Abstract:
An optical system, preferably used in an optical data recorder, has a laser light source emitting a light beam having an ovate or elliptical cross-section. An astigmatic optical element, such as a hemicylinder or cylindrical lens, disposed at 45 degrees with respect to the major axis, also termed the beam ellipse axis, in front of a focus detector eliminates focus offset errors caused by beam ellipticity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical systems employing lasers, and more particularly to focusing systems adaptable for use with optical information-bearing signal recorders. 
     2. Description of the Prior Art 
     Most optical systems, including optical information-bearing signal recorders, require that the light beam be focused at a predetermined focal point. In optical information-bearing signal recorders, it is required that the laser beam be focused at the recording surface of the optical record member. There are several schemes available for detecting that focus condition which create an error signal suitable for driving an optical system to a desired focus. One scheme for focus detection is to introduce astigmatism into the optical path immediately in front of a focus detector. In such a system, a focus detector usually includes a plurality of closely spaced-apart photodetectors. Quite commonly, a so-called quadrant detector consisting of four photodetectors disposed as a rectangular array is employed. The astigmatism is often introduced through the use of a cylindrical lens mounted at the quadrant detector. The astigmatism causes changes in shape of the beam impinging upon the focus detector in accordance with the direction and degree of out-of-focus condition. Generally, the astigmatic axis of the cylindrical lens is disposed at 45 degrees with respect to the axes of the quadrant detector and a tangential length of a record track being scanned. The below listed U.S. patents are exemplary of such a focus detection system, including disposing a cylindrical lens at 45 degrees with respect to the axes of a quadrant detector: 4,280,215; 4,296,316; 4,163,149; 4,293,944; 4,123,652; 4,290,132; 4,273,998; and 4,287,413. 
     The precise operation contemplated in astigmatic focusing systems, as above described, contemplates a circularly cross-sectioned light beam. For example, in FIG. 3b of U.S. Pat. No. 4,280,215, the illustrated gas lasers emit circularly cross-sectioned light beams; however, solid-state or semiconductive lasers tend to emit ovate-shaped light beams. Further, such semiconductive lasers emit ovately, cross-sectioned light beams that vary in their respective aspect ratios; that is, the length of the major axis with respect to the length of the minor axis varies from laser to laser. The optical asymmetry associated with an ovately, cross-sectioned-shaped light beam results in offset errors of focus; that is, a somewhat out-of-focus condition will be indicated as being in focus when an ovate-shaped light beam is used in focusing arrangements. As a result, the light beam emitted from solid-state lasers have been made circular in cross-section, apparently to avoid the offset problem. For example, see U.S. Pat. Nos. 4,411,500; 4,235,507; 4,272,651; 4,334,173; 4,397,527, and others. The circularization of an ovate light beam has not always provided a true circle. As a result, some offset errors still occur. While in many optical information-bearing signal recorders a small focus offset error may be acceptable, as optical recorders employ higher and higher areal recording densities such offset errors in focus become unacceptable. Further, the circularization of light beams require higher emitted energy from the laser than without circularization, i.e., many circularization techniques truncate the peripheral energy levels of the light beam during circularization. It is also desired to enable maximal laser energy to be used throughout an optical system, therefore such truncation of energy levels should be avoided. 
     U.S. Pat. No. 4,464,741 shows a pair of astigmatic lenses disposed at 90 degrees with respect to each other for accommodating asymmetry in the response of the focus detector. That is, in one direction of out-of-focus condition, focus detector could indicate for a given degree of out-of-focus condition different focus error signals. The crossed astigmatic lenses re-establish symmetry. The resulting symmetry apparently results because the images of the pupil of the objective system and its two focal planes are not symmetrical relative to the plane of the detectors. Apparently, this latter system requires precise spacing of the crossed lenses from the detector plane. 
     Therefore, it is still desired to maximize energy throughput of an optical system by accommodating an ovate-shaped light beam having various varying aspect ratios with a minimal number of optical components. It is desired that the construction of such an optical system be insensitive to precise positioning along the light beam path as well as accommodate ovate light beams having different aspect ratios. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an optical system that maximizes usage of emitted light beams and yet reduce offset errors caused by non-circular, cross-sectionally-shaped beams. 
     In accordance with the invention, in an optical system, astigmatism along an axis disposed at 45 degrees with respect to an ellipse axis of a light beam being processed by optical elements is introduced into the optical system for removing offset errors introduced by ellipticity of the light beam. In a simple form of the invention, a hemicylinder lens is introduced into the optical system having an ellipse axis disposed at 45 degrees with respect to the astigmatic axis of the light beam being processed in the optical system. 
     The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompany drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 diagrammatically illustrates an optical data recorder employing the present invention. 
     FIG. 2 graphically shows the variations in focus operation of the FIG. 1 illustrated recorder by practicing the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawing, optical information-bearing signal data recorder 10 records optical signals onto and recovers optical signals from optical data record or disk 11. The form of recording on disk 11 can be any optically sensible recording, such as ablative using pits or bubbles, phase change which causes reflectance changes in an active recording layer, surface magneto-optic type, etc. The recording and recovery of optical signals can use any known technique. Numeral 12 diagrammatically represents a head-supporting arm of a disk-type optical signal recording. Such a head arm generally is movable radially with respect to record disk 11, and faces the optical surface of the record disk 11 and spaced from the disk at a distance 13, referred to as the focus distance &#34;D&#34;. 
     A solid-state or semiconductive laser 14 emits an ovate cross-section light beam 17 diagrammatically illustrated at 15 along an axis 16 toward optical system 12. While the shape of the emitted beam may not be a perfect ellipse, for purposes of analysis and for practicing the present invention, the ovate shape 17 has a major axis 18 (also referred to as an ellipse axis) and a minor axis 19. It has been found that by disposing an astigmatic element, later described, at 45 degrees with respect to either the major or minor axes of the ovate-shaped beam 15, that substantially all focus offset errors are accommodated irrespective of the aspect ratio or size of the oval, i.e., the relative length of major axis 18 to the length of minor axis 19. Such an arrangement eliminates the need for precisely calibrating each and every recorder constructed and eliminates the need for changing the shape of the beam from ovate to circular. 
     Optical system 12 is constructed in the usual manner. It includes a beam splitter 25 optically coupled to an objective lens 26. Splitter 25 and objective lens 26 direct the laser 14 emitted beam 15 to record disk 11 along an axis 28 as an ovate-shaped beam 29. Quarter-wave plate 27 is optically interposed between splitter 25 and objective lens 26 for providing optical isolation between the beam reflected from record disk 11 to optical system 12 and for directing the reflected beam along an optical axis 35 for detecting the focus condition of beam 28 at disk 11. The light beam travelling along axis 35 maintains its ovate shape. In a magneto-optical recorder (not shown), the arrangement of optical elements is substantially different, as is known. 
     Hemicylinder lens 36, an astigmatic optical element, is disposed symmetrically with respect to axis 35 and has an astigmatic axis 37 disposed at about 45 degrees as measured in a plane (not shown) disposed orthogonally to axis 35 with respect to the major axis 18 of the ovate-shaped beam projected along axis 35. Disposed close to hemicylinder lens 36 is a so-called quad or quadrant detector 40 having four photodetectors A, B, C, and D disposed in a plane orthogonal to axis 35. These four photodetectors can be of any geometric shape. Quad detector 40 includes an axis 41 disposed along track line 42 of a record track (not shown) on record disk 11. In this manner, quad detector 40 can also be used for track following, as is known. Quad detector 40 also includes a minor axis 43. The intersection of axes 41, 43 is aligned with axis 35. Detector circuits 44 are electrically coupled to quad detector 40 in a usual manner for detecting focus, track-following signals, and for detecting data. Of course, separate detectors can be used. 
     The operation of the invention is best understood by referring to the graph of FIG. 2. Abscissa 50 represents the distance D between the effective focal center of objective lens 26 and the optical recording surface of record disk 11. Ordinate 51 represents the focus condition representing focus error signal amplitude and polarity. The intersection of the ordinate and the abscissa represent an in-focus condition while points above the abscissa represent focus error signals indicating a too-close, out-of-focus condition, while points below the abscissa represent focus error signals indicating a too far, out-of-focus condition. An ideal focus error curve 52 is disposed symmetrically about the ordinate and abscissa and intersects the ordinate and abscissa at their intersection. When the astigmatic optical element 36 is disposed at 45 degrees with respect to the major axis 18, and hence, the minor axis 19, focus error signals follow curve 52 and as indicated by the numeral 45°. Measurements have shown that when the lens 36 is disposed at 25 degrees with respect to the astigmatic axis 18, curve 53 is generated as the focus error signal. This results in an offset indicated by double-headed arrow 54 such that a false focus condition is indicated. It should be noted that curve 53 corresponds to lens 36 being disposed at 45 degrees with respect to axis 41 of quad detector 40. In a similar manner, measurements have shown that when astigmatic lens 36 is disposed at 75 degrees with respect to the major axis 18, the focus error signal is indicated by curve 55. The offset error is in the reverse direction as indicated by double-headed arrow 56. Accordingly, it is seen that even disposing the astigmatic optical element at precisely 45 degrees with respect to the axis 41 of quad detector 40 as taught in the prior art but not necessarily at 45° with respect to the major axis of an ovate light beam does not remove the offset errors 54, 56 required for high-areal density, optical data recording. The invention applies equally to reflective or transmissive record disks 11 and other optical targets. 
     It is preferred that astigmatic lens 36 be glued to detector 40, then the unit assembly of lens 36-detector 40 can be adjusted so that the lens 36 axis is disposed at 45° to major axis 18. Using this subassembly approach further reduces offset errors. For example, whenever the detector 40 inadvertently moves transversely with respect to axis 35 an offset error is introduced; i.e., the center of detector 40 which is the intersection of its two axes 41, 43 is not aligned. It has been found that if both lenses 36 and detector 40 move together, any resulting offset error is reduced. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.