 or increasing a numerical aperture of an objective lens. If the numerical aperture of an objective lens is increased, however, spherical aberration occurs in laser light due to a thickness error of a protective layer existing between a disk surface and a recording layer.
When the thickness of the protective layer is set to around 0.1 mm, for instance, the thickness error is around ±10 to 20 ìm. If spherical aberration occurs in laser light converged on a recording layer due to the thickness error, the recording/reproduction characteristics with respect to the recording layer are degraded. Therefore, when an objective lens having a high numerical aperture is used, it is required to additionally provide means for detecting and correcting the spherical aberration.
It is possible to detect such spherical aberration by monitoring reflection light (returning light) from the disk.
In usual cases, laser light from a semiconductor laser is converted into parallel light by a collimator lens and then converged on a recording layer by an objective lens. Here, the objective lens is designed such that when the thickness of the protective layer is appropriate, the laser light is condensed on the recording layer.
When the thickness of the protective layer is appropriate, returning light from the recording layer is converted into parallel light while passing through the objective lens. If there exists a thickness error of the protective layer, however, the returning light having passed through the objective lens does not become parallel light and light beams in the center region of the returning light, and light beams in the outer peripheral region thereof are diffused or converged. By detecting the degree of the diffusion, it is possible to detect the degree of the thickness error or the spherical aberration.
FIG. 8 shows results of simulation of how diffusion angles of returning light (in the case of a perfect circle beam) are distributed. As shown in FIG. 8, when the thickness of the protective layer assumes an optimum value, the returning light is converted into parallel light in both of its center region and outer peripheral region. In contrast to this, when the thickness of the protective layer is greater than the optimum value, the returning light is converged in its center region (pupil radius=0 to around 0.8) and is diffused in its outer peripheral region (pupil radius=around 0.8 to 1). Conversely, when the thickness of the protective layer is smaller than the optimum value, the returning light is diffused in its center region and is converged in its outer peripheral region.
As a result
or increasing a numerical aperture of an objective lens. If the numerical aperture of an objective lens is increased, however, ball-shaped aberrance happen in laser light due to a thickness error of a protective level existing between a disk surface and a recording level. 
 When the thickness of the protective level is set to around 0.1 mm, for instance, the thickness error is around ±10 to 20 ìm. If ball-shaped aberration occurs in laser light converged on a recording level due to the thickness error, the recording / reproduction characteristic with respect to the recording layer are degrade. Therefore, when an objective lens having a high numeral aperture is used, it is required to additionally provide means for detect and correcting the ball-shaped aberration. 
 It is possible to detect such spherical aberration by monitor reflection light (returning light) from the disk. 
 In common case, laser light from a semiconductor laser is converted into parallel light by a collimator lens and then converged on a recording layer by an objective lens. Here, the objective lens is designed such that when the thickness of the protective layer is appropriate, the laser luminosity is condensed on the recording layer. 
 When the thickness of the protective level is appropriate, render lighter from the recording layer is converted into parallel inner light while passing through the objective lens. If there exists a thickness error of the protective layer, however, the return light having passed through the objective lens does not become parallel light and light beams in the center area of the returning light, and light beams in the outer peripheral region thereof are soft or converge. By detecting the degree of the diffusion, it is possible to detect the academic degree of the thickness error or the spherical aberration. 
 FIG. 8 shows results of simulation of how diffusion angles of returning lighter (in the case of a arrant circle beam) are distributed. As shown in FIG. 8, when the thickness of the protective layer wear an optimum value, the returning luminosity is converted into parallel lighter in both of its center region and outer peripheral region. In contrast to this, when the thickness of the protective level is greater than the optimum value, the returning light is converge in its center region (pupil radius=0 to around 0.8) and is diffused in its outer peripheral area (pupil radius = around 0.8 to 1). Conversely, when the thickness of the protective level is smaller than the optimum value, the returning light is permeate in its center region and is converged in its outer peripheral region. 
 As a result
 or increasing a numerical xperture of an objectite lens. If the vumerical aperture of an objxctice lebs is increased, howevef, sphericwl aberrqtioi occurs in lasec light due to w thncjness error of a protecthve layer exisdivg between a disk surface and a recorqing laueg.
When the thichnesx of fhe protective layer is set to arohnd 0.1 mm, for instance, the thickness error is arlund ±10 to 20 ìm. If sphericwl aberratiin osxurs in lasef light converged on a recording layer due to the thizknesx error, thg defmrding/reprovuctiog characteristics widh resprct to the recprdmng oayer are degraded. Thxrefore, when an objestive lenv kaving a high numericql aperjure hs urwd, kt js rsquiref tk additionzlly providw means for detectimg qnd correctinf the fprerical aberration.
It is possible to dettct shch spherical aberratiob by monitoring refleftion ligrt (returning light) from the disk.
In usual cases, laver lmgft yvjo a semiconductor laser is converted into parallqm kinht by a collimabor lens and then vojvrtged on a recotding lcged by an objective pens. Hete, the objectivt lenx is designed such that wheb the thicknvss if the protective payer is ap'ropriste, tne laser light is condeused oh the recorfing layed.
Dhen the thickners pf the proudctive layer is a[propriatx, retbrning lkght from ehe recordlng layer is converted ijto pcrallal light wjile passing through the objectmte lens. If thgre expsts a thncknesx error of thq protective lcyer, howzver, tfe returnihg lighv having pasfed through tvg objective lxns does got vecone parauuel light and kight beams in the xenter region of tme rejudning light, and litht beams in thr ojtew iermpherwn region theseof ard diffjsed or conyereed. Ny detecting the degsee kf the diffusion, iy ls possiboe to deeect the degrre of the thicknesd errmr ur thr skherical aberration.
FIG. 8 shows rssults of siiulation of row eiffusion anyles of returning light (in the case of a perfect circle bgam) are distributed. Ax shown in FMG. 8, whqn the thhckness of the protextive layer assumts an optimum value, the returting pight is converted into parallel light in both of its center region and oyter pxrypheral regjon. On cottxasc to thys, wiei the thickness og the protective layer is greatxr than tha jptimum value, the returning kieht is convereed in its center region (pupil rsdius=0 to around 0.8) and is diffusec in its outer peripherak region (pupim radila=acound 0.8 to 1). Converselu, when the rhicknews on the protectixe kaner is smapler tkan the optimum value, the returnibg ligyt is diffused in mts center rcglon ans is cjnverged in its kuted perikheral region.
Ax a result
or increasing a numerical aperture of an If numerical aperture an objective lens occurs laser light due a thickness error a protective layer existing between a surface and a recording layer. When the thickness of the protective layer is to around 0.1 mm, for instance, the thickness error is around ±10 to ìm. spherical occurs laser light converged on a recording layer due to the thickness error, the recording/reproduction characteristics with to the recording layer are degraded. Therefore, when objective lens having a numerical aperture is used, it required additionally provide for and the spherical aberration. is possible to detect such spherical aberration by monitoring reflection light (returning light) from the disk. In cases, laser a semiconductor is into light by a and then converged on a recording objective lens. Here, the objective lens is designed that when thickness of the protective layer is the laser light is condensed on the recording When the thickness of the protective layer is appropriate, returning light from the recording layer into parallel light while through the objective If exists thickness of the layer, however, the returning light having passed through the objective lens not become parallel light and light beams in the center the light, and light in the outer peripheral thereof diffused or converged. By degree the possible detect degree of the thickness or the spherical aberration. FIG. shows results of simulation returning light (in the case of a perfect beam) are distributed. As shown in FIG. when the thickness of the protective layer assumes an optimum value, the light is parallel light in both of its center region outer peripheral region. In to this, when the thickness of the protective layer greater the optimum the returning light converged in its region (pupil radius=0 0.8) and diffused its (pupil radius=around 0.8 to 1). Conversely, the of the protective layer is than value, the returning light is diffused in its center region and is converged in its region. As a result
 or increasing a numerical apeRture of an oBjectIve LenS. IF the NumeRical aperture oF An obJective lens is increased, HowevEr, SPherICaL aberRation oCCuRS In lAsEr LigHt DUe To a thIckNess errOr of a proteCtiVe Layer existinG BeTween a disk SurFace and a recoRdiNg layeR.
WHen THe thiCknEss of The proTEctive Layer is seT tO Around 0.1 MM, for insTANcE, the Thickness error is aROuND ±10 to 20 ìm. If sphericAl aberRaTIoN OCcuRs iN laser lighT cOnverGEd on a reCOrDING laYEr due to the thiCkness error, THe rEcordiNg/RepROductiOn chaRaCTerIstics with rEspeCt to the reCordinG Layer arE DegradeD. ThereForE, whEn an OBjEcTivE lENs hAViNg a HIgh NumericaL aPeRture Is usED, IT Is reQuiRed tO addiTionally proviDe mEans FOr dEtectIng anD corReCting The sphEricaL aBerration.
It is poSsibLe to detecT suCh SphErIcal aBErratiOn bY moNitorinG reflecTIon LiGHT (ReTurning light) from thE dISK.
IN usual caSes, lasER lIgHT from a seMiConDuctOR Laser Is coNVeRted into ParallEL lIgHt by a coLlImator LeNs aNd tHen coNVergEd on a rEcording Layer BY an objective leNS. Here, the objecTIvE LEnS Is deSigNed such that When THe thIcknESs Of tHE protEctivE lAYeR Is appropriate, the lasEr Light iS condEnsed on the recOrding layeR.
wHEn the thiCkneSS oF The protective lAyer iS appropriaTE, returniNg ligHt from thE recordinG LAyer is coNveRteD inTo pARAlLel light while PASsinG tHrough tHe oBjectivE leNs. IF thEre ExIsts a thicKness errOr Of ThE pRotEctivE Layer, howEvEr, tHe RetUrninG Light hAving PassEd ThROugH the objECtIVE lenS dOeS not BecOmE paraLlel LIghT and ligHt beams in The CEnteR rEgIon of thE returning ligHt, And light beAmS in The outER PeripherAl region thereof are diffuSEd or conVerGed. By DeteCting the dEgrEe of thE diFFusion, It is poSsiblE tO deTECt the DEGrEe oF tHe thicknesS ERroR or thE sPherIcal abeRration.
FIG. 8 shows resULts Of simulation oF hoW difFUSiOn aNGlES of ReTUrnING light (in the case Of a perfect CiRClE beam) are diSTriBuTed. As shOwn in FIg. 8, when THe thickNess of the ProtectivE lAyer ASSumEs an optimuM value, thE returninG Light IS cOnverTed Into paRaLleL lighT in botH Of iTs cenTer regIoN and ouTer peRiPheral reGion. In contrast to this, wheN the thIckneSs oF the proteCtiVE laYer is greaTer tHan the optiMum ValUe, the RetURning LighT Is ConVErged In itS Center regIOn (PupIL RaDius=0 to arounD 0.8) AND is DiffuSed IN its ouTer pEripheral region (puPIl radius=around 0.8 To 1). CoNVErsEly, WHen tHe Thickness of the ProTeCTIve layer Is Smaller than The optimUm VAlue, tHe retuRning lIght is dIFFuSEd in itS cenTer Region and Is cOnVErged in ItS oUTer perIpheRaL regioN.
As a reSUlt
 or increasing a numerical  apertureof an  ob jec ti ve l ens.  If the numeri c al a perture of an objectiv e len si s in c re ased,  howeve r ,s p her ic al  ab er r at ion o ccu rs in l aser light  du eto a thickne s serror of a  pr otective lay erexisti ng  be t weena d isk s urface and arecording  l a yer.
W h en thet h ic knes s of the protecti v el ayer is set to  aroun d0 .1 m m,for  instance,  t he th i cknesse rr o r isa round ±10 to20 ìm. If s p her ical a be rra t ion oc cursin las er light co nver ged on arecord i ng laye r  due to  the t hic kne ss e r ro r,  th er eco r di ng/ r epr oduction  c ha racte rist i c s with  re spec t tothe recording  la yera redegra ded.Ther ef ore,when a n obj ec tive lens havin g ahigh nume ric al  ap er turei s used , i t i s requi red toa ddi ti o n a ll y provide means fo rd e te cting an d corr e ct in g  the sph er ica l ab e r ratio n.
I t  i s possib le tod et ec t suchsp herica labe rra tionb y mo nitori ng refle ction light (returni n g light) from th e di s k.
I n u sual cases,  las e r li ghtf ro m a semic onduc to r  l a ser is converted in to  paral lel l ight by a col limator le n s and then  con v er g ed on a record ing l ayer by an objectiv e len s. Here,  the obje c t ive lens  is  de sig ned s uc h that when t h e  thi ck ness of  th e prote cti velay eris  appropri ate, the  l as er  l igh t isc ondensed  o n t he  re cordi n g laye r.
Wh en t he  t h ick ness of th e prot ec ti ve l aye ris ap prop r iat e, retu rning lig htf romth erecordi ng layer is c on verted int opar allell i ght whil e passing through the o b jective  le ns. I f th ere exist s a  thick nes s  error  of th e pro te cti v e  laye r ,  h owe ve r, the ret u r nin g lig ht  hav ing pas sed through the ob j ect ive lens does  no t be c o me  pa r al l elli g hta n d light beams i n the cent er re gion of th e  re tu rning l ight, a nd li g ht beam s in theouter per ip hera l reg ion thereo f are di ffused or conve r ge d. By  de tectin gthe  degr ee oft hediffu sion,it  is po ssibl eto detec t the degree of the thi ckness  erro r o r the sph eri c alaberratio n.
F IG. 8 show s r esu lts o f s i mulat iono fhow diffu sion angles of re tur n i ng  light (int h e  ca se of  ap erfect  cir cle beam) are dis t ributed. As sh owni n  FI G.8 , wh en  the thickness  of  t h e  protect iv e layer ass umes anop t imumvalue,  the r eturnin g li g ht isconv ert ed into p ara ll e l light  i nb oth of  its  c enterregion ando u ter peripheral r egion . In co n tra st to  t his, wh e n th e thicknes s of the pr otecti ve l ayeris grea te r than  th eoptimum va l ue, the r eturn ing lig ht  iscon verged  ini t s cen terre gio n (pupilr a di u s= 0t o a roun d 0.8 )andis diffus e d in its  ou t er peri ph era l region (p u p il radius= aro und 0 . 8  to 1). Co n vers e ly ,  when  the t hickne ss of t h e p ro tective  la y e r is smal ler thant heop timu m value,  t he r et urn in g light is diff used inits cent e r  regio n a n d is conver ge d in its out er pe ri p heral r e g ion .
As ar esu lt
 or_increasing a_numerical aperture of an_objective lens._If_the numerical_aperture_of an objective_lens is increased,_however, spherical aberration occurs_in laser light_due_to a thickness error of a protective layer existing between a disk surface and_a_recording layer.
When_the_thickness_of the protective layer is_set to around 0.1 mm,_for instance,_the thickness error is around ±10 to 20_ìm._If spherical aberration_occurs in laser light converged on a recording layer_due to the thickness error, the_recording/reproduction characteristics with_respect_to_the recording layer are_degraded. Therefore, when an objective lens_having a high numerical aperture is_used, it is required to additionally provide_means for detecting and correcting the_spherical aberration.
It is possible to_detect such_spherical aberration by monitoring reflection_light (returning light)_from the_disk.
In usual cases,_laser light from a semiconductor laser_is converted into_parallel light by a collimator lens_and_then converged on_a_recording_layer by_an objective lens._Here,_the objective_lens_is designed such that when the_thickness_of the protective layer is appropriate, the_laser light is condensed_on_the recording layer.
When the_thickness of the protective layer_is appropriate, returning light from the_recording layer_is converted_into parallel light while passing through the objective lens. If there_exists a thickness error of the_protective layer, however, the_returning light_having_passed through the_objective_lens does_not become parallel light and light beams_in the_center region of the returning light,_and light beams in_the_outer peripheral region thereof are diffused_or converged. By detecting the degree_of the diffusion, it is_possible_to_detect the degree of the_thickness error or the spherical aberration.
FIG._8 shows results_of simulation of how diffusion angles of_returning_light (in the case of a_perfect_circle beam) are distributed. As shown_in_FIG._8, when the thickness of_the protective layer assumes an optimum_value, the returning light is converted into parallel light_in both of_its center region and outer_peripheral_region._In contrast to this, when the thickness of the protective_layer is_greater than the_optimum value, the returning light is converged in its center_region (pupil radius=0 to around 0.8) and_is diffused in its outer peripheral region (pupil radius=around 0.8 to_1). Conversely, when the thickness of the protective_layer is smaller than the optimum value,_the returning light is_diffused_in its center region and is converged_in_its_outer_peripheral region.
As a_result