United States Patent
`Katayama
`
`[19]
`
`[54] OPTICAL HEAD APPARATUS FOR
`DIFFERENT TYPES OF DISKS
`
`[75]
`
`Inventor:. Ryuidd Katayama, Tokyo, Japan
`
`[73] Assignee: NEC Corporation, Japan
`
`[21] AppL No.: 658,373
`
`[22] Filed: Jml. 5, 1996
`
`[30]
`
`Foreign Application Priority Data
`
`Jun. 5, 1995 [JP] Japan .................................... 7-137675
`Aug. 15, 1995 [JP] Japan ................................... 7-208026
`Sep. 7, 1995
`Japan .................................... 7-230099
`[JP]
`Mar. 7, 1996
`Japan ................................... 8-049781
`[JP]
`
`Int. CL6 ....................................................... GllB 7/00
`[51]
`[52] U.S. C! ............................................. 369/112; 3691109
`[58] Field of Searda ..................................... 3691112, 103,
`3691109, 110, 44.12, 120, 121
`
`[561
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`IlilnllNHlmllnimlllllllllNIIIlil
`5,696,750
`Dec. 9, 1997
`
`US005696750A
`111] Patent Number:
`[45] Date of Patent:
`
`5,450,378
`
`9/1995 Hekker .................................... 369/103
`
`OTHER PUBLICATIONS
`
`Y. Komma et al., "Dual Focus Optical Head for 0.6 ram and
`1.2 mm Disk~’, Optical Review voL 1, No. 1, pp. 27-29,
`1994.
`
`Primary Examiner--Nabil I-Iindi
`Agorae3 Agent, or Firm--Hayes, Soloway, Hennessey,
`Grossman & Hage, P.C.
`
`[57]
`
`ABSTRACT
`
`In an optical head apparatus for two or more different types
`of discs, there are provided a first fight source for a first
`wavelength light beam, a second light souxce for a second
`wavelength light beam, and an objective lens for leading the
`first and second wavelength light beans to one of the discs.
`A holographic optical element is provided to converge or
`diverge only one of the first and second wavelength light
`beams. Or, an aperture limiting element is provided to adjust
`an effective numerical aperture of the objective lens for only
`one of the first and second wavelength light beams.
`
`5,2O2,860
`
`4/1993 Takahashi ct al ....................... 369/110
`
`24 Claims, 34 Drawing Sheets
`
`B
`
`:.A
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`12
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`LG Electronics, Inc. et al.
`EXHIBIT 1002
`IPR Petition for
`U.S. Patent No. RE43,106
`
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`U.S. Patent
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`Dec. 9, 1997
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`Dec. 9, 1997
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`Dec. 9, 1997
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`5,696,750
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`1
`OPTICAL HEAD APPARATUS FOR
`DIFFERENT TYPF~ OF DISKS
`
`BACKGROUND OFTHEINVENTION
`
`1. Field of the Invention
`The present invention relates to an optical head apparatus
`for different types of disks which have different thicknesses
`and/or different densities.
`2. Description of the Related Art
`
`A first prior art optical head apparatus has been known for
`two types of disks, i.e., a high density thin disk such as a 0.6
`mm thick digital video disk and a low density thick disk such
`as a 1.2 mm thick compact disk (see: Y. Komma et al., "Dual
`Focus Optical Head for 0.6 mm and 1.2 mm Disks", Optical
`Review Vol. 1, No. 1, pp. 27-29, 1994). This first prior art
`optical head apparatus includes a single light source for a
`light beam, an objective lens for leading the light beam to
`one of the disks, and a holographic optical element for
`splitting the light beam into a zeroth order light beam
`(transmission light beam) and a +lst order diffraction light
`beam. As a result, the transmission light beam passes
`through the objective lens, so that this light beam ean be
`focused at one type of the disks. On the other hand, the +lst
`order diffraction light beam passes through the objective
`lens, so that this light beam can be focused at anothez type
`of the disks. This will be explained later in detail
`In the above-described first prior art apparatus, howevex,
`since the incident light beam is split into the transmission
`light beam and the +lst order diff~ction light beam, the
`efficiency of the light beam is low, this reducing the signal-
`to-noise (S/N) ratio.
`A second prior art optical head apparatus has been known
`for two types of disks, i.e., a high density thick disk such as
`a 1.2 mm thick digital video disk and a low density thick
`disk such as a 1.2 mm thick compact disk. This second prior
`art optical head apparatus includes a single light source for
`a light beam and an objective lens for leading the light beam
`to one of the disks. This will be explained later in detail
`In the above-described second prior art apparatus,
`however, it is actually impossible to read the two types of
`disks whose densities are different.
`
`SUMMARY OF THE INVENTION
`
`It is an object of the present invention to provide an
`optimum optical head apparatus for different types of disks.
`According to the present Invention, in an optical head
`apparatus for two or more di~ercnt types of disks, there are
`provided a first light source for a first wavelength light
`beam, a second light source for a second wavelength light
`beam, and an objective lens for leading the first and second
`wavelength light beams to one of the disks. A holographic
`optical dement is provided to converge or diverge only one
`of the first and second wavelength light beams. Or, an
`aperture limiting element is provided to adjust an effective
`numerical aperture of the objective lens for only one of the
`first and second wavelength light beams.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The present invention wili be more clearly understood
`from the description as set forth below, in comparison with
`the prior art, with reference to the accompanying drawings,
`wherein:
`FIG. I is a diagram illusWating a first prior art optical head
`apparatus;
`
`2
`FIG. 2 is a plan view of the holographic optical element
`of FIG. 1;
`FIGS. 3A and 3B arc cross-sectional views of the holo-
`graphic optical element of FIG. !;
`5 FIG. 4 is a diagram illustrating a second prior art optical
`head apparatus;
`FIG. $ is a diagram illustrating a first embodiment of the
`optical head apparatus according to the present invention;
`
`10 FIGS. 6Aand 6B are oross-sectional views ofapart of the
`holographic optical etement of FIG. 5;
`FIG. 7 is a cross-sectiona! view of the entire holograplfic
`optical element of FIG. 5;
`
`FIG. 8 is a diagram illuslrating a second embodiment of
`15 the optical head apparatus according to the present inven-
`tion;
`FIG. 9 is a cross-sectional view of the entire holographic
`optical element of FIG. 8;
`
`20 FIGS. 10A, 10B, HA, llB, 12A, 1213, 13A and 13B are
`diagrams of the interfezence filter of FIGS. 5 and 8;
`
`FIG. 14 is a detailed diagram of a first example of the
`module of FIGS. 5 and 8;
`FIGS. 15A and 15B are enlarged diagrams of the laser
`25 diode and the photodetector of FIG. 14;
`FIG. 16A is an enlarged plan view of the grating of FIG.
`14;
`FIG. 16B is an enlarged plan view of the holographic
`optical element of FIG. 14;
`
`3o
`
`FIG. 17 is an enlarged plan view of the photodetector of
`FIG. 14;
`FIG. 18 is a detailed diagram of a second example of the
`module of FIGS. 5 and 8;
`35 FIGS. 19 is an enlarged cross-sectional view of the
`polarizing holographic optical element of FIG. 18;
`FIG. 20 is an enlarged plan view of. the polarizing
`holographic optical element of FIG. 18;
`FIG. 21 is an enlarged plan view of the photodetector of
`40 FIG. 18;
`
`FIG. 22 is a detailed diagram of a third example of the
`modnle of FIGS. $ and 8;
`FIG. 2,3A is an enlarged plan view of the polarizing
`45 grating of FIG. 22;
`FIG. 2,3B is an enlarged plan view of the holographic
`optical element of FIG. 22;
`FIG. 24 is a cross-sectional view of the mieroprism of
`FIG. 22;
`5o FIG. 25 is an enlarged plan view of the photodetector of
`FIG. 22;
`FIG. 26 is a diagram illustrating a third embodiment of the
`optical head apparatus according to the present invention;
`FIG. 27 is a diagram illustrating a fourth embodiment of
`55 the optical head apparatus according to the present inven-
`tion;
`FIG. 28 is a diagram illustrating a fifth embodiment of the
`optical head apparatus according to the present invention;
`FIG. 29Ais a plan view of a first example of the aperture
`limiting element of FIG. 28;
`FIG. 29B is a cross-sectional view talon along the llne
`B--B of FIG. 29A;
`FIG. 30A is a plan view of a second example of the
`65 altorture limiting element of FIG. 28;
`FIG. 30B is a cross-sectional view taken along the line
`B---B of FIG. 30A;
`
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`FIG. 31A is a plan view of a third example of the aperture
`limiting element of FIG. 28;
`FIG. 31B is a cross-sectional view taken along the line
`B--B of FIG. 31A;
`FIG. 32 is a diagram iLlustrating a sixth embodiment of
`the optical head apparatus according to the present inven-
`tion;
`FIG. 33 is a cross-sectional view of a first example of the
`aperture limiting holographic optical dement of FIG. 32;
`FIG. 34A is a front plan view of the element of FIG. 33;
`FIGS. 3415 is a rear plan view of the element of FIG. 33;
`FIG. 35 is a cross-sectional view of a second example of
`the aperture limiting holographic optical dement of FIG. 32;
`FIG. 36A is a front plan view of the element of FIG. 35;
`and
`FIGS. 36B is a rear plan view of the dement of FIG. 35.
`
`DESCRIPTION OF THE P~D
`EMBODIMENTS
`
`Before the description of the preferred embodiments,
`tx’ior art optical head apparatuses will be explained with
`reference to FIGS. 1, 2, 3A, 3B and 4.
`In FIG. 1, which illustrates a first prior art optical head
`apparatus (see: Y. Komma et al, "Dual Focus Optical Head
`for 0.6 mm and 1.2 mm Disks", Optical Review, VoL 1, No.
`1, pp. 27-29, 1994), reference A designate a high density
`thin disk such as is an about 0.6 mmthick digital video disk,
`and B is a low density thick disk such as an about 1.2 mm
`thick compact disk-recordable (CD-R). Note that only one
`of the disks A and B is mounted on the apparatus.
`In FIG. 1, reference numeral 1 designates a laser diode for
`emitting a light beam to a half mirror 2, and reference
`numeral 3 designates a quadrant photodetector. Provided
`between the half mirror 2 and the disk A (or B) are a
`collimator lens 4, a holographic optical element 5 and an
`objective lens 6. Also, provided between the half mbror 2
`and the photodetector 3 is a concave lens 7.
`About half of the light beam emitted from the laser diode
`1 is reflected at the half mirror 7 and is incident to the
`collimator lens 4 which generates a collimated light beam.
`The collimated light beam is incident to the holographic
`optical dement $.
`A moth order light beam (transmission light beam) of the
`holographic optical element 5 is incident as a collimated
`light beam to the objective lens 6, mad then, is focused on the
`disk A. A reflected light beam from the disk A is again
`incident via the objective bus 6 to the holographic optical
`element $, and is split into a zeroth order light beam
`(transmission light beam) and a +1st order diffraction fight
`beam at the holograpkic optical element 5.
`On the other hand, a +lst order diffiaction light beam of
`the holographic optical element 5 is incident as a divergent
`fight beam to the objective lens 6 and then, is focused on the
`disk B. A reflected light beam from the disk B is again
`incident via the objective lens 6 to the holographic optical
`element 5, and is split into a zeroth order light beam
`(transmission light beam) and a +lst order diffraction light
`beam at the holographic optical element 5.
`The transmission light beam at the holographic optical
`element 5 of the reflected light beam from the disk A and the
`+lst order diffraction light beam at the holographic optical
`element 5 of the reflected light beam from the disk B are
`incident as collimated light beams to the collimator lens d.
`Half of the light beam passed through the collimator lens
`4 passes through the half mirror 2, and further passes
`through the concave lens 7 to reach the photodetector 3.
`
`4
`In the photodetector 3, a focusing eater signal is detected
`by an astigmatism method using astigmatism generated at
`the half mirror 2, and a lracking error signal is detected by
`a push-pull method. Also, an information signal is detected
`5 by a sum of the four outputs of the photode~ 3.
`The objective lens 6 has a spherical aberration capable of
`compensating for a spherical aberration caused when the
`outgoing transmission light beam of the objective lens 6 is
`incident to the disk A and returns therefrom, Therefore, the
`I0 zeroth order light beam (transmission light beam) of the
`holographic optical element 5 can be focused at the disk A
`without aberrations. On the other hand, the holographic
`optical element 5 has a spherical aberration capable of
`compensating for a sun of a spherical aberration caused
`15 when the outgoing +lst order diffraction light beam of the
`holographic optical element 5 is incident to the disk B and
`returns therefrom and a spherical aberration of the objective
`lens 6 caused when the outgoing +lst order diffraction light
`beam of the holographic optical element $ is incident to the
`2o objective lens 6 and returns therefrom. Therefore, the +lst
`order diffxaction light beam of the holographic optical
`element $ can be focused at the disk B without abe=ations.
`In FIG. 2, which is a plan view of the holographic optical
`element 5 of FIG. I, the holographic optical dement 5
`25 includes concentric interference fringes. Therefore, the holo-
`
`graphic optical element $ can compensate for the above-
`described spherical aberration of the +lst order diffraction
`fight beam, and also can serve as a concave lens for the +lst
`order diffraction light beam. As a result, the focal point of
`3o the +1st order diffraction light beam at the disk B is far from
`
`35
`
`the focal point of the Wansmission light beam at the diskA,
`so that the distance between the surface of the disk A and the
`objective lens 6 can be about the same as the distance
`between the surface of the disk B and the objective lens 6.
`In FIG. 3A, which is a cross-sectional view of the
`holographic optical element S of FIG. 1, the grating shape is
`rectangular, and as a remit, a spurious -lst order diffraction
`light beam as wdl as the +lst order diffraction light beam is
`40 generated, and in this case, the intensity of the -lst order
`diffraction light beam is about the same as that of the +lst
`order diffraction light beam. Thus, the efficiency of fight is
`reduced.
`
`In FIG. 3B, which is also a cross-sectional view of the
`45 holographic optical element $ of FIG. 1, the grating shape is
`blazed, and as a resul~ the intensity of the +lst order
`diffraction fight beam is increased while the intensity of the
`-lst order diffraction light beam is decreased. In this case,
`ff the height of the saw-tooth portion is 2h, the refractive
`5o index is n, and the wavelength of the incident light is ~, the
`transmittance rio and the diffraction efficiency ll+x of +lst
`order diffTaction light are represented by
`
`55
`
`n÷~=(s.m2¢y(¢_nf
`
`(2)
`
`where ¢=2x(n-1)h/L
`If q~rd2,11o--11+1--0.405, so that the efficiency of the going
`and returning light is 11o2--q+12---0.164. Therefore, the
`6o amount of light received by the photodetector 3 is only 0.164
`times as compared with a conventional optical head
`apparatus, which reduces the S/N ratio of an information
`signal. In other words, if the power of the laser diode I is
`Increased to 6.10 times as compared with the conventional
`65 laser diode for a non-dual focus optical head apparatus, the
`photodetector 3 receives light whose amount is the same as
`the conventional photodetector. Further, in order to write the
`
`

`
`¯ : ..
`
`5,696,750
`
`6
`5
`diskA or B, the power of the laser diode 1 has to be further
`On the other hand, the 785 nm wavelength fight beam
`increased; however, it is actually impossible to furthex
`emitted from the laser diode of the module 12 is reflected by
`increase the power of the laser diode 1.
`the interference filter 13 and is incident to the collimator lens
`Further, in the first prior art optical head apparatus, the
`4 which generates a collimated light beam. The collimated
`wavelength of the laser diode 1 for the disk A which is, in
`5 fight beam is incident to the holographic optical element $’.
`Then, a -lst order diffraction fight beam of the holographic
`this case, a digital video disk is 635 to 655 nm to obtain a
`small focused spot. On the other hand, the wavelength of the
`optical dement $’ is incident as a divergent light beam to the
`laser diode 1 for the disk B which is, in this case, a compact
`objective lens 6’, and then, is focused on the disk B. A
`disk-reeordable (CD-R) using organic dye material has to be
`reflected light beam from the disk B is again ineideut via the
`785 rim, so that a refleetivity higher that 70% is obtained. If
`l0 objective lens 6’ to the holographic optical element 5’. The
`the wavelength of the laser diode 1 is 635 to 655 mm a
`-lst order diffraction light beam is ~eflected by the inter-
`reflectivity of only about 10% is obtained, so that it is
`ference filter 13 and reaches the photodetector of the module
`Lmposs~le to read the CD-R.
`12.
`In FIG. 4, which illustrates a second pri~ art optical head
`In FIG. 5, the objective lens 6’ has a spherical aberration
`apparatus, A’ is a high density disk such as an about 1.2 nm
`Is capable of compensating for a spherical aberration caused
`thick digital video disk, and the disk B is a low density thick
`when the 635 nm wavelength outgoing transmission light
`disk such as an about 1.2 mm thick CD-R. Note that one of
`beam of the objective lens 6’ is incident to the diskA and
`the disks A’ and B is mounted on the apparatus.
`returns thereflom Therefore, the zeroth order 635nm wave-
`In FIG. 4, the holographic optical element $ of FIG. 1 is
`length fight beam (transmission light beam) of the holo-
`not provided, since the disks A’ and B have the same
`graphic optical element 5’ can be focused at the disk A
`thickness. Note that the radius of the focused spot at the disk
`without aberrations. On the other hand, the holographic
`A’ or B is generally inversely proportional to the numerical
`optical element 5’ has a spherical aberration capable of
`aperture (NA) of the objective lens 6 and is proportional to
`compensating for a sum of a spherical aberration caused
`the wavelength of the laser diode 1.
`when the 785 nm wavelength outgoing-lst order diffraction
`For the disk A’, in order to decrease the focused spot in
`response to the high density, the numerical aperture of the
`25 light beam of the holographic optical element 5’ is incident
`to the disk B and returns therefrom and a spherical aberra-
`objective lens 6 is made large, for example, 0.52 to 0.6, and
`tion of the objective lens 6’ caused when the outgoing -lst
`the wavelength of the laser diode I is made small, for
`order diffraction 785 am wavdength light beam of the
`example, 635 to 655 nnx
`holographic optical element 5’ is inddeut to the objective
`On the other hand, for the disk B, since the tolerance of
`the disk tilt has to be large, so that, it is necessary to suppress
`3o lens 6’ and returns therefrorm Therefore, the -lst order
`diffraction 785 nm wavelength light beam of the holographic
`the generation of coma caused by the disk tilt, the numerical
`optical element U can be focused at the disk B without
`aperture of the objective lens 6 is made small~ for example,
`aberrations.
`0.45. In addition, the CD-R type disk B is designed so that
`Also, the holographic optical dement ~ includes eoneen-
`a reflectivity of higher than 70% can be obtained for a 785
`nm wavelength, the wavelength of the laser diode 1 is made
`35 tric interference hinges as illuslrated in FIG. 2. Therefore,
`the holographic optical element 5’ can compensate for the
`about 785 nm.
`above-described spherical aberration of the -lst order dif-
`Thus, in the optical head apparatus of FIG. 4, two kinds
`fraction 785 nm wavdength light beam, and also can serve
`c£ numerical apertures are required for the objective lens 6
`as a concave lens for the -lst order diffraction 785 nm
`in order to read the two kinds of disks A’ and B; therefore,
`it is actually impossible for the optical head apparalms of
`4o wavelength light beanx As a result, the focal point of the
`-1st order diffraction light benin at the diskB is far from the
`FIG. 4 to read the two kinds of disks A’ and B.
`focal point of the transmission light beam at the disk A, so
`In FIG. 5, which illustrates a first embodiment of the
`that the distance between the surface of the disk A and the
`present invention, modules 11 and 12 and an interference
`objective lens 6’ can be about the same as the distance
`falter 13 are provided instead of the laser diode 1, the half
`mirror 2, the photodetector 3 and the concave lens 7 of FIG.
`45 between the surface of the disk B and the objective lens 6’.
`In FIGS. 6A and 6B, which are cross-sectional views of
`1. Also, the holographic optical element 5 and the objective
`a part of the holographic optical element 5’ of FIG. 5, the
`lens 6 of FIG. 1 are modflied into a holographic optical
`grating shape is staireuse. For example, in order to form the
`dement 5’ and an objective lens 6’, respectively.
`holographic optical element 5’ of FIG. 6A, a first silicon
`The module 11 includes a laser diode for a 635 nm
`wavelength fight beam and a photodetector, while the mod-
`5o oxide layer is deposited on a glass substrate 501, and then a
`silicon oxide pattern 502 is formed by a photolithography
`ule 12 includes a laser diode for a 785 nm wavelength light
`process. Next, a second silicon oxide layer is deposited on
`beam and a photodetector. The interference filter 13 trans-
`the silicon oxide pattern 502, and then, silicon oxide patterns
`mits most of the 635 nm wavelength light beam
`503 and 504 are simultanoously formed by a photolithog-
`therethrough, while the interference ftlter 13 reflects most of
`the 785 um wavelength light beam.
`55 raphy process. On the other hand, in order to form the
`holographic optical element 5’ of FIG. 6B, a glass substrate
`The 635 nm wavelength light beam emitted from the laser
`505 is etched by a photolithography process to form a
`diode of the module 11 passes through the interference filter
`groove 506 within the glass subsla’ate 505. Next, the glass
`13 and is incident to the collimator lens 4 which generates
`substrate 505 is etched by a photolithography process to
`a collimated light beam. The collimated light beam is
`incident to the holographic element 5’. Then, a zeroth order
`6o form grooves 507 and 508 within the glass suhstrate $05.
`In FIGS. 6A and 6B, ff the height of the staircase portion
`light beam (transmission light beam) of the holographic
`is h/2, the refractive index is n, and the wavelength of the
`optical element 5’ is incident as a collimated light beam to
`incident light is ~L, the transmittance 11o, the diffraction
`the objective lens 6’ and then, is focused on the disk A. A
`efficiency Tl+l of the +lst order diffraction light and the
`reflected light beam from the disk A is again incident via the
`65 diffraction efficiency 11_1 of the -lst order diffraction fight
`objective lens 6’ to the holographic optical dement 5’. The
`are represented by
`zeroth order fight beam passes through the interference filter
`13 to reach the photodetector of the module 11.
`
`~o--o~(¥2) co~(¥4) (~)
`
`

`
`5,696,750
`
`(4)
`
`(5)
`
`where gb=2n(n-l)ldL
`For example, ff h=2.76 Inn and n=1.46, then
`n for L=635 ILrn. Therefore,
`11o =1, 11+x=0 and 11_x=0
`As a result, the efficiency of the going and returning 635 nm
`wavelength light is
`
`~02=1.
`
`Thus, the S/N ratio of the information signal at the photo-
`detector of the module 11 is almost the same as that in the
`conventional non-dual focus optical head apparatus. This 15
`also makes it possible to write the disk A.
`On the other hand,
`(cid:128)~=3.23~ for L=785 nm. Therefore,
`~1o=0.0851, 11+t=0.023 and Iq_a=0.685
`As a result, the etticiency of the going and returning 785 nm
`wavelength fight is
`
`20
`
`8
`objective lens 6", and then, is focused on the disk A. A
`reflected fight beam from the disk A is again incident via the
`objective lens 6" to the holographic optical dement 5". The
`+lst order dit~action light beam passes through the inter-
`5 ference filter 13 to reach the photodetector of the module 11.
`On the other hand, the 785 nm wavelength light beam
`emitted from the laser diode of the module 12 is reflected by
`the interference filter 13 and is incident to the collimator lens
`4 which generates a collimated light beam. The collimated
`10 light beam is inddent to the holographic optical element $".
`Then, a zerolh order Hght beam (transmission light beam) of
`the holographic optical element 5" is incident as a collimated
`light beam to the objective lens 6", and then, is focused on
`the disk B. A reflected light beam from the disk B is again
`incident via the objective lens 6" to the holographic optical
`element 5". The zeroth order Hght beam is reflected by the
`interference filter 13 and reaches the photodetector of the
`module 12.
`In FIG. 8, the objective lens 6" has a spherical abelration
`capable of compensating for a spherical aberration caused
`when the 785 am wavelength outgoing transmission light
`beam of the objective lens 6" is incident to the disk B and
`returns therefrom. Therefore, the zeroth order 785 am wave-
`length light beam (transmission light beam) of the holo-
`25 graphic optical element 5" can be focused at the disk B
`without abexrafions. On the other hand, the holographic
`optical element $" has a spherical aberration capable of
`compensating for a sum of a spherical aberration caused
`when the 635 am wavelength outgoing +lst order diffraction
`30 light beam of the holographic optical dement 5" is incident
`to the diskA and returns therefrom and a spherical aberration
`of the objective lens 6" caused when the outgoing +lst order
`diffraction 635 am wavelength light beam of the holographic
`optical element 5" is incident to the objective lens 6" and
`35 returns therefrom. Therefore, the +lst order dil~actiou 635
`am wavelangth light beam of the holographic optical ele-
`ment $" can be focused at the disk A without aberrations.
`Also, the holographic optical element 5" includes con-
`centric interference fringes as illustrated in FIG. 2.
`4o Therefore, the holographic optical element 5" can compen-
`sate for the above-described spherical abe~ation of the +1st
`order diffraction 635 nm wavelength light beam, and also
`can serve as a convex lens for the +lst order diffxaetion 635
`nm wavelength light beam. As a result, the focal point of the
`45 +lst order diff~ction light beam at the disk A is near from
`the focal point of the transmission fight beam at the disk B,
`so that the distance between the surface of the diskA and the
`objective lens 6" can be about the same as the distance
`between the surface of the disk B and the objective lens 6".
`50 The holographic optical element $" of FIG. 8 is also
`illustrated in FIGS. 6A and 6B. For example, in the above-
`mentioned formulae (3), (4) and (5), ff h=3.45 pm