JPS62229203A - Grating lens - Google Patents

Abstract

PURPOSE:To extend the minimum pitch of an unequal interval diffraction grating and widen the effective angle of field by forming the concentric circle-shaped unequal interval diffraction grating on a convex of an optically transparent body whose one face at least is formed into the convex. CONSTITUTION:A concentric circle-shaped unequal interval diffraction grating 2 whose section is saw tooth-shaped is formed on the convex of a planoconvex lens 3 whose one face is plane and the other is convex, and the parallel rays of luminous flux entering from the plane side are converged on a focus F by the lens 3. When the minimum pitch of the diffraction grating 2 is extended, the angle of diffraction of light due to the diffraction grating 2 is narrowed, and as the result, the effective numerical aperture of the lens is reduced. Simultaneously, the effective angle of field is widened.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a grating lens used, for example, as an objective lens or collimating lens of an optical information processing device.

(Prior Art) Generally, an optical pickup head that reads information from an information recording medium, such as an optical disk, converts a light beam from a light source such as a semiconductor radar into a parallel beam using a collimating lens, and then converts the light beam from a light source such as a semiconductor radar into a parallel beam using an objective lens onto the information recording surface of the optical disk. It is configured to converge, scan a fine pit row, and detect the reflected light beam from there with a photodetector. The most important component in such an optical pickup head is an objective lens that focuses the light beam onto the information recording surface of the optical disk to a diameter of about 1 to 2 μm.

Conventionally, this objective lens used a compound lens made by combining multiple spherical single lenses.
This is because it is necessary to eliminate various lens aberrations such as spherical aberration, coma aberration, field curvature, and distortion as much as possible. However, even with a compound lens, it is impossible to completely eliminate lens aberration. In addition, high-performance compound lenses,
Polishing and assembly inspection were difficult, and since multiple glass lenses were used, the vehicle size was large.

In order to solve these problems, so-called grating lenses that utilize diffraction of light waves have been proposed. As shown in FIG. 4, this grating lens is constructed by forming concentric diffraction gratings 2 with rectangular or sawtooth cross sections and non-uniformly spaced diffraction gratings 2 whose pitch gradually narrows toward the periphery on a glass substrate 1, for example, as shown in FIG. It has a lens function that uses the diffraction of light waves by the non-uniformly spaced diffraction grating 2 to converge parallel light to one point or convert divergent light into parallel light.

The minimum pitch of such a grating lens is given by λ/NA, where NA is the numerical aperture and λ is the wavelength of light. Therefore, for example, if λ = 0.78 nm, N
For a collimator lens with A=0.1, the minimum pitch is 7.
~8 μm.

(Problem to be Solved by the Invention) However, in the case of an objective lens for an optical pickup that requires a numerical aperture of 0.45 or more, the minimum pitch is 1.6 to 1.8 μm, and the objective lens There was a problem in that ultra-fine processing was required in order to manufacture grating lenses for use.

Additionally, grating lenses produce extremely large aberrations for light incident at angles other than the designed incidence angle, and the usable angular range (hereinafter referred to as effective viewing angle) is much wider than glass combination lenses. It has the disadvantage of being narrow. The amount of aberration increases as the numerical aperture increases; for example, when NA = 0.1, the angle of incidence is ±1 degree and the aberration is approximately λ/14 (
RMS value) aberration occurs, whereas NA = 0.4
7, the tilt incident angle is ±0.1 degrees and the amount of aberration is λ/10(R
H3 value).

Mainly for these two reasons, grating lenses have not been put to practical use as objective lenses for optical pickups.

On the other hand, in order to solve these problems, it has been proposed to use a combination of a spherical single lens and a grating lens. By using such a combination lens, the effective numerical aperture of the grating lens can be reduced,
The required minimum pitch of the concentric non-uniformly spaced diffraction gratings is considerably increased, and the effective viewing angle can also be increased.

However, since such a combination lens requires alignment of the two lenses, there is a problem in that productivity is poor. Furthermore, since the four lens surfaces are separated, it is necessary to form an antireflection film on each lens surface in order to suppress reflection loss on each lens surface, which has the disadvantage of increasing production costs.

The present invention was made in response to the above-mentioned circumstances, and it is possible to greatly increase the minimum pitch of the unevenly spaced diffraction grating, widen the effective viewing angle, and achieve excellent productivity and reduce production costs. The purpose is to provide low-priced grating lenses.

[Structure of the Invention] (Means for Solving the Problems) The grating lens of the present invention is an optically transparent object whose at least one surface has a convex spherical shape. It is formed by forming a spaced diffraction grating.

(Function) In the grating lens of the present invention, compared to a conventional grating lens in which diffraction gratings are formed at uneven intervals on a flat substrate, the minimum pitch can be increased, and at the same time, the effective viewing angle can be widened. be able to.

(Example) Hereinafter, the present invention will be described in detail with reference to the example shown in the drawings.

FIG. 1 shows an embodiment of the present invention, in which a concentric unequal-spaced diffraction grating with a sawtooth cross section is placed on the convex spherical surface of a plano-convex lens 3 whose one surface is flat and the other surface is a convex spherical surface. 2 is formed, and the parallel light beam incident from the plane side is converged to a focal point F by this grating lens.

By the way, in this lens, the m-th annular radius rI, l of the unevenly spaced diffraction grating 2 formed on the convex spherical surface is determined by the following equation.

(1) where f is the distance from the tip of the convex spherical surface to the focal point F, R is the radius of curvature of the convex spherical surface, and n is the refractive index of the plano-convex lens.

Generally, f>r, ffl, and R>rtll, so the following equation can be approximately derived from equation (1).

Therefore, from equation (2), the minimum pitch P of the unevenly spaced diffraction grating
is given by the following equation.

λ Here, NA is the numerical aperture of the lens, and 2rIIlax is the effective aperture of the lens.

Here, if the formation surface of the unevenly spaced diffraction grating is a plane as shown in FIG. 4, then in equations (2) and (3), R→
■ By setting r, = 2mλf ・・・・・・・・・・・・・・・・・・
......(2)'.

As is clear from the comparison between equation (3) and equation (3)',
The radius of curvature R is selected so that R>n rmax /NA, S; as a result, the minimum pitch of an unevenly spaced diffraction grating formed on a convex spherical surface is much larger than that when formed on a flat surface. It is possible to do so.

As described above, if the minimum pitch of the unevenly spaced diffraction grating becomes large, the diffraction angle of light by the unevenly spaced diffraction grating becomes smaller, and as a result, the effective numerical aperture of the grating lens becomes smaller. This also means that the effective viewing angle can be widened.

FIG. 2 shows another embodiment of the present invention, in which a concentric unequal-spaced diffraction grating 2 with a sawtooth cross section is formed on one side of a biconvex lens 4 having convex spherical surfaces on both sides. In such a grating lens 5, the refraction of light on the other convex spherical surface can also be used, so the minimum pitch of the unevenly spaced diffraction grating 2 is made wider than when the unevenly spaced diffraction grating is formed on a plano-convex lens. It is also possible to obtain a larger effective viewing angle.

FIG. 3 shows an example of an optical pickup head using the grating lens 5 shown in FIG. , the reflected light beam therefrom is configured to be detected by a photodetector 10 via a beam splitter 8 and an error detection optical element 9. Since the objective lens is likely to be exposed to the outside of the apparatus, in order to protect the surface on which microfabrication has been performed, it is arranged so that the surface on which the unevenly spaced diffraction grating is formed is on the light emitting side.

The grating lens of the present invention can be obtained by forming, on the convex spherical surface of a convex lens made of glass, an unevenly spaced diffraction grating made of a photocurable resin whose refractive index after curing is almost the same as the refractive index of the glass. However, in addition to this, it is also possible to integrally mold the convex lens and the unevenly spaced diffraction grating using optically transparent plastic in a plastic mode.

[Effects of the Invention] As is clear from the above explanation, in the present invention, the unevenly spaced diffraction grating is formed on a convex spherical surface, so even when used for an objective lens with a large numerical aperture, the unevenly spaced diffraction grating is The minimum pitch does not need to be so small, and the effective viewing angle can be widened, resulting in excellent productivity and economy.

[Brief explanation of drawings]

Figures 1 and 2 are cross-sectional views showing embodiments of the present invention, Figure 3 is a cross-sectional view showing an example of an optical pickup head using the grating lens shown in Figure 2 as an objective lens, and Figure 4. is a sectional view showing a conventional grating lens. 2...... Unequally spaced diffraction grating 3...
......Plano-convex lens 4...
...Biconvex lens 5...Grating lens 6...Semiconductor laser 7
......Optical disc 8...
・・・・・・Beams Brick 9・・・・・・・・・・・・
・Error detection optical element 10...Photodetector Applicant Toshiba Corporation Representative Patent attorney Sasu Suyama - Figure 1 Figure 2

Claims (4)

[Claims] (1) A grating lens characterized in that a concentric irregularly spaced diffraction grating is formed on the convex spherical surface of an optically transparent object having at least one convex spherical surface. (2) The grating lens according to claim 1, wherein the optically transparent object having at least one convex spherical surface is a plano-convex lens. (3) The grating lens according to claim 1, wherein the optically transparent object having at least one convex spherical surface is a biconvex lens. (4) The grating lens according to any one of claims 1 to 3, wherein the unevenly spaced diffraction grating is made of a photocurable resin.