JP3254284B2 - Optical pickup - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup.

[0002]

2. Description of the Related Art A laser beam radiated from a semiconductor laser is converted into a parallel beam by a coupling lens, a desired beam cross section is formed by a beam shaping prism, and the parallel beam is passed through a beam splitter, and is converted into an optical information recording medium by an objective lens. Focus on the recording surface as a light spot,
Conventionally, an optical pickup configured to guide a reflected light beam from the recording surface to a detection system via the objective lens and the beam splitter has conventionally been used to write, reproduce, or erase information on a compact disk or a magneto-optical disk. It is widely known as a device for performing the above.

For example, FIG. 4 shows a typical example of an optical pickup for writing, reproducing, and erasing information on a magneto-optical disk 50. Semiconductor laser 10
Is emitted from the coupling lens 1
2 is converted into a parallel light beam. As is well known, the far-field pattern of a light beam emitted from a semiconductor laser is
It has an elliptical shape whose major axis is the direction orthogonal to the bonding surface of the semiconductor laser, and the light beam cross-sectional shape of the laser beam converted into a parallel light beam by the coupling lens 12 also has an elliptical shape.

[0004] The major axis direction of an elliptical cross-sectional shape of a laser beam converted into a parallel beam is a direction orthogonal to the drawing of FIG. The collimated light beam is then applied to the beam shaping prism 14.
A enters the incident surface 14a of A in an oblique direction, and is refracted upward in the figure. In this refraction, the parallel light beam is not affected by refraction in a direction perpendicular to the drawing (long axis direction), but the light beam diameter is enlarged in a plane parallel to the drawing due to the influence of the refraction.

For this reason, the beam cross-sectional shape of the parallel light beam whose direction has been changed upward in FIG. 4 by refraction is "beam-shaped" from a previous elliptical shape to a circular shape or an elliptical shape close to a circular shape. The beam shaping prism 14A has an incident surface 1
A right-angle prism 14B is bonded to the surface opposite to the surface 4a, and a light semi-permeable film 14C is formed on the bonding surface. Light semi-permeable membrane 1
The light beam transmitted through 4C is incident on the objective lens 16, becomes a convergent light beam, and is condensed as a light spot on the recording surface 51 of the magneto-optical disk 50.

The light beam reflected by the recording surface 51 enters the right-angle prism 14B via the objective lens 16, is reflected by the light semi-permeable film 14C, and is separated to the "detection system" side.
That is, the beam shaping prism 14A, the light semi-permeable film 14C, and the right-angle prism 14C constitute a “beam splitter”. Although the beam shaping prism and the beam splitter can be configured separately from each other, as shown in this example, one optical function having two optical functions, that is, a “beam shaping function” and a “beam splitting function” is used. It can also be integrated as the element 14.

The light beam separated to the detection system side is first condensed by the condensing lens 18 and travels while converging.
Approximately half of the light beam is reflected by the reflecting prism 20, and the rest is further converged to generate a focusing error signal.
The light is incident on the divided light receiving elements 22 to generate a focusing error signal by the well-known “knife edge method”. That is, the ridge portion of the reflecting prism 20 that has entered the converged light beam forms a “knife edge”.

The light beam reflected by the reflecting prism 20 is transmitted through the half-wave plate 24, is rotated by 45 degrees on the polarization plane, is incident on the polarization beam splitter 26, and is transmitted light of P polarization and reflected light of S polarization. Is separated into These transmitted light and reflected light enter the light receiving elements 28 and 30, respectively. The light receiving element 28 is a two-divided light receiving element, and generates a tracking error signal by a well-known push-pull method.

The reproduction signal is composed of "the difference between the sum of outputs from the light receiving elements of the light receiving element 28 and the output of the light receiving element 30", and the pre-pit signal is, for example, "the output from each light receiving part of the light receiving element 28". And the sum of the output of the light receiving element 30 ".

As is well known, the semiconductor laser 1
The wavelength of the laser beam emitted from 0 varies depending on the ambient temperature. In order to prevent this wavelength fluctuation, the temperature of the semiconductor laser may be controlled to be constant, but an expensive temperature controller is required, and the light source itself is undesirably complicated and large.

In order to prevent the wavelength fluctuation due to the temperature change from affecting the operation of the optical pickup, first, it is necessary to correct the chromatic aberration of the coupling lens 12, the objective lens 16, and the condenser lens 18. The corrected lens is expensive and cannot be made of a single material, so the lens itself becomes large.

Recently, the objective lens 16 and the coupling lens 1 have been used to reduce the cost and size and weight of the optical pickup.
For example, an aspherical single lens has been used as a second lens. However, even if a low dispersion type glass material is used as a material for these lenses, the chromatic aberration cannot be completely corrected by the single lens. In addition, since a high focusing performance is not required for the focusing lens 18, a single spherical lens is generally used as the focusing lens 18.

If the lens included in the optical pickup has chromatic aberration as described above, the focusing control by the knife edge method is affected. This effect will be described for each of the condenser lens, the coupling lens, and the objective lens.

FIG. 5 shows that the objective lens 16,
The optical path of the reflected light flux reaching the light receiving element 22 via the condenser lens 18 is linearly developed. In the drawing, the light beam indicated by the solid line is for the design reference wavelength: λ, and in the state shown in FIG.
Light is converged on 1.

When the wavelength shifts to λ ₁ (> λ) due to the temperature change of the semiconductor laser 1, the convergent light beam by the condenser lens 18 becomes as shown by the broken line in FIG. The light condensing position is shifted to the rear of the light receiving element 22. Then, although the light spot is actually focused on the recording surface correctly, the generated focus error signal is generated as if “the recording surface 51 has approached the objective lens 16” and an appropriate Impedes focusing control.

Conversely, when the wavelength shifts to λ ₂ (<λ) due to a temperature change of the semiconductor laser 1, the convergent light beam by the condenser lens 18 becomes as shown by a broken line in FIG. Shifts forward of the light receiving element 22. Then
Although the light spot is actually correctly focused on the recording surface, the generated focusing error signal is generated as if "the recording surface 51 has moved away from the objective lens 16", and proper focusing control has been performed. Hinder.

FIG. 7 is a diagram for explaining the effect of chromatic aberration of the coupling lens 12 on focusing control. In order to avoid complication, the beam shaping prism is omitted, and the configuration of the optical pickup is simplified. As in FIG. 5, the solid-line light beam is for the reference wavelength: λ, and the light spot is correctly focused on the recording surface 51 in the state shown in FIG.

When the wavelength shifts to λ ₁ (> λ) due to the temperature change of the semiconductor laser 1, the light beam transmitted through the coupling lens 12 becomes slightly divergent as indicated by a broken line, while the light beam which should be a parallel light beam is shown. The focus position of the light spot by the objective lens 16 is shifted to a position (shown by a broken line) behind the recording surface 51 shown by a solid line. Therefore, when the recording surface 51 is at the position shown by the broken line, the light spot is properly focused on the recording surface 51, but the light beam reflected by the recording surface at the position shown by the broken line slightly converges when transmitted through the objective lens 16. And is further converged by the condenser lens 18,
The condensing position is shifted to the front of the light receiving element 22 as shown by a broken light beam. Then, although the light spot is actually focused on the recording surface (indicated by the broken line) 51 correctly,
The generated focus error signal is generated as if “the recording surface 51 has moved away from the objective lens 16” and hinders proper focusing control.

Conversely, if the wavelength shifts to λ ₂ (<λ) due to a temperature change of the semiconductor laser 1, the coupling lens 1
The light beam transmitted through 2 becomes slightly convergent, and the condensing position of the light spot is shifted to the objective lens side from the recording surface 51 shown by the solid line. When the light spot is correctly converged on the recording surface shifted as described above, the condensing position of the convergent light beam by the condensing lens 18 is shifted to the rear of the light receiving element 22, and the light spot is actually correctly focused on the recording surface. Despite being in focus, a focusing error signal is generated as if “the recording surface 51 has approached the objective lens 16”, preventing proper focusing control.

FIG. 8 is a diagram for explaining the effect of the chromatic aberration of the objective lens 16 on the focusing control. For simplicity, as in FIG. 7, the beam shaping prism is omitted and the configuration of the optical pickup is simplified. As in FIG. 5, the solid light beam is the reference wavelength:
The light spot is correctly focused on the recording surface 51 in the state shown in FIG.
(A)).

When the wavelength shifts to λ ₁ (> λ) due to a change in the temperature of the semiconductor laser 1, the convergence point of the light spot is changed due to the chromatic aberration of the objective lens 16, as shown by the broken line in FIG. It shifts to the rear of the recording surface 51. As described above, in a state where the condensing position of the light spot and the recording surface 51 are shifted, the light beam reflected by the recording surface 51 is
After passing through 6, the light becomes slightly divergent as indicated by a broken line, and is condensed by the condenser lens 18 and then condensed behind the light receiving element 22.

Therefore, the generated focusing error signal is a signal indicating that "the recording surface 51 has moved away from the objective lens". Based on this focusing error signal, the focusing servo mechanism moves the objective lens 16 toward the light source by the distance: x in the figure, and shifts the condensing position of the light spot on the recording surface 51 as shown in FIG. Position. In this state, the light beam reflected by the recording surface 51 is focused on the light receiving element 22. Similarly, when the wavelength is shifted to λ ₂ (<λ), defocus due to chromatic aberration of the objective lens 16 is corrected by ordinary focusing control. That is, the chromatic aberration of the objective lens 16 does not cause an error in the focusing error signal. Therefore, when the wavelength of the semiconductor laser fluctuates, it is the chromatic aberration of the coupling lens 12 and the condenser lens 18 that affects the focusing control.

As described above, the error of the focusing error signal caused by the chromatic aberration generated in the coupling lens 12 and the condenser lens 18 due to the wavelength fluctuation is called "focusing offset caused by chromatic aberration".

As described above, when the wavelength shifts to λ ₁ (> λ), the chromatic aberration caused by the coupling lens 12 has the effect of shifting the condensing position of the convergent light beam by the condensing lens 18 toward the light receiving element 22. On the contrary, the chromatic aberration of the condenser lens 18 has the effect of shifting the focusing position to the rear of the light receiving element 22. Conversely, if the wavelength shifts to λ ₂ (<λ), the chromatic aberration caused by the coupling lens 12 has the effect of shifting the condensing position of the convergent light beam by the condenser lens 18 to the rear of the light receiving element 22. Conversely, the chromatic aberration has the effect of shifting the focus position to the front side of the light receiving element 22.

Therefore, when the wavelength of the semiconductor laser changes, the influence of the chromatic aberration of the coupling lens 12 and the effect of the chromatic aberration of the condenser lens 18 give opposite errors to the focusing error signal.

However, generally, a low-dispersion glass material is used for the coupling lens 12 and the chromatic aberration is relatively small. On the other hand, an inexpensive glass material such as BK7 and SF11 is used for the condenser lens 18 and the chromatic aberration is reduced. In general, the above-mentioned “focusing offset caused by chromatic aberration” in an optical pickup is mainly affected by chromatic aberration of the condenser lens 18 and the wavelength is λ ₁ (> λ).
, The condensing position of the convergent light flux by the condensing lens 18 shifts to the rear of the light receiving element 22, and conversely, the wavelength becomes λ ₂ (<
When the position shifts to λ), the light condensing position shifts to the rear of the light receiving element 22.

An error in the focusing error signal due to the wavelength fluctuation occurs in addition to the chromatic aberration. That is, since the refractive index of the transparent body generally changes depending on the wavelength due to dispersion, the “refraction angle” of the incident surface 14a of the beam shaping prism 14A also changes due to the wavelength change.

FIG. 6 conceptually shows the influence of the change in the refractive index on the focusing control. The broken line shows the light beam for the aforementioned reference wavelength: λ. Wavelength is λ
₁ (> λ), the refractive index of the beam shaping prism 14A becomes relatively small, the refraction angle increases, and the refracted chief ray travels as shown by the solid line in FIG. The principal ray of the reflected light beam also travels as indicated by the solid line. As a result, even if the light spot is properly focused on the recording surface, the focusing error signal is generated as if "the recording surface was close to the objective lens". Conversely, when the wavelength shifts to λ ₂ (<λ), the refractive index of the beam shaping prism 14A becomes relatively large, the refraction angle becomes small, and the refracted principal ray is as shown by the solid line in FIG. The principal ray of the light beam reflected by the recording surface also advances as indicated by the solid line. As a result, even if the light spot is properly focused on the recording surface, the focusing error signal is generated as if "the recording surface has moved away from the objective lens".

As described above, the error of the focusing error signal caused by the change in the refractive index caused in the beam shaping prism 14A due to the wavelength change is called "focusing offset caused by the change in the refraction angle".

[0030]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has effectively reduced an error in a focusing error signal due to a variation in oscillation wavelength of a semiconductor laser regardless of the chromatic aberration of a lens. The purpose is to provide an optical pickup.

[0031]

According to an optical pickup of the present invention, a laser beam emitted from a semiconductor laser is converted into a parallel beam by a coupling lens, a desired beam cross section is formed by a beam shaping prism, and the parallel beam is converted into a beam splitter. An optical pickup configured to converge a light spot on a recording surface of an optical information recording medium as an optical spot by an objective lens through the objective lens and guide a reflected light beam from the recording surface to a detection system via the objective lens and the beam splitter. "
In the detection system, the “focusing error signal detection system is a detection system based on the knife edge method.” The light beam from the beam splitter side is converted into a convergent light beam before the knife edge, and the light receiving element of the detection system is used. And a condenser lens that leads to

[0032] The optical pickup according to claim 1 is an object pickup.
Laser beam from the side opposite to the detection system with respect to the optical axis of the lens
An incident surface for incidence is provided on the beam shaping prism,
Knife edge on the same side as the optical information recording medium with respect to the optical axis of the
Bead caused by the wavelength change of the semiconductor laser
Angle change in beam shaping prism: Δθ and beam shaping
Light path from the entrance surface of the prism to the light receiving surface of the light receiving element
Length: The product of L: Δθ · L is caused by the wavelength change
Focusing offset caused by chromatic aberration in the lens
The absolute value of the set is approximately equal to the absolute value of the set . "

In the optical pickup according to the second aspect, the " object "
Laser beam from the same side as the detection system with respect to the optical axis of the lens
The beam-shaping prism is provided with an incident surface where
With respect to the optical axis of the lens
Arrange the wings.

Then, due to the wavelength change of the semiconductor laser,
Refraction angle change in the beam shaping prism that occurs: Δ
θ and the light receiving element from the entrance surface of the beam shaping prism
The product of the optical path length to the surface: L and Δθ · L depends on the wavelength change.
Focusing due to chromatic aberration in the above lens
Should be approximately equal to the absolute value of the sing offset.
And features.

[0035] In the optical pickup according to the claim 1 or 2, may be separate members from the beam shaping prism and a beam splitter, which may be integrated as one optical element (claim 3) .

[0036]

As described above, both the focusing offset caused by the change in the refraction angle and the focusing offset caused by the chromatic aberration depend on whether the change in the oscillation wavelength occurs on the long wavelength side or on the short wavelength side. Plus or minus) is determined. Further, both focusing offsets are generated independently of each other.

Accordingly, it is possible to determine the arrangement of the optical system so as to generate each focusing offset so that these two focusing offsets are reduced to each other, and ideally are canceled each other.

[0038]

EXAMPLES Specific examples will be described below. FIG.
(A) shows an embodiment in which the inventions of claims 1 and 3 are applied to an optical pickup of the type described with reference to FIG. In order to avoid complication, the same reference numerals as those in FIG. 4 are used for those which do not seem to be confused. The description of the parts other than the focusing error signal detection system in the detection system is omitted. The configuration of the omitted part is the same as the configuration of FIG.

The difference between the optical arrangement of the embodiment shown in FIG. 1 and the optical arrangement of FIG. 4 is that, in the embodiment of FIG. 1, the beam shaping prism 14A1, the right-angle prism 14B and the light semi-permeable film 14C are used.
The direction of the incident surface 14a1 of the beam shaping prism 14A1 in the optical element 140 integrating the
4 is set opposite to the case of FIG. 4, and the semiconductor laser 10 and the coupling lens 12 are disposed on the left side of the optical axis.

FIGS. 1B and 1C show a semiconductor laser 10.
5 shows a case where the oscillation wavelength at the time has changed due to a temperature change. The broken line in the figure indicates a light ray (principal ray) for the reference wavelength: λ, and the solid line indicates the principal ray when the wavelength changes. FIG. 1B shows a case where the wavelength is shifted to λ ₁ (> λ), and the “focusing offset caused by a change in the refraction angle” occurs as if “the recording surface has moved away from the objective lens”. . FIG. 1C conversely shows that the wavelength is λ ₂ (<
λ), the “focusing offset due to the change in the refraction angle” occurs as if “the recording surface was closer to the objective lens”.

Therefore, when a wavelength change occurs, the "focusing offset caused by the change in the refraction angle" is generated so as to cancel out the "focusing offset caused by the chromatic aberration". (The sum of the difference caused by the chromatic aberration) is effectively reduced.

The “focusing offset caused by a change in the refraction angle” is defined by the incident surface 1 of the beam shaping prism 14A1
Since L is the optical path length from 4a1 to the light receiving surface of the light receiving element 22 and Δθ is the change in the refraction angle, it is given by approximately Δθ · L, so that this is equal to the absolute value of “focusing offset caused by chromatic aberration”. By setting the optical path length: L, both focusing offsets can be canceled each other to prevent the focusing offset.

FIG. 2A shows an embodiment in which the first and third aspects of the present invention are applied to an optical pickup of the type described with reference to FIG. In order to avoid complication, in FIG. 2, the same reference numerals as those in FIG. 4 are used for those which do not seem to be confused, and the description of the detection system other than the focusing error signal detection system is omitted.

The difference between the optical arrangement of the embodiment shown in FIG. 2 and the optical arrangement of FIG. 4 is that the beam shaping prism 14A2, the right-angle prism 14B1 and the light semi-permeable
Light semi-permeable film 14 in optical element 141 integrating C1
The point is that the direction of C1 is set to “reverse to the case of FIG. 4 with respect to the optical axis of the objective lens 16”, and the detection system is provided on the left side of the optical axis.

FIGS. 2B and 2C show the semiconductor laser 10.
5 shows a case where the oscillation wavelength at the time has changed due to a temperature change. The broken line in the figure indicates the light ray (principal ray) for the reference wavelength: λ, and the solid line indicates the principal ray when the wavelength changes. FIG. 2B shows a case where the wavelength is shifted to λ ₁ (> λ), and the “focusing offset caused by a change in the refraction angle” occurs as if “the recording surface has moved away from the objective lens”. On the contrary, FIG. 2C shows that the wavelength is λ ₂ (<
λ), and the “focusing offset due to the change in the refraction angle” occurs as if “the recording surface was closer to the objective lens”.

Therefore, when a wavelength change occurs, the "focusing offset caused by the change in the refraction angle" is generated so as to cancel out the "focusing offset caused by the chromatic aberration". Therefore, the focusing offset based on the wavelength change is effectively reduced. Become smaller.

The “focusing offset caused by a change in the refraction angle” is approximately the same as Δθ ·
Since L is given by L, if the optical path length: L is set so that this becomes equal to the absolute value of “focusing offset due to chromatic aberration”, both focusing offsets can be canceled out to prevent the focusing offset.

FIG. 3A shows an embodiment in which the invention according to claims 2 and 3 is applied to an optical pickup of the type described with reference to FIG. Also in FIG. 3, the same reference numerals as those in FIG. 4 are used for those which do not seem to be confused, and the description of the detection system other than the focusing error signal detection system is omitted.

The difference between the optical arrangement of the embodiment shown in FIG. 3 and the optical arrangement of FIG. 4 is that, in the embodiment of FIG. 3, the reflecting prism 21 as a knife edge member whose ridge acts as a knife edge. “With respect to the optical axis of the condenser lens 18, FIG.
On the opposite side to the case of the above.

FIGS. 3B and 3C show the semiconductor laser 10.
5 shows a case where the oscillation wavelength at the time has changed due to a temperature change. The broken line in the figure indicates the light ray (principal ray) for the reference wavelength: λ, and the solid line indicates the principal ray when the wavelength changes. FIG. 3B shows the case where the wavelength is shifted to λ ₁ (> λ), and the “focusing offset caused by the chromatic aberration of the optical lens 18” is as if “the recording surface was closer to the objective lens”. appear. FIG. 1 (c) shows that the wavelength is λ
₂ (<λ), and the “focusing offset due to chromatic aberration” occurs as if “the recording surface was far from the objective lens”.

Therefore, when a wavelength change occurs, the "focusing offset caused by the change in the refraction angle" is generated so as to cancel out the "focusing offset caused by the chromatic aberration". Therefore, the focusing offset based on the change in the wavelength is effectively reduced and becomes small. Become.

The “focusing offset caused by the change in the refraction angle” is approximately the same as Δθ ·
Since it is given by L, if the optical path length: L is set so that this becomes equal to the absolute value of “focusing offset caused by chromatic aberration”, both focusing offsets can be canceled each other, and the focusing offset can be prevented.

[0053]

As described above, according to the present invention, a novel optical pickup can be provided. Since the optical pickup of the present invention is configured as described above, regardless of the chromatic aberration of the lens in the knife-edge type focusing error signal detection system, the focusing offset due to the fluctuation of the oscillation wavelength of the semiconductor laser is effectively reduced. , Good focusing control can be realized. As the knife edge member, a known appropriate member can be used in addition to the above-described reflection prism.

It goes without saying that the present invention can be implemented not only as an optical pickup for a magnetic recording disk described in the embodiment but also as an optical pickup for a compact disk.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an embodiment of the invention described in claims 1 and 3 ;

FIG. 2 is a view for explaining one embodiment of the invention described in claims 1 and 3 ;

FIG. 3 is a diagram for explaining one embodiment of the invention described in claims 2 and 3 ;

FIG. 4 is a diagram for explaining a conventional technique.

FIG. 5 is a diagram for explaining chromatic aberration of a condenser lens and an error in focusing control.

FIG. 6 is a diagram for explaining a focusing offset caused by a change in a refraction angle.

FIG. 7 is a diagram for explaining chromatic aberration of a coupling lens and an error in focusing control.

FIG. 8 is a diagram for explaining that chromatic aberration of an objective lens does not affect focusing control.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Semiconductor laser 12 Coupling lens 14A1 Beam shaping prism 14B Right angle prism 14C Light semi-permeable film 16 Objective lens 18 Condensing lens 20 Reflection prism 22 Two-part light receiving element 50 Magneto-optical disk 51 Recording surface

Continuation of front page (56) References JP-A-4-305824 (JP, A) JP-A-4-34740 (JP, A) JP-A-4-281227 (JP, A) JP-A-1-287827 (JP) JP-A-4-291027 (JP, A) JP-A-2-292735 (JP, A) JP-A-4-291026 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) G11B 7/09-7/22

Claims (3)

(57) [Claims] 1. A laser beam emitted from a semiconductor laser is converted into a parallel beam by a coupling lens, a desired beam cross section is formed by a beam shaping prism, and the parallel beam is recorded on an optical information recording medium by an objective lens via a beam splitter. An optical pickup configured to converge a light spot on a surface and guide a reflected light beam from the recording surface to a detection system via the objective lens and a beam splitter, wherein a focusing error signal in the detection system is provided. The detection system is a detection system based on the knife edge method, and the light beam from the beam splitter is convergent on the front side of the knife edge.
Having a condensing lens that converts the light into the light receiving element of the detection system, and from the side opposite to the detection system with respect to the optical axis of the objective lens.
The incident surface on which the laser beam is incident is defined by the beam shaping
Rhythm, and the same as the optical information recording medium with respect to the optical axis of the condenser lens.
The knife edge is arranged on one side, and the beam shaping caused by the wavelength change of the semiconductor laser
Refraction angle change in prism: Δθ and the above beam shaping
The optical path from the entrance surface of the prism to the light receiving surface of the light receiving element
Length: product of L: Δθ · L is generated by the above wavelength change
Focusing error due to chromatic aberration in the above lens
An optical pickup characterized in that the absolute value of the offset is substantially equal to the absolute value of the offset . 2. A laser beam emitted from a semiconductor laser is converted into a parallel beam by a coupling lens, a desired beam cross section is formed by a beam shaping prism, and the parallel beam is recorded on an optical information recording medium by an objective lens via a beam splitter. An optical pickup configured to converge a light spot on a surface and guide a reflected light beam from the recording surface to a detection system via the objective lens and a beam splitter, wherein a focusing error signal in the detection system is provided. The detection system is a detection system based on the knife edge method, and the light beam from the beam splitter is convergent on the front side of the knife edge.
Having a condenser lens that converts the light into a light-receiving element of the detection system, and from the same side as the detection system with respect to the optical axis of the objective lens.
The incident surface on which the laser beam is incident enters the beam shaping
Rhythm, opposite to the optical information recording medium with respect to the optical axis of the condenser lens
The above-mentioned knife edge is arranged on the side, and the above- mentioned beam shaping caused by the wavelength change of the semiconductor laser
Refraction angle change in prism: Δθ and the above beam shaping
The optical path from the entrance surface of the prism to the light receiving surface of the light receiving element
Length: product of L: Δθ · L is generated by the above wavelength change
Focusing error due to chromatic aberration in the above lens
An optical pickup characterized in that the absolute value of the offset is substantially equal to the absolute value of the offset . 3. The optical pickup according to claim 1, wherein the beam shaping prism and the beam splitter are integrated as one optical element.