JPH0460931A - optical pickup - Google Patents

Abstract

PURPOSE:To reduce the thickness of an optical head by moving a base body in such a manner that diffracted light is condensed to the surface of an optical disk according to the detection signal detected by a photodetecting element. CONSTITUTION:The light reflected from the surface of the optical disk is subjected to wave front conversion by a hologram element 4 for splitting beams and is introduced to the photodetecting element 5 by which the servo signals of tracking and focusing are detected. The entire part of the base body 1 is moved by an actuator 6 according to these servo signals, by which the laser beam is so controlled as to be condensed as a microspot to the tracks of the optical disk 7. The diameter of the condensed spot by the 1st hologram element 3 is determined by the aperture diameter of the hologram element and, therefore, a specified or larger size is necessary, but the installation of the hologram element at a shallow angle to the base of the base body by designing of a grating is possible. The optical pickup contributing to the thickness reduction of the base body is obtd. in this way.

Description

[Detailed Description of the Invention] Industrial Field of Application The present invention relates to an optical pickup for recording and reading information stored in an optical storage medium such as an optical disk and detecting a servo signal using a laser beam. In particular, the present invention provides a method for simplifying the optical system and reducing the size and thickness of the optical system.

Prior Art FIG. 6 shows an example of a conventional optical pickup in which information stored on an optical disk is read using a laser beam. A semiconductor laser 131 as a light source, a collimation lens 132 that converts the light emitted from this semiconductor laser into a parallel beam, and a large NA (approximately 0.45) to condense the light into a spot diameter of 1 μm or less on the surface of the optical disk 134. a condensing lens 133 and a beam splitter 135 that separates a portion of the beam reflected by the optical disk 134; a cylindrical lens 136 and a lens 137 for imparting astigmatism to the wavefront and guiding it to a light receiving element (detector) 138;
It is configured to include a four-part light receiving element 138 for detecting recording signals, and servo signals. Light receiving element 13
The condenser lens 133 moves in response to a servo signal detected through the optical disc 8, and a minute spot focused almost to the diffraction limit is always formed on the surface of the optical disc 134. Then, as described above, a recorded signal is read out from the intensity of reflected light detected by the light receiving element. In this way, a normal optical pickup uses a beam splitter to guide the laser light reflected on the surface of the optical disc to the light receiving element, and the reflected light has astigmatism, etc., which causes the focus shift and tracking shift of the light focused on the optical disc. An optical element is required to enable detection and form an image on the photodetector, which increases the number of optical components and makes it difficult to align the optical axis, making manufacturing expensive. Therefore, the optical system of the optical pickup was made smaller {t.
For simplicity, a hologram head has been proposed in which the beam splitter and prism are replaced with a single hologram. The outline of this hologram-type optical surprise is explained in Section 7.
As shown in the figure. The light emitted from the semiconductor laser 141
The optical disc 14 passes through the hologram 142 and is bent in the traveling direction by the reflecting mirror 146.
The light is focused on the surface of 4. Light that is reflected on the surface of the optical disk 144 and spreads with the amount of reflected light modulated according to the recorded information (
The force is again focused by the objective lens 143 and a portion returns to the semiconductor laser 141 (a portion is generated by the two regions (Rl and R
2) The compact disc is divided into two directions by the divided hologram element 142 and focused on the four-divided light receiving element 145, and a focus error signal is detected by the Naifezi method and a tracking servo signal is detected by the push-pull method. signal is detected. The objective lens moves in response to a servo signal, and a minute spot narrowed down to approximately the diffraction limit is formed on the surface of the optical disk 144.

Problems to be Solved by the Invention However, even if a compact optical pickup is realized using such a hologram, the thickness of the optical pickup is determined by the sum of the diameter of the optical beam near the condenser lens and the thickness of the lens. In particular, in the above-mentioned hologram head with a moving condenser lens, in order to suppress off-axis aberrations of the lens as much as possible, it is necessary to use a thick aspherical lens with a large diameter of about 5 mm and a thickness of several P in order to obtain a large NA. Therefore, there was a limit to how thin the optical head could be made, i.e., thickness 1
It is almost impossible to manufacture an optical pickup with a thickness of 0 mm or less.9 Reducing the thickness of the optical head is extremely important in terms of miniaturizing the device and increasing the capacity of the same size device. This is an important problem.The present invention overcomes these problems and provides a thin (eg, 3 mm or less thick) optical pickup for optical disks.

Means for Solving the Problems The present invention solves the above problems by the following means.

The present invention includes a semiconductor laser, a first hologram element having a function of diffracting light emitted from the semiconductor laser and propagating in space while being diverging, and condensing the diffracted light to one point on an optical disk; A plurality of optical elements including a second hologram element that has the function of splitting a part of the light propagating in the opposite direction on the optical path, converting the wavefront, and guiding it to the photodetection element, and a photodetection element that detects the signal are on the same base. The present invention provides an optical pickup in which the base body is moved so that the diffracted light is focused on the surface of an optical disk in accordance with a detection signal detected by the photodetection element.

The present invention also provides a semiconductor laser, a first hologram element having a function of diffracting light emitted from the semiconductor laser and propagating in space while diverging and converting it into parallel light, and condensing the diffracted light onto one point on an optical disk. a condensing lens having a function of splitting a part of the light reflected from the optical disk and propagating in the opposite direction along the optical path, a second hologram element having a function of performing wavefront conversion and guiding it to a photodetecting element, and detecting a signal. A plurality of optical elements including a photodetecting element are arranged on the same base, and the base is moved so that the diffracted light is focused on the surface of the optical disk in response to a detection signal detected by the photodetecting element. Provides optical pickup.

The present invention also provides a semiconductor laser, a third hologram element that diffracts the light emitted from the semiconductor laser and propagating in space while diverging, and changes the direction of travel of the light, and the diffracted light is focused at one point on an optical disk by a diffraction effect. a first hologram element having a function of emitting light, a second hologram element having a function of splitting a part of the light reflected from the optical disk and propagating in the opposite direction along the optical path, converting the wavefront, and guiding it to the photodetecting element; A plurality of optical elements including a light-receiving element for detecting a signal are arranged on the same base, and the base is arranged so that the diffracted light is focused on the surface of the optical disk according to the detection signal detected by the light-detecting element. Provides a moved optical pickup.

Also provided is a structure in which the third hologram element is formed on the same substrate as the second or third hologram element.

Next, the present invention provides means for using a wavelength stabilized semiconductor laser as a semiconductor laser.

The present invention also provides a first and second hologram element force (a reflection type hologram element having an uneven grating formed on the surface thereof, and the groove direction of the grating is substantially perpendicular to the polarization direction of the semiconductor laser. Provide the means to be placed.

Further? Q: The present invention also provides a structure in which the third hologram element is formed on the same substrate as the first or second hologram element.

=9- O- Effect The present invention focuses the diverging light emitted from a semiconductor laser directly onto an optical disk using a reflective light-collecting diffraction grating, and moreover, the reflective light-collecting diffraction grating By setting the incident angle of the element to the semiconductor laser at 45 degrees or less, the optical pickup can be made thinner.

Examples The details of the present invention will be explained using examples. A first embodiment of the present invention is shown in FIG. I'o A semiconductor laser 2, a condensing hologram element 3 (first hologram element), a beam splitting hologram element 4 (second hologram element), and a light receiving element 5 are formed on a base 1. First, the light emitted from the semiconductor laser 2 is reflected by the focusing hologram element 3 and focused near the surface of the optical disk 7.The light reflected from the optical disk surface is wavefront-converted by the beam splitting hologram element 4. The servo signal for tracking and focusing is detected by the light receiving element 5.According to this servo signal, the entire base 1 is moved by the actuator 6, and the laser beam is focused on a minute spot on the track of the optical disk 7. The signal recorded on the optical disc is then accurately detected.

Here, the first hologram element 3 is installed at a constant angle with respect to the bottom surface of the base, and the shape of its grating is formed from a group of concentric circles with gradually different pitches. It has the function of focusing light on a single point (optical disc). The diameter of this condensed spot is determined by the aperture diameter of the hologram element, so a force greater than a certain size (equivalent to the distance between the hologram element and the optical disk) is required (for example, due to the design of the grating, the hologram element is moved against the bottom surface of the substrate). It is possible to install it at a shallow angle of about 20 degrees.

As a result, a thin optical pickup with a thin substrate of about 3 mm in thickness is possible. In this example, a reflective hologram element with an uneven grating formed on the ζ main surface is used (
Also, since the groove direction of the grating is arranged almost perpendicular to the polarization direction of the semiconductor laser, the radiation angle of the semiconductor laser can be configured to be small in the thickness direction, resulting in high light utilization efficiency. As shown in FIG. 8, the second hologram element 4 is formed of areas a and ao formed by two sets of Fresnel zone plates, and areas S and b' formed by two sets of linear gratings, and generates a focus error signal. The spot size is detected by the diffracted light from the regions a and ao.The tracking error signal is detected by a push-pull method using the diffracted light from the regions b and b'. In the above embodiment, the second hologram element was placed above the first hologram element, but it does not necessarily need to be placed above it, and may be placed between the semiconductor laser and the first hologram. The first hologram element and the second hologram element can be formed on the same substrate, and the spot size detection method is used as the focusing servo signal detection method. It is also possible to use a method.

A second embodiment of the invention is shown in FIG. +-Semiconductor laser 22 and collimation hologram element 2 on base 21
3 (first hologram element), a condensing lens 24, a beam splitting hologram element 25 (second hologram element), and a light receiving element 26. First, the light ζ emitted from the semiconductor laser 22 is reflected by the collimation hologram element 23 and focused near the surface of the optical disk 27 through the condenser lens 24 . The light reflected from the surface of the optical disk passes through the condenser lens 24 in the opposite direction, and travels toward the light source by the hologram element 23. Next, the beam is converted by the beam splitting hologram element 25, and guided to the light receiving element 26 for tracking and focusing. Cushing's servo signal is detected. In response to this servo signal, the entire base 21 is moved by the actuator 27 and controlled so that the laser beam is focused on a minute spot on the track of the optical disk 28. The signals recorded on the optical disc can then be accurately detected.

In the same way as the first embodiment, it is possible to make the base thinner, that is, to make the optical pickup thinner. Similarly to the first embodiment, the second hologram element 25 is formed of an area formed by two sets of Fresnel zone plates and an area formed by two sets of linear gratings, and the focus error signal is generated by spot size detection. The human tracking error signal is detected using a push-pull method. In this embodiment, the second hologram element is installed between the first hologram element and the semiconductor laser. The hologram element and the second hologram element may be formed on the same substrate. In addition, the focusing servo signal detection method uses a spot size detection method (not necessarily limited to this, it is also possible to use a knife method that can accommodate astigmatism).

FIG. 3 (Dai) This is an improved version of the embodiment shown in FIG. Therefore, in order to stabilize the wavelength of the semiconductor laser, a condensing grating mirror 9 is installed and an external resonator type configuration is adopted.

The embodiment shown in FIG. 4 is an improvement on the embodiment shown in FIG. That is, the collimation hologram element 23 is used to stabilize the wavelength of the semiconductor laser, since the direction of light emission changes due to wavelength fluctuations of the semiconductor laser, and the degree of collimation deteriorates, making it difficult to form a minute spot with a condenser lens. A condensing grating mirror 29 is installed in the external resonator type configuration. Still another embodiment of the invention is shown in FIG. On the base 31/semiconductor laser 32
, hologram element 33 (third hologram element), condensing hologram element 34 (first hologram element),
A beam splitting hologram element 35 (second hologram element) and a light receiving element 36 are formed. First, the light emitted from the semiconductor laser 32 is transmitted to the hologram element 33
The light is then reflected by the hologram element 34 and focused near the surface of the optical disk 7 . The light reflected from the surface of the optical disk undergoes wavefront conversion by a beam splitting hologram element 35 and is guided to a light receiving element 36 where tracking and focusing servo signals are detected. In response to this servo signal, the entire base 1 is moved by the actuator 38, and the laser beam is controlled to be focused on a minute spot on the track of the optical disk 7. The signals recorded on the optical disc can then be accurately detected. Here, the -th and 20th hologram elements 33 and 34 are installed at a constant angle with respect to the bottom surface of the base, and the shapes of their respective gratings are formed from a group of concentric circles with gradually different pitches, and diverge from the -point (semiconductor laser). The light is focused on another point (the optical disk) by the diffraction effect of the two holograms. This focused spot diameter is determined by the aperture diameter of the hologram element, so it requires a certain size or more (equivalent to the distance between the hologram element and the optical disk).The grating design allows the hologram element to be attached to the bottom of the base, for example. It is possible to install it at a shallow angle such as 20 degrees.

As a result, it is possible to make the substrate thinner, that is, to make the optical pickup thinner. In addition, the wavelength of the semiconductor laser changes depending on the injected current and temperature, and the diffraction angle of the light in the third and first hologram elements changes accordingly. This is compensated by diffraction, and as a result, even if the oscillation wavelength of the semiconductor laser changes, the deterioration of the light focusing property by the second hologram element is alleviated. In this embodiment, a reflection type hologram element having a grating with irregularities formed on its surface is used, and the groove direction of the grating is arranged almost perpendicular to the polarization direction of the semiconductor laser. The radiation slope angle can be configured to be small in the thickness direction, resulting in high light utilization efficiency. Second hologram element 3
5 is formed by an area formed by two sets of Fresnel zone plates and an area formed by two sets of linear gratings, and a focus error signal is detected by spot size detection and a tracking error signal is detected by a push-pull method. In the above embodiments, the second hologram element 35 was installed above the third hologram element 34, but it does not necessarily have to be installed above. It may be placed between the element and the third hologram element, or it may be formed on the same substrate as the first or third hologram element). Furthermore, the focusing servo signal detection method is not necessarily limited to the spot size detection method, but it is also possible to use the astigmatism method or the knife method.

As described above, the present invention provides a thin optical pickup that could not be realized with conventional optical pickups.
It has great value.

[Brief explanation of the drawing]

Figure 1 is a cross section of the main part of the first embodiment of the thin pickup of the present invention @ Figure 2 is the cross section of the main part of the second embodiment of the thin pickup of the present invention A cross-section of the main parts of an example of a thin pickup that is an improved version of the example.
The figure shows a cross section of the main part of an embodiment of a thin pickup that is an improvement on the embodiment shown in Fig. 2. Figure 5 shows a cross section of the main part of the first other embodiment of the thin pickup of the present invention. Figure 6 shows a conventional example. C of
FIG. 7 shows a schematic structure of a conventional CD hologram pickup head. FIG. 8 shows a wavefront conversion hologram used in this embodiment. DESCRIPTION OF SYMBOLS 1... Base 2... Semiconductor laser 3... First hologram element, 4... Second hologram element,
5... Light-receiving element, 6... Acqueue 7...
·optical disk. Name of agent: Patent attorney Shigetaka Awano

Claims (1)

Scope of Claims: (1) a semiconductor laser; a first hologram element having a function of diffracting light emitted from the semiconductor laser and propagating in space while diverging; and condensing the diffracted light to a single point on an optical disk; a plurality of optics including a second hologram element having a function of splitting a part of the light reflected from the optical disk and propagating in the opposite direction along the optical path, performing wavefront conversion, and guiding it to a photodetecting element; and a light receiving element detecting a signal; The elements are arranged on the same base, and the base is moved so that the diffracted light is focused on the surface of the optical disk according to the detection signal detected by the photodetector. Optical pickup. (2) a semiconductor laser; a first hologram element having a function of diffracting the light emitted from the semiconductor laser and propagating in space while diverging and converting it into parallel light; and condensing the diffracted light onto one point on the optical disk. a condensing lens having a function, a second hologram element having a function of splitting a part of the light reflected from the optical disk and propagating in the opposite direction along the optical path, performing wavefront conversion and guiding it to a photodetecting element, and detecting a signal. A plurality of optical elements including a light-receiving element are arranged on the same base, and the base is moved so that the diffracted light is focused on the surface of the optical disk in response to a detection signal detected by the light-detecting element. An optical pickup characterized by: (3) The optical pickup according to claim 1 or 2, wherein the semiconductor laser is an oscillation wavelength stabilized laser. (4) A wavelength-stabilized semiconductor laser focuses only light of a plurality of specific wavelengths out of diverging synchrotron radiation emitted from one end surface of the active layer directly onto one end surface of the active layer of the semiconductor laser. an external resonator mirror that has the function of performing optical feedback and is formed on a flat substrate and is formed of a reflective optical diffraction element, and a semiconductor laser that oscillates at the specific wavelength. The optical pickup according to claim 3, which is an external cavity type semiconductor laser. (5) a semiconductor laser, a third hologram element that diffracts the light emitted from the semiconductor laser and propagating through space while diverging, and changes the traveling direction of the light; the diffracted light is focused on one point on the optical disk by the diffraction effect; a first hologram element having the function of splitting a part of the light reflected from the optical disk and propagating in the opposite direction along the optical path, performing wavefront conversion, and guiding it to the photodetecting element; A plurality of optical elements including a photodetection element for detecting a signal are arranged on the same substrate, and the diffracted light is focused on the surface of the optical disk in accordance with the detection signal detected by the photodetection element. An optical pickup characterized in that a base body is moved. (6) The first and third hologram elements are reflective hologram elements having uneven gratings formed on their surfaces, and the groove direction of the gratings is substantially perpendicular to the polarization direction of the semiconductor laser. The optical pickup according to claim 1, 2, or 5, characterized in that the optical pickup is arranged as follows. (7) The optical pickup according to claim 1, 2, or 5, wherein the first hologram element and the second hologram element are formed on the same substrate. (8) The optical pickup according to claim 5, wherein the second hologram element is formed on the same substrate as the first or third hologram element.