Optical pickup and information reproducing apparatus

An optical pickup for reading information recorded on information tracks of a recording medium by using a main light beam and a subsidiary light beam includes a diffraction device for diffracting an original light beam to generate the main light beam and the subsidiary light beam. The main light beam is applied to a reading track which is one of the information tracks on which the information to be read is recorded. The subsidiary light beam is applied to the information track adjacent to the reading track. The diffracting device is made of an optical material having coefficient of linear expansion .beta., which is determined as follows: EQU .alpha.-.DELTA..alpha..sub.1.ltoreq..beta..ltoreq..alpha.+.DELTA..alpha.. sub.2, .alpha.=.DELTA..lambda./.lambda..sub.0, EQU .DELTA..alpha..sub.1 ={(1+.alpha..times..DELTA.t.sub.max).times..DELTA.L.sub.max }/{(L.sub.0 +.DELTA.L.sub.max).times..DELTA.t.sub.max }, EQU .DELTA..alpha..sub.2 ={(1+.alpha..times..DELTA.t.sub.max).times..DELTA.L.sub.max }/{(L.sub.0 -.DELTA.L.sub.max).times..DELTA.t.sub.max }, where .lambda..sub.0 is a wavelength of the original light beam at a design temperature, .DELTA..lambda. is an amount of a change of the wavelength of the original light beam per unit temperature, .DELTA.t.sub.max is a predetermined permissible amount of a change of an ambient temperature, L.sub.0 is a spacing between an irradiation position of the subsidiary light beam and an irradiation position of the main light beam at the design temperature, and .DELTA.L.sub.max is a predetermined permissible amount of a change of the spacing.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an optical pickup for reproducing
 information recorded on a recording medium such as an optical disk, and in
 particular, to an optical pickup for reproducing information while
 removing crosstalk caused by adjacent information tracks, and an
 information reproducing apparatus having such an optical pickup.
 2. Description of the Related Art
 An optical disk has information tracks formed on its surface. Information
 is recorded on the information tracks. When reproducing information
 recorded on an information track, a light beam is applied to an
 information track, reflected light of the light beam is received, and
 information is extracted from the reflected light thus received. When a
 light beam is applied to an information track on which information to be
 reproduced has been recorded, the irradiation range of the light beam
 extends over not only the information track to which the light beam should
 be applied but also other information tracks adjacent to the information
 track to which the light beam should be applied. Because of demand of
 higher density of recorded information, this phenomenon is more remarkable
 in a recording medium having a small interval between information tracks.
 If the irradiation range of the light beam extends over other information
 tracks, crosstalk is caused thereby. The term "crosstalk" means that
 information on adjacent tracks gets mixed in information on the
 information track to be originally reproduced. If crosstalk occurs, it
 becomes difficult to accurately reproduce information to be reproduced.
 As a method for removing crosstalk, the three beam method is known. In the
 three beam method, a light beam is applied to an information track from
 which information should be originally reproduced. In addition, a light
 beam is also applied to each of tracks adjacent to the information track
 from which information should be originally reproduced. In other words, a
 total of three light beams are applied toward an optical disk. From a
 light receiving signal based upon reflected light from an information
 track to be reproduced, light receiving signals based upon reflected light
 from two adjacent information tracks are subtracted to remove the
 crosstalk.
 In a typical reproducing apparatus employing the three beam method, one
 light beam is divided into three light beams by using a diffraction
 grating. Among three beams resulting from the division, one is used as a
 main light beam whereas two remaining beams are used as subsidiary light
 beams for removing the crosstalk. The main light beam is applied to the
 information track having information to be reproduced, whereas the
 subsidiary light beams are applied to the adjacent tracks.
 The subsidiary light beams are respectively applied to information tracks
 located on either side of the information track having information to be
 reproduced so as to be adjacent thereto. This means that irradiation
 positions of the two subsidiary light beams are disposed so as to be
 displaced respectively leftward and rightward from the irradiation
 position of the main light beam, assuming the extension direction of
 information tracks on which the irradiation position of the main light
 beam is located to be a center line. In other words, the subsidiary light
 beams are disposed so as to be displaced in the radial direction of the
 optical disk.
 In addition, irradiation positions of the two subsidiary light beams are
 disposed so as to be equally displaced before and behind the irradiation
 position of the main light beam in the extension direction of information
 tracks. In other words, the irradiation positions of the subsidiary light
 beams are disposed so as to be displaced not only in the radial direction
 of the optical disk but also in the circumferential direction of the
 optical disk. If the three light beams are disposed so as to line up in a
 single file in the radial direction of the optical disk, then irradiation
 ranges of the light beams interfere with each other, and information on
 each information track cannot be detected accurately. In order to prevent
 this, the irradiation positions of the subsidiary light beams are disposed
 so as to displaced in the circumferential direction of the optical disk.
 If the irradiation positions of the three light beams are disposed so as to
 be displaced in the circumferential direction of the optical disk, then
 light receiving signals obtained from respective light beams diverges in
 time. The temporal divergence of the light receiving signals can be
 removed by electrically delaying the light receiving signals.
 It is now assumed that the spacing between the irradiation position of the
 main light beam and the irradiation position of each of the subsidiary
 light beams in the circumferential direction of the optical disk is L. The
 distance L can be represented as
EQU L=F.times.(.lambda./D) (1)
 where .lambda. is the oscillation wavelength of the light source, D is the
 pitch of the diffraction grating, and F is the focal length of an
 objective lens for focusing each light beam onto an information track
 corresponding thereto.
 As for an optical material used for the diffraction grating, an optical
 material having the least possible coefficient of linear expansion .beta.
 is used from the viewpoint of stability against a change of the ambient
 temperature.
 On the other hand, as the light source for emitting a light beam, a laser
 diode is used in many cases. The laser diode typically has poor
 temperature characteristics. As the ambient temperature changes, the
 oscillation length .lambda. of the laser diode changes. By the way, since
 an optical material having the least possible coefficient of linear
 expansion .beta. is used for the diffraction grating, a change of the
 pitch D of the diffraction grating caused by a change of the ambient
 temperature is small. As a result, a change of the oscillation frequency
 .lambda. of the laser diode caused by a change of the ambient temperature
 changes the spacing L (see the equation (1)). If the spacing L changes,
 time spacing between light receiving signals actually obtained from the
 reflected light of a light beam becomes different from time spacing preset
 in a removing circuit in order to electrically remove a time difference of
 an obtained light receiving signal. This results in a problem that the
 crosstalk cannot be removed accurately and the stability is poor provided
 that the ambient temperature changes.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide an optical pickup for
 removing crosstalk by using a plurality of light beams, capable of
 removing crosstalk accurately and stably even if the oscillation
 wavelength of a light source is changed by a change of the ambient
 temperature and capable of reproducing information accurately and stably,
 and provide an information reproducing apparatus having such an optical
 pickup.
 An optical pickup in accordance with the present invention is a device for
 reading information recorded on a plurality of information tracks of a
 recording medium by using at least a main light beam and a subsidiary
 light beam. The main light beam is applied to a reading track which is one
 of the plurality of information tracks on which the information to be read
 is recorded. The subsidiary light beam is applied to an adjacent track
 which is different one of the plurality of information tracks adjacent to
 the reading track. The optical pickup includes: a light source for
 emitting an original light beam; a diffraction device for diffracting the
 original light beam to divide the original light beam into the main light
 beam and the subsidiary light beam, so that the main light beam and the
 subsidiary light beam are applied to the reading track and the adjacent
 track, respectively; and a receiving device for receiving the main light
 beam and the subsidiary light beam reflected by the recording medium and
 generating a main signal corresponding to the received main light beam and
 a subsidiary signal corresponding to the received subsidiary light beam.
 The diffracting device is made of an optical material having coefficient
 of linear expansion .beta., which is determined as follows:
EQU .alpha.-.DELTA..alpha..sub.1.ltoreq..beta..ltoreq..alpha.+.DELTA..alpha..
 sub.2,
EQU .alpha.=.DELTA..lambda./.lambda..sub.0,
EQU .DELTA..alpha..sub.1
 ={(1+.alpha..times..DELTA.t.sub.max).times..DELTA.L.sub.max }/{(L.sub.0
 +.DELTA.L.sub.max).times..DELTA.t.sub.max },
EQU .DELTA..alpha..sub.2
 ={(1+.alpha..times..DELTA.t.sub.max).times..DELTA.L.sub.max }/{(L.sub.0
 -.DELTA.L.sub.max).times..DELTA.t.sub.max }, (2)
 where .lambda..sub.0 is a wavelength of the original light beam at a design
 temperature of the optical pickup, .DELTA..lambda. is an amount of a
 change of the wavelength of the original light beam per unit temperature,
 .DELTA.t.sub.max is a predetermined permissible amount of a change of an
 ambient temperature at which the optical pickup is used, L.sub.0 is a
 spacing between an irradiation position of the subsidiary light beam and
 an irradiation position of the main light beam at the design temperature,
 and .DELTA.L.sub.max is a predetermined permissible amount of a change of
 the spacing between the irradiation position of the subsidiary light beam
 and the irradiation position of the main light beam.
 Since the coefficient of linear expansion .beta. of the optical material of
 the diffracting device is determined according to the aforementioned
 equations (2), a change of the spacing between the irradiation position of
 the main light beam and the irradiation position of the subsidiary light
 beam can be restricted within a permissible range, even if a change of the
 wavelength of the original light beam is caused by a change of the ambient
 temperature. Therefore, the crosstalk can be removed accurately and
 stably. Accordingly, the information recorded on the recording medium can
 be read accurately and stably.
 In the aforementioned optical pickup, it is preferable that the coefficient
 of linear expansion .beta. is equal to the value .alpha.. That is to say,
 it is preferable that the coefficient of linear expansion .beta. satisfies
 the equation:
EQU .beta.=.alpha.. (3)
 If the coefficient of linear expansion .beta. is equal to the value
 .alpha., the spacing between the irradiation position of the main light
 beam and the irradiation position of the subsidiary light beam can be held
 constant, even if a change of the wavelength of the original light beam is
 caused by a change of the ambient temperature. Accordingly, the crosstalk
 can be removed accurately and stably.
 As an optical material of the diffracting device, diethylene glycol
 bisallyl carbonate, poly-4-methyl pentene-1, poly methyl methacrylate or
 poly carbonate may be used, because each material satisfies at least the
 aforementioned equations (2).
 The aforementioned optical pickup in accordance with present invention
 reads the information recorded on the recording medium using at least two
 light beams. Another type of the optical pickup in accordance with the
 present invention uses three light beams. Hereinafter, this type of the
 optical pickup will be described.
 The optical pickup reads information recorded on a plurality of information
 tracks of a recording medium by using a main light beam, a first
 subsidiary light beam and a second subsidiary light beam. The main light
 beam is applied to a reading track which is one of the plurality of
 information tracks on which the information to be read is recorded. The
 first subsidiary light beam is applied to a first adjacent track which is
 different one of the plurality of information tracks adjacent to one side
 of the reading track. The second subsidiary light beam is applied to a
 second adjacent track which is further different one of the plurality of
 information tracks adjacent to another side of the reading track. A
 spacing between an irradiation position of the main light beam and an
 irradiation position of the first subsidiary light beam is equal to a
 spacing between an irradiation position of the main light beam and an
 irradiation position of the second subsidiary light beam. The optical
 pickup includes: a light source for emitting an original light beam; a
 diffraction device for diffracting the original light beam to divide the
 original light beam into the main light beam, the first subsidiary light
 beam and the second subsidiary light beam, so that the main light beam,
 the first subsidiary light beam and the second subsidiary light beam are
 applied to the reading track, the first adjacent track and the second
 adjacent track, respectively; and a receiving device for receiving the
 main light beam, the first subsidiary light beam and the second subsidiary
 light beam reflected by the recording medium and generating a main signal
 corresponding to the received main light beam, a first subsidiary signal
 corresponding to the received first subsidiary light beam and a second
 subsidiary signal corresponding to the received second subsidiary light
 beam. The diffracting device is made of an optical material having
 coefficient of linear expansion .beta., which is determined by the
 aforementioned equations (2). In addition, the spacing between the
 irradiation position of the first subsidiary light beam and the
 irradiation position of the main light beam at the design temperature
 (L.sub.0) is equal to the spacing between the irradiation position of the
 second subsidiary light beam and the irradiation position of the main
 light beam at the design temperature.
 According to this type of optical pickup, each of a change of the spacing
 between the irradiation position of the main light beam and the
 irradiation position of the first subsidiary light beam and a change of
 the spacing between the irradiation position of the main light beam and
 the irradiation position of the second subsidiary light beam can be
 restricted within a permissible range, even if a change of the wavelength
 of the original light beam is caused by a change of the ambient
 temperature. Therefore, the crosstalk caused by both the first adjacent
 track and the second adjacent track can be removed accurately and stably.
 Accordingly, the information recorded on the recording medium can be read
 accurately and stably.
 As the optical pickup uses three light beams, it can be used as an optical
 pickup using the three-beam method. In this case, the main light beam, the
 first subsidiary light beam and the second subsidiary light beam are moved
 along the respective information tracks in a predetermined direction, the
 irradiation position of the first subsidiary light beam is located ahead
 of the irradiation position of the main light beam in the predetermined
 direction, and the irradiation position of the main light beam is located
 ahead of the irradiation position of the second subsidiary light beam in
 the predetermined direction. Since both a change of the spacing between
 the irradiation position of the main light beam and the irradiation
 position of the first subsidiary light beam and a change of the spacing
 between the irradiation position of the main light beam and the
 irradiation position of the second subsidiary light beam can be restricted
 within the permissible range, the accuracy of reproduction of the
 information recorded on the recording medium using the three beam method
 can be improved.
 The nature, utility, and further feature of this invention will be more
 clearly apparent from the following detailed description with respect to
 preferred embodiments of the invention when read in conjunction with the
 accompanying drawings briefly described below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments of the present invention will now be described by referring to
 the drawing.
 The spacing L between the main light beam for reproducing information and
 the subsidiary light beams for detecting crosstalk is represented as
 L=F.times.(.lambda./D) (4)
 where D is the pitch of the diffraction grating, F is the focal length of
 an objective lens, and .lambda. is the wavelength of the light beams
 emitted from the light source. Even if the oscillation length .lambda. of
 the light beams is changed by a change of the ambient temperature, the
 spacing L can be kept constant provided that the term (.lambda./D) of the
 equation (4) can be kept constant.
 Letting the oscillation wavelength at a design temperature of the optical
 pickup be .lambda..sub.0 and the pitch of the diffraction grating at the
 design temperature be D.sub.0, the spacing L.sub.0 between the irradiation
 position of the main light beam and each of the irradiation positions of
 the subsidiary light beams at this time becomes as represented by the
 following equation.
EQU L.sub.0 =F.times.(.lambda..sub.0 /D.sub.0) (5)
 It is now assumed that the oscillation wavelength increases (decreases) by
 .DELTA..lambda. when the ambient temperature has risen (fallen) by
 1.degree. C. It is also assumed that when the ambient temperature rise
 (fall) of 1.degree. C. causes expansion or contraction of the diffraction
 grating and thereby increases (decreases) the pitch by .DELTA.D. When the
 ambient temperature has changed by t.degree.C., the spacing L(t) between
 the irradiation position of the main light beam and each of the
 irradiation positions of the subsidiary light beams becomes as represented
 by the following equation.
EQU L(t)
EQU =F.times.{(.lambda..sub.0 +.DELTA..lambda..times.t)/(D.sub.0
 +.DELTA.D.times.t)}
EQU =F.times.[[.lambda..sub.0.times.{1+(.DELTA..lambda./.lambda..sub.0)
 .times.t}]/[D.sub.0.times.{1+(.DELTA.D/D.sub.0).times.t}]] (6)
 Therefore, the spacing between the irradiation position of the main light
 beam and each of the irradiation positions of the subsidiary light beams
 can be kept constant irrespective of the change of the ambient
 temperature, provided that
EQU .DELTA.D/D.sub.0 =.DELTA..lambda./.lambda..sub.0. (7)
 Typically, a change factor of the pitch (.DELTA.D/D.sub.0) is represented
 by the coefficient of linear expansion .beta. of the diffraction grating.
 If the change factor of the oscillation wavelength
 (.DELTA..lambda./.lambda..sub.0) of the light source caused by a change of
 the ambient temperature is equal to the coefficient of linear expansion
 .beta. of the diffraction grating, therefore, the spacing between the
 irradiation position of the main light beam and each of the irradiation
 positions of the subsidiary light beams can always be kept constant.
 In the present invention, the spacing between the irradiation position of
 the main light beam and each of the irradiation positions of the
 subsidiary light beams is kept constant by setting the coefficient of
 linear expansion .beta. of the diffraction grating for generating the main
 light beam and the subsidiary light beams equal to a change value of the
 oscillation length of the light source caused by the temperature.
 FIG. 1 shows an information reproducing apparatus S according to an
 embodiment of the present invention. The information reproducing apparatus
 S is an apparatus for reproducing information recorded on an optical disk
 1 which is a disklike recording medium having information tracks taking
 the shape of concentric circles or a spiral. On the optical disk 1,
 information is formed on the information tracks as a sequence of pits.
 As shown in FIG. 1, the information reproducing apparatus S includes a
 laser diode 7 serving as a light source, a diffraction grating 6 serving
 as a diffraction means, a beam splitter 4, an objective lens 2, a detector
 5 serving as a light receiving means, three A/D converters 8, delay
 circuits 9 and 10, a subtracter 11 serving as a removing means, a
 demodulator 12 serving as a reproducing means, a zero cross sample
 extractor (ZCS extractor) 13, a filter coefficient computation circuit
 (FCC circuit) 14, and variable coefficient filters (VC filter) 15 and 16
 formed of transversal filters.
 In this configuration, an optical pickup PU of an embodiment includes the
 laser diode 7, the diffraction grating 6, the beam splitter 4, the
 objective lens 2, and the detector 5.
 Recorded information on the optical disk 1 has been recorded by using pits
 P having a plurality of kinds of lengths corresponding to recorded
 information and lining up in a single file in their longitudinal direction
 to form an information track as shown in FIG. 1. Furthermore, the rotation
 speed of the optical disk 1 in the tangential direction is V.sub.L.
 When reproducing information recorded on the optical disk 1, the laser
 diode 7 first emits a light beam B which is laser light serving as an
 original light beam. The diffraction grating 6 divides the light beam B
 into a main light beam BM, a first subsidiary light beam BS.sub.1, and a
 second subsidiary light beam BS.sub.2. Subsequently, the beam splitter 4
 transmits a part of each of the main light beam BM, the first subsidiary
 light beam BS.sub.1, and the second subsidiary light beam BS.sub.2 to make
 it arrive at the objective lens 2. The objective lens 2 applies the main
 light beam BM, the first subsidiary light beam BS.sub.1, and the second
 subsidiary light beam BS.sub.2 which have been transmitted through the
 beam splitter 4 to the optical disk 1.
 At this time, the main light beam BM is applied to an information track on
 which information to be reproduced has been recorded (hereafter referred
 to as reproduction track). As a result, a central light spot LS2 is formed
 on the reproduction track. Furthermore, the first subsidiary light beam
 BS.sub.1 is applied to an information track located immediately inside the
 reproduction track (hereafter referred to as inner track). As a result, an
 inner light spot LS1 is formed on the inner track. Furthermore, the second
 subsidiary light beam BS.sub.2 is applied to an information track located
 immediately outside the reproduction track (hereafter referred to as outer
 track). As a result, an outer light spot LS3 is formed on the outer track.
 Thereafter, the main light beam BM, the first subsidiary light beam
 BS.sub.1, and the second subsidiary light beam BS.sub.2 applied to
 respective information tracks are modulated in intensity by pits P formed
 on respective information tracks. Furthermore, the plane of polarization
 of each of the beams BM, BS.sub.1 and BS.sub.2 is rotated, when it is
 reflected by the optical disk 1. And the beams BM, BS.sub.1 and BS.sub.2
 arrive at the beam splitter 4 again via the objective lens 2. Furthermore,
 the beams BM, BS.sub.1 and BS.sub.2 are reflected by the beam splitter 4
 so as to proceed to a light receiving face of the detector 5.
 The detector 5 receives separately and independently the main light beam
 BM, the first subsidiary light beam BS.sub.1, and the second subsidiary
 light beam BS.sub.2 inputted thereto, and converts the beams BM, BS.sub.1
 and BS.sub.2 respectively to electric signals. In other words, the main
 light beam BM, the first subsidiary light beam BS.sub.1, and the second
 subsidiary light beam BS.sub.2 are converted respectively to a central
 detected signal Scent, an inner detected signal Sin, and an outer detected
 signal Sout by the detector 5. These three signals are inputted to the
 three A/D converters 8, and converted to digital signals.
 The digitized outer detected signal Sout is inputted to the variable
 coefficient filter 16 as it is. On the other hand, the digitized central
 detected signal Scent is delayed in the delay circuit 9 by a delay value
 DL, and then inputted to the subtracter 11. Furthermore, the digitized
 inner detected signal Sin is delayed in the delay circuit 10 by a delay
 value (DL.times.2), and then inputted to the variable coefficient filter
 15.
 The delay value DL is derived by
EQU DL=L/V.sub.L (8)
 where L is the distance in a direction along the information tracks of the
 optical disk 1 between the inner light spot LS1 and the central light spot
 LS2, and between the central light spot LS2 and the outer light spot LS3
 (see FIG. 1). In the case where the optical disk 1 is a DVD, L is in the
 range of approximately 10 to 20 .mu.m. V.sub.L is the rotation velocity of
 the optical disk 1 in the tangential direction.
 Delay value setting in the delay circuits 9 and 10 will now be described.
 In the present embodiment, the inner detected signal Sin is subtracted in
 the subtracter 11 from the central detected signal Scent. As a result, the
 crosstalk caused in the central detected signal Scent by the inner track
 is removed. In addition, the outer detected signal Sout is subtracted from
 the central detected signal Scent. As a result, the crosstalk caused in
 the central detected signal Scent by the outer track is removed.
 The inner light spot LS1, the central light spot LS2, and the outer light
 spot LS3 are disposed at intervals of L in the extension direction of
 information tracks in order to eliminate interference between the light
 spots. Among the three light spots, a light spot located at the head of
 others in the direction of advance of the light beams is the inner light
 spot LS1.
 The inner light spot LS1 is located 2L ahead of the outer light spot LS2.
 Therefore, the inner detected signal Sin generated from the inner light
 spot LS1 is located 2DL ahead of the outer detected signal Sout generated
 from the outer light spot LS3. The central light spot is located L ahead
 of the outer light spot LS2. Therefore, the central detected signal Scent
 generated from the central light spot LS2 is located DL ahead of the outer
 detected signal Sout.
 In the information reproducing apparatus S, therefore, the inner detected
 signal Sin is delayed by the delay value (DL.times.2), and the central
 detected signal Scent is delayed the delay value DL. As a result, it is
 possible to simultaneously obtain three detected signals containing
 information recorded on three information tracks which are located on a
 straight line along the radial direction of the optical disk 1 and which
 are adjacent to each other. At the time when the outer detected signal
 Sout has been generated in the information reproducing apparatus S, the
 three detected signals are simultaneously inputted to the subtracter 11
 and the variable coefficient filters 15 and 16.
 Even during this delay processing interval, the optical disk 1 itself moves
 at the velocity V.sub.L. Eventually, therefore, detected signals based
 upon pits P lining up in a single file in the radial direction of the
 optical disk 1 are simultaneously inputted to the subtracter 11 by the
 delay processing. As a result, each crosstalk can be removed accurately.
 By using filter coefficients described later and contained in a filter
 coefficient signal Sci supplied from the filter coefficient computation
 circuit 14, the variable coefficient filter 15 conducts filter processing
 on the inputted inner detected signal Sin. The variable coefficient filter
 15 thus generates an inner crosstalk signal Sfi corresponding to a
 crosstalk component caused by the inner track, and supplies the inner
 crosstalk signal Sfi to the subtracter 11.
 On the other hand, the variable coefficient filter 16 conducts filter
 processing on the inputted outer detected signal Sout by using filter
 coefficients described later and contained in a filter coefficient signal
 Sco supplied from the filter coefficient computation circuit 14. The
 variable coefficient filter 16 thus generates an outer crosstalk signal
 Sfo corresponding to a crosstalk component caused by the outer track, and
 supplies the outer crosstalk signal Sfo to the subtracter 11.
 Operation of the zero cross sample extractor 13 and the filter coefficient
 computation circuit 14 accompanying the operation of the variable
 coefficient filters 15 and 16 will be described in detail later.
 On the basis of the inner crosstalk signal Sfi, the central detected signal
 Scent, and the outer crosstalk signal Sfo inputted to the subtracter 11,
 the subtracter 11 subtracts the value of the inner crosstalk signal Sfi
 and the value of the outer crosstalk signal Sfo from the central detected
 signal Scent. Thus the subtracter 11 removes the crosstalk caused by the
 inner track and the crosstalk caused by the inner track, generates a
 removed signal Sd, and supplies the removed signal Sd to the demodulator
 12 and the zero cross sample extractor 13.
 As a result, the demodulator 12 demodulates the removed signal Sd, and
 generates a reproduced signal Ss which corresponds to information recorded
 on the optical disk 1 to be reproduced.
 The configuration of the variable coefficient filters 15 and 16 will now be
 described by referring to FIG. 2. The variable coefficient filters 15 and
 16 have the same basic configuration. In the ensuing description,
 therefore, the configuration of the variable coefficient filter 15 will be
 described on behalf of them.
 As shown in FIG. 2, the variable coefficient filter 15 is formed as a
 transversal filter. To be concrete, the variable coefficient filter 15
 includes n D-flip-flop circuits D1 through Dn connected in series,
 coefficient multipliers M0 through Mn, and an adder AD.
 Operation of the variable coefficient filter 15 will now be described
 together with the operation of the zero cross sample extractor 13 and the
 filter coefficient computation circuit 14.
 It is now assumed among three consecutive samples in the removed signal Sd
 that each sample value changes from positive to negative or from negative
 to positive. In this case, the zero cross sample extractor 13 extracts the
 center sample of the three samples, i.e., a zero cross sample, and
 supplies its value to the filter coefficient computation circuit 14 as an
 error signal Se.
 On the basis of the inner detected signal Sin and the error signal Se, the
 filter coefficient computation circuit 14 computes filter coefficients (Co
 through Cn in FIG. 2) of the variable coefficient filter 15. The filter
 coefficient computation circuit 14 generates a filter coefficient signal
 Sci containing the filter coefficients Co through Cn, and supplies it to
 the variable coefficient filter 15.
 In parallel therewith, the filter coefficient computation circuit 14
 computes filter coefficients of the variable coefficient filter 16 on the
 basis of the outer detected signals Sout and the error signal Se,
 generates a filter coefficient signal Sco containing the filter
 coefficients, and supplies it to the variable coefficient filter 16.
 To be concrete, the filter coefficient computation circuit 14 successively
 updates the filter coefficients of each of the variable coefficient
 filters 15 and 16 so as to make the error signal Se converge to "0" by
 using, for example, the LMS (Least Mean Square) adaptive algorithm. The
 filter coefficient computation circuit 14 thus generates the corresponding
 filter coefficient signals Sci and Sco.
 The D-flip-flop circuits D1 through Dn in the variable coefficient filter
 15, as shown in FIG. 2, successively takes in a sequence of input sample
 values corresponding to the digitized and inputted inner detected signal
 Sin while shifting the sequence.
 On the other hand, the coefficient multiplier M0 in the variable
 coefficient filter 15 multiplies the input sample value sequence
 corresponding to the inner detected signal Sin by the filter coefficient
 CO, and supplies a resultant product to the adder AD.
 Furthermore, the coefficient multipliers M1 through Mn in the variable
 coefficient filter 15 multiply output signals of the D-flip-flop circuits
 D1 through Dn by the filter coefficients C1 through Cn respectively, and
 supplies resultant products to the adder AD respectively.
 The adder AD in the variable coefficient filter 15 adds all output signals
 of the coefficient multipliers M0 through Mn, and outputs a sequence of
 output sample values corresponding to the crosstalk caused by the inner
 track. The sequence of output sample values is supplied as the above
 described inner crosstalk signal Sfi to the subtracter 11.
 The operation of the variable coefficient filter 16 using the filter
 coefficients contained in the filter signal Sco is the same as that of the
 above described variable coefficient filter 15 except that concrete values
 of the inputted filter coefficients are different. Therefore, description
 of details of the variable coefficient filter 16 will be omitted.
 An optical material forming the diffraction grating 6 in the optical pickup
 PU of an embodiment will now be described.
 As described above, in the present embodiment, the coefficient of linear
 expansion .beta. of the optical material forming the diffraction grating 6
 is set equal to the change value of the oscillation wavelength of the
 laser diode 7 caused by a temperature change (i.e., the change value of
 the oscillation wavelength caused when the ambient temperature has changed
 by 1.degree. C.). Thereby, the spacing L between the irradiation position
 of the main light beam BM and the irradiation position of the first
 subsidiary light beam BS.sub.1 or the irradiation position of the second
 subsidiary light beam BS.sub.2 (see FIG. 1) is kept constant.
 Therefore, it is most desirable that the coefficient of linear expansion
 .beta. of the optical material forming the diffraction grating 6 is set
 equal to the change value .DELTA..lambda./.lambda..sub.0 of the
 oscillation wavelength caused by a temperature change (see equation (7)).
 Actually, however, kinds of the optical material which can be used as the
 diffraction grating 6 are limited. On the other hand, the change value of
 the oscillation length caused by a temperature change varies according to
 the material or the like forming the laser diode 7.
 As a result, it is not easy to set the coefficient of linear expansion
 .beta. strictly equal to the change value .DELTA..lambda./.lambda..sub.0
 of the oscillation wavelength of the laser diode 7 caused by a temperature
 change. In the diffraction grating 6 of the embodiment, therefore, a
 permissible range of the coefficient of linear expansion .beta. is
 determined so as to make the coefficient of linear expansion .DELTA.
 substantially equal to the change value .DELTA..lambda./.lambda..sub.0
 caused by a temperature change, so long as the information reproduction is
 not hindered. By using an optical material having the coefficient of
 linear expansion .beta. in this permissible range, the diffraction grating
 6 is formed.
 The permissible range of the coefficient of linear expansion .beta. is
 determined by
EQU .alpha.-.DELTA..alpha..sub.1.ltoreq..beta..ltoreq..alpha.+.DELTA..alpha..
 sub.2 (9)
 where
EQU .alpha.=.DELTA..lambda./.lambda..sub.0
EQU .DELTA..alpha..sub.1
 ={(1+.alpha..times..DELTA.t.sub.max).times..DELTA.L.sub.max }/{(L.sub.0
 +.DELTA.L.sub.max).times..DELTA.t.sub.max }
EQU .DELTA..alpha..sub.2
 ={(1+.alpha..times..DELTA.t.sub.max).times..DELTA.L.sub.max }/{(L.sub.0
 -.DELTA.L.sub.max).times..DELTA.t.sub.max }
 At this time, .lambda..sub.0 is the oscillation wavelength of the laser
 diode 7 at the design temperature of the optical pickup PU.
 .DELTA..lambda. is the change value of the oscillation wavelength per unit
 temperature. .DELTA.t.sub.max is a predetermined permissible change value
 of the ambient temperature at which the optical pickup PU is used. L.sub.0
 is the spacing between the irradiation position of the first subsidiary
 light beam BS.sub.1 or the irradiation position of the second subsidiary
 light beam BS.sub.2 and the irradiation position of the main light beam BM
 on the optical disk 1 at the design temperature. .DELTA.L.sub.max is a
 predetermined permissible change value of the spacing on the optical disk
 1.
 Among the above described parameters, ".lambda..sub.0 " and
 ".DELTA..lambda." are predetermined according to the material or the like
 of the laser diode 7. Furthermore, "L.sub.0 " is set so that the light
 beams will not interfere with each other, according to the area of the
 irradiation range of each of the first subsidiary light beam BS.sub.1, the
 second subsidiary light beam BS.sub.2, and the main light beam BM on the
 optical disk 1. In the case where the optical disk 1 is a DVD, "L.sub.0 "
 is set equal to a value in the range of approximately 10 to 20 .mu.m.
 As for the parameter .DELTA.t.sub.max, it is set by considering the ambient
 temperature at which the optical pickup PU might be installed. For
 example, if the design temperature is 25.degree. C., the parameter
 .DELTA.t.sub.max is set equal to approximately 60.degree. C. (In this
 case, the above described range of considered ambient temperature is the
 range of -35.degree. C. to 85.degree. C.)
 Setting of the parameter .DELTA.L.sub.max will now be described.
 As described above, the parameter .DELTA.L.sub.max is a permissible change
 value of the spacing between the irradiation position of the first
 subsidiary light beam BS.sub.1 or the irradiation position of the second
 subsidiary light beam BS.sub.2 and the irradiation position of the main
 light beam BM on the optical disk 1. In the crosstalk removing method of
 the embodiment, in order to correctly reproduce contents of the
 information actually read out by respective light beams, the value of the
 parameter .DELTA.L.sub.max is set on the basis of the range of such
 spacing between the light beams that information can be reproduced without
 errors.
 In addition to this, the parameter .DELTA.L.sub.max is set by also
 considering the so-called number of taps of the transversal filter in the
 variable coefficient filter 15 or 16.
 This will now be described. In the variable coefficient filter 15 or 16
 shown in FIG. 2, the delay value in one D-flip-flop circuit is set equal
 to the delay value of 1T where T is the unit of the length of the pits P
 on the optical disk 1. The unit "T" is popularly used in the technical
 field of DVD.
 Even if an error is contained in the output of one D-flip-flop circuit due
 to a change of spacing between the irradiation position of the first
 subsidiary light beam BS.sub.1 or the irradiation position of the second
 subsidiary light beam BS.sub.2 and the irradiation position of the main
 light beam BM, a correct detected value corresponding to a pit P to be
 read out is contained in outputs of other D-flip-flop circuits in the case
 where the number of taps is large. By adding outputs of D-flip-flop
 circuits in the adder AD, therefore, the probability that the inner
 crosstalk signal Sfi or the outer crosstalk signal Sfo can be generated
 more correctly is raised.
 As the number of taps in the delay circuit 9 or 10 increases, the parameter
 .DELTA.L.sub.max becomes large as evident from the foregoing description.
 To be more concrete, it is now assumed that the optical disk 1 is the
 above described DVD. When the number of taps is only one, the parameter
 .DELTA.L.sub.max is set equal to approximately .+-.1T. When the number of
 taps is three and three consecutive taps are used (n=3 in FIG. 2), the
 parameter .DELTA.L.sub.max is set equal to approximately .+-.2T. When the
 number of taps is five and five consecutive taps are used (n=5 in FIG. 2),
 the parameter .DELTA.L.sub.max is set equal to approximately .+-.3T.
 Concrete examples of the optical material used as the diffraction grating 6
 will now be described by referring to FIGS. 3A and 3B.
 In a first example, the parameters are set as follows: .lambda..sub.0 =0.4
 .mu.m, .DELTA..lambda.=0.08 nm/.degree. C., .DELTA.t.sub.max =60.degree.
 C., .DELTA.L.sub.max =1T (=0.08 .mu.m), and L.sub.0 =15.0 .mu.m. From the
 equation (9), the possible range of the coefficient of linear expansion
 .beta. becomes
EQU 11.06.times.10.sup.-5 (.degree.
 C.sup.-1).ltoreq..beta..ltoreq.28.95.times.10.sup.-5 (.degree. C..sup.-1)
 (10)
 In this case, diethylene glycol bisallyl carbonate (trade name CR-39)
 having a value of coefficient of linear expansion .beta. equal to
 11.7.times.10.sup.-5 (.degree. C.sup.-1) or poly-4-methyl pentene-1(trade
 name TPX) having the same value of coefficient of linear expansion .beta.
 equal to 11.7.times.0.sup.-5 (.degree. C.sup.-1) can be used as the
 material of the diffraction grating 6.
 FIG. 3A shows the change of spacing between the irradiation position of the
 first subsidiary light beam BS.sub.1 or the second subsidiary light beam
 BS.sub.2 and the irradiation position of the main light beam BM caused by
 a temperature change. In the case where the diffraction grating 6 is made
 of diethylene glycol bisallyl carbonate or poly-4-methyl pentene-1 (as
 indicated by a solid line in FIG. 3A), the change value is within the
 permissible range (i.e., the range of 2.times..DELTA.L.sub.max), unlike
 the case where the diffraction grating 6 is made of glass (the coefficient
 of linear expansion .beta.=0) as indicated by a broken line in FIG. 3A.
 In a second example, the parameters are set as follows: .lambda..sub.0 =0.4
 .mu.m, .DELTA..lambda.=0.08 nm/.degree. C., .DELTA.t.sub.max =60.degree.
 C., .DELTA.L.sub.max =1T (=0.08 .mu.m), and L.sub.0 =10.0 .mu.m. From the
 equation (9), the possible range of the coefficient of linear expansion
 .beta. becomes
EQU 6.61.times.10.sup.-5 (.degree. C.sup.-1).ltoreq..beta..ltoreq.
 33.39.times.10.sup.-5 (.degree. C.sup.-1) (11)
 In this case, PMMA (Poly Methyl Methacrylate) having a value of coefficient
 of linear expansion .beta. equal to 7.times.10.sup.-5 (.degree. C..sup.-1)
 or PC (Poly Carbonate) having a value of coefficient of linear expansion
 .beta. equal to 8.times.10.sup.-5 (.degree. C.sup.-1) can also be used as
 the material of the diffraction grating 6.
 FIG. 3B also shows the change of spacing between the irradiation position
 of the first subsidiary light beam BS.sub.1 or the second subsidiary light
 beam BS.sub.2 and the irradiation position of the main light beam BM
 caused by a temperature change. In the case where the diffraction grating
 6 is made of diethylene glycol bisallyl carbonate or poly-4-methyl
 pentene-1 (as indicated by a solid line in FIG. 3B), the case where the
 diffraction grating 6 is made of PMMA (as indicated by a dotted line in
 FIG. 3B), and the case where the diffraction grating 6 is made of PC (as
 indicated by a dotted line in FIG. 3B), the change value is within the
 permissible range, unlike the case where the diffraction grating 6 is made
 of glass (as indicated by a broken line in FIG. 3B).
 Even if the oscillation length of the laser diode 7 is changed by a change
 of the ambient temperature, therefore, this can be canceled and the change
 of spacing between the irradiation position of the first subsidiary light
 beam BS.sub.1 or the second subsidiary light beam BS.sub.2 and the
 irradiation position of the main light beam BM can be restricted in the
 permissible range.
 In the configuration of the information reproducing apparatus S of the
 embodiment, the permissible range of the coefficient of linear expansion
 .beta. of the optical material forming the diffraction grating 6 is
 determined as heretofore been described by
EQU .alpha.-.DELTA..alpha..sub.1.ltoreq..beta..ltoreq..alpha.+.DELTA..alpha..
 sub.2
 where
EQU .alpha.=.DELTA..lambda./.lambda..sub.0
EQU .DELTA..alpha..sub.1
 ={(1+.alpha..times..DELTA.t.sub.max).times..DELTA.L.sub.max }/{(L.sub.0
 +.DELTA.L.sub.max).times..DELTA.t.sub.max }
EQU .DELTA..alpha..sub.2
 ={(1+.alpha..times..DELTA.t.sub.max).times..DELTA.L.sub.max }/{(L.sub.0
 -.DELTA.L.sub.max).times..DELTA.t.sub.max }
 where .lambda..sub.0 =the oscillation wavelength of the laser diode 7 at
 the design temperature of the optical pickup PU, .DELTA..lambda.=the
 change value of the oscillation wavelength per unit temperature,
 .DELTA.t.sub.max =a preset permissible change value of the ambient
 temperature at which the optical pickup PU is used, L.sub.0 =the spacing
 between the irradiation position of the first subsidiary light beam
 BS.sub.1 or the irradiation position of the second subsidiary light beam
 BS.sub.2 and the irradiation position of the main light beam BM on the
 optical disk 1 at the design temperature, and .DELTA.L.sub.max =a preset
 permissible change value of the spacing on the optical disk 1.
 Even if the oscillation length of the laser diode 7 is changed by a change
 of the ambient temperature, therefore, the change of spacing between the
 irradiation position of the main light beam BM and the irradiation
 position of the first subsidiary light beam BS.sub.1 or the second
 subsidiary light beam BS.sub.2 can be restricted in the permissible range.
 Accordingly, crosstalk can be removed accurately and stably.
 Furthermore, in the case where the coefficient of linear expansion .beta.
 can be set equal to
EQU .beta.=.DELTA..lambda./.lambda..sub.0,
 the spacing between the irradiation position of the main light beam BM and
 the irradiation position of the first subsidiary light beam BS.sub.1 or
 the second subsidiary light beam BS.sub.2 can be kept constant, even if
 the oscillation length is changed by a change of the ambient temperature.
 Accordingly, the crosstalk can be removed more accurately and stably.
 Furthermore, even if the oscillation length of the laser diode 7 is changed
 by a change of the ambient temperature, information can be reproduced
 while removing the crosstalk accurately and stably.
 In the above described embodiment, crosstalk has been removed by using two
 subsidiary light beams. Besides, however, the present invention can also
 be applied to the case where information from three or more information
 tracks is detected by using two or more subsidiary light beams and
 crosstalk should be removed by using them. In this case, the spacing
 between the irradiation position of each of subsidiary light beams and the
 irradiation position of the main light beam is restricted into a
 permissible range.
 The invention may be embodied in other specific forms without departing
 from the spirit or essential characteristics thereof. The present
 embodiments are therefore to be considered in all respects as illustrative
 and not restrictive, the scope of the invention being indicated by the
 appended claims rather than by the foregoing description and all changes
 which come within the meaning and range of equivalency of the claims are
 therefore intended to be embraced therein.
 The entire disclosure of Japanese Patent Application No. 10-252196 filed on
 Sep. 7, 1998 including the specification, claims, drawings and summary is
 incorporated herein by reference in its entirety.