CA1303739C - Magneto-optic records - Google Patents
Magneto-optic recordsInfo
- Publication number
- CA1303739C CA1303739C CA000566559A CA566559A CA1303739C CA 1303739 C CA1303739 C CA 1303739C CA 000566559 A CA000566559 A CA 000566559A CA 566559 A CA566559 A CA 566559A CA 1303739 C CA1303739 C CA 1303739C
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- Canada
- Prior art keywords
- layer
- record according
- optical record
- substrate
- optic
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B11/00—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor
- G11B11/10—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field
- G11B11/105—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing
- G11B11/10582—Record carriers characterised by the selection of the material or by the structure or form
Abstract
ABSTRACT OF THE DISCLOSURE
A magneto-optic (MO) record for recording in re-sponse to radiation intensity modulation in the pre-sence of a magnetic field. In one embodiment, radia-tion is incident on a given side of the record and it includes a layered structure having erasable MO mater-ial. A substrate supports the MO material on the side of the material away from the given side and is fabri-cated from a material having preferred support charac-teristics that are selected without regard for the transmissive optical characteristics thereof. A sup-port spaces a window for forming an air gap on the side of the MO material toward the given side of the record. The window is optically transparent and has minimal birefringence so that the radiation incident on the given side of the record is transmitted through the window with minimal optical retardation. As a result, the difference of the effective Kerr angle of polarization rotation corresponding to different mag-netizations (up or down) of the MO recording layer re-lates primarily to data recorded and not to optical retardation from the window. In other embodiments, transparent adhesive is provided in the air gap, and a tuned multi-layer interference structure (including MO
material) is used in place of the previous layered structure to enhance and optimize the difference of the effective Kerr angle of polarization rotation cor-responding to different magnetizations of the MO
recording layer. Two of the records can be placed with their substrates back-to-back (or sharing a common substrate) to form a double sided MO record.
A magneto-optic (MO) record for recording in re-sponse to radiation intensity modulation in the pre-sence of a magnetic field. In one embodiment, radia-tion is incident on a given side of the record and it includes a layered structure having erasable MO mater-ial. A substrate supports the MO material on the side of the material away from the given side and is fabri-cated from a material having preferred support charac-teristics that are selected without regard for the transmissive optical characteristics thereof. A sup-port spaces a window for forming an air gap on the side of the MO material toward the given side of the record. The window is optically transparent and has minimal birefringence so that the radiation incident on the given side of the record is transmitted through the window with minimal optical retardation. As a result, the difference of the effective Kerr angle of polarization rotation corresponding to different mag-netizations (up or down) of the MO recording layer re-lates primarily to data recorded and not to optical retardation from the window. In other embodiments, transparent adhesive is provided in the air gap, and a tuned multi-layer interference structure (including MO
material) is used in place of the previous layered structure to enhance and optimize the difference of the effective Kerr angle of polarization rotation cor-responding to different magnetizations of the MO
recording layer. Two of the records can be placed with their substrates back-to-back (or sharing a common substrate) to form a double sided MO record.
Description
~03739 Hoe 87/K 093 MAGNETO-OPTIC RECORDS
Speclficatlon BACKGROUND OF THE INVENTION
Field of the Invention The present inventlon relates to the fleld of erasable magneto-optic recordlng and more particularly to the fleld of record structureæ that allow a higher _ signal-to-noise ratio of signals output in response to reading of the record.
Descrlptlon of the Prlor Art `10Magneto-optic (MO) recording is sometimes re-ferred to as thermomagnetic recording, or thermo-mag-neto-optlc recordlng. These terms refer to technology that is being actively developed in many laboratories around the world because it promlses to comblne the capaclty and packing density of optical recording to-gether with the erasability of conventional magnetic recordlng. The basic prlnciples of MO recording are set forth in U.S. Patent #3,949,387 issued April 6, 1976 for ~EAM ADDRESSABLE FILM USING AMORP~OUS MAG-NETIC MATERIAL to Chaudhari, et al. Distinct fromconventional optlcal recording in which an optical coating is ablated, for example, to produce reflec-tance variations ln a laser beam that reads the re-cord, MO recording is accomplished by inducing and detecting relatively small rotations of the plane of polarization of a linearly polarized read beam re-flected off the record. This is sometimes referred to as Kerr MO recording, and such rotatlon of the plane of polarization as the "polar Kerr magneto-optlc ef-fect", or, in context, as ~ust the "Kerr effect." Theamount of such polarization rotation of the read beam is most desirably dependent only on the magnetization of the MO thin film recording structure at the parti-~:~03739 cular locatlon on which the read beam ls lncident on the record.
As understood, the normal practlce at the present state of the art of MO recordlng is to address the MO
record for reading, erasing, and wrlting by means of a focussed laser beam that is dlrected through a trans-parent disk of glass or plastic which serves as a sub-strate for the thin film MO recording structure. Such addressing for wrlting or erasing ls done in the pre-sence of a magnetic fleld. The substrate of the MO
) record, typlcally about 1.2 mm thick, also serves to provlde defocussing of dust that may be on the surface of the substrate, and may be provlded wlth grooves or other features to provide informatlon for tracklng, etc. One problem with this MO record ls caused by thebirefringence of such substrate.
Birefringence refers to the phenomenon in which an optical materlal, ln a state of stress, for exam-ple, becomes non-homogeneous and non-isotropic with respect to the index of refraction. That is, light havlng different states of polarization sees sllghtly different lndices of refraction in such optical mater-ial. Since readout of recorded lnformation by means - of the Kerr effect is based on detecting relatively small rotations of the plane of polarization (of the order of one degree), and since the read beam of light makes a double pass through such substrate, the opti-cal retardatlon caused by rotatlon of the plane of polarlzatlon due to blrefrlngence is a very serious problem for magneto-optic recording. In particular, for state of the art MO records, a tolerance of 30 na-nometers for double pass optlcal retardatlon and a further tolerance on the gradient of the retardation of about 30 nanometers/cm may be requlred to assure satisfactory performance of such substrate with re-spect to birefringence.
It has proven qulte dlfflcult to achleve thls low level of blrefrlngence ln preferred optlcal plastlcs, such as polycarbonate (PC) t used for such substrates.
Thus, in the current llterature lt ls recognlzed that from most polnts of vlew, bisphenol-a-polycarbonate ls a good candldate for such MO disk substrates. See, for example, Alan B. Marchant, "Retardation Effects in Magneto-optic Readout", SPIE Proc. of Optical Mass Data Stora~e II, (San Diego, CA, August, 1986) 695.
Marchant noted that despite years of research which have resulted in such substrates wlth a very low op-tical retardation for normally-incldent light, the problem ls substantlal with lncreases in the angle of lncldence relatlve to normal. Marchant also noted that even vertlcal blrefrlngence of such sub6trates ln a dlfferentlal detectlon system ls a serlous problem.
~ ased on read-only dlgltal audlo disk technology, for example, the most economical means of providing a grooved transparent substr~te is in~ection moldlng of a suitable optical plastic. Thls is the method by whlch substrates are lmpressed wlth grooves and digl-tal audlo lnformatlon. The commerclal success of such read-only dlgltal audlo dlsks has lead to attempts to ln~ectlon mold such optlcal plastlcs as polymethyl-methacrylate (PMMA) and PC as substrates for use assubstrates ln MO records. However, the stress result-lng from such moldlng leads to blrefrlngence problems when PC ls the materlal used for such MO record sub-strates. Although adequate performance wlth respect to substrate blrefrlngence has been achleved wlth PMMA, PMMA has undesirable permeablllty to water vapor and expands ln a humld envlronment. Thls dlmensional lnstability causes cracklng and peellng of the thln fllm MO recordlng structure.
Desplte the problems of blrefrlngence noted ln the Marchant article, there have been continued at-1;~03739 tempts to use such conventlonal substrates both to support the thin film MO recording structure and to transmlt the record and read beams to and from the thln film MO recordlng structure. This ls lndlcated by an artlcle publlshed in August, 19~, by Treves and Bloomberg, entitled, "Effect of Birefringence on Opti-cal Memory Systems", in SPIE Proc. of Optlcal Mass Da-ta Stora~e II, (San Diego, CA). They reported on such conventional substrates and concluded that optical bi-refringence must be controlled since substantial re-duction in the signal-to-noise ratio is attributed to the birefringence of the substrate. As recently as March 13, 1987, Toda, et al., in an article entitled "Analysis of Signal to Noise Ratio in Magneto-Optical Disk Using a Polarlzation Slmulator", Topical Meetin~
on OPtical Data Stora~e, OPtical Societ~ of America Tech. Di~est Series, Vol. 10, (Statellne, Nevada, 3/11-13/87) pp. 34-37, also reported on such conven-tional substrates. They concluded that the reta~da-tion of a PC substrate showed complicated dependenceon the lncident angle of the laser beam. Since MO
recording, llke all optical recording methods, uses a focussed beam of light for writing and reading, and since focussed beams include rays of all angles of in-cidence out to a limiting angle set by the numericalaperture of the recording obJective lens, readout of MO signals through PC substrates is vulnerable to these effects.
Such continued attempts to use such conventlonal substrates to perform the above-noted multiple func-tions of support (for the thin film MO recording structure) and beam transmission have apparently ig-nored disclosures relating to types of optical record-ing other than MO recordlng. In those other types of 3~ optical recordlng physical changes such as melting or evaporation, or chemical changes such as decomposi-. . .
130373~
tion, effect the recording function. In one form of such recording by physical changes, uslng ablative write-once media, mark formatlon is created either by e~ecting material from the viclnity of the mark, or by causlng material to withdraw from the mark region lnto a surroundlng rim, or both. In elther case, a free surface unconstralned by any overlayer i6 required, or is said to be beneflcial. For example, in U.S. Patent #4,074,282 lssued February 14, 1978 for RADIATION-SENSITIVE RECORD WITH PROTECTED SENSITIVE SURFACE to - ! Balas, Jr., et al., llght is lncident upon an ablative recording medium deposited on the inner surface of a substrate dlsk, which ls one of two disks that have an air gap between them. The light is incident on and is transmitted through the other disk, which is a window disk, across the air gap and directly onto the abla-tive recordlng medium. However, such disclosure rec-ognizes that in a number of situations, such as when the ablated material would coat the window, the inci-dent light should be transmitted through the substratedisk and not through the window disk. Since the above-referenced conventional use of substrates in MO
recording conforms to this latter teaching of the 4,074,282 Patent, it appears that such conventional MO
substrate practices and the earlier non-MO recording practices (e.g. Patent 4,074,282) both teach the use of the substrate for both the support and beam trans-mission functlons. In view of the above-referenced articles, however, when such teachings are followed there are substantial, unsolved problems with bire-fringence that detract from the overall performance of such MO records.
SUMMARY OF THE INVENTION
In contrast to the prior art in which an incident laser beam has been transmitted through a substrate .. .... .. ..
1303'739 that supports a thln fllm M0 recordlng structure, mak-ing lt necessary to contlnue efforts to reduce the bl-refrlngence of such substrate fabricated from materi-als havlng the undesirable blrefrlngence, the M0 re-cord of the present lnventlon provldes a high quallty,durable, economlcal, erasable optlcal record structure havlng mlnimal blrefrlngence problems, such that the slgnal-to-nolse ratlo ls enhanced relative to that of the conventional structures for erasable magneto-optlc records.
A record for sensing radlatlon lntenslty modula-tlon in the presence of a magnetlc fleld accordlng to the prlnclples of the present inventlon separates the functlon of supportlng the thin fllm M0 recordlng structure of the substrate from the substrate's optl-cal transmlsslon functlon so that the ldeal substrate can be selected wlthout regard to the transmlsslve optlcal properties thereof.
An M0 record deslgned ln accordance wlth the prlnciples of the present lnventlon has a substrate that may have very poor blrefrlngence propertles, yet the record provides lmproved slgnal-to-nolse ratlos ln read beams slnce neither the record beam nor the read beam ls transmltted through the substrate.
In an M0 record of the present invention, the substrate ls effectlvely the bottom of the record, wlth a thln film M0 recording structure on the top surface of the substrate, where such top surface sup-ports a wlndow through whlch recordlng energy ls transmitted ln the presence of a magnetic blas fleld for M0 recording.
Wlth these and other obJects in mind, a flrst preferred embodlment of an M0 record for recordlng ln response to radlatlon lntensity modulatlon in the pre-sence of a magnetlc fleld accordlng to the present ln-vention is responslve to radlatlon lncldent on a given side thereof. The record lncludes erasable material for M0 recording in response to such inten~ity modu-lated radlatlon and to the magnetlc fleld. A sub-strate is provlded for supporting the erasable mag-neto-optic material on the side of the material away from the glven slde of the record. The 6ubstrate is fabricated from a material havlng preferred support characterlstics that are selected without regard for the transmisslve optlcal characterlstic6 thereof. A
support structure overlles at least a portlon of the \ substrate for provldlng at least one support surface spaced from the erasable magneto-optlc material. A
wlndow ls mounted on the support surface for formlng an alr gap on the slde of the erasable magneto-optlc material toward the given slde of the record. The window has flat surfaces and ls fabricated from a ma-terial that is optically transparent and that has mln-imal birefrlngence so that the radlatlon incldent on the glven side of the record is transmitted through the wlndow with mlnimal optical retardation and is not transmitted through the substrate before being lncl-dent on the erasable materlal for M0 recording ln the presence of the magnetlc field.
A second preferred embodiment of the present ln-ventlon relates to a record for recordlng ln response to radiation intenslty modulation in the presence of a magnetic field, where the radiation is incident on a given side of the record. The record includes a sub-strate and a wlndow. The window forms the glven side of the record. The wlndow is flat and fabricated from a material that is opticall~ transparent and has mini-mal birefringence so that the incident radiation is first transmitted therethrough with minimal optical retardation. A transparent adhesive (such as optlcal cement) is provided between the surface of the window opposlte to such given side of the record and a thin ~303'739 film M0 recording structure deposlted on the sub-strate. The thln film M0 recordlng structure lncludes a magneto-optic layer sandwiched between a transparent layer and a protectlve layer. The protectlve layer lles between the substrate and the magneto-optlc lay-er, and the transparent layer overlies the magneto-optic layer and therefore is adJacent to both the mag-neto-optic layer and the transparent adheslve. The optically transparent layer and the protectlve layer are formed from hermetlc materlal and the magneto-! optic layer records ln response to such radlatlon lntenslty modulatlon ln the presence of a magnetlc field. The substrate ls provlded for supportlng the protectlve layer and thus the remainder of the record.
The substrate ls fabricated from a material having lm-proved film support and moisture impermeability char-acteristics, eg. dlmenslonal stability, without regard for the transmlssive optlcal characterlstlcs thereof since the intenslty modulated radiatlon ls not trans-mitted through the substrate.
A thlrd preferred embodiment of the present ~n-ventlon provldes for the lncorporation of the erasable magneto-optic layer into a tuned multl-layer interfer-ence structure to further enhance and optlmize the ~5 difference of the effect~ve Kerr angle of polarization rotatlon corresponding to different magnetlzations (up or down) of the magneto-optic recordlng layer. The tuned multi-layer interference structure is a four layer structure comprised of a reflective layer over-lying the substrate, a transparent dielectrlc spacer protective layer overlying the reflective layer, a magneto-optic recording layer overlylng the spacer protective layer, and a protective overlayer of trans-parent dlelectric overlying the magneto-optic layer.
A fourth preferred embodiment of the present in-vention provides two of the records of the first, se-, ~ _ ... . . ... ...... . .. ... .
`` 1303739 cond, or third embodlments, wlth the substrates back-to-back or sharlng a common substrate to form a double sided MO record.
BRIEF DESCRIPTION OF THE DRAWINGS
Other ob~ects, features and advantages of the present lnvention will be apparent from an examination of the following detailed descriptions which include the attached drawings in which:
FIG. lA is a vertlcal cross-sectlonal vlew of a flrst preferred embodiment of a record for recordlng in response to radiatlon lntenslty modulatlon in the presence of a magnetlc fleld accordlng to the present lnventlon, showlng a magneto-optic recordlng layer between a substrate and a wlndow;
FIG. lB is an enlarged vlew of a portion of the record shown ln FIG. lA, lllustratlng the magneto-optic layer between protectlve layers;
FIG. 2A is a vertical cross-sectional vlew of a portlon of a second preferred embodiment of a record for recordlng in response to radiation lntensity modu-latlon ln the presence of a magnetic fleld accordlng to the present invention, where a spacer for separat-ing a window from a substrate is in the form of trans-parent adhesive;
FIG. 2B ls an enlarged vlew of a portion of the record shown ln FIG. 2A, lllustrating a thin fllm mag-neto-optic recording structure between the adhesive and the substrate;
FIG. 3A is a vertical cross-sectional view of a fourth preferred embodlment showing a two-sided mag-neto-optlc record for recording in response to radia-tlon intensity modulation in the presence of a mag-netic field according to the present lnventlon;
FIG. 3B is an enlarged vlew of a portlon of the two-sided record shown ln FIG. 3A; snd ............................... .. .. .
~303739 FIG. 4 is an enlarged vlew of a third embodlment of a magneto-optlc record showing a tuned multi-layer lnterference structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Summar.V of the Preferred Embodiments The first preferred embodlment of the present ln-ventlon ls shown ln FIG. lA as a record 10 for recor-! dlng ln the presence of a magnetlc fleld (represented by arrows 11) and ln response to lntenslty modulatlon of energy such as radlatlon ln the form of a laser beam lndicated by arrow 12. The record 10 according to the present invention records in response to such radiation 12 incident on a given side 13 thereof. The record 10 lncludes a thin film magneto-optic (MO) re-cording structure 14 that includes a layer 15 of eras-able material for MO recordlng. A substrate 16 ls provlded for supportlng the thln fllm MO recordlng structure 14 on a slde 17 thereof opposlte to the gl-ven side 13 of the record 10. The substrate 16 is fabricated from materlal havlng preferred support characterlstlcs that are selected wlthout regard for the transmissive optlcal characteristics thereof. A
support structure 18 in the form of concentric rlngs 19-19 overlles at least a portlon of the substrate 16 for providing at least one support surface 20 spaced from the erasable magneto-optic layer 15. A window 21 ls mounted on the support surfaces 20 for formlng an alr gap 22 on a side 23 of the erasable magneto-optic layer 15 toward the glven slde 13 of the record 10.
The wlndow 21 has flat surfaces 24-24 and is fabri-cated from a material that ls optlcally transparent and that has mlnlmal blrefringence so that the light radiation 12 incident on the given side 13 of the re-1;~03'739 cord 10 is transmltted through the wlndow 21 wlth mlnlmal optical retardatlon and ls not transmitted through the substrate 16 before belng incldent on the erasable layer 15 for M0 recordlng.
A second preferred embodlment of the present in-vention ls shown ln FIGs. 2A and 2B as a record 25 for recordlng ln response to lntenslty modulatlon of the radlation 12 in the presence of the magnetlc fleld 11, where the radiatlon 12 ls lncldent on a given side 13' of the record 25. The record 25 includes a substrate 26 and a window 27. The wlndow 27 forms the glven slde 13' of the record 25. The wlndow 27 ls flat and fabrlcated from a materlal that ls optlcally trans-parent and has minimal blrefringence so that the in-cldent radlatlon 12 ls flrst transmltted therethrough wlth mlnimal optlcal retardatlon. A transparent ad-hesive 28 (such as optical cement) is provlded between a surface 29 of the window 27 opposite to such given side 13l of the record 25 and a thin fllm M0 recordlng - 20 structure 30 deposited on the substrate 26. The thin fllm M0 recording structure 30 lncludes a layer 31 of magneto-optlc material sandwlched between a transpar-ent layer 32 and a protectlve layer 33. The protec-tive layer ~3 lies between the substrate 26 and the magneto-optlc la~er 31, and the transparent layer 32 overlles the magneto-optlc layer 31 and therefore ls ad~acent to both the magneto-optic layer 31 and the transparent adheslve 2~. The optlcally transparent layer 32 and the protectlve layer 33 are formed from hermetic materlal and the magneto-optic layer 31 is responslve to such radlatlon 12 ln the presence of the magnetic field 11 for recording the intensity modula-tion of the radiatlon 12. The substrate 2~ is provl-ded for supporting the protective layer 33 and thus the remainder of the record 25. The substrate 2~ ls fabricated from a material havlng excellent fllm sup-.... .
13~3739 port and moisture impermeablllty characterlstlcs with-out regard for the transmissive optlcal characteris-tics thereof slnce the radlation ls not transmitted through the substrate 26.
A thlrd preferred embodlment of the present ln-vention is shown in FIG. 4 as provldlng a tuned multl-layer lnterference structure 34 instead of the thln film recordlng structures 14 and 30 of the respectlve records 10 and 25 and lnstead of the comparable thln film recording structure of the fourth embodiment shown in FIGs. 3A and 3B. In place of such struc-tures, the structure 34 lncludes a reflectlve layer 35 directly on the substrate 16 of the first embodiment, for example, for reflecting the lncldent radlatlon 12.
A transparent dlelectrlc spacer protective layer 36 overlies the reflective layer 35. An erasable mag-neto-optic layer 37 similar to that of the first em-bodiment is provided on the spacer protectlve layer 36. A protectlve overlayer 38 of transparent dielec-tric material overlies the magneto-optic recording layer 37. The thicknesses of the layers 35, 36, 37 and 38 of the interference structure 34 are selected so that the interference structure 34 is tuned to further enhance and optimlze the difference of the effectlve Kerr angle of polarlzatlon rotatlon corres-ponding to different dlrectlons of magnetization (up or down) of the magneto-optic recording layer 37. If the interference structure 34 ls provlded on the sub-strate 16 of the flrst embodlment (of the record 10), slnce the lntenslty modulated radlation 12 ls not transmitted through the substrate 16, the substrate material is selected without regard for the trans-missive optlcal characterlstics thereof.
A fourth preferred embodlment of the record of the present invention is shown in FIGs. 3A and 3B as a double-sided optical record 40 including dual thin 1;~03739 fllm magneto-optic recording structures 41-41. Each dual structure 41-41 includes a layer 42-42 of eras-able magneto-optic materlal for recording ln the pre-sence of the magnetic field 11 in response to inten-sity modulation of the radiation 12 incident on respe-ctive first and second sides 43 and 44 of the record 40.
The record 40 also includes a substrate 45 for supporting the dual thin film magneto-optic recordlng structures 41-41 on opposite sides 46 and 47 thereof ! so that the M0 layer 42 shown as the upper layer 48 (FIG. 3B) is toward the first side 43 of the record 40 and the M0 l~yer 42 shown as the lower layer 49 (FIG.
3B) is toward the second side 44 of the record 40. The substrate 45 ls fabrlcated from a materlal having sup-port characterlstics selected wlthout regard for the transmlsslve optlcal properties thereof. Spacers such as rings 50-50 overlie at least a portion 51 (FIG. 3A) of each such opposlte slde 46 and 47 of the substrate 45 for providing support surfaces 52-52 (FIG. 3A) spaced from each erasable magneto-optlc layer 42-42.
Wlndows 53-53 are mounted on the support surfaces 52-52 for forming an alr gap 54 on a slde 55 ~FIG.3B) of 3 the upper layer 48 toward the flrst slde 43 of the record 40 and for formlng the alr gap 54 on a slde 56 of the lower layer 49 toward the second slde 44 of the record 40. The wlndows 53-53 have flat surfaces 57 and are fabrlcated from a material that ls optlcally transparent and has minlmal blrefringence so that the incident radlation 12 ls transmltted therethrough wlth mlnlmal optlcal retardatlon. The thin film magneto-optic recording structures 41-41 can be ln the form of the lnterference structure 34 of the thlrd embodiment.
Also, the air gaps 54-54 can be replaced b~ the trans-parent adheslve 28 of the second embodlment depending on what operational characteristics are desired for KD-10~1 -13-the dual record 40.
Detailed Description of Record 10 Referrlng agaln to FIGs. lA and lB, the record 10 is shown in detail as including the substrate 16. The substrate 16 is formed in the shape of a rigid dlsk from material that has thermal and humidity coeffi-cients of expansion that are compatlble wlth the thin film magneto-optic recording structure 14. Signifl-cantly, the rigid structural and e~pansion character-lstics of the materlal of the substrate 16 mag be se-lected wlthout regard to the transmissive optical characteristics thereof since the incident radiation 12 is not transmitted through the substrate 16. In addition, the material from which the substrate 16 is formed is selected for having characteristics suitable for having grooves 58 or other features for tracking, etc. molded therein. In a preferred embodiment of the record 10, the substrate 16 ls fabricated from po,ly-carbonate materlal that has been in~ection molded to ZO form the grooves 58. Such substrates 16 are avail-able, for example, from Idemitsu Petrochemical Co., _ Ltd., New .York, New York. The substrate 16 of the preferred embodiment has a diameter of 130 mm., a pltch of 1.6 microns of the grooves 58 and a thickness of 1.2 mm. As shown in FIGs. lA and lB, the grooves 58 are provided on the side 59 (FIG. lB) of the sub-strate 16 that ls toward the given slde 13 of the record 10 on whlch the radlation 12 is incldent.
Optionally, a layer 60 of hermetic sealing mater-lal may be provlded on a lower surface 61 of the sub-strate 16. The lower surface 61 is opposite tG the glven side 13 on which the radiation 12 is incident, thus the hermetic sealing material may be selected without regard to the transmissive optical properties thereof. Thus, the material of the hermetic sealing layer 60 may be metal, and i6 prefer~bly alumlnum.
As shown ln detall ln FIG. lB, on the grooved surface 59 of the substrate 16, the thln fllm magneto-optlc recordlng structure 14 ls shown. The structure 14 includes a lower protectlve layer 62 that ls ad~a-cent the surface 59 of the substrate 16. The protec-tlve layer 62 ls formed from a materlal that ls selec-ted for lts hermetlc seallng characterlstlcs and rel-atlvely strong adheslon to the materlal of the sub-strate 16. However, since the radiatlon 12 is not ) transmltted through the protectlve layer ~2, such se-lectlon of the material for the protectlve lower layer 62 ls wlthout regard to the transmlsslve opt~cal pro-pertles thereof. In a preferred embodlment of the re-cord 10, the protectlve lower layer 62 ls formed from slllcon nltride havlng a thlckness of about 90 nano-meters.
The erasable magneto-optic layer 15 ls deposited over the protectlve layer 62. For recordlng ln re-sponse to the radlation 12 using the Kerr effect, the magneto-optlc layer 15 is formed from an alloy of rare earth elements and transltlon metals. E~amples of _~ such materlals are dlsclosed ln U.S. Patent #3,949,387 for BEAM ADDRESSABLE FILM USING AMORPHOUS MAGNETIC
MATERIAL, lssued on Aprll 6, 1976 to Chaudharl, et al.
In partlcular, such alloy may be selected from the groups conslstlng of the elements Tb, Fe, and Co; Gd, Tb and Fe; Gd, Tb, Fe and Co; and Tb and Co. Such alloy may also be formed from such groups, whereln each such group further lncludes one or more elements selected from the group conslstlng of Pt, Ti and Cr.
In a preferred embodlment of the record 10, the alloy of rare earth elements and trans~tion metals forming the magneto-optlc layer 15 ls formed from 22~ Tb, 70 Fe and 8~ of Co, wlth 2~ Pt [(Tb22 Fe70 C8)98 Pt2]
The thickness of the preferred alloy is 90 nanometers .. . .
10~. The alloy may be deposlted onto the protective lower layer 62 by sputterlng ln a well known manner.
Still referrlng to FIG. lB, the thln fllm struc-ture 14 ls shown lncludlng an optically transparent layer 63 provlded on the magneto-optlc layer 15. The transparent layer 63 ls selected for havlng charac-terlstics of optlcal transparency, hlgh inde~ of re-fraction and hermetic seallng properties. In a pre-ferred embodlment of the record 10, the optically transparent layer 63 may be formed from sillcon ni-i tride havlng a thlckness of 90 nanometers + 10~.
Referring agaln to FIG. lA, the support structure18 ls shown formed from the rings 19-19 whlch are an-nular and concentric. The rlngs 19-19 overlle annular portlons 64 (FIG. lA) of the substrate 16. The rlngs 19-19 are provlded wlth the support surfaces 20 that support and malntaln the wlndow 21 spaced from a sur-face 65 (FIG. lB) of the optlcally transparent layer 63 to form the air gap 22. Preferably the rings 19-19 are bonded directly to the annular portions 64 of the substrate 16. The corresponding portions of the M0 layer 15 and the layers 62 and 63 are masked off dur-_ ing depositlon for thls purpose. The rings 19-19 may i be formed from material having a uniform but non-cri-tical thickness of .05 to .5 mm in the dlrectlon in which the radlation 12 is lncident on the side 13 of the record 10. However, lf the materials from which the window 21 and the substrate 16 are made differ substantially with respect to expansion, eg. thermal expanslon, -the spacer rings 19-19, partlcularly the outer ring 19, should be made of a compllant, elas-tomeric material.
The air gap 22 between the upper surface 65 of the optically transparent layer 63 and the bottom flat surface 24 of the window 21 thus h~s a thickness of about .05 to .5 mm. An advantage of providlng the ... ...
~303739 alr gap 22 ln magneto-optlc recordlng relates to the larger Fresnel reflectlon coef~iclent associated with the air/transparent layer 63 interface, as compared with that of a plastlc substrate/protectlve layer interface. The Fresnel coefficlent for interfaces between non-absorblng dlelectrlcs ls glven by (n0-nl)/(n0+nl) where n0 and nl are the lndlces of re-fractlon on either side of the interface. Slnce the lndex of air ls l.O and is much lower than the index of plastics (typically of the order of 1.5) the mag-i nitude of the Fresnel coefflcient ls much larger at an alr/dlelectrlc interface than at a plastic/dielectric interface. Thls results ln a better anti-reflectlon condition, and therefore a better "bloomlng" enhance-ment of the Kerr effect and playback slgnal for alr incldence than for substrate incidence of the light.
See, for example, the discussion of blooming at page 67 of the text "Magnetlc Domalns and Technlques For Their Observation" by R. Carey and E.D. Isaac, Aca-demic Press, 1963.
Further, "optically smooth" surfaces of sub-strates have some degree of micro-roughness. Such _ micro-roughness is usually expressed in terms of the "root mean square roughness" of the surface; i.e., the r.m.s. devlation of the actual surface proflle from an ideal perfectly smooth flat surface. This micro-roughness can be measured by various means, a conven-ient one being a measurement of "total integrated scatter" (TIS) of llght reflected from the surface.
See for e~ample the article, "Comparlson of Techniques for Measuring the Roughness of Optical Surfaces" by Jean M. Bennett, OPtical En~ineerin~, Vol. 24, No. 3, (May-June 1985), pp. 380-385 and references cited therein.
This micro-roughness induced scatter is one source of noise in a slgnal derived from an optical KD-lOOl -17-i303739 record. For a glven roughness of the substrate sur-face, the nolse ln the slgnal due to thls roughness will be conslderably less for alr lncldence of the radlation 12 onto the thln film structure 14 than for substrate incldence onto ~uch structure 14 (as in the prlor art). In partlcular, accordlng to the above ~ennett reference, the total lntegrated scatter (TIS) _ (4~ )2 where ~ ls the r.m.s. roughness, ~
is the wavelength of the llght, and TIS ls the total scattered fractlon of the reflected llght. In a medlum of lndex of refractlon, n, the optical thlck-ness of a given scattering surface displacement, ~, becomes n~ whereas the wavelength becomes ~/n. There-fore, TIS in a medlum of lndex n becomes:
(TIS)n ~ ¦4~n~ /n)]2 ~ n4(4~ )2 b n4(TIS) i Thus, in terms of llght scattering, a given surface ls n4 times worse from the substrate-lncident side as compared to the air-incident side. Therefore, to the extent that micro roughness is a factor in determining the signal-to-noise ratlo of a slgnal derlved from an M0 optical record, it ls better for the optical-beam to address the recordlng structure from an alr amblent rather than from the amblent of the substrate. Slnce the alr gap 22 is ad~acent the upper surface 65 of the transparent layer 63, the effects of microroughness are reduced in the M0 records 10 and 40.
As shown in FIG. lA, the window 21 is mounted on the support surfaces 20 of the rings 19-19. The win-dow 21 has optimum transmissive optical characteris-tics including the flat surfaces 24. Further, thewindow 21 is fabricated from a material that is opti-cally transparent and that has mlnlmal blrefringence so that the incldent radiatlon 12 ls transmitted through the wlndow 21 wlth mlnimal optlcal retarda-~303739 tlon. Such characterlstlcs are provided by polymeth-ylmethacrylate, polymethylpentene and polyolefln. In a preferred embodlment of the record 10, the wlndow 21 ls formed from chemlcally tempered sheet glass havlng a diameter of 130 mm and a thlckness of 1.2 mm. Such wlndows are available from Glaverbel, Brussels, Bel-gium. As a result of such chemical tempering, the flat surfaces 24 are ln a state of compressive stress yet retain the desired minlmal birefringence.
10Prior to deposition of the thin films that form i the hermetlc seallng layer 60 on the lower surface 61 of the substrate 16 and the thln fllm structure 14 on the grooved side 59 of the substrate 16, the surfaces 59 and 61 of the substrate 16 are treated by ion beam 15 bombardment to promote good adhesion of such films thereto.
Detailed Disclosure -Fourth Embodiment - Record 40 Referring now to FIGs. 3A and 3B, it may be 20 understood that the record 40 includes the substrate 45 that ls provlded with grooves 66 that are slmilar to the grooves 58 formed ln the substrate 16. The J substrate 45 may be formed by a single in~ection mol-ded member (as shown) or by two members 45' (FIG. 3A
25 ~olned along a llne 45ll, each member 45~ being slmi-lar to the substrate 16 of the record 10. The record 40 is symmetrical with respect to a plane 67 (FIG. 3B) that ls parallel to the flrst and second sides 43 and 44. Thus, the sides 43 and 44 of the record 40 are 30 constructed and function the same. Therefore, the de-scription of one of the sides 43 and 44 applies to the other side.
Each dual thin film magneto-optic recording structure 41-41 formed on the substrate 45 is simllar 35 to the thin film magneto-optic recordlng structure 14 of the record 10, and lncludes the protectlve lower layer 62 and the optlcally transparent layer 63 provl-ded on the record 10. The M0 layer 42 of the struc-ture 41-41 ls the same as the MO layer 15. Further, the spacers 50-50 shown ln FIG. 3A are simllar to the rings 19-19 provlded in the record 10. The spacers 50-50 thus form the air gap 54 whlch has the same ad-vantages as the air gap Z2 of the record 10. The spacers 50-50 also support the windows 53-53 in op-posite confronting relationship with respect to the `~ upper surface 65 of the transparent layer 63 in a man-ner similar to the rings 19-19 of the record 10. The rings 50-50 are slmilarly sealed to the substrate 45 to form the air gap 54 as an annularly-shaped space.
Simllarly, each of the wlndows 53-53 ls formed from materlal selected for desired transmissive optical characterlstlcs, as in the manner of the window 21 of the record 10. It may be understood then that the re-cord 25 provides the ability to record on both sides 43 and 44, which increases the amount of data that can be stored on a single record 40.
The record 40 shown ln FIGs. 3A and 3B may alter-natively be provided with the tuned multi-layer inter-.J ference structure 34 shown in FIG. 4 lnstead of the thin film magneto-optic recording structure 41-41.
The advantages of using such structure 34 are dis-cussed below.
Detailed DescrlPtion - Record 25 Referring ln detail to FIGs. 2A and 2B, it may be understood that the substrate 26 may be designed with the same criteria in mind as used for the substrate 16 of the record 10, for example. Similarly, the respec-tive transparent layer 32, the magneto-optic layer 31 and the protective layer 33 are selected in a manner 3~ slmilar to that for the respectlve protective layer 63, the magneto-optlc layer 15 and the optlcally transparent layer 62 of the record 10. The trans-parent adhesive layer 28 may be an ultraviolet light cured optlcal photopolymer. Thls has characterlstlcs 5 as a cement and is also optlcally transparent. The thlckness of the transparent adheslve layer 28 ls not crltlcal, but ls about 20 mlcrometers ln a preferred embodlment of the record 25. The wlndow 27 adheres dlrectly to the transparent adheslve layer 28 and ls 10 selected wlth the same crlterla ln mlnd as the wlndows 21 and 53-53 of the records 10 and 40.
Referrlng to FIGs. 2A and 2B, lt may be observed that the record 25 embodles certain of the advantages of the records 10 and 40. In particular, the sub-15 strate 26 of the record 25 may be designed wlth the same obJectlves ln mlnd as used ln the design of the substrate 16 and the substrate 45 of the respectlve records 10 and 40. In partlcular, the functlons of the substrate 26 may be separated from that of the 20 functlon of the window 27, such that the ldeal charac-teristics of the substrate 26 may be obtained without regard to the transmlssive optlcal characterlstlcs thereof. Similarly, because of such separation of r functlons, the characteristics of the window 27 can be 25 selected from the standpoint of optlcal transmlsslon wlthout blrefringence. This advantage is believed to more than off-set the results of elimlnatlng the air gap from the record 25 and the resultant loss of the "blooming" and surface roughness effect.
.
30Detailed Di~closure - Structure 34 The third preferred embodiment of the present inventlon ls shown in FIG. 4 as provlding the tuned multi-layer interference structure 34 lnstead of the thln fllm recording structures 14, 30 and 41-41 of the 35respective records 10, 25 and 40, and instead of the thln film recordlng structure ~1-41 of the record 40 shown ln FIGs. 3A and 3B. The structure 34 lncludes the reflectlve layer 35 that is deposited dlrectly on the substrate 16 of the record 10 or the record 40, for example, for reflectlng the lncident radlatlon 12.
The reflectlve layer 35 ls preferably alumlnum. The transparent dielectrlc spacer protectlve layer 36 overlles the reflectlve layer 35. The spacer layer 3~
ls preferably silicon nl~rlde. The erasable magneto-optic layer 37 may be slmilar to that of the record 10and ls provided on the spacer protective layer 36.
For use in the interference structure 34, the magneto-optic recordlng layer 37 is preferably an alloy com-posed of [(Tb22 Fe70 Cog)g~ Pt2]. The protective15 overlayer 38 of transparent dlelectric material is preferably also silicon nitride and overlies the mag-neto-optlc recordlng layer 37. The thicknesses of the layers 35, 36, 37 and 38 of the lnterference structure 34 are designed using well known techniques which allow for some variatlon in thicknesses while still achieving the desired tuning to enhance and optimize the difference of the effective Kerr angle of polari-zation rotation corresponding to different directions of magnetization (up or down) of the magneto-optic re-cording layer 37. Such enhancement is in addition tothat resulting from the use of the alr incidence structure of the records 10 and 40. In a preferred embodiment of the interference structure 34, the thl- es~e re as follows:
.
Layer Thlckness Thlckness La~er(nanometers) Reflectlve 35100 Spacer 36 50 magneto-optic 37 24 Overlayer 38140 With the lnterference structure 34 provided on the substrate 1~ of the record 10, since the lntenslty ~~ 10 modulated radlatlon 12 is not transmitted through the substrate 16~ the substrate materlal may be selected without regard for the transmlssive optlcal charac-terlstlcs thereof.
Charac_eristics of Records 10, 25, and 40 Referrlng to FIGs. lAj 2A, and 3A, it may be un-derstood that the records 10, 25, and 40 are provided with a window, such as the wlndow 21 of the record 10, that is on the side of the records 10, 25, or 40 on which the radiation 12 is lncldent. As shown ln FIGs.
1~, 2B and 3B, the preferred angle of lncldence of a central ray 6g of the radiation 12 onto the respective '. windows 21, 27 and 53 is 90. The typical radiation 12 is a focussed beam of light having a numerical aperture ln the range of .5 to .65. The magnetic 25 field 11 is produced by a coil 69 (FIGs. 1 and 2), a similar coll 69 belng used with the record 40. The magnetlc fleld 11 is perpendicular to the plane formed by the flat surfaces of the records 10, 25 and 40, such as the flat surfaces 24-24 (FIG. lA). The radia-tion 12 is transmitted through the window 21 of the record 10, through the window 27 of the record 25 and through the windows 53-53 of the record 40. The radi-ation 12 is then transmltted through the air gaps 22 and 54-54 respectiYely, or in the case OI the record i303739 25, through the transparent adheslve layer 28, and ls then transmitted through the optlcally transparent layers 63 of the records 10, 25 and 40. When the ln-tenslty of the radlation 12 is of a relatively hlgh value, the radiation 12 ls referred to as a "record beam" slnce the record 10 records in the presence of the magnetic fleld 11 when the lntenslty ls hlgh. In particular, when the modulatlon of the intensity of the radiatlon 12 results ln the hlgh intenslty record beam, the materlal of the respective magneto-optlc layers 15, 31 or 42-42 ls locally heated by the fo-cussed radiatlon 12, causing the coerclvity of such material to fall below the value of the magnetic field 11, thus-enabling the direction of magnetization of the material of the respective magneto-optic layers 15, 31 and 42-42 to be locally reversed from their original unrecorded conditlon by becoming aligned with the magnetic field 11. During recording, the magnetic field 11 ls oriented in a dlrection opposite to the direction of magnetization of the material of the un-recorded magneto-optlc layers 15, 31 and 42-42. For e~ample, if the dlrection of magnetization of the mag-neto-optic layers 15, 31 and 42-42 is initially up in FIGs. lA, 2A and 3A, then at the particular location on whlch the record beam of the radiation 12 is inci-dent on the magneto-optic layers 15, 31 or 42-42, the direction of the magnetlzatlon wlll be reversed and will be downward. Thus, although the magneto-optic layers 15, 31 and 42-42 do not record in response to only the record beam, the record beam of radiation 12 and the magnetic field 11 result in recording of the radiation intensity modulation in the form of the re-versal of the magnetization at the locatlons at which the M0 layers 15, 31 or 42-42 are heated as described.
In the case of the magneto-optic recording layer 37 of the lnterference structure 34, there is greater heat-, . _ .. . ... . .
1~03739 ing of the magneto-optlc recordlng layer 37 due to the lnterference structure.
To read the record thus produced by the radlatlon 12 in the form of the record beam, radlation ln the form of a plane polarlzed read beam of focussed llght energy 12 having a substantially reduced level of in-tensity is dlrected perpendicularly to the respective sides 13, 13' and 43 of the records 10, 25 and 40.
The read beam 12 is transmitted through the respective windows 21, 27 and 5~-53 and across the respective alr gaps 22 and 54-54 or through the adheslve layer 2~ and ls transmitted through the optlcally transparent lay-ers 63 and onto the respective magneto-optic layers 15, 31 and 42-42. At locations on whlch the record beam of radlation 12 was dlrected onto the magneto-optic layers 15, 31 or 42-42, the resulting reversal of the magnetization at such locations will, according to the polar Kerr magneto-optic effect, rotate the plane of polarization of the read beam 12 in a direc-tion opposite to that of non-reversed locations. The read beam 12 is reflected off the magneto-optic layers 15, 31 and 42-42 as indicated by the arrows 70 (FIGs.
--, lA, 2A and 3A) and the rotations of the plane of polarization are detected. In the case of the mag-neto-optic recordlng layer 37 of the lnterference structure 34, the read beam of radlation 12 is reflec-ted ln part off both the recordlng layer 37 and the reflectlve layer 35, such parts belng approximately out of phase by vlrtue of proper cholce of thlckness of the transparent dlelectrlc spacer protectlve layer 36.
The advantages of the structure of ths records 10 and 40, both of which include alr incldence of the read optical radlation 12 onto the thin film magneto-optic recording structures 14 and 41-41, respectively.
whether ln the form shown in respective FIGs. lA or 3A
or uslng the lnterference structure 34 shown ln FIG.
4, lnclude better anti-reflectlon "blooming" enhance-ment of the Kerr slgnal. Further, such records 10 and 40 avoid the necesslty of uslng an optlcal quallty grooved substrate having stringent tolerances on blre-frlngence and blrefrlngence gradlent. Instead, the substrates 16 and 45, respectlvely, may be used wlth-out regard for transmlsslve optlcal characterlstlcs.
The effect of micro-roughness of the substrates 16 and 45 is conslderably reduced for alr lncldence as com-pared to substrate lncldence of the read beam of radl-ation 12. Slnce the substrates 16 and 45 need not be transparent, non-transparent materials such as metals can be used on elther slde of the substrates 16 and 45, elther to seal out molsture from the bulk of the substrates, or to lmprove the adhesion or barrler pro-perties of the protectlve layer 62. The substrate surfaces 46 and 47 (FIG. 3B) and 59 (FIG~ 1~) may be vigorously treated by plasma or by ion bombardment to cross llnk and harden such surfaces, to activate such surface for improved adhesive bondlng of the thin film recording structures 14 and 41-41 thereto (as to the respective records 10 and 40) or for improved adhesive bonding of the reflective layer 35 to such substrate surfaces, and to seal low molecular welght fragments of polymer, or plastlclzer awa~ from such surfaces, without regard to transmlsslve optlcal propertles.
Dust defocusslng ls provlded by the transparent windows 21, 27 and 53-53 having the flat surfaces, ~0 such as the surfaces 24 and 57-57 of the respectlve records 10 and 40.
E~perimentally, it has been observed that inter-ference effects assoclated wlth the alr gaps 22 and 54-54 are not a serlous problem; thls ls most llkely due to the compensatlng effect of reflectance and Kerr angle factors contrlbuting to the Kerr slgnal.
KD-1001 -2~-It has also been experlmentally observed that the carrler-to-noise ratlo of a test slgnal observed through the respective wlndows 21 and 53-53 and across the air gaps 22 and 54-54 ls conslstently better than that observed when the read beam radlatlon 12 is transmltted through a substrate as in the above-de-scrlbed prlor art M0 records. The lmprovement has been about 3.7 dB ln carrler-to-nolse ratio.
Whlle the preferred embodlments have been de-scribed ln order to lllustrate the fundamental rela-`~ tlonships of the present lnventlon, lt should be understood that numerous variations and modlflcatlons may be made to these embodiments without departing from the teachings and concepts of the present lnven-tion. Accordlngly, lt should be clearly understood that the form of the present lnventlon descrlbed above and shown in the accompanylng drawlngs ls lllustratlve only and ls not lntended to limit the scope of the invention to less than that descrlbed ln the following claims.
Speclficatlon BACKGROUND OF THE INVENTION
Field of the Invention The present inventlon relates to the fleld of erasable magneto-optic recordlng and more particularly to the fleld of record structureæ that allow a higher _ signal-to-noise ratio of signals output in response to reading of the record.
Descrlptlon of the Prlor Art `10Magneto-optic (MO) recording is sometimes re-ferred to as thermomagnetic recording, or thermo-mag-neto-optlc recordlng. These terms refer to technology that is being actively developed in many laboratories around the world because it promlses to comblne the capaclty and packing density of optical recording to-gether with the erasability of conventional magnetic recordlng. The basic prlnciples of MO recording are set forth in U.S. Patent #3,949,387 issued April 6, 1976 for ~EAM ADDRESSABLE FILM USING AMORP~OUS MAG-NETIC MATERIAL to Chaudhari, et al. Distinct fromconventional optlcal recording in which an optical coating is ablated, for example, to produce reflec-tance variations ln a laser beam that reads the re-cord, MO recording is accomplished by inducing and detecting relatively small rotations of the plane of polarization of a linearly polarized read beam re-flected off the record. This is sometimes referred to as Kerr MO recording, and such rotatlon of the plane of polarization as the "polar Kerr magneto-optlc ef-fect", or, in context, as ~ust the "Kerr effect." Theamount of such polarization rotation of the read beam is most desirably dependent only on the magnetization of the MO thin film recording structure at the parti-~:~03739 cular locatlon on which the read beam ls lncident on the record.
As understood, the normal practlce at the present state of the art of MO recordlng is to address the MO
record for reading, erasing, and wrlting by means of a focussed laser beam that is dlrected through a trans-parent disk of glass or plastic which serves as a sub-strate for the thin film MO recording structure. Such addressing for wrlting or erasing ls done in the pre-sence of a magnetic fleld. The substrate of the MO
) record, typlcally about 1.2 mm thick, also serves to provlde defocussing of dust that may be on the surface of the substrate, and may be provlded wlth grooves or other features to provide informatlon for tracklng, etc. One problem with this MO record ls caused by thebirefringence of such substrate.
Birefringence refers to the phenomenon in which an optical materlal, ln a state of stress, for exam-ple, becomes non-homogeneous and non-isotropic with respect to the index of refraction. That is, light havlng different states of polarization sees sllghtly different lndices of refraction in such optical mater-ial. Since readout of recorded lnformation by means - of the Kerr effect is based on detecting relatively small rotations of the plane of polarization (of the order of one degree), and since the read beam of light makes a double pass through such substrate, the opti-cal retardatlon caused by rotatlon of the plane of polarlzatlon due to blrefrlngence is a very serious problem for magneto-optic recording. In particular, for state of the art MO records, a tolerance of 30 na-nometers for double pass optlcal retardatlon and a further tolerance on the gradient of the retardation of about 30 nanometers/cm may be requlred to assure satisfactory performance of such substrate with re-spect to birefringence.
It has proven qulte dlfflcult to achleve thls low level of blrefrlngence ln preferred optlcal plastlcs, such as polycarbonate (PC) t used for such substrates.
Thus, in the current llterature lt ls recognlzed that from most polnts of vlew, bisphenol-a-polycarbonate ls a good candldate for such MO disk substrates. See, for example, Alan B. Marchant, "Retardation Effects in Magneto-optic Readout", SPIE Proc. of Optical Mass Data Stora~e II, (San Diego, CA, August, 1986) 695.
Marchant noted that despite years of research which have resulted in such substrates wlth a very low op-tical retardation for normally-incldent light, the problem ls substantlal with lncreases in the angle of lncldence relatlve to normal. Marchant also noted that even vertlcal blrefrlngence of such sub6trates ln a dlfferentlal detectlon system ls a serlous problem.
~ ased on read-only dlgltal audlo disk technology, for example, the most economical means of providing a grooved transparent substr~te is in~ection moldlng of a suitable optical plastic. Thls is the method by whlch substrates are lmpressed wlth grooves and digl-tal audlo lnformatlon. The commerclal success of such read-only dlgltal audlo dlsks has lead to attempts to ln~ectlon mold such optlcal plastlcs as polymethyl-methacrylate (PMMA) and PC as substrates for use assubstrates ln MO records. However, the stress result-lng from such moldlng leads to blrefrlngence problems when PC ls the materlal used for such MO record sub-strates. Although adequate performance wlth respect to substrate blrefrlngence has been achleved wlth PMMA, PMMA has undesirable permeablllty to water vapor and expands ln a humld envlronment. Thls dlmensional lnstability causes cracklng and peellng of the thln fllm MO recordlng structure.
Desplte the problems of blrefrlngence noted ln the Marchant article, there have been continued at-1;~03739 tempts to use such conventlonal substrates both to support the thin film MO recording structure and to transmlt the record and read beams to and from the thln film MO recordlng structure. This ls lndlcated by an artlcle publlshed in August, 19~, by Treves and Bloomberg, entitled, "Effect of Birefringence on Opti-cal Memory Systems", in SPIE Proc. of Optlcal Mass Da-ta Stora~e II, (San Diego, CA). They reported on such conventional substrates and concluded that optical bi-refringence must be controlled since substantial re-duction in the signal-to-noise ratio is attributed to the birefringence of the substrate. As recently as March 13, 1987, Toda, et al., in an article entitled "Analysis of Signal to Noise Ratio in Magneto-Optical Disk Using a Polarlzation Slmulator", Topical Meetin~
on OPtical Data Stora~e, OPtical Societ~ of America Tech. Di~est Series, Vol. 10, (Statellne, Nevada, 3/11-13/87) pp. 34-37, also reported on such conven-tional substrates. They concluded that the reta~da-tion of a PC substrate showed complicated dependenceon the lncident angle of the laser beam. Since MO
recording, llke all optical recording methods, uses a focussed beam of light for writing and reading, and since focussed beams include rays of all angles of in-cidence out to a limiting angle set by the numericalaperture of the recording obJective lens, readout of MO signals through PC substrates is vulnerable to these effects.
Such continued attempts to use such conventlonal substrates to perform the above-noted multiple func-tions of support (for the thin film MO recording structure) and beam transmission have apparently ig-nored disclosures relating to types of optical record-ing other than MO recordlng. In those other types of 3~ optical recordlng physical changes such as melting or evaporation, or chemical changes such as decomposi-. . .
130373~
tion, effect the recording function. In one form of such recording by physical changes, uslng ablative write-once media, mark formatlon is created either by e~ecting material from the viclnity of the mark, or by causlng material to withdraw from the mark region lnto a surroundlng rim, or both. In elther case, a free surface unconstralned by any overlayer i6 required, or is said to be beneflcial. For example, in U.S. Patent #4,074,282 lssued February 14, 1978 for RADIATION-SENSITIVE RECORD WITH PROTECTED SENSITIVE SURFACE to - ! Balas, Jr., et al., llght is lncident upon an ablative recording medium deposited on the inner surface of a substrate dlsk, which ls one of two disks that have an air gap between them. The light is incident on and is transmitted through the other disk, which is a window disk, across the air gap and directly onto the abla-tive recordlng medium. However, such disclosure rec-ognizes that in a number of situations, such as when the ablated material would coat the window, the inci-dent light should be transmitted through the substratedisk and not through the window disk. Since the above-referenced conventional use of substrates in MO
recording conforms to this latter teaching of the 4,074,282 Patent, it appears that such conventional MO
substrate practices and the earlier non-MO recording practices (e.g. Patent 4,074,282) both teach the use of the substrate for both the support and beam trans-mission functlons. In view of the above-referenced articles, however, when such teachings are followed there are substantial, unsolved problems with bire-fringence that detract from the overall performance of such MO records.
SUMMARY OF THE INVENTION
In contrast to the prior art in which an incident laser beam has been transmitted through a substrate .. .... .. ..
1303'739 that supports a thln fllm M0 recordlng structure, mak-ing lt necessary to contlnue efforts to reduce the bl-refrlngence of such substrate fabricated from materi-als havlng the undesirable blrefrlngence, the M0 re-cord of the present lnventlon provldes a high quallty,durable, economlcal, erasable optlcal record structure havlng mlnimal blrefrlngence problems, such that the slgnal-to-nolse ratlo ls enhanced relative to that of the conventional structures for erasable magneto-optlc records.
A record for sensing radlatlon lntenslty modula-tlon in the presence of a magnetlc fleld accordlng to the prlnclples of the present inventlon separates the functlon of supportlng the thin fllm M0 recordlng structure of the substrate from the substrate's optl-cal transmlsslon functlon so that the ldeal substrate can be selected wlthout regard to the transmlsslve optlcal properties thereof.
An M0 record deslgned ln accordance wlth the prlnciples of the present lnventlon has a substrate that may have very poor blrefrlngence propertles, yet the record provides lmproved slgnal-to-nolse ratlos ln read beams slnce neither the record beam nor the read beam ls transmltted through the substrate.
In an M0 record of the present invention, the substrate ls effectlvely the bottom of the record, wlth a thln film M0 recording structure on the top surface of the substrate, where such top surface sup-ports a wlndow through whlch recordlng energy ls transmitted ln the presence of a magnetic blas fleld for M0 recording.
Wlth these and other obJects in mind, a flrst preferred embodlment of an M0 record for recordlng ln response to radlatlon lntensity modulatlon in the pre-sence of a magnetlc fleld accordlng to the present ln-vention is responslve to radlatlon lncldent on a given side thereof. The record lncludes erasable material for M0 recording in response to such inten~ity modu-lated radlatlon and to the magnetlc fleld. A sub-strate is provlded for supporting the erasable mag-neto-optic material on the side of the material away from the glven slde of the record. The 6ubstrate is fabricated from a material havlng preferred support characterlstics that are selected without regard for the transmisslve optlcal characterlstic6 thereof. A
support structure overlles at least a portlon of the \ substrate for provldlng at least one support surface spaced from the erasable magneto-optlc material. A
wlndow ls mounted on the support surface for formlng an alr gap on the slde of the erasable magneto-optlc material toward the given slde of the record. The window has flat surfaces and ls fabricated from a ma-terial that is optically transparent and that has mln-imal birefrlngence so that the radlatlon incldent on the glven side of the record is transmitted through the wlndow with mlnimal optical retardation and is not transmitted through the substrate before being lncl-dent on the erasable materlal for M0 recording ln the presence of the magnetlc field.
A second preferred embodiment of the present ln-ventlon relates to a record for recordlng ln response to radiation intenslty modulation in the presence of a magnetic field, where the radiation is incident on a given side of the record. The record includes a sub-strate and a wlndow. The window forms the glven side of the record. The wlndow is flat and fabricated from a material that is opticall~ transparent and has mini-mal birefringence so that the incident radiation is first transmitted therethrough with minimal optical retardation. A transparent adhesive (such as optlcal cement) is provided between the surface of the window opposlte to such given side of the record and a thin ~303'739 film M0 recording structure deposlted on the sub-strate. The thln film M0 recordlng structure lncludes a magneto-optic layer sandwiched between a transparent layer and a protectlve layer. The protectlve layer lles between the substrate and the magneto-optlc lay-er, and the transparent layer overlies the magneto-optic layer and therefore is adJacent to both the mag-neto-optic layer and the transparent adheslve. The optically transparent layer and the protectlve layer are formed from hermetlc materlal and the magneto-! optic layer records ln response to such radlatlon lntenslty modulatlon ln the presence of a magnetlc field. The substrate ls provlded for supportlng the protectlve layer and thus the remainder of the record.
The substrate ls fabricated from a material having lm-proved film support and moisture impermeability char-acteristics, eg. dlmenslonal stability, without regard for the transmlssive optlcal characterlstlcs thereof since the intenslty modulated radiatlon ls not trans-mitted through the substrate.
A thlrd preferred embodiment of the present ~n-ventlon provldes for the lncorporation of the erasable magneto-optic layer into a tuned multl-layer interfer-ence structure to further enhance and optlmize the ~5 difference of the effect~ve Kerr angle of polarization rotatlon corresponding to different magnetlzations (up or down) of the magneto-optic recordlng layer. The tuned multi-layer interference structure is a four layer structure comprised of a reflective layer over-lying the substrate, a transparent dielectrlc spacer protective layer overlying the reflective layer, a magneto-optic recording layer overlylng the spacer protective layer, and a protective overlayer of trans-parent dlelectric overlying the magneto-optic layer.
A fourth preferred embodiment of the present in-vention provides two of the records of the first, se-, ~ _ ... . . ... ...... . .. ... .
`` 1303739 cond, or third embodlments, wlth the substrates back-to-back or sharlng a common substrate to form a double sided MO record.
BRIEF DESCRIPTION OF THE DRAWINGS
Other ob~ects, features and advantages of the present lnvention will be apparent from an examination of the following detailed descriptions which include the attached drawings in which:
FIG. lA is a vertlcal cross-sectlonal vlew of a flrst preferred embodiment of a record for recordlng in response to radiatlon lntenslty modulatlon in the presence of a magnetlc fleld accordlng to the present lnventlon, showlng a magneto-optic recordlng layer between a substrate and a wlndow;
FIG. lB is an enlarged vlew of a portion of the record shown ln FIG. lA, lllustratlng the magneto-optic layer between protectlve layers;
FIG. 2A is a vertical cross-sectional vlew of a portlon of a second preferred embodiment of a record for recordlng in response to radiation lntensity modu-latlon ln the presence of a magnetic fleld accordlng to the present invention, where a spacer for separat-ing a window from a substrate is in the form of trans-parent adhesive;
FIG. 2B ls an enlarged vlew of a portion of the record shown ln FIG. 2A, lllustrating a thin fllm mag-neto-optic recording structure between the adhesive and the substrate;
FIG. 3A is a vertical cross-sectional view of a fourth preferred embodlment showing a two-sided mag-neto-optlc record for recording in response to radia-tlon intensity modulation in the presence of a mag-netic field according to the present lnventlon;
FIG. 3B is an enlarged vlew of a portlon of the two-sided record shown ln FIG. 3A; snd ............................... .. .. .
~303739 FIG. 4 is an enlarged vlew of a third embodlment of a magneto-optlc record showing a tuned multi-layer lnterference structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Summar.V of the Preferred Embodiments The first preferred embodlment of the present ln-ventlon ls shown ln FIG. lA as a record 10 for recor-! dlng ln the presence of a magnetlc fleld (represented by arrows 11) and ln response to lntenslty modulatlon of energy such as radlatlon ln the form of a laser beam lndicated by arrow 12. The record 10 according to the present invention records in response to such radiation 12 incident on a given side 13 thereof. The record 10 lncludes a thin film magneto-optic (MO) re-cording structure 14 that includes a layer 15 of eras-able material for MO recordlng. A substrate 16 ls provlded for supportlng the thln fllm MO recordlng structure 14 on a slde 17 thereof opposlte to the gl-ven side 13 of the record 10. The substrate 16 is fabricated from materlal havlng preferred support characterlstlcs that are selected wlthout regard for the transmissive optlcal characteristics thereof. A
support structure 18 in the form of concentric rlngs 19-19 overlles at least a portlon of the substrate 16 for providing at least one support surface 20 spaced from the erasable magneto-optic layer 15. A window 21 ls mounted on the support surfaces 20 for formlng an alr gap 22 on a side 23 of the erasable magneto-optic layer 15 toward the glven slde 13 of the record 10.
The wlndow 21 has flat surfaces 24-24 and is fabri-cated from a material that ls optlcally transparent and that has mlnlmal blrefringence so that the light radiation 12 incident on the given side 13 of the re-1;~03'739 cord 10 is transmltted through the wlndow 21 wlth mlnlmal optical retardatlon and ls not transmitted through the substrate 16 before belng incldent on the erasable layer 15 for M0 recordlng.
A second preferred embodlment of the present in-vention ls shown ln FIGs. 2A and 2B as a record 25 for recordlng ln response to lntenslty modulatlon of the radlation 12 in the presence of the magnetlc fleld 11, where the radiatlon 12 ls lncldent on a given side 13' of the record 25. The record 25 includes a substrate 26 and a window 27. The wlndow 27 forms the glven slde 13' of the record 25. The wlndow 27 ls flat and fabrlcated from a materlal that ls optlcally trans-parent and has minimal blrefringence so that the in-cldent radlatlon 12 ls flrst transmltted therethrough wlth mlnimal optlcal retardatlon. A transparent ad-hesive 28 (such as optical cement) is provlded between a surface 29 of the window 27 opposite to such given side 13l of the record 25 and a thin fllm M0 recordlng - 20 structure 30 deposited on the substrate 26. The thin fllm M0 recording structure 30 lncludes a layer 31 of magneto-optlc material sandwlched between a transpar-ent layer 32 and a protectlve layer 33. The protec-tive layer ~3 lies between the substrate 26 and the magneto-optlc la~er 31, and the transparent layer 32 overlles the magneto-optlc layer 31 and therefore ls ad~acent to both the magneto-optic layer 31 and the transparent adheslve 2~. The optlcally transparent layer 32 and the protectlve layer 33 are formed from hermetic materlal and the magneto-optic layer 31 is responslve to such radlatlon 12 ln the presence of the magnetic field 11 for recording the intensity modula-tion of the radiatlon 12. The substrate 2~ is provl-ded for supporting the protective layer 33 and thus the remainder of the record 25. The substrate 2~ ls fabricated from a material havlng excellent fllm sup-.... .
13~3739 port and moisture impermeablllty characterlstlcs with-out regard for the transmissive optlcal characteris-tics thereof slnce the radlation ls not transmitted through the substrate 26.
A thlrd preferred embodlment of the present ln-vention is shown in FIG. 4 as provldlng a tuned multl-layer lnterference structure 34 instead of the thln film recordlng structures 14 and 30 of the respectlve records 10 and 25 and lnstead of the comparable thln film recording structure of the fourth embodiment shown in FIGs. 3A and 3B. In place of such struc-tures, the structure 34 lncludes a reflectlve layer 35 directly on the substrate 16 of the first embodiment, for example, for reflecting the lncldent radlatlon 12.
A transparent dlelectrlc spacer protective layer 36 overlies the reflective layer 35. An erasable mag-neto-optic layer 37 similar to that of the first em-bodiment is provided on the spacer protectlve layer 36. A protectlve overlayer 38 of transparent dielec-tric material overlies the magneto-optic recording layer 37. The thicknesses of the layers 35, 36, 37 and 38 of the interference structure 34 are selected so that the interference structure 34 is tuned to further enhance and optimlze the difference of the effectlve Kerr angle of polarlzatlon rotatlon corres-ponding to different dlrectlons of magnetization (up or down) of the magneto-optic recording layer 37. If the interference structure 34 ls provlded on the sub-strate 16 of the flrst embodlment (of the record 10), slnce the lntenslty modulated radlation 12 ls not transmitted through the substrate 16, the substrate material is selected without regard for the trans-missive optlcal characterlstics thereof.
A fourth preferred embodlment of the record of the present invention is shown in FIGs. 3A and 3B as a double-sided optical record 40 including dual thin 1;~03739 fllm magneto-optic recording structures 41-41. Each dual structure 41-41 includes a layer 42-42 of eras-able magneto-optic materlal for recording ln the pre-sence of the magnetic field 11 in response to inten-sity modulation of the radiation 12 incident on respe-ctive first and second sides 43 and 44 of the record 40.
The record 40 also includes a substrate 45 for supporting the dual thin film magneto-optic recordlng structures 41-41 on opposite sides 46 and 47 thereof ! so that the M0 layer 42 shown as the upper layer 48 (FIG. 3B) is toward the first side 43 of the record 40 and the M0 l~yer 42 shown as the lower layer 49 (FIG.
3B) is toward the second side 44 of the record 40. The substrate 45 ls fabrlcated from a materlal having sup-port characterlstics selected wlthout regard for the transmlsslve optlcal properties thereof. Spacers such as rings 50-50 overlie at least a portion 51 (FIG. 3A) of each such opposlte slde 46 and 47 of the substrate 45 for providing support surfaces 52-52 (FIG. 3A) spaced from each erasable magneto-optlc layer 42-42.
Wlndows 53-53 are mounted on the support surfaces 52-52 for forming an alr gap 54 on a slde 55 ~FIG.3B) of 3 the upper layer 48 toward the flrst slde 43 of the record 40 and for formlng the alr gap 54 on a slde 56 of the lower layer 49 toward the second slde 44 of the record 40. The wlndows 53-53 have flat surfaces 57 and are fabrlcated from a material that ls optlcally transparent and has minlmal blrefringence so that the incident radlation 12 ls transmltted therethrough wlth mlnlmal optlcal retardatlon. The thin film magneto-optic recording structures 41-41 can be ln the form of the lnterference structure 34 of the thlrd embodiment.
Also, the air gaps 54-54 can be replaced b~ the trans-parent adheslve 28 of the second embodlment depending on what operational characteristics are desired for KD-10~1 -13-the dual record 40.
Detailed Description of Record 10 Referrlng agaln to FIGs. lA and lB, the record 10 is shown in detail as including the substrate 16. The substrate 16 is formed in the shape of a rigid dlsk from material that has thermal and humidity coeffi-cients of expansion that are compatlble wlth the thin film magneto-optic recording structure 14. Signifl-cantly, the rigid structural and e~pansion character-lstics of the materlal of the substrate 16 mag be se-lected wlthout regard to the transmissive optical characteristics thereof since the incident radiation 12 is not transmitted through the substrate 16. In addition, the material from which the substrate 16 is formed is selected for having characteristics suitable for having grooves 58 or other features for tracking, etc. molded therein. In a preferred embodiment of the record 10, the substrate 16 ls fabricated from po,ly-carbonate materlal that has been in~ection molded to ZO form the grooves 58. Such substrates 16 are avail-able, for example, from Idemitsu Petrochemical Co., _ Ltd., New .York, New York. The substrate 16 of the preferred embodiment has a diameter of 130 mm., a pltch of 1.6 microns of the grooves 58 and a thickness of 1.2 mm. As shown in FIGs. lA and lB, the grooves 58 are provided on the side 59 (FIG. lB) of the sub-strate 16 that ls toward the given slde 13 of the record 10 on whlch the radlation 12 is incldent.
Optionally, a layer 60 of hermetic sealing mater-lal may be provlded on a lower surface 61 of the sub-strate 16. The lower surface 61 is opposite tG the glven side 13 on which the radiation 12 is incident, thus the hermetic sealing material may be selected without regard to the transmissive optical properties thereof. Thus, the material of the hermetic sealing layer 60 may be metal, and i6 prefer~bly alumlnum.
As shown ln detall ln FIG. lB, on the grooved surface 59 of the substrate 16, the thln fllm magneto-optlc recordlng structure 14 ls shown. The structure 14 includes a lower protectlve layer 62 that ls ad~a-cent the surface 59 of the substrate 16. The protec-tlve layer 62 ls formed from a materlal that ls selec-ted for lts hermetlc seallng characterlstlcs and rel-atlvely strong adheslon to the materlal of the sub-strate 16. However, since the radiatlon 12 is not ) transmltted through the protectlve layer ~2, such se-lectlon of the material for the protectlve lower layer 62 ls wlthout regard to the transmlsslve opt~cal pro-pertles thereof. In a preferred embodlment of the re-cord 10, the protectlve lower layer 62 ls formed from slllcon nltride havlng a thlckness of about 90 nano-meters.
The erasable magneto-optic layer 15 ls deposited over the protectlve layer 62. For recordlng ln re-sponse to the radlation 12 using the Kerr effect, the magneto-optlc layer 15 is formed from an alloy of rare earth elements and transltlon metals. E~amples of _~ such materlals are dlsclosed ln U.S. Patent #3,949,387 for BEAM ADDRESSABLE FILM USING AMORPHOUS MAGNETIC
MATERIAL, lssued on Aprll 6, 1976 to Chaudharl, et al.
In partlcular, such alloy may be selected from the groups conslstlng of the elements Tb, Fe, and Co; Gd, Tb and Fe; Gd, Tb, Fe and Co; and Tb and Co. Such alloy may also be formed from such groups, whereln each such group further lncludes one or more elements selected from the group conslstlng of Pt, Ti and Cr.
In a preferred embodlment of the record 10, the alloy of rare earth elements and trans~tion metals forming the magneto-optlc layer 15 ls formed from 22~ Tb, 70 Fe and 8~ of Co, wlth 2~ Pt [(Tb22 Fe70 C8)98 Pt2]
The thickness of the preferred alloy is 90 nanometers .. . .
10~. The alloy may be deposlted onto the protective lower layer 62 by sputterlng ln a well known manner.
Still referrlng to FIG. lB, the thln fllm struc-ture 14 ls shown lncludlng an optically transparent layer 63 provlded on the magneto-optlc layer 15. The transparent layer 63 ls selected for havlng charac-terlstics of optlcal transparency, hlgh inde~ of re-fraction and hermetic seallng properties. In a pre-ferred embodlment of the record 10, the optically transparent layer 63 may be formed from sillcon ni-i tride havlng a thlckness of 90 nanometers + 10~.
Referring agaln to FIG. lA, the support structure18 ls shown formed from the rings 19-19 whlch are an-nular and concentric. The rlngs 19-19 overlle annular portlons 64 (FIG. lA) of the substrate 16. The rlngs 19-19 are provlded wlth the support surfaces 20 that support and malntaln the wlndow 21 spaced from a sur-face 65 (FIG. lB) of the optlcally transparent layer 63 to form the air gap 22. Preferably the rings 19-19 are bonded directly to the annular portions 64 of the substrate 16. The corresponding portions of the M0 layer 15 and the layers 62 and 63 are masked off dur-_ ing depositlon for thls purpose. The rings 19-19 may i be formed from material having a uniform but non-cri-tical thickness of .05 to .5 mm in the dlrectlon in which the radlation 12 is lncident on the side 13 of the record 10. However, lf the materials from which the window 21 and the substrate 16 are made differ substantially with respect to expansion, eg. thermal expanslon, -the spacer rings 19-19, partlcularly the outer ring 19, should be made of a compllant, elas-tomeric material.
The air gap 22 between the upper surface 65 of the optically transparent layer 63 and the bottom flat surface 24 of the window 21 thus h~s a thickness of about .05 to .5 mm. An advantage of providlng the ... ...
~303739 alr gap 22 ln magneto-optlc recordlng relates to the larger Fresnel reflectlon coef~iclent associated with the air/transparent layer 63 interface, as compared with that of a plastlc substrate/protectlve layer interface. The Fresnel coefficlent for interfaces between non-absorblng dlelectrlcs ls glven by (n0-nl)/(n0+nl) where n0 and nl are the lndlces of re-fractlon on either side of the interface. Slnce the lndex of air ls l.O and is much lower than the index of plastics (typically of the order of 1.5) the mag-i nitude of the Fresnel coefflcient ls much larger at an alr/dlelectrlc interface than at a plastic/dielectric interface. Thls results ln a better anti-reflectlon condition, and therefore a better "bloomlng" enhance-ment of the Kerr effect and playback slgnal for alr incldence than for substrate incidence of the light.
See, for example, the discussion of blooming at page 67 of the text "Magnetlc Domalns and Technlques For Their Observation" by R. Carey and E.D. Isaac, Aca-demic Press, 1963.
Further, "optically smooth" surfaces of sub-strates have some degree of micro-roughness. Such _ micro-roughness is usually expressed in terms of the "root mean square roughness" of the surface; i.e., the r.m.s. devlation of the actual surface proflle from an ideal perfectly smooth flat surface. This micro-roughness can be measured by various means, a conven-ient one being a measurement of "total integrated scatter" (TIS) of llght reflected from the surface.
See for e~ample the article, "Comparlson of Techniques for Measuring the Roughness of Optical Surfaces" by Jean M. Bennett, OPtical En~ineerin~, Vol. 24, No. 3, (May-June 1985), pp. 380-385 and references cited therein.
This micro-roughness induced scatter is one source of noise in a slgnal derived from an optical KD-lOOl -17-i303739 record. For a glven roughness of the substrate sur-face, the nolse ln the slgnal due to thls roughness will be conslderably less for alr lncldence of the radlation 12 onto the thln film structure 14 than for substrate incldence onto ~uch structure 14 (as in the prlor art). In partlcular, accordlng to the above ~ennett reference, the total lntegrated scatter (TIS) _ (4~ )2 where ~ ls the r.m.s. roughness, ~
is the wavelength of the llght, and TIS ls the total scattered fractlon of the reflected llght. In a medlum of lndex of refractlon, n, the optical thlck-ness of a given scattering surface displacement, ~, becomes n~ whereas the wavelength becomes ~/n. There-fore, TIS in a medlum of lndex n becomes:
(TIS)n ~ ¦4~n~ /n)]2 ~ n4(4~ )2 b n4(TIS) i Thus, in terms of llght scattering, a given surface ls n4 times worse from the substrate-lncident side as compared to the air-incident side. Therefore, to the extent that micro roughness is a factor in determining the signal-to-noise ratlo of a slgnal derlved from an M0 optical record, it ls better for the optical-beam to address the recordlng structure from an alr amblent rather than from the amblent of the substrate. Slnce the alr gap 22 is ad~acent the upper surface 65 of the transparent layer 63, the effects of microroughness are reduced in the M0 records 10 and 40.
As shown in FIG. lA, the window 21 is mounted on the support surfaces 20 of the rings 19-19. The win-dow 21 has optimum transmissive optical characteris-tics including the flat surfaces 24. Further, thewindow 21 is fabricated from a material that is opti-cally transparent and that has mlnlmal blrefringence so that the incldent radiatlon 12 ls transmitted through the wlndow 21 wlth mlnimal optlcal retarda-~303739 tlon. Such characterlstlcs are provided by polymeth-ylmethacrylate, polymethylpentene and polyolefln. In a preferred embodlment of the record 10, the wlndow 21 ls formed from chemlcally tempered sheet glass havlng a diameter of 130 mm and a thlckness of 1.2 mm. Such wlndows are available from Glaverbel, Brussels, Bel-gium. As a result of such chemical tempering, the flat surfaces 24 are ln a state of compressive stress yet retain the desired minlmal birefringence.
10Prior to deposition of the thin films that form i the hermetlc seallng layer 60 on the lower surface 61 of the substrate 16 and the thln fllm structure 14 on the grooved side 59 of the substrate 16, the surfaces 59 and 61 of the substrate 16 are treated by ion beam 15 bombardment to promote good adhesion of such films thereto.
Detailed Disclosure -Fourth Embodiment - Record 40 Referring now to FIGs. 3A and 3B, it may be 20 understood that the record 40 includes the substrate 45 that ls provlded with grooves 66 that are slmilar to the grooves 58 formed ln the substrate 16. The J substrate 45 may be formed by a single in~ection mol-ded member (as shown) or by two members 45' (FIG. 3A
25 ~olned along a llne 45ll, each member 45~ being slmi-lar to the substrate 16 of the record 10. The record 40 is symmetrical with respect to a plane 67 (FIG. 3B) that ls parallel to the flrst and second sides 43 and 44. Thus, the sides 43 and 44 of the record 40 are 30 constructed and function the same. Therefore, the de-scription of one of the sides 43 and 44 applies to the other side.
Each dual thin film magneto-optic recording structure 41-41 formed on the substrate 45 is simllar 35 to the thin film magneto-optic recordlng structure 14 of the record 10, and lncludes the protectlve lower layer 62 and the optlcally transparent layer 63 provl-ded on the record 10. The M0 layer 42 of the struc-ture 41-41 ls the same as the MO layer 15. Further, the spacers 50-50 shown ln FIG. 3A are simllar to the rings 19-19 provlded in the record 10. The spacers 50-50 thus form the air gap 54 whlch has the same ad-vantages as the air gap Z2 of the record 10. The spacers 50-50 also support the windows 53-53 in op-posite confronting relationship with respect to the `~ upper surface 65 of the transparent layer 63 in a man-ner similar to the rings 19-19 of the record 10. The rings 50-50 are slmilarly sealed to the substrate 45 to form the air gap 54 as an annularly-shaped space.
Simllarly, each of the wlndows 53-53 ls formed from materlal selected for desired transmissive optical characterlstlcs, as in the manner of the window 21 of the record 10. It may be understood then that the re-cord 25 provides the ability to record on both sides 43 and 44, which increases the amount of data that can be stored on a single record 40.
The record 40 shown ln FIGs. 3A and 3B may alter-natively be provided with the tuned multi-layer inter-.J ference structure 34 shown in FIG. 4 lnstead of the thin film magneto-optic recording structure 41-41.
The advantages of using such structure 34 are dis-cussed below.
Detailed DescrlPtion - Record 25 Referring ln detail to FIGs. 2A and 2B, it may be understood that the substrate 26 may be designed with the same criteria in mind as used for the substrate 16 of the record 10, for example. Similarly, the respec-tive transparent layer 32, the magneto-optic layer 31 and the protective layer 33 are selected in a manner 3~ slmilar to that for the respectlve protective layer 63, the magneto-optlc layer 15 and the optlcally transparent layer 62 of the record 10. The trans-parent adhesive layer 28 may be an ultraviolet light cured optlcal photopolymer. Thls has characterlstlcs 5 as a cement and is also optlcally transparent. The thlckness of the transparent adheslve layer 28 ls not crltlcal, but ls about 20 mlcrometers ln a preferred embodlment of the record 25. The wlndow 27 adheres dlrectly to the transparent adheslve layer 28 and ls 10 selected wlth the same crlterla ln mlnd as the wlndows 21 and 53-53 of the records 10 and 40.
Referrlng to FIGs. 2A and 2B, lt may be observed that the record 25 embodles certain of the advantages of the records 10 and 40. In particular, the sub-15 strate 26 of the record 25 may be designed wlth the same obJectlves ln mlnd as used ln the design of the substrate 16 and the substrate 45 of the respectlve records 10 and 40. In partlcular, the functlons of the substrate 26 may be separated from that of the 20 functlon of the window 27, such that the ldeal charac-teristics of the substrate 26 may be obtained without regard to the transmlssive optlcal characterlstlcs thereof. Similarly, because of such separation of r functlons, the characteristics of the window 27 can be 25 selected from the standpoint of optlcal transmlsslon wlthout blrefringence. This advantage is believed to more than off-set the results of elimlnatlng the air gap from the record 25 and the resultant loss of the "blooming" and surface roughness effect.
.
30Detailed Di~closure - Structure 34 The third preferred embodiment of the present inventlon ls shown in FIG. 4 as provlding the tuned multi-layer interference structure 34 lnstead of the thln fllm recording structures 14, 30 and 41-41 of the 35respective records 10, 25 and 40, and instead of the thln film recordlng structure ~1-41 of the record 40 shown ln FIGs. 3A and 3B. The structure 34 lncludes the reflectlve layer 35 that is deposited dlrectly on the substrate 16 of the record 10 or the record 40, for example, for reflectlng the lncident radlatlon 12.
The reflectlve layer 35 ls preferably alumlnum. The transparent dielectrlc spacer protectlve layer 36 overlles the reflectlve layer 35. The spacer layer 3~
ls preferably silicon nl~rlde. The erasable magneto-optic layer 37 may be slmilar to that of the record 10and ls provided on the spacer protective layer 36.
For use in the interference structure 34, the magneto-optic recordlng layer 37 is preferably an alloy com-posed of [(Tb22 Fe70 Cog)g~ Pt2]. The protective15 overlayer 38 of transparent dlelectric material is preferably also silicon nitride and overlies the mag-neto-optlc recordlng layer 37. The thicknesses of the layers 35, 36, 37 and 38 of the lnterference structure 34 are designed using well known techniques which allow for some variatlon in thicknesses while still achieving the desired tuning to enhance and optimize the difference of the effective Kerr angle of polari-zation rotation corresponding to different directions of magnetization (up or down) of the magneto-optic re-cording layer 37. Such enhancement is in addition tothat resulting from the use of the alr incidence structure of the records 10 and 40. In a preferred embodiment of the interference structure 34, the thl- es~e re as follows:
.
Layer Thlckness Thlckness La~er(nanometers) Reflectlve 35100 Spacer 36 50 magneto-optic 37 24 Overlayer 38140 With the lnterference structure 34 provided on the substrate 1~ of the record 10, since the lntenslty ~~ 10 modulated radlatlon 12 is not transmitted through the substrate 16~ the substrate materlal may be selected without regard for the transmlssive optlcal charac-terlstlcs thereof.
Charac_eristics of Records 10, 25, and 40 Referrlng to FIGs. lAj 2A, and 3A, it may be un-derstood that the records 10, 25, and 40 are provided with a window, such as the wlndow 21 of the record 10, that is on the side of the records 10, 25, or 40 on which the radiation 12 is lncldent. As shown ln FIGs.
1~, 2B and 3B, the preferred angle of lncldence of a central ray 6g of the radiation 12 onto the respective '. windows 21, 27 and 53 is 90. The typical radiation 12 is a focussed beam of light having a numerical aperture ln the range of .5 to .65. The magnetic 25 field 11 is produced by a coil 69 (FIGs. 1 and 2), a similar coll 69 belng used with the record 40. The magnetlc fleld 11 is perpendicular to the plane formed by the flat surfaces of the records 10, 25 and 40, such as the flat surfaces 24-24 (FIG. lA). The radia-tion 12 is transmitted through the window 21 of the record 10, through the window 27 of the record 25 and through the windows 53-53 of the record 40. The radi-ation 12 is then transmltted through the air gaps 22 and 54-54 respectiYely, or in the case OI the record i303739 25, through the transparent adheslve layer 28, and ls then transmitted through the optlcally transparent layers 63 of the records 10, 25 and 40. When the ln-tenslty of the radlation 12 is of a relatively hlgh value, the radiation 12 ls referred to as a "record beam" slnce the record 10 records in the presence of the magnetic fleld 11 when the lntenslty ls hlgh. In particular, when the modulatlon of the intensity of the radiatlon 12 results ln the hlgh intenslty record beam, the materlal of the respective magneto-optlc layers 15, 31 or 42-42 ls locally heated by the fo-cussed radiatlon 12, causing the coerclvity of such material to fall below the value of the magnetic field 11, thus-enabling the direction of magnetization of the material of the respective magneto-optic layers 15, 31 and 42-42 to be locally reversed from their original unrecorded conditlon by becoming aligned with the magnetic field 11. During recording, the magnetic field 11 ls oriented in a dlrection opposite to the direction of magnetization of the material of the un-recorded magneto-optlc layers 15, 31 and 42-42. For e~ample, if the dlrection of magnetization of the mag-neto-optic layers 15, 31 and 42-42 is initially up in FIGs. lA, 2A and 3A, then at the particular location on whlch the record beam of the radiation 12 is inci-dent on the magneto-optic layers 15, 31 or 42-42, the direction of the magnetlzatlon wlll be reversed and will be downward. Thus, although the magneto-optic layers 15, 31 and 42-42 do not record in response to only the record beam, the record beam of radiation 12 and the magnetic field 11 result in recording of the radiation intensity modulation in the form of the re-versal of the magnetization at the locatlons at which the M0 layers 15, 31 or 42-42 are heated as described.
In the case of the magneto-optic recording layer 37 of the lnterference structure 34, there is greater heat-, . _ .. . ... . .
1~03739 ing of the magneto-optlc recordlng layer 37 due to the lnterference structure.
To read the record thus produced by the radlatlon 12 in the form of the record beam, radlation ln the form of a plane polarlzed read beam of focussed llght energy 12 having a substantially reduced level of in-tensity is dlrected perpendicularly to the respective sides 13, 13' and 43 of the records 10, 25 and 40.
The read beam 12 is transmitted through the respective windows 21, 27 and 5~-53 and across the respective alr gaps 22 and 54-54 or through the adheslve layer 2~ and ls transmitted through the optlcally transparent lay-ers 63 and onto the respective magneto-optic layers 15, 31 and 42-42. At locations on whlch the record beam of radlation 12 was dlrected onto the magneto-optic layers 15, 31 or 42-42, the resulting reversal of the magnetization at such locations will, according to the polar Kerr magneto-optic effect, rotate the plane of polarization of the read beam 12 in a direc-tion opposite to that of non-reversed locations. The read beam 12 is reflected off the magneto-optic layers 15, 31 and 42-42 as indicated by the arrows 70 (FIGs.
--, lA, 2A and 3A) and the rotations of the plane of polarization are detected. In the case of the mag-neto-optic recordlng layer 37 of the lnterference structure 34, the read beam of radlation 12 is reflec-ted ln part off both the recordlng layer 37 and the reflectlve layer 35, such parts belng approximately out of phase by vlrtue of proper cholce of thlckness of the transparent dlelectrlc spacer protectlve layer 36.
The advantages of the structure of ths records 10 and 40, both of which include alr incldence of the read optical radlation 12 onto the thin film magneto-optic recording structures 14 and 41-41, respectively.
whether ln the form shown in respective FIGs. lA or 3A
or uslng the lnterference structure 34 shown ln FIG.
4, lnclude better anti-reflectlon "blooming" enhance-ment of the Kerr slgnal. Further, such records 10 and 40 avoid the necesslty of uslng an optlcal quallty grooved substrate having stringent tolerances on blre-frlngence and blrefrlngence gradlent. Instead, the substrates 16 and 45, respectlvely, may be used wlth-out regard for transmlsslve optlcal characterlstlcs.
The effect of micro-roughness of the substrates 16 and 45 is conslderably reduced for alr lncldence as com-pared to substrate lncldence of the read beam of radl-ation 12. Slnce the substrates 16 and 45 need not be transparent, non-transparent materials such as metals can be used on elther slde of the substrates 16 and 45, elther to seal out molsture from the bulk of the substrates, or to lmprove the adhesion or barrler pro-perties of the protectlve layer 62. The substrate surfaces 46 and 47 (FIG. 3B) and 59 (FIG~ 1~) may be vigorously treated by plasma or by ion bombardment to cross llnk and harden such surfaces, to activate such surface for improved adhesive bondlng of the thin film recording structures 14 and 41-41 thereto (as to the respective records 10 and 40) or for improved adhesive bonding of the reflective layer 35 to such substrate surfaces, and to seal low molecular welght fragments of polymer, or plastlclzer awa~ from such surfaces, without regard to transmlsslve optlcal propertles.
Dust defocusslng ls provlded by the transparent windows 21, 27 and 53-53 having the flat surfaces, ~0 such as the surfaces 24 and 57-57 of the respectlve records 10 and 40.
E~perimentally, it has been observed that inter-ference effects assoclated wlth the alr gaps 22 and 54-54 are not a serlous problem; thls ls most llkely due to the compensatlng effect of reflectance and Kerr angle factors contrlbuting to the Kerr slgnal.
KD-1001 -2~-It has also been experlmentally observed that the carrler-to-noise ratlo of a test slgnal observed through the respective wlndows 21 and 53-53 and across the air gaps 22 and 54-54 ls conslstently better than that observed when the read beam radlatlon 12 is transmltted through a substrate as in the above-de-scrlbed prlor art M0 records. The lmprovement has been about 3.7 dB ln carrler-to-nolse ratio.
Whlle the preferred embodlments have been de-scribed ln order to lllustrate the fundamental rela-`~ tlonships of the present lnventlon, lt should be understood that numerous variations and modlflcatlons may be made to these embodiments without departing from the teachings and concepts of the present lnven-tion. Accordlngly, lt should be clearly understood that the form of the present lnventlon descrlbed above and shown in the accompanylng drawlngs ls lllustratlve only and ls not lntended to limit the scope of the invention to less than that descrlbed ln the following claims.
Claims (45)
1. In an optical record including erasable means for magneto-optic recording, said erasable means including a recording layer that records in the pre-sence of a magnetic field in response to recording radiation incident on a given side of said record, the improvement comprising:
substrate means for supporting said erasable magneto-optic means on the side thereof away from said given side of said record, said substrate means being fabricated from a material having support characteris-tics selected without regard for the transmissive optical characteristics thereof;
means overlying at least a portion of said substrate for providing at least one support surface spaced from said erasable magneto-optic means; and window means mounted on said support surface for forming an air gap on the side of said erasable magneto-optic means toward said given side of said record, said window having flat surfaces and being fabricated from a material that is optically trans-parent and has minimal birefringence so that said incident energy is transmitted therethrough with mini-mal optical retardation.
substrate means for supporting said erasable magneto-optic means on the side thereof away from said given side of said record, said substrate means being fabricated from a material having support characteris-tics selected without regard for the transmissive optical characteristics thereof;
means overlying at least a portion of said substrate for providing at least one support surface spaced from said erasable magneto-optic means; and window means mounted on said support surface for forming an air gap on the side of said erasable magneto-optic means toward said given side of said record, said window having flat surfaces and being fabricated from a material that is optically trans-parent and has minimal birefringence so that said incident energy is transmitted therethrough with mini-mal optical retardation.
2. An optical record according to Claim 1, in which:
said support surface means is an optically transparent adhesive material that fills said air gap.
said support surface means is an optically transparent adhesive material that fills said air gap.
3. An optical record according to Claim 1, in which:
said erasable magneto-optic recording means is formed from said recording layer and from a protec-tive layer on each side thereof so that one of said protective layers is adjacent said air gap, said adjacent protective layer being optically transparent.
said erasable magneto-optic recording means is formed from said recording layer and from a protec-tive layer on each side thereof so that one of said protective layers is adjacent said air gap, said adjacent protective layer being optically transparent.
4. An optical record according to Claim 3, in which:
said erasable magneto-optic means is formed from first and second ones of said protective layers located on opposite sides of said recording layer, said first protective layer being nearest said sub-strate means and being formed from a material selected for its hermeticity and relatively strong adhesion to said substrate means but without regard to the trans-missive optical properties thereof.
said erasable magneto-optic means is formed from first and second ones of said protective layers located on opposite sides of said recording layer, said first protective layer being nearest said sub-strate means and being formed from a material selected for its hermeticity and relatively strong adhesion to said substrate means but without regard to the trans-missive optical properties thereof.
5. An optical record according to Claim 3, in which:
said erasable magneto-optic means also in-cludes a reflective layer on said substrate means for reflecting said recording radiation transmitted through said protective layers and said recording layer; and said recording layer, said protective layers, and said reflective layer forming a multi-layer inter-ference structure, the thicknesses of said layers of said multi-layer interference structure being selected to enhance and optimize the effective Kerr polariza-tion rotation difference corresponding to different magnetization directions of said recording layer.
said erasable magneto-optic means also in-cludes a reflective layer on said substrate means for reflecting said recording radiation transmitted through said protective layers and said recording layer; and said recording layer, said protective layers, and said reflective layer forming a multi-layer inter-ference structure, the thicknesses of said layers of said multi-layer interference structure being selected to enhance and optimize the effective Kerr polariza-tion rotation difference corresponding to different magnetization directions of said recording layer.
6. An optical record according to Claim 5, in which:
the temperature of said recording layer increases in response to said radiation; and said recording layer is fabricated from an alloy of Tb, Co, Fe and Pt that has a relatively high Curie point and a relatively large Kerr polarization rotation angle.
the temperature of said recording layer increases in response to said radiation; and said recording layer is fabricated from an alloy of Tb, Co, Fe and Pt that has a relatively high Curie point and a relatively large Kerr polarization rotation angle.
7. In a double-sided optical record including dual-erasable means for magneto-optic recording, said dual-erasable means including first and second record-ing layers that record in the presence of a magnetic field in response to recording radiation incident on first and second sides of said record, the improve-ment comprising:
substrate means for supporting one of said dual-erasable magneto-optic means on each opposite side of said substrate means so that said first layer is toward said first side of said record and said second layer is toward said second side of said re-cord, said substrate means being fabricated from a material having support characteristics selected without regard for the transmissive optical charac-teristics thereof;
means overlying at least a portion of each said opposite side of said substrate for providing at least one support surface spaced from each of said dual-erasable magneto-optic means; and window means mounted on each said support surface for forming an air gap on the side of said first layer toward said first side of said record and for forming an air gap on the side of said second layer toward said second side of said record, said window means having flat surfaces and being fabricated from a material that is optically transparent and has minimal birefringence so that said incident radiation is transmitted through each said window means to said respective first and second layers with minimal opti-cal retardation.
substrate means for supporting one of said dual-erasable magneto-optic means on each opposite side of said substrate means so that said first layer is toward said first side of said record and said second layer is toward said second side of said re-cord, said substrate means being fabricated from a material having support characteristics selected without regard for the transmissive optical charac-teristics thereof;
means overlying at least a portion of each said opposite side of said substrate for providing at least one support surface spaced from each of said dual-erasable magneto-optic means; and window means mounted on each said support surface for forming an air gap on the side of said first layer toward said first side of said record and for forming an air gap on the side of said second layer toward said second side of said record, said window means having flat surfaces and being fabricated from a material that is optically transparent and has minimal birefringence so that said incident radiation is transmitted through each said window means to said respective first and second layers with minimal opti-cal retardation.
8. An optical record according to Claim 7, in which:
said support surface means is formed from optically transparent adhesive material that fills each said air gap.
said support surface means is formed from optically transparent adhesive material that fills each said air gap.
9. An optical record according to Claim 7, in which:
each said dual-erasable magneto-optic record-ing means is formed from said recording layer and from a protective layer on each side thereof so that one of said protective layers is adjacent said air gap, said adjacent protective layer being optically transparent;
each said dual-erasable magneto-optic means also includes a reflective layer on said substrate means for reflecting said recording radiation trans-mitted through said protective layers and said recor-ding layer; and each said recording layer, protective layers and reflective layers forming a multi-layer structure the thicknesses of said layers of said multi-layer interference structure being selected to enhance and optimize the effective Kerr polarization rotation difference corresponding to different magnetization directions of said recording layer.
each said dual-erasable magneto-optic record-ing means is formed from said recording layer and from a protective layer on each side thereof so that one of said protective layers is adjacent said air gap, said adjacent protective layer being optically transparent;
each said dual-erasable magneto-optic means also includes a reflective layer on said substrate means for reflecting said recording radiation trans-mitted through said protective layers and said recor-ding layer; and each said recording layer, protective layers and reflective layers forming a multi-layer structure the thicknesses of said layers of said multi-layer interference structure being selected to enhance and optimize the effective Kerr polarization rotation difference corresponding to different magnetization directions of said recording layer.
10. A magneto-optic record, including:
a rigid substrate fabricated from a material having improved support and moisture impermeability characteristics without regard for the transmissive optical characteristics thereof a transparent disk-shaped window, said window being flat and fabricated from a material that is optically transparent and that has minimal birefrin-gence so that radiation is transmitted therethrough with minimal optical retardation;
a first surface of said substrate and a first surface of said window being in opposite confronting relationship;
a pair of coaxial sealing rings sealed bet-ween said opposite confronting surfaces of said sub-strate and window and forming an annularly-shaped space therebetween; and a recording layer overlying said first sur-face of said substrate that is in confronting rela-tionship with said window and said recording layer being fabricated from an erasable, magneto-optic material that records in the presence of a magnetic field and in response to radiation transmitted through said window and across said annularly-shaped space.
a rigid substrate fabricated from a material having improved support and moisture impermeability characteristics without regard for the transmissive optical characteristics thereof a transparent disk-shaped window, said window being flat and fabricated from a material that is optically transparent and that has minimal birefrin-gence so that radiation is transmitted therethrough with minimal optical retardation;
a first surface of said substrate and a first surface of said window being in opposite confronting relationship;
a pair of coaxial sealing rings sealed bet-ween said opposite confronting surfaces of said sub-strate and window and forming an annularly-shaped space therebetween; and a recording layer overlying said first sur-face of said substrate that is in confronting rela-tionship with said window and said recording layer being fabricated from an erasable, magneto-optic material that records in the presence of a magnetic field and in response to radiation transmitted through said window and across said annularly-shaped space.
11. An optical record according to Claim 1, in which:
said substrate means is provided with grooves that are formed without regard for the transmissive optical properties of said substrate means.
said substrate means is provided with grooves that are formed without regard for the transmissive optical properties of said substrate means.
12. An optical record according to Claim 1, in which:
said substrate material is polycarbonate.
said substrate material is polycarbonate.
13. An optical record according to Claim 1, in which:
said window means material is polymethyl-methacrylate.
said window means material is polymethyl-methacrylate.
14. An optical record according to Claim 1, in which:
said window means material is polymethylpen-tene.
said window means material is polymethylpen-tene.
15. An optical record according to Claim 1, in which:
said window means material is polyolefin.
said window means material is polyolefin.
16. An optical record according to Claim 1, in which:
said recording layer is fabricated from a magneto-optic alloy of rare earth elements and transi-tion metals.
said recording layer is fabricated from a magneto-optic alloy of rare earth elements and transi-tion metals.
17. An optical record according to Claim 16, in which:
said alloy is taken from the groups con-sisting of the elements Tb, Fe, and Co; Gd, Tb and Fe;
Gd, Tb, Fe and Co; and Tb and Co.
said alloy is taken from the groups con-sisting of the elements Tb, Fe, and Co; Gd, Tb and Fe;
Gd, Tb, Fe and Co; and Tb and Co.
18. An optical record according to Claim 17, in which:
each of said groups further include one or more elements taken from the group consisting of Pt, Ti and Cr.
each of said groups further include one or more elements taken from the group consisting of Pt, Ti and Cr.
19. An optical record according to Claim 4, in which:
said first protective layer is aluminum.
said first protective layer is aluminum.
20. An optical record according to Claim 1, in which:
said window means material is chemically tem-pered glass such that said flat surfaces are in a state of compressive stress yet said window means retains said minimal birefringence.
said window means material is chemically tem-pered glass such that said flat surfaces are in a state of compressive stress yet said window means retains said minimal birefringence.
21. An optical record according to Claim 11, in which:
said grooves are formed on the side of said substrate means toward said given side of said record.
said grooves are formed on the side of said substrate means toward said given side of said record.
22. An optical record according to Claim 11, in which:
a hermetic sealing layer is provided on the surface of said substrate means away from said given side of said record.
a hermetic sealing layer is provided on the surface of said substrate means away from said given side of said record.
23. An optical record according to Claim 22, in which:
said hermetic sealing layer is a metal.
said hermetic sealing layer is a metal.
24. An optical record according to Claim 23, in which:
said hermetic sealing layer is aluminum.
said hermetic sealing layer is aluminum.
25. An optical record according to Claim 3, in which:
each of said protective layers is formed from silicon nitride.
each of said protective layers is formed from silicon nitride.
26. An optical record according to Claim 1, in which:
said substrate means is formed from injection molded material.
said substrate means is formed from injection molded material.
27. An optical record according to Claim 3, in which:
said adjacent protective layer is formed from silicon nitride.
said adjacent protective layer is formed from silicon nitride.
28. An optical record according to Claim 7, in which:
each said dual-erasable magneto-optic record-ing means is formed from one of said first and second layers and from a protective layer on each side there-of so that one of said protective layers is adjacent said air gap, each said adjacent protective layer being optically transparent.
each said dual-erasable magneto-optic record-ing means is formed from one of said first and second layers and from a protective layer on each side there-of so that one of said protective layers is adjacent said air gap, each said adjacent protective layer being optically transparent.
29. An optical record according to Claim 7, in which:
said substrate means is provided with grooves that are formed without regard for the transmissive optical properties of said substrate.
said substrate means is provided with grooves that are formed without regard for the transmissive optical properties of said substrate.
30. An optical record according to Claim 7, in which:
said substrate means material is polycar-bonate.
said substrate means material is polycar-bonate.
31. An optical record according to Claim 7, in which:
said window means material is polymethyl-methacrylate.
said window means material is polymethyl-methacrylate.
32. An optical record according to Claim 7, in which:
said window means material is polymethylpen-tene.
said window means material is polymethylpen-tene.
33. An optical record according to Claim 7, in which:
said window means material is polyolefin.
said window means material is polyolefin.
34. An optical record according to Claim 7, in which:
said window means material is chemically tem-pered glass such that said flat surfaces are in a state of compressive stress, yet said window means retains said minimal birefringence.
said window means material is chemically tem-pered glass such that said flat surfaces are in a state of compressive stress, yet said window means retains said minimal birefringence.
35. An optical record according to Claim 29, in which:
said grooves are formed on each of said opposite sides of said substrate means.
said grooves are formed on each of said opposite sides of said substrate means.
36. An optical record according to Claim 28, in which:
each said dual-erasable magneto-optic means is formed from first and second ones of said protec-tive layers located on opposite sides of a given one of said first and second recording layers, said first protective layers being nearest said substrate means and being formed from a material selected for its hermeticity and relatively strong adhesion to said substrate means but without regard to the transmissive optical properties thereof.
each said dual-erasable magneto-optic means is formed from first and second ones of said protec-tive layers located on opposite sides of a given one of said first and second recording layers, said first protective layers being nearest said substrate means and being formed from a material selected for its hermeticity and relatively strong adhesion to said substrate means but without regard to the transmissive optical properties thereof.
37. An optical record according to Claim 36, in which:
said first protective layers are aluminum.
said first protective layers are aluminum.
38. An optical record according to Claim 7, in which:
said recording layers are fabricated from a magneto-optic alloy of rare earth elements and transi-tion metals.
said recording layers are fabricated from a magneto-optic alloy of rare earth elements and transi-tion metals.
39. An optical record according to Claim 38, in which:
said alloy is selected from the group con-sisting of A, B, C and D, wherein A consists of the elements Tb, Fe, and Co; B consists of the elements Gd, Tb and Fe; C consists of the elements Gd, Tb, Fe and Co; and D consists of the elements Tb and Co.
said alloy is selected from the group con-sisting of A, B, C and D, wherein A consists of the elements Tb, Fe, and Co; B consists of the elements Gd, Tb and Fe; C consists of the elements Gd, Tb, Fe and Co; and D consists of the elements Tb and Co.
40. An optical record according to Claim 39, in which:
each said group further includes one or more elements selected from the group consisting of Pt, T1 and Cr.
each said group further includes one or more elements selected from the group consisting of Pt, T1 and Cr.
41. An optical record according to Claim 28, in which:
said protective layers are silicon nitride.
said protective layers are silicon nitride.
42. An optical record according to Claim 7, in which:
said substrate means is formed from injection molded material.
said substrate means is formed from injection molded material.
43. An optical record according to Claim 28, in which:
said adjacent protective layer is fabricated from silicon nitride.
said adjacent protective layer is fabricated from silicon nitride.
44. An optical record according to Claim 9, in which:
the temperature of said recording layer in-creases in response to said recording radiation; and said recording layer is fabricated from an alloy of Tb, Co, Fe and Pt that has a relatively high Curie point and a relatively large Kerr polarization rotation angle.
the temperature of said recording layer in-creases in response to said recording radiation; and said recording layer is fabricated from an alloy of Tb, Co, Fe and Pt that has a relatively high Curie point and a relatively large Kerr polarization rotation angle.
45. An optical record according to Claim 7, in which:
said support surface means is an optically transparent adhesive material that fills said air gap.
said support surface means is an optically transparent adhesive material that fills said air gap.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US5745087A | 1987-06-02 | 1987-06-02 | |
| US057,450 | 1987-06-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1303739C true CA1303739C (en) | 1992-06-16 |
Family
ID=22010623
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000566559A Expired - Fee Related CA1303739C (en) | 1987-06-02 | 1988-05-12 | Magneto-optic records |
Country Status (6)
| Country | Link |
|---|---|
| EP (1) | EP0293797B1 (en) |
| JP (1) | JPS63311647A (en) |
| AT (1) | ATE93337T1 (en) |
| CA (1) | CA1303739C (en) |
| DE (1) | DE3883290D1 (en) |
| ES (1) | ES2043727T3 (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4877666A (en) * | 1987-08-11 | 1989-10-31 | Kerdix, Inc. | Magneto-optic substrates |
| US5667862A (en) * | 1989-03-15 | 1997-09-16 | Sony Corporation | Magneto-optical disk |
| JP2880723B2 (en) * | 1989-03-15 | 1999-04-12 | ソニー株式会社 | Magneto-optical disk |
| CS335190A3 (en) * | 1989-07-05 | 1992-11-18 | Mitsui Petrochemical Ind | Magneto-optical recording medium |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS6052944A (en) * | 1983-09-01 | 1985-03-26 | Ricoh Co Ltd | Optomagnetic recording medium |
| JPS60212849A (en) * | 1984-04-06 | 1985-10-25 | Ricoh Co Ltd | Photomagnetic recording medium |
| DE3519070A1 (en) * | 1984-05-29 | 1985-12-05 | Ricoh Co., Ltd., Tokio/Tokyo | Magneto-optical recording medium |
| JPS6171436A (en) * | 1984-09-14 | 1986-04-12 | Hitachi Ltd | Optomagnetic recording medium |
| JPS61255546A (en) * | 1985-05-09 | 1986-11-13 | Kokusai Denshin Denwa Co Ltd <Kdd> | Photomagnetic recording medium |
-
1988
- 1988-05-12 CA CA000566559A patent/CA1303739C/en not_active Expired - Fee Related
- 1988-05-30 AT AT88108604T patent/ATE93337T1/en not_active IP Right Cessation
- 1988-05-30 ES ES88108604T patent/ES2043727T3/en not_active Expired - Lifetime
- 1988-05-30 DE DE8888108604T patent/DE3883290D1/en not_active Expired - Fee Related
- 1988-05-30 EP EP88108604A patent/EP0293797B1/en not_active Expired - Lifetime
- 1988-06-01 JP JP63132910A patent/JPS63311647A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| EP0293797A2 (en) | 1988-12-07 |
| JPS63311647A (en) | 1988-12-20 |
| ES2043727T3 (en) | 1994-01-01 |
| DE3883290D1 (en) | 1993-09-23 |
| EP0293797A3 (en) | 1990-12-05 |
| EP0293797B1 (en) | 1993-08-18 |
| ATE93337T1 (en) | 1993-09-15 |
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