JPH0684223A - Optical pickup - Google Patents

Abstract

PURPOSE:To make an optical pickup compact and light in weight much more by effectively using duplex diffraction grating structure at a maximum. CONSTITUTION:On the optical path of a light source 21 and a condensing means 23, an optical element 25 for separation is provided while forming a diffraction grating 25b on a substrate 25a, and an optical element 24 for polarized light separation is provided while forming a transmitting area 24b to simply transmit light from the light source and high-density diffraction grating areas 24c and 24d to diffract/transmit respective reflected beams, which are reflected from a magneto-optical recording medium 22 and separated by the optical element 25 for separation, on a substrate 24a. Further, a signal detecting means is provided to detect a magneto-optical signal based on difference between the quantity of diffracted light and the quantity of transmitted light in the high-density diffraction grating areas 24c and 24d, to detect a focusing error signal based on the light diffracted or transmitted by the high-density diffraction grating areas 24c and 24d, and to detect a track error signal based on the light diffracted or transmitted by the high-density diffraction grating areas 24c and 24d.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup of a magneto-optical recording / reproducing apparatus which irradiates a magneto-optical recording medium with light from a light source to record or reproduce information.

[0002]

2. Description of the Related Art Generally, in this type of optical information recording / reproducing apparatus, it is important to reduce the size and weight of an optical head (optical pickup) in order to shorten the access time. In this respect, if track error signal detection, focus error signal detection, and magneto-optical signal detection are performed by separate optical systems as in the conventional case, many optical components are required, and it becomes complicated,
It also becomes heavy and the access time becomes slow.

Therefore, it is considered to use a high-density diffraction grating (grating) in the optical pickup to reduce the size and weight of the optical pickup.
However, in the case of this method, since it is a high-density diffraction grating,
The diffraction angle of the diffracted light largely changes due to the wavelength variation of the semiconductor laser used as the light source. Further, the light receiving element for detecting the focus error signal and the light receiving element for detecting the track error signal are widely separated, which makes assembly and adjustment difficult.

In view of the above, since the optical signal detecting element is formed into a composite integrated structure, a double diffraction grating (dual grating) that can significantly reduce the change in the diffraction angle even if the wavelength of the semiconductor laser changes is used as the optical element. The present applicant has proposed an optical pickup used as a separation optical element. The outline will be described with reference to FIG. This is based on the point that one lens serves as both the collimator lens and the detection lens. First, the laser light emitted from the semiconductor laser 1 enters the polarization beam splitter 2, is reflected and deflected, and travels toward the collimator lens 3 side. Here, the polarization beam splitter 2 is for imparting polarization characteristics such that all polarization components due to Kerr rotation in the magneto-optical disk 4 are guided to the detection optical system, and increases the apparent Kerr rotation angle ( , If it is not expected to increase the apparent Kerr rotation angle, a non-polarizing beam splitter having almost no polarization characteristics may be used). The light deflected by the polarization beam splitter 2 and collimated by the collimator lens 3 is further shaped by the beam shaping prism pair 5 so that the beam cross section becomes a circle (see the cross-sectional shape shown next to the beam shaping prism pair 5). ), The objective lens 6 focuses and irradiates the magneto-optical disk 4.

The reflected light from the magneto-optical disk 4 passes through the objective lens 6, the beam shaping prism pair 5 and the collimating lens 3 again, and further through the polarizing beam splitter 2, and enters the dual grating 7. The dual grating 7 has a high-density diffraction grating 8 with an equal pitch on the incident side and a diffraction grating 9 with a modulation pitch on the emitting side.
Is a transmissive type integrally formed on both sides of the same substrate 10, and is arranged at a predetermined angle on the optical axis of reflected light from the magneto-optical disk 4 side (on the optical axis of the objective lens 6). It is a thing. Here, when the reflected light from the magneto-optical disk 4 is incident on the dual grating 7, it is transmitted through the half-wave plate 11 so that the transmitted light and the diffracted light in the dual grating 7 have the same intensity. If so, the magneto-optical signal can be obtained most effectively.

On the exit side of such a dual grating 7, a two-divided light receiving element 13 for detecting a track error signal for receiving the first-order light = diffracted light 12 and a zero-order light = transmitted light 1
A four-division light receiving element 15 for detecting a focus error signal for receiving four is provided (in the drawing, these light receiving elements 13 and 15 are shown in a plan view for the sake of convenience in order to clearly show the divided state).

In such a structure, the incident light on the dual grating 7 is converged in one direction by the diffracted light 12 used for detecting the track error signal by the push-pull method and the modulation pitch of the diffraction grating 9 to be subjected to astigmatism. And the transmitted light 14 used for detection of the focus error signal by the method. Therefore, the focus error signal is detected by the astigmatism method using the four-division light receiving element 15 that receives the transmitted light 14 from the dual grating 7. Further, the track error signal is detected by the push-pull method using the two-division light receiving element 13 that receives the diffracted light 12. Further, the magneto-optical signal on the magneto-optical disk 4 is transmitted to the light receiving elements 1
It is obtained by taking the difference between the detection outputs of 3 and 15.

According to the system or the configuration using the dual grating 7 as described above, there are the following advantages. First, even if the laser light of the semiconductor laser 1 has a wavelength variation, the diffracted light 12 emitted from the dual grating 7 hardly changes and is hardly affected by the wavelength variation. Secondly, the two-division light receiving element 13 for detecting the track error signal and the four-division light receiving element 15 for detecting the focus signal can be arranged very close to each other. At the same time, the arrangement distance between these light receiving elements 13 and 15 can also be set and adjusted by appropriately changing and adjusting each lattice constant.
It can be appropriately variably set. Therefore, at the time of assembly, the adjustment is performed only once by adjusting only the other light receiving element 15 or 13 with respect to the light receiving element 13 or 15 prepared in advance. Thirdly, there is no possibility that the light receiving element 13 is positioned in the vertical direction with respect to the paper surface, and the light receiving element 13 and the light receiving element 15 can be arranged so as to be positioned on the paper surface, which is also advantageous. Fourth, it is a compact and lightweight optical pickup with a small number of parts, which enables high-speed operation. Fifth, dual grating 7
Since the cross-sectional shape of the light incident on is circular, the focus sensitivity is improved, and the distance to the light receiving element 15 can be shortened. This not only contributes to downsizing of the optical pickup, but also means that the width of the positional fluctuation of the light spot on the light receiving element 15 due to the change of the diffraction angle caused by the wavelength fluctuation becomes smaller.

As the optical system configuration, the position of the semiconductor laser 1 and the position after the dual grating 7 may be interchanged as shown in FIG. 11, and the point is to determine it according to the layout of the optical disk device.

[0010]

However, even in the optical pickup configuration using the dual grating 7 as described above, an optical element such as the polarization beam splitter 2 is required between the objective lens 6 and the dual grating 7. Therefore, there is still room for improvement in terms of the number of parts and miniaturization.

[0011]

In an optical pickup of a magneto-optical recording / reproducing apparatus for irradiating a magneto-optical recording medium with light from a light source through a converging means, the light source and the condensing light are collected. A separation optical element having a diffraction grating formed on a substrate, a transmission region for simply transmitting light from the light source, and a reflection optical element separated from the magneto-optical recording medium on the optical path with the means. Diffract each reflected light
A polarization separating optical element having a high-density diffraction grating region to be transmitted formed on a substrate is provided, and a magneto-optical signal is detected based on a light amount difference between at least one diffracted light and the transmitted light by the high-density diffraction grating region, Signal detecting means for detecting a focus error signal based on at least one diffracted light or transmitted light by the high-density diffraction grating region and detecting a track error signal based on at least one diffracted light or transmitted light by the high-density diffraction grating region. And.

[0012]

Since the separation optical element and the polarization separation optical element that substantially form the double diffraction grating have not only the function for the signal detection system but also the beam splitter function, the required optical components are The number is small, the size and weight can be further reduced, and the access speed can be improved.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described with reference to FIGS. First, a semiconductor laser 21 serving as a light source is provided on the optical axis of an objective lens (focusing means) 23 with respect to a magneto-optical disk (magneto-optical recording medium) 22, and on the optical path between these semiconductor laser 21 and objective lens 23. A polarization separation function diffraction grating (polarization separation optical element) 24 and a beam splitter function diffraction grating (separation optical element) 25, which characterize this embodiment, are arranged in order. The beam splitter function diffraction grating 25 includes the diffraction grating 2 on the substrate 25a.
5b (grating pitch Λb) is formed.
Here, the diffraction grating 25b and the semiconductor laser 21 are aligned so that the polarization direction of the light from the semiconductor laser 21 with respect to the diffraction grating 25b forms 45 ° as shown in FIG. 2 (a).

Further, the polarization separation function diffraction grating 24 is shown in FIG.
As shown in (b), high-density diffraction grating regions 24c and 24d (having a grating pitch Λp) are formed on both sides, leaving a transmission region 24b that simply transmits light from the semiconductor laser 21 at the center of the substrate 24a. It is a thing. These high-density diffraction grating regions 24c and 24d are used for the magneto-optical disk 2 described above.
The diffracted lights reflected from the beam No. 2 and separated by the diffraction grating 25b are formed at positions where they can respectively enter. Therefore, the high-density diffraction grating regions 24c and 24d generate four separated lights of diffracted light and transmitted light. The light receiving elements 26a to 26d are provided for the respective separated lights, and the signal detecting means 26 is provided.
Is configured. Here, the light receiving element 26a receives the transmitted light from the high-density diffraction grating region 24c.
As shown in the figure, it is a four-division light receiving element. The light receiving element 26b receives the diffracted light from the high-density diffraction grating region 24d, and is a two-divided light receiving element as shown in FIG. The light receiving element 26c receives the diffracted light from the high-density diffraction grating region 24c, and is a two-divided light receiving element as shown in FIG. The light receiving element 26d receives the transmitted light from the high-density diffraction grating region 24d, and is a four-division light receiving element as shown in FIG.

In such a structure, the semiconductor laser 2
The light emitted from No. 1 passes through the transmission region 24b of the polarization separation function diffraction grating 24, and then enters the beam splitter function diffraction grating 25. The light transmitted through the beam splitter function diffraction grating 25 (a part of the light is diffracted by the diffraction grating 25b, but this diffracted light is not used) is the objective lens 2
The light is focused and irradiated onto the magneto-optical disk 22 by the laser beam No. 3. It should be noted that the collimator lens may collimate the light before it enters the objective lens 23.

Since information is written on the magneto-optical disk 22 based on the principle of the Kerr effect or the like, the plane of polarization of the reflected light from the magneto-optical disk 22 is rotated according to the information. The reflected light including such information passes through the objective lens 23 again, and then the beam splitter function diffraction grating 25.
To the high-density diffraction grating regions 24c and 24d of the polarization separation function diffraction grating 24.
Incident on. Here, the Kerr rotation angle is about ± 1 °, so that the light enters with a plane of polarization of about 45 ° and has both components of S-polarized light and P-polarized light. These diffracted lights are separated into diffracted lights and transmitted lights by the high-density diffraction grating regions 24c and 24d, respectively. At this time, due to the characteristic of the high-density diffraction grating, the S-polarized light is dominant in the diffracted light and the P-polarized light is dominant in the transmitted light, so that the high-density diffraction grating regions 24c and 24d separate the polarized light. The diffracted light or transmitted light thus polarized and separated is received and detected by each of the light receiving elements 26a to 26d, and is used for magneto-optical signal detection, focus error signal detection, and track error signal detection.

Now, the order of the diffracted light is m, the wavelength is λ, the diffraction angle from the beam splitter function diffraction grating 25 is θ ₁ , and the diffraction angle from the polarization separation function diffraction grating 24 is θ ₂ (see FIG. 4).
Then, the relationship of the diffraction angle etc. is expressed by the following equations (1) and (2) sin θ ₁ = m · λ / Λb …………………… (1) sin θ ₁ + sin θ ₂ = λ / Λp ……………… ... (2) For example, m = 1, λ = 830 nm, Λb =
If 2.43 μm and Λp = 0.7 μm, θ ₁ = 20
And θ ₂ = 57.5 °.

Therefore, the P-polarized light is predominant in the light incident on the light receiving elements 26a and 26d, and the light receiving elements 26b and 26c.
S-polarized light is predominant in the light incident on. Now, the respective light receiving regions of these light receiving elements 26a~26d as shown in FIG. _{3, A 1, B 1,} C 1, D 1, E 2, F 2, E 3, F 3,
Assuming A ₄ , B ₄ , C ₄ , and D ₄ , the magneto-optical signal is Mo ₁ = (E ₂ + F ₂ + E ₃ + F ₃ ) − (A ₁ + B ₁ + C ₁ + D ₁
+ A ₄ + B ₄ + C ₄ + D ₄ ) Mo ₂ = (E ₂ + F ₂ ) − (A ₁ + B ₁ + C ₁ + D ₁ ).

Further, the track error signal may be obtained by the push-pull method using the detection signal of the light receiving element 24b or 24c having the two-divided light receiving element structure. The focus error signal may be obtained by the astigmatism method using the detection signal of the light receiving element 24a or 24d having the four-division light receiving element configuration.

As described above, according to the present embodiment, the combined structure of the polarization splitting function diffraction grating 24 and the beam splitter function diffraction grating 25 exhibiting the double diffraction grating function is applied to the detection system having only the polarization splitting function. In addition to contribution, it has a normal separation function of separating incident light and reflected light, so that the detection system and the light source system are arranged on the same side, and the size and weight can be further reduced. Therefore, the access speed of the optical pickup can be further improved.

Next, an embodiment of the invention described in claim 2 will be described with reference to FIGS. The same parts as those shown in the above-mentioned embodiments are designated by the same reference numerals (the same applies to the following embodiments). This embodiment clarifies the detection method of the focus error signal in the above embodiment. As described above, the astigmatism method, which is well known in the related art, is used to detect the focus error signal, and the principle thereof will be described with reference to FIGS. 5 and 6. After the reflected light from the optical disk a passes through the objective lens b and then is received by the four-division light receiving element e through the detection lens c and the obliquely arranged parallel flat plate d, astigmatism is obtained depending on the focused or unfocused state. Aberration occurs. That is, when the optical disk a approaches the objective lens b, the optical disk a has an extended shape as shown in FIG. 6A, and in the focused state, it has a substantially circular shape as shown in FIG. 6B.
Is flattened away from the objective lens b, as shown in FIG. Therefore, if each light receiving area of the four-divided light receiving element e is indicated by A, B, C, and D, the focus error signal Fo can be calculated by Fo = (A + C)-(B + C).

Considering the case where such an astigmatism method is applied to this embodiment, the optical disk a is replaced by the magneto-optical disk 2.
2, the objective lens b is the objective lens 23, the detection lens c is the diffraction grating 25b, and the parallel plate d is the substrate 25a or 24a.
The divided light receiving element e may be replaced with the light receiving element 26a or 26d. As a result, the focus error signal Fo is detected by the astigmatism method in the novel optical pickup configuration using the polarization separation function diffraction grating 24 and the beam splitter function diffraction grating 25.

Next, an embodiment of the invention described in claim 3 will be described. This embodiment is a modification of the above embodiment. That is, in the case of the above embodiment, the diffraction grating 25
The convergent light from b utilizes the astigmatism generated by passing through the parallel flat plate made of the substrate 25a or 24a, and does not require a special structure, but in order to optimize the astigmatism state, It is necessary to control the thickness of the substrate 25a or 24a, and high manufacturing accuracy is required. In consideration of such a point, in the present embodiment, a method using a cylindrical lens, which is known as an astigmatism generation method, is applied. For this purpose, the parallel plate d in FIG. 5 may be replaced with a cylindrical lens. Considering FIG. 1, the pitch distribution of either the diffraction grating 25b or the high-density diffraction grating regions 24c and 24d may be subjected to a chirping process so as to have a cylindrical lens function. At this time, the focal length may be set according to the degree of chirping that can be arbitrarily designed and manufactured by the two-beam interference method, the electron beam drawing method, or the like.
In addition, astigmatism causes the substrate 25a or 24 as described above.
Since the thickness of a naturally contributes, astigmatism can be effectively generated by both the cylindrical lens function and the function of the present embodiment. The focus error signal Fo itself may be obtained by the above-mentioned formula. Also in this case, only one of the light receiving elements 26a and 26d may be used as the light receiving element.

Further, an embodiment of the invention described in claims 4 and 5 will be described with reference to FIGS. 8 and 9. The present embodiment is devised so that the detection system is strong against wavelength fluctuations when the influence of fluctuations of the diffraction angle on the focus error signal Fo cannot be ignored because the wavelength fluctuations of the semiconductor laser 21 are large. To this end, the arrangement configuration of the light receiving elements 26a to 26d (combination of which is to be non-divided / divided / divided / divided) may be set as shown in FIG. That is, the light receiving elements 26a to 26d are sequentially divided into two, four, four,
It may be a two-divided light receiving element. As is already known as a characteristic of the double diffraction grating, this is because the light diffracted twice has a smaller variation in the diffraction angle due to the wavelength variation than the light diffracted only once. The light-receiving elements 26b and 26c located at the light-receiving position of 4 are divided into four light-receiving elements.

As for the tracking error signal Tr, it is better to use the diffracted light twice than the diffracted light only once. Therefore, when the detection of the tracking error signal Tr is also taken into consideration. As shown in FIG. 6B, one of the light receiving elements 26b and 26c, for example, the light receiving element 26b side is used as a two-divided light receiving element, and the track error signal Tr
May be used for detection. In this case, the light receiving elements 26a and 26d may be of the undivided type.

Considering the track error signal, however, even if there is wavelength variation, the spot movement due to wavelength variation occurs in the direction of the dividing line of the two-divided light receiving element f, as shown in FIG. Since it is considered that it has almost no effect as compared with the signal, the arrangement configuration shown in FIG.

Further, an embodiment of the invention described in claim 6 will be described. Generally, in an optical pickup for an optical information recording medium such as a write-once, originally, the polarization separation property is not required, but since the magneto-optical signal Mo is reproduced and detected in the magneto-optical disk 22 which is an object of the present invention. Has
Sufficient polarization separation is required. For this purpose, it is necessary to make the high-density diffraction grating have a higher density. However, for example, when m = 1, θ ₁ cannot be increased, so that the grating pitch Λp of the high-density diffraction grating cannot be decreased. This is θ ₁
This is because when θ is increased, θ ₂ becomes too large, and the light receiving element that receives this diffracted light is arranged far away from the other other light receiving elements, which violates the desired reduction in size and the like. Therefore, the lattice pitch Λp cannot be reduced, and
There is a limit to further densification.

Therefore, in the present embodiment, the secondary light generated in the diffraction grating 25b is made to enter the high-density diffraction grating regions 24c and 24d, thereby making it possible to increase the density of the high-density diffraction grating. Even if there is, the diffracted light can be received at an appropriate position without being separated from each other, and the miniaturization can be maintained.

For example, when m = 2, λ = 830n
When m and Λb = 2.43 μm, the angle θ ₂ when m = 1
= 57.5 °, the lattice pitch Λp is 0.544.
It can be seen that the density can be further increased and the density can be further increased. This ensures a sufficient polarization separation function.

[0030]

According to the first aspect of the present invention, a separating optical element having a diffraction grating formed on a substrate is provided on an optical path between the light source and the light collecting means, and a transmitting region for simply transmitting light from the light source. A high-density diffraction grating region for diffracting / transmitting each reflected light reflected from the magneto-optical recording medium and separated by the separating optical element, and a polarization separating optical element formed on a substrate are provided. Since it is configured to detect the magneto-optical signal, the focus error signal, and the track error signal by the signal detection means using the transmitted light or the transmitted light, the double diffraction grating has not only the function for the signal detection system but also the beam splitter function. Therefore, the number of required optical components can be minimized, the size and weight of the optical pickup can be further reduced, and the access speed can be improved. .

In this case, according to the second aspect of the invention, the focus error signal is detected by the signal detecting means, and the diffracted light by the diffraction grating of the separating optical element passes through the substrate or the substrate of the polarization separating optical element. Since the astigmatism caused thereby is detected by the astigmatism method with the light receiving element, a simple focus error signal detection method suitable for the optical pickup according to claim 1 can be established.

In particular, according to the third aspect of the invention, the focus error signal detection by the signal detecting means is chirped in at least one of the diffraction grating region of the separation optical element or the high-density diffraction grating region of the polarization separation optical element. Since the astigmatism method that detects the astigmatism caused by the cylindrical lens function obtained by processing is detected by the light receiving element, the degree of chirping can be designed and produced by the two-beam interference method or the like for the focal length. The focus error signal detection system suitable for the optical pickup according to the first aspect can be highly optional.

According to the invention described in claim 4, in these inventions, the focus error signal detection by the signal detection means is performed by the diffraction grating of the separation optical element and the high-density diffraction grating of the polarization separation optical element. Since the diffracted light is used, a stable focus error signal detection system can be established even if the wavelength of the light source varies.

Similarly, according to the invention described in claim 5, in these inventions, the track error signal detection by the signal detecting means is performed by the diffraction grating of the separation optical element and the high-density diffraction grating of the polarization separation optical element. Since the light diffracted by is used, it is possible to establish a stable track error signal detection system even if the wavelength of the light source varies.

Further, according to the invention described in claim 6, in these inventions, the light incident from the diffraction grating of the separation optical element to the high-density diffraction region of the polarization separation optical element is
Since the diffracted light of the second or higher order of the diffraction grating is used, high-density diffraction can be performed without difficulty, and a sufficient polarization separation function can be ensured.

[Brief description of drawings]

FIG. 1 is a schematic perspective view conceptually showing an embodiment of the invention according to claim 1. FIG.

FIG. 2 is a plan view showing a state of a diffraction grating and a grating in a high-density diffraction grating region.

FIG. 3 is a plan view showing an arrangement and the like of light receiving elements.

FIG. 4 is a schematic front view for explaining a diffraction angle.

FIG. 5 is a principle configuration diagram of an astigmatism method for explaining an embodiment of the invention described in claim 2;

FIG. 6 is a plan view showing a light receiving state of the light receiving element.

FIG. 7 is a schematic perspective view showing an application form of the astigmatism method to the present embodiment.

FIG. 8 is a plan view showing an arrangement of light receiving elements and the like showing an embodiment of the invention described in claims 4 and 5;

FIG. 9 is a plan view showing spot movement of a track error signal.

FIG. 10 is a configuration diagram of an optical system showing a conventional example.

FIG. 11 is an optical system configuration diagram showing another configuration example thereof.

[Explanation of symbols]

 Reference Signs List 21 light source 22 magneto-optical recording medium 23 condensing means 24 polarization separation optical element 24a substrate 24b transmission region 24c, 24d high density diffraction grating region 25 separation optical element 25a substrate 25b diffraction grating

Claims (6)

[Claims] 1. An optical pickup of a magneto-optical recording / reproducing apparatus for irradiating a magneto-optical recording medium with light from a light source through a converging means to record or reproduce information. On the road, a separating optical element having a diffraction grating formed on a substrate, a transmissive region that simply transmits light from the light source, and each reflected light reflected by the magneto-optical recording medium and separated by the separating optical element. A high-density diffraction grating region for diffracting / transmitting light, and a polarization separation optical element formed on a substrate, and at least 1 of the high-density diffraction grating region is provided.
A magneto-optical signal is detected based on a light amount difference between two diffracted light beams and a transmitted light beam, and a focus error signal is detected based on at least one diffracted light beam or transmitted light beam by the high-density diffraction grating region. An optical pickup comprising signal detection means for detecting a track error signal based on at least one diffracted light or transmitted light. 2. The astigmatism generated by the diffracted light by the diffraction grating of the separating optical element passing through the substrate or the substrate of the polarization separating optical element is detected by the light receiving element in the focus error signal detection by the signal detecting means. The optical pickup according to claim 1, wherein the astigmatism method is used. 3. A cylindrical lens function obtained by performing a chirping process on at least one of the diffraction grating of the separation optical element or the high-density diffraction grating area of the polarization separation optical element, in order to detect the focus error signal by the signal detection means. 2. The optical pickup according to claim 1, wherein the astigmatism caused by the astigmatism is detected by a light receiving element. 4. The focus error signal detection by the signal detection means uses light diffracted by the diffraction grating of the separation optical element and the high-density diffraction grating of the polarization separation optical element. Item 1. The optical pickup according to Item 1, 2 or 3. 5. The tracking error signal detection by the signal detection means uses light diffracted by the diffraction grating of the separation optical element and the high-density diffraction grating of the polarization separation optical element. The optical pickup according to item 1, 2, 3 or 4. 6. The light incident from the diffraction grating of the separation optical element to the high-density diffraction region of the polarization separation optical element is diffracted light of the second or higher order of the diffraction grating. The optical pickup described in 2, 3, 4 or 5.