Abstract:
A magneto optic or optical disk drive comprises a laser source for providing laser beam, and a lens or compound lens for focussing the laser onto a currently inserted recording disk with specific overcoat thickness. In one embodiment, the lens is a compound lens that can focus aberration-free spots on different types of disk media having recording layers located under overcoats of different thicknesses. When a specific disk is inserted, only one spot in focussed onto the current recording layer. Furthermore, if the disk spins rapidly, it can wobble. The lens or compound lens comprises different regions where the focal length may vary slightly and continuously or discretely. Because of this, as the disk wobbles, the lens can nonetheless continue to focus laser light onto a small spot on the recording media, providing increased depth of focus in the vicinity of the currently inserted recording media. Furthermore, the increased depth of focus will also remedy problems related to the change in focal length of the lens with changes in wavelength of the laser source. Because some of these changes in wavelength occur abruptly, this is also a case in which the limited performance of the auto-focus tracking system would be overly burdened.

Description:
BACKGROUND OF THE INVENTION 
     This invention pertains to lenses. In particular, this invention pertains to lenses that can be used to focus a laser beam on a small spot, or on two spatially separated small spots, but simultaneously provide large depth of focus. Such a lens is particularly useful in optical and magneto-optical disk drives, and drives that read and write to different disks with different overcoat thicknesses. 
     Referring to FIG. 1A, a prior art optical or magneto-optic disk drive typically comprises a laser source  1  for providing a laser beam  2 , an optical or magneto-optic data recording disk  3 , and a lens  4  for focussing the laser on a small spot on disk  3 . A motor  5  rotates disk  3  so that laser beam  2  can scan over a data recording track of the disk. In such applications, the depth of focus is an important parameter which is preferably maximized. In conventionally designed optics, the depth of focus D is given by the following relationship: 
     
       
           D =0.8λ/( NA ) 2   
       
     
     where NA is the numerical aperture of the lens and λ is the laser light wavelength. 
     In optical and magneto-optic data storage, one also generally strives for high data recording density. This means that the spot size produced by the focussing lens should be as small as possible. To reduce the spot size of a given design, one increases the NA of the lens. However, as can be seen from the above equation, this has the detrimental effect of reducing the depth of focus D. This detrimental effect is especially important in the case of drives in which the disk spins rapidly, e.g. 40× drives. (The nomenclature “40×” is part of a numbering system which is well known in optical storage and refers to a rotation rate 40 times that of standard audio CD players. An audio CD player spins at 1×.) Because disk  3  spins rapidly, it can sometimes wobble, e.g. moving in the direction of arrow A, causing disk  3  to move closer to or further from lens  4 , and causing the laser spot to be out of focus. In such drives, a shallow depth of focus puts an unmanageable burden on the frequency response of the auto-focus system of the drive. Accordingly, it would be desirable to provide a lens with a relatively large depth of focus and a small spot size. 
     There are several types of optical and magneto-optic media known in the art. FIG. 1B shows a prior art laser beam  6  passing through a lens  7  to read data from a DVD disk  8 . DVD disk  8  comprises a transparent substrate  9  over a recording layer  10 . Typical transparent substrates  9  on DVD recording media have a thickness T 1  of about 600 microns. 
     FIG.  1 B′ shows laser beam  6  passing through a lens  11  to read data from a CD disk  12 . CD disk  12  comprises a transparent substrate  13  over a recording layer  14 . Typical transparent substrates  13  on a CD disk have a thickness T 2  of about 1200 microns. Thus, in order to read both CD disks and DVD disks, an optical disk drive must have two lenses—one for reading CD media and one for reading DVD media. Such drives typically include a mechanism for switching lenses. Such a mechanism is cumbersome and adds expense to the disk drive. Further, it is difficult to substitute one lens for the other while continuing to maintain precise spacing between the lens and the media. Accordingly, it would be desirable to provide a lens capable of reading data from (or writing data to) different types of recording media having different substrate thicknesses. 
     SUMMARY 
     A lens in accordance with the invention comprises regions having at least two different optical phase functions implementing two different focal lengths, which are spatially multiplexed over the lens aperture. The lens is used to produce from a single incoming laser beam at least two converging beams, but produce only one extended depth of focus spot on a currently inserted rotating data recording disk, for a given overcoat thickness. Thus, even if the disk wobbles, and moves closer to or further from the lens, the lens will continue to focus one aberration-free small spot onto the recording layer of the current disk inserted in the drive. 
     In one embodiment, a lens in accordance with our invention accommodates types of recording media having transparent substrates thereon of different thicknesses. For example, in one embodiment, the lens has a first region having a first focal length for accommodating recording media having a transparent substrate of a first thickness. The lens also has at least a second region having a second focal length for accommodating recording media having a transparent substrate of a second thickness. Thus, one can use one lens to read data from (or write data to) different types of recording media. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1A shows a lens focussing a laser beam on a recording medium in accordance with the prior art. 
     FIGS.  1 B and  1 B′ show a lens focussing a laser beam on two different types of optical recording media having different transparent overcoat thicknesses. 
     FIG. 2 illustrates a blazed diffractive lens constructed in accordance with the teachings of PCT Patent Application WO 98/52101. 
     FIG. 3 schematically illustrates in plan view a lens constructed in accordance with a first embodiment of our invention having different regions of various focal lengths. FIG. 3 shows how these regions can be arranged within the lens aperture. 
     FIG. 4 illustrates in plan view a blazed lens constructed in accordance with the first embodiment of our invention, comprising a centrally obscured area and 16 circumferential periods of a continuously varying focal length. 
     FIG. 4A is a magnified view of fringes within the lens of FIG.  4 . 
     FIG. 5A is a perspective view of the lens of FIG.  4 . 
     FIG. 5B shows a binary (two phase levels) phase lens constructed in accordance with another embodiment of our invention. 
     FIG. 5C shows a four phase level lens constructed in accordance with another embodiment of our invention. 
     FIG. 6 shows a lens constructed in accordance with another embodiment of our invention in which portions of a transparent substrate are doped to locally alter the refractive index of the doped portions. 
     FIGS. 7A to  7 C illustrate in cross section the lens of FIG. 6 during manufacturing. 
     FIGS. 8A and 8B shows a binary amplitude lens constructed in accordance with another embodiment of our invention. 
     FIG. 9 shows a numerically reconstructed intensity pattern along the optical axis for a blazed Fresnel zone plate lens. 
     FIG. 10 shows a numerically reconstructed intensity pattern along the optical axis for a blazed lens constructed in accordance with the teachings of PCT Patent Application WO 98/52101. 
     FIG. 11 shows a numerically reconstructed intensity pattern along the optical axis for a blazed lens constructed in accordance with the first embodiment of our invention. 
     FIGS. 12A and 12B show how a single lens constructed in accordance with the invention can be used in conjunction with two different types of media having two different protective overcoat thicknesses. 
     FIGS. 13A and 13B schematically show in plan view and side view, respectively, a doughnut spatial multiplexing geometry for implementing a compound lens that focuses two aberration-free spots. 
     FIGS. 14A and 14B schematically show in plan view and side view, respectively, a pie chart multiplexing geometry for implementing a compound lens that focuses two aberration-free spots without side lobes. 
     FIGS. 15A and 15B schematically show in plan view and side view, respectively, a dartboard multiplexing geometry for implementing a compound lens that focuses two aberration-free spots without side lobes and with a high Strehl ratio. 
     FIG. 16A shows a low NA lens and FIG. 16B shows a high NA lens within the dartboard multiplexing geometry lens schematically shown in FIGS.  15 A and  15 B. 
    
    
     DETAILED DESCRIPTION 
     A lens constructed in accordance with the invention comprises regions of different focal lengths. In one embodiment, the lens includes abrupt changes in focal length between the regions. In another embodiment, the focal length changes smoothly and slowly from one region to the next. The lens can be constructed using a number of techniques. In one embodiment, the manufacturing steps for constructing the lens can be as described in PCT Patent Application WO 98/52101, incorporated herein by reference. The lens can also be constructed using the manufacturing steps described in U.S. patent application Ser. No. 08/833,608, entitled “Optical Head with a Diffractive Lens”, or U.S. patent application Ser. No. 09/059,778, entitled “Method for Making an Optical or Magneto-Optic Head and the Resulting Head”. The &#39;608 and &#39;778 Applications are incorporated herein by reference. 
     FIG. 2 illustrates a diffractive lens  20  constructed in accordance with the above-mentioned PCT Patent Application WO 98/52101. This lens comprises a structure known as a blazed zone plate. In particular, lens  20  comprises a transparent substrate  22 . Etched into substrate  22  is a set of grooves  24 . The upper surface  26  of substrate  22  thus contains grooves  24  and a set of ridges  28  that diffract light and cause this structure to serve as a diffractive focussing lens. Lens  20  exhibits a focal length L. Thus, lens  20  causes laser light striking the lens to be focussed onto a small spot S at length L from lens  20 . Length L depends upon the spacing of grooves  24  and ridges  28 . If lens  20  is used in conjunction with recording media such as media  30 , and media  30  wobbles due to high rotational velocity, media  30  will occasionally move away from spot S. 
     FIG. 3 schematically illustrates in plan view a modified lens  32  in accordance with our invention, comprising regions R 1  through Rn. Of importance, the ridges and grooves are spaced in region R 1  so that region R 1  of lens  32  exhibits a first focal length L 1 . In region R 2 , the ridges and grooves are spaced so that region R 2  exhibits a second focal length L 2 . Similarly, in region R 3 , the ridges and grooves are spaced so that region R 3  exhibits a third focal length L 3 , and so forth. As a result of this novel structure, if media  30  wobbles, the different regions R 1  to Rn ensure that regardless of such wobbling, laser light will continue to be focussed on a small spot on media  30 . 
     In one embodiment, there are discrete breaks between the focal length of regions R 1  to Rn. In other words, there is an abrupt discontinuity in the spacing of the ridges and grooves in region R 1  compared to region R 2 . Similarly, there is an abrupt discontinuity in the spacing of the ridges and grooves in region R 2  compared to region R 3 , and so forth. These discontinuities are typically of very low magnitude. In another embodiment, the spacing of the grooves and ridges in the lens can vary continuously from region R 1  to Rn. Furthermore, the variation from R 1  to Rn can be reversed in order to periodically vary from the spacing of region R 1  to the spacing of region Rn and then again to R 1 . This periodic variation can be carried out once, or a number of times through the entire lens aperture. Two periods are depicted in FIG.  3 . 
     FIGS. 4 and 5A illustrate in plan view and perspective view, respectively, an example of such a lens. In FIG. 4, as one proceeds in a circumferential direction around lens  32 , the ridges and grooves move inwardly (towards the center of the lens) and outwardly (away from the center of the lens). Those portions of lens  32  in which the grooves and ridges are closer to the center of the lens exhibit a smaller focal length, whereas those portions of lens  32  in which the grooves and ridges are further from the center of lens  32  exhibit a larger focal length. The net result of this unique structure is that if the lens is used in conjunction with recording media that wobbles, lens  32  can continue to focus laser light onto a small spot on the media. 
     FIG. 4 also shows that a centrally located portion  34  of lens  32  is opaque and occluded. This causes a reduction in the laser spot size provided by the lens, for reasons discussed in the above incorporated &#39;608 application. 
     The pattern of ridges and grooves are typically formed by applying a mask on a transparent substrate, patterning the mask to form the pattern of ridges and grooves in the mask, and transferring that pattern of ridges and grooves from the mask into the substrate. Details concerning such a manufacturing process are described in the above-mentioned &#39;608 and WO 98/52101 applications 
     As mentioned above, a lens in accordance with our invention can be formed in several ways. For example, instead of being formed with a blazed zone plate, e.g. as shown in FIG. 5A (see also FIGS.  4  and  4 A), the lens can be formed as a binary zone plate or a multilevel phase relief zone plate, e.g. as shown in FIG. 5B (a binary version having two phase levels) and FIG. 5C (a quaternary version, or four phase levels). Referring to FIG. 5B, lens  100  comprises alternating recessed and non-recessed regions  102  and  104  formed in a transparent substrate  106 . These alternating recessed and non-recessed regions form a lens as described in the above-incorporated &#39;608 application. However, the lateral spacing of these recessed and non-recessed regions varies as one proceeds about the circumferential direction of the lens. Because of this, different portions of lens  100  exhibit different focal lengths. The focal length of lens  100  varies continuously and smoothly (here 8 times for one circumferential revolution). In another embodiment, the variation of focal length can take discrete jumps instead of continuously increasing or decreasing, and thus produce abrupt phase discontinuities as one moves around the lens in the circumferential direction. 
     FIG. 5C is similar to FIG. 5B except that lens  100 ′ of FIG. 5C comprises regions having four phase levels (i.e. alternating bands of four different heights). These phase levels also form a diffractive lens. However, the lateral spacing of these phase levels varies as one proceeds about the circumferential direction of the lens. Because of this, different portions of lens  100 ′ exhibit different focal lengths in the same manner as the lens of FIG.  5 B. As with lens  100  of FIG. 5B, the focal length of lens  100 ′ of FIG. 5C varies continuously and smoothly. However, in another embodiment, the variation of focal length can be abrupt as well. 
     The lenses of FIGS. 5B and 5C function because light striking the regions of different heights (or depths) emerge from the lens at different phases. The light from the different regions forms an interference pattern that effectively focuses light onto a small spot on data recording media. 
     The lenses of FIGS. 5B and 5C can be manufactured by applying a mask to a transparent substrate, patterning the mask to expose portions of the substrate, and etching the exposed portions of the substrate. The mask is then removed, leaving a set of etched (recessed) and non-etched (unrecessed) regions. (See the &#39;608 and &#39;778 applications.) In the case of FIG. 5C, several masking and etching steps are required. 
     In an alternate embodiment, instead of altering the phase of light emerging from the lens by etching recessed regions into substrate  106 , the phase of the light emerging from the different portions of the lens can be modified by diffusing a dopant into portions of the substrate to locally increase the refractive index at those portions of the substrate. An example of such a lens is lens  150  of FIG.  6 . 
     Referring to FIG. 6, lens  150  comprises regions  152  and  154  in a transparent substrate  156 . Regions  152  comprise intrinsic substrate material, i.e. substrate material that is not doped. However, regions  154  comprise substrate material that has been doped with an impurity that locally increases the refractive index of the substrate material. Therefore, instead of achieving phase modulation of the light emerging from the lens by forming a relief pattern in the substrate as shown in FIGS. 5B and 5C, in FIG. 6, this phase modulation is achieved by doping portions of the transparent substrate. In one embodiment, the substrate is formed from glass, and the dopants used to achieve this effect are a silver-containing material, e.g. silver or a salt such as silver nitrate. 
     FIGS. 7A to  7 C illustrate the manner in which lens  150  can be manufactured. In particular, in FIG. 7A, transparent glass substrate  156  is covered with a layer of e-beam resist  158 . E-beam resist layer  158  is patterned in a conventional manner by subjecting resist  158  to an e-beam and then developing the resist, thereby exposing portions of substrate  156 . Dopants are then introduced into the exposed portions of substrate  156 , e.g. by ion implantation, to locally modify the refractive index at portions  154  of substrate  156  (FIG.  7 B). E-beam resist layer  158  is then removed (FIG.  7 C). 
     In an alternative embodiment, a Cr layer is sputtered onto substrate  156  before depositing e-beam resist layer  158 . After e-beam resist layer  158  is patterned, the pattern in e-beam resist layer  158  is transferred to the Cr layer. The Cr layer serves as a mask during the above-mentioned ion implantation step. After ion implantation, the Cr layer and e-beam resist layer  158  are both removed. (In lieu of Cr, other masking materials can be used as well.) 
     In yet another embodiment, the silver-containing dopant can be introduced into substrate  156  using a method other than ion implantation. For example, in one embodiment, substrate  156  (along with a patterned mask formed thereon) can be immersed in a heated bath containing a silver salt, such as silver nitrate. Silver can diffuse into the exposed portions of the substrate from the bath to locally increase the refractive index of the substrate. The bath can be under pressure in this embodiment. 
     FIGS. 8A and 8B illustrate a diffractive lens  160  constructed in accordance with another embodiment of our invention comprising alternating transparent and opaque regions  162  and  164 . (Opaque regions  164  of lens  160  are formed by depositing an opaque material such as Cr on a transparent substrate  166  and lithographically patterning the Cr so that the Cr covers regions  164 .) Of importance, regions  162  and  164  form a diffraction pattern that causes lens  160  to focus light onto a small spot. However, the spacing of regions  162  and  164  increases and decreases as one proceeds circumferentially around lens  160 . Thus, different regions of lens  160  exhibit different focal lengths. Lenses comprising alternating opaque and transparent regions are described in more detail in the above-incorporated &#39;608 application. 
     FIG. 9 represents a numerical reconstruction of the focal spot in a plane along the optical axis of a lens. A collimated laser beam  71  is diffracted by lens  72  and focused into a spot  73  onto recording media  74 . The lens considered in FIG. 9 is a simple Fresnel lens  72 . The depth of focus is short, and thus wobbling of the media  74  can only occur with small amplitude along the arrow A 1 , or the lens will no longer provide a small spot on the media. 
     FIG. 10 represents a numerical construction of the focal spot in a plane along the optical axis for a lens  82  constructed in accordance with the above mentioned PCT Patent Application WO 98/52101. Referring to FIG. 10, the depth of focus of the resulting spot  83  is somewhat increased compared to FIG. 9, allowing the media  74  to wobble within a greater range along arrow A 2 . 
     In FIG. 11, lens  92  has been constructed in accordance with our invention, and produces a spot  93  with extended focal depth, thus allowing media  74  to wobble along arrow A 3  without adversely affecting the reading/writing performance of the disk drive. The focal length variation in the last case (FIG. 11) is of about 0.5%, thus enjoying a focal depth of about 7 microns for a mean focal length of 1500 microns. The focal length variation should not be increased too much since this would result in both a dramatic intensity drop at the recording media, and creation of successive spots along the optical axis. 
     In one embodiment, the lens provides a spot of laser light having a diameter of less than about 0.5 microns, and preferably between 0.25 and 0.45 microns. However, other spot sizes can be used in accordance with our invention. 
     The lens can be used in conjunction with different kind of lasers. In one embodiment, laser light having a wavelength of 650 nm is used. Alternatively, a laser diode having a wavelength of 405 or 407 nm can be used. (As explained in the above incorporated PCT and &#39;608 applications, if a diffractive lens is used in conjunction with our invention, the spacing of the various ridges and grooves, recessed and non-recessed regions, or opaque and transparent regions depends upon the wavelength of the laser light.) 
     As is known in the art, laser wavelengths produced by a diode laser can vary over a given range. Since the focal length of a diffractive lens is a function of the laser wavelength, the variation of focal length of different portions of the lens allows the diffractive lens to accommodate such variation in laser wavelength without losing the ability to provide a small focused spot of laser light on the media. 
     We have now explained how one can increase the focal depth of a single focal spot, and how the phase function of a diffractive lens can be defined by forming appropriately spaced ridges and grooves, recessed and non-recessed regions, opaque and transparent regions, or regions of different refractive indices. 
     In another embodiment of our invention, an optical compound lens produces not only one spot, but two or more focal spots in order to be able to read and write over several different disk media types which may have different substrate thicknesses. 
     FIG. 12 shows how a compound lens  180  can receive a laser beam  181  and generate two separate spots S 1 , S 2  from beam  181 . Spots S 1 , S 2  can be used to read from and write to optical or magneto-optic media, e.g. disks  182 ,  183 , with substrates  184 ,  186  of different thicknesses. T 1 , T 2  over recording layers  188 ,  190  respectively. Lens  180  compensates for two different spherical aberrations associated with substrates  184 ,  186 . (Spherical aberration occurs when converging light from a lens passes through a planar structure such as the substrates of disks  182 ,  183 . Thus, there is a certain amount of spherical aberration associated with each of the substrates on disks  182 ,  183 .) Lens  180  of FIG. 12 uses spatial multiplexing of two separate lens functions that have been calculated individually. In other words, the gratings on one portion of lens  180  compensate for the spherical aberration for substrate thickness T 1 , while the gratings on the other portion of lens  180  compensate for the spherical aberration for the other substrate thickness T 2 . Both of these lens functions can use the previously described technique to increase the depth of focus of each spot individually. 
     In one embodiment a computer software package can be used to calculate the position of the gratings. One such package is called Code V, available from Optical Research Associates in Pasadena, Calif. Code V can be used to optimize the lens aspherical phase function. Another software package is DOE-CAD, available from Diffractive Solutions of Strasbourg, France. DOE-CAD can be used to calculate the position of the gratings and generate an e-beam lithography file for lens fabrication. 
     (It should be noted that as used herein, the term “gratings” includes diffraction structures that are curved, circular, or have other shapes.) 
     Several different geometries can be used in order to spatially multiplex the two different lens functions. The doughnut shaped aperture geometry  201  in FIG. 13 can handle both a small NA lens  202  within a central region and a high NA lens  203  in an outer region. Small NA lens  202  can be used in order to accommodate a thick overcoat  205  and the high NA lens  203  can be used to accommodate a thin overcoat  204 . Thus, the combination of the two lenses within a single circular aperture produces a compound lens (or dual lens) that can focus a single collimated, diverging or converging beam  210  into two aberration-free spots  206  and  207  on recording media through two different overcoat thickness  204  and  205 . Of importance, the spacing of the ridges and grooves of lenses  202 ,  203  (for the case of a blazed lens) can vary to increase the depth of focus for each of these lenses. (For the case in which lenses  202 ,  203  are phase or zone lenses, the recesses or opaque regions can likewise be varied to increase the depth of focus for these lenses.) Lenses  202 ,  203  are formed on a single transparent substrate (typically simultaneously) using a manufacturing process as described above. 
     Of importance, if the structure of FIG. 13 is used in conjunction with recording media having a relatively thick substrate, the focal spot of lens  202  is located above and sufficiently far from the media recording layer so as to not interfere with reading and writing to the recording layer. Similarly, if the structure of FIG. 13 is used in conjunction with recording media having a relatively thin substrate, the focal spot of lens  203  will be below and sufficiently far from the media recording layer so as to not interfere with reading and writing to the recording layer. 
     FIG. 14 shows another geometry  301  of spatial multiplexing for two lenses that resembles a pie chart, where different sectors of a central pie chart have a low NA lens function  302  and the interleaved sectors and the external portion have a high NA lens function  303 . The compound lens operation is similar to the one described in FIG.  13 . This geometry has however the advantage of reducing the side lobes of the high NA spot which is caused by the corresponding doughnut shaped lens described in FIG.  13 . (These side lobes are undesired areas of illumination provided by the lens near the lens focal spot.) In this case both high NA and low NA use all the spatial frequencies in the radial direction to produce aberration-free spots  306  and  307 . 
     The term “spatial frequencies” refers to the spacing of the gratings (e.g., ridges and grooves) of the diffractive lens. Closely spaced gratings have a high spatial frequency, i.e. a high number of gratings per unit length. Distantly spaced gratings have a low spatial frequency. For optimal focusing of a spot, it is desirable to use most (or, preferably all) of the spatial frequencies of the grating. The angle of the gratings also varies over the surface of the lens. It is also desirable to use most (or preferably all) of the angular positions of the gratings as well to diffract the laser light wavefront into a focal spot. In the pie chart embodiment, (FIG.  14 ), all spatial frequencies contribute to both the focal spots, but not all the angular positions. In the dart board embodiment (FIG. 15, described below), all of the spatial frequencies as well as the angular positions of the gratings contribute to both focal spots. 
     Another geometry that can be used for spatially multiplexing two different lenses into a circular aperture resembles a dart board geometry  401  in FIG.  15 . Again, the compound lens operation is similar to the one described in FIG.  13  and FIG.  14 . However, this geometry has the advantage of not only reducing the side lobes of the high NA spot  406 , but also reducing the asymmetry and increasing the Strehl ratio of that same spot  406  by using all the spatial frequencies in the radial and in the circumferential directions, for both high NA and low NA lenses. In this case the lenses  402  and  403  are not only interleaved in the circumferential direction as in the pie chart case, but also in the radial direction. The performance increase of the high NA lens producing spot  406  does not decrease the performance of the low NA lens producing spot  407 . 
     An example of spatial multiplexing in a dart board geometry of two different lenses calculated to produce two aberration-free spots through two different substrate thicknesses is shown in FIG. 16A for the low NA lens  401 , and in FIG. 16B for the high NA lens  402 . Very low focal length variation has also been introduced within both lenses in order to produce extended depth of focus within each focal spot as described above. As a result, a lens in accordance with our invention can read from and write to two or more disk media with various substrate thicknesses, and at the same time allow wobbling of the disk media much more than with prior art lenses. 
     While FIGS. 13 to  15  show a compound lens with two regions having two different focal lengths, other embodiments of the invention can have more than two regions having more than two different focal lengths. These regions are typically formed on a common transparent substrate as described above. 
     While the invention has been described with respect to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention. For example, the single or compound lens can be used in applications other than magneto-optic or optical disk drives. The lens can be refractive, diffractive or graded-index. Also, although the focal length can vary as one proceeds circumferentially about the lens, in other embodiments the focal length can change as one moves radially about the lens. Further, the position of the lens having small focal length variations can be arranged in other ways as well. Similarly, spatial multiplexing of two or more lenses within a single aperture, can be performed in numerous ways, and is not constrained to doughnut, pie chart and dartboard geometries. Also, the overall aperture of the compound lens can have shapes other than circular. A disk drive incorporating a lens in accordance with our invention can focus light from a single laser source or two different laser sources with different wavelengths. It is known in the art that there are different types of disks designed for lasers of different wavelengths. For example, some optical media are designed for use with a 650 nm wavelength laser beam whereas other media are designed for use with a 780 nm wavelength laser beam. In one embodiment, a compound lens in accordance with our invention comprises a first region for accommodating the first type of media and a 650 nm laser, and a second region for accommodating the second type of media and a 780 nm laser. An optical drive employing a lens in accordance with our invention can contain both a 650 nm laser and a 780 nm laser. (Accommodating the first and second types of media also includes being able to accommodate the overcoat thicknesses associated with each type of media.) The laser sources could be switched on or off, depending upon which type of media is inserted into the drive. Alternatively, a shutter or filter could block one laser or the other, depending upon which type of media is inserted into the disk drive. 
     While the embodiments of FIGS. 13 to  15  comprise relatively low and high NA (numerical aperture) regions, these regions need not have different NAs. The regions in the embodiments of FIGS. 13 to  15  need only have different optical design characteristics so as to accommodate different substrate thicknesses or laser wavelengths. 
     The lens can be used in conjunction with media manufactured with different techniques. For example, the transparent layer over the recording layer is typically the substrate on which the recording layer is deposited. However, the transparent layer over the recording layer can be transparent material deposited on the recording layer as well. In addition, the lens of the present invention can be used in conjunction with a disk lacking a transparent layer. 
     In one embodiment, a lens in accordance with our invention produces one spot with extended depth of focus. In another embodiment, the lens produces a plurality of spots without extended depth of focus. In yet another embodiment, the lens produces one or more spots with extended depth of focus, and one or more spots without extended depth of focus. 
     While the lens is used in the above-described embodiments in conjunction with data storage media, our lens can be used in conjunction with other applications as well, e.g. any time it is desirable to focus a beam of light, such as a laser, on any workpiece that moves toward or away from the lens, or any other application requiring extended depth of focus. The lens can also be used in any application in which it is desired to provide more than one focal spots from one incoming light beam. (The spots can be displaced from one another either in the direction of the optical axis or in a direction perpendicular to the optical axis.) Accordingly, all such changes come within our invention.