JP2002208159A - Light beam irradiating optical system and optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an easily manufactured light beam irradiating optical system emitting a diverging beam whose cross section is nearly circular. SOLUTION: The optical beam irradiating optical system is composed of a light source emitting the diverging beam whose cross section is elliptic and a reshaping element having cylindrical surfaces whose curvature central axis are parallel to each other as an incidence surface and an exit surface. The cross section of the light beam after transmitting the reshaping element is formed into a nearly circle by paralleling the curvature central axis of the reshaping element to the major axis of the cross section of the light beam from the light source and increasing a degree of divergence in the direction of the minor axis. The reshaping element is set so that an astigmatic difference does not occur entirely in the light beam.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for emitting a divergent beam having a substantially circular cross section, and an optical pickup of an optical recording / reproducing apparatus for recording / reproducing information using light.

[0002]

2. Description of the Related Art An optical pickup of an optical recording / reproducing apparatus for recording information on a recording medium or reading recorded information by light converges a light source and light from the light source to a minute recording area of the recording medium. An objective lens, a detector that detects reflected light from the recording medium for reading information, and guides light from the light source to the objective lens, and guides reflected light from the recording medium that has passed through the objective lens to the detector. It has an optical system.

In general, a laser diode, which is a semiconductor laser, is used as a light source of an optical pickup. However, since a laser diode emits a laser beam from the end face of a thin band-shaped oscillation region, the emitted laser beam becomes a divergent beam having an elliptical cross section. If the diverging beam is converged by the objective lens as it is, the beam is irradiated only to a part of the circular recording area or to the outside of the recording area, and the accuracy of recording and reproduction is reduced. This leads to a decrease in the use efficiency of the laser. Therefore, it is necessary to shape the beam so that the cross section on the recording medium is circular.

In recent years, the recording density of a recording medium, as represented by a blue DVD, has become extremely high, and C / N
From the viewpoint of characteristics as well, it is required that the beam be shaped into a circular cross section to improve the laser use efficiency.

[0005] Beam shaping is usually performed by an anamorphic prism. However, an anamorphic prism must be used in a parallel beam and cannot directly shape the diverging beam from a laser diode. For this reason, an element for forming a parallel beam is required, and the laser diode and the anamorphic prism cannot be arranged close to each other, which increases the size of the optical system for shaping.

When beam shaping is performed using an anamorphic prism, an optical system for guiding light reflected by a recording medium to a detector becomes complicated and large. The light that is turned into a convergent beam by the objective lens and reflected by the recording medium is
This is because a parallel beam is formed by passing through the objective lens again, and an element having power is required to converge the beam on the detector.

In order to eliminate such inconvenience and reduce the size of the optical pickup, an element for directly shaping a divergent beam, that is, an shaping element usable in a finite system has been proposed. Such a shaping element is shown in FIG. Shaping element 50
In this example, both the incident side surface 51 and the exit side surface 52 are anamorphic surfaces having different curvatures in two orthogonal directions. In order to suppress the occurrence of aberration, both surfaces 51 and 52 are aspherical. By using the shaping element 50, the laser beam from the laser diode 53 can be a beam having a substantially circular cross section with almost no occurrence of aberration.

[0008]

However, the shaping element 5
0 is not suitable for mass production although it has high performance.
First, since the shapes of the surfaces 51 and 52 are complicated, it is difficult to accurately produce a mold for molding. It is also difficult to transfer the surface shape with high precision during molding. Furthermore, since the surfaces 51 and 52 are both anamorphic, it is not easy to align the molds necessary to make the optical axes coincide.

Therefore, even if this shaping element can be realized, it is inevitable that the cost will increase. Optical systems that use this shaping element to provide a light beam having a substantially circular cross section are naturally also expensive.

The present invention has been made in view of the above problems, and has as its object to provide a light beam irradiation optical system which emits a divergent beam having a substantially circular cross section and which is easy to manufacture. I do. It is another object of the present invention to provide a small optical pickup using such a light beam irradiation optical system.

[0011]

According to the present invention, there is provided a light source for emitting a diverging light beam having an elliptical cross section, and a light beam transmitted by transmitting the light beam emitted from the light source. In a light beam irradiation optical system comprising a shaping element for making a cross section of a beam substantially circular, the first surface on the incidence side and the second surface on the emission side of the shaping element are arranged only in the short axis direction of the cross section of the light beam. A spherical or aspherical cylindrical surface having a curvature, and the optical axis of the shaping element in the minor axis direction of the cross section of the light beam coincides with the principal ray of the light beam.
Satisfies the relationship

1.2 · s / (n + 1) ≦ R1 ≦ 0.85 · s / (n + 1) Equation 1 R2 = R1 ｛{t / (ns) −1} ² + ｛t / (n ・) s) -1｝ · (t−t / n) Formula 2 Here, s is the distance on the optical axis from the light source expressed as a negative value to the first surface, and R 1 is the light on the first surface. Radius of curvature near the axis,
R2 is the radius of curvature of the second surface near the optical axis, t is the thickness of the shaping element on the optical axis, and n is the refractive index of the shaping element.

The first and second surfaces of the shaping element are cylindrical surfaces, and have power only in the short-axis direction of the cross section of the light beam from the light source. Since the distance s is a negative value, the radius of curvature R1 is also negative and the first surface is concave.
Therefore, the shaping element can increase the degree of divergence of the light beam in the short-axis direction on the first surface, whereby the light beam having an elliptical cross section is converted into a light beam having a substantially circular cross section. .

In general, a concave surface having a radius of curvature R provided on the surface of a medium having a refractive index of n has a relation R = s / (n + 1) with respect to light from a point light source separated by a distance s. , An aplanatic surface free of any aberration.
Even if this relationship does not hold strictly, by satisfying the relationship of Expression 1, the first surface of the shaping element can be made approximately an aplanatic surface, and the occurrence of various aberrations can be suppressed well. Is possible.

By transmitting through a shaping element having a higher refractive index than air, the diverging light beam apex moves from the position of the light source toward the shaping element. Here, since the degree of divergence is increased only in the short-axis direction, usually, a difference occurs between the movement amount of the vertex in the short-axis direction and the movement amount of the vertex in the long-axis direction. , Simply referred to as astigmatic difference). However, when the radius of curvature R2 of the second surface satisfies Expression 2, the amount of movement of the vertex in the short axis direction is equal to the amount of movement of the vertex in the long axis direction, and the astigmatic difference occurs. Can be prevented.

In order to achieve the above object, the present invention also provides a light source for emitting a diverging light beam having an elliptical cross section, and a light beam emitted from the light source for transmitting the light beam. In a light beam irradiation optical system including a shaping element that forms a circle, a first surface near the light source of the shaping element and a second surface far from the light source have a spherical surface having a curvature only in a short-axis direction of a cross section of the light beam. It is an aspherical cylindrical surface, and the optical axis of the shaping element in the minor axis direction of the cross section of the light beam coincides with the principal ray of the light beam, and satisfies the following equations 3, 4, and 5. .

R 1 = s / (n + 1) Equation 3 R 2 = (s−t / n) · (s−n · t) / {s · (n + 1)} Equation 4 0.2 ≦ | s | / t .Ltoreq.1.5 Equation 5 s, R1, R2, t, and n are as described above.

The principle of converting a light beam having an elliptical cross section into a light beam having a substantially circular cross section is the same as that of the above-described optical system set so as to satisfy Expressions 1 and 2. By satisfying Expression 3, the first surface is strictly an aplanatic surface with respect to the light beam from the light source, and no aberration occurs at the first surface. Also,
By satisfying Expression 4, the moving amount of the vertex in the long axis direction becomes equal to the moving amount of the vertex in the short axis direction, and no astigmatic difference occurs.

When | s | / t = 1, the incident angle of the light beam with respect to the second surface in the minor axis direction can be set to 0 °. In this case, the second surface does not cause any refraction of the light beam in the minor axis direction, and no spherical aberration occurs on the second surface. | S | / t =
If 1 does not hold, the angle of incidence of the light beam on the second surface cannot be reduced to 0 ° and the occurrence of spherical aberration cannot be avoided, but | s | / t must be within the range of Expression 5. Thus, spherical aberration can be suppressed favorably.

Each of the above light beam irradiation optical systems is very simple because it has only a shaping element having two cylindrical surfaces in addition to the light source. Moreover, since the directions in which the two cylindrical surfaces have the curvature coincide, the fabrication of the shaping element is easy, and the positioning of the shaping element and the light source is also easy. Further, a light beam having a substantially circular cross section can be obtained simply by determining the radius of curvature of the first surface of the shaping element according to the ratio of the long axis to the short axis of the light beam emitted by the light source. Therefore, there is almost no restriction on the light source, and a light source having a small to large ratio of the major axis to the minor axis of the light beam can be used.

Here, it is preferable that the light source is a semiconductor laser, which is fixed to the shaping element. An optical system capable of providing a laser beam having a substantially circular cross section is provided, and a single optical member facilitates handling.

In order to achieve the above object, the present invention further provides an optical pickup for irradiating a recording medium with a light beam and detecting the light beam reflected by the recording medium. An objective lens for converging the light beam from the light beam irradiation optical system onto the recording medium, a detector for detecting the light beam from the recording medium transmitted through the objective lens, and a light beam from the light beam irradiation optical system. A branch optical system that guides the light beam from the recording medium that passes through the objective lens to the detector.

The branch optical system is located between the light beam irradiating optical system and the objective lens, and guides the divergent light beam from the light beam irradiating optical system to the objective lens and converges from the recording medium transmitted through the objective lens. Light beam to the detector. Therefore, there is no need to separately provide an optical system for converging the light beam on the detector, and the optical pickup has a simple configuration.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a light beam irradiation optical system and an optical pickup according to the present invention will be described with reference to the drawings. Light beam irradiation optical system 1 of the first embodiment
1 is schematically shown in FIG. The light beam irradiation optical system 1 includes a light source 10 and a shaping element 20. Light source 1
Numeral 0 denotes a laser beam which emits a divergent beam whose cross section perpendicular to the principal ray is elliptical, for example, a laser diode.

Hereinafter, the axis coincident with the principal ray of the light beam emitted from the light source 10 is the Z axis, the axis parallel to the short axis of the cross section of the light beam is the X axis, and the axis parallel to the long axis of the cross section of the light beam is Defined as the Y axis. The X, Y, and Z axes are orthogonal to each other. In FIG. 1, (a) is an XZ section including the X axis and the Z axis,
(B) is a YZ section including the Y axis and the Z axis.

The shaping element 20 transmits the light beam emitted from the light source 10 and makes the cross section of the transmitted light beam substantially circular. Cross section S0 of the light beam before entering the shaping element 20
FIG. 2 schematically shows the cross section S1 of the light beam after passing through the shaping element 20. Here, the cross section S0 before incidence is shown.
Although the long axis (diameter) of the cross section S1 after transmission is equal to the long axis of the cross section S1 after transmission, the light beam is still a divergent beam even after transmission through the shaping element 20, and the cross section S1 after transmission is the same as that before incidence. It is larger than the cross section S0.

The shaping element 20 has a first surface 21 on which the light beam from the light source 10 is incident, and a second surface 22 on which the incident light beam is emitted. The first surface 21 and the second surface 22 are both cylindrical surfaces having a curvature only in the X-axis direction and no curvature in the Y-axis direction. That is, each of the first surface 21 and the second surface 22 has power only in the short axis direction of the cross section of the light beam from the light source 10 and has no power in the long axis direction. Further, the first surface 21 is a concave surface, and the second surface 22 is a convex surface.

The central axis of curvature of the first surface 21 and the second surface 2
The central axis of the curvature of 2 is parallel, and the principal ray of the light beam from the light source 10 is orthogonal to the central axis of the curvature of the surfaces 21 and 22.
That is, the optical axes of the first surface 21 and the second surface 22 in the X-axis direction coincide with each other, and also coincide with the principal ray of the light beam. Since the central axes of the curvatures of the surfaces 21 and 22 are parallel, no eccentricity occurs in the direction along the central axis, and the shaping element 20 can be easily manufactured. Further, the position of the light source 10 with respect to the shaping element 20 can be easily set.

The concave first surface 21 increases the degree of divergence of the light beam having an elliptical cross section from the light source 10 in the minor axis direction. The shaping element 20 is thereby
By making the degree of divergence in the short axis direction substantially equal to the degree of divergence in the long axis direction, the cross section of the transmitted light beam is made substantially circular.
The degree of divergence of the light beam from the light source 10 in the major axis direction decreases at the time of incidence and increases at the time of emission due to the difference in refractive index between the shaping element 20 and air. However, the first surface 21 and the second surface 22 The shaping element 2 is parallel in the Y-axis direction.
It does not change even after transmitting 0.

Hereinafter, the refractive index of the shaping element 20 is n, the radius of curvature of the first surface 21 near the optical axis is R1, the radius of curvature of the second surface 22 near the optical axis is R2, and the first light source 10 The distance on the optical axis to the surface 21 is represented by s, and the thickness of the shaping element 20 on the optical axis is represented by t. However, the distance s from the light source 20 to the first surface 21 is determined to be negative. The thickness t of the shaping element 20 is positive.

The first surface 21 which controls the shaping function is provided on the light source 10.
In the vicinity of the optical axis, an aplanatic surface that does not generate any aberration with respect to light from the lens. That is, the radius of curvature R1 of the first surface 21, the distance s from the light source 10 to the first surface 21, and the refractive index n of the shaping element 20 are set so as to satisfy the relationship of Expression 3. R1 = s / (n + 1)
… Equation 3 (reprinted)

Since the degree of divergence is increased by the first surface 21, the position of the vertex (vertex in the XZ plane) of the light beam transmitted through the shaping element 20 in the minor axis direction is determined by the light source 10.
From the position to the shaping element 20. Further, although the degree of divergence does not change, the optical path length is shortened by transmitting through the shaping element 20, so that the vertex (vertex in the YZ plane) in the long axis direction of the light beam transmitted through the shaping element 20 is determined. The position also moves toward the shaping element 20 from the position of the light source 10. The post-movement position of the vertex in the short axis direction is P
x represents the position of the vertex after movement in the long axis direction as Py
And shown in FIG.

The moving amount of the vertex in the short axis direction is the first.
Radius R1 of the surface 21 and radius R2 of curvature of the second surface 22
And the refractive index n of the shaping element 20 and the distance s from the light source 10 to the first surface 21. On the other hand, the moving amount of the vertex in the major axis direction depends on only the refractive index n of the shaping element 20 and the thickness t of the shaping element, and is t / (1−n).

If the moving amount of the vertex in the short axis direction is not equal to the moving amount of the vertex in the long axis direction, the moved vertices will have different positions Px and Py, and the shaping element 2
An astigmatic difference is generated in the light beam transmitted through 0. When the astigmatic difference occurs, astigmatism occurs when the light beam is subsequently converged by an ordinary optical system having axial symmetry. In the light beam irradiation optical system 1, the amount of movement of the vertex in the short axis direction is made equal to the amount of movement of the vertex in the long axis direction in order to prevent astigmatism.

This is because the radius of curvature R2 of the second surface 22 is:
This is realized by making the refractive index n of the shaping element, the distance s from the light source 10 to the first surface 21, and the thickness t of the shaping element satisfy the relationship of Expression 4. R2 = (s−t / n) · (s−n · t) / {s · (n + 1)} Equation 4 (again)

In the light beam irradiation optical system 1, the distance s from the light source 10 to the first surface 21 and the thickness t of the shaping element 20 are set so as to satisfy the relationship of Expression 5. This is to reduce the angle of incidence of the light beam on the second surface 22 in the minor axis direction, thereby suppressing spherical aberration. 0.2 ≦ | s | /t≦1.5 Equation 5 (again)

When | s | / t = 1, as shown in FIG. 1A, the incident angle of the light beam on the second surface 22 in the short axis direction is set to 0 ° (perpendicularly incident). be able to. In this case, the second surface 22 does not cause any refraction of the light beam in the short axis direction, and therefore, there is no aberration on the second surface 22. That is, the aberration due to the spherical component of the shaping element 20 is completely eliminated. At this time, the movement amount of the vertex in the short axis direction is | s | / (1-n), which is equal to the movement amount t / (1-n) of the vertex in the long axis direction.

Although the shaping element 20 is a flat plate having no curvature in the Y-axis direction, it causes spherical aberration in the major axis direction of the diverging light beam. Since the occurrence of this spherical aberration is unavoidable, it will be removed later by another optical system that handles the light beam, but with an ordinary optical system having axial symmetry, without introducing new aberration in the short axis direction It is impossible to remove spherical aberration in the major axis direction. Therefore, the second surface 22 is made not a spherical surface but an aspherical surface so as to generate a spherical aberration in the short axis direction of the light beam which is substantially the same as that in the long axis direction.
The light beam after shaping is given a substantially axially symmetric spherical aberration.

In general, the magnitude of the spherical aberration on the image plane is expressed as an even-order polynomial of the image height (distance from the principal ray). Term becomes dominant. Therefore, the second surface 2
In 2, the second-order term causes a dominant spherical aberration.

By setting the distance s from the light source 20 to the first surface 21 and the thickness t of the shaping element 20 so as to satisfy Equation 5, the aberration due to the spherical component is sufficiently reduced, so that the second-order term is obtained. It is possible to determine the aspherical component of the second surface 22 necessary to generate a dominant spherical aberration. When the relationship of Expression 5 is no longer satisfied, the fourth order to the aberration in the X-axis direction,
The contribution of higher-order terms such as the sixth order increases, and the second surface 22
Is not an aspheric surface, the spherical aberration in the major axis direction and the spherical aberration in the minor axis direction of the light beam cannot be equalized.

Table 1 shows configuration data of a first specific example of this embodiment, and FIG. 3 shows a cross-sectional view at the setting. In Table 1, RDX is the radius of curvature (mm) near the optical axis in the X-axis direction, RDY is the radius of curvature (mm) near the optical axis in the Y-axis direction, T is the distance between surfaces on the optical axis (mm), and MED. Is a medium. The shaping element 20 is made of glass, and has a refractive index n of 1.90641, 1.91342, and 1.92124 for wavelengths of 415, 405, and 395 nm, respectively.

The aspherical surface is defined by Equation 6. ^{Z = (X 2 / RDX +} Y 2 / RDY) / [1 + {(1- (1 + KX) · X 2 / RDX 2 - (1 + KY) · Y 2 / RDY 2} 1/2] + AR · {(1-AP) ^{· X 2 + (1 + AP} ) · Y 2} 2 + BR · {(1-BP) · X 2 + (1 + BP) · Y 2} 3 + CR · {(1-CP) · X 2 + (1 + CP) · Y 2 ｝ ⁴ + DR ｛(1−DP)） X ² + (1 + DP) ・ Y ² ｝ ⁵ ... Equation 6

<Table 1> Surface RDX RDY T MED 10 ∞ ∞ 1.0000 Air 21 -0.34324 ∞ 1.0000 Glass 22 -1.52262 非 Aspherical surface coefficient of surface 22 KX = 0 KY = 0 AR = 0 .23469 × 10 ⁻² BR = 0.95831 × 10 ⁻³ CR = 0 DR = 0 AP = −0.10000 × 10 ¹ BP = −0.10000 × 10 ¹ CP = 0 DP = 0

In this setting example, | s | / t = 1. The RMS of wavefront aberration when the shaped light beam is sufficiently converged by an optical system having axial symmetry is 0.002λ when the wavelength of the light beam is represented by λ, and the spherical aberration is sufficiently large. It is suppressed.

Table 2 shows the configuration data of the second setting example.
In this setting example, the shaping element 20 is made of a polycarbonate resin, and its refractive index n is set at wavelengths 415, 405, and 3
1.6148 and 1.618 respectively for 95 nm
46, 1.62255.

<Table 2> Surface RDX RDY T MED 10 {} 1.0000 Air 21-0.38190 {1.0000 Resin 22-1.6787} {Aspherical surface coefficient of surface 22 KX = 0 KY = 0 AR = 0 .27685 × 10 ⁻² BR = 0.95107 × 10 ⁻³ CR = 0 DR = 0 AP = −0.10000 × 10 ¹ BP = −0.10000 × 10 ¹ CP = 0 DP = 0

Also in this setting example, | s | / t = 1. The RMS of the wavefront aberration when the shaped light beam is sufficiently converged by an optical system having axial symmetry is 0.00.
1λ, and the spherical aberration is sufficiently suppressed.

The light beam irradiation optical system 2 according to the second embodiment will be described. The light beam irradiation optical system 2 of the present embodiment is a modification of the light beam irradiation optical system 1 of the first embodiment, and the components, the shaping principle, and the principle of generating no astigmatic difference are the same. Only the differences will be described.

In the light beam irradiation optical system 1, the first surface 21 of the shaping element 20 is an aplanatic surface which satisfies the expression (3). 21 can be favorably suppressed. The light beam irradiation optical system 2 satisfies the relationship of Expression 1. 1.2 · s / (n + 1) ≦ R1 ≦ 0.85 · s / (n + 1) Equation 1 (again)

When the radius of curvature R1 of the first surface 21 exceeds the upper limit of the expression 1, the symmetry of the spherical aberration decreases. If the radius of curvature R1 does not reach the lower limit of Expression 1, the symmetry of spherical aberration is reduced, and the effect of increasing the degree of divergence of the light beam, that is, the shaping magnification is also reduced. By satisfying the relationship of the expression 1, while suppressing the occurrence of various aberrations due to the first surface 21, the light beam having a ratio of the major axis to the minor axis of the cross section of about 3 is converted into a substantially circular cross section and the spherical aberration is also reduced. A substantially axially symmetric light beam can be obtained.

In the case where the relationship of the expression 3 is not satisfied, it is necessary to satisfy the relationship of the expression 2 instead of the expression 4 in order to prevent the astigmatic difference from being generated. Is set to R2 = R1 ｛{t / (ns) -1} ² + {t / (ns) -1}｝ (tt / n) Equation 2 (represented)

Table 3 shows the configuration data of a specific setting example of this embodiment. In this setting example, the shaping element 20 is made of glass, and the refractive index n thereof is 415, 405, 3
1.90641, 1.913 respectively for 95 nm
42, 1.92124.

<Table 3> Surface RDX RDY T MED 10 {} 1.0000 Air 21-0.32608 {1.0000 Glass 22-1.48284} Aspherical surface coefficient of surface 22 KX = 0 KY = 0 AR = 0 .23277 × 10 ⁻² BR = 0.85384 × 10 ⁻³ CR = 0 DR = 0 AP = −0.10000 × 10 ¹ BP = −0.10000 × 10 ¹ CP = 0 DP = 0

In each of the above embodiments, the first surface 21 of the shaping element 20 is a spherical surface.
May be an aspherical surface. Since the first surface 21 and the second surface 22 are cylindrical surfaces whose central axes of curvature are parallel to each other, the shaping element 2 can be formed even if the first surface 21 is aspheric.
0 and alignment of the light source 10 with respect to the shaping element 20 does not become difficult. Here, in order to facilitate handling of the shaped light beam, the second surface 22 is made aspherical to intentionally generate spherical aberration. However, the shaped light beam is anamorphic. When handling with an optical system, the second surface 22 may be a spherical surface.

The light source 10 and the shaping element 20 are integrated by defining their relative positions so as to satisfy Equations 1 and 2 or Equations 3 to 5. Thus, the light beam irradiation optical systems 1 and 2
A single optical member facilitates use in various devices.

FIG. 4 schematically shows the structure of the optical pickup 3 according to the third embodiment. The optical pickup 3 is provided in an optical recording / reproducing apparatus using a magneto-optical disk for recording information by magneto-optical as a recording medium M. The optical pickup 3 includes a laser irradiation optical system 31, an objective lens 32, a composite prism 33, a PDIC (light detection integrated circuit) plate 34 for reproduction, a diffraction grating 35, and a photodetector 36 for monitoring.

The laser irradiation optical system 31 is the above-described light beam irradiation optical system 1 or 2 and includes a laser diode as the light source 10. The laser diode 10 and the shaping element 20 are arranged in the positional relationship described in the first embodiment or the second embodiment, and the laser irradiation optical system 31 includes a laser beam having an elliptical cross section emitted from the laser diode 10. Is emitted as a laser beam having a substantially circular cross section and no astigmatic difference.

The objective lens 32 includes a laser irradiation optical system 31.
Are converged on a small circular recording area of the recording medium M as a convergent beam. Objective lens 3
2 is arranged such that its optical axis coincides with the principal ray of the laser beam from the laser irradiation optical system 31. However, the objective lens 32 is set to be displaceable relative to the laser irradiation optical system 31 in a direction parallel to a principal ray of the laser beam and a direction perpendicular to the laser irradiation optical system 31 for focusing and tracking on the recording medium M.

Since the laser beam emitted from the laser irradiation optical system 31 has no astigmatic difference, the cross section of the beam on the recording medium M is circular even if the objective lens 32 is displaced in the optical axis direction for focusing. And only slightly changes in size. Even if the objective lens 32 is displaced in the direction perpendicular to the optical axis for tracking, the cross-sectional shape of the beam on the recording medium M hardly changes.

The composite prism 33 has three prisms 33.
a, 33b, 33c. Polarization separation (PB) is applied to the joint surface between the prisms 33a and 33b.
S) A film 41 is provided, and a semi-transmissive (half mirror) film 42 is provided on the joint surface between the prisms 33b and 33c. Further, a reflection film 43 is provided on the surface of the prism 33c. Polarization separation film 41, semi-transmissive film 4
2. The bonding surface or surface on which the reflection film 43 is provided is parallel to each other. The composite prism 33 includes the laser irradiation optical system 3
Polarization separation film 41 for the principal ray of the laser beam from
Irradiate the laser irradiation optical system 3 so that
1 and an objective lens 32.

The laser beam from the laser irradiation optical system 31 enters the composite prism 33 as a divergent beam, and a part of the laser beam is reflected by the polarization separation film 41 and enters the monitor photodetector 36. The photodetector 36 detects the intensity of the incident laser beam and outputs a signal indicating the intensity. This signal is given to a control circuit (not shown) for controlling the light emission of the laser diode 10, and is used to keep the laser intensity constant.

The laser beam transmitted through the polarization splitting film 41 enters the objective lens 32 as a divergent beam, and is guided to the recording medium M as a convergent beam. This laser beam is reflected by the recording medium M and becomes a divergent beam containing the MO signal. The laser beam from the recording medium M passes through the objective lens 32, becomes a convergent beam, enters the composite prism 33, and is reflected by the polarization splitting film 41. The laser beam reflected by the polarization splitting film 41 travels through the composite prism 33 as a convergent beam and reaches the semi-transmissive film 42, and is partially reflected by the PDIC plate 34.
It is led to.

The PDIC board 34 has two photodetectors 34
a and 34b are provided, and a circuit for controlling the objective lens 32 and processing the MO signal is formed. The laser beam reflected by the semi-transmissive film 42 is
converges on a. The PDIC board 34 controls focusing and tracking of the objective lens 32 with respect to the recording medium M based on the intensity and shape of the laser beam detected by the light detection unit 34a. Since the laser beam emitted from the laser irradiation optical system 31 has no astigmatism, the light detection unit 3
4a does not produce astigmatism and therefore
The PDIC board 34 can obtain information necessary for focusing and tracking by a beam size method.

The laser beam transmitted through the semi-transmissive film 42 is
The light is reflected by the reflection film 43 and enters the diffraction grating 35. The diffraction grating 35 polarizes and separates the incident laser beam into a plurality of laser beams, and guides the plurality of laser beams to the light detection unit 34 b of the PDIC plate 34. The PDIC board 34 detects the MO signal accurately from the intensities of the plurality of laser beams and detects the detected MO signal.
The signal is amplified and output. The diffraction grating 35 is provided near the convergence position of the laser beam from the recording medium M,
The diameter of the laser beam incident on the diffraction grating 35 is small.
Therefore, the diffraction grating 35 and the light detecting portion 34b of the PDIC plate 34 are small. Note that a Wollaston prism or a birefringent crystal can be used instead of the diffraction grating 35.

In the optical pickup 3 that directs the divergent beam from the laser irradiation optical system 31 to the objective lens 32 as it is, the laser beam from the recording medium M passes through the objective lens 32 and becomes a convergent beam.
It is not necessary to provide an optical system or element having power to converge the laser beam from the recording medium M. This makes it possible to use a composite prism 33 in which a plurality of prisms are integrated, and as a result, the overall configuration is also reduced in size. In addition, the laser irradiation optical system 31 is a single optical member that integrates the laser diode 10 and the shaping element 20, so that the optical pickup 3 can be easily assembled.

Although the example in which the magneto-optical disk is used as the recording medium M has been described here, the optical system of the optical pickup 3 is as follows.
It can be used for any optical recording / reproducing device.
For example, when an optical disk is used as the recording medium M, the internal circuit of the PDIC board 34 may be changed while keeping the optical system as it is.

[0067]

The light beam irradiation optical system according to the present invention has a very simple configuration, a substantially circular cross section, and
A divergent beam without astigmatism can be provided. Also, it is easy to manufacture and suitable for mass production.

In the optical pickup of the present invention, since the branching optical system is arranged in a finite system between the light beam irradiation optical system and the objective lens, an element for converging the light beam from the recording medium on the detector is used. There is no need to provide separately, and miniaturization is easy.

[Brief description of the drawings]

FIG. 1 is an XZ sectional view (a) and a YZ sectional view (b) schematically showing a configuration and an operation principle of a light beam irradiation optical system according to first and second embodiments.

FIG. 2 is a diagram schematically illustrating a cross section of a light beam before being incident on a shaping element of the light beam irradiation optical system according to the first and second embodiments and a cross section of a light beam after passing through the shaping element.

FIGS. 3A and 3B are an XZ sectional view (a) and a YZ sectional view (b) of a specific setting example of the light beam irradiation optical system according to the first embodiment.

FIG. 4 is a sectional view schematically showing a configuration of an optical pickup according to a third embodiment.

FIG. 5 is a perspective view schematically showing a configuration of a conventional shaping element.

[Explanation of symbols]

 1, 2 light beam irradiation optical system 10 light source 20 shaping element 21 first surface (incident surface) 22 second surface (outgoing surface) 3 optical pickup 31 laser irradiation optical system 32 objective lens 33 compound prism 33a, 33b, 33c Prism 34 Photodetection integrated circuit board 34a, 34b Photodetection unit 35 Diffraction grating 36 Photodetector 41 Polarization separation film 42 Semi-transmission film 43 Reflection film

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) G11B 7/125 G11B 7/125 A

Claims (4)

[Claims] 1. A light beam irradiation device comprising: a light source that emits a light beam diverging with an elliptical cross section; and a shaping element that transmits the light beam emitted by the light source and makes the cross section of the light beam after transmission substantially circular. In the optical system, the first surface on the entrance side and the second surface on the exit side of the shaping element are spherical or aspherical cylindrical surfaces having a curvature only in the minor axis direction of the cross section of the light beam, and the light beam The optical axis of the shaping element in the minor axis direction of the cross section of the cross section coincides with the principal ray of the light beam, the distance on the optical axis from the light source expressed by a negative value to the first surface is s, and the light on the first surface When the radius of curvature near the axis is R1, the radius of curvature near the optical axis of the second surface is R2, the thickness of the shaping element on the optical axis is t, and the refractive index of the shaping element is n, 1.2 · s /(N+1)≦R1≦0.85·s/(n+
1), and R2 = R1 ｛{t / (ns) −1} ² + ｛t / (ns) −
An optical system for irradiating a light beam, which satisfies a relationship of 1｝ · (t−t / n). 2. A light beam irradiation device comprising: a light source that emits a light beam diverging with an elliptical cross section; and a shaping element that transmits the light beam emitted by the light source and makes the cross section of the light beam after transmission substantially circular. In the optical system, the first surface near the light source of the shaping element and the second surface far from the light source are spherical or aspherical cylindrical surfaces having a curvature only in the short-axis direction of the cross section of the light beam, and the light beam The optical axis of the shaping element in the minor axis direction of the cross section of the cross section coincides with the principal ray of the light beam, the distance on the optical axis from the light source expressed by a negative value to the first surface is s, and the light on the first surface When the radius of curvature near the axis is R1, the radius of curvature near the optical axis of the second surface is R2, the thickness of the shaping element on the optical axis is t, and the refractive index of the shaping element is n, R1 = s / ( n + 1), R2 = (s−t / n) · (s−n · t) / ｛s · (n +
1) A light beam irradiation optical system, which satisfies the relationship of｝ and 0.2 ≦ | s | /t≦1.5. 3. The light beam irradiation optical system according to claim 1, wherein the light source is a semiconductor laser, and is fixed to the shaping element. 4. An optical pickup for irradiating a recording medium with a light beam and detecting a light beam reflected by the recording medium, comprising: a light beam irradiation optical system according to claim 1; An objective lens for converging the light beam from the light beam irradiation optical system onto the recording medium, a detector for detecting the light beam from the recording medium that has passed through the objective lens, and an object for detecting the light beam from the light beam irradiation optical system. An optical pickup comprising: a branch optical system that guides a light beam from a recording medium that has passed through an objective lens to a lens and guides the light beam to a detector.