WO1999031658A1

WO1999031658A1 - Integrated optical pickup system for use with optical disks of different thicknesses

Info

Publication number: WO1999031658A1
Application number: PCT/KR1998/000420
Authority: WO
Inventors: Yang Oh Choi
Original assignee: Daewoo Electronics Co., Ltd.
Priority date: 1997-12-16
Filing date: 1998-12-11
Publication date: 1999-06-24

Abstract

An optical pickup system operates in one of electrically switchable first and second modes in accordance with an optical disk loaded on a disk tray. The optical pickup system comprises a substrate having a top surface and an inclined side surface, a waveguide, formed on the top surface of the substrate and provided with a light source, an optical detector and a coupling lens, and an optical device attached to the inclined side surface and provided with a first and a second regions, the first region reflecting a first linearly polarized component of the light beam from the light source to the coupling lens and the second region operating in one of electrically switchable first and second modes in accordance with the type of the optical disk. In the system, the light incident to the second region is transmitted to outside of the optical pickup system in the first mode and is reflected to the coupling lens by the second region in the second mode. The reflected light is focused on the optical disk by the coupling lens and reflected back thereto to travel in a reverse direction to the optical detector.

Description

INTEGRATED OPTICAL PICKUP SYSTEM FOR USE WITH OPTICAL DISKS OF DIFFERENT THICKNESSE

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an integrated optical pickup system; and, more particularly, to an optical pickup system incorporating therein an optical device including a liquid crystal for reading a thin and a thick optical disks.

BACKGROUND ART

As is well known, a short wavelength light source and a large numerical aperture (NA) are essential in optical pickup heads for realizing the reproduction of data from a high density optical storage medium. Therefore, a large NA, e.g., 0.6, lens is preferably employed in an optical head for use with a high density DVD(digital video disk) having a thickness of, e.g., 0.6mm. However, if such an optical head for reading the thin optical disk is used to read a thick conventional 1.2 mm CD (compact disk), the spherical aberration caused by the difference in the thickness of the optical disk must be corrected.

One of the optical heads introduced to solve the problem is a dual focus optical head with a holographic optical element (HOE) shown in Fig. 1.

Referring Fig. 1, there is illustrated a conventional dual focus optical head 100 capable of reproducing information signals stored in optical disks of different thicknesses, as further described in Kanda and Hayashi, "Dual Focus Optical Head for 0.6mm and 1.2mm Disks", SPIE Vol. 2338 Optical Data Storage (1994) /283. The dual focus optical head 100 comprises: a light source 126 for generating a light beam, a beam splitter 106, a collimate lens 108, a HOE 110, an objective lens 112, a cylindrical lens 104 and a detector 102 provided with four photoelectric cells. A light beam from the collimate lens 108 is split into a 0th order and a 1st order diffracted light beam by the HOE 110, which are then focused by the objective lens 112, wherein the focal length of the 1st order diffracted light beam 128 is greater than that of the 0th order diffracted light beam 124.

In the optical head 100, the 0th order diffracted light beam 124 is utilized for reproducing the information signal off a recording surface 118 of a thin optical disk 116. The light beam emitted from the light source 126, e.g., a laser diode, enters the HOE 110 via the beam splitter 106 and the collimate lens 108, wherein the beam splitter 106 partially reflects the light beam by a surface incorporated therein and the collimate lens 108 makes the light beam from the beam splitter 106 parallel. The 0th order diffracted light beam 124 is focused onto the recording surface 118 of the thin optical disk 116 by the objective lens 112. The HOE 110 simply plays the role of a parallel plate for the 0th order diffracted light beam 124 of the parallel light beam. When the 0th order diffracted light beam 124 is reflected from the thin optical disk 116 to the HOE 110 via the objective lens 112, the HOE 110 also plays the role of a parallel plate. The reflected 0th order diffracted light beam 124, after passing through the collimate lens 108 and the beam splitter 106, becomes astigmatic after passing through the cylindrical lens 104, thereby allowing the detector 102 to read the information signal off the recording surface 118 of the thin optical disk 116.

Meanwhile, in order to reproduce the information signal off a recording surface 120 of a thick optical disk 122, the 1st order diffracted light beam 128 transmitted through the HOE 110 is used. In this case, the HOE 110, in combination with the objective lens 112, functions as a lens for focusing the 1st order diffracted light beam 128 onto the recording surface 120 of the thick optical disk 122. Therefore, the optical head 100 for use with the thin optical disk 116 is also capable of reproducing the information signal off the recording surface 120 of the thick optical disk 122.

One of the major shortcomings associated with the above-described optical head 100 is that the optical head

100 requires numerous bulky discrete components, rendering the assembly and alignment thereof rather complex.

DISCLOSURE OF THE INVENTION

It is, therefore, a primary object of the present invention to provide an optical pickup system of a reduced size with a simpler assembly and alignment thereof .

In accordance with the present invention, there is provided an optical pickup system for reading an information signal off an optical disk, the system comprising: a substrate having a top surface and an inclined side surface; a waveguide, formed on the top surface of the substrate and provided with a light source, an optical detector and a coupling lens; and an optical device attached to the inclined side surface and provided with a first and a second regions, the first region reflecting a first light from the light source to the coupling lens and the second region operating in one of electrically switchable first and second modes in accordance with the type of the optical disk, wherein the first light incident to the second region is transmitted to outside of the optical pickup system in the first mode and is reflected to the coupling lens by the second region in the second mode, the reflected light being focused on the optical disk by the coupling lens and reflected back thereto to travel in the reverse direction to the optical detector.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects and advantages will become apparent from the following description of preferred embodiments, when given in conjunction with the accompanying drawings, wherein: Fig. 1 represents a schematic diagram of the prior art optical pickup system;

Fig. 2 shows a schematic diagram of the inventive optical pickup system;

Fig. 3 displays a explanatory diagram representing an optical path of a light beam in accordance with the inventive integrated optical pickup system shown in Fig. 2;

Fig. 4A explains the operation of the optical device shown in Fig. 2 when a voltage Vcc is applied thereto;

Figs. 4B and 4C provide explanatory diagrams of a second optical element and an upper transparent electrode in the optical device shown in Fig. 4A, respectively; and

Fig. 4D illustrates the operation of the optical device shown in Fig. 2 when the Vcc is not applied thereto.

MODES OF CARRYING OUT THE INVENTION

Referring to Figs. 2 to 4, there are illustrated optical pickup systems in accordance with preferred embodiments of the present invention. It should be noted that like parts appearing in Figs. 2 to 4 are represented by like reference numerals.

In Fig. 2, there is a schematic diagram of an optical pickup system 200 operating in either one of a first and a second mode, which are electrically switchable therebetween in accordance with the type of an optical disk. The optical pickup system 200 comprises a glass substrate 202 having a top surface and an inclined side surfaces; an optical device 230 provided with a central and a peripheral regions 230-1, 230-2 and attached to the inclined side surface of the glass substrate 202; a waveguide layer 204 provided with an end, a top and a bottom surfaces, a twin focusing beam splitter 220, a reflection collimating lens 250 and a transmission off-axis diffractive objective lens 260 integrated on the top surface of the waveguide layer 204; a light source 210, e.g., a semiconductor laser, for generating a light beam, which can be of a first linearly polarized, e.g., P-polarized, light beam with a wavelength λ and a first and a second optical detectors being attached to the end surface of the waveguide layer 204. It is preferable that an angle between the top and the inclined side surfaces is less than 90 degrees. In the system 200, the light beam emitted from the light source 210 impinges onto the end surface of the waveguide layer 204 which is disposed adjacent to the glass substrate 202 so that it propagates through the waveguide layer 204 as a guided light beam. The guided light beam, after passing through the twin focusing beam splitter 220, is substantially collimated by the reflection collimating lens 250, which then reflects the collimated light beam to the optical device 230 through the glass substrate 202. When a thick optical disk 280 having, e.g., a thickness of 1.2mm, is loaded on a disk tray (not shown) , the optical pickup system 200 operates in the first mode, wherein a predetermined voltage Vcc is applied to the optical device 230. In the first mode, the central region 230-1 of the optical device 230 is reflective and serves as an aperture, whereas the peripheral region 230- 2 thereof is transmissive to the collimated light beam impinged thereonto, as will be described in detail hereinafter.

When the collimated light beam falls onto the optical device 230, a portion of the collimated light beam impinging onto the central region 230-1 of the optical device 230 is reflected to the transmission off- axis diffractive objective lens 260. In the preferred embodiment, the transmission off -axis diffractive objective lens 260 has a number of gratings as shown in Fig. 3. Each of the gratings is of an elliptic shape to thereby make the collimated light beam incident onto the transmission off-axis diffractive objective lens 260 at an oblique angle converge onto a focal point on the optical disk 280 along an optical axis formed by a central point of the transmission off-axis diffractive objective lens 260 and a convergence point thereof. This type of the transmission off-axis diffractive objective lens 260 is well known and so the detailed description thereof is omitted. The portion of the collimated light beam is focused on the loaded optical disk 280 by the transmission off-axis diffractive objective lens 260 and reflected back thereto to travel to the twin focusing beam splitter 220 along the same optical path in a reverse direction. The reflected light beam entering the twin focusing beam splitter 220, after passing therethrough, splits into a first and a second divided light beams, respectively. The twin focusing beam splitter 220 includes a first and a second parts 222, 224, each part being made to have a separate focal point, as shown in Fig. 3. The first and the second optical detectors 290, 292 are placed at the focal points of the first and the second parts 222, 224 of the twin focusing beam splitter 220, respectively. The first and the second divided light beams impinge onto the first and the second optical detectors 290, 292 so as to be converted into electric signals by means of the first and the second optical detectors 290, 292. The peripheral region 230-2 of the optical device 230 transmits the remaining portion of the collimated light beam falling thereon to outside of the optical pickup system 200, whereby the remaining portion which is represented by solid lines m Fig. 2 is not used for reading information signals off the loaded optical disk 280.

Referring to Fig. 4A, there is shown a diagram illustrating the operation of the optical device 230 shown in Fig. 2 when the Vcc is supplied to a lower and an upper transparent electrodes 233, 235 in the optical device 230 by closing a switch 238. The optical device 230 includes a first optical element 232, the electrodes 233, 235 made of, e.g., ITO (indium tin oxide) or the like, a liquid crystal 234, and a second optical element 236 provided with a first and a second portions 236A, 236B. In the preferred embodiment, the lower transparent electrode 233 is formed on top of the first optical element 232, while the liquid crystal 234, the upper transparent electrode 235 and the second portion 236B of the second optical element 236 are mounted on a peripheral region of the lower transparent electrode 233, sequentially, so that the peripheral region 230-2 corresponds to a region of the optical device 230 under the second portion 236B of the optical element 236. In Figs. 4B and 4C, there are shown explanatory diagrams of the second optical element 236 and the upper transparent electrode 235 in the optical device 230 shown in Fig. 4A, respectively. The second optical element 236 is divided into the first and the second portions 236A, 236B by a division circle 236C. The second portion 236B is in the form of an annular disk. The first portion 236A of the second optical element 236 transmits a polarization component of a light beam, e.g., an S- polarized light beam, and reflects another, e.g., a P- polarized light beam, which is linearly polarized and perpendicular to the S-polarized light beam, whereas the second portion 236B transmits a P-polarized light beam, and reflects an S-polarized light beam. The upper transparent electrode 235 is in the form of an annular disk whose inner circle has a radius same as that of the second portion 236B, as shown in Fig. 4C. Referring back to Fig. 4A, the portion of the collimated light beam incident to the first portion 236A of the second optical element 236 is reflected to the transmission off-axis diffractive objective lens 260, whereas the remaining portion of the collimated light beam incident to the second portion 236B of the second optical element 236 enters the liquid crystal 234 which is made of a doubly refracting crystal such as a nematic liquid crystal or the like.

In the first mode, the remaining portion of the collimated light beam remains unchanged after passing through the liquid crystal 234, since a fraction of the liquid crystal under the upper transparent electrode 235 serves as a material transparent to a light beam impinged thereto. The remaining portion passing through the liquid crystal 234 is transmitted to the outside of the optical pickup system 200 through the lower transparent electrode

233 and the first optical element 232, whereby the central region 230-1 of the optical device 230 serves as an aperture for shaping a cross section of a light beam impinging onto the transmission off-axis diffractive objective lens 260.

Referring back to Fig. 2, if a thin optical disk 282, e.g., a thickness of 0.6mm, is loaded on the disk tray, the optical pickup system 200 operates in the second mode. In the second mode, no voltage is applied to the optical device 230 so that both of the central and the peripheral region 230-1, 230-2 are reflective, wherein solid lines represent the optical paths of the light beam for use in detecting information signals recorded on the loaded optical disk 282. Therefore, the whole area of the optical device 230 functions as an aperture for shaping a cross section of a light beam impinging onto the transmission off-axis diffractive objective lens 260, as will be described in detail hereinafter .

When the light beam emanating from the light source 210 falls onto the optical device 230, a portion of the collimated light beam impinging onto the central region 230-1 in the second mode behaves similarly to that of the collimated light beam in the first mode. However, the remaining portion of the collimated light beam impinging onto the peripheral region 230-2 behaves differently from that of the collimated light beam in the first mode.

Referring to Fig. 4D, there is shown a diagram of the operation of the optical device 230 shown in Fig. 2 when the Vcc is applied to the lower and the upper transparent electrodes 233, 235 in the optical device 230 by closing a switch 238. In the second mode, the remaining portion of the collimated light beam impinging onto the peripheral region 230-2 is transmitted to the liquid crystal 234 through the second portion 236B of the second optical element 236. In the preferred embodiment, the thickness of the liquid crystal 234 is set to a thickness value which introduces a phase difference of a magnitude of an odd number multiple of λ/2 to the light beam impinged thereonto, after passing therethrough, when the Vcc is not applied to the electrodes 233, 235. Therefore, the remaining portion of the collimated light beam transmitted through the liquid crystal 234 is converted into an S-polarized light beam.

The converted S-polarized light beam is reflected back to the liquid crystal 234 by the first optical element 232. The converted S-polarized light beam from the second optical element 232 is converted into a P- polarized light beam after passing through the liquid crystal 234. Thereafter, the converted P-polarized light beam is transmitted to the transmission off-axis diffractive objective lens 260 through the upper transparent electrode 235 and the second portion 236B of the second optical element 236 so that the peripheral region 230-2 is reflective. Therefore, the whole area of the optical device 230 functions as the aperture in the second mode . While the present invention has been described with respect to the preferred embodiments, other modifications and variations may be made without departing from the spirit and scope of the present invention as set forth in the following claims.

Claims

what is claimed is:

1. An optical pickup system for reading an information signal off an optical disk, the system comprising- a substrate having a top surface and an inclined side surface; a waveguide, formed on the top surface of the substrate and provided with a light source, an optical detector and a coupling lens; and an optical device attached onto the inclined side surface and provided with a first and a second regions, the first region reflecting a light of a first linearly polarized component from the light source to the coupling lens through the waveguide and a substrate and the second region operating m one of electrically switchable first and second modes m accordance with the type of the optical disk, wherein the light incident to the second region is transmitted to outside of the optical pickup system in the first mode and is reflected to the coupling lens by the second region m the second mode, the reflected light being focused on the optical disk by the coupling lens and reflected back thereto to travel m a reverse direction to the optical detector.

2. The optical pickup system of claim 1, wherein a top surface of the first region is coated with a material which reflects a first linearly polarized component of a light beam impinging thereonto and transmits a second linearly polarized component of the light beam impinging thereonto.

3. The optical pickup system of claim 2, wherein the second region of the optical device includes : a top portion for transmitting the first linearly polarized component of the light beam impinged thereonto; a bottom portion for reflecting the second linearly polarized component; and a liquid crystal device, disposed between the top and the bottom portions, for transmitting the first linearly polarized component of the light beam from the top portion in the first mode and changing the first linearly polarized component of the light beam into the second linearly polarized component of the light beam in the second mode, wherein the second linearly polarized component of the light beam is reflected back to the liquid crystal device by the bottom portion and converted into the first linearly polarized component, thereby the converted light of the first linearly polarized component being transmitted to the coupling lens through the top portion.

4. The optical pickup system of claim 3, wherein the liquid crystal includes: an upper and a lower transparent electrodes; and a liquid crystal, disposed between the electrodes, for serving as a ╬╗/2 plate when an electric signal is not applied to the electrodes and for serving as a plate transparent to the light beam incident thereto when the electric signal is applied to the electrodes, thereby allowing the optical device to operate in one of the switchable first and second modes.

5. The optical pickup system of claim 4, wherein a top portion of the second region is coated with a material which reflects the second linearly polarized component of the light beam impinging thereonto and transmits the first linearly polarized component of the light beam impinging thereonto.

6. The optical pickup system of claim 5, wherein a bottom portion of the second region is coated with a material which reflects the second linearly polarized component of the light beam impinging thereonto.

7. The optical pickup system of claim 4, wherein one of the electrodes is in the form of an annular disk.

8. The optical pickup system of claim 7, wherein the top portion is of the form of an annular disk whose inner circle has a radius same as that of the liquid crystal.

9. The optical pickup system of claim 6, wherein, if one of the optical disks which is thinner than the other optical disk is loaded on a disk tray, the optical device operates m the second mode.

10. The optical pickup system of claim 9, wherein, if the other optical disk is loaded on the disk tray, the optical device operates m the first mode.

11. The optical pickup system of claim 1, further comprising means for collimating the first linearly polarized component of the light beam from the light source and reflecting the collimated light to the optical device through the substrate.

12. An optical pickup system for reading an information signal off an optical disk, the system comprising: a light source for generating a light beam; a detection unit; an objective lens; and an optical device provided with an optical element having a first and a second parts and an optical member for transmitting the light beam, the first part reflecting a portion of the light beam from the light source to the objective lens and the second part operating in one of electrically switchable first and second modes in accordance with the type of the optical disk, wherein a remaining portion of the light beam incident to the second part is transmitted to outside of the optical pickup system m the first mode and is reflected to the objective lens in the second mode, the reflected light being focused on the optical disk by the objective lens and then reflected back thereto to travel in a reverse direction to the detection unit.

13. The optical pickup system of claim 12, wherein the detection unit includes: a first and a second optical detectors; and a twin focusing beam splitter for splitting the light beam reflected back to the objective lens into a first and a second divided light beams and focusing each of the divided light beams on a separate focal point.

14. The optical pickup system of claim 13, wherein each of the optical detectors is placed at the separate focal point .

15. The optical pickup system of claim 14, wherein the twin focusing beam splitter is disposed between the light source and the objective lens.

16. The optical pickup system of claim 15, further comprising means for collimating the light beam from the light source through the waveguide and reflecting the collimated light beam to the optical device through the substrate.

17. The optical pickup system of claim 16, wherein collimating means is located between the twin focusing beam splitter and the objective lens.

18. The optical pickup system of claim 17, wherein the optical device is in a facing relationship with the objective lens and the collimating lens.