JPH06101128B2 - Optical head - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improvement of an optical head for focusing a light beam for writing or reading information on an information recording medium such as an optical disk.

[Technical Background of the Invention] In recent years, it has been used in read-only information storage media such as DAD CDs (compact discs) and video discs, image files, still image files, COM (computer output memory), etc. Information is recorded by causing a state change such as making a hole in the recording layer by the focused light.
In addition, various optical information recording / reproducing devices have been developed, in which information is optically written to and read from an information storage medium which can be reproduced from the information storage medium and an erasable information storage medium (hereinafter simply referred to as a disk). . In these information recording / reproducing devices, it is required that the light beam focused by the objective lens is always focused on the optical disk whether it is writing or reading. That is, it is required that the beam waist of the light beam is irradiated onto the optical disc and the minimum beam spot is formed on the optical disc. For this reason, the optical head includes a focus detection device that detects the focus state of the light beam, and various focus detection devices have been proposed and developed in the past. For example,
Japanese Patent Laid-Open No. 57-44236 discloses an example of a focus detection method.

That is, as shown in FIGS. 1 (a), (b), and (c), in the middle of the reflection optical path c of the light beam reflected from the recording layer or the light reflection layer a and passing through the objective lens b, with respect to this optical axis. A light extraction member (knife wedge) d that asymmetrically extracts a light beam, a lens e, and a photodetector h having two photodetection cells f and g are provided, and a spot on the photodetector as before is provided. Instead of detecting defocus by size,
It has been considered that the defocusing effect is detected by detecting the defocus as the movement of the beam spot i (in the direction of arrow j) on the photodetector h.

The defocus detection signal is as shown by the solid line in FIG.
When there is a focus, the level becomes “0”, and when the objective lens b and the recording layer or the light reflection layer a approach each other, the beam spot i hits the upper photodetection cell g. , A negative signal is generated, and the objective lens b and the recording degree or the light reflection layer a are too far apart from each other, and the beam spot i hits the lower photodetecting cell f to generate a positive signal. (Note that in the figure, the right direction from the 0 point indicates the direction in which the objective lens moves away, and the left direction indicates the direction in which the objective lens moves closer.) [Problems of the Background Art] By the way, in FIG. Is ｜ δor
If | becomes larger than |, | δof |, the beam spot i is projected from the photodetector cells f and g on the photodetector, so that the amount of detected light is decreased and the defocus detection signal becomes small.

This directly leads to the problem that defocus detection becomes unstable if the distance between the signal inversion units δor and δof in FIG. 2 is too small. Further, when the servo loop gain (amplification factor of the objective lens position control negative feedback circuit) on the electrical defocusing correction circuit is small, the objective lens b is not always completely stopped at the in-focus position, and the in-focus position is reduced. It has been finely moved in the vicinity. And if it shifts out of this signal inverting part δor, δof,
Since the defocus detection signal is reduced, the servo loop gain is substantially reduced, control for defocus correction becomes impossible, and there is a problem that the focus is more likely to be defocused.

[Object of the Invention] The present invention has been made based on the above circumstances, and an object of the present invention is to provide an optical head capable of detecting a focus with high reliability and sufficient sensitivity.

SUMMARY OF THE INVENTION According to the present invention, in order to achieve such an object, there is provided a device for writing information by irradiating a light beam on an information recording medium having a recording layer on which information is recorded. An objective lens that collects the light beam from the recording layer and collects the light beam from the recording layer, and a light that extracts a part of the light beam from the light beam collected by the objective lens asymmetrically with respect to the optical axis of the light beam. The extracting means and the light detecting area for detecting the light beam, and the light detecting means for receiving the light beam extracted by the light detecting means are provided, and the light is detected based on the detection result by the light detecting means. In the optical head that detects the defocus of the light beam irradiated on the information recording medium by detecting the movement of the beam spot on the detecting means, F: Focusing from the main point of the objective lens near the information recording medium To the point Where y is the radius of the exit pupil of the objective lens or the radius of the aperture of the objective lens, m is the imaging magnification on the photodetector, and δcrit is the allowable defocus amount, the photodetection means is , Radius r shown below However, here However, an optical head is provided in which NA is a numerical aperture λ of an objective lens and has a light detection region having a size larger than a circle of a used wavelength.

Embodiments of the Invention Hereinafter, embodiments of the optical head of the present invention will be described with reference to FIGS.

An information recording / reproducing apparatus to which the optical head of the present invention is applied will be schematically described with reference to FIG. The optical disc 2 includes a pair of disk-shaped transparent plates 4 and 6 and inner and outer spacers 8 and 1.
Formed by laminating through 0, its transparent plate 4,6
Recording layers or light reflecting layers (hereinafter simply referred to as light reflecting layers) 12 and 14 are formed on the inner surfaces of the respective layers by vapor deposition. A tracking guide 16 is helically formed on each of the light reflecting layers 12 and 14, and information is recorded on the tracking guide 16 in the form of pits. A hole is bored in the center of the optical disk 2, and when the optical disk 2 is placed on the turntable 18, the center spindle 20 of this turntable 18 is inserted into the hole of the optical disk 2, and the turntable 18 and the optical disk The two centers of rotation are aligned. A chuck device 22 is further mounted on the center spindle 20 of the turntable 18.
The optical disk 2 is fixed on the turntable 18 by 22. The turntable 18 is rotatably supported by a support (not shown), and is rotated at a constant speed by a drive motor 24.

An optical head 26 is provided so as to be movable in the radial direction of the optical disk 2 by a linear actuator 28 or a rotating arm. Inside the optical head 26, a laser device 30 for generating a laser beam is provided. Optical disc 2
A laser beam whose light intensity is modulated according to the information to be written is generated from the laser device 30 when writing to the optical disk, and when reading information from the optical disc 2, a laser beam having a constant light intensity is emitted. Generated from device 30. The laser beam generated from the laser device 30 is diverged by the concave lens 32, modulated into a parallel light beam by the convex lens 32, and directed to the polarization beam splitter 36. The parallel laser beam reflected by the polarization beam splitter 36 passes through the quarter-wave plate 38 and is incident on the objective lens 40, which is focused by the objective lens 40 toward the light reflection layer 14 of the optical disc 2. . The objective lens 40 is movably supported by the voice coil 42 in the optical axis direction, and when the objective lens 40 is positioned at a predetermined position, the beam waist of the focused laser beam emitted from the objective lens 40. Are projected onto the surface of the light reflecting layer 14 and a minimum beam spot is formed on the surface of the light reflecting layer 14. In this state, the objective lens 40 is kept in focus, and writing and reading of information becomes possible.
When writing information, pits are formed in the tracking guide 16 on the light reflection layer 14 by the light intensity-modulated laser beam, and when reading information, the laser beam having a constant light intensity is The light intensity is modulated by the pits formed in the guide 16 and reflected.

The divergent laser beam reflected from the light reflection layer 14 of the optical disc 2 is converted into a parallel light flux by the objective lens 40 at the time of focusing, passes through the 1/4 wavelength plate 38 again, and is returned to the polarization beam splitter 36. Be done. By reciprocating the laser beam through the quarter-wave plate 38, the laser beam has a polarization plane of 90 degrees as compared with when it is reflected by the polarization beam splitter 36.
A laser that rotates the polarization polarization plane by 90 degrees.
The beam will not pass through the polarizing beam splitter 36, but will pass through the polarizing beam splitter 36. The laser beam that has passed through the polarization beam splitter is split into two systems by the half mirror 44, one of which is
The convex lens 46 illuminates the first photodetector 48. The first signal detected by the first photodetector 48, including the information recorded on the optical disc 2, is sent to the signal processing device and converted into digital data. Half mirror 44
With respect to the other laser beam divided by, only the component passing through the region separated from the optical axis 53 is taken out by the light shielding plate 52 as the light extracting means, passes through the projection lens 54, and is reflected by the mirror 56. It is incident on the second photodetector 58. Here, the light shielding plate 52 may be composed of any of a prism, an aperture slit, a knife edge, or the like. The signal detected by the second photodetector 58 is processed by the focus signal generator 60, and the focus signal generated by this focus signal generator 60 is given to the voice coil drive circuit 62. Voice coil drive circuit 62
Will drive the voice coil 42 according to the focus signal to maintain the objective lens 40 in focus.
In order to accurately trace the tracking guide 16 formed on the light reflection layer 14 of the optical disk 2, the push-pull method is used to process the signal from the second photodetector 48 and the linear actuator is processed. 28 may be activated, and
The objective lens 40 may be moved laterally, or a galva mirror (not shown) may be operated.

The optical system for detecting the in-focus state shown in FIG. 3 is shown in a simplified manner as shown in FIG. 4, and the locus of the laser beam relating to the focus detection in this optical system is shown in FIG. 5 (a). ~
(D). When the objective lens 40 is in focus, a beam waist is projected on the light reflection layer 14,
A minimum beam spot, or beam waist spot 64, is formed on the light reflecting layer 14. Usually laser device 30
Since the laser beam incident on the objective lens 40 from the parallel light beam is parallel, the beam waist is formed on the focal point of the objective lens 40. However, the laser device 30 is attached to the objective lens 40.
A beam waist is formed near the focal point of the objective lens 40 when the laser incident from is slightly divergent or focused. 3, 4 and 5 (a)
In the optical systems shown in (d) to (d), the light receiving surface of the photodetector 58 is arranged on the image forming surface of the beam waist spot 64 in the focused state. Therefore, at the time of focusing, the image of the beam waist spot 64 is formed at the center of the light receiving surface of the photodetector 58. That is, as shown in FIG. 5A, a beam waist spot 64 is formed on the light reflection layer 14, and the laser beam reflected by the light reflection layer 14 is converted into a parallel light flux by the objective lens 40. Light shield
Turned to 52. Only the light component passing through the area separated from the optical axis 53 is taken out by the light shielding plate 52, focused by the projection lens 54, narrowed down to the minimum on the photodetector 58, and the beam
A waist spot image is formed on it. Next, when the objective lens 40 approaches the light reflecting layer 14, the beam
The waist is generated when the laser beam is reflected by the light reflecting layer 14 as shown in FIG. That is, the beam waist is generated between the objective lens 40 and the light reflection layer 14. In such an out-of-focus state, the beam waist usually occurs within the focal length of the objective lens 40. The laser beam reflected and emitted from the objective lens 40 is converted into a divergent laser beam by the objective lens 40. Since the laser beam component that has passed through the shading plate 52 is also divergent, even if this laser beam component is focused by the projection lens 54, it is not narrowed down to the minimum on the light receiving surface of the photodetector 58 and the light detection is performed. Container 58
It will be focused toward a point farther away. Therefore,
The laser beam component is projected from the center of the light receiving surface of the photodetector 58 upward in the figure, and a pattern larger than the beam spot image is formed on the light receiving surface. Further, when the objective lens 40 is separated from the light reflecting layer 14 as shown in FIG. 5C, the laser is reflected by the reflecting layer 14 after forming the beam waist. In such an out-of-focus state, the normal beam waist is
Since it is formed outside the focal length of the objective lens 40 and between the objective lens 40 and the reflection layer 14, the reflected laser beam from the objective lens 40 toward the light shielding plate 52 has a converging property. Therefore, the laser beam component that has passed through the shading plate 52 is further focused by the projection lens 54, forms a focal point, and is then projected onto the light receiving surface of the photodetector 58. As a result, the photodetector 58
A pattern larger than the image of the beam waist spot is formed on the light-receiving surface of 1 from the center downward in the figure. Further, as shown in FIG. 5D, the objective lens 40 is a light reflection layer.
When it is largely separated from 14, it is focused at a point closer to the light shielding plate 52. Therefore, as described above in the conventional example, the laser beam is directed upward from the center of the light receiving surface of the photodetector 58 in the figure.
The beam component is projected.

The change of the laser trajectory described above, that is, the change of the ray trajectory is
Geometrically described as follows, the value h ₃ at which the laser beam component is deflected on the photodetector 58 can be determined. The geometrical imaging system of the objective lens 40 can be represented as shown in FIG. Where f ₀ is the objective lens 40
In addition, δ represents the moving distance of the objective lens 40, that is, the light reflection layer 14 of the optical disc 2 from the time of focusing to the time of non-focusing, and the ray trace shown by the solid line in FIG.
It shows what is emitted from the beam waist, passes through a point on the main surface of the objective lens 40, which is separated from the optical axis 53 by a distance h ₀ , and is focused. At the time of focusing shown in FIG. 5 (a), it is clear that δ = 0, and FIG. 5 (b)
When the object is out of focus as shown in FIG. 3, the optical disk 2 is close to the objective lens 40 by the distance δ, and the beam waist is the light reflection layer 14
The beam waist is twice as close to the objective lens 40 as the beam waist. (In the case of close proximity, δ <0.) In the non-focus state shown in FIG. 5C, the optical disc 2 is separated from the objective lens 40 by the distance δ, and after forming the beam waist, the laser beam is formed. -Because the beam is reflected from the light reflection layer 14,
Substantially the same as the beam waist being formed behind the light reflecting layer 14, the beam waist is separated from the objective lens by 2δ. Assuming that the beam waist is formed at the focal position of the objective lens 40 at the time of focusing, when the optical disc 2 moves by δ, the beam waist and the objective lens 40 are mainly moved as shown in FIG. The distance between the surfaces is represented by (f ₀ + 2δ). If the beam waist is regarded as a light spot, the angles β ₀ and β in FIG.
₁ is represented by the following equations (1) and (2).

Also, from the coupling formula of the lens Therefore, Substituting into equation (1) FIG. 7 shows a ray trace in the optical system of the projection lens 54, in which the projection lens 54 is a pair of combination lenses 54-1 and 54-2.
It is treated as consisting of.

Here, the lenses 54-1 and 54-2 have focal lengths f ₁ and f ₂ , respectively, and the light shielding plate 52 is arranged at a position separated from the main surface of the objective lens 40 by a, and The main surface of the lens 54-1 is arranged at a position separated from the surface by L, and further, the main surface of the lens 54-2 is separated from the main surface of the lens 54-1 by H and the main surface of the lens 54-2 is separated from the main surface by e. It is assumed that the light receiving surface of detector 58 is aligned. A ray trace shown by a solid line in the drawing shows a light beam which is converged by the objective lens 40 and which is a light transmitting surface of the light shielding plate 52 and is separated from the optical axis 53 by y.

The distance y is represented by the following equation (3).

y = h ₀ −aβ _{1 When the} equation (2) is substituted, Here, if F (δ) = (f ₀ + f ₀ ² / 2δ) ⁻¹ , β ₁ is represented by β ₁ = h ₀ · F (δ), and equation (3) is It is represented by.

y = h ₀ (1-aF (δ) (4) Therefore, Further, the position h ₁ from the optical axis 53 where the light ray passes on the main surface of the lens 54-1 is expressed by the equation (6).

Substituting h ₁ = y- (La) β ₁ β ₁ = h ₀ · F (δ), h ₁ = y−h ₀ (La) · F (δ) Further substituting equation (5), (2) be determined likewise the angle beta ₂ and wherein the angle beta ₂ is
It is expressed by equation (7).

Similarly to the equation (2), β ₂ is represented by the following equation.

In this equation, f ₁ represents the focal length of the lens 54-1. Substituting β ₁ = h ₀ · F (δ), (5) and (6) into this equation, Similarly, the position h _{2 of the} ray passing through the principal surface of the lens 54-2 from the optical axis 53 and the position h ₂ of the incident angle β _{3 of the} ray incident on the light receiving surface of the photodetector 58 from the optical axis 53. ₃ That is, the deviation amounts are expressed by the equations (8) to (10), respectively.

h ₂ = h ₁ −Hβ ₂ By substituting the equations (6) and (7), Similarly to the equation (2), β ₃ is represented by the following equation.

Here, f ₂ represents the focal length of the lens 54-2.

Substituting equations (7) and (8), If h ₃ is expressed in the same manner as h ₂ , h ₃ = h ₂ − (l−H) β ₃ (8) and (9) are substituted, The optical system shown in FIGS. 6 and 7 is such that, as described above, at the time of focusing, that is, at δ = 0, the light beam on the detector 58 is h ₃ =
Since it is focused on 0, under these conditions,
F (0) = 0, and the equation (10) is represented by the following equation.

Further, since only the light passing through the light beam outside the optical axis 53 is taken out by the light shielding plate 52, y ≠ 0. Therefore, By simplifying equation (10) with this equation, equation (13) or (1
Equation 4) is obtained.

That is, Substituting F ⁻¹ (δ) = f ₀ + f ₀ ² / 2δ, Or When δ is sufficiently small (δ << f ₀ ² ), (a−f ₀ )
≪f ₀ ² / 2δ, so Next, at the time of focusing (δ = 0), the lateral magnification m of the beam waist image formed on the light receiving surface of the photodetector 58 with respect to the beam waist on the light reflection layer 14 of the optical disc 2 is as follows ( It is expressed by equation 16).

m = −β ₀ / β ₃ (inverted image) where β _{0 at} δ = ₀ is If δ = 0 in the equation (1), h ₀ = y, and Under this condition, β ₁ = 0 and β ₂ is Furthermore, in the equation (8), when δ = 0, F (δ) = 0, so h ₂ = y (1-H / f ₁ ) Therefore, β ₃ is m is represented by the following equation.

By eliminating f ₂ from equation (16) in equation (12), Considering the case where an erect image is formed in the same manner (m
= Β ₀ / β ₃ ), f ₁ is expressed by the following equation.

Therefore, When f ₂ is calculated from the equation (12), By substituting equation (20) into equation (13) and expressing h ₃ by the lateral magnification m, equations (21) and (22) are obtained.

However, the (a-f ₀₎ in most cases does not change (y-h ≒ 0) is ray trajectories between «f ₀ ² / 2δ light shielding plate 52 and the objective lens 40, the distance a, and a = 0 It may be assumed that equation (21) is expressed as follows. In an actual optical system, a parallel light flux is directed from the objective lens 40 to the light shielding plate at the time of focusing, and y−h = 0 holds. When the lens is out of focus, the light-shielding plate 52 is more objective than the other optical elements.
Since it is arranged relatively close to 40, y−h≈0 holds, and thus a≈0 holds.

Therefore, Since h ₀ / (f ₀ + 2δ) = β ₀ from the equation (1), h ₃ = ± 2mδβ ₀ (23) In the optical system shown in FIG. 6, the beam waist is the focus of the objective lens 40. However, when a divergent or converging laser beam is incident on the objective lens 40, the beam waist is formed by deviating from the focus by b. Therefore, regarding the entire optical system as one compound lens, if 2δ = 2δ ′ + b is placed in the equation (1) showing the value of β _{0 in} the equation (23) and the equation (21) is rewritten, Quantity h ₃ is required.

Here, when a = 0, If f ₀ + b >> 2δ ′, Becomes Here, if f ₀ + b = F Becomes

Next, let's examine the maximum allowable value for defocus.

Further, when recording is performed by changing the state such as making a hole in the light reflecting layer 14 of the information storage medium, defocusing occurs and the spot on the light reflecting layer 14 becomes large, which makes recording difficult. The spot size al on the light reflection layer 14 at the time of focusing is given by al = 0.82λ / NA (27) (where λ is the wavelength of the laser and NA is the numerical aperture).

Usually, the spot on the light reflection layer 14 has a light intensity distribution having a sharp peak called an Airy disk pattern as shown in FIG. The diameter of the spot having such a distribution is defined as the center intensity, that is, the diameter a1 of the ring body having 1 / e ² (e is the base of the natural logarithm) of the maximum intensity. This diameter al is usually expressed by equation (27).

If the beam spot having such a size corresponds to the beam waist formed by the objective lens 40, the size of the image of the beam waist formed on the light receiving surface of the photodetector 58 at the time of focusing. A _d is the lens system 40,5
It is represented by a lateral magnification of 4.

The intensity distribution at this time is considered to be similar to the Gaussian distribution, and the radius ω on the recording layer when the focus is defocused by Z
(Z) Let ω ₀ be the radius of the ring body at which the intensity at the beam waist is 1 / e ^{2 of the} central intensity, and let ω (Z) be the radius at a point Z away from it. Becomes Therefore, from equation (29) Here, since ω ₀ is the radius of the ring, The spot center intensity I at this time is Decrease to. Assuming that the minimum recordable center intensity is Imin, the spot center intensity I is required to be larger than this minimum intensity Imin.

From equation (30) Since λ = 0.83 μm and NA = 0.6, Imin = 0.7, so Since λ = 0.85 μm and NA = 0.5, usually Imin = 0.7 In other words, the allowable defocus amount δcrit is Imin = 0.
It is calculated by substituting 7.

Given in.

Therefore, the allowable defocus amount is ± 3 in anticipation of the margin.
It can be set to 0 μm.

Therefore, an optical system as shown in FIGS. 5 (a) to 5 (d) (including an optical system using a prism, a slit, an aperture, a mirror, a photodetector or the like instead of the knife edge), that is, an objective lens A light detector 58 is provided at at least one of a light condensing point (beam waist position) of the light condensed by 40 and an image forming position with respect to the light reflecting layer (recording layer) 14 of the information storage medium, and the objective Lens 40
Convergence point of the light focused by and the light reflection amount of the information recording medium 14
In the optical system of the optical head capable of detecting defocus by projecting a left-right asymmetric light amount on the photodetector 58 according to the amount of positional deviation of When the focus defocuses by ± 3.0 μm from the formula (6), the radius r (≈h ₃ ) on the photodetector 58 as shown in FIG. A millimeter circular beam spot 70 is created in which a shadow 71 of the knife edge 52 appears. Where F is the distance from the principal point of the objective lens near the information recording medium to the focal point (beam waist position), y is the radius of the exit pupil of the objective lens or the radius of the objective lens aperture, m : Imaging magnification (lateral magnification) on the photodetector, δcrit: Allowable defocus amount (allowable deviation amount from standard objective lens / light reflecting layer distance).

At this time, if the beam spot 70 does not protrude from the photodetection cells 58a and 58b as the two photodetection regions, the above requirements can be satisfied. That is, the radius r of the photodetector 58 is It is necessary to have a detection area larger than the circle of, and the present invention is set to a size that satisfies this.

Although the recording / reproducing apparatus has been described in the above-described embodiments, it is obvious that the present invention is not limited to this and can be applied to a recording- or reproducing-only apparatus.

In addition, it goes without saying that the present invention can be variously modified without departing from the spirit of the present invention.

EFFECT OF THE INVENTION As described above, according to the present invention, between the signal inversion parts of the defocus detection signal with respect to the defocus amount (from δor to δof in FIG. 8).
Even if the focus deviates a little, the optical detection signal does not become small but the servo loop gain for defocus detection remains large, which makes the defocus correction system more stable. .

[Brief description of drawings]

FIGS. 1 (a), (b), and (c) are explanatory diagrams showing the basic principle of detecting defocus by moving the beam spot, and FIG. 2 is light detection accompanying the movement of the beam spot on the photodetector. FIG. 3 to FIG. 11 are explanatory views showing the changing state of the amount, and FIG. 3 shows an embodiment of the present invention. FIG. 3 shows an information recording / reproducing apparatus equipped with an optical head according to an embodiment of the present invention. FIG. 4 is a block diagram, FIG. 4 is a diagram showing the optical system shown in FIG. 3, and FIGS. 5 (a) to 5 (d) are lasers for focusing and non-focusing.
FIG. 6 is an explanatory view showing the trajectory of the beam, FIG. 6 is a diagram for analyzing the trajectory of the light beam passing through the objective lens shown in FIG. 4, and FIG. 7 is a diagram showing the light beam passing through the projection lens shown in FIG. FIG. 8 is a diagram for analyzing the locus, FIG. 8 is an explanatory diagram showing the state of the defocus detection signal with respect to the defocus amount, and FIG. 9 is the sum of the light amounts detected by the defocus detection detector with respect to the defocus amount. Explanatory drawing, FIG. 10 is a view showing a light intensity distribution in a beam waist formed by an objective lens, and FIG. 11 is a view showing a beam spot at the time of defocusing on a photodetector. 2 ... Information storage medium, 12, 14 ... Recording layer or light reflection layer, 4
0 ... Objective lens, 52 ... Light extraction means, 58 ... Photodetector.

Claims (1)

[Claims] 1. An apparatus for writing information by irradiating a light beam to an information recording medium having a recording layer on which information is recorded, wherein the light beam is focused on the recording layer and light from the recording layer is collected. An objective lens for collecting the beam, a light extraction unit for extracting a part of the light beam asymmetrically with respect to the optical axis of the light beam from the light beam collected by the objective lens, and a light detection region for detecting the light beam are provided. And a photodetector that receives the light beam extracted by the photodetector, and detects the movement of the beam spot on the photodetector based on the detection result of the photodetector. In the optical head that detects the defocus of the light beam applied to the information recording medium, F: the distance from the principal point of the objective lens closer to the information recording medium to the focal point, y: the radius of the exit pupil of the objective lens Or pair Radius of the aperture of the lens, m: image forming magnification of the optical detector, Derutacrit: when the defocusing amount allowed, the light detecting means, the radius r shown below However, here However, NA is an optical head characterized in that it has a light detection area whose numerical aperture λ of the objective lens is larger than the circle of the used wavelength.