JPH10239508A - Relief type diffraction optical element, optical system using it, and die for manufacturing relief type diffraction optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high intensity at a converging point and facilitate the manufacture of a diffraction optical element, by providing an unequally spaced grating pattern, providing a zone group of inclined cross section with a place different in the inclination of a relief face and a zone group of rectilinear cross section constant in the inclination, and including a zone largest in pitch in the former zone group. SOLUTION: This relief type diffraction optical element 1 is a diffraction lens converging parallel luminous flux of wavelength λ at focus distance (f), and one face of optical glass, for instance, of parallel plate shape with both faces polished in parallel is provided with a relief face with a relief pattern formed of an unequally spaced grating pattern 2 on concentric circle structure rotation-symmetric with respect to an optical axis. The relief face is provided with a first zone group showing inclined cross section with a place different in the inclination of the relief face in one zone, and a second zone group showing linear cross section in which the inclination of the relief face in one zone is constantly inclined, and is provided with cross section shape including a zone largest in pitch in the first zone group.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a relief type diffractive optical element having a predetermined structure on its surface, an optical system using the same, and a mold for manufacturing a relief type diffractive optical element.

[0002]

2. Description of the Related Art In recent years, attention has been paid to a relief type diffractive optical element such as a diffractive lens in response to a demand for miniaturization and weight reduction of an optical system. The application in various fields is expected from the point that it can have an aspherical action.

[0003] Here, the diffractive lens is a lens that realizes the function of a conventional refraction lens, for example, a spherical lens, a cylindrical lens, an anamorphic lens, and the like by using a diffraction action. A diffractive lens corresponding to a spherical lens having a lattice pattern and a diffractive lens corresponding to a cylindrical lens having a linear lattice pattern are known. Hereinafter, in this specification, each of the grooves of the grating pattern of the diffractive lens is referred to as a zone, and is counted as the first zone and the second zone in order from the optical axis of the lens toward the periphery. To

When a parallel light beam is condensed at one point by a lens, the phase shift function of the lens is, as shown in FIG. 9, φ (r) = − πr ² / (λf) (1) r: The distance from the optical axis is represented by λ: wavelength f: focal length. When this φ (r) is transformed into a 2π phase structure, the phase shift function φ _d (r) of the diffractive lens is given by the following equation (2). _{φ d (r) = φ (} r) + 2π (i-1) ··· (2) r i-1 <r <r i (i = 1,2,3, ---) This is the first-order diffracted light This is equivalent to optimizing the diffraction efficiency. Here, r _i is the outer radius of the i-th zone. The difference between the outer radii of adjacent zones is called a pitch.

As shown in FIG. 10, when a diffractive lens is realized by a relief structure, the height t (r) of the relief structure at a radius r is t (r) = t _g · [｛φ _d (r) / 2π｝ +1] (3) Here, t _g is the maximum groove depth of the relief, and t _g = λ / (n−1) (4).

The above equation (2) represents the phase shift function of a diffractive lens when a single diffractive lens focuses a parallel light beam on a single point. When a diffractive lens is used in combination with another optical element, for example, a refractive lens element, the phase shift function of the diffractive lens is generally represented by a high-order even-order polynomial.

[0007] As a conventional relief type diffractive optical element,
For example, as shown in JP-A-1-250902, the oblique sides of all the zones are formed in a curved shape corresponding to the phase shift function so that the original optical performance can be sufficiently exhibited. And, Mitsushi surgery contact Vol.26,
As shown in No. 3, pp. 208 to 212, there is one in which the oblique sides of all the zones are approximated by straight lines to improve the manufacturability.

As one of the methods for manufacturing a relief type diffractive optical element, there is a processing method using a mold. In this method, the mold has a shape in which the cross-sectional shape of the diffractive optical element to be manufactured is inverted, and this mold is pressed against a material such as glass or plastic to invert the shape of the mold and transfer or inject. The shape of the mold is inverted and transferred by a molding method or a photopolymer method. As such a mold for manufacturing a relief type diffractive optical element, for example, the above-mentioned optical technology contact Vol. 26, No. 3, p211 includes:
In the figure, the hypotenuses of all the zones are approximated by straight lines to improve the manufacturability of the mold.

[0009]

The cross-sectional shape of the relief type diffractive optical element is desirably a shape derived from a phase shift function as shown in FIG. 10, for example. In this case, the oblique side of each zone is generally a curve. However, forming the cross-sectional shape into a curved shape as shown in FIG. 7 has various problems in processing technology, processing time, labor for inspecting the manufactured product, and the like. For example, if the cross section is to be formed into an ideal curved shape by cutting, the cutting will be performed at the tip of the cutting edge of the cutting tool. It will be difficult to do.

For this reason, conventionally, as shown by a broken line in FIG. 11, the oblique side of the cross-sectional shape 4 derived from the phase shift function is often processed into a cross-sectional shape 3 approximated by a straight line as shown by a solid line. Done. This structure has great advantages in manufacturing and inspection. For example, when the cross-sectional shape is formed into a linear shape by cutting, machining can be performed with the main cutting edge of the cutting tool, so that machining data can be simplified,
There is an advantage that the surface roughness of the workpiece can be improved.
However, as described below, there is a problem that the optical performance is deteriorated when the oblique side portion is approximated by a straight line. In particular, when the number of zones is small, the effect is remarkable.

Here, in a diffractive lens that converges a parallel light beam having a wavelength λ at a focal length f, the intensity at the converging point when the cross-sectional shapes of all the zones are approximated by straight lines will be considered. FIG. 12 shows the intensity ratio calculated by the following equation (5) at the same wavelength and the same focal length with respect to the number of zones.

(Equation 1)

As is clear from FIG. 12, when the number of zones is large, there is almost no effect of approximating the oblique side of the cross-sectional shape with a straight line. However, as the number of zones decreases, the intensity at the converging point decreases. The drop is significant. This phenomenon is
For example, there is a problem that cannot be ignored when the number of zones through which a light beam passes is small, such as in a state where the aperture is stopped down by using a micro lens or an imaging optical system.

For this reason, when an optical system is configured using such a relief type diffractive optical element, when a light beam passes through only a small number of zones of the relief type diffractive optical element, the intensity decreases at the converging point, In other words, there is a problem that the condensed spot is spread, the MTF is reduced, and the resolution is reduced.

When a mold used for manufacturing a relief type diffractive optical element is manufactured, the sectional shape of the mold may have a curved shape corresponding to the phase shift function of the relief type diffractive optical element to be manufactured. desirable. However, since it is actually difficult to have a curved shape, the hypotenuse may be approximated by a straight line. In this case, the relief type diffractive optical element manufactured using the mold also has a shape in which the oblique side is approximated by a straight line, and the optical performance is deteriorated.

Further, when processing a mold with a lathe or the like, it is possible to make the cross-sectional shape into a curve by cutting using the cutting edge of a cutting tool, but generally, the workability of the mold material is poor. In cutting using the cutting edge of a cutting tool, it is difficult to finish the surface of a workpiece to a mirror surface. For this reason, when a relief type diffractive optical element is formed using the mold, even if the cross-sectional shape is formed to an almost ideal shape derived from the phase shift function, the surface roughness is large, so that the incident light flux In addition to the problem that scattering occurs in some parts and the light use efficiency is reduced, the problem that the scattered light flux becomes stray light and the optical performance is reduced is generated.

A first object of the present invention is to provide a relief type diffractive optical element which has a high intensity at a light condensing point and which is appropriately constructed so as to be easily manufactured. It is something to offer.

A second object of the present invention is to reduce the surface roughness of the relief type diffractive optical element to be manufactured, thereby effectively preventing the scattering of incident light and improving the light use efficiency. Producing a relief-type diffractive optical element appropriately configured so that a mold for producing a relief-type diffractive optical element capable of effectively preventing a decrease in optical performance due to scattering and having a high intensity at a focal point can be easily manufactured. To provide a type for

Further, a third object of the present invention is to reduce a decrease in intensity at a converging point when a light beam passes through only a small number of zones of a relief type diffractive optical element, thereby effectively preventing a decrease in resolution. An object of the present invention is to provide an optical system using a relief type diffractive optical element appropriately configured as possible.

[0019]

In order to achieve the first object, the present invention relates to a relief type diffractive optical element having a non-uniformly spaced grating pattern, in which a relief angle of a relief surface in one zone is different. A first zone group showing an inclined cross section having a first zone group, and a second zone group showing a straight cross section having a constant inclination angle of a relief surface in one zone, and the zone having the largest pitch is It has a cross-sectional shape included in the first zone group.

Further, in order to achieve the second object,
The present invention relates to a mold for manufacturing a relief type diffractive optical element having a non-uniform lattice pattern, a first zone group showing an inclined cross section having a portion where a tilt angle of a relief surface in one zone is different, A second zone group showing a straight-line cross section in which the inclination angle of the relief surface in one of the zones is constant, and the zone having the largest pitch has a cross-sectional shape included in the first zone group. It is a feature.

Further, in order to achieve the third object,
According to the present invention, an optical system includes the relief type diffractive optical element according to the first aspect.

Here, the influence of the cross-sectional shape of the zone of the relief type diffractive optical element on the intensity at the focal point will be described.
A description will be given of an example of a diffractive lens that collects a parallel light beam having a wavelength λ at a focal length f. According to the study by the inventor,
As shown below, when the number of zones is small, compared to the case where the hypotenuses of all the zones are approximated by straight lines, the center of the
It was found that forming only the second zone in a shape derived from the phase shift function dramatically improved the intensity at the focal point.

FIG. 13 shows a diffractive lens having a structure in which only the first zone is formed in a shape derived from the phase shift function and the hypotenuses of all other zones are approximated by straight lines. Shows the intensity ratio at the focal point with respect to the number of zones, calculated by

(Equation 2)

As is apparent from FIG. 13, there is almost no decrease in intensity at the light converging point due to approximation of a zone other than the first zone with a straight line. For example, when FIG. 12 and FIG. 13 are compared for the case where the number of zones is 10, when the oblique sides of all the zones are approximated by straight lines, the intensity ratio at the light condensing point is reduced to about 0.9. On the other hand, when the first zone is formed in a shape derived from the phase shift function, the intensity ratio at the focal point is approximately 1.0.

As described above, if the cross-sectional shape of the center zone is formed to have a shape derived from the phase shift function, it is possible to substantially prevent the intensity from decreasing at the converging point. That is, as can be seen from FIG. 11, the deviation from the ideal shape when the hypotenuse portion of the zone is approximated by a straight line, which corresponds to the error amount of the phase shift, is the largest in the central zone, and this degrades the overall optical performance. It is a major cause of lowering. Therefore, if the shape of the central zone is formed to be a shape derived from the phase shift function, it is possible to greatly improve the overall optical performance.

Therefore, in a relief type diffractive optical element having a non-uniformly spaced grating pattern, a zone having the largest pitch is defined by an inclined cross section having a portion where the inclination angle of the relief surface is different in the zone. If the shape is formed, it becomes possible to approximate an ideal shape. In this case, as already described with reference to FIGS. 12 and 13, the intensity at the converging point is improved as compared with the case where the hypotenuses of all the zones are approximated by straight lines. The effect increases. As a matter of course, for some of the zones other than the zone having the largest pitch, if the cross-sectional shapes of the zones are formed into inclined cross-sectional shapes having portions where the inclination angle of the relief surface is different in one zone, the light condensing is performed. The strength at the point will be further improved.

With this configuration, it is possible to realize a relief type diffractive optical element having a high intensity at the light-converging point regardless of the number of zones through which the light beam passes without greatly increasing the manufacturing burden. Become.

In one embodiment of the present invention, in the relief type diffractive optical element according to claim 1, the first zone group has a sectional shape derived from a phase shift function,
The second zone group has a shape obtained by approximating the oblique side of the cross-sectional shape derived from the phase shift function with a straight line.

As described above, in the relief-type diffractive optical element having the irregularly-spaced grating pattern, if the cross-sectional shape of the zone having the largest pitch is made to be the cross-sectional shape derived from the phase shift function, the configuration shown in FIGS. As described above, the intensity at the light condensing point is more improved than when the hypotenuses of all the zones are approximated by straight lines, and the effect is particularly large when the number of zones is small. In this case, too, it is a matter of course that, for some zones other than the zone having the largest pitch, if the cross-sectional shape is changed to a cross-sectional shape derived from the phase shift function, the intensity at the focal point can be further improved. Becomes possible.

With this configuration, it is possible to realize a relief type diffractive optical element having a higher intensity at the light condensing point regardless of the number of zones through which the light beam passes without greatly increasing the manufacturing burden.

Further, in one embodiment of the present invention, the relief type diffractive optical element is a diffractive lens, and the number of zones included in the first zone group is set to not more than half of the total number of zones.

As described above, in the relief type diffractive optical element having the irregularly-spaced grating pattern, if the cross-sectional shape of the zone having the largest pitch is made to be a cross-sectional shape derived from the phase shift function, the oblique sides of all the zones are formed. The intensity at the focal point can be improved as compared with the case of approximating with a straight line.Furthermore, for some zones other than the zone having the largest pitch, their cross-sectional shapes are changed to the cross-sectional shape derived from the phase shift function. Then, it is possible to further improve the intensity at the focal point. However, as the number of zones in the cross-sectional shape derived from the phase shift function increases, the burden on manufacturing also increases. Therefore, the number of zones included in the first zone group is at most half of the total number of zones. It is desirable to do the following.

According to this structure, it is possible to realize a relief type diffractive optical element having a high intensity at the converging point regardless of the number of zones through which the light beam passes, without greatly increasing the manufacturing burden. .

Further, in one embodiment of the present invention, the first zone group includes only a central zone.

That is, in a diffractive lens, generally, 1
Since the pitch of the center zone, which is the second zone, is the largest, if only the zone having the largest pitch has a shape derived from the phase shift function, it is possible to obtain a practically sufficient effect as described above. .

Further, in one embodiment of the present invention, the above-mentioned relief surface has a shape in which a surface having a refracting action is superimposed on a surface having a refracting action so as to simultaneously perform both a diffracting action and a refracting action. Form.

That is, irregularly spaced grating patterns are formed on a spherical surface, and the power required for the relief type diffractive optical element as a whole is shared between the spherical surface, which is a refraction surface, and the irregularly spaced grating pattern, which is a diffraction surface. In such a case, the power shared by the unequally-spaced lattice patterns tends to be small, and the pitch of each zone tends to be large. As described above, when the pitch is increased, the deviation from the ideal shape due to approximation of the hypotenuse portion of the zone with a straight line increases, so if the cross-sectional shape of the zone having the largest pitch is ideally formed as described above, The effect is great.

According to the mold of the second aspect, the relief type diffractive optical element manufactured by using the mold has a zone having the largest pitch in a portion where the inclination angle of the relief surface in one zone is different. The other zones have straight cross-sectional shapes in which the inclination angle of the relief surface in one zone is constant. Therefore, since the zone having the largest pitch can be made closer to the ideal cross-sectional shape, as described above,
It is possible to manufacture a relief type diffractive optical element having a high intensity at the focal point regardless of the number of zones through which the light beam passes. Also, needless to say, in some zones other than the zone having the largest pitch, if their cross-sectional shapes are formed to have a cross-sectional shape derived from the phase shift function, the intensity at the converging point can be further improved. It becomes possible.

In one embodiment of the mold according to the present invention, the first zone group has a sectional shape derived from the phase shift function, and the second zone group has a sectional shape derived from the phase shift function. The oblique side of the shape is configured to have a shape approximated by a straight line.

With such a configuration, a relief type diffractive optical element having a zone having the largest pitch having a cross-sectional shape derived from the phase shift function and other zones having a shape in which the oblique side of the cross-sectional shape is approximated by a straight line is obtained. can get. Such a relief type diffractive optical element, as described above,
The intensity at the focal point increases regardless of the number of zones through which the light beam passes. Also, needless to say, for some zones other than the zone with the largest pitch, if their cross-sectional shapes are made to be cross-sectional shapes derived from the phase shift function,
As a manufactured relief type diffractive optical element, it is possible to further improve the intensity at the focal point.

Further, in one embodiment of the mold according to the present invention, the mold is used for producing a diffractive lens, and the number of zones included in the first zone group is set to be equal to or less than half of the total number of zones.

By using such a mold, it is possible to obtain a diffractive lens having a cross-sectional shape in which only the center zone is derived from the phase shift function and a cross-sectional shape in which the other side is approximated by a straight line. Here, since the diffractive lens generally has the largest pitch of the center zone, the diffractive lens manufactured using this mold has a condensing point regardless of the number of zones through which the light beam passes, as described above. Is high in strength. Also, needless to say, in some zones other than the zone having the largest pitch, if their cross-sectional shapes are made to be cross-sectional shapes derived from the phase shift function, the intensity at the focal point can be further improved. This is possible, but this also increases the manufacturing burden of the mold. For this reason, it is desirable that the number of zones included in the first zone group is not more than half of the total number of zones at the maximum.

Further, in one embodiment of the mold according to the present invention, the first zone group includes only a central zone.

By using such a mold, it is possible to obtain a diffractive lens having a cross-sectional shape in which only the central zone is derived from the phase shift function and the other zones have cross-sectional shapes in which the oblique sides are approximated by straight lines. Here, the diffractive lens generally has the largest pitch of the center zone as described above,
If only the center zone having the largest pitch has a cross-sectional shape derived from the phase shift function, a practically sufficient effect can be obtained.

Further, in one embodiment of the mold according to the present invention, the above-mentioned relief surface is provided with a surface having a diffractive action so as to be able to perform both a diffractive action and a refracting action at the same time. It is formed in an overlapping shape.

That is, a relief type diffractive optical element in which unequally spaced grating patterns are formed on a spherical surface, and the required power as a whole is shared between the spherical surface as a refraction surface and the unevenly spaced grating pattern as a diffraction surface. In the case of manufacturing, there is a tendency that the power shared by the unequally-spaced grid pattern becomes small and the pitch of each zone becomes large. As described above, when the pitch increases, the deviation from the ideal shape due to approximation of the hypotenuse portion of the zone with a straight line increases. Therefore, similarly to the above-described relief type diffractive optical element, the cross section of the zone having the largest pitch If the shape is ideally formed, the effect is increased.

According to the optical system of the third aspect, in the relief type diffractive optical element included in the optical system, the sectional shape of the zone having the largest pitch is a shape derived from the phase shift function. Since a high intensity is obtained at the focal point regardless of the number of zones through which the light beam passes, it is possible to effectively prevent the occurrence of flare and a reduction in resolution of the entire optical system.

In one embodiment of the optical system according to the present invention,
Further, it has an aperture stop having a variable aperture area.

By providing the aperture stop in this manner, it is possible to control the brightness and the depth of focus of the optical system, and to reduce the aperture area of the aperture stop, thereby providing a relief type diffractive optical element. Even if the number of zones through which the luminous flux passes decreases, high intensity can be obtained at the converging point as described above, so that it is possible to effectively prevent the occurrence of flare and a decrease in resolution of the entire optical system. .

Further, in one embodiment of the present invention, an imaging optical system for an endoscope is constituted by using the above-mentioned relief type diffractive optical element.

For example, in a medical endoscope, as the diameter of the insertion section is reduced, the outer diameter of the imaging optical system provided at the distal end is also reduced. Therefore, if such an endoscope imaging optical system is configured by using the above-described relief type diffractive optical element, even if the number of zones of the relief type diffractive optical element through which the light beam passes is small, the light is focused at the converging point. Since high strength is obtained, it is possible to effectively prevent the occurrence of flare and the reduction in resolution.

[0052]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 and FIG. 2 are diagrams for explaining a first embodiment of the present invention. The relief type diffractive optical element 1 is a diffractive lens that converges a parallel light beam having a wavelength λ at a focal length f. It has a relief surface on which a relief pattern composed of unequally spaced lattice patterns 2 having a symmetrical concentric structure is formed. It should be noted that the drawings of the relief type diffractive optical element including FIG. 1 are conceptual diagrams, and do not accurately show the actual shape.

FIG. 2 is an enlarged view near the optical axis of the relief type diffractive optical element 1 shown in FIG. In the irregularly-spaced grating pattern 2, the diffraction efficiency is optimized for the first-order diffracted light, and the maximum groove depth t _g of each zone at that time is t _g = λ.
/ (N-1). Here, n is the refractive index for the wavelength λ of the substrate material of the relief type diffractive optical element 1. In FIG. 2, a solid line is a cross-sectional shape 5 in this embodiment, and a broken line is a cross-sectional shape 6 derived from a phase shift function. The first central zone 7 is formed in a cross-sectional shape 6 derived from the phase shift function. In FIG. 2,
Although the cross-sectional shape 6 derived from the phase shift function is slightly shifted for the sake of clarity, the cross-sectional shape 5 and the cross-sectional shape 6 derived from the phase shift function coincide in the central zone 7. The other zones have a shape in which the oblique side is approximated by one straight line.

In a diffractive lens, the center zone generally has the largest pitch. Therefore, if only the zone having the largest pitch has a cross-sectional shape derived from the phase shift function as in this embodiment, as described above, it is easy to manufacture, and the hypotenuses of all the zones are approximated by straight lines. The intensity at the focal point can be increased as compared with the case. In particular, by adopting such a structure, even when the number of zones is small, or when the number of zones as a whole of the diffractive optical element is large, but the number of zones through which the light beam actually passes is small, the light beam at the light-converging point A significant decrease in strength can be effectively prevented.

In the embodiment shown in FIG. 1, the case where the substrate type of the relief type diffractive optical element 1 is a parallel plate has been described. For example, as shown in FIG. The same effect can be obtained even if a non-uniform grid pattern is formed. That is, in the case of a diffractive lens having a non-uniform lattice pattern on a spherical surface, the power required for the entire diffractive lens is designed to be shared between the spherical surface as the refracting surface and the non-uniform lattice pattern as the diffractive surface. Then, the power shared by the unequally-spaced lattice patterns tends to decrease, and the pitch of each zone tends to increase. Therefore, in such a case, if the first zone at the center is formed in an ideal shape, the intensity at the converging point can be increased, and the effect is large. In addition, the unequally-spaced lattice pattern 2 having the configuration shown in FIG. 1 can be effectively applied to a case where the lattice pattern 2 is formed not only on one side of the substrate but also on both sides.

Further, the zones having the cross-sectional shape derived from the phase shift function can be not only the first zone 7 at the center but also, for example, the first and second zones from the center. With such a configuration, the performance can be optically improved as compared with the case where only the first central zone 7 is formed in an ideal shape. However, practically, if only the first zone at the center is formed into a shape derived from the phase shift function, a sufficient effect can be obtained.

In FIG. 1, the unequally-spaced grid pattern 2
Has been optimized for the first-order diffracted light, but may be optimized for other-order light. In this case, assuming that the diffraction order to be optimized is m, t _g = mλ / (n−1).

Further, the above irregularly-spaced grid pattern 2
The configuration is not limited to the transmission type, but applies to the reflection type as well.
can do. However, in the case of the reflection type, the maximum groove depth
T _gIs t_g= Mλ / 2 (m: reflection diffraction order).

The irregularly-spaced lattice pattern 2 is not limited to a concentric circle, but may be formed into a linear pattern corresponding to a cylindrical lens or an elliptical pattern. Further, the substrate material constituting the relief type diffractive optical element 1 is:
Not limited to optical glass, plastics, optical crystals, metals and the like can also be used.

FIG. 4 is a diagram for explaining a second embodiment of the present invention. This embodiment shows a mold 10 for manufacturing a relief type diffractive optical element used for manufacturing a diffractive lens having a convex action. The mold 10 is provided with an unequally-spaced lattice pattern 11 having a shape obtained by inverting the unequally-spaced lattice pattern of a diffraction lens to be manufactured, on one surface of a parallel plate-shaped mold material made of, for example, WC (tungsten carbide). And the other surface is polished smoothly. The unequally-spaced lattice pattern 11 is concentric, and its cross section is formed so that the diffractive lens to be manufactured has a groove depth and shape optimized for primary diffracted light.

The zone at the center of the unequally spaced grating pattern 11 has a shape obtained by inverting the shape derived from the phase shift function of the diffractive lens to be manufactured, and the other zones have shapes obtained by approximating the oblique sides with straight lines. The shape is inverted. In addition,
When the material processed by the mold 10 contracts in the processing step, the mold 10 is formed so as to correct the contraction.

The mold 1 for producing this relief type diffractive optical element
According to 0, only the center zone having the largest pitch has a shape obtained by inverting the cross-sectional shape derived from the phase shift function, so that it can be easily manufactured. When a diffractive lens is manufactured using the mold 10 for manufacturing a relief type diffractive optical element, the center zone has an ideal cross-sectional shape derived from the phase shift function, and the other zones have oblique sides. Since a diffractive lens having a shape approximated by a straight line can be obtained, a diffractive lens having good optical characteristics similar to that described in the first embodiment can be manufactured.

The material of the mold 10 for manufacturing the relief type diffractive optical element is not limited to the above-described WC, and other mold materials, for example, SiC (silicon carbide), NiP, and the like can be used. In particular, when glass is used as the material of the relief type diffractive optical element, W is used as the material of the mold 10.
It is preferable to use C and SiC, and when plastic is used as the material of the relief type diffractive optical element, it is preferable to use NiP as the material of the mold 10.
Also, the mold for manufacturing the relief type diffractive optical element described above is:
The present invention is not limited to the manufacture of a relief type diffractive optical element having a convex action, but may be configured for the manufacture of a relief type diffractive optical element having a concave action. Further, the irregularly-spaced lattice pattern 11 is not limited to a concentric shape, and may be a linear pattern or an elliptical pattern according to a relief type diffractive optical element to be manufactured.

In FIG. 4, the irregularly-spaced lattice pattern 11 is formed on a parallel plate-shaped substrate. However, as shown in FIG. 5, for example, as shown in FIG. Can obtain the same effect. That is, as described above, in the case of a diffractive lens having a non-uniform lattice pattern on a spherical surface, the power required for the entire diffractive lens is defined as a spherical surface which is a refraction surface and a non-uniform lattice pattern as a diffraction surface. If they are designed to share the power, the power shared by the unequally-spaced lattice patterns tends to be small, and the pitch of each zone tends to be large. Therefore,
When such a diffractive lens is manufactured, if the first zone at the center of the mold is formed in an ideal shape, the intensity of the obtained diffractive lens at the focal point can be increased. The effect is great.

FIG. 6 is a view for explaining a third embodiment of the present invention, and schematically shows an imaging optical system such as a camera including a relief type diffractive optical element. The imaging optical system includes a stop 12, a refractive lens 13, a diffraction lens 14 as a relief type diffraction optical element, and an image plane 15. The diffractive lens 14 has a non-equidistant lattice pattern formed on the surface on the stop 12 side, and its cross-sectional shape is such that the first zone at the center is derived from the phase shift function as described in the first embodiment. The other zones have a curved shape, and the other zones have a cross-sectional shape in which the hypotenuse is approximated by a straight line.

In this optical system, when the amount of light incident from an object (not shown) increases and the diameter of the aperture 12 is reduced accordingly, the number of zones through which the light flux of the diffractive lens 14 passes decreases. In this case, if a diffractive lens having a shape in which the oblique sides of all the zones are approximated by a straight line as described in the conventional example is used, the intensity at the focal point is greatly reduced due to the decrease in the number of zones. become. However, in this embodiment, the diffractive lens 14 having the structure described in the first embodiment, that is, the one having the zone having the largest pitch formed into a shape derived from the phase shift function is used, Even if the number of zones of the diffraction grating decreases, it is possible to effectively prevent a significant decrease in intensity at the focal point, and thus to effectively prevent a decrease in resolution in the imaging optical system.

Although FIG. 6 shows an optical system in which the refracting lens 13 and the diffractive lens 14 are combined, the optical system can be constructed without using the refracting lens 13. Further, an optical system that does not use the stop 12 can be configured. When the diaphragm 12 is not used, it is preferable that the number of zones is small, for example, about 30 or less, as the diffractive lens 14 in terms of manufacturing, optical characteristics, and the like.
Further, the optical system including the relief type diffractive optical element is not limited to the transmission type described above, but may be configured to be a reflection type.

FIG. 7 is a view for explaining a fourth embodiment of the present invention, and shows the entire configuration of an endoscope apparatus including a so-called rigid endoscope of a direct-view type as an endoscope. The endoscope device 20 includes an endoscope 28 having an insertion section 22, a camera 24, a monitor 25, a light guide cable 26, and a light source device 27. Tip 21 of insertion section 22
, An imaging optical system, an illumination optical system, a relay optical system, and the like are arranged. An eyepiece optical system (not shown) is arranged on the base 23 of the endoscope 28, and after the eyepiece optical system,
A camera 24 as an imaging unit is attached. Here, the base 23 and the camera 24 of the endoscope 28 are configured as an integral type or a detachable type. Also,
The subject imaged by the camera 24 is finally
Is displayed so as to be observable to an observer as an endoscope image.

In this embodiment, the tip 2 of the insertion section 22
The imaging optical system arranged in 1 is configured using a relief type diffractive optical element. That is, as shown in the sectional view of FIG. 8, a front negative lens 32 is provided on the end surface of the outer cylinder 31, and a light beam incident through the front negative lens 32 is connected to an image plane 34 via a relief type diffractive optical element 33. Make it image.
The relief type diffractive optical element 33 has an aspheric surface on the front negative lens 32 side, an unequally-spaced lattice pattern 35 on a plane on the image plane 34 side, and an ideal center zone whose center zone is derived from a phase shift function. The other zones have a cross-sectional shape, and the other zones are formed so as to have a cross-sectional shape in which the hypotenuse portion is approximated by a straight line. Since the illumination optical system and the relay optical system can be realized by known techniques, their illustration and description are omitted here.

In the above configuration, the outer diameter of the tip portion 21 is
Although it depends on the observation target, it may be about several millimeters when it is thin. In such a case, the relief type diffractive optical element 3
The outer diameter of 3 is inevitably small, and the total number of zones may be several tens or less. However, as in this embodiment, a relief zone having a non-equidistant lattice pattern 35 in which the center zone has an ideal cross-sectional shape derived from the phase shift function and the other zones have a cross-sectional shape in which the hypotenuse portion is approximated by a straight line. If the imaging optical system is configured by using the diffractive optical element 33, even when the number of all zones of the relief type diffractive optical element 33 is small, it is possible to effectively prevent the decrease in intensity at the converging point as described above. As a result, a good image can be obtained.

In this embodiment, the relief type diffractive optical element 33 is not limited to the one in which the unequally spaced grating pattern 35 is formed on a plane, but may be the one formed on a spherical surface or another curved surface. it can. This embodiment can be effectively applied not only to a direct-view type rigid endoscope but also to an imaging optical system of a flexible endoscope or a perspective type endoscope.

Additional Items 1. The relief type diffractive optical element according to claim 1,
The first zone group has a cross-sectional shape derived from a phase shift function, and the second zone group has a shape in which a hypotenuse portion of the cross-sectional shape derived from the phase shift function is approximated by a straight line. Relief type diffractive optical element. 2. 2. The relief type diffractive optical element according to claim 1, wherein the relief type diffractive optical element is a diffractive lens, and the number of zones included in the first zone group is equal to or less than half of the total number of zones. A relief type diffractive optical element characterized by the following. 3. The relief type diffractive optical element according to claim 1, wherein the first zone group includes only a central zone. 4. The relief type diffractive optical element according to any one of claims 1 to 3, wherein the relief surface is refracted to a surface having a diffractive action so as to perform both a diffractive action and a refraction action at the same time. A relief type diffractive optical element characterized in that surfaces having functions are superposed. 5. 3. A mold for manufacturing a relief type diffractive optical element according to claim 2, wherein said first zone group has a sectional shape derived from a phase shift function, and said second zone group has a sectional shape derived from a phase shift function. A mold for manufacturing a relief-type diffractive optical element, wherein the mold has a shape in which a hypotenuse portion of the shape is approximated by a straight line. 6. The mold for manufacturing a relief type diffractive optical element according to claim 2 or 5, wherein the number of zones included in the first zone group is equal to or less than half of the total number of zones. A mold for manufacturing a relief type diffractive optical element, characterized in that it is provided. 7. The relief type diffractive optical element according to any one of claims 2 and 5, wherein the first zone group includes only a central zone. Mold for manufacturing optical elements. 8. The mold for manufacturing a relief-type diffractive optical element according to any one of claims 2 and 5 to 6, wherein the relief surface has a diffractive action so as to simultaneously perform both a diffractive action and a refraction action. A mold for manufacturing a relief-type diffractive optical element, characterized in that a surface having a refracting action is superimposed on a surface to be formed. 9. An optical system comprising the relief type diffractive optical element according to any one of additional items 1 to 4. 10. In the optical system according to claim 3 or claim 9,
An optical system having an aperture stop with a variable aperture area. 11. An imaging optical system for an endoscope, comprising the relief type diffractive optical element according to claim 1.

[0073]

According to the relief type diffractive optical element of the first aspect, the unequally-spaced grating pattern includes a zone having the largest pitch and a slope having a portion where the tilt angle of the relief surface in one zone is different. And the second zone group having a straight cross-sectional shape in which the inclination angle of the relief surface in one zone is inclined at a constant angle. High strength and can be easily manufactured.

According to the mold for manufacturing a relief type diffractive optical element according to the second aspect, the unevenly-spaced grating pattern includes a zone having the largest pitch and a portion having a different inclination angle of a relief surface in one zone. Since the first zone group having the inclined cross-sectional shape and the second zone group having the straight cross-sectional shape in which the inclination angle of the relief surface in one zone is inclined at a constant level, the mold is formed. In addition to being able to easily manufacture itself, as a relief type diffractive optical element manufactured using the same, it is possible to reduce the surface roughness of the surface, thereby effectively preventing scattering of incident light and improving light use efficiency. In addition, it is possible to effectively prevent the optical performance from deteriorating due to scattering, and to increase the intensity at the focal point.

According to the optical system of the third aspect, the first aspect.
Since the relief type diffractive optical element described is used, a decrease in intensity at a focal point when a light beam passes through only a small number of zones of the relief type diffractive optical element can be reduced, and thus a decrease in resolution can be effectively prevented. .

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a first embodiment of the present invention.

FIG. 2 is a partially enlarged view of FIG.

FIG. 3 is a diagram showing a modification of the first embodiment.

FIG. 4 is a diagram for explaining a second embodiment of the present invention.

FIG. 5 is a diagram showing a modification of the second embodiment.

FIG. 6 is a diagram for explaining a third embodiment of the present invention.

FIG. 7 is a diagram for explaining a fourth embodiment.

8 is a diagram showing a configuration of a main part of a distal end portion of the insertion section shown in FIG.

FIG. 9 is a diagram illustrating a phase shift function of a diffraction surface when a parallel light beam is condensed at one point by a diffraction lens.

FIG. 10 is a diagram showing the relationship between the radius of the relief type diffractive lens derived from the phase shift function and the height of the relief structure.

FIG. 11 is a view for explaining a conventional relief type diffractive optical element.

FIG. 12 is a diagram showing an intensity ratio at a light-converging point with respect to the number of zones in a conventional relief-type diffractive optical element.

FIG. 13 is a diagram showing an intensity ratio at a light converging point with respect to the number of zones in the relief type diffractive optical element according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Relief type diffractive optical element 2 Irregularly-spaced grating pattern 3,5 Cross-sectional shape of diffraction grating 4,6 Cross-sectional shape derived from phase shift function 7 First zone at center 10 Mold for manufacturing relief-type diffractive optical element 11 Equally spaced grating pattern 12 Aperture 13 Refractive lens 14 Diffractive lens 15 Image plane 20 Endoscope device 21 Tip 22 Insertion unit 23 Base 24 Camera 25 Monitor 26 Light guide cable 27 Light source device 28 Endoscope 31 Outer cylinder 32 Forward negative Lens 33 Relief type diffractive optical element 34 Image plane 35 Irregularly spaced grating pattern

Claims (3)

[Claims] 1. A relief type diffractive optical element having unequally spaced grating patterns, wherein: a first zone group showing an inclined cross section having a portion where a tilt angle of a relief surface in one zone is different; And a second zone group showing a linear cross section in which the inclination angle of the relief surface is constant. The zone having the largest pitch has a cross-sectional shape included in the first zone group. Relief type diffractive optical element. 2. A mold for manufacturing a relief type diffractive optical element having a non-uniform lattice pattern, comprising: a first zone group showing an inclined cross section having a portion where a tilt angle of a relief surface in one zone is different; A second zone group showing a linear cross section in which the inclination angle of the relief surface in one zone is inclined at a constant angle, and a zone having the largest pitch has a cross-sectional shape included in the first zone group. A mold for manufacturing a relief type diffractive optical element, characterized by comprising: 3. An optical system comprising the relief type diffractive optical element according to claim 1.