WO2006043208A1 - Large sized detector for optimized read-out from an optical data carrier. - Google Patents

Large sized detector for optimized read-out from an optical data carrier. Download PDF

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Publication number
WO2006043208A1
WO2006043208A1 PCT/IB2005/053348 IB2005053348W WO2006043208A1 WO 2006043208 A1 WO2006043208 A1 WO 2006043208A1 IB 2005053348 W IB2005053348 W IB 2005053348W WO 2006043208 A1 WO2006043208 A1 WO 2006043208A1
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Prior art keywords
detector
data carrier
distance
read
optical data
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PCT/IB2005/053348
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French (fr)
Inventor
Marcello Balistreri
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Koninklijke Philips Electronics N.V.
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Priority to EP04300691.5 priority
Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2006043208A1 publication Critical patent/WO2006043208A1/en

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    • GPHYSICS
    • G11INFORMATION STORAGE
    • G11BINFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B7/00Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
    • G11B7/12Heads, e.g. forming of the optical beam spot or modulation of the optical beam
    • G11B7/13Optical detectors therefor

Abstract

The invention relates to a read-out device (10) for reading out data from an optical data carrier (22, 70) by detecting radiation (34, 35) emitted by the optical data carrier (22, 70) in response to an exciting beam (32). In order to provide a read-out device (10) which allows a better collection efficiency and thus a better read-out of the optical data carrier, it is proposed to provide a detector (21, 51, 61) which is, at least during operation, arranged at a predetermined distance (40, 41, 55, 56) in an axial direction to a surface (36, 37, 71, 72) of the optical data carrier (22, 70), wherein the detector (21, 51, 61) has a detecting area with a diameter in the range of 8 to 20 times this distance (40, 41, 55, 56).

Description

LARGE SIZED DETECTOR FOR OPTIMIZED READ-OUT FROM AN OPTICAL DATA CARRIER

FIELD OF THE INVENTION The invention relates to a read-out device for reading out data from an optical data carrier by detecting radiation emitted by said optical data carrier in response to an exciting beam.

BACKGROUND OF THE INVENTION An optical data carrier is read out by applying an irradiation beam. This irradiation beam is either reflected or causes the optical data carrier , e.g. an optical disc such as CD, DVD or BD, to emit radiation in response to the exciting beam, for example due to fluorescence. The radiation exiting the optical data carrier is to be detected by the read-out device. A read-out device for fluorescent multi-layer storage is disclosed, for example, in WO 2004/023459 A2. The read-out device comprises an objective lens for projecting an exciting beam in a layer of the data carrier and for collecting radiation emitted in response and a detector unit for detecting the radiation collected by the objective lens.

For reflective storage systems the efficiency of collection or detection by the read-out device by means of an objective lens is always 100% because the light path for the incoming and outgoing light is reversible.

In the case of fluorescent storage, the reversibility of the light path for incoming and outgoing is not true anymore, as in fact not the same photons are re-emitted from the disc. The emitted light is incoherent compared to the coherent, reflected light in the case of reflective storage. While this in fact has several advantages, a disadvantage is the emission of the light under a larger solid angle than that defined by the incoming beam. Therefore, a significant amount of signal intensity is lost. By simple geometrical considerations it can be shown that the collection efficiency for isotropic emission is

Figure imgf000002_0001
with n as the refractive index of the substrate. For a numerical aperture (NA) of the lens of 0.6 and a refractive index of 1.62 of the polycarbonate (PC) substrate at 405 nm, this results in only 3.6% of the collected light. The refractive index of PC limits the maximal achievable collection efficiency of 10,7% using a lens with a NA of 1, which is three times larger. However, for the "standard" case of isotropic fluorescent emission, with a NA of 0.6 and a refractive index of 1.62 of the PC substrate at 405 nm, 21.4% of the emitted photons leave the disk on both sides and which results for a collection efficiency of 3.6% in a loss of 17.8%. At the top side of the disc 7.1% of the emitted photons are lost, because the limited NA of the objective lens can not collect them. At the bottom side of the disc 10.7% of the emitted photons leaving the disc are lost. This means that for this "standard" case still a maximal signal strength enhancement with a factor 6 is possible by collecting these lost emitted photons.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a read-out device which allows a better collection efficiency and thus a better read-out of the optical data carrier.

In order to achieve this object a read-out device as described in the opening paragraph is proposed, which comprises a detector which is, at least during operation, arranged in a predetermined distance in an axial direction to a surface of said optical data carrier, wherein said detector has a detecting area with a diameter in the range of 8 to 20 times said distance.

It is thus proposed to use a large sized detector, positioned close to the air/disc interface of the optical data carrier, in order to collect the emitted photons, which leave the disc and which are normally lost. A large part of the emitted photons are collected by this detector which can not be collected due the limited NA of the objective lens. This will increase the collection efficiency significantly and thus the signal strength. The amount to collected photons is determined by the outer diameter of the detector and the distance of the detector from the data carrier. For a given distance of the detector from the data carrier an outer diameter for collecting a predetermined ratio of the photons exiting the data carrier at its upper or lower surface can be calculated depending on the angle of beam spread of the emitted radiation corresponding to said ratio.

The invention is based on the insight that it is not necessary to use a system comprising of a detector and a lens to detect the radiation emitted in response to the exciting beam although the lens may still be necessary to focus the exciting beam itself. The emitted radiation can be detected directly by the detector without an additional focusing by any lens. A sufficient efficiency can be achieved by giving the detector a sufficient size depending on the distance between the detector and the surface of the optical data carrier. In a preferred embodiment of a read-out said detector comprises at least two detector elements being arranged on opposite sides of said optical data carrier. Since photons are emitted directed to both surfaces of the optical data carrier, with a detector only on one side a large part of the emitted photons would leave undetected. By providing detector elements on both sides of the optical data carrier the efficiency of collecting the emitted photons is almost doubled. It has to be noted that the term "distance between the detector and the data carrier" refers to the distance between the detector itself - in the case of only one detector element comprised by the detector - as well as to the distances of said at least two detector elements to the respective surface of the data carrier which is nearest to them. It is possible that the distance from a first detector element to the data carrier differs from the distance between a second detector element and the data carrier.

In another embodiment of a read-out device the distance in axial direction between said optical data carrier and said detector is, at least during operation, larger than 20 nm, preferably in a range from 0.5 mm to 2.0 mm. In a further embodiment the read-out device comprises a distance actuator for adjusting a distance in an axial direction between said detector and said optical data carrier. While inserting the data carrier into the read-out device it is preferred that the distance between the data carrier or the space where the data carrier is to be inserted and the detector is increased to avoid damages. During operation the distance actuator allows to change the distance between said detector and said data carrier so that the detector can be put as near as possible to the data carrier without risking a collision between the detector and the data carrier. There is an axial displacement of the data carrier during rotation of maximal 1 - 2 mm, typically 100 - 500 μm for a 12 cm disc which should be taken into consideration when adjusting the distance. In another preferred embodiment the read-out device further comprises a distance sensor for detecting said distance between said detector and said optical data carrier and a control means for controlling, at least during operation, said distance actuator according to the detected distance for adjusting said distance to a predetermined value, in particular a value above 20 nm, preferably in a range from 0.5 to 2.0 mm. Said distance sensor, said control means and said distance actuator work together as a system for adjusting said distance by measuring the distance and using this measurement to control the distance in order to keep the distance at a predetermined value or at least in a predetermined range. By controlling said distance it is possible to bring the detector nearer to the data carrier while reducing the risk of a collision which could damage either the data carrier or the read-out device. When the detector is very close to the data carrier it will detect almost all of the radiation exiting the data carrier at its respective surfaces but the risk of a collision is high. That risk becomes low when the detector is arranged at a greater distance which leads to a lower detecting efficiency since a large part of the radiation will not impinge on the detector. The range given above gives a compromise with a sufficient safety from collisions and a sufficient detecting efficiency.

In yet another embodiment of the read-out device said detector has a substantially circular shape and a diameter larger than 240 nm, in particular in the range of 10 to 15 times the distance between said detector and said surface of said optical data carrier during operation, preferably 12 times said distance. The radiation emitted in response to the exciting beam has a substantially circular cross section. A detector with a corresponding cross section would therefore efficiently collect the emitted photons without unused detecting area which would increase the costs of the read-out device unnecessarily. In a further embodiment the read-out device further comprises a focusing means for focusing said exciting beam on said optical data carrier, wherein said detector is attached to said focusing means. Since a focusing means is necessary for focusing the exciting beam by adjusting the distance between the data carrier and a lens or lens system said focusing means can be used as said distance actuator. Thus, there is no need for an additional distance actuator which would increase the complexity of the read-out device and the manufacturing costs.

In another embodiment of the read-out device said detector is attached coaxial to said focusing means and comprises a central aperture for allowing said exciting beam to pass through said detector. By arranging the detector in such way that its center is located on the centerline of the exciting beam it is also centered with respect to the emitted radiation.

These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein:

Fig. 1 shows a known read-out device; - Fig. 2 schematically shows a sectional view of an embodiment of a read-out device according to the present invention; - Fig. 3 shows a part of the read-out device shown in Fig. 2 in greater detail;

- Fig. 4 shows a plan view of a part of the read-out device shown in Figs. 2 and 3; and

- Figs. 5 - 7 show further embodiments of a detector of a read-out device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows a read-out device 1 for multi-layer optical storage for reading out a multi-layer optical data carrier 2. Such a read-out device 1 comprises a exciting source 3, a dichroic mirror 4, an objective lens 6, an imaging lens 8 and a detector 9. From the exciting source 3 an exciting beam 5 is generated and redirected by said mirror 4. The beam 5 is focused by lens 6 on a layer of the data carrier 2. An excited radiation 7 is generated in response to said exciting beam 5, passes through said objective lens 6 and said imaging lens 8 and is detected by said detector 9.

Fig. 2 schematically shows a sectional view of an embodiment of a read-out device according to the present invention. The read-out device 10 comprises a detector 21 arranged

- at least during operation - adjacent to an optical data carrier 22. Said detector 21 comprises two detector elements 23, 24 which are arranged on opposite sides of said data carrier 22 in predetermined distances 40, 41 form said data carrier 22. Said read-out device 10 further comprises distance actuators 25, 26 and distance sensors 27, 28, which are connected to a control means 29. A focusing means 30 is arranged in the center of said detector element 23, and an exciting source 31, e.g. a radiation source such as a laser diode, is provided on the side of said focusing means 30 opposite to said data carrier 22. Said detector elements 23, 24, said focusing means 30 and a part of said data carrier 22 are shown in greater detail in Fig. 3.

An exciting beam 32 is generated by said exciting source 31 and focused by said focusing means 30 to a focus spot 33. At said focus spot 33 a radiation is emitted in response to said exciting beam 32. The largest part of said radiation is emitted in lateral direction and can not be detected. An upper part 34 and a lower part 35 of said radiation is emitted in vertical direction and exits said data carrier 22 at its upper and lower surface 36, 37. Said parts of radiation 34, 35 are refracted at said surfaces 36, 37. Said refraction determines what amount of radiation can exit the data carrier at said surfaces 36, 37. Said refraction depends on the refractive index n. of the material of the data carrier 22 and of the surrounding material which is normally air with a refractive index of 1. The Brewster angle of commonly used polycarbonate is 38° (n = 1.62). Thus, only 21.4 % of the photons emitted in response to said exciting beam 32 leave the data carrier 22 through said surfaces 36, 37. The two detector elements 23, 24, e.g. active area Si PIN (positive intrinsic negative) photodiodes, PDICs (photo detector integrated circuit), APDs (avalanche photo diode) or PMTs (photon-multiplier-tube), with a diameter of, for example, 6.22 mm, detect the radiation 34, 35 exiting the data carrier 22. If said detector elements 23, 24 are arranged at a distance 40, 41 of 0.5 mm from the data carrier 22 approximately 96% of the emitted radiation 34, 35 is detected, giving an enhancement compared to known read-out devices with a factor of 5.8. Such an efficiency enhancement is obtained for a diameter of the detector elements 23, 24 being 12 times the distance 40, 41 of said detector elements 23, 24 from said data carrier 22. In the case of a near field actuator the smallest distance between the detector elements 23, 24 and the data carrier 22 would be approximately 20 nm. In the case of near field optical recording the practical distance is 40 - 200 nm. The maximum distance would be approximately 5 mm in order to fit the read-out device 10 inside a common 5.25" drive format.

The data carrier 22 is rotated during operation around an axis 38. This rotation leads to an axial displacement in case of a 12 cm disc of maximal 1 - 2 mm, typically 100 - 500 μm. To keep the detector elements 23, 24 at a substantially constant distance 40, 41 of, for example, 0.5 mm said distance actuators 25, 26 are controlled by said control means 29 based on the distance between said data carrier 22 and said detector elements 23, 24 detected by said distance detectors 27, 28, respectively. Said distance actuators may also be used to change the lateral position of the detector 21. There is commonly an actuator provided for said focusing means 30. Said detector element 23 may be attached to said focusing means 30, so said focusing actuator replaces said distance actuator 25 or vice versa. As shown in Fig. 4, said detector element 23 and said focusing means 30 are arranged in such a manner that said focusing means 30 is positioned in an central aperture of said detector element 23. The radiation emitted in response to said exciting beam 32 is generated in focus spot 33 which is located along the center line of said beam 32. If the emitted photons propagate symmetrically the best shape of the detector elements 23, 24 is substantially circular and said detector elements 23, 24 should be coaxially aligned with said beam 32.

In general, the minimum distance between said detector elements 23, 24 and said data carrier 22 is determined by the typical disc displacement during rotation. The maximum applicable size of the detector elements 23, 24 is restricted to approximately 25 mm in order to fit the read-out device 10 inside a common 5.25" drive format.

The free working distance between lenses 6 of known read-out devices is around 0.6 mm. If thin and light weight detector elements 23, 24 are used, for example of a thickness of approximately 0.01 mm, this distance may be kept virtually unchanged. If the detector elements 23, 24 are thicker, for example 0.01 to 0.4 mm, or the distance is reduced for other reasons the rate of detected radiations increases at the expense of robustness of the system.

Figs. 5 - 7 show further embodiments of a detector 51, 61, 71 of a read-out device according to the present invention in a view similar to that of Fig. 3.

Detector 51 shown in Fig. 5 comprises two detector elements 52, 53 arranged on opposite sides of a data carrier 22 made of polycarbonate with a refractive index n of 1.62. The first detector element 52 is attached to a focusing means 54, in particular an achromatic lens 54, for example by gluing. As described above, an exciting beam 32 is focused by said lens 54 and applied to said data carrier 22, wherein radiation 34, 35 is generated in response to said exciting beam 32. Said lens 54 is adapted for an exciting beam 32 having a wavelength of 405 nm and for a response radiation having a wavelength in the range of 460 to 540 nm and has a numerical aperture of 0.6. Said detector elements have a substantially circular shape with a diameter of 6.22 mm and a thickness of 0.25 mm and are arranged in a distance 55, 56 of 0.5 mm. Said first detector element 52 comprises a central aperture with a diameter of 1.23 mm in order to let said exciting beam 32 pass through said lens 54 and said detector element 52.

The data carrier 22 is shown with a thickness of 1.2 mm and the focus spot 33 is shown having an equal distance to both surfaces 36, 37. The focus spot 33 may be at any other position depending on the type of data carrier used, for example a multi- layer optical data carrier. Due to refraction only a part of the total radiation generated in response to said exciting beam 32 can exit the data carrier 22 at its upper or lower surface 36, 37. With the surrounding air having a refractive index n of 1.0 and polycarbonate having a refractive index n of 1.62 - which leads to a Brewster angle of approximately 38° - only photons emitted within said Brewster angle to the axis of the data carrier 22 will pass through said upper or lower surface. So only 21% of the emitted photons may be detected above or below said data carrier 22. With a known read-out device only 17% of said photons are detected or collected while 96% - corresponding to an angle of approximately 79.3° — may be detected by the detector 51 shown in Fig. 5. So a gain factor of 5.8 can be achieved in comparison to the known read-out device.

A detector 61 similar to that shown in Fig. 5 is shown in Fig. 6, wherein the achromatic lens 54 is replaced by a carrier lens 64 with a numerical aperture of 0.6. Further, detector 51 shown in Fig. 5 is also shown in Fig. 7 together with an optical data carrier 70 having an upper surface 71 and a lower surface 72 made of a material with a lower refractive index than the commonly used polycarbonate, in this case with an refractive index n of 1.2. The lower refractive index leads to a higher Brewster angle and thus a larger ratio, i.e. 45% instead of 21% in the case of polycarbonate, of photons exiting the data carrier 70 through its upper or lower surface 71, 72. It has to be noted that there is a number of possible modifications to the read-out device described above. For example, it is not necessary to provide a second distance sensor 28 with a focus and tracking sensor. The second actuator 26 may be controlled on the basis of the distance measured by the first distance sensor 27. It is possible to collect light from the data carrier by a small lens in the center of the detector element 24 to generate a servo signal without the use of a second light source. Further, the actuator 26 can be simplified by removing the servo part and using it only for lateral, radial actuation. A second detector element 24 is proposed with a diameter of 25 mm at a distance of 2 mm from the data carrier 22. Such a distance is large enough to prevent any contact between the detector element 24 and the data carrier 22. The variation of this distance due to rotation of the disc will lead only to a negligible distortion of the data retrieval. The second detector element 24, distance actuator 26 and distance sensor 28 could further be omitted completely, which would reduce the enhancement but, however, significantly simplify the read-out device while maintaining a considerable enhancement. It has to be noted that the distances 40, 41 of said detector elements 23, 24 may differ from each other. If actuator 26 is not provided for changing said distance 41 but only for lateral actuation distance 41 is set to a fixed predetermined value while distance 40 may be changed by means of distance actuator 25. Even if both actuators 25, 26 are provided for axial actuation the distances 40, 41 may be controlled by means of said control unit 29 to have different values.

If a common read-out device is modified by adding the additional components described above an improved read-out device is obtained which is still compatible with known data carrier. Namely, if there is a detector 9 provided on the side of said focusing means 30 opposite to said data carrier 22 data carriers of the reflective system can still be read out by the improved read-out device.

According to the present invention a read-out device is proposed which is improved in detection efficiency compared with known read-out devices for fluorescent optical storage systems. At least one detector element which is large in comparison with the focusing means is provided in the proximity of the data carrier. Almost any radiation exiting the data carrier at is upper and lower surface can be detected which leads to an detection efficiency increased by a factor of 6 or 3, respectively. Any reference sign in the following claims should not be construed as limiting the claim. It will be obvious that the use of the verb "to comprise" and its conjugations does not exclude the presence of any other elements besides those defined in any claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

1. A read-out device (10) for reading out data from an optical data carrier (22, 70) by detecting radiation (34, 35) emitted by said optical data carrier (22, 70) in response to an exciting beam (32), characterized in that it comprises a detector (21, 51, 61) which is, at least during operation, arranged at a predetermined distance (40, 41, 55, 56) in an axial direction to a surface (36, 37, 71, 72) of said optical data carrier (22, 70), wherein said detector (21, 51, 61) has a detecting area with a diameter in the range of 8 to 20 times said distance (40, 41, 55, 56).
2. A read-out device (10) as claimed in claim 1, characterized in that said detector (21, 51, 61) comprises at least two detector elements (23, 24, 52, 53) being arranged on opposite sides of said optical data carrier (22, 70).
3. A read-out device (10) as claimed in claim 1, wherein, at least during operation, the distance (40, 41, 55, 56) in axial direction between said optical data carrier (22, 70) and said detector (21, 51, 61) is larger than 20 nm, preferably in a range from 0.5 mm to 2.0 mm.
4. A read-out device (10) as claimed in claim 1, further comprising a distance actuator (25, 26) for adjusting a distance (40, 41, 55, 56) in an axial direction between said detector (21, 51, 61) and said optical data carrier (22, 70).
5. A read-out device (10) as claimed in claim 4, further comprising a distance sensor (27, 28) for detecting said distance (40, 41, 55, 56) between said detector (21, 51, 61) and said optical data carrier (22, 70) and a control means (29) for controlling, at least during operation, said distance actuator (25, 26) according to the detected distance (40, 41, 55, 56) for adjusting said distance (40, 41, 55, 56).
6. A read-out device (10) as claimed in claim 1, wherein said detector (21, 51, 61) has a substantially circular shape and a diameter larger than 240 nm, in particular in the range of 10 to 15 times the distance (40, 41, 55, 56) between said detector (21, 51, 61) and said surface (36, 37, 71, 72) of said optical data carrier (22, 70) during operation, preferably 12 times said distance (40, 41, 55, 56).
7. A read-out device (10) as claimed in claim 1, further comprising a focusing means (30, 54, 64) for focusing said exciting beam (32) on said optical data carrier (22, 70), wherein said detector (21, 51, 61) is attached to said focusing means (30, 54, 64).
8. A read-out device (10) as claimed in claim 7, wherein said detector (21, 51, 61) is attached coaxial to said focusing means (30, 54, 64) and comprises a central aperture for allowing said exciting beam (32) to pass through said detector (21, 51, 61).
PCT/IB2005/053348 2004-10-19 2005-10-12 Large sized detector for optimized read-out from an optical data carrier. WO2006043208A1 (en)

Priority Applications (2)

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EP04300691.5 2004-10-19

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1999023647A1 (en) * 1997-11-05 1999-05-14 Omd Devices, L.L.C. Focus error correction apparatus
US6115344A (en) * 1995-05-23 2000-09-05 Opticom Asa Device and method for optical data storage having multiple optical states
WO2001006501A2 (en) * 1999-07-15 2001-01-25 Trid Store Ip, L.L.C. Optical data storage system having combined fluorescent three-dimensional information carrier
US6529463B1 (en) * 1998-06-05 2003-03-04 Massachusetts Institute Of Technology Very-high-density memory device utilizing a scintillating data-storage medium
US20040081033A1 (en) * 2001-02-06 2004-04-29 Yoel Arieli Multiple layer optical storage device

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6115344A (en) * 1995-05-23 2000-09-05 Opticom Asa Device and method for optical data storage having multiple optical states
WO1999023647A1 (en) * 1997-11-05 1999-05-14 Omd Devices, L.L.C. Focus error correction apparatus
US6529463B1 (en) * 1998-06-05 2003-03-04 Massachusetts Institute Of Technology Very-high-density memory device utilizing a scintillating data-storage medium
WO2001006501A2 (en) * 1999-07-15 2001-01-25 Trid Store Ip, L.L.C. Optical data storage system having combined fluorescent three-dimensional information carrier
US20040081033A1 (en) * 2001-02-06 2004-04-29 Yoel Arieli Multiple layer optical storage device

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