I IMMIIIIIIIIMMIIIIIIIIIIIIIIIIIIIIIIIIIIIIUlIIIIIIIIIIIIIIIUl
`
`US005349471A
`[111 Patent Number:
`[45] Date of Patent:
`
`5,349,471
`Sep. 20, 1994
`
`United States Patent
`Morris et al.
`
`[19]
`
`[54] HYBRID REFRACtIVE/DIFFRACTIVE
`ACHROMATIC LENS FOR OPTICAL DATA
`STORAGE SYSTEMS
`
`[75] Inventors: G. Michael Morris, Fairport; David
`Kay, Rochester, both of; Dale
`Buralli, both of Rochester, all of
`N.Y.; David Kubalak, Somerville,
`Mass.
`
`[73] Assignee: The University of Rochester,
`Rochester, N.Y.
`
`[21]
`[22]
`1511
`[52]
`[58]
`
`156]
`
`Appl. No.: 17,712
`
`Filed:
`
`Feb. 16, 1993
`
`Int. CI.5 .......................... G02B 3/08; G02B 5/18;
`G02B 27/44
`U.S. CI ..................................... 359/565; 359/566;
`359/569; 359/571
`Field of Search ............... 359/355, 356, 357, 565,
`359/566, 569, 571; 369/109
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,768,183 8/1988 Ohnishi et al..
`5,044,706 9/1991 Chen.
`5,078,513 1/1992 Spaulding et al..
`5,117,306 5/1992 Cohen.
`5,117,433 5/1992 Tatsuno et al..
`5,148,314 9/1992 Chert ................................... 359/565
`5,151,823 9/1992 Chen ................................... 359/565
`5,155,553 10/1992 Chen ................................... 359/565
`5,157,555 10/1992 Reno ................................... 359/565
`5,161,040 11/1992 Yokoyama et al. ................. 359/565
`5,161,057 11/1992 Johnson.
`5,229,880 7/1993 Spencer et al ...................... 359/357
`
`FOREIGN PATENT DOCUMENTS
`
`WO91/12551 8/1991 PCT Int’l Appl. .
`
`OTHER PUBLICATIONS
`
`Goto et al, Proe. Int. Syrup. on Optical Memories-Japa-
`nese J. AppL Phys, vol. 26 (1987), p. 135.
`Tanaka et al, J.P. Nat. Toeh. Repts, vol. 35, No. 2, (Apr.
`1989).
`Primary Examiner--Martin Lerner
`Attorney Agent, or Fi..,’m--M° LuKaeher
`ABSTRAUr
`[57]
`A diffraefwe/refraetive hybrid lens for use in an oprieal
`data storage system as an objective is provided by a
`convex-piano singlet having a refractive element de-
`freed by piano-convex surfaces and a diffractive de-
`ment defined by a Fresnel zone-like pattern on the plano
`surface which together provide the total power of the
`lens. The refractive lens is made of a high index, high
`dispersion glass so that the curvature and thickness of
`the refractive lens is minimized while providing a large
`numerical aperture (at least 0.45) at the expense of in-
`creased longitudinal chromatic aberration, which are
`compensated by the diffractive element and without the
`need for one or more additional curved surfaces as in
`low index biaspherie glass objective lenses for ehro-
`marie and mono-ehromatie aberration reduction, which
`increases the thielmess and curvatures of the lens. The
`invention enables longitudinal chromatic aberration to
`be corrected for at least a 10 nm band width around a
`center wavelength over a 20 nm range, as results when
`different lasers are used and as laser power varies during
`optical data storage on an optical data storage device
`(an optical disk). The thin, light weight low curvature
`aehromat has maximum tolerance for various possible
`manufacturing errors such as decentering, variations in
`thickness of the lens, tilt and focal length especially for
`on-axis field of view less than 2* while providing a very
`high quality spot (Strehi ratio of at least 0.9.).
`
`6 C]Rims, 4 Drawing Sheets
`
`/6
`
`, 3 - D/FFRACT/ VE ,..qUt~ FACE
`
`5URFAC£
`
`, .’z--. [ Iop TICAL AXIS
`
`HYBRID
`LEN,~
`
`SUwgSTRAT, E
`
`LG Electronics, Inc. ct al.
`EXHIBIT 1011
`IPR Petition for
`U.S. Patent No. RE43,106
`
`

`
`U.S. Patent
`
`Sep. 20, 1994
`
`Sheet 1 of 4
`
`5,349,471
`
`SURFACE
`
`/E SURFACE
`
`FIG. 1
`
`L R£CORDIlV6
`LAYER,
`
`FIG. 2
`
`INCIDENT
`
`I ~ \ \ .P÷mAo
`
`~ FIG. 3
`
`

`
`U.S. Patent
`
`Sep. 20, 1994
`
`Sheet 2 of 4
`
`5,349,471
`
`F
`
`I
`
`FIG. 4
`
`--~/0o
`80
`
`r~ 6o
`zlo
`
`V , , ,
`
`0.9
`0.8
`0.7’
`0.6
`0.5
`0.,,(cid:128) ~ 0
`/NCIDENT- WAVELENGTH, ,~ (’,u.m ,)
`FIG. 5
`
`

`
`U.S. Patent
`
`Sep. 20, 1994
`
`Sheet 3 of 4
`
`5,349,471
`
`i.o0o-
`
`i i i
`
`i
`
`.995-
`
`o99o~
`
`0.985-
`
`0.9800 o
`
`¯
`
`",%
`WAVELENOTH~
`~~
`"---. 780nm +/-" 4rim
`"’,~x
`---.79ohm +/- ~m
`’~
`
`’ ’
`0.4 0.6
`
`!
`0.8
`
`I
`
`1.0
`
`f.00-
`
`0.99
`
`0.96
`
`0..94
`
`0.92-
`
`0..90
`
`.(cid:128)
`
`FIG. 6
`
`0° FIELD ANGLE
`
`WAVE/_EIV’GTHS ~" "f~:~ V" DII~ECTION
`........ 7701~m ÷/- 4rim
`78Ohm ÷/-.4nm
`.... 7"9Ohm ÷/-,4nm
`
`|
`
`-20
`
`|
`
`I
`
`I
`
`|
`
`eO
`dO
`0
`- fO
`DE CEN TERING ("N m)
`
`FIG. 7
`
`

`
`U.S. Patent
`
`Sep. 20, 1994
`
`Sheet 4 of 4
`
`5,349,471
`
`SURFACE
`l
`
`RADIUS(.,,.) TmCK~ESS(,~.,) ~
`2. 12209
`2.49707
`
`,,,,,,
`
`o. o
`
`INF" 1 10059
`
`f. 7~5
`
`DIFF. SURFACE
`
`i ,
`
`1.0
`
`4
`
`IIVF"
`
`Z ~0000
`
`IIVF O. 0
`
`, .,,,,..
`
`FIG. 8
`
`¯ FIG. 9
`
`

`
`5,349,471
`
`HYBRID REFRACTIVE/DIFFRACNVE
`ACHROMATIC LENS FOR OPTICAL DATA
`STORAGE SYSTEMS
`
`The present invention relates to hybrid refractive/-
`diffractive lenses and particularly to a refractive/dif-
`fractive achromat which is especially suitable for use in
`optical data storage or recording systems which use
`laser light from a laser diode to write and read data on
`an optical storage medium such as an optical disk.
`In optical data storage systems, the spot of light
`needed to write and read must be extremely small and
`the lens that focuses the light must be well corrected for
`aberrations. It is not enough for a lens to be corrected
`for its own aberrations, the lens must also be corrected
`for wavelength shift in the laser. For each color or
`wavelength of light incident on the lens, the lens will
`have a different focal length. This is referred to Iongitu.
`dinal chromatic aberration. While automatic refocusing
`mechanisms of optical data storage systems can correct
`for slow wavelength changes, individual lasers, espe-
`cially laser diodes used in optical data storage systems
`shift wavelength almost instantaneously. For example,
`the laser beam shifts in wavelength with output power
`by approximately 0.08 nm/mW. Laser diodes also mode
`hop with power changes. This effect is especially signif-
`icant during digital recording. In addition, laser diodes
`operating at constant output power tend to mode hop
`(the longitudinal modes) with very small amounts of
`optical feedback, as low as a few tenths of a percent.
`While such mode hopping may be tolerated when the
`system is used for reading (in read optical heads), in
`writing or recording, the focus tolerance is tighter and
`focus shifts affect the quality (introduce errors) in the
`recorded data.
`Current designs for optical recording objective lenses
`use low dispersion, low index of refraction glass to
`minimize longitudinal chromatic aberration. Low dis-
`persion biaspherie glass objective lenses are used in
`lower cost systems. Multi-element lenses are used to
`provide chromatic aberration correction in multi-ele-
`ment more costly lens systems and the multi-element
`lenses generally have shorter working distances (the
`distance between the lens and the optical disk surface).
`Moreover, in order to reduce the difficulty of correct-
`ing for chromatic aberration, lens designs have been
`driven in the direction of reducing dispersion (using low
`index, high Abbe number glass). Higher curvature,
`thicker lenses have therefore been required. Such lenses
`are more difficult to manufacture, since they are more
`sensitive to variations in lens thickness, wedge, and
`deeentering. Such lenses can also sacrifice weight and
`size in order to achieve low chromatic aberration, for
`example singlet lenses require strong biaspberic sur-
`faces.
`In an effort to alleviate mono-ehromatie aberration
`without a second surface with power, it has been pro-
`posed to use a grating. See K. Goto, et al., "Spherical
`Grating Objective Lenses for Optical Disk Pick-Ups",
`Proe. Int. Sym. on Optical Memories (1987)--Japanese
`Journal of Applied Physics, Vol. 26 (1987) Supplement
`26-4. The singlet described in the Goto et al. article
`utilizes moderate optical material (plastic) for 0.45-0.5
`N.A. lenses that require hundreds of zones in the dif-
`fractive element. The grating is not designed to correct
`for chromatic aberration.
`
`2
`Although various patents and publieatious have dis-
`cussed the use of diffractive elements to compensate for
`chromatic aberration (See Olmishi, U.S. Pat. No.
`4,768,183, Aug. 30, 1988; International Patent Publica-
`5 tion WO 91/12551, Aug. 22, 1991; U.S. Pat. No.
`5,117,433, May 26, 1992; U.S. Pat. No. 5,044,706, Sep. 3,
`1991; U.S. Pat. No. 5,078,513, Jan. 7, 1992, U.S. Pat. No.
`5,117,306, May 26, !992; U.S. Pat. No. 5,161,057, Nov.
`3, 1992), designs for optical rreording objective lenses
`10 have consistently used low index, low dispersion glass
`in order to minimize chromatic aberration. See also
`Tanaka, et al., "A Spherical Molded Glass Lens of
`Super-low Chromatic Aberration", JP Nat. Teeh.
`Repts. Vol. 35, No. 2, April 1989.
`15 The present invention deviates from the conventional
`wisdom in the field of optical data storage/recording
`objective lenses and utilizes high index high dispersion
`glass, thereby loosening manufacturing tolerances with
`respect to alignment of the lens in the data storage sys-
`20 tern (tilt, etc.) and in the lens itself (glass element thick-
`ness, focal length, etc.), and obtaining a thinner, lens. It
`has been discovered in accordance with the invention
`that the increased chromatic aberration resulting from
`the use of the high index high dispersion material for the
`25 refractive element of the lens can be corrected by a
`diffractive element which is incorporated into the lens
`thereby providing an improved hybrid refractive/dif-
`fractive achromatic for optical data storage applica-
`tions.
`30 The invention provides an improved optical data
`storage objective lens which is suitable not only for
`reading but also writing information on an optical data
`storage media which has: (a) high NA; Co) high Strehl
`ratio (high quality spot) at the recording surface; (c)
`35 chromatic correction; (d) a long working distance (air
`space) between the front surface of the lens and the
`optical recording medium, (e) small size and light
`weight to facilitate mechanical focusing and tracking of
`the head; and (f) on axis field of view of less than 2*.
`40 Laser diodes used in optical recording applications
`are subject to wavelength shifts of for example up to 10
`nm band-width, over a 20 um wavelength range (e.g.,
`±5 nm from 770 nm to 790 nm). A hybrid refractive/-
`diffractive aehromat objective in accordance with the
`45 invention provides chromatic correction over such a
`range.
`The foregoing and other objects, features and advan-
`tages of the invention will become more apparent from
`a reading of the following description in couneetion
`50 with the accompanying drawings in which:
`FIG. 1 is a schematic diagram of a diffractive/refrac-
`tive hybrid lens in accordance with the invention shown
`adjacent to an optical disk, the lens having a curved
`surface 1 and a grating or diffractive surface 3, the
`55 features on the diffractive surface being too small to be
`seen on the scale of the Figure;
`FIG. 2 is a sectional view of the lens shown in FIG.
`1 but the diffractive surface features are greatly magni-
`fied;
`60 FIG. 3 is a diagrammatic, perspective view of a Fres-
`nei zone pattern which may be formed as by blazing on
`the diffractive surface of the lens shown in FIGS. 1 and
`2, where ~ is the design wavelength, m is an integer
`greater than 0, f is the focal length and F designates the
`65 focal point;
`FIG. 4 is a greatly enlarged side view of the diffrac-
`tive surface of the lens shown in FIGS. 1 and 2 showing
`the surface blaze profde of a few zones, the actual thick-
`
`

`
`5,349,471
`
`4
`Light travels in waves, which can interfere. If the
`waves interfere such that the peaks and valleys coin-
`cide, the energy in the two waves adds to each other;
`this is referred to as constructive interference. Note that
`5 if one of the waves is delayed exactly one or more
`wavelengths behind the other, then it is once again in
`phase, and they will interfere constructively. If the
`waves line up out of phase, the energy in one wave will
`cancel the energy in the other; this is referred to as
`10 destructive interference.
`To design the diffractive surface, a Fresnel zone pat-
`tern is used, as shown in FIG. 3. A focal point, F, is
`designated at a distance, f, from the center of the pat-
`tern. This distance is equal to the focal length. The
`15 rings, or zones, are spaced such that the edge of each
`zone is exactly one wavelength further away from the
`point F. This way, light passing through the pattern at
`the edges of the zones will be in phase and construe-
`tively interfere at the point F.
`20 Using right triangles, an equation can be derived that
`gives the radii of the zones as a function of the focal
`length (distance from the pattern to F) and the wave-
`length of light used to design the pattern:
`
`3
`ness being of the order of microns and the spacing be-
`tween the zones actually being of the order of tens of
`microns;
`FIG. 5 is a plot of the efficiency of the lens for design
`wavelength 80 a of 780 rtm and showing the boundaries
`of a 20 run wavelength range;
`FIG. 6 is a plot of the polyehromatic Strehl ratio for
`an 8 nm band-width around three possible center wave-
`lengths of laser diodes, the center wavelengths and the
`___4 nm wavelengths all being weighted equally;
`FIG. 7 is a plot of the polychromatie Strehl ratio as a
`function of diffractive surface decentering with decent-
`ering in the y direction, the x and y directions being
`orthogonal to each other in a plane perpendicular to the
`optical axis;
`FIG. 8 is a table showing a lens design for the sur-
`faces indicated in FIG. 1; and
`FIG. 9 is a table showing exemplary surface data for
`surface 1, the aspheric surface and surface 3, the diffrac-
`tive surface shown in FIG. 1. The phase coefficients A
`are from equation (3) and the aspherie coefficients D are
`from equation (11), the units being defined in FIG. 9 and
`E being an exponent.
`Referring to FIG. 1 there is shown a diffractive/re-
`fractive hybrid lens 10 for use in an optical data storage 25
`system having an optical disk substrate 14 of transparent
`plastic, such as polycarbonate. The lens 10 is a piano
`convex singlet having a curved surface 1 and a Fresnel
`zone pattern on the piano surface 3 of the lens body
`which is the surface opposite to the curved surface. 30
`Both surfaces are perpendicular at the optical axis of the
`lens. The refractive lens is made from optically trans-
`missive material having a high index of refraction of at
`least 1.65. Suitable material is flint glass, sueh as LaF
`glass. FIG. 2 shows the lens 10 and emphasizes its
`curved surface which defines the refractive element as
`well as the Fresnel zone pattern which defines the sur-
`face 3 and the diffractive element. An annular ring 16 is
`part of the lens body and is merely for attachment and
`location in a barrel of an optical head used in the optical
`data storage system. The overall thickness of the lens
`may be about 2.12 mm (millimeters). The radius of the
`curved surface may be about 2.5 mm to a point along
`the optical axis on the fight of that surface. Exemplary
`dimensions and spacings are set forth in FIG. 8. In FIG.
`8 the index is at the center of range (nm); being mea-
`sured at 780 nm. The depth of echelons in the zones is
`exaggerated in FIG. 2 and may be of the order of a
`micron and the spacing between the zones on the order
`of tens of microns.
`Although the diffractive element can be added to the
`curved surface I, it is preferable that the diffractive
`elements be formed on the piano surface.
`The achromat is effectively a singiet in size, but is in
`effect a doublet in that the refractive and diffractive
`elements work together to add to the final total power.
`Table I, which will be discussed in greater detail
`below, shows that 95% of the total power of the achro-
`mat is in the refractive element. It therefore behaves
`much like a singlet. The diffractive surface compensates
`for longitudinal chromatic aberration, but also, because
`of introduction of higher order terms (4th order, 6th
`order, 8th order and 10th order--see FIG. 9) in the
`phase function (equation (3)) of the diffractive surface 3,
`monochromatic aberrations, such as spherical aberra- 65
`tion and coma are also substantially corrected.
`Consider the design of the diffractive surface 3. The
`design takes advantages of the wave nature of light.
`
`using the fact that the wavelength of light is much
`smaller than the focal length, Eq. (1) can be reduced to
`
`r2ra=2mhaf.
`
`(2)
`
`From Eq. (2), it can be seen that the diffractive sur-
`face has a strong dependence on the wavelength of light
`used to construct the zones. If the wavelength of light
`35 incident on the diffractive surface deviates from the
`design wavelength, the focal length also changes. This
`is an important property when the diffractive surface is
`used to achromatize the refractive element.
`Although the light at the edge of zone is in phase
`40 when it gets to the focal point F, the light coming
`through the middle of each of the zones is not yet in
`phase, and therefore will not interfere constructively.
`To correct this problem, material is added in the middle
`of the zones to delay the phase jnst enough so that at the
`45 point F, all the light coming through the surface con-
`structively interferes. This blaze is shown in FIG. 4.
`In the center of the zone pattern, where the material
`is the thickest, the light is delayed exactly one wave-
`length. Moving away from the center of the pattern, the
`50 distance from the focal point increases so that less mate-
`rial is needed. The material is gradually thinned to a
`minimum at the edge of the first zone, where no addi-
`tional delay is needed, beeanse the distance at the edge
`of the first zone is one wavelength further from the
`55 focal point than the center of the ring pattern. Again
`material is added to delay the light exactly one wave-
`length, but the light is still in phase. Since the material
`is once again thick, the process starts over. This way all
`the light passing through the diffractive surface will be
`60 in phase and constructively interfere at the focal point.
`In general, the phase delay introduced by such a surface
`can be described with Eq. (3):
`
`2~-
`q~ = ~ (A2r2 W A4~ q- -46t6 q- ASr8 + A10rl0).
`
`(3)
`
`For now, A2=l/(2F), and A4, A6, etc.=0. Giving
`value to the higher order phase terms, At, A6, etc., has
`
`

`
`5,349,471
`5 6
`the same advantage as introducing aspherical terms for
`This number has very important ramifications when
`a glass surface, and is useful for minimizing monoehro-
`achromatizing lenses.
`mie aberrations. Whenever dp is equal to an integer
`The power of the refractive element is balanced with
`multiple of 2¢r, r is the radius of a new zone.
`Using F-x!. (3), it is possible to design a diffractive 5 the power of the diffractive surface. The power of each
`surface that is nearly 100% efficient at the desired focal
`element add to equal the desired power of the achro-
`mat, and the powers are also of the correct proportion
`point. The efficiency changes, however, with change in
`the wavelength of incident light, in a manner described
`so that the longitudinal chromatic aberration is zero.
`These power are given by
`by
`
`10
`
`sin ~r - 1 (a) q~ref v,ef- vaiff ua~ff- vrey
`
`(10)
`
`)2 "
`
`*r
`
`-- 1
`
`15 tive and diffracted elements, (cid:128)~refand~diffare the powers
`
`where ko is the design wavelength and X is the incident
`wavelength. For a design wavelength of ko=0.780 i~m,
`the efficiency as a function of wavelength is plotted in
`FIG. 5.
`To account for variations in the center wavelength of
`780 nm laser diodes, optical data storage lenses should
`work well for light at a wavelength of 780+ 10 urn.
`Undiffraeted light, or light not diffracted into the right
`focal point, becomes unwanted stray light at the focal
`plane. As seen in FIG. 5, a 20 nm bandwidth centered
`on 780 nm does not appreciably decrease the diffraction
`efficiency.
`By equating Eqs. (5a) and (Sb), for the powers of a
`thin glass lens and a diffractive lens, F-xt. (6) is obtained.
`Equation (6) is used to find the index of refraction with
`light at wavelengths other than the design wavelength.
`
`of the two elements, and ~btot is the total power of the
`lens.
`Once the required focal length for the diffractive
`20 surface is found, the zone spacing which results in this
`focal length is then determined as discussed in connec-
`tion with FIGS. 3 and 4. Once the proper zone locations
`are determined, they are then fine tuned to minimize
`field (mono-ehromatie) aberrations, as discussed in con-
`25 nection with FIG. 9.
`Eq. (9), shows the ~ number for a diffractive lens is
`vd=--3.45. Since the lowest v number for glasses is
`about 20, the diffractive lens is found to be much more
`dispersive than any refractive lens. It is also seen to be
`30 negative, where all glasses are positive. Equations (10a)
`and (10b) e.an again be used to design a diffractive/re-
`fractive hybrid aehromat with a focal length of 3 ram.
`Typical values for this lens are shown in Table 1 (typi-
`cal values for a 3 man focal length).
`
`35
`
`TABLE 1
`(5) % of total
`power
`~b (~ 0.333/nm)
`
`x
`(a) q,t* = (n(x) - 1)e (b) 4,a~y = Fx0
`
`n(’h)
`
`[(nx0) + 1]
`
`(6)
`
`One measure of how dispersive glasses are, that is,
`how much their index changes with change in wave-
`length, is the Abbe ~J-number formula, Eq. (7). Three
`wavelengths are picked, and their indices axe substi-
`tuted into Eq. (7),
`
`(n,~ - 1)
`v- (ns- nl)
`
`(7)
`
`where ns, nm and nl are the indices of refraction for the
`short, middle, and long wavelengths. When evaluating
`glasses, the wavelengths chosen are usually
`hs=0.48613, ~.m=0.58756, and k1=0.65627. For all
`glasses, the v number for these three wavelengths is
`between 20 and 90. The lower the v number, the further
`ns and nl are from each other, and the glass is more
`dispersive.
`If Eq. (6) is substituted into Eq. (7), the Abbe v-num-
`ber for a diffractive lens is found to be
`
`Xm
`uaiff = xs - ~.I "
`
`(8)
`
`element
`
`nm
`
`v number
`
`power d?
`
`LASF3
`40 diffractive
`
`1.808
`10,001
`
`40.724
`--3.45
`
`0.3073
`0.0260
`
`92.2
`7.8
`
`Because of the unusually low, and negative, v number
`for the diffractive surface, Table 1 shows a weak, posi-
`45 five diffractive element can be used to achromatize the
`lens. Since the powers of the two elements are no longer
`working against each other, the power in the refractive
`element is greatly reduced by using the diffractive sur-
`face. The advantage of having less power in the refrac-
`50 tive element is a thinner lens with lower surface curva-
`tures.
`Because the refractive element is thinner, there is no
`bulky negative element. The diffractive/refractive hy-
`brid achromat is heretofore much lighter than an achro-
`55 mat made from only glass. AdditionaUy, because the
`surface eurvatare (FIG. 1) is not steep, the refractive
`element in the hybrid introduces lower monochromatic
`aberrations than would be present in an all glass aehro-
`mat and high NA lenses are now practical.
`60 The aspheric coefficients for surface 1, defined by Eq.
`(il),
`
`cp2
`
`-t- D4p4 -b D6p6 W DSps + DI0P10,
`
`(11)
`
`If ks, Xm, and kl are chosen as above, then using Eq. 65
`(8), the v number for a diffractive lens is found to be
`
`¯ ’a~ -3.45
`
`(9)
`
`where z is the surface sag from a x-y plane tangent to
`the surface, C is the surface curvature, D4, D6, etc. are
`
`

`
`5
`
`35
`
`7
`the fourth, sixth, etc. aspheric coefficients, and p is a
`radial coordinate in the lens. An example of suitable
`coefficients is given in FIG. 9, along with the phase
`coefficients for the diffractive surface, defined in F--xl.
`(3).
`The lens shown in FIG. 1 focuses incoming light
`down to a spot and the measure of quality for the lens
`will be Strehl ratio. This is a ratio of the amount of
`energy at the center of the spot of light produced by the 10
`actual lens, to the amount of energy at the center of the
`spot if it were produced by an aberration-free leas. A
`Strehl ratio of 1.0, therefore, is theoretically the best
`possible.
`Consider design wavelengths ks-----770 rim, hrn=780 15
`Inn, and k1=790 nm, and that the lens will be forming
`a point focus through a disc substrate 14 (FIG. 1), a
`plate of polyearbonate material. This is the disk sub-
`strate which must be taken into account with the lens 20
`design.
`The laser diode may have center wavelength, for
`example, anywhere in the range between 770 nm and
`790 nm. The lens must be achromatized for an 8 nm
`bandwidth around the center wavelength of each par- 25
`titular laser so that random wavelength fluctuations in
`the laser do not affect performance of the optical re-
`cording system. For example, the ratio of the focal
`length change to wavelength change should be less than
`0.1/~m/nm. To account for possible errors in mounting 30
`the lens, the lens accommodates at least a one degree, 1"
`field of view. The Strehl ratio of the whole system is
`maintained equal to or above 0.9, and the lens itself
`above 0.96 over the full field of view.
`The hybrid diffractive/refractive lens 10 is achroma-
`tized such that the focal length change per change in
`incident wavelength is less than 0.032 /xm/nm. This
`exceeds the requirement for optical data storage.
`FIG. 6 shows the lens design meets this requirement 40
`out to the full field, with a Strehl ratio well above 0.96,
`giving a margin for manufacturing tolerances.
`In addition to mounting alignment error, errors can
`also arise in lens manufacture. If the diffractive surface
`is not lined up exactly with the glass element, for exam- 45
`pie, the Strehl ratio will decrease in a manner shown in
`FIG. 7. FIG. 7 shows plots for both 0* and 1" field of
`view. For 0", the Strehl ratio stays above 0.96 beyond
`20 /~m of decentering. At 1", however, decentering 50
`must be kept under 20 pro.
`Other possible manufacturing errors include glass
`element thickness, diffractive surface tilt, and diffrac-
`tive surface focal length tolerance. The results for each
`tolerance taken separately, along with field angle and 55
`deeentering, are shown in Table 2, and the characteris-
`tics of the exemplary lens are shown in Table 3.
`
`TABLE 2
`
`glass
`
`element
`decentering thickness
`
`diffractive
`surface tilt
`
`60
`
`diffractive
`
`surface
`focal length
`
`---+20 ~tm -----35 pm
`
`±0.2*
`
`__-3 mm
`
`65
`
`eITor
`
`limit to
`maintain
`Strchl > 0.96
`at 10 field
`of view
`
`5,349,471
`
`8
`TABLE 3
`Pupil
`Lens
`diameter
`thickness
`
`Working
`distance
`
`Number of
`rings
`
`Focal
`N.A." length
`
`0.55 2.99 mm
`
`3.3 mm
`
`2.12 mm
`
`1.1 mm
`
`30
`
`From the foregoing description, it will be apparent
`that the invention provides a diffractive/refractive hy-
`brid achromat designed for use in an optical data stor-
`age head. Utilizing the thin and light characteristics of a
`diffractive lens, along with a dispersion opposite of
`ordinary glasses, a small light diffractive/refractive
`hybrid with an extremely high NA is obtained. The
`hybrid achromatic has tolerance limits within the re-
`quirements of optical recording, including a very small
`focal length shift per wavelength shift of incoming
`light. The diffractive surface minimize longitudinal
`chromatic aberration and spherical and coma in refrac-
`tive element without the use of bi-aspheres. Variations
`and modifications within the scope of the invention will
`undoubtedly suggest themselves to those skilled in the
`art. Accordingly, the foregoing description should be
`taken as illustrative and not in a limiting sense.
`We claim:
`1. A hybrid refractive and diffractive achromat lens
`for optical data storage systems using a laser beam hav-
`ing a wavelength which can vary over a 20 nm wave-
`length range, which lens comprises a body of optically
`transmissive material having an index of refraction at a
`wavelength approximately in the center of said range of
`at least 1.65, said body having first and second surfaces
`on opposite sides thereof, said first and second surfaces
`being intersected successively by an optical axis of said
`lens which extends in a longitudinal direction, at least
`one of said surfaces being curved to provide a converg-
`ing refractive element having power and longitudinal
`chromatic aberration, said lens having a converging
`diffractive element having power which substantially
`achromatizes said lens for said longitudinal chromatic
`aberration of said refractive element over said 20 nm
`range; and wherein said lens has an Abbe v number, vrel;,
`of less than 50, and said diffractive element has an Abbe
`v-number, Vdiffwhieh is negative.
`2. The lens according to claim 1, wherein said lens
`has a thickness not exceeding about 3.2 mm between
`said first and second surfaces, said at least one of said
`surfaces having a radius not exceeding about 4.0 ram
`and said lens having a focal length =<_ 5 ram.
`3. The leas according to claim 2, wherein Vdiffis about
`-- 3.45.
`4. The lens according to claim 1, wherein said curved
`surface is said first surface and said second surface has a
`diffractive zone pattern det’ming said diffractive de-
`ment.
`5. The lens according to claim 1 wherein said lens had
`a numerical aperture (NA) of at least 0.45.
`6. A hybrid refractive and diffractive achromat lens
`for optical data storage systems using a laser beam hav-
`ing a wavelength which can vary over a 20 um wave-
`length range, which lens comprises a body of optically
`transmissive material having an index of refraction at a
`wavelength approximately in the center of said range of
`at least 1.65 said body having first and second surfaces
`on opposite sides thereof, said first and second surfaces
`being intersected successively by an optical axis of the
`said lens which extends in a longitudinal direction, at
`least one of said surfaces being curved to provide a
`converging refractive element having power and longi-
`
`

`
`5,349,471
`9 10
`~refof the diffractive and refractive elements as follows:
`tudinal chromatic aberration, said lens having a con-
`verging diffractive element having power which sub-
`stantially achromatizes said lens for said longitudinal
`chromatic aberration of said refractive element over
`said 20 nm range, and wherein the power of the refrae- 5
`rive dement Orefand the power of the diffractive de-
`ment dpd~ffadd to provide the total power dptotof the lens,
`are proportional such that the longitudinal chromatic
`aberration is approximately zero and are longitudinal 10
`chromatic aberration is approximately zero and are
`defined in accordance with the Abbe ~number -ddiffand
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65