibits the same optical response regardless of the wavelength of light unless the incident light is dispersed by the transparent board  20 . Therefore, it is possible to realize a lens with small chromatic aberration by choosing a material with small dispersion as the board material. 
     According to the optical device manufacturing method of the present invention, it is also possible to form a reflection type device using total reflection at the interface of the board. As an example, assuming that the transparent board  20  is a quartz glass, the refractive index of the transparent board  20  is approximately 1.46. When the refractive index of the hollow structure  21  is set to 1, the total reflection critical angle is approximately 43°. In a case where an angle of the incident light with respect to the interface exceeds the critical angle all the time, in principle, an optical device with efficiency of 100% can be realized 
       FIG. 13  illustrates an example in which a concave mirror is formed by the hollow structure  21 . In the example illustrated in  FIG. 13 , a concave surface which is obliquely inclined with respect to the incident parallel light (light) is formed. The concave surface is formed such that the interface has an angle exceeding the critical angle with respect to the incident light all the time, and light is reflected at the interface. Since the interface is a concave surface, the incident parallel light is condensed to a certain point. The device can be used, for example, when coupling incident light to a waveguide which extends in a direction different from the incident direction. When an angle of the interface with respect to incident light decreases below the critical angle, efficiency of the device is reduced. However, in use in which reduction of efficiency is not a problem, the device may be used as a reflection type device. The device can also be used as a device such as a beam splitter by using the fact a part of the incident light is transmitted. 
     It is possible to form an optical device using an effect other than the effect in that a direction of a light beam is changed by total reflection at the interface. For example, in a case where a Fresnel rhomb-shaped structure is formed by providing a plurality of cavity structures, it is possible to form a broadband wavelength plate similar to the Fresnel rhomb. 
       FIG. 14  is a diagram illustrating a configuration example of an optical system using the optical device illustrated in  FIG. 10( b ) . In  FIG. 14 , when light is coupled from a light source to a waveguide  23 , the lens illustrated in  FIG. 10( b )  is used. For example, in a case where divergent light beams are emitted from the light source, it is necessary to collect the light beams and couple the light beams to the waveguide  23 . Assuming that the light source has a plurality of wavelengths, a case where light is coupled to a waveguide  23  capable of propagating the light is considered. In this case, preferably, the chromatic aberration of the lens is as small as possible. When a board material has small dispersion, the chromatic aberration can be suppressed. Thus, the lens formed by the manufacturing method according to the present invention is suitable for such a use. 
     Modification Example of Present Invention 
     The present invention is not limited to the above-described embodiments, and includes various modification examples. The above-described embodiments have been described in detail for a better understanding of the present invention, and are not necessarily limited to those including all the configurations described above. In addition, a part of the configuration of an embodiment can be replaced by the configuration of another embodiment, and the configuration of an embodiment can be added to the configuration of another embodiment. Further, in a part of the configuration of each embodiment, addition of another configuration, omission, substitution can be made. 
     In the above embodiments, it has been described that an optical function is realized by controlling the shape of a single hollow structure  21 . It is desirable that the size of the hollow structure  21  be sufficiently larger (preferably, 10 times or more) than the wavelength of light incident on the optical device. 
     In the above embodiments, an example in which the hollow structure  21  and the denaturation region  22  are respectively formed on a flat surface orthogonal to the irradiation axis of the laser beam is described. However, the hollow structure  21  and the denaturation region  22  may be formed at positions different from each other in a direction along the irradiation axis. Accordingly, it is possible to adjust the shape of the hollow structure  21  in a direction along the irradiation axis. 
     REFERENCE SIGNS LIST 
     
         
           20 : transparent board 
           21 : hollow structure 
           22 : denaturation region 
           100 : optical device manufacturing apparatus 
           101 : control device 
           102 : short pulse laser 
           103 : laser beam 
           104 : optical shutter 
           105 : attenuator 
           106 : objective lens 
           107 : automatic stage 
           108 : optical branch device 
           109 : mirror 
           110 : optical shutter 
           111  attenuator 
           112 : irradiation timing control device 
           113 : spatial pattern control device 
           114 : mirror 
           115 : multiplexer