WO2006121966A2

WO2006121966A2 - Light focusing systems with extended depth of focus

Info

Publication number: WO2006121966A2
Application number: PCT/US2006/017609
Authority: WO
Inventors: Gregory E. Johnson; Wade Thomas Cathey, Jr.; Edward Raymond Dowski, Jr.; Hans Brandon Wach
Original assignee: Cdm Optics, Inc.
Priority date: 2005-05-06
Filing date: 2006-05-08
Publication date: 2006-11-16
Also published as: WO2006121966A3

Abstract

A system and method record data on an optical recording medium with a recording threshold. An illumination source creates an electromagnetic radiation beam. Optics, with a pupil phase function, image the beam onto the optical recording medium. The pupil phase function modifies the phase of the beam to form an aerial image at a point on a surface of the optical recording medium. A spot of the recording medium is modified at the point when the intensity of the aerial image is above the recording threshold, the intensity of the aerial image at the spot and area of the spot being substantially constant over an extended depth of focus.

Description

LIGHT FOCUSING SYSTEMS WITH EXTENDED DEPTH OF FOCUS

RELATED APPLICATIONS

[0001] This application claims priority to provisional application serial number 60/678,315, filed May 6, 2005, incorporated herein by reference.

BACKGROUND

[0002] Focusing of a light beam to provide a very small spot of light is essential in a variety of applications. For example, a tightly focused light beam is crucial in the recording and playback of compact disks and digital video disks ("CD" and "DVD," or collectively "CD/DVD"). Normally, a master copy of a CD or a DVD is used for replicating the CD or DVD. Optical recording of the master copy advantageously uses a high resolution master lens to focus a light beam into a small data spot on an optical recording medium, ensuring maximum data density and recording quality. (The term "optical recording medium" herein includes media that contain information that is optically readable, whether the information was written to the media optically, or by non-optical means as discussed below.) Providing a small data spot can require expensive lenses that have high-precision elements and tight tolerances. For example, a CD master lens may cost approximately $20,000, and a DVD master lens may cost approximately $40,000.

[0003] Once a master CD or DVD (a "master disk") has information written thereon, it may be reproduced by mechanical or other means. For example, one or more stamping tools may be created by transferring mechanical impressions from the master disk, and such stamping tools may stamp impressions onto optical recording media of individual CDs or DVDs that are intended for distribution.

[0004] A read/write CD and/or DVD system, which may be used to write non-master CDs or DVDs, may have a subsystem called an optical pickup unit

("OPU") that focuses a light beam to an adequately small data spot to record or read a CD or DVD. The adequately small data spot produced by the OPU facilitates reliable recording and reading quality in generally uncontrolled environments. OPUs may be, for example, opto-mechanical subsystems with modest-precision elements and tight tolerances. The cost, weight, size, and power consumption of CD/DVD read/write systems may be increased by tight tolerances and fast servo subsystems that may be required to maintain focusing and tracking.

[0005] Misfocus of a light beam used for reading or writing may occur due to changes in distance between the focusing optics and CD/DVD; these changes in distance may be caused by, for instance, thermal effects, imperfect flatness of the disk, loose tolerances on a mechanism that holds and/or rotates the disk, or vibrations. Due to the wavelengths used in CD/DVD writing and the high numerical aperture ("NA") of lenses in OPUs, the depth of focus ("DOF") of the OPU may be very small, resulting in very tight tolerance requirements in the position control subsystem and/or a need for very flat disks.

[0006] In making a master recording, the positioning servo must move the master lens (1) rapidly, to support a high data recording rate, and (2) precisely, to keep the data spot focused and at the right position on the disk. Reducing the weight and/or the number of elements used in a CD or DVD master lens mitigates these challenges faced by the positioning servo. Increasing the DOF of the master lens may further ameliorate positioning requirements because the tolerance on the position required to keep the master lens focused on the disk can be relaxed, thereby resulting in a more reliable and less expensive CD/DVD recording system. [0007] For example, FIG. 1 shows a graph 10 illustrating point spread functions ("PSFs") derived from simulation of a traditional CD/DVD writing subsystem while in focus and with misfocus. The simulation used an illumination wavelength of 193 nanometers (nm) and an imaging system with a numerical aperture of 0.7. Graph 10 includes cross sections of simulated PSFs for different values of misfocus.

[0008] The depth of focus of the system, as demonstrated in FIG. 1, is determined by comparing the normalized intensities of a peak PSF value at various values of misfocus. The depth of focus shown by the PSFs in graph 10 is so small that given mechanical tolerances within the CD/DVD writing subsystem and the thresholds for writing and reading CDs and/or DVDs, traditional CD/DVD writing subsystem may not successfully record data (e.g., by producing a pit or other intended Docket No: 446773

change in an optical recording medium thereof). In particular, in FIG. 1, the normalized intensity of the peak PSF value approaches a value of 1 for misfocus distances of 0 and +100nm, but is greatly reduced for misfocus values of -lOOnm, - 300nm and +400nm. The width of the PSF is shown in nanometers. For example, if an optical recording medium has a recording threshold equivalent to the normalized intensity value of 0.7 in graph 10, it is readily seen that as the misfocus value changes from zero to -300nm or from zero to +400nm, the size of the PSF area of intensity above the normalized intensity value 0.7 (i.e., the recording threshold of the optical recording medium in this example) first decreases and then disappears entirely. In other words, outside of a narrow range of allowed misfocus values, the traditional CD/DVD writing subsystem simulated in FIG. 1 may not function effectively to record data on a CD/DVD. This range of allowed misfocus values is generally referred to as the depth of focus.

[0009] FIG. 2 shows a graph 20 illustrating PSFs derived from simulation of another traditional CD/DVD writing subsystem while in focus and with misfocus. The simulated CD/DVD writing subsystem was similar to the simulated CD/DVD writing subsystem that produced the PSFs of FIG. 1, except that the NA has been increased to 0.8. In comparing graph 20 with graph 10 of FIG. 1, it may be noted that the depth of focus of the system simulated in graph 20 is even smaller than that determined from graph 10, as demonstrated by the greatly reduced PSF peaks for misfocus values of -300 nm and +400 nm. In general, it is recognized that the depth of focus of an imaging system decreases quadratically with increase in NA.

[0010] While lenses for OPUs in read/write CD/DVD systems may exhibit high resolution, the resolution requirements for such systems are not as high as for CD/DVD master lenses. OPUs for read/write CD and/or DVD systems are advantageously lighter, less expensive and have a larger DOF than CD/DVD master lenses.

[0011] In traditional imaging systems, and as noted above, DOF varies inversely with the square of NA and directly with wavelength of illumination; that is, there is a tradeoff between resolution and depth of focus such that increasing resolution results in a decreased DOF. Small DOF may impose high demands on a mechanical focus control subsystem used therewith, and may result in tracking or writing/reading errors in the case of a CDfDVD system.

[0012] In reading systems, vibrations (e.g., due to use in moving vehicles, use by a walking user, or use in high-vibration environments) can create errors or reduced confidence in a tracking signal. Additionally, crosstalk may inject information intended for writing in one track into an adjacent track or read information from a track adjacent to the intended track.

SUMMARY

[0013] In one embodiment, therefore, systems and methods are provided hereinbelow that maintain a narrow shape of an in-focus PSF above a threshold of an optical recording medium over a large range of misfocus, thereby preserving resolution of a system even when tolerances of assembly and/or tolerances on a position control subsystem are relaxed.

[0014] In another embodiment, the numerical aperture of a system may be increased without reducing DOF, resulting in a smaller spot size without decreased DOF. Existing lenses or lens designs may be modified to operate at a higher numerical aperture than an original design permitted, thereby increasing resolution.

[0015] In another embodiment, the numerical aperture of the system may be increased without reducing the DOF while simultaneously tailoring the spatial frequency transfer function of the imaging system to match the spatial frequency content of a generalized photomask or reticle being used in place of a pinhole, to generate a smaller spot size without the usual large decrease in the DOF.

[0016] In another embodiment, the number of lens elements is reduced (as compared to traditional systems) by using an increase in the DOF to accommodate for focus-related aberrations such as spherical aberration, curvature of field, astigmatism, and/or chromatic aberration. This permits use of less expensive and lighter lenses that have the same resolution as the traditional lenses with more lens elements. This is especially critical in read heads, where weight is a problem.

[0017] In another embodiment, the complexity of a traditional OPU is reduced by using an increase in DOF to simplify generation of a focus error signal and a tracking error signal. This new OPU has fewer detector elements, fewer optical elements and fewer electrical connections as compared to the traditional OPU.

[0018] In another embodiment, an optical subsystem has an illumination source, a pupil plane phase function added to the lens, optics, and an optical recording medium, such that the narrow shape of the point spread function is maintained above the threshold of the optical recording medium over a range of misfocus that is greater than the range of misfocus without the phase function added to the lens.

[0019] In another embodiment, an optical subsystem has an illumination source, a resolution-enhancing mask for a point object (resolution enhancement techniques, or RETs, are for example used in photolithography); optics with a pupil plane phase function; and an optical recording medium such that a point spread function maintains a narrow shape above the threshold of the optical recording medium over a range of misfocus that is greater than the range of misfocus without the phase function applied to the optics. [0020] In another embodiment, a system records data on an optical recording medium with a recording threshold and includes an illumination source for creating an electromagnetic radiation beam; optics with a pupil phase function for imaging the beam onto the optical recording medium, the pupil phase function modifying the phase of the beam to form an aerial image at a point on the optical recording medium; wherein a spot of the recording medium is modified at the point when the intensity of the aerial image is above the recording threshold, the intensity of the aerial image at the spot and area of the spot being substantially constant over an extended depth of focus.

[0021] In another embodiment, a subsystem for reading data from an optical recording medium includes an illumination source for creating an electromagnetic radiation beam, optical elements for imaging the beam onto a point on a surface of the optical recording medium, the optical elements creating a point spread function, a pupil plane phase function for modifying phase of light transmitted by the optical elements to create a modified point spread function at the surface of the optical recording medium, such that a portion of the modified point spread function above a decision threshold is substantially unaffected by a range of misfocus resulting from variation in distance between the optical elements and the optical recording medium as compared to the unmodified point spread function, and a detector having the decision threshold for detecting light reflected from the optical recording medium and generating a signal representative of the data. [0022] In another embodiment, an apparatus increases the depth of focus in a CD or DVD mastering system having an illumination source and optical elements for imaging onto an optical recording medium with a recording threshold. The apparatus includes pupil plane phase function optics in an imaging path of the system that alters phase of light transmitted by the optical elements, thereby creating an altered point spread function of the system, such that a portion of the altered point spread function that is above the recording threshold is substantially insensitive over a range of distance between the source and the optical recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 shows a simulation of a PSF vs. misfocus graph for a traditional CD/DVD writing system.

[0024] FIG. 2 shows a simulation of a PSF vs. misfocus graph for another traditional CD/DVD writing system.

[0025] FIG. 3A schematically shows one CD/DVD writing system with extended depth of focus, in accord with an embodiment. [0026] FIG. 3B shows exemplary detail of a portion of a disk including a layer of an optical recording medium.

[0027] FIG. 4 shows a three-dimensional representation of one pupil-plane phase function that can be used to extend the depth of focus of the CD/DVD writing system of FIG. 3 A. [0028] FIG. 5 shows a simulation of PSFs produced over varying values of misfocus, by using the phase function of FIG. 4 in the subsystem of FIG. 3A.

[0029] FIG. 6 schematically shows a CD/DVD reading subsystem with extended depth of focus, in accord with an embodiment.

[0030] FIG. 7 shows a simulation of PSFs of a system with an NA of 0.8. [0031] FIG. 8 schematically shows a subsystem for reading a CD/DVD with extended depth of focus, in accord with an embodiment.

[0032] FIG. 9 schematically shows a subsystem for reading a CD/DVD with extended depth of focus, in accord with an embodiment.

[0033] FIG. 10 shows an exemplary phase surface that may be used to increase depth of focus.

[0034] FIG. 11 shows contour plots of four additional phase function profiles suitable for increasing depth of focus.

[0035] FIG. 12 shows a contour plot of another exemplary phase function profile suitable for increasing depth of focus. [0036] FIG. 13 shows a cross sectional plot of the phase profile of the phase function of FIG. 12 measured along a radial direction.

[0037] FIG. 14 shows a cross sectional plot of the phase profile of the phase function of FIG. 12 measured along a transverse direction.

[0038] FIG. 15 shows a contour plot of another exemplary phase function profile suitable for increasing depth of focus.

[0039] FIG. 16 shows a modulation transfer function resulting from use of the phase function profile of FIG. 15.

[0040] FIGS. 17 and 18 show numerically generated plots of other phase function profiles suitable for increasing depth of focus.

DETAILED DESCRIPTION OF THE DRAWINGS

[0041] The quantity of data that may be recorded on a compact disk and digital video disks ("CD" and "DVD," or collectively "CD/DVD") is related to the resolution of a recording lens and the stability of a mechanical subsystem that holds and maneuvers the recording lens. Therefore, increasing the resolution of the focusing optics in a CD/DVD writing subsystem (without making the subsystem impractical to fabricate or use) is advantageous. Maintaining and/or increasing resolution of the focusing optics of a CD/DVD writing subsystem while reducing cost and/or power consumption are also advantageous.

[0042] By improving a tracking signal used within a CD/DVD reader, the probability of read errors may be reduced. Also, increasing the DOF of an optical system within the CD/DVD reader may also reduce the probability of errors induced by opto-mechanical variations in disks during use (e.g., due to temperature variations, vibration, moisture or pressure). It would also be advantageous to reduce crosstalk in CD/DVD read/write subsystems. [0043] It should be noted that, while the following description concentrates on an example of light focusing in an embodiment including a CD/DVD writing system, systems and methods herein may be equally applicable in other applicatiosn requiring the focusing of light into a tightly focused spot. That is, the embodiments including a CD/DVD writing subsystem, as described hereinafter, should be considered exemplary and not limiting.

[0044] FIG. 3A schematically shows a CD/DVD writing subsystem 100 with extended depth of focus. FIG. 3A may not be drawn to scale. Subsystem 100 has imaging optics 110 that include a wavefront coding (WFC) pupil plane phase function 120 and a variety of optical elements 130 (e.g., lenses and/or mirrors). An electromagnetic radiation beam 60 is generated by an illumination source 50 and imaged by imaging optics 110 such that an aerial image 140 is created at a point 158 near a surface of a disk 150. Disk 150 is configured so as to be sensitive to light imaged thereon such that aerial image 140 is recorded onto disk 150. The term "light" herein denotes electromagnetic radiation that may be within the visible spectrum or may be in other regions of the electromagnetic spectra such as infrared, ultraviolet, X-rays, etc. Electromagnetic radiation beam 60 may be modulated such that data 52 is encoded onto electromagnetic radiation beam 60 and subsequently recorded onto disk 150. That is, disk 150 may be rotated and/or moved relative to all or part of subsystem 100 such that data 52 may thereby be recorded to specific areas (e.g., tracks) of disk 150.

[0045] Imaging optics 110 are maintained at a distance '/ ' from disk 150, as shown in FIG. 3. Imaging optics 110 may be designed, for example, to produce optimal performance of subsystem 100 when/is an optimal distance, denoted herein as "in focus." Deviations of distance/from the optimized distance are denoted herein as "misfocus," as shown, for example, as the x-axis in the plots shown in FIGS. 1 and 2. [0046] FIG. 3B shows a portion of disk 150 illustrating an optical recording medium 155 formed as a layer upon disk 150. FIGS. 3A and 3B are best viewed together with the following description. As noted above, the term "optical recording medium" herein includes media that contain information that is optically readable, whether the information was written to the media optically, or by non- optical means; however in the example of FIG. 3, the information is also optically written. Optical recording medium 155 has a non-linear response to incident light, such as electromagnetic radiation beam 60. When light intensity of one or more parts of aerial image 140 is below a recording threshold of optical recording medium 155, little or no change occurs in optical recording medium 155 (i.e., information encoded onto electromagnetic radiation beam 60, and thereby aerial image 140, is not completely recorded onto optical recording medium 155). In contrast, when light intensity of one or more parts of aerial image 140 is above the recording threshold, optical recording medium 155 changes at the location that part of the aerial image is incident on the optical recording medium (for example, the optical recording medium is consequently ablated, or the index of refraction of the optical recording medium is altered, a magnetic domain is switched, or the reflectivity or transmissivity of the optical recording medium is modified).

[0047] Assuming intensity of light generated by illumination source 50 is constant, the recording threshold of optical recording medium 155 may be expressed as a certain intensity level of the PSF of subsystem 100. hi portions of the PSF of subsystem 100 where the intensity level remains below the recording threshold of optical recording medium 155, variations in the PSF intensity level resulting from changes in distance/ (i.e., misfocus) have no adverse effect upon data recorded to optical recording medium 155. Specifically, since adjacent tracks are written at different times, exposure resulting from overlap of multiple aerial images 140 has no cumulative effect.

[0048] Phase function 120 causes subsystem 100 to generate a PSF such that, even though there may be misfocus in subsystem 100, the part of the PSF of subsystem 100 above the recording threshold does not change appreciably with this misfocus. Phase function 120 may be implemented by a discrete optical element located at or near a pupil plane of subsystem 100, or may be effected by modification of a lens surface that is, for example, close to the pupil plane. Phase function 120 may alternatively be effected, for example, by a spatial light modulator, a digital mirror modulator, or a hologram. Still alternatively, phase function 120 may be implemented by aspheric features of one or more optical elements 130.

[0049] Use of phase function 120 to generate a PSF that does not change appreciably with misfocus provides advantages for subsystem 100, as compared to the prior art. For example, focus-related aberrations such as spherical aberration, curvature of field, astigmatism and/or chromatic aberration may be reduced through the use of phase function 120. Thus, the number of lens elements in imaging optics 110 may be reduced, as compared to prior art systems, thereby reducing cost and/or weight. Use of phase function 120 may also allow tolerances related to mechanical positioning, vibration, disk flatness and/or disk warp to be relaxed without reducing performance of subsystem 100. [0050] FIG. 4 shows a three-dimensional representation 300 of one exemplary pupil-plane phase function for extending the depth of focus of subsystem 100, FIG. 3A (e.g., for extending the depth of focus of CD/DVD subsystems). The equation of the surface of the phase function as shown in FIG. 4 is:

^^XJ ⁱ Equation 1 where n = \, ... 9, radius \r\ < 1.0,

Θ is a polar angle 0 < θ < 2π,

«„ = [ 4.6967 -2.7162 1.7921 -0.7771 -0.5688 -1.3528 0.8717

0.2985 0.0236 ], element radius Y = 119 mm and normalizing radius R = 35,000 mm.

The phase function of Equation 1 may, for example, be implemented as phase function 120, FIG. 3 A. Equation 1 and the associated parameters result in a numerical aperture of 0.7. A numerical aperture of 0.8 may be achieved by using an element radius value of Y = 130 mm and normalizing radius R = 45,000 mm [0051] FIG. 5 shows a graph 310 illustrating PSFs produced over varying values of misfocus when the phase function of Equation 1 is implemented by phase function 120 within subsystem 100. In an upper part 312 of graph 310, where normalized intensity is above 0.7, PSFs vary little with misfocus. In a lower part 314 of graph 310, where normalized intensity is below 0.7, PSFs may vary more than, and have higher intensity levels than, equivalent PSFs of a traditional system with the same NA (see, for example, PSFs of graph 10, FIG. 1). However, if optical recording medium 155 has a recording threshold equivalent to normalized intensity 0.7 of graph 310, light with lower intensity levels (e.g., as indicated by PSFs in portion 314 of graph 310) causes little or no effect upon optical recording medium 155 during the reading/writing process.

[0052] Graph 310 demonstrates that a misfocus tolerance of subsystem 100 using the phase function of Equation 1 is at least -300 nm to +400nm, which is a misfocus range of 700nm. That is, using a normalized intensity of 0.7 as the recording threshold, a 'spot' (or data bit area) with a diameter 'D' (as shown in FIG. 5) is recorded onto recording medium 155. This spot (data bit area) substantially maintains the same size between -300nm to +400nm of misfocus. The inclusion of the phase function of Equation 1 in imaging optics 110 therefore allows relaxation of misfocus related tolerances (such as mechanical positioning tolerances, disk flatness and/or warp tolerances, and vibration tolerances) by a factor of at least 2 compared to the misfocus tolerance of a traditional system (about ±lOOnm) shown in graph 10, FIG. 1. The large range of misfocus over which the PSF of subsystem 100 maintains its narrow shape is sometimes denoted as extended depth of focus ("extended DOF") herein. This range of misfocus is larger than the DOF of like optical systems that do not employ an especially designed phase function (e.g., phase function 120).

[0053] In certain applications it may be desirable to increase the resolution of a CD/DVD writer without increasing the NA of a lens or decreasing the wavelength of light used. In these applications, it is possible to image a mask with resolution-enhancing techniques, much as is done in lithography. [0054] FIG. 6 schematically shows another CD or DVD writing subsystem

100' with extended depth of focus. FIG. 6 may not be drawn to scale. Subsystem 100' includes the same or similar components as subsystem 100, and further includes a mask (or reticle) 80 with one or more apertures that are smaller than the resolution limit of the lens, and therefore do not image. However, these apertures do affect diffraction that occurs around a portion of the mask that represents a desired spot size. The result is a PSF that is smaller than a traditional diffraction limit of an imaging system. These techniques may be combined with the extended DOF designs described herein (e.g., system 100, FIG. 3A).

[0055] Use of a separate mask 80 enables the modification of the illumination in order to further reduce the size of the data spot. Mask 80 may thus be modified such that aerial image 140' of the PSF does not change appreciably

(including the portions of the PSF that fall below an exposure threshold), by applying the equivalent of image processing to the object rather than to the image. Such image processing is disclosed, for example, in US patent 5,748,371, incorporated herein by reference. [0056] In one embodiment, an optical system designed to use a lens having a first NA may be modified to use a lens of higher NA by adding an optical element (or by modifying the surface of an existing optical element) to incorporate pupil plane phase functions such as the phase function of Equation 1. Such a modification increases the system's depth of focus and, consequently, reduces effects of misfocus-related aberrations.

[0057] FIG. 7 shows a graph 320 of PSFs resulting from a simulation of a CD/DVD writer subsystem similar to subsystem 100, FIG. 3 A, except imaging optics 110 are modified to have an NA of 0.8. That is, the subsystem producing the PSFs shown in graph 320 is similar to the subsystem producing the PSFs in graph 310, except that the NA is increased to 0.8 without any other changes; in other words, aberrations have not been re-balanced for the larger NA. Graph 320 shows that resolution has been increased by increasing NA, as expected. Note the change of scale on the horizontal axis in comparison with that of FIG. 1 and FIG. 5. Graph 320 shows that, with modifications of the surface or index of refraction of an existing lens (or the introduction of a separate optical element in the pupil plane) to provide a phase function (e.g., the phase function of Equation 1), NA of imaging optics 110 may be increased to 0.8 while achieving a depth of focus that is greater than the depth of focus of a traditional system with an NA of 0.7. It is also noted that the portion of the PSFs that is above the recording threshold of optical recording medium 155 (i.e., corresponding to a normalized intensity equivalent of 0.7 in graph 320) is substantially similar to the in-focus PSF of graph 310, FIG. 5.

[0058] FIG. 8 schematically shows a subsystem 200 for reading a CD or DVD with extended depth of focus. FIG. 8 may not be drawn to scale. Subsystem 200 has imaging optics 210 that include a WFC pupil plane phase function 220 and a variety of optical elements 230 (e.g., lenses and/or mirrors). Phase function 220 may, for example, be effected by a discrete optical element located at or near a pupil plane of subsystem 200, or may be implemented by modification of a surface of one of optical elements 230 close to the pupil plane. Phase function 220 may also be generated, for example, by a spatial light modulator, a digital mirror modulator, or a hologram. [0059] An illumination source 205 generates an electromagnetic radiation beam 260 that is imaged by imaging optics 210 onto a (CD or DVD) disk 250. In particular, beam 260 illuminates data previously recorded onto an optical recording medium 255 at a point 258 of disk 250. (As previously noted, the term "optical recording medium" herein includes media that contain information that is optically readable, whether the information was written to the media optically, or by non- optical means; the means for writing information into optical recording medium 255 is irrelevant to the example of FIG. 5.) Imaging optics 210 are maintained at a distance f i along an optical path of beam 260 away from disk 250, as shown. Light 260 is reflected from disk 250 as reflected light 260', which is changed in some manner (e.g., changed in intensity, polarization, or phase) by this reflection depending upon data encoded at point 258. A detector 270 detects reflected light 260' and subsequently generates an electronic signal representing data 272 based upon changes detected in reflected light 260'. For example, electronics within detector 270 may compare an electronic signal representing reflected light 260' against a decision threshold to determine data being read from point 258. That is, if reflected light 260', as detected at detector 270, exhibits a change in intensity, polarization or phase, for instance, above a specified threshold, detector 270 may generate a first data value (e.g., a "1" in binary notation). If reflected light 260' does not exhibit such a change or if the change is below the specified threshold, detector 270 may generate a second data value (e.g., a "0" in binary notation). Detector 270 may use other means of determining data 272 without departing from the scope hereof. Illumination source 205, optics 210 and detector 270 may collectively form an OPU 240.

[0060] Imaging optics 210, through use of phase function 220, generate a PSF such that the part of the PSF above a recording threshold remains substantially the same regardless of misfocus. Thus, most of beam 260 falls in a consistently sized illuminated area at point 258 with a consistent spot size (i.e., size of the illuminated area) that does not change appreciably with misfocus. Consistency in the spot size over a range of values of misfocus may enable advantages such as: (1) relaxation of tolerances on mechanical positioning of OPU 240 with respect to disk 250; (2) relaxation of vibration requirements of OPU 240 and/or disk 250; (3) reduction in complexity and/or weight of optics 210; and/or (4) relaxation of tolerances on thickness, flatness and/or warping of disk 250. Phase function 220 may represent, for example, the phase function of Equation 1.

[0061] FIG. 9 schematically shows another exemplary subsystem 300 for reading a CD or DVD with extended depth of focus. FIG. 9 may not be drawn to scale. Subsystem 300 is similar to subsystem 200 of FIG. 8 but includes additional optics 262, described below, and an optional mask 280. Optics 210 image beam 260, generated by illumination source 205, onto (CD or DVD) disk 250 to illuminate a point 306 containing data previously recorded onto optical recording medium 255 of disk 250. Optics 210 are maintained at a distance fi along an optical path of beam 260 from disk 250, as shown. Disk 250 reflects beam 260 as reflected light 304, which is changed in some manner (e.g., changed in intensity, polarization, or phase) depending on data encoded at point 306. Illumination source 205, optional mask 280, optics 210, optional optics 262, and detector 270 may form an OPU 302.

[0062] Optics 262 includes one or more imaging lenses 264 and a pupil plane phase function 266. Optics 262 image an area at point 306 of disk 250 upon detector 270; the optical paths of optics 210 and optics 262 may be aligned upon the same area at point 306. The image formed upon detector 270 by optics 262 may be optimized for a distance f₂ between disk 255 and optics 262. However, phase function 266 may be designed such that the image of point 306 upon detector 270 is less sensitive to misfocus (i.e., changes in distance f₂) than if phase function 266 were omitted.

[0063] When included, optional mask 280 may have one or more apertures that are smaller than the resolution limit of optics 210, and therefore do not image. These apertures do, however, affect diffraction that occurs around a portion of mask 280 that represents a desired spot size. Mask 280 may result in a PSF that is smaller than a traditional diffraction limit of an imaging system. Mask 280 may be used with or without phase functions 220 and/or 266.

[0064] The phase function of Equation 1 is only one example of a phase function that produces an extended depth of focus. FIG. 10 shows an optical surface type 350 that may be used to generate phase functions that increase depth of focus within an optical system. Optical surface type 350 exemplifies a "cubic" family of phase function profiles. Surface type 350 may be implemented on a discrete optical element, or may modify an existing surface (e.g., a surface of lenses 130, 230, 264). More information about designing phase functions to produce extended depth of focus, and about other systems and methods useful in CD/DVD reading and writing systems with extended depth of focus, may be found in commonly-owned and copending U.S. Patent Applications Serial Nos. 10/376,924 and 10/858,337 entitled "Optimized Image Processing for Wavefront Coded Imaging Systems" and "Lithographic Systems And Methods With Extended Depth Of Focus," respectively; these applications are incorporated herein by reference. [0065] Additional examples of suitable pupil plane phase functions (e.g., pupil plane phase function 266) for increasing depth of focus are shown in FIG. 11. The contour lines shown in FIGS. H(A) - H(D) correspond to paths of constant phase.

[0066] One example of a design progression for tailoring a phase function to achieve a desired modulation transfer function (MTF) is illustrated in FIGS. 12-16. FIG. 12 shows a contour plot 500 of an exemplary phase function 500. Contour plot 500 includes a plurality of paths of constant phase 502. The phase function contours may also be described, for example, in terms of variation in a radial direction from the center and variation along a path at a given distance from the center. For instance, the phase function corresponding to contour plot 500 may be measured along a dashed, radial line 504 and along a vertical line 506. The phase function profile along radial line 504 is shown in FIG. 13 in a plot 520. Similarly, the phase function profile along line 506 is shown in FIG. 14.

[0067] One example of a contour plot of an exemplary phase function for providing extended depth of focus is shown in FIG. 15. The use of a mask implementing the exemplary phase function of FIG. 15 in a subsystem results in a tailored MTF as shown in FIG. 16.

[0068] FIGS. 17 and 18 show additional phase functions suitable for use with the subsystems described above. FIG. 17 shows a cosine form profile 580 generated using numerical modeling software. FIG. 18 shows a higher order cubic profile (similar to that of FIG. 10, but with a flatter center), also generated using numerical modeling software.

[0069] In the systems and methods disclosed, certain advantages may be achieved as compared to the prior art, such as: o tolerance on focus control of a CD or DVD reading systems may be relaxed by extending the depth of focus. o tolerance on focus control of a CD or DVD read/write systems may be relaxed by extending the depth of focus. o tolerance on focus control of a CD or DVD mastering systems may be relaxed by extending the depth of focus. o resolution of disk reading systems may be increased by increasing the numerical aperture of the system without reducing the depth of focus, o resolution of disk writing systems may be increased by increasing the numerical aperture of the system without reducing the depth of focus, o tolerances on disk flatness may be relaxed by a factor of at least 2. o the two-dimensional spatial frequency transfer function of the writing system may be optimized with respect to the two-dimensional spatial frequency spectra of the spot that are recorded. o the two-dimensional spatiaL frequency transfer function of the reading system may be optimized with respect to the two-dimensional spatial frequency spectra of the spot that are read. o resolution may increase by the addition of a phase function and without a decrease in DOF. o the optical recording medium may be magneto-optic, pits in a medium, phase changes in the optical recording medium, changes of reflectivity, mechanisms for producing changes in phase of the reflected light. [0070] Changes may be made in the above systems without departing from the scope hereof. For example, a CD/DVD read/write system may include one or more optical subsystems (e.g., subsystems 200, 300 above) that are designated for reading and other subsystems (e.g., subsystems 100, 100' above) that are designated for writing. Alternatively, a CD/DVD read/write system may include one or more subsystems that are capable of both reading and writing. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall there between.

Claims

CLAIMSWhat is claimed is:

1. A system for recording data on an optical recording medium with a recording threshold, comprising: an illumination source for creating an electromagnetic radiation beam; optics with a pupil phase function for imaging the beam onto the optical recording medium, the pupil phase function modifying the phase of the beam to form an aerial image at a point on the optical recording medium; wherein a spot of the recording medium is modified at the point when the intensity of the aerial image is above the recording threshold, the intensity of the aerial image at the spot and area of the spot being substantially constant over an extended depth of focus.

2. The system of claim 1, wherein the beam is modulated based upon the data to be recorded to the optical recording medium.

3. The system of claim 2, wherein the optical recording medium is supported on a disk, and wherein the disk moves relative to the optics and illumination source such that data is recorded in one or more tracks.

4. The system of claim 1, wherein the pupil plane phase function is generated by one of a discrete optical element, modification of a surface close to the pupil plane of a lens within the optics, a spatial light modulator, a digital mirror modulator, and a hologram.

5. The system of claim 1, wherein the electromagnetic radiation of the beam is in one or more of: the visible spectrum, infrared spectrum, ultraviolet spectrum, X-ray spectrum.

6. The system of claim 1 , further comprising a mask located between the illumination source and the optics.

7. A subsystem for reading data from an optical recording medium, comprising: an illumination source for creating an electromagnetic radiation beam; optical elements for imaging the beam onto a point on a surface of the optical recording medium, the optical elements creating a point spread function; a pupil plane phase function for modifying phase of light transmitted by the optical elements to create a modified point spread function at the surface of the optical recording medium, such that a portion of the modified point spread function above a decision threshold is substantially unaffected by a range of misfocus resulting from variation in distance between the optical elements and the optical recording medium as compared to the unmodified point spread function; and a detector having the decision threshold for detecting light reflected from the optical recording medium and generating a signal representative of the data.

8. The subsystem of claim 7, wherein an upper portion of the modified point spread function when the subsystem is in a range of misfocus is essentially the same as the upper portion of the unmodified point spread function when the subsystem is in focus.

9. The subsystem of claim 8, wherein the pupil plane phase function is generated by one of a discrete optical element, modification of a lens surface close to the pupil plane, a spatial light modulator, a digital mirror modulator, and a hologram.

0. Apparatus for increasing the depth of focus in a CD or DVD mastering system having an illumination source and optical elements for imaging onto an optical recording medium with a recording threshold, comprising: pupil plane phase function optics in an imaging path of the system that alters phase of light transmitted by the optical elements, thereby creating an altered point spread function of the system, such that a portion of the altered point spread function that is above the recording threshold is substantially insensitive over a range of distance between the source and the optical recording medium.