Optical scanning device with coma correction for improved focus tracking signal

An optical scanning device (1) includes: a radiation source (4) for supplying a radiation beam (8); a lens system (5) for transforming the radiation beam to a scanning spot (14); and a detection system (6) that includes a quadrant detector (20), an astigmatism generating element (18) for generating a first amount of coma (W31a) so as to transform the radiation beam to a first astigmatic radiation beam (21), and a coma correcting element (19) for generating a second amount of coma (W31b) so as to compensate the first amount of coma. The coma correcting element includes a correction surface (19A) having a shape defined by a function “H(r, θ)” that includes the term “A.r3.cos(θ)” wherein: “H” is the position of the correction surface along the optical axis of the lens system, “r” and “θ” designate polar coordinates, and “A” designates a first constant dependent on the second amount of coma.

The invention relates to an optical scanning device for scanning an information layer of an optical record carrier, the device including:a radiation source for supplying a radiation beam,a lens system for transforming said radiation beam to a scanning spot in the position of said information layer, this system having an optical axis, anda detection system including:an astigmatism generating element for transforming said radiation beam to a first astigmatic radiation beam, this element further generating a first amount of coma so that the first astigmatic radiation beam includes coma aberration,a coma correcting element for generating a second amount of coma so as to compensate said first amount of coma, thereby transforming said first astigmatic radiation beam to a second astigmatic radiation beam that is substantially free from coma aberration, anda quadrant detector for transforming said second astigmatic radiation beam to an electrical signal.

“Scanning an information layer” refers to scanning by a radiation beam for: reading information from the information layer (“reading mode”), writing information in the information layer (“writing mode”), and/or erasing information from the information layer.

Generally speaking, in a conventional optical scanning device of the type described in the opening paragraph, a “focus error signal” is used for maintaining the scanning spot in focus in the information layer which is to be scanned. This signal is commonly formed from the well-known “astigmatic method” which is known from, inter alia, G. Bouwhuis, J. Braat, A. Huijser et al, “Principles of Optical Disc Systems,” 75–80 (Adam Hilger 1985) (ISBN 0-85274-785-3). The astigmatic method is based upon the voluntary introduction of an optical aberration, astigmatism, in the optical path of the radiation beam. Typically, a plane parallel plate that is tilted with an angle of 45 degrees with respect to the optical axis is used as the astigmatism generating element: when the radiation beam traverses this element, astigmatism is generated.

A problem encountered with such a conventional optical scanning device resides in that the plane parallel plate generates, apart from astigmatism, an additional aberration, coma. Coma can be expressed in the form of the Seidel coefficient W31. The following equation gives the “root-mean-square” W31rms, normalized with respect to the wavelength λ, of the coefficient W31:

W31⁢rms=-n2⁡(n2-1)·sin⁡(α)·cos⁡(α)2·(n2-sin2⁢α)52·dλ·NA372(1)
wherein “d” designates the thickness of the plane parallel plate, “n” designates the refractive index of the plane parallel plate, “α” designates the angle of the plane parallel plate with the optical axis (preferably 45 degrees), and “NA” designates the numerical aperture of the radiation beam that is incident to the plane parallel plate. For further information, see e.g. M. Born and E. Wolf, “Principles of Optics,” 469–470 (Pergamon Press) (ISBN 0-08-026482-4).

The presence of such a coma aberration is not desired since it affects the focus tracking signal, because the spot on the center of the quadrant-detector is not symmetrical due to the amount of coma generated by the plane parallel plate.

A first solution to this problem consists in reducing the thickness d of the plane parallel plate, since the generated amount of coma (that may be expressed in the form of W31rmsas given in Equation (1)) varies proportionally with the thickness d.

However, the first solution has the following drawbacks. Firstly, the thickness of the plane parallel plate also affects the amount of astigmatism generated by the plate, since the plate generates an amount of astigmatism, expressed in the form of the Seidel coefficient W22. The following equation gives the “root-mean-square” W22rms, normalized with respect to the wavelength λ, of the coefficient W22:

W22⁢rms=(n2-1)·sin2⁡(α)2·(n2-sin2⁢α)32·dλ·NA324(2)
wherein “d,” “n,” “α” and “NA” are the same as those defined in Equation (1). Secondly, even a thin plate generates a substantial amount of coma. For instance, calculations show that a plane parallel plate with a 1.1 mm thickness generates an amount of coma that equals 71 mλrms in the case where NA=0.135, α=45 degrees, n=1.51. Thirdly, the thickness of the plate is preferably larger than 1 mm because of mechanical constraints, especially bending limitations on the carrier.

A second solution consists in reducing the numerical aperture NA of the radiation beam that is incident to the plane parallel plate, since the generated amount of coma (that may be expressed by W31rmsas given in Equation (1)) varies with the numerical aperture NA. However, the second solution has the drawback that it also reduces the laser power associated with the radiation source, which also depends on the numerical aperture NA.

Other solutions to the aforementioned problem exist in the state of the art.

U.S. Pat. No. 4,709,139 describes an optical scanning device of the type described in the opening paragraph, wherein the coma correcting element is formed by a plane parallel plate that is tilted oppositely to the astigmatism generating element in respect of the optical axis of the lens system. As a result, the astigmatism generating element and the coma correcting element generate equal amounts of astigmatism that are directed in the same direction, as well as equal amounts of coma that are directed in opposite directions: therefore, the amount of coma generated by the astigmatism generating element is eliminated by the amount of coma generated by the coma correcting element.

However, the device as described in U.S. Pat. No. 4,209,139 has the drawback that it needs an additional plane parallel plate, taking up space, generating an extra amount of astigmatism, and preventing focus adjustment of the spot with the quadrant detector.

U.S. Pat. No. 5,496,993 describes an optical scanning device of the type described in the opening paragraph, wherein the astigmatism generating element is formed by a plane parallel plate and the coma correcting element is formed by a wedge-shaped optical element. The wedge-shaped element has an entrance surface that is inclined in a direction diametrically opposite to the direction of inclination of the plate, thereby producing an amount of coma that is directed diametrically opposite to the direction of the amount of coma generated by the plate. In other words, the amount of coma generated by the wedge-shaped element compensates the amount of coma generated by the plate.

However, the device as described in U.S. Pat. No. 5,496,993 has drawbacks. In particular, the coma correcting element generates, apart from coma, an extra amount of astigmatism.

An object of the invention is to provide an optical scanning device that remedies the aforementioned disadvantages.

In accordance with the invention, these objects are achieved by an optical scanning device as described in the opening paragraph, which is characterized said coma correcting element includes a correction surface having a shape defined by a function “H(r, θ)” that includes the term “A.r3.cos(θ)” wherein: “H” is the position of the correction surface along the optical axis of the lens system, “r” and “θ” designate polar coordinates in a cross-section of the first astigmatic radiation beam, and “A” designates a first constant dependent on the amount of coma generated by the coma correction element.

In a preferred embodiment of the optical scanning device according to the invention, the function “H(r,θ)” is defined by:
H(r,θ)=A.r3.cos(θ)+B.r+C.r2.cos2(θ−θo)
wherein “B,” “C” and “θo” designate a second constant, a third constant and a fourth constant, respectively.

An advantage of such a coma correcting element is that it generates the second amount of coma for compensating the first amount of coma, without generating an additional amount of astigmatism and without affecting the amount of astigmatism generated by the astigmatism generating element.

Another advantage of such a coma correcting element is that it reduces the radial-to-focus cross-talk, since the cross-section of the second astigmatic radiation beam is no longer deformed by coma aberration.

Another advantage of such a coma correcting element is that the second astigmatic radiation beam is not tilted, thereby avoiding a shift of the position of the quadrant detector and consequently avoiding an increase in the free working distance.

An advantage of such a coma correcting element is that it may be formed as the entrance surface of a servo lens; the exit surface of the servo lens may be advantageously used for forming, for instance, a negative spherical lens. This advantageously results in allowing focus adjustment of the second astigmatic radiation beam with the quadrant detector, by moving the servo lens in the direction of the optical axis of the lens system and, in the case where the servo lens is not formed by an aspherical lens, by moving the quadrant detector.

Another advantage of such a coma correcting element is that it may be used for focus adjustment of the spot with the quadrant detector.

FIG. 1is a schematic illustration of the optical components of an optical scanning device1according to the invention for scanning an information layer2of an optical record carrier3.

By way of illustration, the optical record carrier3includes a transparent layer60on one side of which the information layer2is arranged. The side of the information layer facing away from the transparent layer is protected from environmental influences by a protective layer61. The transparent layer acts as a substrate for the optical record carrier by providing mechanical support for the information layer. Alternatively, the transparent layer may have the sole function of protecting the information layer, while the mechanical support is provided by a layer on the other side of the information layer, for instance by the protection layer or by an additional information layer and transparent layer connected to the uppermost information layer. The information layer2is a surface of the carrier3containing tracks. A “track” is a path to be followed by a focused radiation on which optically-readable marks that represent information are arranged. The marks may be, e.g., in the form of pits or areas with a reflection coefficient or a direction of magnetization different from the surroundings. In the case where the optical record carrier3has the shape of a disc, the following is defined with respect to a given track: the “radial direction” is the direction between the track and the center of the disc and the “tangential direction” is the direction that is tangential to the track and perpendicular to the “radial direction”.

During scanning, the record carrier3rotates on a spindle (not shown inFIG. 1) and the information layer2is then scanned through the transparent layer60.

As shown inFIG. 1, the optical scanning device1includes a radiation source4, a lens system5having an optical axis OO′, a beam splitter18and a detection system6. The optical scanning device1preferably further includes a servocircuit6A, a focus actuator6B, a radial actuator6D, and an information processing unit6C for error correction.

The radiation source4is arranged for supplying a radiation beam8. Preferably, the radiation source4includes at least one semiconductor laser that emits the radiation beam8at a selected wavelength λ. For instance, in the case where the optical record carrier3is of a DVD format, the wavelength λ of the radiation beam8is between 620 and 700 nm and, preferably, equals 660 nm. More preferably, the optical scanning device1further includes a grating structure9for forming first and second satellite radiation beams10and11as the −1 and +1 order diffracted beams from the central radiation beam, that is, the radiation beam8.

The beam splitter18is arranged for reflecting the radiation beam8(as well as the satellite radiation beams10and11) toward the lens system5. Preferably, the beam splitter18is formed by a plane parallel plate that is tilted with respect to the optical axis OO′ so as to form an angle α with respect to this axis. Preferably, the angle α equals to 45 degrees. Notably, the plane parallel plate is used, apart from its function of beam splitter, for generating astigmatism as described below.

The lens system5is arranged for transforming the radiation beam8(as well as the satellite radiation beams10and11) to a focused radiation beam13so as to form a scanning spot14in the position of the information layer2. The lens system5includes a first objective lens15; it preferably further includes a collimator lens7and a second objective lens16. Preferably, the second objective lens16is used together with the first objective lens15in the case where the numerical aperture of the radiation beam8approximately equals 0.85, while only the first objective lens15is used in the case where the numerical aperture of the radiation beam8is comprised between 0.45 and 0.65.

The collimator lens7is arranged for transforming the radiation beam8(as well as the satellite radiation beams10and11) into a substantially collimated beam12.

The first objective lens15is arranged for transforming the collimated radiation beam12to a converging radiation beam17. Furthermore, the objective lens15is preferably aspherical.

The second objective lens16is arranged for transforming the converging radiation beam17to the focused radiation beam13. It may be formed by a plano-convex lens having a convex surface15athat faces the objective lens15and a flat surface15bthat faces the position of the information layer2. Notably, the objective lens16forms, in cooperation with the objective lens15, a doublet-lens system that advantageously has a larger tolerance in mutual position of the optical elements than the single-lens system. Furthermore, the objective lens16is preferably aspherical.

By way of illustration, in the case where the optical record carrier3is of a DVD format, the selected numerical aperture of the focused beam13approximately equals to 0.60 for the “reading mode” and to 0.65 for the “writing mode.”

Thus, during scanning, the focused radiation beam13reflects on the information layer2, thereby forming a reflected beam which returns on the optical path of the forward converging beam17. The lens system5transforms the reflected radiation beam to a first converging reflected radiation beam63. Finally, the beam splitter18separates the forward radiation beam8from the reflected radiation beam17by transmitting at least part of the reflected radiation beam63towards the detection system6, in the form of a radiation beam21.

The detection system6is arranged for capturing the radiation beam21and the satellite radiation beams and converting them to one or more electrical signal. One of the signals is an information signal Idata, the value of which represents the information scanned on the information layer2. The information signal Idatais processed by the information processing unit6C for error correction. Other signals from the detection system6are a focus error signal Ifocusand a radial tracking error signal Iradial. The signal Ifocusrepresents the axial difference in height (along the optical axis of the lens system5) between the scanning spot14and the position of the information layer2. Preferably, this signal is formed from the “astigmatic method” as described above. The radial tracking error signal Iradialrepresents the distance in the plane of the information layer2between the scanning spot14and the center of a track in the information layer2to be followed by the scanning spot14. Preferably, this signal is formed from the “radial push-pull method” as described above.

The servocircuit6A is arranged for, in response to the signals Ifocusand Iradial, providing servo control signals Icontrolfor controlling the focus actuator6B and the radial actuator6D, respectively. The focus actuator6B controls the position of the objective lenses15and16along the optical axis of the lens system5, thereby controlling the position of the scanning spot14such that it coincides substantially with the plane of the information layer2. The radial actuator6D controls the position of the objective lenses15and16in a direction perpendicular to the optical axis of the lens system5, thereby controlling the radial position of the scanning spot14such that it coincides substantially with the center line of the track to be followed in the information layer2.

FIG. 2is a schematic illustration of the detection system6ofFIG. 1; it shows in detail that the detection system6includes an astigmatism generating element (preferably formed by the beam splitter18), a coma correcting element19and a quadrant detector20. As a matter of purely arbitrary choice, the reference “Z-axis” designates the direction of the optical axis OO′ of the lens system5, and the references “X-axis” and “Y-axis” designate the two directions of the quadrant detector12that corresponds to the radial and tangential directions, respectively.

The astigmatism generating element18is arranged for transforming the radiation beam8to a first astigmatic radiation beam21. Furthermore, the astigmatism generating element18generates a first amount of coma so that the astigmatism radiation beam21includes coma. In the preferred case where the astigmatism generating element is formed by the plane parallel plate that forms the beam splitter18, the plate forms the angle α with respect to the Z-axis, that preferably equals 45 degrees, as described above. In this preferred case, said first amount of coma may be expressed in the form of the Siedel coefficient W31rmsasimilarly to that given by Equation (1).

The coma correcting element19is arranged for generating a second amount of coma so as to compensate said first amount of coma (that is, in the preferred case, W31rmsa), thereby transforming the astigmatic radiation beam21to a second astigmatic radiation beam22that is substantially free from coma aberration (the term “substantially” being explained below). In the following, said second amount of coma may be expressed in the form of the root-mean-square W31rmsbof the Seidel coefficient W31b. Furthermore, the coma correcting element19is formed by a correction surface19A having a shape defined by a function “H(r, θ)” that includes the term “A.r3.cos(θ)” wherein: “H” is the position of the correction surface along the optical axis of the lens system, “r” and “θ” designate polar coordinates in a cross-section of the astigmatic radiation beam21, and “A” designates a first constant dependent on the amount of coma generated by the coma correction element19(that is, on the normalized root-mean-square W31rmsbas described below in further detail).

In a preferred embodiment of the optical scanning device1, the function “H(r,θ)” is defined by:
H(r,θ)=A.r3.cos(θ)+B.r+C.r2.cos2(θ−θo)   (3)
wherein: “H,” “r,” “θ,” are those defined above, and “B,” “C” and “θo” designate a second constant, a third constant and a fourth constant, respectively.

In a preferred embodiment of the optical scanning device1, the correction surface19A is formed as the entrance surface of a lens19′, thereby the exit surface19B of the lens19′ to provide an additional optical function. For instance, the exit surface19B may be spherically curved in order to form a spherical lens. Three embodiments of the lens19′ that include three embodiments of the correction surface19A are described in detail below.

The quadrant detector20is arranged for converting the astigmatic radiation beam22to the signals Idata, Ifocusand Iradial. In order to generate the signal Ifocusaccording to the “astigmatic method,” the quadrant detector20includes: (a) four first radiation-sensitive detection elements C1through C4(as shown inFIG. 2) for providing four detection signals IC1, IC2, IC3and IC4, respectively, and (b) a first electronic circuit for, in response to the signals IC1through IC4, providing the signals Idataand Ifocus. In order to generate the radial-tracking error signal Iradialaccording to the well-known “radial push-pull method,” the detection system39includes second radiation-sensitive detection elements and a second electronic circuit for, in response to the output signals of these detection elements, providing the signal Iradial.

Three embodiments of the lens19′ that include three embodiments of the correction surface19A are now described in detail.FIGS. 3 through 5show the cross-sectional views of first, second and third embodiments of the lens19′ that include first second and third embodiments of the correction surface19A, respectively. The reference numerals19′1,19′2and19′3designate the first, second and third embodiments of the lens19′, respectively, the reference numerals19A1,19A2and19A3designate the first, second and third embodiments of the correction surface19A, respectively, and the reference numerals19B1,19B2and19B3designate the first, second and third embodiments of the exit surface of the lenses19′1,19′2and19′3, respectively.

With reference toFIG. 3, in the case where A≠0, B=0 and C=0 in Equation (3), the correction surface19A1corresponds to the following equation:
H1(r,θ)=A1.r3.cos(θ)   (4)
wherein “A1” corresponds to the constant “A” defined with respect to Equation (3). As previously stated, A1 depends on W31rmsb; more specifically, it is given by the following equation:

A1=W31⁢rmsb·λ·72L3·NA3·(nlens-1)(5)
wherein: “W31rmsb” is the “root-mean-square” value associated with the amount of coma W31bgenerated by the correction surface19A1; “λ” is the wavelength of the astigmatic radiation beam22; “L” is the distance from the correction surface19A1to object of the spherical lens19B1(as shown inFIG. 3); “NA” is the numerical aperture of the astigmatic radiation beam22; and “nlens” is the refractive index of the lens19′1. Furthermore, in order to compensate the amount of coma generated by the plane parallel plate18, the value W31rmsbin Equation (5) is to ideally equal the “root-mean-square” value W31rmsagiven by Equation (1).

By way of illustration only, if d=1.1 mm, NA=0.135, α=45 degrees and n=1.51 in Equation (1), the value W31rmsaequals to 71 mλrms. And, if nlens=1.57, NA=0.16, λ=790 nm, L=2.8 mm, an ideal value A1idealof the constant A1 is finally known from Equation (5) where W31rmsbideally equals 71 mλrms. In practice, the ideal value A1idealcannot be obtained and the actual value A1actualof the parameter results in compensating the first amount of coma W31aso as to form the second astigmatic radiation beam22that is substantially free from coma. For instance, if A1actual=0.1 mm−2, the resulting value W31rmsbequals to 71 mλrms. In other words, a difference of 5 mλrms between the ideal and actual values may be tolerate; in the description, “substantially free from coma” means that the “root-square-mean” value of the resulting amount of coma in the astigmatic radiation beam22is less than 10 mλrms.

With reference toFIG. 4, in the case where A≠0, B≠0 and C=0 in Equation (3), the correction surface19A2corresponds to the following equation:
H2(r,θ)=A2.r3.cos(θ)+B2.r
wherein “A2” and “B2” correspond to the constants “A” and “B,” respectively, defined with respect to Equation (3). In the case where the astigmatism generating element18is formed by a plate, the constant A2 may be calculated as described with respect toFIG. 3. The constant B2 represents a constant tilt of the correction surface19A2for compensating the average tilt of the correction surface curved by the term “A.r3.cos(θ)” in the Y-direction; it typically is less than one degree. Notably, the constant tilt has no significant effect on the coma correction; however, it makes the correction surface19A2advantageous with respect to the surface19A1, in terms of mould making. Another advantage of forming such a cylindrical surface is that it generates a second amount of astigmatism in addition to the amount generated by the plane parallel plate.

With reference toFIG. 5, in the case where A≠0, B≠0, C≠0 and θo=0 in Equation (3), the correcting surface19A3corresponds to the following equation:
H3(r,θ)=A3.r3cos θ+B3.r+C3.r2.cos2(θ)
wherein “A3,” “B3” and “C3” correspond to the constants “A,” “B” and “C”, respectively, defined with respect to Equation (3). Notably, the “C3.r2cos2(θ)” term represents a cylindrical surface that generates an additional amount of astigmatism that is added to the amount of astigmatism generated by the plate18, i.e. W22as expressed in Equation (2); the coefficient C3 then may be expressed as follows:

C3≈12·Rcyl
wherein “Rcyl” is the cylinder radius associated with the cylindrical surface.

Alternatively, in the case where C≠0 and θo≠0, the azimuth of the corresponding cylindrical surface forms an angle θo with respect to the Y-direction. This advantageously results in rotating the focal lines of the astigmatic radiation beam21that emerges from the plane parallel plate18.

As a matter of illustration, the operation of the optical scanning device1is described below—in particular, the effect of the coma correcting element19on the radiation beam21.FIGS. 6A and 6Bare schematic representations of the cross-sections of the radiation beam22on the quadrant detector20, with and without correction according to the invention, respectively. AndFIGS. 7A and 7Bshow the focus S-curves (that have been produced by simulation) with respect to the radiation beam22that is incident to the quadrant-detector, with and without correction according to the invention, respectively.

FIGS. 6A and 6Bshow dots that represent the intersection of rays of the radiation beam22with the plane of the quadrant detector20. With reference toFIG. 6A, the astigmatic radiation beam22, with coma correction according to the invention, is substantially symmetrical with respect to the center of the beam. With reference toFIG. 6B, the astigmatic radiation beam22, without correction according to the invention, is affected by the first amount of coma and is consequently symmetrical with respect to the center of the beam.

In relation toFIG. 7AandFIG. 7B, the normalized value FE of the focus-S curves (with respect to the maximum value of the signal Ifocus) are derived from a measurement of the detection signals IC1through IC4according to the following equation:

With reference toFIG. 7A, the focus-S curve61associated with the astigmatic radiation beam22, with correction according to the invention is substantially symmetrical with respect to the point that corresponds to FE=0.

With reference toFIG. 7B, the focus-S curve62associated with the astigmatic radiation beam22, without correction according to the invention, has a peak63that is affected by the presence of coma in the radiation beam22. The skilled person notes that a disadvantage of the presence of such a peak is that the optical scanning device is particularly sensitive to chocks.

An advantage of such a coma correcting element is that, when adjusting the position of the lens19′ with respect to the quadrant-detector20for focus adjustment purposes, it results in an insignificant change in the amount of coma generated by the correcting element. For instance, calculations have shown that a change of 0.2 mm in the position of the lens19′ along the axis OO′ results in a change of 5 mλrms in the generated amount of coma which is considered not to be significant with respect to 71 mλrms.

It is to be appreciated that numerous variations and modifications may be employed in relation to the embodiments described above, without departing from the scope of the invention which is defined in the appended claims.

As an alternative, the coma correcting element may be combined with any astigmatism generating element other than a plane parallel plate, that further generates coma aberration.

As an alternative, the optical scanning device may be of the type capable to performing simultaneous multi-track scanning. This results in improving data read-out in the “reading mode” and/or write speed in the “writing mode” as described, for example, in U.S. Pat. No. 4,449,212. The description of the multi-tracking arrangement according to U.S. Pat. No. 4,449,212 is incorporated herein by reference.