JP4380593B2 - Fresnel lens - Google Patents

Description

  In the present invention, a plurality of annular zones are set on the disc-shaped optical substrate having light transmittance, and each annular zone pitch is gradually narrowed from the center toward the outer peripheral side. In the Fresnel lens in which each triangular diffraction grating is formed, each triangular diffraction grating has a bottom surface whose base length is set in accordance with each annular zone pitch, and an inclined surface that is inclined toward the outer peripheral side facing the bottom surface. And a second diffraction grating part having an inclined surface facing the bottom side and inclined toward the inner peripheral side, with respect to each annular pitch of the first and second diffraction grating parts. The occupancy ratio is set to be substantially constant, and the height of each triangular diffraction grating is varied for each annular region so that each annular region has the required light transmittance with the base as the height reference. By adjusting the light transmittance for incident light for each annular zone, It relates Fresnel lens configured so as to be able to equalize the light intensity distribution at the peripheral portion and the outer peripheral portion.

  Recently, Fresnel lenses, which are a type of diffractive optical element, can be used in optical pickups for optical disk devices and optical systems for optical communication devices because of the fact that optical aberrations can be set to small values when combined with aspherical lenses. It is used a lot.

As an example of this type of Fresnel lens, a Fresnel lens having a right-angled triangular diffraction grating is manufactured by heat compression molding with a mold using a glass material, or by injection molding with a mold using a transparent resin material. In this case, a technical idea of manufacturing a die for a Fresnel lens by cutting with a diamond tool is disclosed (Non-Patent Document 1).
Optical Technology Contact Vol. 26, No3 (1988), P208-212.

14 (a) and 14 (b) are a plan view and a longitudinal sectional view showing a Fresnel lens having a right-angled triangular diffraction grating as a conventional example,
FIG. 15 is a longitudinal sectional view schematically showing a state in which when the conventional Fresnel lens shown in FIG. 14 is molded by a mold, the mold for the Fresnel lens is cut by a diamond tool,
FIG. 16 is a diagram schematically showing a state in which a conventional Fresnel lens is irradiated with a laser beam having a high light intensity at the central portion and a low light intensity at the outer peripheral portion.

  As shown in FIGS. 14A and 14B, the conventional Fresnel lens 100 is formed on a disc-like optical substrate (Optical Base Plate) OBP using a light-transmitting glass material or transparent resin material. In addition, a plurality of ring zone regions (Ring Zone) RZ draw a ring-shaped concentric circle around the center 0, and each ring zone pitch is gradually narrowed from the center 0 toward the outer periphery, A right-angled triangular diffraction grating 101 is formed in each annular zone RZ. At this time, the diffraction grating 101 described above includes a base 101a virtually set on the side on which the laser light L, which is incident light, is incident on the optical substrate OBP, and an inclined surface that faces the base 101a and becomes an incident light diffraction surface. It is formed in a right triangle shape by being surrounded by 101b and a vertical surface 101c that is a phase turn-up point surface.

  The lens function is provided by using the diffraction effect of each diffraction grating 101 formed in each annular zone RZ.

  By the way, when the conventional Fresnel lens 100 described above is used as a diffractive optical element of a pickup of an optical disc device (not shown), for example, downsizing as the element and beam power of laser light incident on the Fresnel lens 100 are performed. Both high diffraction efficiency and high diffraction efficiency are required to make effective use of.

  In order to realize these requirements, as described above, the conventional Fresnel lens 100 is formed by heat compression molding with a mold using a glass material or by injection molding with a mold using a transparent resin material. At this time, as shown in FIG. 15, the Fresnel lens mold 150 is formed on the base 150a virtually set on the metal mold base material by the diamond tool 151, and the base 150a. It is fabricated by cutting into a right triangle surrounded by the opposed inclined surface 150b and the vertical surface 150c.

  Specifically, as shown in FIG. 15, while rotating the metal mold base material for the Fresnel lens, the diamond bit 151a attached to the tip of the diamond tool 151 is moved to form a right triangle triangular diffraction grating pattern. By the method of linearly approximating the phase distribution of the diffraction grating 101 {FIG. 14 (b)}, advantages such as shortening the machining time and maintaining a good surface roughness on the cut surface can be obtained.

  Here, in the mold 150 for the Fresnel lens, the phase shape for one annular zone is a substantially right triangle, and corresponds to the inclined surface 101b {FIG. 15B} that is the incident light diffraction surface of the Fresnel lens 100. The inclined surface 150b is substantially linear connecting the highest cutting height to the lowest cutting height at the annular zone pitch, and is a vertical surface 101c {FIG. 15 (b)} serving as a phase turning point surface of the Fresnel lens 100. The corresponding vertical surface 150c has a shape that rises substantially perpendicular to the horizontal direction of the mold.

  When the diamond cutting tool 151a attached to the tip of the diamond tool 151 is parallel to the cutting surface, the cutting surface becomes a mirror surface. Therefore, the diamond cutting tool 151a is inclined and the inclined surface 150b has a predetermined inclination angle. On the other hand, the die 150 for the Fresnel lens is manufactured by raising the diamond bit 151a substantially vertically and cutting the vertical surface 150c substantially vertically.

  Thereafter, the above-described conventional Fresnel lens 100 is molded from a glass material or a transparent resin material by using the manufactured Fresnel lens mold 150.

  Incidentally, a semiconductor laser (not shown) used for a light source such as an optical pickup has a light intensity distribution. As shown in FIG. 16, a laser beam L emitted from a semiconductor laser is emitted to a conventional Fresnel lens 100. Is converted into parallel light by a collimator lens (not shown), and generally, the central portion of the light beam of the laser light L having a substantially circular cross-sectional area when sectioned at an appropriate position has a high light intensity. (The light amount is large) and the light flux at the outer peripheral portion tends to have low light intensity (small light amount). When the light beam of the laser beam L having such a non-uniform light intensity distribution (light quantity distribution) is incident on the conventional Fresnel lens 100 and condensed, the light intensity at the outer periphery of the lens is compared with the central part. Since the lens becomes weak, it becomes a phenomenon equivalent to a state where the aperture of the lens is reduced. For this reason, the numerical aperture (NA) of the lens is lowered, and the phenomenon that the condensing spot becomes larger than that of the original lens design occurs, which causes a problem of deterioration of optical characteristics.

  Therefore, in the present invention, the Fresnel lens that condenses the laser light has a function of adjusting the light intensity distribution (light amount distribution) of the laser light source in addition to the condensing function, and uniformly adjusts the incident light to the Fresnel lens. Thus, there is a demand for a Fresnel lens having a structure in which the light amount distribution at the center portion and the outer peripheral portion of the Fresnel lens is made uniform, the above-described reduction in NA is suppressed, and deterioration of the focused spot can be prevented.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and the invention according to claim 1 is that the plurality of annular zones are formed on the optical substrate having light transmittance, and each annular zone pitch is gradually changed from the center toward the outer peripheral side. In a Fresnel lens that is set narrowly to each other and each triangular diffraction grating is formed in each annular region,
Each of the triangular diffraction gratings has a base on which a base length is set in accordance with each ring pitch on a side on which incident light is incident on the optical substrate, and a side on which the incident light transmitted through the base is emitted. A first diffraction grating portion having an inclined surface inclined toward the outer peripheral side and diffracting a specific-order diffracted light with respect to the incident light and condensing it at a predetermined condensing position; and before passing through the base The incident light is emitted on the side from which the incident light exits and is inclined toward the inner peripheral side, and the incident light is diffracted to a diffraction order different from that of the first diffraction grating portion to generate diffracted light other than the specific order. A second diffraction grating portion for adjusting the light transmittance with respect to the incident light,
The occupation ratio of each of the first and second diffraction grating portions to each of the annular zone pitches is set to be substantially constant, and the height of each of the triangular diffraction gratings is set to each of the annular zone regions with the base as a height reference. Is a Fresnel lens characterized in that it is varied for each of the annular zones so as to obtain the required light transmittance.

The invention according to claim 2 is the Fresnel lens according to claim 1,
The triangular diffraction grating formed in the annular region at the center of the lens on the optical substrate is a Fresnel lens having only the first diffraction grating.

  According to the Fresnel lens according to claim 1, in particular, each triangular diffraction grating formed in each annular zone set by gradually narrowing each annular zone pitch from the center toward the outer peripheral side on the optical substrate, A base having a base length set in accordance with each annular pitch, a first diffraction grating portion having an inclined surface facing the base and inclined toward the outer peripheral side, and facing the base toward the inner peripheral side And a second diffraction grating portion having an inclined surface that is inclined, and the occupying ratio of each of the first and second diffraction grating portions to each annular pitch is set to be substantially constant, and each triangle is defined with the base as the height reference. Since the height of the shape diffraction grating is varied for each annular zone so that each annular zone has a required light transmittance, when the Fresnel lens according to the present invention is applied to, for example, an optical pickup, Light intensity distribution on incident light entering the Fresnel lens Even, uniformly corrected light amount distribution with respect to the incident light, it is possible monkey condenses the incident light at a predetermined focusing position, thereby to obtain a focused spot not deteriorated. Furthermore, the Fresnel lens according to the present invention not only makes the light intensity distribution of incident light uniform, but also sets an arbitrary light transmittance according to the characteristics of the optical system to be combined, and an arbitrary light transmitted light amount distribution. Obtainable.

  Further, according to the Fresnel lens of claim 2, the triangular diffraction grating formed in the annular zone at the center of the lens on the optical substrate has only the first diffraction grating portion. When producing a mold for a Fresnel lens, the triangular diffraction grating at the center of the lens can be easily cut.

  Hereinafter, an example of a Fresnel lens according to the present invention will be described in detail with reference to FIGS.

  The Fresnel lens according to the present invention is applied in combination with, for example, an objective lens having an aspheric surface attached to an optical pickup of an optical disk device (not shown). When a light beam having a non-uniform light intensity distribution (light amount distribution) emitted from a laser light source in the optical pickup is incident on the Fresnel lens, the light beam is corrected to a light beam having a uniform light intensity distribution (light amount distribution) by the Fresnel lens. And it is comprised so that it can radiate | emit to the objective lens side.

1 (a) and 1 (b) are a top view, a longitudinal sectional view, showing a Fresnel lens according to the present invention,
FIG. 2 is a diagram for explaining the optical path of the optical system when the Fresnel lens according to the present invention is used;
FIG. 3 is a diagram showing the light intensity distribution and light transmittance of the optical system corresponding to positions X1 to X3 in FIG. 2 when the Fresnel lens according to the present invention is used.

  As shown in FIGS. 1A and 1B, a Fresnel lens 10 according to the present invention includes a plurality of rings on a disc-like optical substrate (Optical Base Plate) OBP using a light-transmitting glass substrate. A band zone (Ring Zone) RZ draws a ring-shaped concentric circle with the center 0 as the center, and each ring zone pitch is gradually narrowed from the center 0 toward the outer peripheral side.

  Further, each triangular diffraction grating 11 is formed in each annular zone RZ of the Fresnel lens 10 from the center 0 toward the outer peripheral side according to each annular zone pitch.

  Each triangular diffraction grating 11 formed in each annular zone RZ of the Fresnel lens 10 has a base 11a virtually set on the side on which the laser light L that is incident on the optical substrate OBP is incident. The first diffraction grating portion 11b is formed with an inclined surface inclined toward the outer peripheral side on the side from which the length is set in accordance with each annular zone pitch and the laser light L transmitted through the base 11a is emitted. In addition, the second diffraction grating portion 11c is formed with an inclined surface inclined toward the inner peripheral side on the side from which the laser light L transmitted through the bottom 11a is emitted.

  In FIGS. 1 (a) and 1 (b), only the first diffraction grating portion 11b is formed in the annular zone at the center of the lens so as to face the base 11a. This will be described later. .

  At this time, the first diffraction grating portion 11b in the triangular diffraction grating 11 diffracts, for example, a first-order diffracted light as a specific-order diffracted light with respect to incident light that has entered the annular zone RZ and has a predetermined focusing position. A lens condensing function for condensing light at (focal position) is provided. That is, the angle formed by the base 11a in the triangular diffraction grating 11 and the first diffraction grating portion 11b increases from the inner circumference side to the outer circumference side in accordance with each annular zone pitch. Since the incident angle of the laser beam L incident in parallel to the grating portion 11b varies depending on each first diffraction grating portion 11b, the refraction angle varies accordingly, and the laser beam L is condensed at a predetermined condensing position (focal position). Can be made.

  On the other hand, the second diffraction grating portion 11c in the triangular diffraction grating 11 diffracts incident light to a diffraction order different from that of the first diffraction grating portion 11b to generate diffracted light other than the specific order (other than the first order diffracted light), And it has the effect | action which adjusts the light transmittance with respect to incident light.

  Accordingly, the first diffraction grating portion 11b in the triangular diffraction grating 11 serves as a first-order diffracted light diffraction surface, while the second diffraction grating portion 11c serves as a diffraction surface for diffracted light other than the first-order diffracted light and a phase folding point surface. .

Furthermore, each triangular diffraction grating 11 formed in each ring-shaped zone region RZ of the Fresnel lens 10, first, all the zones occupied ratio of each ring-shaped zone pitch Rp _n of the second diffraction grating portion 11b, 11c The height H _n (H ₀ , H ₁ , H ₂ , H ₃ ,... From the base 11a to the vertex where the first and second diffraction grating portions 11b, 11c intersect is set with respect to the region RZ. ..., H _n , However, n is a positive integer greater than or equal to 0). In other words, the height H _n of each triangular diffraction grating 11 is set to the light transmittance required by each annular zone RZ with the base 11a as a height reference. The height _Hn is gradually increased from the inner peripheral side toward the outer peripheral side, which will be described in detail later.

Here, in the Fresnel lens 10, the plurality of annular zones RZ are arranged in order from the center 0 of the optical substrate OBP toward the outer peripheral side RZ ₀ , RZ ₁ , RZ ₂ , RZ ₃ ,..., RZ _n (where n : A positive integer greater than or equal to 0), first, a lens radius (annular radius) R _n corresponding to the n-th annular region RZ _n with the center 0 of the optical substrate OBP as a center is expressed by the following equation 1 It can be obtained more.

In this number 1, R _n is the n-th lens radius, lambda is the wavelength of the incident light, the wavelength lambda is using a laser beam of 0.4μm as the incident light. Further, f1 is a focal length of the first-order diffracted light by the first diffraction grating portion 11b in the triangular diffraction grating 11, and is set to f1 = 20 mm in this embodiment. Further, d is the diffraction order. In this embodiment, d = 1 because it is intended for the first-order diffracted light by the first diffraction grating portion 11b.

Therefore, centering on the center 0 of the optical substrate OBP, RZ ₀ , RZ ₁ , RZ ₂ , RZ ₃ ,..., RZ _n corresponding to the lens radii R ₀ , R ₁ , R ₂ , R ₃ ,. _n is determined.

Next, the annular zone pitch Rp ₀ of the annular zone RZ ₀ including the center 0 of the optical substrate OBP can be obtained from the following formula 2.

Then, to the exclusion of the case of n = 0 as described above, n-th ring zone pitch Rp _n can be determined from the number 3 below.

Therefore, from Equations 2 and 3, only the annular zone RZ _{0 at the} center of the lens is set to be circular with the annular zone pitch Rp ₀ as the radius, but the annular zones RZ ₁ , RZ ₂ , RZ ₃ ,. , RZ _n is zonal pitch _{Rp 1, Rp 2, Rp 3} , ......, is set in a ring shape at the pitch of Rp _n.

When the maximum value of n corresponding to the maximum value of the lens radius of the Fresnel lens 10 is set to 140, for example, λ = 0.4 μm, f1 = 20 mm, d while sequentially changing n in the range of 0 to 140. = 1 is substituted into Equation 1, and the lens radius R _n is calculated. Then, according to Equations 2 and 3, for example, the lens radius R _n corresponding to the annular zone RZ _n includes the center 0 of the optical substrate OBP. it 0μm vicinity _{(RZ 0), 500μm (RZ} 15), 1000μm (RZ 62), when obtaining each ring-shaped zone pitch Rp _n in 1500μm located lens outermost portion _{(RZ 140), 126.5μm (Rp} 0) _{, 16.1μm (Rp 15), 8.0μm} (Rp 62), 5.4μm (Rp 140) is obtained, hereinafter, the values of the resulting lens radius _{R n} and annular pitch Rp _n where And use are described.

In the case of designing the Fresnel lens 10 of the embodiment, it may be obtained a lens radius _{R n} and annular pitch Rp _n corresponding to all annular areas _RZ n (n = _0~140).

  Next, when the Fresnel lens 10 according to the present invention having the above-described configuration is used, as shown in FIG. 2, the position X1 is set to a position corresponding to the laser light L as incident light, and the position X2 is set to the Fresnel lens 10. And the position X3 is a position corresponding to the emitted light, and further, the position X4 is the condensing position of the emitted light, the laser light L emitted from a laser light source (not shown) is collimated. The parallel light of the laser light L, which has a substantially circular cross-sectional shape when converted into parallel light by a lens (not shown) and cross-sectioned at an appropriate position, is incident from one surface of the Fresnel lens 10, and the other surface. When the light is emitted after being diffracted by the first and second diffraction grating portions 11b and 11c in the triangular diffraction grating 11 formed in each of the plurality of annular zones RZ, in particular, the first diffraction grating portion 11 The first-order diffracted light diffracted by the second diffraction grating portion 11c is focused on a predetermined condensing position corresponding to the focal length f1 on the optical axis K, and diffracted light other than the first-order diffracted light is It is diffracted to a diffraction position different from the one diffraction grating portion 11b.

  Here, as indicated by the dotted line in FIG. 3, when the parallel light of the laser light L having a substantially circular cross section is incident on the Fresnel lens 10, the light intensity of the incident light beam at the position X1 shown in FIG. Assuming that the distribution is symmetric about the optical axis K, the light intensity at the center radius of 500 μm, 1000 μm, and 1500 μm is 97.degree. It has decreased to 5%, 90.0%, and 77.5%, and has a convex light intensity distribution characteristic in which the central portion protrudes in a convex shape.

  As described above, when the incident light flux having the convex light intensity distribution in the central portion is condensed by the conventional Fresnel lens 100 as described above with reference to FIG. 16, the light amount in the outer peripheral portion of the lens is compared with the central portion. As a result, the phenomenon becomes equivalent to a state where the aperture of the lens is reduced, the numerical aperture (NA) of the lens is lowered, and the condensed spot becomes larger than the original lens design. Wake up and optical properties are degraded.

  In order to solve this problem, in the light transmittance distribution of the light beam on the Fresnel lens 10 at the position X2 in FIG. 2, the lens radii are 0 μm, 500 μm, and 1000 μm as shown by the solid line in FIG. , 1500 μm, the required light transmittance is set to 77.5%, 79.5%, 86.1%, and 100%, respectively, that is, with respect to the convex light intensity distribution characteristic at the position X1. Inverted and reversely corrected so as to have a concave light transmittance distribution characteristic in which the central portion is recessed in a concave shape at the position X2.

  With this setting, when the incident light is transmitted through the Fresnel lens 10 and emitted, the light intensity distribution of the emitted light at the position X3 shown in FIG. 2 is as shown in FIG. 3 using the one-dot chain line. And the Fresnel lens transmittance, resulting in a flat.

  As a result, the amount of light at the outer peripheral portion of the Fresnel lens 10 is substantially equal to the central portion, which is equivalent to a state in which the aperture of the lens does not change, the numerical aperture (NA) of the lens does not change, and the condensing spot is originally Compared with the lens design of this type, the phenomenon of becoming larger does not occur.

  However, when the light intensity distribution of the emitted light becomes flat, the total amount of light decreases. However, if a laser light source with a larger amount of light such as a high-intensity laser is used, the decrease in the amount of light can be prevented.

Next, in the Fresnel lens 10 according to the present invention, each triangular diffraction grating 11 is placed in each annular zone pitch Rp in each annular region RZ (RZ ₀ , RZ ₁ , RZ ₂ , RZ ₃ ,..., RZ _n ). _n (Rp ₀ , Rp ₁ , Rp ₂ , Rp ₃ ,..., Rp _n ) are formed to be inclined toward the outer peripheral side and the inner peripheral side when formed from the center 0 toward the outer peripheral side. the second diffraction grating portion 11b, and occupation ratio for each ring-shaped zone pitch Rp _n and 11c on set substantially constant, the height H _n of each triangular diffraction grating 11 the base 11a as a height reference, each wheel A concave light transmittance at a position X2 as shown by a solid line in FIG. 3 on the Fresnel lens 10 is varied for each annular region RZ so that the belt regions RZ each have a required light transmittance. Structure to compensate for the distribution and For method will be described with reference to FIGS. 4 to 13.

FIG. 4 is an enlarged view for explaining the height of a triangular diffraction grating formed in one annular zone in the Fresnel lens according to the present invention, and (a) shows a triangular diffraction grating. A case of a right triangle is shown, (b) is a diagram showing a case where the maximum cutting angle of the diamond tool is set to, for example, 75 ° with respect to the second diffraction grating portion 11c in the triangular diffraction grating,
FIG. 5 is a diagram showing the relationship between the height of a triangular diffraction grating formed in one annular zone and the first-order diffracted light diffraction efficiency in the Fresnel lens according to the present invention;
FIG. 6 is a diagram showing the height of a triangular diffraction grating within an annular pitch of 5.4 μm at a lens radius of 1500 μm in the Fresnel lens according to the present invention,
FIG. 7 is a diagram showing the diffracted light intensity of a triangular diffraction grating within an annular pitch of 5.4 μm at a lens radius of 1500 μm in the Fresnel lens according to the present invention;
FIG. 8 is a diagram showing the height of a triangular diffraction grating in an annular pitch of 8.0 μm at a lens radius of 1000 μm in the Fresnel lens according to the present invention,
FIG. 9 is a diagram showing the diffracted light intensity of a triangular diffraction grating within an annular pitch of 8.0 μm at a lens radius of 1000 μm in the Fresnel lens according to the present invention;
FIG. 10 is a diagram showing the height of a triangular diffraction grating in an annular pitch of 16.1 μm at a lens radius of 500 μm in the Fresnel lens according to the present invention;
FIG. 11 is a diagram showing the diffracted light intensity of a triangular diffraction grating in an annular pitch of 16.1 μm at a lens radius of 500 μm in the Fresnel lens according to the present invention;
FIG. 12 is a diagram showing the height of a triangular diffraction grating within an annular pitch of 126.5 μm in the vicinity of a lens radius of 0 μm in the Fresnel lens according to the present invention;
FIG. 13 is a diagram showing the diffracted light intensity of the triangular diffraction grating within the annular zone pitch of 126.5 μm near the lens radius of 0 μm in the Fresnel lens according to the present invention.

First, as shown in FIG. 4 (a), the horizontal axis shows the zonal pitch Rp _n, when showing the phase difference λ on the vertical axis, one of the ring-shaped zone pitch Rp _n triangular diffraction formed in Assuming that the lattice 11 is a right triangle, the height H from the base 11a to the vertex of the right triangle can be generally obtained from the following equation (4).

  In Equation 4, H is the height of the right-angled triangular diffraction grating 11, λ is the wavelength of the light source, d is the diffraction order integer of 1 or more, and k is the refractive index of the Fresnel lens 10.

  Since the first diffraction grating portion 11b in the right-angled triangular diffraction grating (11) uses a diffraction grating having a first-order diffraction structure, λ is 0.40 μm, d is 1, and k is 1.46. Then, the height H of the right triangular diffraction grating (11) is 0.869 μm, and the height H of the right triangular diffraction grating (11) has a phase difference of one wavelength λ of incident light. In addition, the diffraction efficiency of the first-order diffracted light of the right-angled triangular diffraction grating (11) is 100%.

  By the way, as described above, when the Fresnel lens 10 is molded with a mold using a glass material, when the mold for the Fresnel lens is manufactured, the processing limit from the tip shape of the diamond tool, or at the time of molding. In consideration of die cutting, the maximum cutting angle of the diamond tool may be set to, for example, 75 ° with the angle of the 90 ° portion of the right triangle being smaller than 90 ° as shown in FIG. 4B.

In this case, the angle formed by the base 11a and the second diffraction grating portion 11c in the triangular diffraction grating 11 is 75 °, and the height H _n of the triangular diffraction grating 11 at which the maximum diffraction efficiency is obtained at this time is the base The height from 11a to the apex where the first and second diffraction grating portions 11b and 11c intersect, in other words, the height of the triangular diffraction grating 11 with the base 11a as the height reference, and the height at this time H _n is naturally lower than the height H of the right triangle. At this time, when the cutting angle of the diamond tool with respect to the second diffraction grating portion 11c in the triangular diffraction grating 11 is reduced, the height H _n of the triangular diffraction grating 11 is reduced.

Further, when the value of the zonal pitch Rp _n for each ring zones RZ _n was set to 1.00 is normalized, the diffraction grating (11) one of the wheels in the right triangle shown in FIG. 4 (a) while only the first diffraction grating portion 11b in the band pitch Rp _n is occupying a first diffraction grating portion 11b in FIG. 4 (b) to indicate triangular diffraction grating 11 a ring-shaped zone in the pitch Rp _n in The second diffraction grating portion 11c occupies each at an occupation ratio of (1.00-α): α, and at this time, α is set to a predetermined value within 0 to 0.1.

Further, in FIG. 4 (b), in a triangular-like diffraction grating 11 formed on one annular in pitch Rp _n, first diffraction grating having an inclined surface opposed to the bottom 11a is inclined toward the outer peripheral side The unit 11b has a function of diffracting only the first-order diffracted light with respect to the incident light and condensing the first-order diffracted light at a predetermined condensing position on the optical axis K (FIG. 2). On the other hand, the second diffraction grating portion 11c having an inclined surface that faces the bottom side 11a and is inclined toward the inner peripheral side diffracts the diffracted light other than the first-order diffracted light with respect to the incident light to diffract the first diffraction grating 11. It has a function of diffracting to different diffraction positions.

Here, in the case where the annular zone pitch is 5.4 μm at the lens radius of 1500 μm at the outermost peripheral portion of the lens that requires the greatest light transmittance in the Fresnel lens 10 (FIG. 1), the right triangle shown in FIG. In the diffraction grating (11), as described above, the first-order diffracted light diffraction efficiency of 100% is obtained. However, as shown in FIG. 4 (b), the second diffraction grating portion 11c in the triangular diffraction grating 11 is used. When the maximum cutting angle of the diamond tool is set to 75 °, for example, and the triangular diffraction grating 11 is formed, the maximum light transmittance at the lens outermost periphery of the Fresnel lens 10 (FIG. 1) decreases, but the Fresnel lens 10 ( The target light transmission amount can be made uniform to the light amount distribution with respect to FIG. 1). At this time, the height H _n of the triangular diffraction grating 11 is necessarily calculated to be 0.833 μm, And 5.4 Occupancy ratio of the first diffraction grating portion 11b relative zonal pitch of m is 0.959, occupation ratio of the second diffraction grating portion 11c is calculated as 0.041, i.e., alpha is 0.041.

  At this time, the first-order diffracted light diffraction efficiency in the annular zone RZ is generally calculated by Fourier transforming the phase distribution of the triangular diffraction grating 11, and the annular zone pitch 5. When the maximum cutting angle of the diamond tool is set to, for example, 75 ° in the case of 4 μm, the first-order diffracted light diffraction efficiency is 91.7%.

  Therefore, as described above, the required light transmittance is 77.5%, 79.5%, 86.1%, and 100% with respect to the Fresnel lens 10 when the lens radius is 0 μm, 500 μm, 1000 μm, and 1500 μm, respectively. However, in consideration of the ease of mold manufacture and the ease of die removal, when the maximum cutting angle of the diamond tool is set to, for example, 75 ° in the case of the above-described lens pitch of 5.4 μm at a radius of 1500 μm. The obtained first-order diffracted light diffraction efficiency of 91.7% is set as a reference correction value, and even when the lens radius is 0 μm, 500 μm, or 1000 μm, the correction is performed based on the case where the lens radius is 1500 μm, that is, the lens radius is 0 μm, The correction value is 0.917 for the required light transmittance of 77.5%, 79.5%, 86.1%, and 100% at 500 μm, 1000 μm, and 1500 μm. The light that is the target for the Fresnel lens 10 (FIG. 1), even though the required corrected light transmittances are 71.0%, 72.9%, 78.9%, and 91.7%, respectively. It is possible to make the transmission amount uniform in the light amount distribution.

Furthermore, the first and second diffraction grating portions 11b in the annular zone pitch obtained when the maximum cutting angle of the diamond tool is set to 75 °, for example, in the case of the annular zone pitch of 5.4 μm at the lens radius of 1500 μm, The height H from the base 11a in the triangular diffraction grating 11 to the vertex where the first and second diffraction grating portions 11b and 11c intersect with the occupation ratio (0.959: 0.041) of 11c fixed. _n, in other words, the height H _n of the triangular diffraction grating 11, so that the light transmittance of the corrected respective ring zones RZ requests respectively (correction light transmittance) the base 11a as a height reference It is made variable for each annular zone RZ.

Accordingly, FIG. 5 shows a triangular diffraction grating in a state where the occupation ratio (0.959: 0.041) of the first and second diffraction grating portions 11b and 11c within the annular zone pitch is fixed. It shows a first order diffracted light diffraction efficiency when changing the height H _n of 11.

In FIG. 5, the horizontal axis indicates the relative value of the height H _n of the triangular diffraction grating 11, the maximum value 1.00 corresponds to 0.833 μm, and the relative value ranges from 0.65 to 1.00. Since it is changed, it is substantially changed from 0.541 μm to 0.833 μm. At this time, the maximum cutting angle of the diamond tool is 75.0 ° when the relative value is 1.00, and the cutting angle is 67.7 ° when the relative value is 0.65. On the other hand, the vertical axis indicates the first-order diffracted light diffraction efficiency (%), and the light intensity in the first-order diffracted light is calculated by Fourier transforming the phase distribution of the triangular diffraction grating 11.

From FIG. 5, the relative value of the height H _n of the triangular diffraction grating 11 is 0.65, 0.80,
When changed to 0.90 and 1.00, the first-order diffracted light diffraction efficiency is 61.3%,
80.6%, 88.7%, 91.7%, and the height H _n of the triangular diffraction grating 11
It can be seen that the diffraction efficiency of the first-order diffracted light can be controlled continuously when V is changed.

Then, as shown in FIG. 5, by having the relationship of the height H _n and 1-order diffracted light diffraction efficiency of the triangular diffraction grating 11 as a table, on the contrary, first order diffracted light diffraction efficiency, i.e. necessary correction of the Fresnel lens 10 it can be determined the height H _n of the triangular diffraction grating 11 corresponding to the light transmittance.

It will be sequentially described specific example for obtaining the height H _n of the triangular diffraction grating 11 in one annular in lens radius R _n and the corresponding zonal pitch Rp _n the region RZ _n.

First, as shown in FIG. 6, on the optical substrate OBP of the Fresnel lens 10 (FIG. 1), as described above, the annular zone pitch at the lens radius of 1500 μm corresponding to the annular zone RZ ₁₄₀ located at the outermost peripheral portion of the lens. Rp ₁₄₀ is 5.4 μm, and the maximum cutting angle of the diamond tool with respect to the second diffraction grating portion 11c in the triangular diffraction grating 11 is 75 ° within this annular zone pitch when a mold for a Fresnel lens is manufactured. The triangular diffraction grating 11 has a configuration in which the occupation ratio of the first diffraction grating portion 11b is 0.959 and the occupation ratio of the second diffraction grating portion 11c with respect to the annular zone pitch of 5.4 μm. It is set to 0.041.

The necessary corrected light transmittance of the triangular diffraction grating 11 required in the annular zone RZ ₁₄₀ is 91.7% as described above. In this case, assuming that the value of the first-order diffracted light diffraction efficiency in the characteristic curve shown in FIG. 5 is equivalent to the required corrected light transmittance value, the first-order diffracted light diffraction efficiency 91 in the characteristic curve shown in FIG. From the value of .7%, it can be read that the height H _n of the triangular diffraction grating 11 is 1.00 as a relative value, so that the height H _n of the triangular diffraction grating 11 corresponds to this relative value 1.00. 0.833 μm.

  At this time, FIG. 7 shows the diffracted light intensity at the diffraction order within the annular zone pitch of 5.4 μm at the lens radius of 1500 μm. In FIG. 7, the horizontal axis indicates the orders from the −9th order to the + 9th order, and the vertical axis indicates the diffracted light intensity normalized with the maximum value being 255. As is apparent from FIG. 7, 91.7% of the diffracted light is concentrated in the + 1st order that is responsible for condensing the lens.

Next, as shown in FIG. 8, on the optical substrate OBP of the Fresnel lens 10 (FIG. 1), as described above, zonal pitch _{Rp 62} in the lens radial 1000μm corresponding with annular region _{RZ 62} is 8.0μm In addition, the configuration of the triangular diffraction grating 11 is the same as in the case of FIG. 6, in which the occupation ratio of the first diffraction grating portion 11b is 0.959 with respect to an annular pitch of 8.0 μm, and the second diffraction grating The occupation ratio of the part 11c is set to 0.041.

The necessary corrected light transmittance of the triangular diffraction grating 11 required in the annular zone RZ ₆₂ is 78.9% as described above. In this case, since the height H _n of the triangular diffraction grating 11 is read as a relative value of 0.791 from the value of the first-order diffracted light diffraction efficiency of 78.9% in the characteristic curve shown in FIG. Corresponding to 0.791, the height H _n of the triangular diffraction grating 11 is 0.659 μm. Further, when a mold for a Fresnel lens is manufactured, the cutting angle to the second diffraction grating portion 11c in the triangular diffraction grating 11 is 63.5 ° within this annular zone pitch.

  At this time, FIG. 9 shows the diffracted light intensity in the diffraction order within the annular zone pitch of 8.0 μm at the lens radius of 1000 μm. As is apparent from FIG. 9, in addition to the main + 1st order diffracted light, there are slightly higher-order lights of 0th order, + 2nd order, and −1st order or more, and these directions are different from the condensing position. The incident light is diffracted to reduce the light transmittance within an annular pitch of 8.0 μm.

Next, as shown in FIG. 10, on the optical substrate OBP of the Fresnel lens 10 (FIG. 1), as described above, zonal pitch _{Rp 15} in the lens radial 500μm corresponding with annular region _{RZ 15} is 16.1μm Further, the configuration of the triangular diffraction grating 11 is the same as in FIGS. 6 and 8, in which the occupation ratio of the first diffraction grating portion 11b is 0.959 with respect to the annular zone pitch of 16.1 μm. The occupation ratio of the two diffraction grating portions 11c is set to 0.041.

The necessary corrected light transmittance of the triangular diffraction grating 11 required in the annular zone RZ ₁₅ is 72.9% as described above. In this case, since the height H _n of the triangular diffraction grating 11 is read as a relative value of 0.743 from the value of the first-order diffracted light diffraction efficiency of 72.9% in the characteristic curve shown in FIG. Corresponding to 0.743, the height H _n of the triangular diffraction grating 11 is 0.619 μm. Further, when a mold for the Fresnel lens is manufactured, the cutting angle to the second diffraction grating portion 11c in the triangular diffraction grating 11 is 43.3 ° within this annular zone pitch.

  At this time, FIG. 11 shows the diffracted light intensity at the diffraction order within the annular zone pitch of 16.1 μm at the lens radius of 500 μm. As apparent from FIG. 11, in addition to the main + 1st order diffracted light, there are more 0th order, + 2nd order, and −1st order or higher order light than in the case of FIG. The incident light is diffracted in a direction different from the position to reduce the light transmittance within the annular zone pitch of 16.1 μm.

Next, as shown in FIG. 12, on the optical substrate OBP of the Fresnel lens 10 (FIG. 1), as described above, zonal pitch Rp ₀ in the lens radial 0μm vicinity corresponding with annular region RZ ₀ 126. As in the case of FIGS. 6, 8, and 10, the configuration of the triangular diffraction grating 11 is 5 μm, and the occupation ratio of the first diffraction grating portion 11 b with respect to the ring pitch of 126.5 μm is 0. 959, the occupation ratio of the second diffraction grating portion 11c may be set to 0.041. However, even if the phase folding point plane by the second diffraction grating portion 11c is deleted at the center of the lens, the light transmittance performance is improved. In order not to affect this, the triangular diffraction grating 11 becomes a right triangle by occupying only the first diffraction grating portion 11b within the annular pitch of 126.5 μm, and a mold for the Fresnel lens is manufactured. In this case, the triangular diffraction grating 11 at the center of the lens can be easily cut.

The necessary corrected light transmittance of the triangular diffraction grating 11 required in the annular zone RZ ₀ is 71.0% as described above. In this case, since the height H _n of the triangular diffraction grating 11 from 1 value of order diffracted light diffraction efficiency 71.0% characteristic curve in shown in FIG. 5 can be read as being 0.713 in relative value, this relative value Corresponding to 0.713, the height H _n of the triangular diffraction grating 11 is 0.594 μm. Further, when the mold for the Fresnel lens is manufactured, the phase turning point surface by the second diffraction grating portion 11c is deleted at the center of the lens, and therefore there is a cutting angle of the diamond tool with respect to the second diffraction grating portion 11c. Instead, only the first diffraction grating portion 11b may be cut.

  At this time, the diffracted light intensity at the diffraction order within the annular zone pitch of 126.5 μm near the lens radius of 0 μm is shown in FIG. 13. As apparent from FIG. 13, in addition to the main + 1st order diffracted light, there are slightly more high order light of 0th order, + 2nd order, and −1st order or more than in the case of FIG. The incident light is diffracted in a direction different from the condensing position, and the light transmittance within the annular zone pitch of 126.5 μm is lowered.

As described above, all of the ring zones RZ _n (where, n = 0 to 140) seeking the lens radius _{R n} and annular pitch Rp _n corresponding to each annular zone in the annular region RZ _n obtains the necessary correction light transmittance of the pitch Rp _n each triangular diffraction grating 11 formed in correspondence with, it may be determined the height H _n of each triangular diffraction grating 11 in each ring-shaped zone pitch Rp _n.

As described above, the Fresnel lens 10 according to the present invention, when forming each triangular diffraction grating 11 in each ring-shaped zone in the pitch Rp _n corresponding to each annular area RZ _n, in each triangular diffraction grating 11 first, second diffraction grating portion 11b, and occupation ratio for each ring-shaped zone pitch Rp _n and 11c on set substantially constant, the height H _n of each triangular diffraction grating 11 the base 11a as a height reference, each ring zones RZ _n is is a variable for each ring zones RZ _n such that the light transmission requests respectively, the Fresnel lens 10 according to the present invention when applied to, for example, an optical pickup, the Fresnel lens 10 Even if there is a light amount distribution in the incident light incident on the light, the light amount distribution can be corrected uniformly with respect to the incident light, and this incident light can be condensed at a predetermined condensing position. Condensation spot It is possible to obtain.

  In the above-described embodiment, the first-order diffraction structure is used for the first and second diffraction grating portions 11b and 11c in the triangular diffraction grating 11, but the first-order diffraction structure is not limited to this. Higher-order diffractive structures such as second-order and third-order can have the same function.

  The application of the Fresnel lens 10 according to the present invention not only makes the light intensity distribution of the incident light uniform in the example, but also sets an arbitrary light transmittance according to the characteristics of the optical system to be combined. In addition, an arbitrary light transmission amount distribution can be obtained.

(A), (b) is the top view and longitudinal cross-sectional view which showed the Fresnel lens based on this invention. It is a figure for demonstrating the optical path of an optical system, when the Fresnel lens which concerns on this invention is used. It is the figure which showed the light intensity distribution and light transmittance of an optical system corresponding to the position X1-X3 in FIG. 2 when the Fresnel lens which concerns on this invention is used. In the Fresnel lens according to the present invention, it is an enlarged view for explaining the height of a triangular diffraction grating formed in one annular zone region, (a) is a triangular diffraction grating of a right triangle. (B) is a diagram showing a case where the maximum cutting angle of the diamond tool is set to, for example, 75 ° with respect to the second diffraction grating portion 11c in the triangular diffraction grating. In the Fresnel lens which concerns on this invention, it is the figure which showed the relationship between the height of the triangular diffraction grating formed in one annular zone area | region, and the 1st-order diffracted light diffraction efficiency. In the Fresnel lens which concerns on this invention, it is the figure which showed the height of the triangular diffraction grating within the ring zone pitch of 5.4 micrometers in the lens radius of 1500 micrometers. In the Fresnel lens which concerns on this invention, it is the figure which showed the diffracted light intensity | strength of the triangular diffraction grating within the ring zone pitch of 5.4 micrometers in the lens radius of 1500 micrometers. In the Fresnel lens which concerns on this invention, it is the figure which showed the height of the triangular diffraction grating within the ring zone pitch of 8.0 micrometers in the lens radius of 1000 micrometers. In the Fresnel lens which concerns on this invention, it is the figure which showed the diffracted light intensity | strength of the triangular diffraction grating within the ring zone pitch of 8.0 micrometers in the lens radius of 1000 micrometers. In the Fresnel lens which concerns on this invention, it is the figure which showed the height of the triangular diffraction grating within the ring zone pitch of 16.1 micrometers in the lens radius of 500 micrometers. In the Fresnel lens which concerns on this invention, it is the figure which showed the diffracted light intensity | strength of the triangular diffraction grating within the ring zone pitch of 16.1 micrometers in the lens radius of 500 micrometers. In the Fresnel lens which concerns on this invention, it is the figure which showed the height of the triangular diffraction grating within 126.5 micrometers of annular zone pitch in the lens radius vicinity of 0 micrometer. In the Fresnel lens which concerns on this invention, it is the figure which showed the diffracted-light intensity | strength of the triangular diffraction grating within the ring zone pitch 126.5micrometer in lens radius 0micrometer vicinity. (A), (b) is the top view and longitudinal cross-sectional view which showed the Fresnel lens which has a right-angled triangular diffraction grating as a prior art example. It is the longitudinal cross-sectional view which showed typically the state which cuts and manufactures the metal mold | die for Fresnel lenses with a diamond tool, when shape | molding the conventional Fresnel lens shown in FIG. 14 with a metal mold | die. It is the figure which showed typically the state into which the light beam of the center part has strong light intensity, and the light beam of the outer peripheral part made weak light intensity to the conventional Fresnel lens.

Explanation of symbols

10 ... Fresnel lens according to the present invention,
11 ... Triangular diffraction grating,
11a ... bottom, 11b ... first diffraction grating part, 11c ... second diffraction grating part,
OBP ... optical substrate, 0 ... center of optical substrate,
K: optical axis, L: incident light (laser light),
RZ (RZ ₀ , RZ ₁ , RZ ₂ , RZ ₃ ,..., RZ _n ) ... annular zone region,
R _n (R ₀ , R ₁ , R ₂ , R ₃ ,..., R _n ) ... lens radius,
_{Rp n (Rp 0, Rp 1} , Rp 2, Rp 3, ......, Rp n) ... ring-shaped zone pitch.

Claims (2)

A plurality of annular zones are set on the optical substrate having optical transparency so that each annular zone pitch is gradually narrowed from the center toward the outer peripheral side, and each triangular diffraction grating is formed in each annular zone. In the Fresnel lens
Each of the triangular diffraction gratings has a base on which a base length is set in accordance with each ring pitch on a side on which incident light is incident on the optical substrate, and a side on which the incident light transmitted through the base is emitted. A first diffraction grating portion having an inclined surface inclined toward the outer peripheral side and diffracting a specific-order diffracted light with respect to the incident light and condensing it at a predetermined condensing position; and before passing through the base The incident light is emitted on the side from which the incident light exits and is inclined toward the inner peripheral side, and the incident light is diffracted to a diffraction order different from that of the first diffraction grating portion to generate diffracted light other than the specific order. A second diffraction grating portion for adjusting the light transmittance with respect to the incident light,
The occupation ratio of each of the first and second diffraction grating portions to each of the annular zone pitches is set to be substantially constant, and the height of each of the triangular diffraction gratings is set to each of the annular zone regions with the base as a height reference. The Fresnel lens is made variable for each of the annular zones so as to obtain the required light transmittance. 2. The Fresnel lens according to claim 1, wherein a triangular diffraction grating formed in the annular zone at the center of the lens on the optical substrate has only the first diffraction grating portion.

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