Optical pickup system with light receiving portion

An optical pickup comprises a light source, a three-way split light diffracting section for dividing light emitted from a light source into one main beam and two subbeams, a hologram for dividing each of the main beam and the subbeams reflected from a recording medium into two predetermined directions, and a light receiving section for receiving the beams divided by the hologram. The hologram includes two regions having a small-pitch grating for diffracting each beam to a predetermined direction, the small-pitch gratings have substantially the same pitch and are symmetrical about a split line separating the two regions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup used in an optical disk system, such as a compact disk, a video disk, etc. More particularly, the present invention relates to an optical pickup comprising a hologram element-incorporating semiconductor laser device.

2. Description of the Related Art

An optical pickup comprising a semiconductor laser device is used to read out information stored in an optical disk, such as a compact disk, etc. In the optical pickup, light emitted from the semiconductor laser device is split by a diffraction grating of a hologram element into one main beam and two subbeams (tracking beams) which are brought onto an optical disk. The main beam and the subbeams are reflected on the optical disk, and each reflected beam is further split by a hologram of the hologram element into two beams, which are brought to a light receiving element or a signal processing integrated circuit with a light receiving element. Thereafter, based on an output signal from the signal processing integrated circuit, a tracking information signal, etc. used for accurately reading out signals recorded in the optical disk can be obtained.

FIG. 4is a schematic diagram showing the optical system of a conventional three-beam hologram optical pickup.

This optical pickup has a semiconductor laser chip (LD)6. Light emitted from the semiconductor laser chip6is split by a tracking beam generating diffraction grating5, provided on the rear side of a hologram element (not shown), into three beams, i.e., two subbeams for tracking and one main beam for reading information signals. This light passes through a hologram4provided on the hologram element as zero-order light, and is then converted by a collimator lens3to parallel light. The parallel light is condensed by an objective lens2onto a disk1. The light condensed onto the disk1is modulated by pits on the disk1and reflected from the disk1. The reflected light from the disk1passes through the objective lens2and the collimator lens3in this order, and is then diffracted by the hologram4and introduced into a five-way split photodiode7as first-order diffracted light.

This five-way split photodiode7has five optical detectors D1to D5. The five-way split photodiode7has a rectangular region which is illuminated by light. The region is divided into three equal parts which are strip regions extending in a longitudinal direction. Two opposite regions are first and fifth optical detectors D1and D5. A middle strip region is further divided into two equal parts in a transverse direction. One of the two regions is a fourth optical detector D4. The other region is further divided into two parts in a longitudinal direction, which are second and third optical detectors D2and D3.

The hologram4has two regions4aand4bwhich have different grating pitches. The main beam of reflected light entering the region4ais condensed onto the splitting line between the second optical detector D2and the third optical detector D3of the five-way split photodiode7. The main beam of reflected light entering the region4bis condensed onto the fourth optical detector D4. Further, the two subbeams of reflected light entering the region4aare condensed onto the opposite first and fifth optical detectors D1and D5, so that two beam spots are formed on each of the optical detectors D1and D5.

As described above, the beam spots of reflected light condensed on the optical detectors D1to D5of the five-way split photodiode7vary depending on the focusing conditions of the light brought onto the disk1as shown inFIGS. 5A to 5C.FIG. 5Ashows spots when light is focused beyond the optical disk1.FIG. 5Bshows spots when light is properly focused on the optical disk1.FIG. 5Cshows spots when light is focused before the disk1.

The outputs of the optical detectors D1to D5of the five-way split photodiodes7are represented by S1, S2, S3, S4and S5, respectively. A focus error signal FES is given by the difference between the outputs of the second optical detector D2and the third optical detector D3:
FES=S2−S3

A tracking error is detected by a so-called three-beam method. The tracking subbeams are condensed onto the optical detectors D1and D5. A tracking error signal TES is given by the difference between the outputs of the optical detectors D1and D5:
TES=S1−S5

A reproduction signal RF is given by the sum of the outputs of the second, third and fourth optical detectors D2,D3and D4:
RF=S2+S3+S4

In a hologram optical pickup using the conventional three-beam method, the hologram4includes two regions4aand4bhaving different grating pitches. Light beams which pass through the regions4aand4bof the hologram4after reflection on the optical disk1have different diffraction angles. Therefore, the light beams which have passed through the regions4aand4bare diffracted at a smaller angle and a larger angle in one direction with respect to the hologram4.

The grating of the hologram4is typically formed of grooves which are formed by patterning using a photoetching technique. When the two regions4aand4bhaving different grating pitches are formed by patterning, the depth of the grooves and the angle of grating vary in each of the regions4aand4b, depending on an etching rate, etc.

If the groove depth and grating angle vary in each of the regions4aand4b, the variations appear as the difference in the intensity of diffracted light between the main beam and the subbeams, i.e., the difference in diffraction efficiency. As a result, the optical intensities of reflected light beams entering the optical detectors D1to D5are unbalanced, so that offset develops in the tracking error signal TES. In this case, characteristics of the hologram optical pickup are likely to be degraded.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an optical pickup comprises a light source; a three-way split light diffracting section for dividing light emitted from a light source into one main beam and two subbeams; a hologram for dividing each of the main beam and the subbeams reflected from a recording medium into two predetermined directions; and a light receiving section for receiving the beams divided by the hologram. The hologram includes two regions having a small-pitch grating for diffracting each beam to a predetermined direction, the small-pitch gratings have substantially the same pitch and are symmetrical about a split line separating the two regions.

In one embodiment of this invention, the light receiving section includes a plurality of light receiving portions, the plurality of light receiving portions being symmetrical about a plane including a split line separating the two regions and perpendicular to the hologram.

In one embodiment of this invention, the light receiving section has a light receiving portion and the light receiving portion is not used to detect the beams.

In one embodiment of this invention, the plurality of light receiving sections of the light receiving portion are substantially the same distance from the hologram.

In one embodiment of this invention, the light source, the three-way split light diffracting section, the hologram and the light receiving section are integrated into the optical pickup.

Thus, the invention described herein makes possible the advantages of providing an optical pickup in which the difference in diffraction efficiency between reflected light beams passing through two regions provided in a hologram is reduced so as to reduce the degradation of a characteristic of a tracking error signal TES.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a schematic diagram showing the optical system of an optical pickup according to the present invention.

This optical pickup comprises a semiconductor laser chip16which emits predetermined light. Light emitted from the semiconductor laser chip16is split by a tracking beam generating diffraction grating15provided on the rear side of a hologram element (not shown) into three beams, i.e., two subbeams for tracking and one main beam for reading information signals. Those three light beams pass through a hologram14provided on the hologram element as zero-order light, and are then converted by a collimator lens13into parallel light. The parallel light is condensed by an objective lens12onto a disk11. The light condensed onto the disk11is modulated by pits formed on the disk11and reflected from the disk11. The reflected light from the disk11passes through the objective lens12and the collimator lens13in this order, and is then diffracted by the hologram14and introduced into a five-way split photodiode17as first-order diffracted light.

The hologram14is in the shape of a circle, and is divided along a split line on a diameter of the circle into two equal semicircular regions14aand14b. The regions14aand14binclude small-pitch gratings having substantially the same grating pitch. The small-pitch gratings provided in the regions14aand14bare arranged to be substantially symmetrical about the split line separating the regions14aand14b. Therefore, the small-pitch gratings of the regions14aand14bhave substantially the same pitch. Therefore, the reflected light beams passing through the regions14aand14bof the hologram14are diffracted at substantially the same angle toward substantially the same direction, so that the light beams are substantially symmetrical about a plane which includes the split line separating the regions14aand14band is perpendicular to the hologram14. Therefore, the light beams passing through the regions14aand14bof the hologram14are separated into two directions as first-order diffracted light beams.

FIG. 2is a plan view showing an arrangement of optical detectors D1to D5of the five-way split photodiode17which reflected light beams enter.

The main beam and the two subbeams are each split into first-order diffracted light beams propagating in two directions by the two regions14aand14bof the hologram14. The optical detectors D1to D5are arranged by considering the incident positions of the first diffracted light beams. The first to fifth optical detectors D1to D5are arranged on the same plane and along a direction orthogonal to the split line splitting the hologram14into the two regions14aand14b. The first and fifth optical detectors D1and D5onto which two subbeams passing through the region14aare brought are positioned on the opposite sides of the photodiode17. A pair of the second and third optical detectors D2and D3, onto which the main beam passing through the region14aof the hologram14is brought, are provided next to the first optical detector D1. The fourth optical detector D4, onto which the main beam passing through the region14bof the hologram14is brought, is provided next to the fifth optical detector D5. The first and fifth optical detectors D1and D5are in the shape of a parallelogram extending in a direction along which the subbeams are diffracted, and are substantially symmetrical about a plane which includes the split line of the hologram14and is perpendicular to the hologram14. The distance between the first and fifth optical detectors D1and D5is increased the further their positions from the hologram14. The fourth optical detector D4provided next to the fifth optical detector D5is in the shape of a parallelogram similar to the fifth optical detector D5. The second and third optical detectors D2and D3are in the shape of parallelograms obtained by dividing a parallelogram, which is substantially symmetrical to the fourth optical detector D4about a plane which includes the split line of the hologram14and is perpendicular to the hologram14, into substantially two equal parts in a longitudinal direction.

Further, an optical detector Dx is provided between the third and fourth optical detectors D3and D4, which is used to prevent the occurrence of stray light, which affects the outputs of the optical detectors D1to D5. The optical detector Dx does not detect the main beam and subbeams. The optical detector Dx is in the shape of a trapezoid such that the width is broadened the further its position from the hologram14. The position of the optical detector Dx is such that the optical detector Dx seemingly buffers a region between the third and fourth optical detectors D3and D4.

The longitudinal length of each optical detector D1to D5is designed to be greater than a range within which the incident position of reflected light varies depending on the wavelength which varies as the temperature of a light source is changed, so that the desired outputs of the optical detectors D1to D5can be obtained. It should be noted that if the lengths of the optical detectors D1to D5are longer than that required, the capacitive components of the optical detectors D1to D5are large, leading to a reduction in response speed. The lengths of the optical detectors D1to D5are determined by considering the precision of positioning. Further, since each of the optical detectors D1to D5is in the shape of a parallelogram, a wastage of space is reduced when compared to the case where each detector is in the shape of a rectangle extending along the same direction as that of the parallelogram. Therefore, peripheral circuits, such as an amplifying circuit, etc., can be easily arranged.

The main beam of reflected light from the disk11is passed through the regions14aand14bof the hologram14so that the main beam is diffracted into directions substantially symmetrical about the split line separating the regions14aand14band the split beams form respective beam spots between the second and third optical detectors D2and D3of the five-way split photodiode17, and the fourth optical detector D4. The two subbeams are passed through the regions14aand14bof the hologram14so that the subbeams are diffracted into directions substantially symmetrical about the split line separating the regions14aand14band the split subbeams form respective beam spots on the first and fifth optical detectors D1and D5on the opposite sides of the five-way split photodiode17.

As described above, in the optical pickup of the present invention, the hologram14, which divides reflected light from the optical disk11into two parts, has two grating regions14aand14bwhose grating pitches are substantially equal to each other. Therefore, the variations in the depth of the small-pitch grating and the angle of the grating between the regions14aand14bcan be reduced, so that the differences in the intensity of the diffracted light (i.e., diffraction efficiency) of the main beam and the subbeams can be reduced between the regions14aand14b. As a result, characteristics of the optical pickup can be prevented from being degraded. Particularly, it is possible to improve the balance characteristic of a tracking error signal TES.

FIG. 3is a schematic diagram showing a specific configuration of an optical pickup according to the present invention.

This optical pickup has a plate-like stem20which is supported via a plurality of lead pins21by an optical disk apparatus body. A semiconductor laser chip16and a five-way split photodiode17are mounted on a surface of the stem20. The semiconductor laser chip16and the five-way split photodiode17are connected via wiring (not shown) to the optical disk apparatus. A hollow cap19is provided on the top side of the stem20f or covering the semiconductor laser chip16and the five-way split photodiode17to block light. A hologram element22is provided on the top wall of the cap19. A tracking beam generating diffraction grating15is provided on the lower side of the hologram element22. A hologram14is provided on the upper side of the hologram element22.

The thus-obtained optical pickup is a package into which the hologram element22, the semiconductor laser16, the photodiode17, etc. are integrated, resulting in miniaturization of the optical pickup. Further, the production processes of such an optical pickup can be simplified.

In the optical pickup of the present invention, the hologram has two regions having a small-pitch grating which diffracts light into a predetermined direction. The small-pitch gratings have substantially the same pitch and are substantially symmetrical about a split line separating the two regions. Therefore, variations in the depth and angle of the small-pitch gratings can be reduced between the two regions. In this case, the differences in the intensity of the diffracted light (i. e., diffraction efficiency) of a main beam and subbeams can be reduced between the two regions. Therefore, characteristics of the optical pickup can be prevented from being degraded. Particularly, it is possible to improve the balance characteristic of a tracking error signal TES.