Apparatus and method for detecting focus errors

Abstract: An apparatus and method is described for detecting focus errors in an optical head by positioning a prism (17) in the optical path of a return light beam (14) reflected from an optical recording medium (16). The prism reduces the beam in one dimension by a factor of M and concurrently increases the divergence/convergence angle associated with a focus-error of the beam by a factor of M in said dimension, thereby desirably enhancing the focus error signal (FES) by a factor of M². A focus error is detected by a segmented photodetector (19) having inner and outer photosensitive regions. The photo­ detector generates an electrical signal indicative of the focus error from the difference in light intensities at the inner and outer regions. The photodetector preferably is segmented in such manner as to also provide a track error signal.

Description: Technical Field This invention relates to methods and means for detecting focus errors in optical heads adapted to read and write data on optical recording media, and more particularly relates to improved means for generating a focus error signal for use as input to a servo system operative to keep an optical beam in focus on a surface. Background of the Invention Focus-error detection methods heretofore used for optical storage applications generally employ a knife edge, an astigmatic lens or a critical angle prism. These techniques require very critical alignment of these optical elements and of a split or segmented photodetector. Published European patent application EP 0164687 on which the two-part form of the appended independent claims is based, discloses a detection technique wherein a laser beam reflected from an optical disk is directed through an objective lens to a prism that reduces beam width in one dimension by a factor M and delivers an elliptical beam to a knife-edge-type focus error detection system. This application, following equation (25), claims to increase the focus error signal by a factor of M. The Digest of the Topical Meeting on Optical Data Storage, October 15-17, 1985 at Washington, D.C., includes Paper THCC2-1 by Yamamoto et al. entitled "Design Considerations of Optical Pregroove Dimension". This paper shows a six-element photodetector to detect the far field spot size variations associated with a focus error. There is a need for a very sensitive focus-error detection technique that will provide, with relatively few components, a significant enhancement of the focus-error signal and provide a large beam size that only requires an uncritical alignment of a segmented photodetector in one dimension. Summary of the Invention The present invention is defined in the attached claims. Toward this end and according to the invention, an apparatus and method are provided for detecting focus errors in an optical head by positioning a prism in the optical path of a return light beam reflected from an optical recording medium. The prism reduces the beam in one dimension by a factor of M and concurrently increases the divergence/convergence angle associated with a focus-error of the beam by a factor of M in said dimension, thereby desirably enhancing the focus error signal by a factor of M². A focus error is detected by a segmented photodetector having inner and outer photosensitive regions, such as shown in the Yamamoto et al. paper. The photodetector generates an electrical signal indicative of the focus error from the difference in light intensities at the inner and outer regions. The photodetector preferably is segmented in such manner as to also provide a track error signal. Brief Description of the Drawings Fig. 1 is a schematic diagram that illustrates the principle of the invention; Figs. 2A and 2B illustrate two preferred configurations of a segmented photodetector, both of which generate a focus error signal and Fig. 2B of which additionally generates a track error signal; Fig. 3 illustrates an optical head according to one embodiment of the invention for use with read-only, write-once or phase-change optical disks; Fig. 4 illustrates an embodiment for use with an optical disk drive employing a swing-arm actuator; and Fig. 5 illustrates an embodiment for use with magneto-optic disks. Description of Preferred Embodiments General The principle of the invention is best illustrated in Fig. 1. The output of a diode laser 10 is passed through a circularizer/collimator system 11 to provide a collimated beam 12. On reflection from a beam splitter 13, beam 12 becomes beam 14. Beam 14 is focused by a lens 15 onto surface 16 of an optical recording medium. When surface 16 is at the focus of lens 15, the collimated beam 14 from beam splitter 13 to lens 15 is retroreflected on itself and, as return beam 14, is transmitted by beam splitter 13 to a prism 17. Prism 17 refracts beam 14 as beam 18, which is directed to a photodetector 19. Beam 18 has an intensity pattern of elliptical configuration, with a major axis equal to D and a minor axis A. Prism 17 reduces the width of beam 14 by a factor of M (which is defined as D/A) to that of refracted beam 18. Assume now that the data on surface 16 is out of focus by an amount δz. Under this assumed condition, the beam 14′ reflected from surface 16 will, after passing through lens 15, diverge with an angle δϑ, in which δϑ = Dδz/f² where f is the focal length of lens 15. If δz is negative, then δϑ will be negative and the beam 14′ will converge. The following analysis considers the case of δz being positive, but when δz is small, the same analysis is valid for a diverging or converging beam 14′. The refracted beam 18′ emerging from prism 17 has a divergence angle of Mδϑ; and the width A′ of said beam on detector 19 is now given by A′ ≃ A + 2lMδϑ where l is the distance from prism 17 to photodetector 19. Using equation (1), one obtains A′ ≃ A + 2lMDδz/f² ≃ A + 2lm²Aδz/f² or A′/A ≃ 1 + 2lM²δz/f² It will thus be seen that, by inserting prism 17 in the path of the return beam 14′ from surface 16, the change in beam width A indicative of a focus error δz is enhanced by a factor M². M is determined by the apex angle of prism 17, and by the refractive index of the material from which it is made. M may be made arbitrarily large; however, the tolerance on alignment of the system becomes increasingly tight as M increases. A practical maximum value for a single prism 17 is approximately M=5; however, several prisms may be used in series and the resultant factor M would then be the product of the values of M for each prism. As illustrated in Fig. 2A, photodetector 24 comprises three photosensitive striped areas 1,2,3. Let I₁,I₂,I₃ be the electrical signals generated by the light incident on these photosensitive areas 1,2,3, respectively. For the configuration shown in Fig. 2A, let S₁ = I₁+I₃ and S₂ = I₂. Let kA be the width of central photosensitive area 2 where k(≃0.25) is chosen such that S₁=S₂ when beam 14 is in focus. If S ′ / 1 and S ′ / 2 are the signals corresponding to a focus error δz, it can be shown that S ′ 1 S₁ ≃ 2(A′-kA) A′+A-2kA S ′ 2 S₂ ≃ 2(1-k)A A′+A-2kA , and FES = S ′ 1 - S ′ 2 S₁ + S₂ ≃ A′ - A A′+A-2kA When A′≃A and k=0.25, FES ≃ 0.67(A′/A-1) ≃ 1.3lM²δz/f² Thus, by way of example, if M=5, l=50 mm and f=4 mm, then a small focus error δz=0.5 µm gives a focus error signal FES=0.05. This large error signal will allow detection of very small focus errors with high signal-to-noise ratio. Signals I₁ and I₃ are summed and amplified at 20 to provide output signal S₁, whereas signal I₂ is amplified at 21 to provide output signal S₂. Signals S₁ and S₂ are supplied to a differential amplifier 22 and a summing amplifier 26 whose outputs are supplied to divider circuit 28. The output of the divider circuit 28 is the focus error signal, FES. The focus error signal is applied to a focus servo control system 23 (Fig. 1). As illustrated, system 23 comprises conventional means (not shown) for adjusting the current in a coil 24 for thereby adjusting the position of lens 15 relative to surface 16. The preferred configuration of photodetector is shown in Fig. 2B. Photodetector 25 includes six photosensitive areas 1-6 for generating electrical signals I₁-I₆, respectively. The focus error signal is given by FES = S ′ 1 -S ′ 2 S₁+S₂ and the track error signal by TES = S₃-S₄ S₃+S₄ where S₁ = I₁ + I₃ + I₄ + I₆ S₂ = I₂ + I₅ S₃ = I₁ + I₂ + I₃ S₄ = I₄ + I₅ + I₆ The manner in which the track error signal is used to adjust track error is conventional and forms no part of the present invention. Fig. 2B is included merely to show that applicants' method and means for detecting and correcting focus errors is compatible with and desirably used in conjunction with a photodetector like 25 that generates both focus error and track error signals. Fig. 3 As illustrated in Fig. 3, the invention according to this embodiment comprises an optical head especially suitable for use with read-only, write-once or phase-change optical disks. A laser 30 emits a beam that is collimated by a lens 32 and circularized by refraction at surface 34 of a prism 46. Surface 34 has a polarizing beam splitter (PBS) coating. The beam 35 is directed through a quarter-wave plate 36 to a beam bender 38 and a lens 40 that focuses said beam onto a selectable track on an optical disk 42. The beam 35′ reflected from disk 42 returns through the wave plate 36 and is reflected as beam 37 from surface 34 toward surface 44 of prism 46. Refraction of beam 37 at surface 44 reduces the width of said beam by a factor of M from D to A and directs this elliptical beam 47 to a photodetector 48 that preferably is segmented as shown in Fig. 2A or 2B. If, as illustrated in Fig. 3, the data on the track of disk 42 is in focus, then the return beam 35′ will coincide with the beam 35, and the beam width at the photodetector 48 will be equal to A. If, however, the data on disk 42 is out of focus, then the return beam 35′ will diverge from 35, and as illustrated in Fig. 1, cause the width of the beam to the photodetector 48 to be greater than A. As described in connection with Fig. 2, photodetector 48 will generate a FES; and the position of lens 40 relative to the tracks on the disk 42 will be adjusted in response to this FES by suitable servo means (not shown) as necessary to put the data in focus. This embodiment requires a desirably low number of components. Fig. 4 Figs. 4A and 4B illustrate another embodiment of an optical head for use with read-only, write-once or phase-change disks. According to this embodiment, the general orientation of the head is parallel to the tracks on a disk 50. This head is therefore most easily adapted to a disk drive that employs a swing-arm actuator. A laser 52 (Fig. 4A) emits a beam that is collimated by a lens 54 and directed to and through a PBS 56 without deflection or reflection. Beam 58 from PBS 56 is elliptical in cross section, with a width D in one dimension and a width A in a direction orthogonal thereto. This beam 58 passes through a quarter-wave plate 60 and is directed to a prism assembly 62. At a first surface 64 of prism assembly 62, beam 58 is circularized, then reflected by total internal reflectance (TIR) from another surface 66 to and off a reflective surface 68 to an objective lens 70 that focuses the resultant beam 72 onto the disk 50. Beam 72 is reflected from a disk track as reflected beam 58′. Beam 58′, which again is elliptical in cross section, is reflected by a surface 74 of the PBS 56 and directed to a photodetector 76 that preferably is segmented as shown in Fig. 2A or 2B. If, as illustrated in Fig. 4, the data on the track of disk 50 is in focus, then the return beam 58′ will coincide with the beam 58, and the beam width at the photodetector 76 will be equal to A. If, however, the data on disk 50 is out of focus, then the return beam 58′ will diverge from 58, and as illustrated in Fig. 1, cause the width of the beam to the photodetector 76 to be greater than A. As described in connection with Fig. 2, photodetector 48 will generate a FES; and the position of lens 70 relative to the tracks on the disk 50 will be adjusted in response to this FES by suitable servo means (not shown) as necessary to put the data in focus. This embodiment requires that the expansion/compression ratio M of prism 66 must be matched to the emission pattern of laser 52. Fig 5 Fig. 5 illustrates still another embodiment of an optical head for use with magneto-optic disks. A laser 80 emits a beam that is collimated by a lens 82, circularized by refraction at surface 84 of a prism 86 and directed towards a beam bender 88 and an objective lens 90. Surface 84 is a partially polarizing beam splitter surface that directs a fraction of the p-polarized component of beam 92′ as reflected from a selected track on disk 94, and substantially all of the s-polarized component of said beam to surface 98 of a prism 96 as a beam 100. Surface 98 is also a partially-polarizing beam splitter surface. Surface 98 refracts a fraction of the p-polarized component of beam 100 as a beam 101 to a servo photodetector 102 preferably segmented as shown in Fig. 2A or 2B. This refraction causes a reduction in one dimension of beam 100 by a factor of M from D to A. Surface 98 also directs a fraction of the p-polarized component of beam 100 as well as substantially all of the s-polarized component of beam 100 as a beam 103 to a Wollaston prism 104, converging lens 106 and data detector 108. The function of the Wollaston prism in detecting the data on a magneto-optic disk is well known in the art and does not form part of the present invention. If, as illustrated in Fig. 5, the data on the track of disk 94 is in focus, then the return beam 92′ will coincide with the primary beam 92, and the beam width at the photodetector 102 will be equal to A. If, however, the data on disk 94 is out of focus, then the return beam 92′ will diverge from 92, and as illustrated in Fig. 1, cause the width of the beam to the photodetector 102 to be greater than A. As described in connection with Fig. 2, photodetector 48 will generate a FES; and the position of lens 90 relative to the tracks on the disk 94 will be adjusted in response to this FES by suitable servo means (not shown) as necessary to put the data in focus. It will now be seen that, in each of the configurations shown in Figs. 3-5, unlike those taught by the prior art, the focus error signal is desirably enhanced by a factor of M².

Claims: An apparatus for detecting focus errors (δz) in an optical head adapted to read and/or write data on an optical recording medium (16), comprising; at least one prism (17) positioned in an optical path of a return light beam (14) reflected from the medium for reducing the beam in one dimension by a factor of M and concurrently increasing the divergence/convergence angle (δϑ) associated with a focus error of said beam by a factor of M in said dimension; and a photodetector (19) located in said optical path for receiving a return light beam from the prism; characterised in that: the return light beam from the prism is unobstructed; the photodetector has an inner photosensitive region in the form of a stripe between two outer photosensitive areas located proximate one another along said dimension of said return beam, such that a focus error signal generated is zero when the signal (I₂) generated by the light incident on the inner region equals the sum of the signals (I₁ + I₃) generated by the light incident on both outer regions; and whereby both the change in said dimension and the increase in said divergence/convergence angle of said return light beam indicative of the focus error of the head is used by the photodetector to enhance the focus error signal by a factor of M². An apparatus as claimed in claim 1 wherein the regions in the photodetector (25) are divided in a direction perpendicular to the stripe to create three upper and three lower photosensitive areas from which another electrical signal indicative of the track error is generated from the difference in light intensities of the upper and lower areas. An apparatus as claimed in claim 1, including: means (10) for providing a primary beam of collimated light; and a focusing lens (15) that directs said primary beam onto the medium (16), such that the return beam coincides with the primary beam when said lens is in focus on the medium. An apparatus as claimed in claim 3, including: means (23,24), including servo means, responsive to the focus error signal resulting from a divergence or convergence of the return beam from the primary beam indicative of a focus error for adjusting the relative position of the lens (15) and medium (16) as necessary to eliminate the focus error. An apparatus as claimed in any of claims 1-4, wherein the optical head is used with an optical disk having a plurality of tracks for storing data, said head comprising: prism means (46) including a polarizing beam splitter for circularizing the beam by refraction; and means including a quarter-wave plate (36), beam bender (38) and lens (40) to focus the circularized beam on a selected track. An apparatus as claimed in claim 1, further comprising: means (52,54) for providing collimated light; a quarter-wave plate (60); said prism (62) having one surface (64) for circularizing the light transmitted through the quarter-wave plate, a second surface (66) from which the circularized light is reflected by total internal reflectance, and a third surface (68) which is reflective; lens means (10) for focusing the beam (12) reflected from said third surface onto the disk (50); and a polarizing beam splitter (56) for directing said return beam (58′) emitted by said prism means to the photodetector (96). An apparatus as claimed in claim 1, further comprising: means (80,82) for providing a beam of collimated light; first prism means (96) for circularizing the beam by refraction; means including a beam bender (88) and lens means (90) via which the circularized beam is directed to a selected track on the disk (94); second prism means (96); said first prism means having a partially polarizing beam splitter surface (84) for directing a fraction of the p- polarized component of the return beam (92′) reflected from the disk and substantially all of the s-polarized component of said return beam to said second prism means; and a photodetector (102) for generating a signal in response to a focus error of said return beam; said second prism means having a partially-polarizing beam splitter surface (98) for refracting a fraction of the p-polarized component of the last-named beam to said photo-detector for causing the last-named fraction to constitute a reduction of the return beam in one dimension by a factor of M and concurrently increasing the divergence/convergence angle (δϑ) associated with a focus error of said beam by a factor M in said dimension, thereby enhancing said signal by a factor of M². A method for detecting focus errors (δz) in an optical head to which a light beam is directed for reading and/or writing data on an optical recording medium (16), the method comprising; positioning at least one prism (17) in an optical path of a return light beam (14) reflected from the medium for reducing the beam in one dimension by a factor of M and concurrently increasing the divergence/convergence and (δϑ) associated with a focus error of said beam by a factor of M in said dimension; and providing a photodetector (19) located in said optical path for receiving a return light beam from the prism; characterised in that: the return light beam from the prism is unobstructed; the photodetector has an inner photosensitive region in the form of a stripe between two outer photosensitive areas located proximate one another along said dimension of said return beam, such that a focus error signal generated is zero when the signal (I₂) generated by the light incident on the inner region equals the sum of the signals (I₁ + I₃) generated by the light incident on both outer regions; whereby both the change in said dimension and the increase in said divergence/convergence angle of said return light beam indicative of the focus error of the head is used by the photodetector to enhance the focus error signal by a factor of M². A method as claimed in claim 8 wherein the regions in the photodetector (25) are divided in a direction perpendicular to the stripe to create three upper and three lower photosensitive areas from which another electrical signal indicative of the track error is generated from the difference in light intensities of the upper and lower areas. A method as claimed in claim 8, including the further step of adjusting the position of a focusing lens within the optical path in response to said signal as necessary to maintain the light intensity detected at said inner region equal to the sum of the light intensities detected at said outer regions and thereby eliminate said focus error.

Inventors: DEWEY ANTHONY GEORGE, LENTH WILFRIED, RUGAR DANIEL, DEWEY, ANTHONY GEORGE, LENTH, WILFRIED, RUGAR, DANIEL
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11B7
IPC codes: G02B7, G11B7