Focus servo circuit in an optical disk system

Abstract: Apparatus useful in maintaining focus in a disk data-­ storage system, comprising: a radiation source (10) for producing a read/write beam of radiation; controllable focus means including an objective lens system (18) for focusing said read/write beam on the disk, a portion of said read/write beam being reflected form the disk to produce a reflected beam (129); an optical element (130) for simultaneously producing from said reflected beam, first (135) and second (136) beams, said first beam at a detector plane having at least a portion non-overlapping in relation to said second beam; first detector means (a,b,c and d) at said detector plane for producing a first signal related to said first beam (135); second detector means (e,f,g and h) to produce a second signal related to said second beam; (136) and electronic means for combining said first signal and said second signal to produce a third signal, said third signal being related to said degree of focus and said third signal being substantially constant in response to optical faults.

Description: The present invention relates to an apparatus useful in maintaining focus in a disk data-storage system, comprising: a radiation source for producing a read/write beam of radiation; controllable focus means including an objective lens system for focusing said read/write beam on the disk, a portion of said read/write beam being reflected form the disk to produce a reflected beam. Disk-type data storage systems, and particularly optically readable disk systems, require proper placement of a focused beam of light, such as a laser beam, with respect to the disk. Placement in the axial direction, i.e. towards or away from the disk surface, involving focusing of the light beam and placement in the radial direction, referred to as tracking, are needed to assure that a light beam of the required size and intensity impinges on the disk at the desired location. Methods for monitoring and controlling tracking and focusing of the beam typically involve detection of characteristics of at least a portion of the beam which is reflected from the disk surface. One such system is described in US-A-4,446,546. This system uses a quadrature detector which produces an "S" curve for use in focus control. A persistent problem with such systems is that commonly-used detecting devices are unable to distinguish between characteristics which result from defocusing or track crossing and characteristics which result from other changes in the optical system such as movement or misalignment of optical components of the system. Because of this inability to distinguish, a misalignment of some components of the optical system can result in the detector system producing a focus control signal or a tracking control signal which is erroneous, i.e. which causes the focused pointed of the light beam to be positioned away from the desired position. A number of methods have been devised to overcome or compensate for the lack of signal discrimination. If the optical detectors are relatively small compared to the size of the beam being detected, movement of the beam axis with respect to the detectors may leave the detectors substantially within the beam and thus substantially unaffected by such movement. However, for many focus correction systems, the beam being detected must be focused on the detector. Producing a focused spot which is sufficiently large to avoid the effects of the axis movement during the anticipated life of the device places constraints on other components, such as lens apertures ad bit density, which make this solution impractical and expensive to implement. The tracking system can be separated from the focus system by, for example, a beam splitting arrangement intended to provide a relatively error-free tracking system. However, by splitting the optical path into two optical paths which pass through different optical elements, events such as optical axis movement in one path may not occur in the other path and thus signals related to one optical path may not be useful in correcting errors in the other optical path. Furthermore, this arrangement is beneficial only to the tracking system and does not solve problems associated with the focus system. Conversely, systems such as that disclosed in US-A-4,123,652 include a focusing system which is intended to overcome certain optical faults. However, this system does not disclose overcoming optical faults in the tracking system. Furthermore, it requires splitting of the optical path thus creating the possibility for independent optical faults in the two optical paths. Accordingly, there is a need for a system for tracking and focusing an optical beam which is insensitive to optical faults such as beam axis movement or component misalignment and which does not involve subjecting split beam to independent optical faults. The present invention provides an apparatus with such system. This apparatus is characterized in that the path of the reflected beam an optical diffractive element is arranged for simultaneously producing from said reflected beam, at least a first beam converging to form a first image on a first image plane and a second beam forming a second image on a second image plane, said first beam at a detector plane having at least a portion non-overlapping relation to said second beam, wherein: (a) said first image changes in a mirror symmetric fashion with respect to changes in said second image when there is a change in said focusing of said read/write beam, (b) said first image changes in a mirror symmetric fashion with respect to changes in said second image when there is a change in the tracking of said read/write beam with respect to the disk, (c) said first image changes in a parallel fashion with respect to changes in said second image when there is a change in the optical axis of said reflected beam; first detector means at said detector plane being responsive to said changes in said first image to provide a first plurality of signals related to said changes in said first image; second detector means being responsive to said changes in said second image to provide a second plurality of signals related to said changes in said second image; electronics means for combining signals comprising: first means for combining said first plurality of signals and second plurality of signals to provide a third signal related to said changes in focus, said third signal being substantially unresponsive to changes in the optical axis of the reflected beam. It is remarked that GB-A-2,101,307 describes a disk data storage system having the precharacterising features of claim 1 and wherein the beam reflected by the disk is splitted up in two beams which are incident on first and second detector means respectively for generating a focus-error signal. However, the beam splitter is a prism and not a diffractive element which provides the said symmetric change in the first and second image upon change of focusing and/or tracking and a parallel change upon change of the optical axis of the reflected beam. A first embodiment of this apparatus is further characterized in that said electronic means for combining signals further comprises: second means for combining said first plurality of signals and said second plurality of signals to provide a fourth signal related to said changes in tracking, said fourth signal being substantially unresponsive to said changes in the optical axis of the reflected beam; and in that first control means for changing the focal plane of said focus means using said third signal; and second control means for changing the radial tracking of said read/write beam relative to the disk using said fourth signal are present. Because the undesired changes resulting from optical faults, for example, optical axis movement, create parallel results in the two images, the two images can be combined in a fashion to nullify such unwanted effects, and thereby to produce a combined signal which reflects substantially only changes in tracking or focusing and which is substantially unresponsive to changes in the optical axis of the reflected beam. This first embodiment may be further characterized in that said optical element further produces a third beam converging to form a third image on a third image plan, and further comprising a third detector means responsive to changes in said third image to provide a signal related to modulation of said read/write beam by the disk. The optical element may be an astigmatizing element. The optical element can be configured to produce astigmatism of the first beam which is mirror symmetrically oriented with respect to the astigmatism of the second beam. In this manner, a single element can produce a desired relationship of two astigmatic focus lines, for example, an orthogonal relationship, without the necessity for separately aligning two different optical elements, such as two lenses. A second embodiment, of the apparatus is further characterized in that said first beam is astigmatically focused by said optical element to an image in an image plane; said second beam being defocused by said optical element at said image plane; said first detector means being arranged substantially in said image plane and providing a first signal varying in response to the shape of said image; said second detector means being arranged substantially in said image plane and located substantially outside said image and producing a second signal in response to said second beam related to said degree of focus; and in that it comprises first control means for changing said degree of focus, responsive to said third signal. The second embodiment may be further characterized in that the first and second beam are coaxial. Two different detecting means are used for the first and second beams. The first detector means detects the focused image from the first beam. The first detector is preferably a quadrature detector which produces an "S" curve from the astigmatically focused first beam. The second detector includes substantial portions outside the image from the first beam and is responsive to the defocused second beam. The second detector, being substantially smaller than the size of the defocused second beam, is substantially unaffected by optical faults such as movement of the optical axis of the defocused beam. The phase relationship detected by the second detector is used to modify the "S" curve produced by the first detector. The resultant signal is related to the degree of focus of the beam on the disk and is substantially unaffected by movement of the optical axis. The second detector is preferably also used in monitoring and controlling tracking of the beam with respect to the disk. As noted, the tracking signal is relatively unaffected by optical faults, such as movement of the optical axis, because, as noted, the second detector is relatively small compared to the size of the defocused second beam at the image plane. The single optical element which accomplishes creation of two beams in both the first and second embodiments is preferably a zone plate. By using a single optical element, as opposed to a combination of elements such as a number of lenses, material expense and production and maintenance costs are reduced. Furthermore, even when a number of optical components are combined to produce the desired effect, the first beam and second beam both pass through the same optical elements so that conditions causing an optical fault in one of these beams also causes an optical fault in the other. For this reason, a signal from a detector of one beam can be combined with a signal from the detector of the other beam to produce a final signal which is relatively free from the influence of optical faults. According to both embodiments of the invention, the detector apparatus can be provided in a single compact device because the detectors are all preferably located in a single image plane and are located close together. By providing a single compact device, material and construction costs are minimized and maintenance requirements are reduced. The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic depiction of a prior-art system for focusing a coherent light beam onto a disk track and for generating focus and tracking signals; Figure 1A is a front view of a quadrant detector taken along Line 1A-1A of Figure 1; Figures 2A-2C depict the quadrant detector of Figure 1A displaying three different focus conditions; Figure 3 is a schematic depiction of another prior-art focus and tracking signal sensing system; Figure 3A depicts the detector taken along Line 3A-3A of Figure 3; Figure 4 is a schematic representation of a detector system for sensing focus and tracking error according to a first embodiment of the invention; Figure 5 is a front view of a detector taken along Line 5-5 of Figure 4; Figures 6A, B, C and D depict the quadrant detector of Figure 4 displaying three different focus conditions and an axis movement; Figures 7A and B depict the diffraction pattern of the refracted beam before and after rotation by the optical element; Figure 8 is a schematic representation of a monitoring apparatus for imaging and focus error signal generation according to the second embodiment of the invention; Figure 8A is a schematic front view of an optical element of Figure 8 taken along Line 8A-8A of Figure 8; Figure 8B is a front view of the composed detector taken along Line 8B-8B of Figure 8; Figure 9 is a phase diagram showing the phase relationship from two detectors in a focusing and tracking detection system according to the first embodiment of the invention; Figure 10 is a schematic depiction of a detector configuration for sensing focus and tracking error signals which is an alternative configuration of the second embodiment of the invention; Figure 10A is a front view of a composed detector taken along Line 10A-10A of Figure 7; Figure 11 is a schematic representation of a point light source and its wavefront at a plane P; and Figure 12 is a diagramatic depiction of various stages in the construction of an elliptical zone plate. Referring to Figures 1 and 1A, depicted therein is a prior art optical disk drive system in which a light beam is produced from a coherent light source 10, such as a semi-conductor laser, preferably with polarization parallel to the plane of incidence of the polarizing beam splitter 14. The beam is directed to a mirror 12, to a polarizing beam splitter 14, and through a quarter wave plate 16. The quarter wave plate 16 changes the linear polarized light which is transmitted by the polarization beam splitter into circular polarized light, as known. The laser beam is then focused by the objective lens 18 to a spot having a diameter of, e.g., about 1 micrometer or less on the surface of a recording medium 20, such as a conventional rotating disk. To achieve such a spot, an objective lens 18 with a large numerical aperture of, e.g., about 0.52 is used. The depth of focus of such an objective lens is small. Therefore, it is desirable that the objective lens be maintained at a preferred distance from the medium surface 20 so that the light beam remains sharply focused on the plane of the information track, the information plane, on the disk 20. In an optical disk drive, the distance between the information plane and the objective lens may tend to vary due to, e.g., the vertical run out of the disk. This requires that some provision be made to detect these variations and to correct the focusing error in response thereto. The disk medium 20 typically has a reflectance of 0.4. Reflected light from the disk 20 passes back through the objective lens 18, the quarter wave plate 16, off the polarizing beam splitter 14, through an astigmatic lens such as a toric lens 22, and is focused on a quad detector 24. The toric lens 22 is fabricated by polishing two curvatures on the same glass piece. This type of lens suffers from spherical abberation and requires accurate centering in order to produce a uniform and well shaped beam for detection. Another method for producing such a lens with two foci is to emboss a thin layer of material with a cylindrical curvature on it. Such a lens is sometimes referred to as a bifocal lens and is of higher quality than a toric lens but at higher cost. The light beam which is focused on the surface of the disk 20 should always remain centered on an information track as defined by track-defining systems such as by pregrooved portions to be read (or written), bit edges or tracking servo bits. Deviations from such centered position can produce a reduction of the modulation depth of the read-out signal and/or cross-talk between adjacent track portions. Centering of the focused beam on the desired track means that the center of the focused beam should coincide with the center of the track. In the case of an off-centered beam, the centers will not coincide. Therefore, the optical drive should include means for detecting the magnitude and the direction of a centering error of the focused beam relative to an information track to be acccessed so that the position of the spot can be corrected. The tracking errors may be obtained by combining detector signals generated by electrical detector zones, a, b, c and d, e.g., by combining in the fashion (1) (a + d) - (b + c) with reference to the quad detector 24 depicted in Figure 1A. If the spot on the disk moves across, e.g., pregrooved data tracks, the intensity of the light on the quad detector 24 will fluctuate, producing a fluctuation in this signal from the detector. The fluctuation of this electrical detector signal is caused by the interference of the diffracted beams produced by the track. The focus error signal may be obtained by combining detector signals generated by electrical detector zones, e.g., by combining in the fashion (2) (a + c) - (b - d) with reference to the quad detector 24 depicted in Figure 1A. The focus error signal is generated using the lens 22 which is usually a conventional toric lens as discussed above. Depending on the condition of the axial position of the focused spot with respect to the information surface of the disk 20, the light pattern on the quadrant detector 24 will have one of the shapes illustrated in Figures 2A through 2C. When the spot is properly focused, the shape of the light is approximately symmetrical, as depicted in Figure 2B. The spot on the quad detector 24 as shown in Figure 2B has equal intensity on all four quadrants of the quad detector 26. On the other hand, when the laser beam is not in proper focus on the information surface of the disk 20, the spot on the quad detector 24 will be elliptical and show preference to diagonally opposed quadrant portions of the quad detector 24, as depicted in Figures 2A and 2B. The electrical signal defined by Equation 2 taken from the quad detector 24 will be reduced (compared to the in-focus signal) when the focus is in error in a positive sense (as depicted in Figure 2A) with the magnitude of the reduction being related to the magnitude of the focus error. The electrical signal obtained according to Equation 2 will be increased (compared to the in-focus signal) when the focus is in error in a negative sense (as depicted in Figure 2C) with a magnitude related to the magnitude of the focus error. In this way, the focus error signal can provide both the direction and the magnitude of the focus errors. Both the tracking and focus systems described suffer from a number of constraints and shortcomings. In the particular configuration of error signal generation described above, the tracking error signal in accordance with Equation 2 can only be accomplished when the spot is close to an optimum focused condition. Moreover, the spot as shown in Figure 2B is rather small, the spot diameter is, for example, about 250 micrometers. As a result, when the drive encounters different thermal environments, the termal expansion of the mechanical parts coupling with stresses built into some of the adjustments parts causes the beam on the quad detector 24 to move from its pre-aligned position. This beam motion produces an electrical offset signal which reduces the accuracy in monitoring the tracking on the disk 20. For example, if the axis of an accurately focused beam shifts so as to move the spot shown in Figure 2B towards quadrant b, the axis shift will cause the tracking error signal defined by Equation 1 to decrease, causing a change in tracking of the beam, even when the beam was properly centered. Such beam motion also affects accuracy of the focusing system. The described axis shift will cause the quantity defined by Equation 2 to decrease and the resulting focus error signal will mimic the signal produced in an out-of-focus situation such as depicted in Figure 2A. Thus, such an axis shift will cause the focus system to defocus a beam which is, in fact, accurately focused. Figure 3 depicts another prior art optical drive system in which the laser 10 directs a beam of coherent light through the polarizing beam splitter 14, the quarter wave plate 16, the objective lens 18 and onto the disk 20. The reflected beam passes back through the objective lens 18, through the quarter wave plate 16 and out of the polarizing beam splitter 14. A second beam splitter 26 divides the return beam into two beams. The tracking signal is sensed by a split detector 28, depicted in Figure 3A. The focus error is sensed by passing the focus beam through a lens 22 to the quadrant detector 24. In this approach, the tracking signal detected on the split detector 28 may become insensitive to the movements of, e.g., the mechanical mounts in the system. However, the focus error detected on the quad detector 24 is still influenced by beam tilt or other motion of the beam caused, e.g., by stresses built into the supporting mechanical mounts. Moreover, because the focus beam passes through the lens 22 while the tracking beam does not, no signal detected by detector 28 could be used to discriminate optical faults which occur only in the focus beam path, such as those that might be caused by misalignment of the lens 22. According to the invention, an optical element which produces two focused beams is used. In a first embodiment of the invention as depicted in Figure 4, an optical element 60 comprising, for example, a lens 62 and a zone plate 64, are placed in the path of at least a portion of the beam 66 reflected from the disk. The characteristics and method of making a zone plate usable in this embodiment are discussed below. Briefly, a preferred zone plate for this embodiment would have raised or opaque regions in the shape of segments of ellipses. The effect of the optical element 60 is to produce two converging beams, a minus first order beam 68 and a plus first order beam 70. Each of these beams converges to an image plate 72 which is preferably the same plane for both beams. The optical element 60 provides the two beams 68, 70 with the characteristic that the shape and diffraction pattern of the beams at the image plane 72 change in a mirror symmetric fashion when there is a change in the focus or tacking of the read/write beam but change in a parallel fashion when there is a change in the optical axis of the reflected beam 66. Further, the optical element 60 imparts astigmatism to the two beams. The first and second beams 68, 70 are detected using two quadrature detectors 74, 76 depicted in Figure 5. The first quadrature detector 74 is positioned to detect the first beam 68 and the second quadrature detector 76 is positioned to detect the second beam 70. The quadrature detectors 74, 76 are located substantially at the image plane 72 of the two beams 68, 70. The optical element 60 can also be configured to provide a third, or zero order, beam 78 which converges to an image plane preferably coplaner with the image plane 72 of the first and second beams 68, 70. In one embodiment of the invention, the diameter of the image at the image plane 72 of the zeroth order light beam 78 is about 10 microns. A third detector 80 is positioned to detect the third beam 78 to provide a signal related to modulation of the read/write beam by the data disk. Figures 6A, 6B, 6C, and 6D schematically depict the pattern of light falling on the detectors 74, 76, 80 in the embodiment depicted in Figure 4 for different focus and optical axis conditions. Figures 6A, B and C depict situations in which the optical axis has not shifted, i.e., in which the first and second beams 68, 70 are centered on the centeres of the quadrature detectors 74, 76, respectively. Figure 6A depicts the light pattern falling on the detectors 74, 76, 80 when the read/write beam is in focus. Figure 6B depicts the light pattern falling on the detectors 74, 76, 80 when the read/write beam is out-of-focus with respect to the data disk in the positive sense. Figure 6C depicts the light pattern falling on detectors 74, 76, 80 when the read/write beam is out-of-focus with respect to the data disk in the negative sense. Figure 6D depicts the pattern falling on the detectors 74, 76, 80 when the read/write beam is in focus with respect to the data disk but the axis of the reflected beam 66 has shifted from the preferred or centered position. According to this embodiment of the invention, the focus error signal is obtained by combining the outputs from the quadrature detectors k, l, m, n, p, q, r and s in the following manner: (3) [(k + m) - (l + n)] + [(q + s) - (p + r)]. The tracking error signal, according to this invention, is obtained by combining the signals from the quadrature detectors in the following fashion: (4) [(k + n) - (l + m)] + [(q + r) - (p + s)]. By combining the signals from the two quadrature detectors in this manner, both the focus error signal and the tracking error signal are sensitive to respectively the focus and tracking conditions of the read/write beam with respect to the data disk, but are ach relatively insensitive to optical faults such as changes in the optical axis of the reflected beam 66. As with the first embodiment, the ability to combine signals from two beams to produce a signal which is insensitive to optical faults is related to the fact that the two beams follow non-independent paths, i.e. each beam passes through the same optical elements. A system generating non-independent beams is further useful in that when the astigmatism of the respective focus beams 68, 70 is produced by a single element 64, the astigmatic focus lines will be produced in a predetermined angular relationship, preferably orthogonal. Thus, there is no need to independently adjust two different optical elements such as cylindrical or toric lens to assure orthologonality of the astigmatic focus lines of the two beams 68, 70. Figure 7A depicts the position of the top and bottom diffraction patterns 81 (denoted T and B in Figure 7A) at a point along the reflected beam 66 and Figure 7B depicts the position of the diffraction patterns 82, 84, related to beams 68 and 70, at the image plan 72. As can be seen from Figure 7, the optical element 60 introduces a ninety degree rotation of the diffraction patterns. The rotations imposed on the diffraction patterns 82, 84 of the first and second beams 68, 70 are mirror symmetric in the sense that when the diffraction pattern 82 of the first beam 68 is rotated 45 degrees in a counter-clockwise direction, the diffraction pattern 84 of the second beam 70 will be rotated 45 degrees in a clockwise direction. Because the diffraction patterns are rotated in opposite directions by the optical element 60, changes in the diffraction patterns which result from tracking events such as track crossing, bit edge detection or servo bit detection will be mirror symmetric at the two detectors 74, 76, while changes in the diffraction patterns which result from optical faults, such as movement of the optical axis of the reflected beam 66, will cause parallel movement of the diffraction patterns at the two detectors 74, 76. In this way, the tracking error signal obtained in accordance with Equation 4 will be relatively insensitive to optical faults. Figure 8 illustrates a second embodiment of the new focusing error and tracking error generation apparatus and method of the present invention. Rather than one optical element that produces two focused beams, this detection system includes an optical element (130) for splitting the reflected beam 129 into two beams, one of which, 135, is astigmatically focused on the plane of the detector 132, the other 137, being defocused at the plane of the detector 132. The optical element 130 can comprise any of a number of components including one or more lenses and one or more diffraction gratings. A preferred component for the optical element is a zone plate element 130. The pattern of this zone plate element 130 can be produced as described more fully below. Briefly, a pattern for the plate is produced originally by a computer on a graphic device such as pen plotter, photoplotter, CRT display or electron beam graphic output device. A preferred pattern for the zone plate element 130 includes elliptically-shaped regions. The computer output is converted into a phase relief type optical element by methods to be discussed below. Figure 8A is an opaque representation of the face of the optical zone plate 130 (not to scale) and is shown here to provide a conceptual visualization thereof. As pointed out hereinbelow, the optical zone plate 130 need not have opaque portions and, in general, better results in regard to light intensity are achieved with a transparent zone plate having a raised or lowered elliptical pattern. Figure 8B depicts the face of a modified detector plate 132 utilized in the optical system represented in Figure 8. One of the beams generated by the optical element 130 is a converging beam 135 similar to the astigmatic beam from a toric lens. This beam is for example the plus first order beam. The second beam 137 therefrom is a defocused, nearly collimated beam with a large beam diameter 1 and is in Figure 8 the zeroth order beam. A third beam 136 may also be produced by the optical element 130, being a diverging beam, and is in Figure 8 the minus first order beam. The convergent beam 135 and the collimated beam 137 are sensed by the detector 132. The divergent beam 136 in the embodiment depicted in Figure 8 does not serve any function, and thus represents some loss of total useful intensity in the detected beams. As in a typical optical drive, the beam diameter of the convergent beam 135 at the quad detector, in one embodiment of the invention, is about 0.25 mm. The diameter of the collimated 137 beam is about 4 mm, and the diameter of a divergent beam 136 at the detector plane is about 8 mm. Since the diameter of the divergent beam is large in comparison to the other two beams, its contribution to the detected signals is small. Figure 8B schematically illustrates the three beams 135, 136 and 137 as these appear on the plane of the detector 132 (not to scale). In a tracking system which uses pregrooves, the inner two beams produce a half moon shaped pattern which relates to the interferences resulting from the pregrooved structure on the recording medium of the disk 20. The detector depicted in Figure 8B can also be used with tracking systems not employing pregrooved structures, such as servo bit tracking systems. According to one embodiment of the invention, the tracking error signal is generated by the electrical detection zones by combining the signals from the detection zones denoted e, f, g and h in Figure 8B in the following fashion: (5) (e + f) - (g + h). Because the collimated beam is large in comparison to the magnitude of deviations of the beam axis produced, e.g., under thermal stress, or other optical faults, the tracking signal thus generated is, to a large extend, stable and not sensitive to optical faults. The small quadrant detector 133 in the center of the composed detector 132 is used to produce the astigmatic focus error signals showing an "S" curve because of the astigmatic nature of the beam 135. This astigmatism is now introduced by the element 130 instead of by conventional astigmatizing means such as a cylindrical or a toric lens. However, as noted above, such error signal is sensitive to optical faults. The present invention, therefore, includes using a signal from the defocused beam detectors e, f, g and h which are relatively insensitive to optical faults, in correcting or compensating for the quadrature detector signals of detector elements a, b, c and d to produce a final signal which is related to the focus, but which is relatively insensitive to optical faults. The usefulness of this aspect of the invention is related to the fact that both the defocused tracking beam 137 and the focused focusing beam 135 pass through the same optical components. Because the focused and defocused beams pass through the same optical components, both beams will be exposed to the same optical faults, enabling a signal from one beam to be used in compensating for the effects of optical faults on the other beam. In the present invention, the phase relationship between the signal from the detector zones e and f of the detector 132, or similar phase relationship between the signal from the detector zones g and h, can indicate the true focus condition of the spot on the medium. Although the following discussion is in terms of using a signal from either pair of detectors (i.e. e and f or g and h), preferably signals from both pairs of detectors are additively combined resulting in a stronger total signal and, thus, a better signal-to-noise ratio. The difference signal of the two detectors indicates the sign of the focus error, while the sum of the two detectors can be used as a phase reference signal. Multiplication of both signals as in the form: (6) (e - f) . (e + f) yields a focus error signal, as described in Braat, et al., "Position Sensing in Video Disk Readout", Applied Optics , Volume 17, pp. 2013-2021. An example of the phase relationship of these signals is illustrated in Figure 9, where the two traces represent idealized signals Se and Sf from detectors e and f or from detectors g and h. The phase , in radians, is the lag Δt of one signal, with respect to the other, divided by the common period T of the signals. The focus error signal as described in the Braat, et al. Applied Optics article, however, has a disadvantage in that the capture range of the e, f, g, h focus signal is relatively small, i.e. before the e, f, g, h focus signal provides meaningful information, the laser beam must already be in a close-to-focused condition, compared to the focus condition required to obtain a meaningful signal from the quadrature detector, which may have a capture range on the order of 100 microns or more. However, as noted above, the quadrature detector is relatively sensitive to optical faults, such as misalignment of optical components, compared to the optical fault sensitivity of the e, f, g, h focus signal. The present invention includes using both the e, f, g, h focus signal and the quadrature detector focus signal. The quadrature detector error signal is used to directly control the focus thus maintaining the focus system sufficiently close to an in-focused condition that the e, f, g, h focus signal is meaningful (i.e. is within the e, f, g, h capture range so as to provide an indication of the true focus condition). When the optical disk drive is first made operational, the components are aligned and adjusted to initially provide the desired focus of the read/write beam with respect to the recording medium. As the system is subjected to such influences as mechanical shock or vibrations or thermal expansion, optical components become misaligned and optical faults occur as a result. As noted, the detector of the defocused beam e, f, g, h is relatively insensitive to such optical faults, while the quadrature detector is relatively more sensitive to optical faults. Thus, when optical faults cause the quadrature detector to erroneously indicate a defocused condition, the true focus condition is nevertheless provided by the e, f, g, h detector, e.g. according to Equation (6) above. According to the present invention, in such a situation, an off-set signal is combined with the quadrature focus error signal to provide a correct signal. The off-set signal is derived using the e, f, g, h signal and has a magnitude and sense such that the corrected "S" curve indicates an in-focus condition if, and only if, the e, f, g, h detector indicates an in-focus condition. The corrected quadrature error signal is relatively insensitive to optical faults because the "S" curve has been off-set so as to indicate correct focus whenever the e, f, g, h signal indicates correct focus. In summary, the process of providing a corrected error signal according to this embodiment of the invention involves the following steps: (1) Initial setup: quadrature detector signal is calibrated such that the "S" curve indicates the true focus condition. (2) During operation the e, f, g, h focus signal is sampled. (3) If the e, f, g, h focus signal and the quadrature detector both indicate an unfocused condition, the focus sustem is controlled using the "S" curve in the usual manner to bring the beam back into the desired focus condition. (4) If the e, f, g, h focus signal and the quadrature detector provide opposite indications of the focused conditions, an off-set signal is combined with the "S" curve to produce a corrected "S" curve. In this manner, a corrected error focus signal is provided which is both relatively insensitive to optical faults and which has a relatively large capture range as about 100 microns or more. Figure 10 illustrates another signal detection principle of the present invention. In this case, the optical element 138 includes a lens 139 in conjunction with an optical element such as zone plate 140 to image the aperture of the objective lens to the detector plane via the minus first order beam 135. The optical element results in splitting a portion of the beam 129 reflected from the disk into first and second beams, and also imparting astigmatism to at least one of two beams. The lens 139 can produce very high contrast interference patterns on the detector 132. Preferably, in the embodiment depicted in Figure 10, the optical element 138 produces little or no intensity of the zeroth order beam, thereby increasing the total useful intensity of the detected plus first order 137 and minus first order 135 beams. Construction of an optical element with the described characteristics is described more fully below and in conjunction with Wai-Hon Lee "High Efficiency Multiple Beam Gratings", Applied Optics , Vol. 18, No. 13, pp. 2152-2158, incorporated herein by reference. The minus first order beam 135 is a converging beam used for obtaining the astigmatic focus error signal in the manner described above in connection with Figure 8. Both beams as they appear on the detector 132 are shown in Figure 10A. A detector 132 identical to the one shown and described for Figures 8 and 8B, can be used in connection with the arrangement depicted in Figure 10. In both the first and second embodiments, the splitting of the reflected beam into first and second beams is preferably accomplished using elliptical (including partially elliptical) zone plates. Such an elliptical zone plate can be made by a photographic method, described below, involving photoreduction and photoresist replication of an elliptical pattern. The pattern can be initially generated by, for example, a computer which drives a pen plotter, photoplotter, electron beam or CRT display or other plotting device. The characteristics of the zones which are to be plotted can be obtained by solving one or more equations which are related to the desired characteristics of the elliptical zone plate. In relation to both embodiments of the present invention, it is well known in optics that a plate consisting of alternating opaque and transparent concentric circular zones can focus a collimated beam of light to a point. This plate is sometimes called a Fresnel zone plate. It can also be interpreted as a hologram of a spherical wave interfering with an online collimated beam. The zone plate of the present invention can be understood with the same general principle. Figure 11 schematically depicts a spherical wave at a distance F from a point source O which can be represented mathematically as follows: where (x,y) is the phase variation of the wavefront of the spherical wave at plane P, λ is the wavelength of the light and r is the radial distance from the origin Q. The radius R n , such that ⊘ (R n ) is equal to 2nπ, is given by: (8) R n = ( n2λ2 + 2nλF) 1/2 . For large F, as in most practical cases, R n can be represented by: (9) R n = (2nλF ) 1/2 . The zone plate with focal lengths F x and F y is obtained by plotting ellipses with major and minor axes given by: (10) R xn = (2nλ F x ) 1/2 . (11) R yn = (2nλ F y ) 1/2 . The difference in radii of curvature along the two axes produces astigmatism similar to that produced by a conventional toric lens. In connection with the first embodiment of this invention, the computer-generated astigmatic element is a combination of a positive cylinder along x direction and a negative cylinder in the y direction. Mathematically the phase of such an element can be written as (12) phase = π (x² - y²)/λf' where f' is the focal length of the astigmatic element. When this element is used in connection with a spherical lens with focal length F'', the beam along x will focus at f x which is determined by (13) f x ⁻¹ = f'⁻¹ + F''⁻¹ and the beam along y is focused at f y given by (14) f y ⁻¹ = -f'⁻¹ + F''⁻¹. Computer-generated optical elements create may diffracted orders. The +1 order will have the phase given above and the -1 order will have its conjugate namely (15) conjugate phase = -π (x² - y²)λf'. These two diffracted orders produce the pattern for the focus error signals as shown in Figure 6. The focal length f' of the astigmatic element is determined from the focus range required by the system. Suppose that F'' is equal to 40 mm. In order to have 3 mm separation between the two astigmatic foci the quations above show that f' is about 1 meter. To obtain the astigmatic pattern in Figure 6 we must rotate the astigmatic element by 45°. The phase in such a case is given by (16) phase = 2πxy/λf' To make such an element it is necessary to determine the interference of such a wavefront with a slight tilted collimated beam. The positions of the fringes are located by solving the following equation (17) 2πd/λF'' + 2πxy/λf' = 2πn where d is the separation of the first order from the zero order. In generating a zone plate usable in connection with the second embodiment of the invention, for the purpose of illustration, it is assumed that in Figure 10 the distance of the readout objective lens is at a distance D' from the lens 138 which has a focal length F'. The zone plate 140 has an average focal length of f. It is also assumed that the detector 132 is located at a distance D from the lens 138. The relationship among all of these variables is given by the following equations: (18) f⁻¹ + F'⁻¹ = 2 D⁻¹, (19) -f⁻¹ + F'⁻¹ = D⁻¹. It is possible to determine the focual lengths F' and f from the information on D. This is by no means the only relationship that exists between F' and f. The astigmatic focal lines in the elliptical zone plate 140 are mainly determined by the range of the focus errors. The distance needed between the two astigmatic focal lines (AD) is approximately given by: (2) AD = 2 K (f/g)², where K is the focus control range, g is the focal length of the objective lens and f is the average focal length of the zone plate. As an example, for K = 30 micrometers, g = 4 mm and f = 30 mm, the AD is calculated to be about 3.4 mm. An advantage of the present invention is the simplicity of fabrication of an elliptical zone plate element 130. The manufacture of this element is similar to the process required in the fabrication of microelectronic components. Fig. 12 depicts the sequence of steps involved in alternate methods for producing the phase relief structure for a zone plate element. A glass substrate 150 is first coated with a light sensitive material 152 such as a photoresist or photopolymer. A mask 154 containing the computer-plotted and usually photographically reduced image of the zone plate pattern that is to appear on the zone plate element 130 or 140 is disposed over the coating material 152. A source of light, such as ultraviolet light, is used to expose the light sensitive material 152 through the mask 154 (Fig. 12A). After exposure and a proper development process for the photosensitive material 152, a structure is derived such as shown in Fig. 12B. This structure can then be subjected to an etching process (such as with hydrofluoric acid or by plasma etching with appropriate gases) to arrive at a surface profile as shown in Fig. 12C. Removing the photosensitive material will produce the phase structure shown in Fig. 12E. As indicated above, it is preferable that the zone plate remain transparent, as shown, but the pattern can also be made opaque in any desired manner. An alternative method to fabricate the phase structure is to deposit a layer 156 of transparent silicon monoxide (SiO), silicon dioxide (SiO), or magnesium fluoride (MgF) on the structure of Fig. 12B to arrive at that shown in Fig. 12D. Removing the photoresist layer from the structure shown in Fig. 12D will produce the phase structure shown in Fig. 12F. As long as the thickness of the relief is correct according to the teaching contained in an article entitled, "High Efficiency Multiple Beam Gratings", by W.H. Lee, Applied Optics , Volume 18, pp 2152-2158, 1979, the structures shown in Figs. 12E and 12F will have identical properties insofar as the ability of such to serve as the zone plate structure described hereinabove. The zeroth order can be suppressed by making the layer which corresponds to the computer generated pattern 154 to have a thickness 158 about the wavelength of the laser light source. The manner of using the present invention will now be described. According to the first embodiment of the invention, depicted in Fig. 4, the beam reflected from the data disk 66 is passed through the optical element 60 described above to produce first and second beams substantially focused on first and second detectors 74, 76. The first and second detectors produce signals k, l, m, n, p, q, r and s related to the amount of light which falls on each quadrant of the respective quadrature detectors. These signals are combined according to Equations 3 and 4 to provide focus and tracking signals respectively which are used to control the tracking and focus of the read/write beam as described above in connection with the first embodiment. According to the second embodiment depicted in Figs. 8 and 10, a portion of a laser beam reflected from a data disc 20 is passed through the optical element 130, 138, as described above, to produce patterns of light on an optical detector 132. Signals a, b, c, d, e, f, g and h from the optical detector, as depicted in Figs. 8B and 10A, are combined to produce a tracking error signal and a focus error signal. The tracking error signal is used to move a tracking device such as a controllable carrier mirror or arm so as to position the focused spot of the read/write beam onto a desired position of the data disk 20. The focus error signal is used to position the objective lens 18 to a position which results in providing a desired degree of focus of the read/write beam on the data plane of the disk 20. In all aspects of the present invention, signals can be combined or compensated whether by way of addition, subtraction, multiplication or other combinations, using well-known electronic circuits, such as adding circuits, difference circuits or the like, or can be combined using computers or microprocessors.

Claims: Focus servo arrangement in an optical disk system, comprising: a radiation source (10) for producing a read/write beam of radiation; controllable focus means including an objective lens system (18) for focusing said read/write beam on the disk (20), a portion of said read/write beam being reflected from the disk to produce a reflected beam (129), wherein in the path of the reflected beam an optical element (60; 130; 138) is arranged for simultaneously producing from said reflected beam, at least a first beam (78; 135) converging to form a first image on a first image plane and a second beam (68; 70; 136; 137) forming a second image on a second image plane, said first beam at a detector plane having at least a portion non-overlapping relation to said second beam; characterised in that the optical element is a diffractive element, whereby (a) said first image changes in a mirror symmetric fashion with respect to changes in said second image when there is a change in said focusing of said read/write beam, (b) said first image changes in a mirror symmetric fashion with respect to changes in said second image when there is a change in the tracking of said read/write beam with respect to the disk, (c) said first image changes in a parallel fashion with respect to changes in said second image when there is a change in the optical axis of said reflected beam; first detector means (74, 133) at said detector plane being responsive to said changes in said first image to provide a first plurality of signals related to said changes in said first image; in that second detector means (76; e, f, g, h) are provided responsive to said changes in said second image to provide a second plurality of signals related to said changes in said second image; and in that electronics means for combining signals are provided comprising: first means for combining said first plurality of signals and second plurality of signals to provide a third signal related to said changes in focus, said third signal being substantially unresponsive to changes in the optical axis of the reflected beam. Arrangement as claimed in Claim 1, characterized in that said electronic means for combining signals further comprises: second means for combining said first plurality of signals and said second plurality of signals to provide a fourth signal related to said changes in tracking, said fourth signal being substantially unresponsive to said changes in the optical axis of the reflected beam; and in that first control means for changing the focal plane of said focus means using said third signal; and second control means for changing the radial tracking of said read/write beam relative to the disk (20) using said fourth signal are present. Arrangement as claimed in Claim 2, characterized in that said optical element further produces a third beam (68; 70) converging to form a third image on a third image plan, and further comprising a third detector means (80) responsive to changes in said third image to provide a signal related to modulation of said read/write beam by the disk (20). Arrangement as claimed in Claims 2 and 3, characterized in that said first detector means (74) comprises a quadrature detector (k, l, m, n) providing first, second, third and fourth detector outputs; said second detector means (76) comprises a quadrature detector (p, q, r, s) providing fifth, sixth, seventh and eighth detector outputs (Figures 4, 5). Arrangement as claimed in Claim 4, characterized in that said first means for combining comprises means for adding the difference of the sums of the first (k) and third detector (m) outputs and second (l) and fourth detector (n) outputs to the difference of the sums of the sixth (q) and eighth detector (s) outputs and the fifth (p) and seventh detector (r) outputs; and said second means for combining comprises means for adding the difference of the sums of said first (k) and fourth (n) detector outputs and said second (l) and third detector (m) outputs to the difference of the sums of said sixth (q) and seventh detector (r) outputs and said fifth (p) and eighth detector (s) outputs. Arrangement as claimed in Claims 2, 3, 4 and 5, characterized in that said single optical element is an astigmatizing element (60) (Figure 4). Arrangement as claimed in Claims 2, 3, 4, 5 and 6, characterized in that said first image plane and said second image plane are coplaner (72; 132). Arrangement as claimed in Claims 1, 2, 3, 4, 5, 6 and 7, characterized in that said optical element (60, 130; 138) comprises a zone plate (64; 130; 140). Arrangement as claimed in Claim 8, characterized in that the zone plate (130) comprises elliptical zone areas (Figure 8A). Arrangement as claimed in Claim 1, characterized in that said first beam (135) is astigmatically focused by said optical element to an image in an image plane (132); said second beam (137) being defocused by said optical element at said image plane; said first detector means (133) being arranged substantially in said image plane and providing a first signal varying in response to the shape of said image; said second detector means (e, f, g, h) being arranged substantially in said image plane and located substantially outside said image and producing a second signal in response to said second beam related to said degree of focus; and in that it comprises first control means for changing said degree of focus, responsive to said third signal (Figures 8, 8A; 10, 10B). Arrangement as claimed in Claim 10, characterized in that said second detector means (e, f, g, h) produces a fourth signal related to radial tracking of said read/write beam. Arrangement as claimed in Claim 11, characterized in that said first monitor beam (135) and said second beam (137) are coaxial. Arrangement as claimed in Claims 11 and 12, characterized in that said optical elements means comprises an elliptical zone plate (130) (Figure 8A). Arrangement as claimed in Claims 10, 11 and 12, wherein: said first and second detector means form a single compact substantially planar detection apparatus comprising first (e, f) and second pairs (g, h) of substantially adjacent optical detectors, and a quadrature detector (133), said quadrature detector lying substantially midway between said first pair (e, f) and said second pair (g, h) of optical detectors.

Inventors: LEE WAI-HON, LEE, WAI-HON
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: G11B7
IPC codes: G02B7, G11B7