WO2001024173A1 - Single laser pickup for use with cd, dvd and cd-r discs - Google Patents

Abstract

An apparatus and method using a single 780 nm laser (780 nm laser diode) is provided for reading DVD and high density CD discs as well as for reading and writing CD-R discs. A first optical pathway directs a first portion of the output beam of the 780 nm laser onto either a CD, DVD or CD-R disc. A second optical pathway is provided for directing a second portion of the 780 nm laser output beam to a photodetector (PDIC). A super-resolution optical element (SUPER-RESOLUTION OBJ) is included in the first optical pathway to achieve a sufficiently small spot size to read DVD discs, high density CD discs and to read and write CD-R discs without requiring the use of a second 650 nm laser output beam. The super-resolution optical element can be a phase plate, an amplitude apodization filter, a diffractive, or both a phase plate and an amplitude filter.

Description

SINGLE LASER PICKUP FOR USE WITH CD. DVD AND CD-R DISCS

CROSS REFERENCE TO RELATED APPLICATION This application claims the benefit of and priority from United States patent applications Serial No. 09/408,736 filed September 29, 1999 and Serial No. 09/553,704 filed April 20, 2000, and United States provisional patent application Serial No. 60/201 ,612 filed May 3, 2000.

BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates to an optical pickup which utilizes a single laser to read DVD and CD discs and to read and write CD-R discs. More particularly, the present invention utilizes a single 780 nm laser together with various super-resolution techniques to facilitate reading DVD and high density CD discs as well as reading and writing CD-R discs. The phrase "CD-R discs," as used herein, is used in a broad sense to include any CD disc that is readable and/or writable. In an optical disc system, the density of the information that can be recorded and retrieved from the disc is a function of the size of the light spot on the disc or the effective spot size. The spot size is generally a function of the wavelength and the N.A. of the lens that focuses the light onto the disc. It has been shown that a spot size smaller than the diffraction limit can be created using a variety of techniques referred to as super-resolution techniques. Super- resolution efforts have been focused primarily on increasing the density of data stored on a disc by decreasing the spot size. Prior art involves the use of a laser source and optical elements to focus and retrieve information from a disc without the practical considerations of system implementation. Prior art compact disc optical systems consist of a laser, a beam splitter, an objective lens and a detector array. DVD optical disc systems have similar components to a compact disc optical system except that the system is optimized to achieve a smaller spot size. This is achieved by decreasing the wavelength of the laser from 780 nm to 650 nm and by increasing the N.A. of the objective lens, and a collimator is often used as well. CD-R discs are different from regular compact discs in that they are based on changing the absorption of the medium which has been optimized for 780 nm. Therefore, in order to read both CD and DVD discs, an optical system according to the prior art would need to have both a 780 nm laser and a 650 nm laser.

It is known in the prior art to use super-resolution optical elements in image forming apparatus such as shown in U.S. patent 5,349,592. However, that patent does not teach or suggest the use of a single laser pickup for use with CD, DVD and CD-R discs. It is also known in the prior art to use a 780 nm laser together with a 650 nm laser to read CD and DVD discs and to read and write CD-R discs. Those prior art systems must utilize relatively complex mechanisms to switch between appropriate lasers depending upon the disc being utilized in the system.

Prior art exists for both dual DVD/CD optical pickup systems and for super-resolution objective lenses. Dual DVD/CD pickup systems in the prior art have used one of the following design concepts. Many of these systems use two totally separate optical systems that are mechanically switched into the optical path. Other prior art systems use two different laser diodes and a selectable aperture stop with a complex objective lens to produce the different spot sizes required for either DVD or CD applications. Other prior art systems use multiple element objective lenses to produce different focal points with a single short- wavelength laser diode. The requirement placed upon all of these systems is to produce an image spot on the data record that is on the order of 0.64 micrometer FWHM (full width at half maximum intensity) for the DVD data record, and on the order of 1.00 micrometer FWHM for the CD data record. Depth of focus must be on the order of +/- 1.0 micrometer for both cases to accommodate the autofocus servomechanism which moves the objective to best axial focus. The first class of prior art dual pickup systems has the obvious disadvantage of very high cost, duplicate optical plus electronic systems, and moving mechanical components for selecting one of the two optical systems. Volume constraints make this prior art concept unusable for laptop or compact systems. US patent 5,236,581 issued to Miyagawa is an example of this prior art concept. Another example is US patent 5,777,959 issued to Nakagama which is directed primarily to writing and recording at different wavelengths on the same disc but can also be applied to reading multiple discs at different wavelengths.

The second class of prior art dual pickup systems use two laser diodes that are made coaxial with a beam combining optic plus a complex objective lens which can produce two different spot sizes for the DVD or CD data record. These prior art patents use different types of complex lenses to operate with one or the other of the two laser diode wavelengths to form the spot size required by either the DVD or CD data record. One variant of this complex lens uses two elements in close proximity, the first element being a holographic or diffractive component and the second element being an aspheric molded component. US patent 5,696,750 issued to Katayama is an example of this prior art pickup concept implemented with a holographic element in the complex objective lens. The holographic or diffractive element typically directs the zero-order diffracted wavefront into the central numerical aperture (NA) of the second element which is used for the CD spot condition of 1.00 micrometer FWHM. The first-order diffracted wavefront is directed into the outer annular NA of the second element which is used for the DVD spot condition of 0.64 micrometer FWHM. The third class of prior art systems uses a single short-wavelength laser diode (typically 640 nm) and a complex objective form for the two different spot sizes required by the DVD and CD data records. US patent 5,768,031 issued to Yang uses a singlet with both surfaces aspheric. The novelty of this design is an annular region on one of the surfaces that operates like a bifocal lens to produce two distinct focal points (one for CD and one for DVD). Another example is US patent 5,281 ,797 issued to Tatsuno. This system uses a 640 nm class laser diode and a fast numerical aperture (0.60 NA) objective with an adjustable aperture stop that can produce either the CD or the DVD spot size. A third example of this prior art is given in US patent 5,734,637 issued to Ootaki. This example uses a liquid crystal variable aperture to adjust the numerical aperture of the objective lens to produce either CD or DVD spot sizes. US patent 5,659,533 issued to Chen appears to be the same as the Ootaki patent where the words "electronic shutter" replace "liquid crystal shutter." US patent 5,748,602 provides another example using polarization to control the NA of an objective with a single laser to produce two different spot sizes. A final example of this prior art is US patent 5,665,957 issued to Lee. This patent describes a 640 nm laser diode used with a 0.60 NA objective with the introduction of an annular obscuration which changes the diffraction-limited spot size to meet either the CD or DVD requirement.

All of the prior art DVD/CD pickup systems either require expensive mechanical displacement mechanisms, a large number of components plus a large volume, or an expensive short-wavelength laser diode plus very short depth of focus (due to the 0.60 NA required to achieve 0.64 micrometer FWHM spot sizes).

A special class of optical pickup is required for readable and/or writable

DVD data records. The spot size for this data record has required the prior art to use very expensive 635 nm laser diodes with a 0.60 NA objective. The use of a super-resolution lens allows the use of a much less expensive 780 nm laser diode and from 0.60 to 0.66 NA.

Prior art also exists for super-resolution objectives. US patent 5,121 ,378 issued to Hirose describes a super-resolution objective for recording data that primarily uses intensity apodization to reduce the spot size but also anticipates the single case of phase apodization by shifting the central aperture zone by pi radians from the outer zone. Katayama in US patent 5,600,614 teaches the use of two polarized components that are phase-shifted by pi radians. Ando in US patent 5,349,592 describes a super-resolution lens that divides the aperture into several concentric zones that have a pi+2n pi phase shift between zones. Other US researchers have described the use of concentric phase-shifting zones combined with intensity apodizing zones, the use of gradient-phase zones, and the use of gradient intensity apodizing zones plus their combination.

Because super-resolution objectives achieve smaller spot sizes at the expense of increased secondary ring intensities ("side-lobes"), digital signal processing (DSP) means to identify the signal in the presence of the noise introduced by the "side-lobes" can be important. Prior art exists in this field as well. For optical pickup systems, the adjacent data pits on the disc being read can be utilized to reject the noise introduced by the "side-lobes."

One embodiment of this invention is the discovery that a 780 nm laser can be combined with a 0.60 to 0.66 NA objective that has an amplitude mask and/or phase mask for super-resolution features for the purpose of reading DVD records, typically requiring a 650 nm laser operating with a 0.60 NA objective. The prior art DVD reader using a 650 nm laser and 0.60 NA objective produces about a 0.55 micron FWHM spot. Prior art super-resolution objectives for DVD reader applications typically use phase masks to reduce the FWHM of the point spread function. This prior art claims that phase masks are more radiometrically efficient in delivering energy to the focused spot compared to classical absorbing amplitude masks. In fact, it is very difficult to make phase masks produce point spread functions that effectively use this retained energy. The retained energy is largely delivered into the surrounding ring structure rather than the central peak and this more intense ring structure can reduce the modulation produced by the DVD pit structure. Amplitude apodization is well known in the art and has been proposed for super-resolution lenses, but its true advantages have not been recognized for DVD reader applications.

Another aspect of the present invention addresses the problem that CD and DVD disc media have different substrate thicknesses which introduce different amounts of spherical aberration.

The present invention in one embodiment uses the discovery that a diffractive structure can be added to the refractive surface of an objective lens to cause the zero-order image to fall at one distance along the optical axis from the lens producing a spot small enough to read the DVD data record while the first-order image will fall at another distance along the optical axis producing a larger spot that can read the CD data record. The objective further uses a super-resolution feature such as a central obscuration or a phase-only profile to achieve the very small spot required for DVD operation while using a 780 nm class diode and a 0.60 to 0.66 numerical aperture.

Prior art has used holograms (on a separate plate from the objective lens) to produce two axially-separated images. How-ever, these prior art solutions use two different laser diodes for the two data media and none uses super-resolution features to permit the use of less-expensive 780 nm class lasers for DVD operation.

According to the present invention, a pickup is provided utilizing a single 780 nm laser which is combined with various super-resolution techniques to be able to read CD and DVD discs and to be able to read and/or write CD-R discs. The present invention also produces and maintains a sufficiently small spot size for reading DVD discs while simultaneously maintaining a long enough depth of focus to reduce actuator strain.

Another significant advantage of the present invention is that existing CD optical systems may be readily modified to facilitate reading of CD and DVD discs.

Another aspect of the present invention is to utilize a super-resolution objective lens with a single 780 nm laser diode to provide resolution that exceeds the Airy diffraction limit in an optical pickup. The result is an optical pickup small enough for use in laptop computers. Digital signal processing (DSP) may be used in this embodiment to reject noise introduced by side-lobes.

Another embodiment of the invention utilizes an amplitude mask applied to the center of a 0.60 to 0.66 NA DVD reader objective, allowing greater manufacturing tolerances for the molded objective.

A further embodiment of the invention provides a super-resolution lens which incorporates a diffractive surface which produces two axial images.

A primary object of the invention is to provide a simplified pickup which may be used in conjunction with CD, DVD and CD-R discs.

A further object of the invention is to provide a simplified pickup using a single 780 nm laser together with super-resolution techniques to facilitate reading of CD and DVD and both reading and writing of CD-R discs.

A further object of the invention is to provide an amplitude mask used on the central portion of a super-resolution lens to increase manufacturing

tolerances of the lens. Another object of the invention is to provide a super-resolution objective lens incorporating a diffractive surface which allows a single laser to read both CD and DVD discs.

A further object is to provide a super-resolution objective lens incorporating a diffractive amplitude mask.

A further object of the invention is to provide a mechanism that may be readily installed in existing CD optical systems to enable reading of DVD discs.

Additional objects and advantages of the invention will become apparent from the following description and the drawings wherein: BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of a prior art system for using super- resolution optical elements when only a CD or DVD disc is used, but not for a plurality of disc types;

Fig. 2 is a schematic representation of a second technique utilizing an apodization filter or mask in the optical pathway between the beam splitter and the photodetector;

Fig. 3 is a schematic representation of super-resolution diffractive lens cross sections that may be utilized in the present invention;

Fig. 4 is a graphical representation showing reflectivity of a CD-R disc; Figs. 5 and 6 are three dimensional and theoretical graphical representations showing the diffraction intensity spread function utilizing 780 nm light and assumed defocusing values for a lens with zero wavefront error and no

super-resolution features;

Figs. 7 and 8 are three dimensional graphical representations of the Figs. 7 and 8 are three dimensional graphical representations of the diffraction intensity spread function using 780 nm wavelength light and the same defocusing values used for Figs. 5 and 6 for a lens with a wavefront shaped by super-resolution features; Fig. 9 is a three dimensional and graphical representation of the intensity point spread function (PSF) of a 0.60 NA classic "diffraction-limited" lens;

Fig. 10 is a three dimensional graphical display showing the PSF using a super-resolution lens having the design characteristics shown in Fig. 11 ;

Fig. 11 is a graphical representation of a phase function lens system to produce the PSF shown in Fig. 10;

Fig. 12 illustrates a pickup system using the super-resolution objective of the present invention;

Fig. 13 shows an alternate embodiment of the invention showing a 0.60 numerical aperture DVD reader objective having an amplitude mask placed in the entrance beam;

Fig. 14 is a graphical representation showing the PSF for the lens shown in Fig. 13;

Fig. 15 is a graphical representation of the FWHM of the central peak as a function of depth of focus for three different lenses; Fig. 16 is a graphical representation of the Strehl ratio for the same three lenses utilized as the starting point for Fig. 15;

Fig. 17 is a graphical representation showing the percentage intensity of the surrounding ring structure for the three lenses utilized to generate the graphs of Figs. 15 and 16; use in the present invention which incorporates a diffractive structure and incorporating a central obscuration as used in conjunction with a DVD disc with a relatively thin substrate;

Fig. 19 is a graphical representation of the super-resolution lens shown in Fig. 18 as utilized in conjunction with a CD disc having a relatively thick substrate;

Fig. 20 is a graphical representation showing the axial displacement between the zero order image and the first order image without the presence of the two disc substrates when utilizing the lens design shown in Figs. 17 and 18; Fig. 21 is a schematic representation of an alternate super-resolution objective for use in the present invention which incorporates a diffractive amplitude mask having two blocking rings or zones;

Fig. 22 is a schematic representation of the lens prescription of the objective of Fig. 21 without the diffractive amplitude mask; Fig. 23 is a graphical description of ring radii and groove depth for the diffractive amplitude mask shown in Fig. 21 ;

Figs. 24 and 25 together form a PSF map for the super-resolution lens using a diffractive amplitude mask illustrated in Fig. 21 ; and

Fig. 26 is a PSF map for the standard, non-super-resolution lens shown in Fig. 22.

DETAILED DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of a system known in the prior art for using super-resolution optical elements in image forming apparatus. That prior art system shown in Fig. 1 utilizes a phase plate to achieve the super-resolution. That patent (U.S. patent 5,349,592) does not teach the use of a single 780 nm pickup for use with CD, DVD and CD-R discs. In fact, this prior art patent cannot be used in optical pickup systems because it only uses (Pi+2N*Pi) phase shifts that always reduce the central peak energy to a very low value and produce side lobes with very high intensity.

Fig. 2 is a schematic representation of a second technique whereby super-resolution is achieved by interposing an apodization filter or mask in the optical pathway between the beam splitter and the photodetector. This arrangement can also serve as a confocal imager to improve the effective spot size.

Fig. 3 is a schematic representation of the cross sections of diffractive surfaces, super-resolution surfaces and super-resolution lenses that combine these diffractive surfaces and super-resolution surfaces, which may be utilized in the present invention. Fig. 4 is a graphical representation of reflectivity of a CD-R disc showing reflectivity percentage on the vertical axis as a function of the wavelength of a light beam directed at the disc, with wavelength being expressed on the horizontal axis. This graph demonstrates clearly that CD-R discs are essentially non-reflective of 650 nm light beams but are highly reflective of 780 nm light beams.

Figs. 5 and 6 are three dimensional graphical representations showing the diffraction intensity spread function utilizing 780 nm wavelength light and defocusing values of -0.001 mm and -0.002 mm, respectively. Figs. 5 and 6 are theoretical graphical representations assuming use of a perfect lens with zero wavefront error. In concert with Fig. 9, these PSF plots show the spot features through the system depth of focus.

Figs. 7 and 8 are three dimensional graphical representations of the diffraction intensity spread function using 780 nm wavelength light and defocusing values of -0.001 mm and -0.002 mm, respectively. These graphical representations are with respect to use of a super-resolution lens and represent the relative intensity values of the spot depicted as the central spike, and the side-lobes depicted as the rings or ripples extending outwardly from the central spike. These graphical representations show the effectiveness of the use of super-resolution techniques in reducing the width of the central peak compared to a theoretically perfect lens. In concert with Fig. 10, these PSF plots show the spot features through the system depth of focus.

Super-resolution lenses use intensity and/or phase variations across their exit pupil to modify the diffraction properties of the system. Prior art lenses that have no optical aberrations and have uniform intensity and phase across their exit pupil will produce the Airy diffraction pattern as the image of a point object.

Classically this condition has been described as a "diffraction-limited" lens. Fig.

9 shows the intensity PSF of a 0.60 NA classic "diffraction-limited" lens. The

PSF consists of a sharp central peak with a weak annular ring that contains about 13% of the energy in the image of a point object.

Fig. 10 shows the PSF of one kind of super-resolution lens. Note that the central peak measured at the FWHM is about 80% of the Airy pattern in Fig. 9.

Note also that the height of the central peak is only about 32% of the Airy pattern, the annular ring side-lobe intensity is much higher, and about 35% of the energy resides outside the central peak. This PSF was produced with an aberration-free lens that had a super-resolution phase function as shown in Fig. 11 placed at its exit pupil. For DVD and CD optical pickup systems, the sharper central peak enhances the ability to read the data tracks. The depth of focus for super-resolution systems can also be increased to make autofocusing the system easier. The stronger annular ring (side-lobe) reduces the modulation of the spot by data pits. DSP (digital signal processing) methods can be used to reject some of the side-lobe contribution to the signal.

Other super-resolution intensity or phase functions can be used to achieve this effect and Fig. 11 is representative of this larger class of super- resolution "transmission functions." The function shown in Fig. 11 is a phase- only arrangement of three concentric rings with zero, +0.5 wave and +0.67 wave phase shift. The annular zones are at 0.40 and 0.70 of the exit pupil radius. The advantage of using super-resolution lenses in DVD optical pickup systems is that a single inexpensive long-wavelength laser diode can be used with an objective that has a numerical aperture of between 0.60 and 0.66 to achieve DVD spot FWHM values. Numerical apertures of between 0.60 and 0.66 are preferred in the present invention, although the invention can operate with up to 0.72 NA. Fig. 12 shows the simple pickup system which can be used for either DVD or CD data records.

Super-resolution lenses decrease the width of the central peak in the spot

image but also reduce the peak intensity and increase the intensity of the secondary ring (side-lobes). DSP can be used to reduce the impact of super-resolution side-lobes in optical pickup systems because the tracks adjacent to the track being interrogated will modulate the energy in the side-lobes differently than the energy in the central peak of the image. When the spot is imaged onto the disc, the image will be large enough so that the "side-lobes" will fall primarily onto the tracks adjacent to the track being interrogated by the central peak of the spot image. Quadrature autofocus detectors divide the energy in the reflected image into four components. Two of these detectors primarily sense the energy from the central peak and from the left-side adjacent side-lobe while the other two detectors sense the energy from the central peak and from the right-side adjacent side-lobe. Because the three tracks have different pit patterns, there are three distinct frequency patterns. The frequency pattern that is present on both the left-side detectors and the right-side detectors represents the desired signal whereas the frequency patterns that are different on the left-side and right- side detectors represent the unwanted side-lobe noise and these signals can be rejected. Other methods can be used that are based on this common-mode detection scheme. In three-beam tracking systems, the frequency patterns in the three tracks interrogated by the three spots can be used for the common- mode detection scheme. Fig. 13 describes a 0.60 NA DVD reader objective with an amplitude mask placed in the entrance beam. This embodiment obscures the central half of the aperture (a loss of 25% of the incident energy). The central obscuration can be placed on the lens itself or on a thin plate preceding the lens. This obscuration has a very important impact on the tolerances of the lens as well as changing the point spread function. Because the central half of the lens is not used in image formation, only the outer annular zone needs to be fabricated with good optical surface figure, good homogeneity and low bi-refringence. Molding this lens is much easier than molding a lens with the full surface and volume contributing to the image formation.

Fig. 14 shows the point spread function at best focus for the lens described in Fig. 13. Note that the ring structure is weak and will not reduce modulation of the DVD pit when scanned by the strong central peak.

Fig. 15 shows the FWHM of the central peak as a function of the depth of focus for three different lenses. The lenses are a perfect diffraction-limited lens without super-resolution features, a lens with amplitude mask super- resolution features, and a lens with phase mask super-resolution features. Fig. 15 shows that both super-resolution masks produce essentially the same FWHM as a function of the depth of focus, and that the central peak is about 85% of a lens without super-resolution features. The smaller central peak is also maintained for 150% of the depth of focus of a lens without super-resolution features.

Fig. 16 shows the Strehl ratio for these three lenses. The Strehl ratio is the peak intensity of the PSF divided by the peak intensity of a diffraction-limited lens. Note that the amplitude mask produces a higher Strehl ratio than the phase mask and, more importantly, this function is centered on the same focal point that the FWHM function is centered. The phase mask Strehl ratio function

is not centered on the FWHM function and this is a non-optimal condition because the peak intensity is not at the same focal point as the best FWHM for the phase mask system.

Fig. 17 shows the percentage intensity of the surrounding ring structure (normalized to the peak intensity of the central peak of the PSF of each lens). This figure shows that the amplitude mask produces about half the ring intensity that the phase mask produces. More importantly, the total energy in the ring structure is much less for the amplitude mask super-resolution lens than for the phase mask lens.

All of the above features show that the simple amplitude mask produces much better super-resolution point intensity functions than the exemplary phase mask. Other phase mask geometries and the combination of amplitude plus phase masks may provide additional improvement in Strehl ratio or reduced side-lobe intensity.

The 780 nm laser operating with a 0.66 NA objective with amplitude super-resolution mask produces about a 0.53 micron FWHM spot. The super- resolution feature offsets the reduced depth of focus associated with normal 0.66

NA objectives and provides a modest manufacturing tolerance margin.

Fig. 18 shows a preferred embodiment of the invention which uses COC (cyclic olefin copolymer) plastic which can be injection molded. PMMA or other plastics can also be used, but minimum bi-refringence and low moisture absorption make COC a preferred material. The surface closest to the disk has

an aspheric profile which incorporates a diffractive structure. The surface furthest from the disk is aspheric. Fig. 18 shows the objective used with a DVD disk which has a 0.60 mm thick substrate and Fig. 19 shows the objective used with a CD disk which has a 1.20 mm thick substrate. The thicker CD substrate introduces much more spherical aberration and this aberration is compensated by spherical aberration of opposite sign that is introduced by the first order image produced by the diffractive structure. Because the diffractive surface does not introduce any optical power in the zero order, the DVD disk operation only uses the spherical aberration correction that is provided by the underlying aspheric surface contour.

Fig. 20 shows the axial displacement between the zero order image and the first order image without the presence of the two disk substrates. This figure illustrates how the image from the unwanted order is blurred at the chosen focal point and does not interfere with the chosen focal point image quality. When a

DVD disk is being read, its surface is axially located at the zero order lens focus and the energy resident in the first order focus is spread out over a very large area so it does not produce a noise background to the wanted DVD signal. The same condition is achieved when the CD disk is being read. Figs. 18 through 20 illustrate how a central amplitude obscuration modifies the PSF of the image. The obscuration itself is not shown in Figs. 18- 20, but may be a coating on the lens itself or a separate obscuration as shown in Fig. 13. It is this amplitude obscuration (or a phase-only modification of the wavefront or their combination) that makes it possible to achieve a PSF with 0.57 micron FWHM with an inexpensive 780 nm laser diode and between a 0.60 and

0.66 Numerical Aperture (NA). This is the super-resolution feature of the lens. This super-resolution feature not only "sharpens" the FWHM, but also improves the depth of focus of the lens. There is a minor performance penalty associated with any super-resolution feature; the side-lobes (the ring around the central core of the PSF) become slightly more intense and introduce more noise background. There is a hidden benefit to the central obscuration (compared to the phase-only mask or a lens without super-resolution features). The central obscuration area implies that the molded lens surface quality is unimportant and this is the part of the lens surface that is more sensitive to molding "sink." The central obscuration therefore allows the molder to optimize the molding process to control the surface figure in the outer annular zone.

Thermal degradation of image quality can be a severe problem with plastic optical systems. The novel design of this invention does not have significant loss of image quality with changes in temperature. The choice of using the zero order diffracted image for the DVD case is critical to this thermal insensitivity. The DVD image must be very small (e.g. 0.57 micron FWHM); the zero order diffraction has no refractive power and is essentially non-existent. Therefore, thermal changes to the diffractive surface will have absolutely no impact on the image quality. The DVD image is thermally modified only by the thermal expansion of the lens and the change in refractive index (dN/dT). The thermal expansion only makes the lens behave as though it were a perfect lens with longer focal length, so that has no effect on image quality. The dN/dT partially causes the lens to behave as though it were a perfect lens with longer focal length and slightly affects the aberration balance. The change in aberration balance is negligible and the DVD image quality remains unchanged over a large thermal range (e.g.-10C to +60C) and the disk is automatically moved to the best focal point location.

The CD image is affected by the first order diffraction. The objective lens only requires a small refractive power in the diffractive structure. This condition is part of the novelty of this invention. The diffractive surface only has about 30 waves of power in order to correct the additional spherical aberration associated with the CD disk. Because the diffractive surface has very little power, changes in temperature (which cause the diffractive groove structure to expand or contract) only changes the amount of power by one or two waves. This change in power is distributed across the surface and most of the change in power is associated with making the lens focal length change rather than changing the aberration balance. Therefore, the CD image quality is also essentially free of the thermal degradation over a large temperature range.

The objective lens diffraction efficiency for the two images is controlled by the depth of the groove structure. To deliver the same amount of energy to both the zero and first order images requires a groove depth of about half the depth which delivers all of the energy to the first order. For the COC material and 780 nm laser wavelength, all of the energy will be delivered to the first order if the groove depth is 1.50 micron. The energy will be almost equally divided between the zero and first order images if the groove depth is about 0.75 micron. A small amount of energy for the latter case will be delivered to other diffraction orders but the resultant images are so blurred that they do not contribute to the noise background. The mathematical relationship between diffraction efficiency and groove depth has been published by Swanson and Veldkamp and is well known

to those skilled in the art of diffractive optical design. For the reasons given above, both the size of the unwanted background images and the diffraction efficiency remain essentially unchanged over a very large thermal range. An additional design variable is available to optimize the energy distribution or to control the CD image size. The diffractive surface may be present over a portion of the optical surface so the NA of the CD image path is slower than 0.65 and the CD image increased in size. The present invention includes a further embodiment using a diffractive amplitude mask shown in Figs. 21-26.

The objective is first optimized (Fig. 22) to produce a diffraction-limited image at the DVD disk data surface. The objective lens has a series of at least three annular zones formed on either lens surface as shown in Fig. 21 (the flattest surface is very strongly preferred for fabrication reasons). Some of the annular zones are clear, but alternating zones act to block the light from that zone from passing to the image. The blocking function can be produced by attenuating material added to the surface (dyes or paints) or blocking can be produced by adding a diffractive structure to the zone. The preferred embodiment shown in Fig. 21 uses a diffractive structure because it can be integrally molded into the surface and adding dyes or paints can distort the local surface region unless great care is exercised. The novelty in the diffractive surface is as follows: The diffractive structure adds strong refractive power to the blocking zones so that these zones focus the light well in front of the DVD disk. For the example case disclosed, the primary image has a 0.56 μM diameter and the unwanted light from the blocking zones is spread

over a 240 μM diameter. The blocking zones represent a small portion of the

total lens surface area, so making the diffractive surface cover only the blocking area will minimize the stray light produced by molded diffractive structures. The reverse arrangement is also within the present invention (the diffractive surface covers the unblocked zones and the greater stray light is removed by electronic signal-processing).

The data on Figs. 21-26 shows the lens prescription, the blocking zone geometry and PSF maps for a standard (non super-resolution) lens plus the super-resolution lens using the diffractive amplitude mask.

The present invention allows the use of a single, relatively low cost laser in conjunction with a relatively low cost super-resolution optical element for the purpose of reading and/or writing a variety of discs having different substrate thicknesses or pit sizes. The invention thereby avoids the use of multiple lasers with different wavelengths and the complex, expensive optics and/or mechanical switching mechanisms inherently required with the multiple lasers. As used in the claims, the phrase "super-resolution means" is used in its broadest sense, and includes without limitation the embodiments disclosed herein, all of which include a single objective lens used in conjunction with one or more super- resolution techniques (amplitude mask, phrase mask and diffractives). The present invention also includes one or more embodiments capable of achieving 2 or more axially spaced images for reading and/or writing discs of different substrate thicknesses. At present, 780 nm lasers are relatively low cost compared to 650 nm lasers. However, in the near future, 650 nm lasers may well become as low cost

as 780 nm lasers and lasers with wavelengths shorter than 650 nm will become

more widely used. The present invention will perform with a single, 650 nm laser to read multiple discs with different substrate thicknesses or pit sizes.

Claims

1. An apparatus using a single laser to read and/or write a plurality of discs, wherein said discs have different substrate thicknesses and different pit sizes, comprising: a single laser producing a first output beam of coherent light, a first optical pathway for directing a first portion of said output beam onto one of said plurality of discs, a second optical pathway for directing a second portion of said first output beam to a photodetector to read said disc, and super-resolution means included in said first optical pathway to produce an image with said first output beam to read and/or write any of said plurality of discs without requiring the use of a second laser output beam.

2. The apparatus of claim 1 wherein said super-resolution means comprises a phase plate.

3. The apparatus of claim 1 wherein said super-resolution means comprises an amplitude apodization filter.

4. The apparatus of claim 1 wherein said super-resolution means comprises both a phase plate and an amplitude filter.

5. The apparatus of claim 1 wherein said super-resolution means comprises a diffractive lens.

6. The apparatus of claim 1 wherein said super-resolution means comprises a diffractive amplitude mask.

7. An apparatus using a single 780 nm laser to read DVD and high density CD discs as well as to read and/or write CD discs, comprising: a 780 nm laser producing a first output beam of coherent light, a first optical pathway for directing a first portion of said output beam onto either a CD, DVD or a read and/or write CD disc, a second optical pathway for directing a second portion of said first output beam to a photodetector to read said CD, DVD or read and/or write CD disc, and super-resolution means included in said first optical pathway to achieve a sufficiently small spot size with said first output beam to read DVD discs, high density CD discs and to read and/or write CD discs without requiring the use of a second, 650 nm laser output beam.

8. The apparatus of claim 7 wherein said super-resolution means comprises a phase plate.

9. The apparatus of claim 7 wherein said super-resolution means comprises an amplitude apodization filter.

10. The apparatus of claim 7 wherein said super-resolution means comprises both a phase plate and an amplitude filter.

11. The apparatus of claim 7 wherein said super-resolution means comprises a diffractive lens.

12. The apparatus of claim 7 wherein said super-resolution means

comprises a diffractive amplitude mask.

13. A method for using a single 780 nm laser to read DVD and high density CD discs without requiring the use of a second, 650 nm laser output beam and to read and/or write CD discs, comprising the steps: producing a first output beam of coherent light from said single 780 nm laser, directing a first portion of said first output beam onto either a CD, DVD or read and/or write CD disc, directing a second portion of said first output beam to a photodetector to read said CD, DVD or read and/or write CD disc, and generating a reduced spot size with said first output beam by super-resolving said first output beam to read DVD discs, high density CD discs, and to read and/or write CD discs, all of which can be done without requiring the use of a second, 650 nm laser output beam.

14. The method of claim 13 wherein super-resolving said first output beam is accomplished by passing said first output beam through a phase plate.

15. The method of claim 13 wherein super-resolving said first output beam is accomplished by passing said first output beam through an amplitude apodization filter.

16. The method of claim 13 wherein super-resolving said first output beam is accomplished by passing said first output beam through a phase plate plus amplitude filter.

17. The method of claim 13 wherein super-resolving said first output beam is accomplished by passing said first output beam through a diffractive

lens.

18. The method of claim 13 wherein super-resolving said first output beam is accomplished by passing said first output beam through a diffractive amplitude mask.

19. An apparatus using a single laser diode to read DVD and high density CD discs as well as to read and write CD-R discs, comprising: a single laser diode with an emission wavelength above 700 nm producing a first output beam of coherent light, a first optical pathway for directing a first portion of said output beam onto either a CD, DVD or CD-R disc, a second optical pathway for directing a second portion of said first output beam to a photodetector to read said CD, DVD or CD-R disc, and super-resolution means included in said first optical pathway to achieve a sufficiently small spot size with said first output beam to read DVD discs, high density CD discs and to read and write CD-R discs without requiring the use of a second, 650 nm laser output beam, said super-resolution means including an objective having a numerical aperture between 0.60 and 0.72.

20. The apparatus of claim 19 where the super-resolution lens incorporates a diffractive surface which serves the function of producing two axial images.

21. The apparatus of claim 19 where the image produced by the diffractive surface zero order lies furthest from the lens, is not modified by the

diffractive surface properties, and yields a very small spot size needed for reading DVD disks, and wherein the image produced by the diffractive surface first order lies closer to the lens and has additional aberration correction that compensates for the thicker CD disk thickness and its larger aberration contribution.

22. The apparatus of claim 19 where the groove depth of the diffractive surface is chosen to deliver the desired proportion of laser energy to the two axially-displaced images.

23. The apparatus of claim 22 where the groove structure extends over only a portion of the full optical surface so that more energy can be directed to the DVD image or the size of the CD image can be made larger to be consistent with the larger data record of the CD media.

24. The apparatus of claim 19 where the super-resolution feature consists of a central obscuration that reduces the image PSF FWHM.

25. The apparatus of claim 19 where the super-resolution feature consists of a phase-only mask that reduces the image PSF FWHM.

26. The apparatus of claim 19 where the super-resolution feature comprises the combination of an obscuration mask plus a phase mask.

27. The apparatus of claim 19 wherein said objective has a numerical aperture between 0.60 and 0.66.

28. The apparatus of claim 19 wherein said objective comprises a diffractive amplitude mask.