Apparatus for optical pick-up

An optical pickup includes a light source configured to emit light, a beam splitter configured to transmit or reflect light, an object lens configured to condense the light transmitted by the beam splitter onto an optical storage, and a diffraction grating having a first grating pattern and a second grating pattern. The first grating pattern is configured to diffract and divide the light reflected by the optical storage into a main beam and two sub beams. The second grating pattern is configured to diffract the light along a different diffracting direction than the first grating pattern. A condensing lens is configured to generate astigmatism to the light diffracted by the first grating pattern, and a light sensor is configured to receive the light via the condensing lens and detect a tracking error signal.

The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2006-0034999, filed on Apr. 18, 2006, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for optical pickup and more particularly, to an apparatus for optical pickup which minimizes offset element occurring between the recorded and non-recorded pattern when object lenses are configured off-axis along the tracks on the optical storage medium is disclosed.

2. Description of the Related Art

Optical storage media such as optical disks optically record data and are in shape of palm-size disk. Those media are loaded on a driver for data to be recorded thereto or to be read therefrom by optical apparatus inside the driver.

There exist various optical storage media, for example, CD (Compact disc), DVD (Digital Versatile Disc), BD (Blu-ray Disc). Also, there are sub groups of DVD, such as DVD-RW, DVD+RW, DVD-RW. The kinds of those media are increasingly diversifying.

In prior arts, various technologies have been developed to trace the track and groove on the optical storage media. Among those technologies, one uses three beams to trace the track and groove. In this case, especially when the three branched-off beams are located off-axis along the tracking direction, there is a problem that offset elements will occur over large areas near the boundary of recording and non-recording pattern of the optical storage media.

In making such various image sensors, efforts are being made to improve these image sensors.

SUMMARY OF THE INVENTION

The present invention relates to an apparatus for optical pickup which effectively minimizes offset elements without being affected by the difference between the times when the beams enter the recording and non-recording area and the difference between the positions where the object lenses are located when those beams are located off-axis along the inner tracks and outer tracks.

To achieve at least one of these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an embodiment of an optical pickup includes a light source configured to emit light, a beam splitter configured to transmit or reflect light, an object lens configured to condense the light transmitted by the beam splitter onto an optical storage, and a diffraction grating having a first grating pattern and a second grating pattern. The first grating pattern is configured to diffract and divide the light reflected by the optical storage into a main beam and two sub beams. The second grating pattern is configured to diffract the light along a different diffracting direction than the first grating pattern. A condensing lens is configured to generate astigmatism to the light diffracted by the first grating pattern, and a light sensor is configured to receive the light via the condensing lens and detect a tracking error signal.

In another aspect of the present invention, there is provided a method for manufacturing a CMOS image sensor including the above described optical pickup.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows an optical pickup100according to the embodiment of the present invention.

Referring toFIG. 1, the optical pickup100according to the first embodiment of the present invention comprises light source110, collimate lens120, beam splitter130, diffraction grating140, wavelength plate150, object lens155, condensing lens160and light detecting means170, and an optical storage180may be placed before the object lens155.

The light source110generate laser beam and the collimate lens120transforms the light generated by light source110into a parallel beam.

The beam splitter130which is a light dividing means transmits the incident light from collimate lens120depending on the polarizing direction of the light or reflects the light reflected by the optical storage180to the condensing lens160.

The transmission light through beam splitter130passes through the diffraction grating140and then is transformed to a circularly polarized light by quarter wavelength plate150.

The circularly polarized light passes through the object lens155and is reflected by the optical storage180having land and groove structure and goes back to the object lens130.

The reflected light is transformed to a parallel beam by the object lens130and then is polarized by quarter wavelength150so that its polarizing direction is reversed. And then the light is diffracted by the diffraction grating140.

The diffraction grating140diffracts and divides the reflected light by the optical storage180into a main beam and two sub beams, in which the diffracted lights of main beam and sub beams form a baseball pattern as shown inFIG. 3.

The diffraction grating140comprises two grating pattern regions which have different polarization direction from each other. The first grating pattern performs diffracting and dividing of the reflected light by the optical storage and excludes the AC signal causing area i.e. the region which the sub beam region and the main beam region overlap. The second grating pattern covers the region which the sub beam region and the main beam region overlap and diffracts the lights to a direction different from case of the first grating pattern. The diffracted light by the second grating pattern does not enter the light detecting means and is excluded from error detecting process.

The detailed structures of the diffraction grating140will be described below with reference toFIG. 4and the following drawings.

The main beam divided out by the diffraction grating140is the 0thbeam and is detected as an MPP signal at the light detecting means170, the sub beams thereof are +1st beam and −1st beam and are detected as first SPP signal and second SPP signal at the light detecting means170respectively.

The diffracted light from the diffraction grating140is reflected by beam splitter130and transmits to the light detecting means170via condensing lens160.

The light detecting means170is a light-to-electrical conversion device such as Photo Diode, receives 0thbeam, +1stbeam −1stbeam and generates MPP signal, first SPP signal and second SPP signal and detects TES (Track Error Signal) therefrom.

FIG. 2aillustrates the diffraction of light by optical storage.

As shown inFIG. 2a, the incident light into optical storage180is diffracted by the track structure of land/groove on the optical storage180with a certain angle θ and forms 0th, 1stand −1stbeams. And the 0th+1stand −1stbeams form circular pattern(S).

The size of circular pattern formed by 0th, +1stand −1stbeams may be equivalent to EPD (Entrance Pupil Diameter) of the object lens placed right before the optical storage and can be calculated by the following equation.
EPD=2×f×NA(Equation 1)

Wherein “f” is focal length of object lens,

FIG. 2billustrates forming of a baseball which is formed by diffracted light at the optical storage180.

As shown inFIG. 2b, when a circular patterned beam is incident into the optical storage180, it is diffracted to form three beams b1, b2, b3.

The circular patterns formed by side beams b1,b3are shifted from the central pattern of the central beam b2, wherein the amount of shift can be calculated by the following equation.
Shift=the dimension of the sub beam×f×λ÷TP

wherein “the dimension of the sub beam is +1 or −1,

“f” is focal length of object lens,

“λ” is the wavelength of the sub beam

“TP” is the track pitch of the optical storage.

Reflected by the optical storage180, three beams b1, b2, b3from a baseball pattern as shown inFIG. 3.

The size and overlapping area is dependent upon the kind of the optical storage180. For example, the overlapping area tends to be relatively large in case of BD (Blu ray disk) or DVD RW.

The baseball pattern P1, P2, P3formed by reflection at the optical storage180is transformed to a parallel beam by object lens155, and passes through wavelength plate150and diffraction grating140.

FIG. 4shows a plane view of the first embodiment of a diffraction grating according to an embodiment of the present invention, which may be used in the optical pickup100inFIG. 1.

In the optical pickup100according to one embodiment of the present invention, the structure of diffraction grating140ais equivalent to circular patterns P1, P2, P3. Beams passing through sub beam (±1stbeam) region (P4, P6) and the overlapped region P7of main beam region P5and sub beam region P4, P6are diffracted to other direction than to the light detecting means170so that they are excluded from the process of detecting TES signal.

The AC signal is caused by the overlapped region of sub beam region and main beam region. By preventing the AC signal from reaching light detecting means170, it enhances detecting the accurate TES.

Referring toFIG. 4, assuming Y axis lies along the track direction and x axis lies along the tangential direction, in the diffraction grating140a, the first grating pattern A1covers the area except sub beam region and main beam region which overlaps with the sub beam regions and the second grating pattern A2covers the rest of the diffraction grating140a.

The second grating pattern A2prevent the ±1stbeams from reaching the optical storage, so that AC signal is excluded from the process of detecting TES signal.

FIG. 5shows a plane view of the second embodiment of a diffraction grating according to the present invention which may be used in the optical pickup100inFIG. 1.

Referring toFIG. 5, similar to the first embodiment, the second embodiment of diffraction grating140bcomprises the first grating pattern A3and the second grating pattern A4. The first grating pattern A3is in the shape of a rectangle, which does not overlap with circular pattern of sub beam P4, P6. The second grating pattern A4covers the rest of the diffraction grating140a.

Preferably, one side of the first grating pattern A3is in contact of the circular pattern of sub beam as shown in theFIG. 5. The width and length of the rectangle is variable within the scope of the present invention.

FIG. 6shows a plane view of the third embodiment of a diffraction grating according to the present invention which may be used in the optical pickup100inFIG. 1.

Referring toFIG. 6, similar to the first embodiment, the third embodiment of diffraction grating140ccomprises the first grating pattern A5and the second grating pattern A6. The first grating pattern A5comprises two horizontally long rectangles, each of the rectangles being placed in the top and the bottom respectively.

Similarly, the first grating pattern A5does not overlap with the circular patterns of sub beams P4, P6and the second grating pattern A6covers the rest of the diffraction grating140a.

The one side of the first grating pattern A5passes through upper or lower two of four intersection points at which the circular pattern of main beam and those of sub beams. The width and length of the rectangle of the first grating pattern is variable within the scope of the present invention.

FIG. 7shows push-pull signals MPP generated by beam 0thand shows SPP1and SPP2generated by ±1stbeams after being reflected by track structure on the optical storage and passing through the first, second or third diffraction grating140a,140b,140cof present invention. Note that there is substantially no AC signal in the push-pull signals generated by ±1stbeam. By transmitting through the diffraction gratings140a,140b,140cof present invention, DC offset is excluded from the signal detected by the light detecting means170. This will be described with reference toFIG. 9aand the following drawings.

Meanwhile, tracking error level from tracking error signal detected by the light detecting means170can be calculated by the following equation. It is noted the tracking error level is improved by the present invention since the AC signal of an SPP signal is excluded before it reaches the optical storage.
Tracking error level=MPPsignal−k×(the firstSPP1 signal+the secondSPPsignal)  (Equation 3)Wherein “k”=DC level of MPP signal÷(2×DC level of SPP signal)

The first grating patterns A1, A3, A5just pass the main beam B2without phase shift, and shift the phase of the sub beams B1, B3, so that the phase-shifted sub beams are excluded from the process of detecting tracking error signal.

The gratings of the second grating patterns A2, A4, A6have different direction from those of the first grating patterns A1, A3, A5, for example, by 90° so that they can divert a beam towards a certain position other than light detecting means170.

FIG. 8shows positions where main beam and sub beams reach over the optical storage according to the present invention.

The main beam B2is on the border line between the unrecorded area U and recorded area R and the sub beams are shifted by about ½ TP from the border line, which corresponds to the track lines on the optical storage.

FIG. 9ashows change of offset voltage dependent on time when there is no radial shift of the object lens andFIG. 9bshows change of offset voltage dependent on time when there is radial shift of the object lens, in both cases the object lenses being placed on-axis. Before time=0 along time axis, offset voltage in the unrecorded area is represented, and after time=0 along time axis, offset voltage in the unrecorded area is represented.

The difference between the unrecorded area U and recorded area R appears as a difference of reflection ratio. When applying DDP method using three beams, comparingFIGS. 9aand9b, there occurs a voltage level difference between the unrecorded area and the recorded area by the radial shift of the object lens, i.e. by moving the object lens over the optical storage.

There is no change of offset voltage level between before and after t=0 when there is no radial shift of the object lens as shown inFIG. 9a, while there occurs a difference of offset voltage level between before and after t=0 when there is radial shift of the object lens as shown inFIG. 9b.

FIG. 10shows change of offset voltage dependent on time when sub beam error occurred in which the object lenses are placed on-axis.

InFIG. 10, sub beam error occurred, so that sub beam was de-tracked towards 0thbeam by 10 TP. As such, offset voltage level change was increased near t=0.

FIG. 11shows change of offset voltage dependent on time when sub beam error occurred and there is radial shift of object lens in which the object lenses are placed on-axis. Note that offset voltage level change was even more increased near t=0.

FIGS. 12aand12bshow change of offset voltage dependent on time when there is radial shift of object lens by 1 au (arbitrary unit) and 5 au respectively in which the object lenses are placed off-axis.

ComparingFIGS. 12aand12b, it is noted that the more radial shift there is, the more serious the offset voltage level change gets.

In on-axis configuration in which two objects are along the radial direction, ±1stbeams are shifted from the 0thbeam by ½ TP and the shift amount is fixed with the object lens moving, so that there occurs relatively small amount of offset change. In contrast, in off-axis configuration in which two objects are along the track direction, ±1stbeams are shifted from the 0thbeam by ½ TP and the shift amount varies depending on the distance from the center of optical storage, so that there can occur very large amount of offset level change.

FIG. 13ashows change of offset voltage dependent on time when there is radial shift of object lens by 5 au and sub beam error occurred in which the object lenses are placed on-axis andFIG. 13bshows change of offset voltage dependent on time when there is radial shift of the object lens by 5 au and sub beam error occurred in which the object lenses are placed off-axis.

ComparingFIG. 13aandFIG. 13b, it is noted that there occurs more serious offset voltage level change near t=0 inFIG. 13bthanFIG. 13a.

FIG. 15shows change of offset voltage dependent on time detected by using the optical pickup according to the present invention, given the same conditions as those ofFIG. 13bmeasured in an optical pickup according the present invention.

ComparingFIG. 14andFIG. 13aor13b, it is noted that the amplitude of AC signal is slightly changed at t=0 but there is substantially no offset voltage level change near t=0.

FIG. 15shows a schematic diagram of an optical pickup200according to the second embodiment of the present invention.

Referring toFIG. 15, the optical pickup200according to the second embodiment of the present invention comprises light source210, collimate lens220, beam splitter230, wavelength plate250, object lens255, diffraction grating240, condensing lens260and light detecting means270, and an optical storage280, which may be placed before the object lens255.

The structure of the second embodiment of the optical pickup ofFIG. 15is similar to the first embodiment ofFIG. 1, but is different in that the diffraction grating240is positioned between beam splitter230and condensing lens260.

The optical pickup according to the first embodiment of the present invention ofFIG. 1can employ all of three embodiment of diffraction gratings140a,140b,140c, while the optical pickup according to the second embodiment of the present invention ofFIG. 15cannot employ second embodiment of diffraction grating140b.

Referring toFIG. 5, when there is radial shift of the object lens, i.e. object lens moves along the radial direction, which corresponds to the horizontal direction of the paper inFIG. 5, the first grating pattern region A3of the second embodiment of diffraction grating140bcan intrude on the second grating pattern region A4. In this case, there may be a significant error in detecting tracking error signal, which makes the second embodiment of the diffraction grating unavailable.

Detailed description regarding other components inFIG. 15will be the same as that ofFIG. 1and is omitted.

The optical pickup according to the present invention provides the following advantages:

The offset voltage level change can be prevented when off-axis configuration is employed and the object lens moves over the border of unrecorded area and recorded area.

It is possible to alleviate the offset voltage level change without being affected by radial shift, sub beam error etc. and without DPP level and light efficiency being degraded when performing tracking servo over HD DVD, DVD-R, DVD-RW, BD which have different track structures.