Optical head device and diffractive element, optical information apparatus, computer, disc player, car navigation system, optical disc recorder and vehicle

Abstract

In an optical head device responding to optical disc of different track pitches, an effect of both obtaining the tracking error signal by the optimum DPP method using a single diffractive element and suppressing the lowering amount in the amplitude of the tracking error signal in a state the object lens is moved by track following is obtained. In the optical head device, the grating phase of left and right regions of a sub-beam generating diffractive element differ from each other by 180 degrees. A central region of the sub-beam generating diffractive element has a grating pattern different from the left and right regions, and is divided into a plurality of regions to form different gratings different from each other.

Description

The tracking error signal by a so-called phase difference method or push-pull method can also be obtained. The tracking signal by the phase difference method is obtained by comparing the phase of the change in temporal signal intensity of 20A+20C and 20B+20D. The tracking error signal by the push-pull method of the main beam (TEPP) is calculated by equation 2.
TEPP=(20A+20B)−(20C+20D)  (Eq. 2)

The light receiving regions 21, 22 receive the sub-beam. The tracking error signal by the differential push-pull method may be detected through calculation with the push-pull signal of the light receiving region 20, the focus error signal by the astigmatism method may be detected from the light receiving regions 21, 22 and calculated with the focus error signal obtained from the light receiving region 20 to remove the cross talk from the track signal.

The tracking error signal TEDPP by the differential push-pull method is calculated by equation 3.
TEDPP=(20A+20B)−(20C+20D)−K1[(21A+21B)−(21C+21D)]−k1[(22A+22B)−(22C+22D)]  (Eq. 3)

where K1 is a constant. The tracking error signal by the push-pull method is prevented from fluctuating when the objective lens moves in the T direction by track following by appropriately defining the constant K1. The cause of fluctuation in the tracking error signal when the objective lens moves in the T direction is the movement of the light beam on the light detector 10 as the far field pattern moves. Since the main beam and the sub-beams are equally subjected to the influence of the movement of the far field pattern, K1 is set so as to cancel the ratio of the light amount of the main beam and the light amount of the sub-beam. Specifically, the optical disc without grooves is prepared, focus control is performed, and the objective lens is forcibly moved in the T direction to define the value of K1 so that the change of TEDPP becomes sufficiently small. The value of K1 must be changed for every track pitch of the optical disc in patent document 3, but preferably, the value of K1 is rather defined according to the light amount distribution of the far field pattern of the light beam and the diffraction efficiency of the diffractive element for sub-beam diffraction. Therefore, K1 is desirably a constant unique to the device such as mounting a half-fixed resistor on the optical head device or the circuit substrate of the optical information apparatus, and shipping after adjusting the amplifier gain.

The focus error signal FED by the differential astigmatism method is calculated by equation 4.
FED=(20A+20C)−(20B+20D)−K2[(21A+21C)−(21B+21D)]−K2[(22A+22C)−(22B+22D)]  (Eq. 4)

where K2 is a constant. The cross talk from the track signal to the focus error signal is removed by appropriately defining the constant K2. In particular, since the push-pull signal of the DVD-RAM is large, K2 is desirably set so that the cross talk from the track signal to the focus error signal becomes a minimum with respect to the DVD-RAM disc. The focus control is performed on the DVD-RAM disc, and the objective lens is forcibly moved in the T direction to define the value of K2 so that the change in the focus error signal FED becomes sufficiently small. Similar to K1, K2 is desirably a constant unique to the device such as mounting a half-fixed resistor on the optical head device or the circuit substrate of the optical information apparatus, and shipping after adjusting the amplifier gain.

In DVD-R, DVD-RW, DVD-ROM, since the push-pull signal is relatively small, a circuit for switching according to the type of optical disc is arranged in the optical information apparatus so that the focus error signal uses FE of equation 1, and uses FED of equation 4 only for the DVD-RAM to switch the signals.

(Other Configuration Examples of Light Detector)

A light detector used in combination with a sub-beam generating diffractive element according to the above embodiment of the present invention capable of reproducing or recording not only the DVD but also on the CD will now be described. The light source 12 in FIG. 1 is assumed to be a two-wavelength light source for radiating not only red light but also infrared light. As disclosed in patent document 6 (Japanese Laid-Open Patent Publication No. 7-98431), with the region divided to the inner periphery and the outer periphery, the objective lens 25 converges the light passing through both inner and outer peripheries for the DVD and converges only the light passing through the inner periphery for the CD. Reproduction and recordation of both the CD and the DVD are realized by using the light detector 1000 shown in FIG. 11. In FIG. 11, the light detection regions 20, 21, 22 act the same as the light detector 10 shown in FIG. 10. The light detector 1000 is further arranged with light detection regions 23, 24, 25 for receiving the infrared light. The light detection region 23 is divided into four, and the light detection regions 24, 25 are divided into two. Similar to when the light receiving region 20 receives the red light, the infrared light is received by the light detection region 23, and the focus error signal by the astigmatism method and the tracking error signal by the push-pull method or the phase difference method are detected. When recording on CD-R or CD-RW, the tracking error signal by the DPP method is detected using the output of the light detection regions 24, 25. The tracking error signal TEDPPCD by the DPP method is detected through calculation of equation 5.
TEDPPCD=(23A+23B)−(23C+23D)−K3(24A−24B)−K3(25A−22B)  (Eq. 5)

where K3 is a constant. The tracking error signal by the push-pull method is prevented from fluctuating when the objective lens moves in the T direction by track following by appropriately defining the constant K3. The cause of fluctuation in the tracking error signal when the objective lens moves in the T direction is the movement of the light beam on the light detector 1000 as the far field pattern moves. Since the main beam and the sub-beam are equally subjected to the influence of the movement of the far field pattern, K3 is set so as to cancel the ratio of the light amount of the main beam and the light amount of the sub-beam. Specifically, the optical disc without grooves is prepared, focus control is performed, and the objective lens is forcibly moved in the T direction to define the value of K3 so that the change of TEDPPCD becomes sufficiently small. The value of K3 is preferably defined according to the light amount distribution of the far field pattern of the light beam and the diffraction efficiency of the diffractive element for sub-beam diffraction. Therefore, K3 is desirably a constant unique to the device such as mounting a half-fixed resistor on the optical head device or the circuit substrate of the optical information apparatus, and shipping after adjusting the amplifier gain. Since the diffractive element described in the previous embodiments is used as the sub-beam generating diffractive element in the present application, a satisfactory DPP signal can be obtained even in CD in which the track pitch greatly differs from the DVD. However, since the effective diameter of the infrared light is about ¾ of the red light, the width of the region between the diffraction grating region 1 and the diffraction grating region 2 of the sub-beam generating diffractive element is desirably less than or equal to 30% of the effective diameter (projection of objective lens) of the red light. As previously described that the width is desirably 10% to 40% in the optical head dedicated for DVD, the width of the region between the diffraction grating region 1 and the diffraction grating region 2 of the sub-beam generating diffractive element is desirably 10% to 30% of the effective diameter (projection of objective lens) of the red light to satisfy both requirements when also reproducing the CD.

(Optical Head Device)

The optical head device capable of reproducing or recording not only DVD but also CD and BD will now be described. In FIG. 12, the direction T is a direction perpendicular to the optical axis of the objective lens 9 and substantially perpendicular to the track groove extending direction of the optical disc (not shown), and direction Z is an optical axis direction of the objective lens 34, 41, that is, focusing direction (direction perpendicular to plane of drawing). The T-axis is a direction of moving the optical head device when recording and reproducing the inner periphery and the outer periphery of the optical disc. Y-axis is a direction perpendicular to the Z-axis and the T-axis and is a direction substantially parallel to the track groove extending direction of the optical disc at the position of the objective lens 9. The configuration of mirror inversion in which the T-axis and the Y-axis are inverted, or rotated by 90°, 180°, 270° may be adopted.

The light beam 2 of linear polarized light radiated from a first short wavelength light source (e.g., blue light source) 1 is reflected by a polarization separation film at the surface of a parallel flat plate 3, and transmitted through a hologram element 4. A reflective hologram (not shown) is formed at a region that does not shield the incident light to the objective lens 9 distant from the optical axis of the hologram element 4, and a monitor signal for stabilizing the light intensity is obtained without increasing the number of components by receiving the reflected diffracted light at the light detector 10 and monitoring the light intensity of the light beam 2.

The light beam 1 transmitted through the hologram element 4 is converted to light flux that is more greatly diverged by a relay lens 5. The relay lens 5 has a concave lens effect, and converts the angle of estimating the light source 1 from the opening portion of the objective lens 9, that is, the light source side numerical aperture (NA) is converted from small NA in the vicinity of the light source to the large NA on the collimator lens 7 side. The parallelism of the light beam 2 is converted to close to parallel by the collimator lens 7, and the optical axis is bent in the Z axis direction perpendicular to the optical disc by the rising mirror 8. The objective lens 9 passes the light beam 2 through the transparent base material of about 0.1 mm, thinner than 0.6 mm, and converges the light beam 2 on the recording surface of the optical disc of high density such as BD. The collimator lens 7 makes the light beams close to be parallel, that is, alleviates the divergence degree, and may be configured by combined two lenses. When moving the collimator lens 7 in the optical axis direction in order to correct spherical aberration, if the collimator lens 7 is configured by two lenses, only one of the two lenses needs to be moved. A quarter wavelength plate 18 changes linear polarized light to circular polarized light. The light beam reflected at the recording surface of the optical disc follows the original light path in the opposite direction, and is made to the linear polarized light in the direction perpendicular to the direction when exit from the light source 1 by the quarter wavelength plate 18, and transmitted through a branching member such as parallel flat plate 3 having the polarization separation film formed on the surface along with one part of the light beam diffracted by the hologram element 4, branched to a direction different from the light source 1, and photoelectric converted by the light detector 1010, so that electrical signals for obtaining information signal and servo signal (focus error signal for focus control, that is, focus servo and tracking signal for tracking control) are obtained.

The light beam 15 radiated from the second light source (e.g., infrared light source) 12 is transmitted through (partial diffraction) the diffractive element 13 for diffracting some light to form a sub-spot on the optical disc, transmitted through a wedge 6 having a cross sectional shape of a wedge shape, and has the parallelism converted by the collimator lens 7 (e.g., to substantially parallel light) and the optical axis bent by the rising mirror 9 in a direction perpendicular to the optical disc such as compact disc (CD) having lower recording density than the BD. The objective lens 9 passes the light beam 14 through the transparent base material of about 1.2 mm and converges the light beam 14 on the recording surface of the optical disc. The light beam reflected by the recording surface of the optical disc follows the original light path in the opposite direction, is branched towards a direction different from the light source 12 by a branching member such as polarizing selection film arranged on the surface on the collimator lens 7 side of the wedge 6, and photoelectric converted by the light detector 1010 similar to the light beam 1, so that electrical signals for obtaining information signal and servo signal (focus error signal for focus control, tracking signal for tracking control) are obtained. If an amplifier circuit is incorporated in the light detector 10, satisfactory information signal having high signal/noise (S/N) ratio is obtained, and miniaturization and thinning of the optical head device are achieved and stability is obtained.

Furthermore, in order to perform reproduction or recordation of a third optical disc (e.g., DVD) having an intermediate recording density of the two types of optical discs, the third red light source is arranged in the vicinity of the light source 12, and a beam splitter arranged in the vicinity of the light source 12 for combining the light paths with the infrared light source may be arranged, but if the light source 12 is a two-wavelength light source for exiting the light beam of two wavelengths of red light and infrared light, the beam splitter is unnecessary and the number of components is reduced.

The light beam 15 of red light radiated from the light source 12 reaches the objective lens 9, similar to the red light, and passes through the transparent base material of about 0.6 mm and converges on the recording surface of the optical disc such as DVD by the objective lens 9. Similar to the red light, the light beam reflected by the recording surface of the optical disc follows the original light path in the opposite direction, and electrical signals for obtaining information signal and servo signal (focus error signal for focus control, tracking signal for tracking control) are obtained by the light detector 10.

Generally, in order to branch the optical path, a cube type beam splitter in which two triangular transparent members are attached may be used, but the number of components can be reduced if parallel flat plate or wedge is used as in the present application, and the material cost is reduced. If the beam splitter of a single member is arranged in a non-parallel light path from the light source to the objective lens to transmit light, the wedge 6 as shown in FIG. 12 is used to prevent production of astigmatism, and the angle of incidence of the optical axis is desirably smaller than 45 degrees. Aberration may still occur due to manufacturing error even if the above are taken into consideration. Thus, in the example shown in FIG. 1 of the present application, the light beam 2 to be converged on the optical disc having the highest density is reflected without transmitting through either of the two beam splitters in the non-parallel light path from the light source 1 to the collimator lens 7. Thus, satisfactory signal reproduction and signal recordation can be realized on the optical disc having the highest density such as BD.

The objective lens 9 is fixed at a predetermined position of the actuator (not shown) for microscopically moving the objective lens. An object lens drive device (objective lens actuator) is able to microscopically move the objective lens 9 in both focusing direction Z orthogonal to the recording surface of the optical disc and tracking direction Y of the optical disc.

When using the objective lens 9 of NA 0.85 or of larger numerical aperture for reproduction or recordation of BD and the like, spherical aberration is significantly produced with respect to the thickness of the transparent base material filled from the surface to which the light enters the optical disc up to the information recording surface when performing recordation or reproduction on the optical disc since the numerical aperture is large. In the present example, the dispersion convergence degree of the light from the collimator lens 7 to the objective lens 9 is changed by moving the collimator lens 7 in the optical axis direction of the collimator lens 7. When the dispersion convergence degree of the light entering the objective lens changes, the spherical aberration changes, and thus the spherical aberration originating from the base material thickness difference is corrected using the same. The optical head device shown in FIG. 12 is arranged with a driving device 11 to move the collimator lens 7 in the optical axis direction of the collimator lens 7. Specifically, stepping motor, brushless motor, or the like is used as the driving device 11. A holder 17 for holding the collimator lens 7, a guide shaft 16 for guiding the movement of the holder 17, and a gear (not shown) for transmitting the driving force of the driving motor 11 to the holder 17 are arranged in the optical head device. The holder 17 for holding the collimator lens 7 may be integrally molded with the collimator lens 7, whereby number of components of the optical head device is reduced if integrally molded.

The collimator lens 7 is prevented from performing an unintended movement with respect to inertia force by acceleration and deceleration speed when moving the entire optical head device in the inner and outer peripheral direction of the optical disc by having the optical axis of the collimator lens 7 non-parallel to the Y axis as in the present application.

The objective lens 25 converges the infrared light 15 at the inner most peripheral portion near the optical axis through the transparent base material of about 1.2 mm of the low density optical disc 28 such as CD. The objective lens 25 also converges the red light 14 up to the middle peripheral portion of a range one size wider than the inner most peripheral portion through the transparent base material of about 0.6 mm of the optical disc 27 such as DVD. The objective lens 25 converges the blue light 2 within the effective diameter through the transparent base material of about 0.1 mm or thinner of the high density optical disc 26 such as BD.

Therefore, in order to converge the respective light passed through the transparent base material of different thicknesses, diffractive element is effectively used, as disclosed in patent document 6 (Japanese Laid-Open Patent No. 7-98431). The diffractive element has the design of the inner most peripheral part, the middle peripheral part, and the outer most peripheral part to be discontinuous, so that the inner most peripheral part converges through the base material of any thickness, and the outer most peripheral part converges only when transmitted through the base material of 0.1 mm or thinner. As one example, the design is more facilitated by using the light source of different wavelengths as described above. The CD uses infrared light, DVD uses red light, and BD uses blue light, and correction of spherical aberration due to base material thickness and switching in limited opening according to the type of disc are achieved by utilizing the fact that the diffraction angle of a primary diffracted light of the diffractive element differs depending on the wavelength.

FIG. 13 shows an example of a division of a light detection region of the light detector 1010 suited for detecting the focus error signal by astigmatism method. In FIG. 13, a pattern seen through from the side opposite to the surface to which the light beam enters of the light detector 1010 is shown. The light receiving regions 20, 21, 22, 23, 24, 25 are denoted with the same reference numbers for the portions acting the same as the light detector 1000 of FIG. 11 with respect to the red light and the infrared light. The light receiving region 20 receives the blue light and the red light. The light receiving region 20 is divided into four, and detects the focus error signal using astigmatism provided by the parallel flat plate 3. The tracking signal by the so-called phase difference method and push-pull method may also be obtained. The light receiving regions 21, 22 receive the sub-beam diffracted when the red light passes through the diffractive element 13. The tracking error signal by the differential push-pull method is detected through calculation with the push-pull signal of the light receiving region 20, the focus error signal by the astigmatism method is detected from the light receiving regions 21, 22 and calculated with the focus error signal obtained from the light receiving region 20 to perform the differential astigmatism method of removing cross talk from the track signal. By arranging the diffractive element between the light source 1 and the parallel flat plate 3, the sub-beam signal can be detected from the light receiving regions 21, 22 in the blue light, similar to in the red light.

The light receiving regions 23, 24, 25 receive infrared light. The inter-center distance of the light receiving regions 20 and 23 is set to the distance obtained by multiplying a magnification ratio realized by the relay lens 5 to the inter-light emitting point distance of the red light and the infrared light in the light source 12. Assuming the inter-center distance of the light receiving regions 20 and 21 is L1, and the inter-center distance of the light receiving regions 23, 24 is L2, the ratio of L1 and L2 is set so as to be equal to the ratio of wavelength of red light: wavelength of infrared light. The light receiving region 23 is divided into four, and the focus error signal is detected using astigmatism provided by the parallel flat plate 3. The tracking signal by the so-called phase difference method and the push-pull method may also be obtained. The light receiving regions 24, 25 receive sub-beam diffracted when the infrared light passes through the diffractive element 13. The tracking signal by the differential push-pull method is detected through calculation with the push-pull signal of the light receiving region 23. In the present application, the diffractive element described in the previous embodiments such as diffractive element 31, 3100 is used as the diffractive element 13. Thus, the effect of satisfying both obtaining the tracking error signal by DPP method suited for the optical disc of different track pitches using a single diffractive element, and suppressing the lowering amount in the amplitude of tracking error signal in a state the objective lens is moved by tracking following is obtained.

The number of components of the semiconductor component is reduced by arranging a plurality of light receiving regions on a single light detector or a single semiconductor chip and photoelectric converting the different wavelengths.

(Optical Information Apparatus)

Furthermore, a configuration example of the optical information apparatus using the optical head device of the present invention is shown in FIG. 14. In FIG. 14, the optical discs 26, 27, 28 are arranged on a turn table 182, and rotated by a motor 164. The optical head device shown in FIG. 12 is used for the optical head device 155. The optical head device 155 is roughly moved by the drive device 151 of the optical head device up to a track where the desired information is present in the optical disc.

The optical head device 155 provides the focus error (focus error) signal or the tracking error signal to an electric circuit 153 in correspondence to the positional relationship with the optical disc 26. The electric circuit 153 provides a signal for microscopically moving the objective lens to the optical head device 155 in response to the focus error signal or the tracking error signal. In response to the signal, the optical head device 155 performs focus servo (control) and the tracking control on the optical disc, and also performs read, write, (record) or erase of the information with respect to the optical head device 155.

The optical information apparatus 157 of the present embodiment is mounted with the optical head device shown in FIG. 12, and thus has an advantage of stably recording or reproducing on a plurality of optical discs having different recording density with a single optical head device.

(Computer)

The computer mounted with the optical information apparatus 167 shown in FIG. 14 will now be described. The computer, the optical disc player, or the optical disc recorder mounted with the optical information apparatus of FIG. 14 or adopting recording and reproducing method described above stably records or reproduces the optical disc of different type, and thus can be used for a wide range of applications.

In FIG. 15, a computer 300 including the optical information apparatus 167 shown in FIG. 14, an input device 365 such as keyboard, mouse, and touch panel to input information, an arithmetic device 364 such as central processing unit (CPU) for performing calculation based on information input from the input device and the information read from the optical information apparatus 167, and an output device 361 such as Braun tube, liquid crystal display device, and printer for displaying information such as result of calculation by the arithmetic device.

(Optical Disc Player)

The schematic form configuration of the optical disc player mounted with the optical information apparatus shown in FIG. 14 is shown in FIG. 16. In FIG. 16, the optical disc player 321 including the optical information apparatus 167 shown in FIG. 14, and an information-image conversion device (e.g., decoder 366) for converting information signal obtained from the optical information apparatus to image is configured. This configuration is also used as a car navigation system by adding a position sensor such as GPS and central processing device (CPU). A mode added with the display device 320 such as liquid crystal monitor is also possible.

(Optical Disc Recorder)

A schematic form configuration of an optical disc recorder mounted with the optical information apparatus shown in FIG. 14 is shown in FIG. 17. The optical disc recorder shown in FIG. 17 includes an optical information apparatus 167 shown in FIG. 14 and an image-information conversion device (e.g., encoder 368) for converting the image information to the information to be recorded on the optical disc by the optical information apparatus. Desirably, the portion that is already recorded can also be reproduced by including the information-image conversion device (decoder 366) for converting information signal obtained from the optical information apparatus to image. The output device 361 such as Braun tube, liquid crystal display device, printer, and the like for displaying information may also be arranged.

(Vehicle)

The vehicle mounted with the optical information apparatus 167 shown in FIG. 14 will now be described. The schematic form configuration of the vehicle mounted with the optical information apparatus shown in FIG. 14 is shown in FIG. 18. In FIG. 18, the optical information apparatus 167 is the optical information apparatus 167 of FIG. 14. The vehicle body 131 is mounted with the optical information apparatus 167. A power generating unit 134 and a fuel storing unit 135 for storing fuel to be supplied to the power generating unit 134 and/or a power supply 136 are also arranged in the vehicle body 131. Therefore, information can be stably obtained from various types of optical disc while in the moving body by mounting the optical information apparatus 167 of the present application on the vehicle body. Alternatively, the information can be recorded. The vehicle body 131 further includes wheels 133 for traveling in the case of train or automobile. In the case of automobile, a handle 130 for changing directions is arranged.

Furthermore, a great number of optical discs can be readily used by mounting a changer 138 or an optical disc accommodating unit 139 on the vehicle body 131. An arithmetic device 164 for processing the information obtained from the optical disc to obtain an image, a semiconductor memory 137 for temporarily storing the information and a display device 142 may be arranged to enable reproduction of picture information from the optical disc. Audio and music can be reproduced from the optical disc by arranging an amplifier 140 and a speaker 141. With arrangement of a positional sensor such as GPS 132, along with the map information reproduced from the optical disc, the current position and advancing direction can be recognized as image displayed on the display device 142 or audio output from the speaker 141. Furthermore, information from the outside can be received and complementary used with information of the optical disc by arranging a wireless communication unit 143.

The output device 361 and the liquid crystal display monitor 320 are shown in FIGS. 15, 16, and 17, but output terminals may be arranged, where the output device 361 and the liquid crystal monitor 320 obviously may be in separately sold product modes without being arranged therein. The input device is not shown in FIGS. 16 and 17, but a product mode equipped with the input device such as keyboard, touch panel, mouse, remote control device and the like is also possible. The input device may be separately sold, and may be in a mode including only the input terminals.

The optical head device according to the present invention can record and reproduce on a plurality of types of optical discs having different base material thickness, responding wavelength, recording density, and the like, and a compatible optical information apparatus using the optical head device can handle optical disc of various standards such as CD, DVD, and BD. Therefore, the application can be expanded to various systems for storing information such as computer, optical disc player, optical disc recorder, car navigation system, edit system, data server, AV component, vehicle, and the like.

The respective advantages are obtained by appropriately combining arbitrary embodiments of the various embodiments described above.

Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.

Claims

1. An optical head device comprising a light source, a diffractive element for branching light exiting from the light source into at least three light fluxes including a main beam that is transmitted without being diffracted and two sub-beams that are diffracted, an objective lens for converging the three light fluxes on a recording surface of an optical disc, and a light detector for receiving the light converged on the recording surface of the optical disc by the objective lens and reflected by the optical disc and photoelectric converting the light to an electrical signal;

wherein the diffractive element is divided into a first diffraction grating region formed with a first diffraction grating, a second diffraction grating region formed with a second diffraction grating having a phase differing by substantially 180 degrees from the first diffraction grating, and a central region sandwiched by the first diffraction grating region and the second diffraction grating region;

wherein the central region is divided into a plurality of divided regions by a virtual dividing line; and

wherein the central region has phases or grating vectors different from the first diffraction grating region and the second diffraction grating region, and the divided regions divided by the dividing line are formed with diffraction gratings having different phases and grating vectors from each other.

2. The optical head device according to claim 1, An optical head device comprising a light source, a diffractive element for branching light exiting from the light source into at least three light fluxes including a main beam that is transmitted without being diffracted and two sub-beams that are diffracted, an objective lens for converging the three light fluxes on a recording surface of an optical disc, and a light detector for receiving the light converged on the recording surface of the optical disc by the objective lens and reflected by the optical disc and photoelectric converting the light to an electrical signal;

wherein the diffractive element is divided into a first diffraction grating region formed with a first diffraction grating, a second diffraction grating region formed with a second diffraction grating having a phase differing by substantially 180 degrees from the first diffraction grating, and a central region sandwiched by the first diffraction grating region and the second diffraction grating region;

wherein the central region is divided into a plurality of divided regions by a virtual dividing line;

wherein the central region has phases or grating vectors different from the first diffraction grating region and the second diffraction grating region, and the divided regions divided by the dividing line are formed with diffraction gratings having different phases or grating vectors from each other; and

wherein the dividing line dividing the central region extends in a direction orthogonal to an extending direction of a track groove of the optical disc.

3. The optical head device according to claim 1, An optical head device comprising a light source, a diffractive element for branching light exiting from the light source into at least three light fluxes including a main beam that is transmitted without being diffracted and two sub-beams that are diffracted, an objective lens for converging the three light fluxes on a recording surface of an optical disc, and a light detector for receiving the light converged on the recording surface of the optical disc by the objective lens and reflected by the optical disc and photoelectric converting the light to an electrical signal;

wherein the diffractive element is divided into a first diffraction grating region formed with a first diffraction grating, a second diffraction grating region formed with a second diffraction grating having a phase differing by substantially 180 degrees from the first diffraction grating, and a central region sandwiched by the first diffraction grating region and the second diffraction grating region;

wherein the central region is divided into a plurality of divided regions by a virtual dividing line;

wherein the central region has phases or grating vectors different from the first diffraction grating region and the second diffraction grating region, and the divided regions divided by the dividing line are formed with diffraction gratings having different phases or grating vectors from each other; and

wherein the central region is divided into three or more divided regions.

4. The optical head device according to claim 3, wherein diffraction gratings having different phases from each other are formed in two divided regions of the three or more divided regions of the central region; and

a diffraction grating of the same grating vector and the same phase as the diffraction grating formed in one of the two divided regions is formed in remaining divided region of the three or more divided regions.

5. The optical head device according to claim 1, An optical head device comprising a light source, a diffractive element for branching light exiting from the light source into at least three light fluxes including a main beam that is transmitted without being diffracted and two sub-beams that are diffracted, an objective lens for converging the three light fluxes on a recording surface of an optical disc, and a light detector for receiving the light converged on the recording surface of the optical disc by the objective lens and reflected by the optical disc and photoelectric converting the light to an electrical signal;

wherein the diffractive element is divided into a first diffraction grating region formed with a first diffraction grating, a second diffraction grating region formed with a second diffraction grating having a phase differing by substantially 180 degrees from the first diffraction grating, and a central region sandwiched by the first diffraction grating region and the second diffraction grating region;

wherein the central region is divided into a plurality of divided regions by a virtual dividing line;

wherein the central region has phases or grating vectors different from the first diffraction grating region and the second diffraction grating region, and the divided regions divided by the dividing line are formed with diffraction gratings having different phases or grating vectors from each other; and

wherein when the phase of the first diffraction grating formed in the first diffraction grating region is 0 degrees as a reference, the phase of a third diffraction grating formed in the divided region obtained by dividing the central region and the phase of a fourth diffraction grating formed in another divided region have opposite polarities but the same absolute value.

6. The optical head device according to claim 5, wherein when the phase of the first diffraction grating formed in the first divided region is 0 degrees as a reference, the phase of the third diffraction grating is 90 degrees, and the phase of the fourth diffraction grating is −90 degrees.

7. The optical head device according to claim 1, An optical head device comprising a light source, a diffractive element for branching light exiting from the light source into at least three light fluxes including a main beam that is transmitted without being diffracted and two sub-beams that are diffracted, an objective lens for converging the three light fluxes on a recording surface of an optical disc, and a light detector for receiving the light converged on the recording surface of the optical disc by the objective lens and reflected by the optical disc and photoelectric converting the light to an electrical signal;

wherein the diffractive element is divided into a first diffraction grating region formed with a first diffraction grating, a second diffraction grating region formed with a second diffraction grating having a phase differing by substantially 180 degrees from the first diffraction grating, and a central region sandwiched by the first diffraction grating region and the second diffraction grating region;

wherein the central region is divided into a plurality of divided regions by a virtual dividing line;

wherein the central region has phases or grating vectors different from the first diffraction grating region and the second diffraction grating region, and the divided regions divided by the dividing line are formed with diffraction gratings having different phases or grating vectors from each other; and

wherein when the phase of the first diffraction grating formed in the first diffraction grating region is 0 degrees as a reference, the phase of each diffraction grating formed in each divided region of the central region is substantially 0 degrees on average.

8. The optical head device according to claim 3, wherein when the phase of the first diffraction grating formed in the first divided region is 0 degrees as a reference, the central region is divided into four or more divided regions, and includes a diffraction grating having a phase of −120 degrees, a diffraction grating having a phase of −60 degrees, a diffraction grating having a phase of +60 degrees, and a diffraction grating having a phase of +120 degrees.

9. The optical head device according to claim 1, wherein the width of the central region is 10% to 40% of a projection diameter on the diffractive element of an effective diameter of the objective lens.

10. The optical head device according to claim 1, wherein the light source is a two wavelength light source for emitting a red light and an infrared light.

11. The optical head device according to claim 10, wherein the width of the central region is less than or equal to 30% of a projection diameter on the diffractive element of an opening effective diameter of the objective lens.

12. The optical head device according to claim 1, further comprising a blue light source.

13. A diffractive element mounted on an optical head device including a light source, an objective lens for converging light exiting from the light source on a recording surface of an optical disc, and a light detector for receiving the light converged on the recording surface of the optical disc by the objective lens and reflected by the optical disc and photoelectric converting the light to an electrical signal, the diffractive element branching the light exiting from the light source into at least three light fluxes including a main beam that is transmitted without being diffracted and two sub-beams that are diffracted;

wherein the diffractive element is divided into a first diffraction grating region formed with a first diffraction grating, a second diffraction grating region formed with a second diffraction grating having a phase differing by substantially 180 degrees from the first diffraction grating and a central region sandwiched by the first diffraction grating region and the second diffraction grating region;

wherein the central region is divided into a plurality of divided regions by a virtual dividing line; and

wherein the central region has phases or grating vectors different from the first diffraction grating region and the second diffraction grating region, and the divided regions divided by the dividing line are formed with diffraction gratings having different phases and grating vectors from each other.

14. The diffractive element according to claim 13, wherein the dividing line dividing the central region extends in a direction orthogonal to an extending direction of a track groove of the optical disc.

15. The diffractive element according to claim 13, wherein the central region is divided into three or more divided regions.

16. The diffractive element according to claim 15, wherein diffraction gratings having different phases from each other are formed in two divided regions of the three or more divided regions of the central region; and

a diffraction grating of the same grating vector and the same phase as the diffraction grating formed in one of the two divided regions is formed in remaining divided region of the three or more divided regions.

17. The diffractive element according to claim 13, A diffractive element mounted on an optical head device including a light source, an objective lens for converging light exiting from the light source on a recording surface of an optical disc, and a light detector for receiving the light converged on the recording surface of the optical disc by the objective lens and reflected by the optical disc and photoelectric converting the light to an electrical signal, the diffractive element branching the light exiting from the light source into at least three light fluxes including a main beam that is transmitted without being diffracted and two sub-beams that are diffracted;

wherein the diffractive element is divided into a first diffraction grating region formed with a first diffraction grating, a second diffraction grating region formed with a second diffraction grating having a phase differing by substantially 180 degrees from the first diffraction grating and a central region sandwiched by the first diffraction grating region and the second diffraction grating region;

wherein the central region is divided into a plurality of divided regions by a virtual dividing line;

wherein the central region has phases or grating vectors different from the first diffraction grating region and the second diffraction grating region, and the divided regions divided by the dividing line are formed with diffraction gratings having different phases or grating vectors from each other; and

wherein when the phase of the first diffraction grating formed in the first diffraction grating region is 0 degrees as a reference, the phase of a third diffraction grating formed in the divided region obtained by dividing the central region and the phase of a fourth diffraction grating formed in another divided region have opposite polarities but the same absolute value.

18. The diffractive element according to claim 17, wherein when the phase of the first diffraction grating formed in the first divided region is 0 degrees as a reference, the phase of the third diffraction grating is 90 degrees, and the phase of the fourth diffraction grating is −90 degrees.

19. The diffractive element according to claim 13, A diffractive element mounted on an optical head device including a light source, an objective lens for converging light exiting from the light source on a recording surface of an optical disc, and a light detector for receiving the light converged on the recording surface of the optical disc by the objective lens and reflected by the optical disc and photoelectric converting the light to an electrical signal, the diffractive element branching the light exiting from the light source into at least three light fluxes including a main beam that is transmitted without being diffracted and two sub-beams that are diffracted;

wherein the diffractive element is divided into a first diffraction grating region formed with a first diffraction grating, a second diffraction grating region formed with a second diffraction grating having a phase differing by substantially 180 degrees from the first diffraction grating and a central region sandwiched by the first diffraction grating region and the second diffraction grating region;

wherein the central region is divided into a plurality of divided regions by a virtual dividing line;

wherein the central region has phases or grating vectors different from the first diffraction grating region and the second diffraction grating region, and the divided regions divided by the dividing line are formed with diffraction gratings having different phases or grating vectors from each other; and

wherein when the phase of the first diffraction grating formed in the first diffraction grating region is 0 degrees as a reference, the phase of each diffraction grating formed in each divided region of the central region is substantially 0 degrees on average.

20. The diffractive element according to claim 16, A diffractive element mounted on an optical head device including a light source, an objective lens for converging light exiting from the light source on a recording surface of an optical disc, and a light detector for receiving the light converged on the recording surface of the optical disc by the objective lens and reflected by the optical disc and photoelectric converting the light to an electrical signal, the diffractive element branching the light exiting from the light source into at least three light fluxes including a main beam that is transmitted without being diffracted and two sub-beams that are diffracted;

wherein the diffractive element is divided into a first diffraction grating region formed with a first diffraction grating, a second diffraction grating region formed with a second diffraction grating having a phase differing by substantially 180 degrees from the first diffraction grating and a central region sandwiched by the first diffraction grating region and the second diffraction grating region;

wherein the central region is divided into a plurality of divided regions by a virtual dividing line;

wherein the central region has phases or grating vectors different from the first diffraction grating region and the second diffraction grating region, and the divided regions divided by the dividing line are formed with diffraction gratings having different phases or grating vectors from each other;

wherein the central region is divided into three or more divided regions;

wherein diffraction gratings having different phases from each other are formed in two divided regions of the three or more divided regions of the central region;

wherein a diffraction grating of the same grating vector and the same phase as the diffraction grating formed in one of the two divided regions is formed in remaining divided region of the three or more divided regions; and

wherein when the phase of the first diffraction grating formed in the first divided region is 0 degrees as a reference, the central region is divided into four or more regions, and includes a diffraction grating having a phase of −120 degrees, a diffraction grating having a phase of −60 degrees, a diffraction grating having a phase of +60 degrees, and a diffraction grating having a phase of +120 degrees.

21. An optical information apparatus comprising:

the optical head device according to claim 1;

a motor for rotating an optical disc; and

an electric circuit for receiving a signal obtained from the optical head device, and controlling and driving the motor, an optical lens, and a laser light source based on the signal.

22. The optical information apparatus according to claim 21, wherein

a push-pull signal obtained by receiving the sub-beam at the light detector and calculating through photoelectric conversion is amplified at an amplification factor K1, and subtracted with a push-pull signal obtained by receiving the sub-beam at the light detector and being processed through photoelectric conversion to be used as a tracking error signal; and

the amplification factor K1 is fixed after being adjusted to a value in which change in tracking control signal becomes smaller when the objective lens is moved in a direction perpendicular to an extending direction of a track groove of the optical disc.

23. The optical information apparatus according to claim 21, wherein a focus error signal obtained by receiving the sub-beam at the light detector and calculating through photoelectric conversion is amplified at an amplification factor K2, and added with a focus error signal obtained by receiving the sub-beam at the light detector and being processed through photoelectric conversion to be used as a focus error signal for focus control.

24. A computer comprising:

the optical information apparatus according to claim 1;

an input device or an input terminal for inputting the information;

an arithmetic device for performing an arithmetic operation based on information input from the input device or information reproduced from the optical information apparatus; and

an output device or an output terminal for displaying or outputting the information input from the input device or the information reproduced from the optical information apparatus, or result of arithmetic performed by the arithmetic device.

25. An optical disc player comprising:

the optical information apparatus according to claim 21; and

an information-image decoder for converting information signal obtained from the optical information apparatus to an image.

26. A car navigation system comprising:

the optical information apparatus according to claim 21;

an information-image decoder for converting information signal obtained from the optical information apparatus to an image; and

a position sensor.

27. An optical disc recorder comprising:

the optical information apparatus according to claim 21; and

an image-information encoder for converting image information to information to be recorded by the optical information apparatus.

28. A vehicle comprising the optical information apparatus according to claim 21, a vehicle body mounted with the optical information apparatus, and a power generating unit for generating power to move the vehicle body.

29. An optical information apparatus comprising:

the optical head device according to claim 2;

a motor for rotating an optical disc; and

an electric circuit for receiving a signal obtained from the optical head device, and controlling and driving the motor, an optical lens, and a laser light source based on the signal.

30. An optical information apparatus comprising:

the optical head device according to claim 3;

a motor for rotating an optical disc; and

an electric circuit for receiving a signal obtained from the optical head device, and controlling and driving the motor, an optical lens, and a laser light source based on the signal.

31. An optical information apparatus comprising:

the optical head device according to claim 4;

a motor for rotating an optical disc; and

an electric circuit for receiving a signal obtained from the optical head device, and controlling and driving the motor, an optical lens, and a laser light source based on the signal.

32. An optical information apparatus comprising:

the optical head device according to claim 5;

a motor for rotating an optical disc; and

an electric circuit for receiving a signal obtained from the optical head device, and controlling and driving the motor, an optical lens, and a laser light source based on the signal.

33. An optical information apparatus comprising:

the optical head device according to claim 6;

a motor for rotating an optical disc; and

an electric circuit for receiving a signal obtained from the optical head device, and controlling and driving the motor, an optical lens, and a laser light source based on the signal.

34. An optical information apparatus comprising:

the optical head device according to claim 7;

a motor for rotating an optical disc; and

an electric circuit for receiving a signal obtained from the optical head device, and controlling and driving the motor, an optical lens, and a laser light source based on the signal.

35. An optical information apparatus comprising:

the optical head device according to claim 8;

a motor for rotating an optical disc; and

an electric circuit for receiving a signal obtained from the optical head device, and controlling and driving the motor, an optical lens, and a laser light source based on the signal.