Method and apparatus for recording and reproducing information in cells using multiple interference

Abstract: An optical information recording medium includes a base, a reflecting layer formed on the base, a second layer formed parallel to the first reflecting layer at a predetermined distance from the same, and a recording layer formed between the first and second reflecting layers. The recording layer has an array of a plurality of cells having different refractive indices which are changed when irradiated with a light beam. The intensity of the light beam is modulated according to information. Irradiating the recording layer with the modulated light beam causes changes in the refractive index according to the intensity of the light beam. In another aspect, the wavelength of the light beam is changed within a predetermined range including at least one wavelength at which multiple interference occurs between the first and second reflecting layers with respect to each cell. The wavelength of light which has effective reflecting multiple interference is detected. The information recorded in the cell is reproduced in accordance with the wavelength detected. The medium may be used in an apparatus for recording and reproducing information.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a medium on which information can be optically recorded and from which recorded information can be optically read out and to a method of recording/reproducing information by using this medium and a related apparatus. Description of the Prior Art Recently, optical information recording mediums such as optical memories including optical disks, optical cards, optical tape, and the like, and recording/reproduction apparatuses using such mediums have notably been developed. Optical recording/reproduction mediums are low-priced in terms of cost per bit compared with magnetic disks or the like and have a high transfer rate, because optical memories are characterized by their capacity for high-density recording. Optical memories are being actively studied and developed in order to further increase their recording density. The recording density of optical memories can be increased by reducing the minimum bit size, multivaluing information recording, or effecting multiple recording of information, and various methods relating to them have been proposed. Almost all well known multivaluing techniques utilize a continuous change in the reflectivity of a dyestuff medium or a phase change type medium. These techniques are based on fractionating an intermediate value between two reflectivity levels ensuring high contrast, and the signal-to-noise ratio (SNR) is therefore sacrificed for multivaluing in accordance with their principle, resulting in an increase in error rate. A method utilizing the reflectivity change is as described below. If the complex indexes of reflection of the base and the medium are N 1 and N 2 , the reflectivity R is, most simply, expressed by the following equation: where n m (m = 1, 2) is a refractive index, K m (m = 1, 2) is an attenuation coefficient, and N m = n m - iK m . For example, in the case of a TeO x phase change medium, N 2 = 3.5 - 0.8i in the amorphous state, and N 2 = 3.9 - 1.3i in the crystallized state and, if N 1 = 1.6, a change ΔR in the reflectivity is small, 0.6 %. It is difficult to uniformly fractionate ΔR, because the change in ΔR with respect to the optical energy necessary for recording is non-linear, and it is also difficult to control this optical energy. A method for increasing SNR, such as the one disclosed in USP 4,183,094, which is not specifically intended for multivaluing, has been proposed. In this method, a high-reflectivity metallic mirror is provided on the reverse side of a recording medium to increase the difference between the reflectivities in the two states by a thin-film multiple interference effect and to thereby improve the contrast. Specifically, in USP 4,183,094, a method of increasing the change in reflectivity by a change in the absorption coefficient is proposed. This method, however, also entails the above-mentioned problems of multivaluing, i.e., the problems of non-linearity and difficulty in controlling optical energy. If this method is applied to multivaluing, it is quite possible that the non-linearity of the change in reflectivity will be increased because the change in absorption spectrum and the multiple interference effect are combined. That is, even if the contrast and, hence, SNR of binary data can be improved by increasing the difference between the two reflectivities corresponding to binary states, it is not always possible to obtain characteristics suitable for multivaluing. In digital memories, if the number of levels of multivaluing on the medium is L, the number M of information recording bits when binary digit data is recorded is expressed by the following equation: M = log 2 L For example, even if 8-level multivaluing (L = 8) on the medium is realized in the above-mentioned reflectivity change multivaluing, the recording density is only trebled since the amount of information which can be recorded by these levels is 3 bits (M = 3). Considering signal processing of an apparatus such as a computer which receives data from a digital memory or an internal processing system of the memory such as a modulation/demodulation system, it can be said that the effect of multivaluing is satisfactory provided that multivalued data of 8 bits or more can be recorded. It is therefore necessary to effect 256 level multivaluing on the medium. If multivaluing is effected by utilizing the reflectivity with respect to the intensity of light as described above and by dividing the change in reflectivity into 256 equal levels assuming ideally the change is linear from 0 to 100 %, the change rate per level is 0.391 %, and it is understood that this multivaluing is very difficult in terms of practice considering the technology for manufacturing the medium, stabilizing the intensity of light from the light source and detecting the changes. A multiple recording method using the same absorption spectrum as USP 4,183,094 and utilizing a photochemical hall burning (PHB) effect has also been proposed. This multiple recording method enables multiple recording in theory. The development of PHB methods, however, is presently at a stage such that experiments for confirming the principle at a very low temperature have been finished. It is technically difficult to use a medium having a multiplicity of narrowband absorption spectra at room temperature with stability. Furthermore, for a sequence of fundamental operations of a memory including recording of information, reproduction and erasing, it is necessary for the light source to have a spectrum width sufficiently narrower than each narrow absorption spectrum width. Many problems other than those due to instability of the mediums are also encountered. For example, wavelength absolute value control is required. As described above, the conventional multivaluing techniques entail many problems, in particular, that of reduction in SNR, and that of limitation of the degree of multivaluing. Although methods for increasing the degree of multivaluing have been proposed, many technical problems are left. US-A-4183094 discloses an optical storage medium comprising first and second parallel reflecting layers. In order to record on the storage medium a light beam is directed on to selected portions of the medium so as to cause heating of the selected portions of the medium and thereby cause permanent changes in the wavelength and intensity of interference fringes produced by light reflected from the medium. Such a medium and method are acknowledged as prior art in the precharacterising portion of the independent claims. EP-A-0290009 discloses an information recording medium with a recording film layer having a thickness which provides a local maximum value of reflectivity of the reproducing light, due to a multiple interference effect, by said recording film. A laser beam is used to form pits in the recording film. The variation of reflectivity over the recording medium enables the information thus stored on the medium to be read out. JP-A-60209940 discloses an optical recording medium comprising an absorptive recording layer between two reflective layers. Interference effects are used to increase the signal to noise ratio for recording signals which are read out using variations in reflectance over the recording medium. EP-A-287 382 discloses optical wavelength division multiple recording and reproduction. The medium is irradiated with a variable wavelength radiation source and the absorption peaks are detected. SUMMARY OF THE INVENTION In view of the above-described problems of the related art, an object of the present invention to provide an optical information recording medium capable of recording information at a high density and reading out recorded information at a high SNR and a recording/reproducing method of using this medium. According to a first aspect of the present invention there is provided an optical information reproducing method comprising: directing a light beam onto an optical information recording medium comprising first and second reflecting layers parallel to and at a predetermined distance from each other, and an information storage layer formed between the first and second reflecting layers, and thereby reproducing the information recorded in said information storage layer; the method being characterized in that: said information storage layer has a plurality of cells, each of which has a refractive index of a different value dependent on multiple value information recorded on the medium, and the method includes the steps of: irradiating a cell of the recording medium with a light beam whose wavelength changes within a range including at least one wavelength at which multiple interference occurs between the first and second reflecting layers in the cell to be reproduced; and detecting the wavelength of the light at which multiple interference occurs, to reproduce thereby the information recorded in said cell. According to a second aspect of the present invention there is provided an apparatus for reproducing information from an optical information recording medium comprising first and second reflecting layers parallel to and at a predetermined distance from each other, and an information storage layer which is formed between the first and second reflecting layers, the apparatus comprising means for irradiating the medium with a light beam and means for detecting the intensity of light reflected by or transmitted through the medium, the apparatus being characterised in that said information storage layer has a plurality of cells, each of which has a refractive index of a different value dependent on multiple value information recorded on a medium, and said light beam includes lights of a plurality of wavelengths including at least one wavelength at which multiple interference occurs between the first and second reflecting layers in each of the cells and the apparatus includes wavelength detecting means for detecting the wavelength at which multiple interference has occurred out of the said plurality of wavelengths from the output of said detecting means and thereby reproducing the information recorded in the cells. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of an embodiment of an optical information recording medium used according to the present invention; Fig. 2A is a schematic diagram of irradiation of the medium with a light beam; Fig. 2B is a diagram of the relationship between the phase difference and the intensity of reflected light with respect to the medium shown in Fig. 1; Fig. 3 is a schematic diagram of a first embodiment of an optical information recording/reproduction apparatus used in accordance with the present invention; Figs. 4A to 4D are time charts of a first embodiment of an information reproduction method of the present invention; Figs. 5A to 5C are time charts of a second embodiment of the information reproduction method of the present invention; Figs. 6A and 6B are time charts of a third embodiment of the information reproduction method of the present invention; Figs. 7A and 7B are time charts of a fourth embodiment of the information reproduction method of the present invention; Figs. 8A and 8B are time charts of a fifth embodiment of the information reproduction method of the present invention; Figs. 9A to 9E are time charts of a sixth embodiment of the information reproduction method of the present invention; Figs. 10A to 10C are time charts of a seventh embodiment of the information reproduction method of the present invention; Fig. 11 is a schematic diagram of a second embodiment of the optical information recording/reproduction apparatus used in accordance with the present invention; Figs. 12A to 12F are time charts of a eighth embodiment of the information reproduction method of the present invention; Figs. 13A to 13C are time charts of a ninth embodiment of the information reproduction method of the present invention; and Figs. 14A to 14C are time charts of an operation for detecting phase difference. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 schematically shows a cross section of an optical information recording medium used in accordance with an embodiment of the present invention. The recording medium 1 shown in Fig. 1 has a double-sided structure and is capable or recording and reproduction on both the obverse and reverse sides. The recording medium 1 is composed of transparent polycarbonate bases 11a and 11b and reflecting layers and medium layers alternately formed on the bases, as described below. First reflecting layers 12a and 12b are formed on the bases 11a and 11b, respectively, and medium layers 13a and 13b whose refractive indices are changed by heat are formed on the surfaces of the reflecting layers 12a and 12b. Second reflecting layers 14a and 14b are formed on the medium layers 13a and 13b and are attached to each other by an adhesive layer 15. These layers are formed parallel to each other to cause multiple interference as described later. Preferably, the medium layers 13a and 13b are formed of an inorganic medium, such as TeOx (x: positive integer), InSeTℓCo, GeTeSbTℓ or GeTeSe, or an organic medium, such as an anthraquinone dielectric, a dioxazine compound or a triphenodithiazine compound. The multilayer reflecting layers 12a, 12b, 14a and 14b are constructed by alternately superposing layers of dielectrics having large and small refractive indices and each having a thickness corresponding to 1/4 of the wavelength of a later-mentioned light beam. SiO 2 , Si 3 N 4 , MgF 4 , Al 2 O 3 and the like may be used as dielectric layer materials. To manufacture this recording medium 1, the first reflecting layer, the medium layer and the second reflecting layer are successively formed by sputtering or coating on each of the bases 11a and 11b forming the two faces of the medium 1. These layers and the bases 11a and 11b are combined by bonding the second reflecting layers 2 to each other with an adhesive (adhesive layer 15), thus forming a recording medium for double side recording. In the recording medium 1 in accordance with this embodiment, a Fabry-Pérot etalon is formed by the arrangement of the first and second layers and the medium layers interposed therebetween. Incident light is repeatedly reflected between the two reflecting layers so that a multiple interference effect is obtained, as described below. According to the present invention, this multiple interference effect is effectively utilized to perform multivalued information recording, as described below in detail. The principle of information recording/reproduction using the above-described recording medium 1 will be described below with reference to Figs. 2A and 2B. In Fig. 2A, a reference number 43 denotes a medium layer which corresponds to each of the medium layers 13a and 13b shown in Fig. 1, and a reference number 42 denotes a first reflecting surface equivalently substituted for the first reflecting surfaces 12a and 12b of Fig. 1. Also a second reflecting surface 44 is equivalently substituted for the second reflecting layers 14a and 14b. A reference number 41 denotes a laser beam which is incident upon the medium layer 43 from above, i.e., from the base shown in Fig. 1. The waist of the laser beam is positioned in the vicinity of the medium layer 43 by the control of an autofocusing mechanism. Accordingly, the laser light can be regarded as substantially parallel rays. To record information, a small converged spot of the light beam is irradiated to the medium so as to locally heat the medium layer 43. At this time, the intensity of the light beam is changed according to the information, and the information is recorded as an array of cells varying in refractive index in accordance with the change in the intensity. To reproduce the information, a small converged spot of laser light is irradiated to the recording medium 1 by a pickup lens as in the case of the ordinary optical head. The information is reproduced from the light reflected by the array of cells. Assuming, for ease of explanation, that air fills both the spaces on the upper and lower sides of the medium layer 43 of Fig. 2A, and that the reflectivities of the two reflecting surfaces are equal, this optical process can be discussed as a case of incidence of parallel light on a simplest Fabry-Pérot etalon. Accordingly, if the intensity reflectivity of the first and second reflecting surfaces is R; the refractive index of the medium layer 43 is n M ; and the thickness of this layer is d, the reflectivity of R E of this etalon determined by the multiple interference effect of repeated reflection can be expressed by an equation (1): R E = Fsin 2 ψ 2 1 + Fsin 2 ψ 2 where F is expressed by the following equation (2), and ψ is a phase difference expressed by the following equation (3) if the wavelength of the incident light is λ. F = 4R (1 + R) 2 ψ = 4πn M d λ F is a parameter corresponding to the fineness and representing the sharpness of interference fringes. If the reflectivity R is greater, the change in reflectivity with respect to the change in phase difference becomes steeper and greater by the effect of multiple interference of the reflected light, as described below with reference to Fig. 2B. That is, the recordable multivalued levels can be increased. The reflectivity is lowered when the following equation (4) obtained from equation (1) is satisfied. ψ = 2πm where m is an integer. Fig. 2B schematically shows the above-mentioned multiple interference. The abscissa represents the phase difference ψ and the ordinate represents the intensity of reflected light. A dotted line 46 in Fig. 2B indicates a case where the reflectivity R is small, and a solid line 45 indicates a case where the reflectivity R is large. It is found that if the reflectivity is increased, the halfwidths of the reflection intensity change with respect to the phase difference, i.e., the widths indicated by WH 1 and WH 2 in Fig. 2B are reduced. A change in the above-mentioned phase difference ψ can be expressed by the following equation (5) from equation (3): Δψ = ∂ψ ∂n M Δn M + ∂ψ ∂λ Δλ + ∂ψ ∂d Δd According to this new multivalued recording method, multivalued information is converted into a refractive index change Δn M and is recorded as a phase difference Δψ on the medium. At the time t of reproduction, a range of wavelength of light is swept and the condition defined by equation (4) is restored at a wavelength corresponding to Δλ which determines a phase difference just cancelling out the phase difference Δψ created by Δn M . That is, when the range of wavelength is swept with respect to time, sharp interference fringes appear at a wavelength matching with the phase difference corresponding to the value of the recorded information, and pulses corresponding to the intensity changes shown in Fig. 2B are thereby obtained as a time waveform. When a wavelength of a laser light is swept within a range of wavelength λ within a period t, pulses corresponding to Δλ are obtained at at time Δt after a start of wavelength sweeping. A position Δt of these pulses on the time base indicates the multiple-value information. The correspondence between the refractive index, wavelength and time is expressed by the following equation (6) when the waveform sweep is linear. Δn M n M = - Δλ λ = - Δt t At the time of the recording mentioned above, the change Δψ in the phase difference expressed by equation (5) is created by controlling Δn M caused by laser heating. At the time of reproduction, light having a variation in its wavelength which cancels out the change in phase difference is introduced and sharp dark fringes are detected. That is, as described above, the range of wavelength is swept, pulses observed as changes in the intensity of light reflected by the medium are detected, and the wavelength corresponding to this detection corresponds to the above phase difference. Since the condition of the appearance of pulses is determined by equation (4), the corresponding wavelength is given by equation (3) and the wavelength satisfying the condition is, of course, discrete in correspondence with the waveform of Fig. 2B. λ m = 2n M d m It is therefore preferred that the change Δψ in the phase difference is a change from ψ m satisfying equation (4) as an initial value. If the initial value is greatly deviated from ψ m , the extent of wavelength sweeping which is required before the appearance of pulses is considerably large, because it is necessary to cancel out the shift of the initial value from ψ m as well as Δψ. From equation (7), λ m+1 - λ m = ( 1 m+1 - 1 m ) 2n M d In a most extreme case where the refractive index n M and the thickness d of the medium layer are reduced so that the interference fringe order m = 1, that is, when the going-and-returning optical path length 2n M d is 1λ 1 from equation (7), λ 2 - λ 1 = ( 1 2 - 1) 2n M d = - 1 2 λ 1 is required to form an interference fringe of m = 2. In this case, therefore, it is necessary to sweep the wavelength λ 1 to half wavelength 1/2λ 1 , and, if the initial value of the phase is at the middle of ψ 2 and ψ 1 , it is necessary to sweep the wavelength by about 50 %. Actually, however, if n M = 5 and d = 0.6 µm, λ m = 0.8 µm when m = 6 or, if d = 1.0 µm, λ m = 0.76 µm when m = 13. It is therefore possible to effect matching between the range of wavelength which can be swept in practice and the optical path length n M d of the medium, as described below. That is, the initial value ψ INT of the phase difference may be set so that, from equations (3) and (4), ψ INT = 2πm = 4πn M d λ m Also, the medium may be formed so that the optical path length is in the vicinity of (n M d) satisfying equation (11): n M d = m λ m 2 To be exact, the meaning of setting in the vicinity of this value is as described below. Ordinarily, the change in refractive index caused by heating is either positive or negative depending upon the kind of medium. The initial value (nM,d) INT is previously shifted in accordance with the direction and range of change in refractive index after multiple-value information recording to conform to the direction and range of possible wavelength sweeping. An example of an information recording/reproduction apparatus using the above-described optical information recording medium will be described below with reference to Fig. 3. The optical system shown in Fig. 3 includes a recording medium 1 on which multiple-value information is recorded as described above, a semiconductor laser 2 which is a first light source for recording information, a collimator lens 3 for making laser light from the semiconductor laser device 2 parallel, another semiconductor laser 4 which is a second light source for reproducing information, a collimator lens 5 for making laser light from the semiconductor laser 4 parallel, beam splitters 6 and 7, a pickup lens 8, a sensor lens 9, and a photodetector 10. Beams of light converted into parallel light by the collimator lenses 3 and 5 are combined by the beam splitter 6. The medium 1 is driven by a motor (not shown) and is scanned with the light beam supplied from the light sources. The recording semiconductor laser 2 is a laser having a comparatively large output and is driven by a recording laser drive drive circuit 31. Words represented by multiple-bit (e.g., 8-bit) binary signals are successively input through a terminal 30. Input words are previously converted into corresponding signal levels in a modulation circuit 32 in accordance with a predetermined table. For example, if each word consists of 8 bits, there are 256 types of words, and a recording signal modulated to 256 levels in correspondence with those words is output from the modulation circuit. These recording signals are input into the recording laser drive circuit 31, and the semiconductor laser 2 outputs a recording beam having intensities in accordance with the levels of the signals. The recording beam is made parallel by the collimator lens 3, transmitted through the beam splitters 6 and 7, and converged as a small spot on the medium layer of the medium 1. The medium layer is heated by being irradiated with this recording beam so that the signal is recorded as a sequence of small recording regions changed in reflectivity. The small recording regions have refractive indices corresponding to the intensities of the irradiated light beam. That is, one word is recorded in one recording region. Outputting the signal from the modulation circuit 32 is effected in certain cycles based on a clock signal output from a clock signal generation circuit 33. Accordingly, the small recording regions are formed on the recording medium 1 at equal pitches in the beam scanning direction. A laser having a comparatively small output is used as the reproducing semiconductor laser 4. The semiconductor laser 4 is driven by a semiconductor laser drive circuit 34. The semiconductor laser drive circuit 34 sweeps the wavelength of the reproducing beam output from the semiconductor laser 4 in certain cycles in synchronization with the clock signal supplied from the clock signal generation circuit 33. This wavelength sweeping can be performed by using, for example, an ordinary double hetero structure (DH) laser and by changing a current injected into this laser. If a tandem electrode type variable wavelength semiconductor laser proposed in Japanese Patent Laid-Open No.63-32985 and so on is employed, it is possible to increase the range and the speed of sweeping. The clock signal generation circuit 33 synchronizes the small recording regions of the medium with the clock signal by, for example, detecting synchronization marks previously recorded on the medium. On the basis of the clock signal, the semiconductor laser 4 is controlled so as to effect wavelength sweeping at least one time within each recording region. The reproducing beam output from the semiconductor laser 4 is reflected by the beam splitter 6, transmitted through the beam splitter 7 and converged as a small light spot on the medium layer of the medium 1. Light reflected at the medium so as to be modulated in accordance with the recorded information is separated from the incident light from the laser by the beam splitter 7, condensed by the sensor lens 9, and received by the photodetector 10. An output from the photodetector 10 is input into a peak detection circuit 35, and a peak of the detection signal is detected therein. A clocking circuit 36 measures, based on the clock signal supplied from the clock signal generation circuit 33, the time between the time point at which wavelength sweeping with the reproducing beam is started and the time point at which a detection signal peak is detected. In accordance with the present invention, as described later, information recorded as changes in the refractive index of the medium layer of the medium 1 is detected by utilizing the optical multiple interference effect, i.e., by being converted into changes in wavelength at which point multiple interference occurs. In the apparatus shown in Fig. 3, the wavelength is swept at a certain rate, and each change in wavelength therefore appears as an elapse of time from the start of sweeping. Accordingly, an information reproduction circuit 37 reproduces the information from the times measured by the clocking circuit 36 and outputs the reproduced information through a terminal 38. The light spot focus positions of the recording light beam and the reproducing light beam are controlled three-dimensionally by autofocusing/autotracking control, although the means for this control is not illustrated. The optical outputs from the above-mentioned two semiconductor lasers elliptically distributed may be converted by a beam shaping prism or the like so as to be generally circularly distributed. In the above-described recording/reproduction apparatus, multiple-value information is recorded by heating the medium with the light from the recording semiconductor laser 2 so that the refractive index n M thereof is changed. In accordance with the present invention, changes in the refractive index are recorded in correspondence with changes in the phase difference of the etalon unlike the conventional method based on the principle of recording utilizing changes in reflectivity caused by changes in the refractive index. For reproduction, the wavelength of the reproducing semiconductor laser 4 is swept, and changes in the wavelength corresponding to the recorded phase difference changes are detected as changes of optical pulses with respect to time by the photodetector 10. Figs. 4A to 4D are diagrams of timing of an information reproducing operation based on wavelength sweeping described above. Fig. 4A shows the state of arrangement of small recording regions (hereinafter referred to as cells) in the direction of a track on the recording medium 1. In this example, cells Ci (i = 1, 2 ...) are arranged in a row in the direction of a track at equal pitches spatially or with respect to time. Cells referred to herein correspond to pits of compact disks or domains of opto-magnetic disks and are basically those used for conventional mark interval recording or mark length recording. That is, the distance between the cells and the length of the cells are unrelated to recorded information. Fig. 4B shows a waveform 16 of sweeping the wavelength of the reproducing semiconductor laser 4, the abscissa representing the time, the ordinate representing the wavelength. The wavelength sweeping waveform 16 is a saw-tooth waveform as illustrated, and has a linear time-wavelength relationship. The wavelength can therefore be detected by measuring the time, so that the construction of the reproduction system can be simplified. Fig. 4C shows the intensity of reflected light used for reading the information from the cells shown in Fig. 4A with a light beam wavelength-swept like the waveform 16 of Fig. 4B, i.e., an waveform 17 output from the photodetector 10. Pulses shown in Fig. 4C are output at wavelengths cancelling recorded phase difference Δψ, and correspond to pulses shown in Fig. 2. The times at which the pulses are output are t ci (i = 1, 2, 3). These times indirectly represent wavelengths. The time t i (i = 1, 2, 3) of wavelength λs corresponding to the initial value is also obtained. The wavelength satisfying the equation of the interference fringe condition (6) with respect to each cell can be obtained by calculating Δt = t ci - t i . The recorded phase differences and changes in refractive index can be obtained by using equations (3) and (6) to reproduce the recorded multiple-value information. Fig. 4D shows a pulse waveform 18 formed from t i and t ci . Each pulse width Δt represents multiple-value information recorded in each cell. As shown in Fig. 4B, the effective range of wavelength sweeping is λs ≤ λ ≤ λe. That is, since the change in wavelength is steep and unstable in the vicinity of peaks and troughs of the wavelength sweeping waveform 16, some marginal portions of the sweeping wavelength range are not used. In this embodiment, a recording medium whose refractive index is reduced by heating is used, the wavelength corresponding to the initial value n MINT of the refractive index and satisfying the equation of the interference fringe condition (1) is λs, and an equation: n MINT d = m·λ s /2 is satisfied. The change in phase difference caused by heating is, from equations (3) and (5), Δψ = 4πd λ m Δn M with respect to λ m . Therefore the phase difference after heating is, from equations (10) and (13), ψ' = ψ INT + Δψ = 4πd λ m (n MINT + Δn M ) = 4πd λ m n M ' A wavelength λ m ' which cancels this change in phase difference and at which another peak of interferences fringes appears is, from equations (3) and (4), 2πm = ψ' = 4πn M 'd λ m ' The change in wavelength is, from equations (11) and (15), Δλ m = - 2d m Δn M having a sign opposite to that of Δn M , as can also be understood from equation (7). The wavelength with respect to the maximum |Δn M | is λ e . Consequently, it is possible to cancel out the maximum reduction |Δn M | at λ e during heating recording by setting n MINT , d, and λs as initial values. Thus, the recording medium has a refractive index and a thickness selected to satisfy the condition n MINT d = m·λ s /2 so that the initial value y INT = 2πm by considering the direction of change of the refractive index with respect to the wavelength sweeping range λ s ≤ λ ≤ λ e , i.e., the range of refractive index changes n MINT ≤ n M ≤ n Me . The need for an unnecessary wavelength sweep is thereby eliminated and the process of reproducing multiple-value information can be optimized. In the above-described embodiment, a reflection type of recording medium is used but, needless to say, a transmission type recording medium is also applicable. If a transmission type medium is used, the photodetector 10 shown in Fig. 3 is arranged to receive light transmitted through the medium 1. Figs. 5A to 5C are time charts of a second embodiment of the information reproduction method of the present invention. A medium and an apparatus constructed as shown in Figs. 1 and 3 are also used in this embodiment. This embodiment is arranged so that the range of wavelength sweeping of reproducing light includes two wavelengths at which multiple interference occurs. This method makes it possible to reduce jitter as described later and to reproduce information with improved reliability even if the absolute value of the reproducing light wavelength varies. Fig. 5A schematically shows an array of cells of recording medium 1. Fig. 5B shows a waveform of sweeping the wavelength of reproducing semiconductor laser 4. This waveform is a sawtooth waveform such that the wavelength is swept linearly in synchronization with the pitch of each cell. The time t c is a time for reading of each cell. Information items in the cells are successively read out. The waveform indicated by a solid line 19 in Fig. 5B is a sweeping waveform in a case where the wavelength at which wavelength sweeping is started is shifted at the time of reading from the third cell C 3 , and the waveform indicated by a broken line 20 is a normal sweeping waveform free from any wavelength shift. Fig. 5C shows the intensity of reflected light from the etalon when the information is read out of the cells with a light beam wavelength-swept as shown in Fig. 5B. The photodetector 10 outputs a corresponding waveform. Reading from the first cell C 1 will be described below with specific reference to Fig. 5C. In this case, two pulses which correspond to a value of multiple-value information appear respectively times 1 t m+1 , and 1 t m after the moment at which wavelength sweeping is started. The reproducing wavelengths at these times are 1 λ' m+1 and 1 λ' m as indicated in Fig. 5B. From equations (3) and (4), if λ'm = 1 λ' m, and λ' m+1 = 1 λ' m+1 at the peak positions of the two pulses with respect to the medium having the refractive index changed by Δn M for recording, the following equations (17) to (20) are satisfied: ψ' m = ψ m + Δψ m 4π λ' m d(n M + Δn M ) = 4π λ' m dn' M ψ' m+1 = ψ m + Δψ m+1 = 4π λ' m+1 d(n M + Δn M ) = 4π λ' m+1 dn' M ψ' m = 2πm ψ' m+1 = 2π(m+1) That is, the two pulses correspond to interference fringes of the orders m and m+1. The wavelength difference therebetween is expressed by equation (20) from equations (17) and (20): λ' m - λ' m+1 = 2n' M d m - 2n' M d m+1 = 2n' M d m(m+1) = λ' m λ' m+1 2n' M d These equations are of course established before the change of Δn M , i.e., with respect to ψ m , n M , and λ m . In equations (17 to (20), the conditions of equations (19) and (20) are satisfied by providing a wavelength change Δλ m cancelling the change Δn M of the new n' M . If λ' m = λ m + Δλ m , that wavelength change corresponds to the refractive index change Δn M obtained by equations (22) and (23): Δλ m = 2dΔn M m = λ m Δn M n M Δλ m+1 = 2dΔn M m+1 = λ m+1 Δn M n M If the wavelength sweeping is linear with respect to time, and if the sweeping rate is "a", the relationship defined by the following equation (24) is satisfied by a change in the pulse position due to the refractive index change Δn M , which change is expressed by the following equations (25) and (26): Δλ = aΔt Δt m = 1 a λ m Δn M n M Δt m+1 = 1 a λ m+1 Δn M n M Accordingly, a change Δt W in the distance t W between the peaks of two pulses is expressed by and equation (27) from equation (21). The pulse interval change Δt W is expressed by the relative value, i.e., the difference between the two wavelengths and is, hence, expressed by the order alone. Δt W = Δt m - Δt m+1 = 1 a Δn M n M (λ m - λ m+1 ) = 1 a 2dΔn M m(m+1) Consequently, it is possible to reproduce the given phase difference, i.e., recorded information by detecting the pulse interval t W and by calculating the change Δt W . Information recorded in the cell C 2 is different from that in the cell C 1 , and two pulses for the cell C 2 appear at times 2 t m and 2 t m+1 different from those in the case of the cell C 1 , i.e., at different wavelengths 2 λ' m and 2 λ' m+1 . The change Δt W in the pulse interval expressed by equation (27) is also different. Thus, recorded multiple-value information can be detected from the time distance between the two pulses. The next cell C 3 has the same recorded information as the cell C 2 , but a wavelength sweeping waveform 19 is shifted from a normal waveform 20 as shown in Fig. 5B. Accordingly, the wavelengths 2 λ'm and 2 λ' m+1 at which two pulses appear are the same as those in the case of the cell C 2 but the times 3 t m and 3 t m+1 are different from 2 t m and 2 t m+1 of C 2 . That is, if the waveform of sweeping the wavelength of the reproducing semiconductor laser 4 is dispersed, the positions of pulses corresponding to information are also dispersed. For example, in the case of a reproduction method wherein the absolute value of the pulse generation time from the wavelength sweeping start time corresponds to the absolute wavelength λ' m cancelling the recorded phase difference Δψ m , the dispersion of the wavelength sweeping waveform directly appears as jitter components of pulses, so that the reliability of data is reduced. In contrast, if information is reproduced based on the time interval between two pulses as in the case of the embodiment of the present invention, there is no variation in the change Δt W even if the absolute value of the wavelength is shifted as in the case of the waveform 19 shown in Fig. 5C, provided that the rate a of wavelength sweeping, i.e., the inclination is not changed, since Δt W is determined by the relative difference between two wavelengths as expressed by equation (27). It is therefore possible to reproduce information without reducing the reliability of the reproduced information, even when the wavelength of the reproducing laser is shifted. For multivaluing of, for example, 8 bits per cell in accordance with the present invention, multiple-value information of 256 levels is recorded on the medium as mentioned above, and the necessary resolution is 1/256 or higher. In this case, assuming that n M = 5; the resolution of n M is 10 -4 ; the range of n M is 3 × 10 -2 ; the thickness d = 0.6 µm; and, if orders m = 6 and m+1 = 7 are used, λ m = 0.80 µm λ m+1 = 0.69 µm from equations (17) to (20), the resolution and range of these wavelength are about 0.2 nm and 5 nm, as determined from equation (6). If the electrical time resolution is 10 nsec., the range is about 2.6 µsec (about 400 kHz). The wavelength sweeping rate a is about 40 nm/µsec. If d = 1.0 µm and m = 13, the wavelength sweeping width is reduced as λ m = 0.76 µm and λ m+1 = 0.71 µm. If the cell size is 1 µm, the relative speed of the spot and the medium is about 0.4 m/sec., lower than that of ordinary optical disks by one place. However, since 8-bit information is recorded in one cell, the transfer rate is about 3 Mbps. Accordingly, if the time resolution and the wavelength sweeping speed are improved, the degree of multivaluing and the transfer rate can be further increased. At the falling portions of the wavelength sweeping waveform 19 shown in Fig. 5B, short pulses are also generated at the corresponding wavelengths, which pulses, though not illustrated, appear in the waveform shown in Fig. 5C. However, such pulses can easily be removed by using a filter for filtering the output from the photodetector 10 or by inserting a gate operating in synchronization with the rise and fall of the sweeping waveform. In the above-described embodiment, two pulses are detected with respect to one cell, but the number of pulses is not limited to two. The intervals between three or more pulses may be detected. In such a case, it is possible to use the two intervals of pulses corresponding to arbitrary orders as well as the interval of pulses of successive orders to reproduce information, and a plurality of pulse intervals may be combined for a further improvement in reliability. Figs. 6A and 6B are time charts of a third embodiment of the information reproduction method of the present invention. A medium and an apparatus constructed as shown in Figs. 1 and 3 are also used in this embodiment. In this embodiment, the wavelength of the reproducing light is changed with respect to the outgoing and returning paths. Each of the operations of sweeping from a shorter wavelength to a longer wavelength and sweeping from the longer wavelength to the shorter wavelength is effected one time during transmission of the reproducing light through one cell. Fig. 6A shows a waveform of sweeping the wavelength of reproducing semiconductor laser 4. Time t 0 is a time for reading one cell. C 1 to C 3 indicate cells defined as small recording regions arranged in the direction of a track. Information items in the cells are successively read out. Information is reproduced in the rising range of a saw-tooth wave 72 shown in Fig. 6A. The wavelength is swept by this saw-tooth wave so that the relationship between the time and the wavelength is linear, thereby simplifying conversion of wavelengths. An operation of converting wavelength information into times is adopted as well as the specific process of the present invention in which recorded phase difference information is converted into wavelengths, thereby enabling the reproduction system to be simplified. A waveform 73 shown in Fig. 6B is an example of the waveform of reflected light from the etalon in a case where information is read out of the cells by the wavelength sweeping waveform shown in Fig. 6A, i.e., an example of the waveform output from the photodetector 10. With respect to the first cell C 1 , a pulse P 1 appears which corresponds to an initial wavelength λ at a low reflectivity, i.e., a sharp dark fringe of the etalon, satisfying equation (4). Since the time between the moment at which the wavelength sweeping saw-tooth wave starts rising and the moment at which the pulse peak appears is 0 sec., this period of time is measured to detect the state in which the wavelength is λ, that is, wavelength change Δλ = 0, phase difference change Δψ = 0, and refractive index variation Δn M = 0. In reading the second cell C 2 , the moment at which the peak of pulse P 2 appears is delayed by t 1 from the moment at which the saw-tooth wave starts rising. The wavelength λ 1 of the reproducing laser can easily be obtained since the sweeping is linear, and Δλ = λ 1 - λ. Then, a phase difference variation Δψ' can be obtained by the following equation: Δψ' = - 4πn M 'd λ 2 Δλ = - 4πn M d λ 2 Δλ - 4πd λ 2 ΔnΔλ ≅ - 4πn M d λ 2 Δλ Assuming that Δψ = Δψ' from equations (13) and (28), the relationship expressed by the following equation is established: Δλ λ = - Δn n M That is, it is possible to detect low-reflectivity peaks with the detector by shifting the wavelength by Δλ. Setting with the refractive index variation Δn M is effected based on the above-mentioned equations (13) and (14). With respect to the third cell C 3 , time t 2 before the appearance of a pulse P 3 is measured and the corresponding wavelength, phase and refractive index are obtained in the same manner. The refractive index of the medium layer is changed in accordance with multiple-value information so that the multiple-value information is recorded as phase differences. For reproduction of the multiple-value information, the phase differences are converted into changes in wavelength to obtain a high-contrast signal, and detection of wavelengths is converted into detection pulses on the time base, thereby enabling enabling the system to be simplified. The relationship between the change in the refractive index of the medium and the final change in wavelength is given as equation (29). Further, the inclination of the rising portion of the saw-tooth wave 72 shown in Fig. 6A is given as equation (30), and the equation (29) is then expressed by equation (31). a = λ t Δn M n M = - Δλ λ = - Δt t Thus, the relationship between the change in refractive index given as multiple-value information and the time interval given as multiple-value information is obtained. For multivaluing of, for example, 8 bits per cell, multiple-value information of 256 levels is recorded on the medium as mentioned above, and the necessary resolution is 1/256 or higher. Assuming that the refractive index is 4; and resolution of the refractive index is 10 -4 ; and the range is 3 × 10 -2 , the corresponding wavelength resolution and range are about 0.2 nm and 5 nm, respectively, with respect to λ = 830 nm, as determined from equation (31). If the electrical time resolution is 10 nsec., the range is about 2.56 µsec (about 400 kHz), and the wavelength sweeping rate is about 2 nm/µsec as determined from equation (30). If the cell size is 1 µm, the relative speed of the spot and the medium is about 0.4 m/sec., lower than that of ordinary optical disks by one place. However, since 8-bit information is recorded in one cell, the transfer rate is about 3 Mbps. Optical power necessary or recording is small according to the small relative speed. Needless to say, if the time resolution and the wavelength sweep speed are improved, the degree of multivaluing and the transfer rate can be further increased. As shown in Fig. 6B, short pulses P 4 are generated at falling portions of the saw-tooth wave of Fig. 6A, because each cell is read during both going and returning of wavelength sweeping. However, such pulses may be removed by using a pulse gate circuit or the like to avoid occurrence of problems. Figs. 7A to 7B are time charts of the reproducing operation of a fourth embodiment of the information reproduction method of the present invention. In this embodiment, certain spacings are provided between recording cells to solve the above-mentioned problem of crosstalk and to set a reference for multiple-value information. Non-recording regions, i.e., regions in which the initial refractive index is maintained are defined in the spacings. In Fig. 7A, such non-recording regions are shown as regions R 1 to R 3 . In this embodiment, for reproduction of information, each information cell is read during the time t 0 for rising of the corresponding one of saw-tooth waves 74 shown in Fig. 7A, and each of reference cells provided between the information cells is read during the time t 0 ' for falling of the wave. A reference pulse P r corresponding to initial information (e.g., denoting naught) is thereby obtained at the wavelength corresponding to the end of each saw-tooth wave, as indicated in a waveform 75 shown in Fig. 7B. Then, times t 1 and t 2 between these pulses and information pulses P D are detected to enable correction of indefinite factors including dispersions of mediums, a spatial dispersion in one medium, a change with time, and a dispersion of the wavelength of the reproducing semiconductor laser 4. In this embodiment, the reliability of data can be improved remarkably although the recording density is reduced to some extent. Fig. 8A and 8B show a fifth embodiment of the information reproduction method the present invention in which the method described with reference to Figs. 7A and 7B is applied to a transmission type medium. If a transmission type medium is used, the complicacy of the apparatus is increased to some extent because of, e.g., a specific arrangement relating to the disposition of the detector and the movement of the detector in synchronization with the optical head. However, upward pulses such as those indicated in a transmission waveform 77 shown in Fig. 8B are basically advantageous in terms of errors due to medium defects, because defects of the medium, scratches, particles, contamination and the like usually absorb or scatter the light and cause downward pulse waveforms in the output from the detector. Other characteristics are the same as the above-described embodiments. Figs. 9A to 9E are time charts of a sixth embodiment of the information reproduction method of the present invention. A medium and an apparatus constructed as shown in Figs. 1 and 3 are also used in this embodiment. In this embodiment, reference cells in which reference information is recorded are provided between cells in which data is recorded. Fig. 9A shows the arrangement of cells in the direction of a track on the recording medium 1. In this example, cells Ci and reference cells Ri are arranged alternately. Reference cells Ri are thus provided between information cells Ci, and are defined as regions where the initial refractive index is maintained to obtain a reference value for multiple-value information. It is thereby possible to reduce non-uniformities of the thickness and the refractive index of the recording medium caused by a change with time or a temperature change, or to jitters due to dispersions of the thickness and the refractive index caused at the time of manufacture, and to thereby improve the error rate. Fig. 9B shows a waveform 22 of sweeping the wavelength of reproducing semiconductor laser 4, the abscissa representing the time, the ordinate representing the wavelength. The wavelength sweeping waveform 22 is a saw-tooth wave swept with respect to each of information cells Ci and reference cells Ri. The relationship between the time and the wavelength is made linear by using such a saw-tooth wave for wavelength sweeping. The wavelength can therefore be detected by measuring the time, thus simplifying the wavelength detection. Also, an operation of converting wavelength information into times is adopted as well as the specific process of the present invention in which recorded phase difference information is converted into wavelengths, thereby enabling the reproduction system to be further simplified. Fig. 9C shows a waveform 23 of the intensity of reflected light from the etalon, i.e., a waveform output from the photodetector 10 when a spot of a light beam whose wavelength is changed in accordance with the waveform 22 is scanned in the direction of arrangement of the cells. The abscissa represents the time while the ordinate represents the intensity of reflected light (the value of output from the photodetector 10). The initial value is recorded in each reference cell Ri as described above. The etalon has a low reflectivity (sharp dark fringe) at the wavelength λ = λ R such as to satisfy equation (4). Pulses therefore generate at the wavelength λ = λ R , as shown in Fig. 9C. The times at which these pulses generate are t = t Ri = t Ri '. With respect to each information cell Ci, a pulse is generated at the wavelength λ cancelling the recorded phase difference Δψ' in accordance with equations (28) and (29). The time at which this pulse is generated is t = t Ci . The wavelength shift Δλ = λ Ci - λ R can be obtained by detecting Δt = t Ci - t Ri ', or Δt i = t Ci - t Ri or Δt i = t Ri+1 - t Ci . The correspondence between the recorded phase difference Δψ' and the refractive index change Δn M is then determined to reproduce the recorded information. However, the change in the wavelength of the reproducing semiconductor laser 4 is steep in regions in the vicinity of the peaks and troughs of the saw-tooth wave, and stability of the wavelength is low in these regions. Moreover, the peak and trough points correspond to the boundaries between the information cells and the reference regions, and the crosstalk from the information cell is large at each point. For this reason, the reference times t R i and t Ri ' obtained from the reference cells are easily dispersed so that jitters in the reproduced signal are increased. Figs. 9D and 9E are diagrams of a method proposed to solve this problem. Fig. 9D shows a waveform 24 of sweeping the wavelength of reproducing semiconductor laser 4, and Fig. 9E shows a waveform 25 of an output from the photodetector 10 (the intensity of reflected light) at the time of scanning in accordance with the waveform 24. In this embodiment, as is clear from Fig. 9D, the wavelength sweeping range is increased so that the reference wavelength λ R is included in the wavelength sweeping range. By this wavelength sweeping, the reference wavelength λ R (t = τ Ri ) at which reference pulses are output correspond to a generally central position in each reference cell. That is, the reference pulses are output not at the boundaries between the cells but in the range corresponding to the reference cell regions, and the desired data can be read from regions of the reference cell where the influence of the crosstalk from the information cells is small. From the information cells Ci, pulses (t = τ Ci ) corresponding to the recorded information are obtained, and multiple-value information in which jitters are limited can be reproduced by calculating Δτ = τ Ci - τ Ri or Δτ = τ Ri+1 - τ Ci . In the method of Fig. 9D and 9E, there is a need to increase the range of wavelength sweeping in comparison with method of Figs. 9A and 9B, and the wavelength sweeping rate or the reading time is therefore increased to some extent. This method, however, enables an improvement in the reliability of information reproduction. Figs. 10A to 10C are time charts of a seventh embodiment of the information reproduction method of the present invention. A medium and an apparatus constructed as shown in Figs. 1 and 3 are also used in this embodiment. The embodiment of Figs. 9 is arranged to increase both the wavelength sweeping rate and range, but the characteristics of this sweeping is mainly determined by the wavelength sweeping characteristics of the reproducing semiconductor laser 4. For this reason, in some cases, it is difficult to increase both the rate and range. The embodiment shown in Figs. 10A to 10C is arranged to solve this problem. In this embodiment, it is not necessary to increase the sweeping rate, while extension of the wavelength sweeping range is required. Fig. 10A shows the arrangement of information cells Ci and reference cells Ri on recording medium 1. This arrangement is the same as those of the above-described embodiments. Fig. 10B shows a waveform 24 of sweeping the wavelength of reproducing semiconductor laser 4. The wavelength at which sweeping for the information cells is started and the wavelength at which sweeping for reference cells is started are different. That is, with respect to reference cells Ri, sweeping is effected so that the middle point of the wavelength sweeping range correspond to reference wavelength λ R . A reference pulse is thereby output when the center of each reference cell Ci is scanned (time t = τ Ri ). It is thereby possible to reduce the crosstalk from the information cells and to reproduce the reference signal in a sweeping range where the stability of the wavelength is high. With respect to information cells Ci, the same wavelength sweeping as that shown in Fig. 9D is effected according to the range of the multiple values of the recorded information. The effective wavelength sweeping time of this wavelength sweeping can be extended in comparison with that of the sweeping shown in Fig. 9D. Correspondingly, the accuracy with which information cell pulses are detected can be improved. Fig. 10C shows a signal waveform 27 obtained from pulses formed by reading the information from the cells with a wavelength-swept light beam. The rise of the signal waveform 27 corresponds to τ Ri obtained from reference cell Ri, and the fall corresponds to τ Ci of multiple-value information obtained from information cell Ci. Consequently, the information can be reproduced from Δt = τ Ci - τ Ri or Δt = τ Ri+1 - τ Ci . That is, since pulses at Δt represent the information, the information can be reproduced from each cell by the above-described operations. To be exact, some shifts, e.g., variations in the reference cell size and time-lags are included in Δt, and the multiple-value information is reproduced based on equations (3) and (31) while correcting such shifts. Fig. 11 is a schematic diagram of a second embodiment of the optical information recording/reproduction apparatus in accordance with the present invention. In Fig. 11, the same components as those shown in Fig. 3 are indicated by the same reference characters, and the description for them will not be repeated. The apparatus shown in Fig. 11 is different from the embodiment shown in Fig. 3 in that a dispersing prism 40 is placed in front of the sensor lens 9, and that the photodetector 10 is replaced with a unidimensional charge-coupled device (CCD). In this embodiment, the light receiving system is constituted by a spectroscopic system so that the wavelength is spatially developed and separated. Occurrence of jitters is thereby prevented even if the wavelength sweeping waveform varies, as explained later. Figs. 12A to 12F are diagrams of an eighth embodiment of the information reproduction method of the present invention using the apparatus shown in Fig. 11. Fig. 12A shows the state of arrangement of cells in the direction of a track on the recording medium 1. Fig. 12B shows a waveform of sweeping the wavelength of reproducing semiconductor laser 4, the abscissa representing the time, the ordinate representing the wavelength. In Fig. 12B, a wavelength sweeping waveform 57 having a variation in the inclination, a non-linear undulation and a parallel shift is illustrated along with an ideal wavelength sweeping waveform 58 indicated by the broken line. A saw-tooth wave is used as the wavelength sweeping waveform 57. This is because it is a simplest and convenient sweeping wave, and because, since the relationship between the time and the wavelength is linear, the wavelength density per unit time is constant so that the wavelength can be distributed generally uniformly when dispersed by a spectrograph. The effective range of wavelength sweeping is set as λs ≤ λ ≤ λe. Some marginal portions of the sweeping wavelength range in the vicinity of the minimum and maximum wavelengths are not used. Fig. 12C shows changes in the intensity of reflected light from the recording medium 1 when the information on the medium is read out with a light beam wavelength-swept by waveforms 57 and 58, the abscissa representing the time, the ordinate representing the intensity of reflected light. The line 50 represents the intensity of reflected light at the time of reading with the light beam swept by the wavelength sweeping waveform 57, and the line 60 represents the intensity of reflected light at the time of reading with the light beam swept by the ideal sweeping waveform 58. Pulse-like changes are caused in the intensity of reflection light at the wavelengths cancelling the recorded phase differences Δψ (t = t i ). These pulses correspond to interference fringes such as those shown in Fig. 2B. If the wavelength varies as in the wavelength sweeping waveform 57 at the time of reading of cells C 3 , C 4 , and C 5 , the times at which optical pulses appear are t 3 ', t 4 ', and t 5 ' which are shifted from the correct pulse appearance times, thereby causing jitters. Fig. 12D shows an example of a drive with a unidimensional CCD 39. To reduce crosstalk between the cells, an exposure time texp shorter than the time for scanning each cell Ci is set. Fig. 12E shows a waveform 22 of an output from the unidimensional CCD 39. The abscissa represents the time, while the ordinate represents the voltage. There are variations in the angle at which the light incident upon the dispersing prism 40 is dispersed at the wavelength changing with time. If this angle is (1); and the focal length of the sensor lens 10 is f, the position x on the imaging plane, i.e., on the unidimensional CCD 39 is expressed by x = f(λ) When the change A(λ) in the angle due to wavelength change Δλ is small, A(λ) oc Δλ can be established. Then, Δx oc fΔλ Thus, the face of the unidimensional CCD 39 is scanned with a light spot by wavelength sweeping. This scanning is repeated for each of the cells Ci shown in Fig. 12A. Scanning with the reproducing light spot over each cell Ci on the recording medium 1 is based on the relative mechanical movement between the medium and the light spot (optical axis), and scanning with the light spot over the unidimensional CCD 39 is effected purely spectroscopically. There is therefore no crosstalk between these scannings in theory and, hence, no cause of jitters. In more detail, the spatial position of the wavelength on the unidimensional CCD 39, i.e., the number of bits on the array sensors of the unidimensional CCD 39 is definitely fixed according to the relationship expressed by equations (32) and (33. Even if the time position of the interference fringe pulses in the reflected light intensity 59 fluctuates, the position of the wavelength on the unidimensional CCD 39 is not changed and the output from the unidimensional CCD 39 does not fluctuate, as shown as an output waveform 62. A reproduction signal free from jitters can therefore be obtained. Fig. 12F shows a pulse waveform 63 obtained from the output from the unidimensional CCD 39 shown in Fig. 12E, which waveform indicate positions of the wavelengths corresponding to the interference fringes. The width of each pulse represents multiple-value information recorded in the corresponding cell. Figs. 13A to 13C are diagrams of a ninth embodiment of the information reproduction method of the present invention using the apparatus shown in Fig. 11. In the above-described embodiments, the range of wavelength of light from the light source is swept to detect wavelengths at which interference fringes occur. In contrast, in this embodiment, a reproducing light source having a spectrum width corresponding to that wavelength range is employed without effecting wavelength sweeping. Since there is no need for wavelength sweeping, the construction of the apparatus can be simplified. As a reproducing light source, a light emitting diode (LED) or a white light source is used instead of the reproducing semiconductor laser 4 shown in Fig. 11. However, it is necessary for the light source to have a sufficiently large spectrum width in order to obtain the desired spectrum width. A band-pass filter for transmission of the desired spectrum may be interposed between the light source and the beam splitter 6. In this arrangement, there is no need to modulate the wavelength of the light source. The light source may by driven by a DC current. In a case where this type of reproducing light source is used, reflected light from the recording medium 1 is dispersed by the dispersing prism 40 as in the case of the above embodiment, and its spectrum is developed on the unidimensional CCD 39 by the sensor lens 9. Figs. 13A to 13C are diagrams of the information reproducing operation of this embodiment. Fig. 13A shows the spectrum of the light source, the abscissa representing the time, the ordinate representing the intensity of light. Fig. 13B shows an example of a waveform 65 of the output from the unidimensional CCD 39, the abscissa representing the time, the ordinate representing the voltage. The dispersing prism 40 and the unidimensional CCD 39 are disposed and adjusted so that light having wavelengths out of the necessary range λ 1 ≤ λ ≤ λ 2 is inhibited from being incident upon the unidimensional CCD 39. In this embodiment, as well as the above embodiment, the number of elements of the unidimensional CCD 39 may be selected in accordance with the degree of multivaluing, e.g., at least 256 bits when 256 levels are required. If in this case spatial sampling is effected superfluously by one wavelength, i.e., N bits per level, the accuracy can be further improved. However, the resulting resolution is limited to the resolution determined by the dispersing prism 40 and the sensor lens 9. The spectral distribution selected with respect to the exposure time texp of the unidimensional CCD 39 appears in the output waveform 65. Interference fringes such as those shown in Fig. 2B exist at wavelengths cancelling recorded phase differences, and corresponding pulses are generated. Fig. 13C shows a pulse waveform 66 which indicates the positions of wavelengths corresponding to the interference fringes and which is obtained from the output waveform shown in Fig. 13B. As in the case of the above embodiment, each pulse width represents multiple-value information of the corresponding cell. Accordingly, the multiple-value information can be reproduced by measuring the pulse width. In this embodiment, the spatial coherence and the time coherence of the light source are ordinarily inferior in comparison with the above embodiment, so that the light spot on the medium is substantially large. There are therefore a possibility of a reduction in spatial resolution and a possibility of a deterioration in visibility of interference fringes. However, since there is no need to sweep the wavelength of the light source, the construction of the apparatus can be remarkably simplified. In the above embodiments, a dispersing optical system is used which includes a dispersing prism, a sensor lens, and a unidimensional CCD. However, this is not exclusive and, for example, a grating can be used in place of the dispersing prism. The grating may be a phase type grating for improving the efficiency of diffraction at a certain order or may be a reflection type grating. It is possible to simplify the apparatus or reduce the size of the apparatus by using a dispersing prism having an imaging function. It is also possible to adopt a recessed surface grating, a grating lens, a hologram or the like. A unidimensional CCD has been described as a sensor but the senor is not limited to this, and a split photodiode having an array of elements or a two-dimensional sensor array can of course be used. In the above-described apparatus or method of the present invention, if a spatial thickness non-uniformity Δd of the medium is included in the phase difference Δψ expressed by equation (5), it finally appears as a jitter pulse component and may cause an error, resulting in a reduction in reliability and a deterioration of the high-density recording performance. According to the present invention, a recording medium having such a thickness non-uniformity Δd can be changed into a recording medium improved in reliability by initialization described below. The change in phase difference due to thickness non-uniformity Δd is expressed by the following equation (34) from equation (3). ψ + Δψ = 4πn M λ (d + Δd) = 4πn M λ d' where d' = d + Δd. In accordance with the present invention, a phase difference is provided as a change in refractive index such as to cancel out this phase difference non-uniformity, thereby enabling the medium to be uniformly initialized so that the phase difference is made uniform. A change Δψ' in phase difference due to a change Δn M in refractive index is expressed by equation (28) from equation (34). Accordingly, from equations (28) and (34) and Δψ - Δψ', equation (35) is obtained: Δd d = - Δn M n M That is, Δn M satisfying equation (35) is given to cancel out Δd. To record multiple-value information, the medium is heated by the light from the recording semiconductor laser 2 so that its refractive index n M is changed. In accordance with the present invention, the change in refractive index is recorded in correspondence with the change in the phase difference of the etalon unlike the conventional method based on the principle of recording using a change in reflectivity due to a change in refractive index. For reproduction, the change in wavelength corresponding to the change in phase difference is detected. In this embodiment, the change in refractive index Δn M cancelling out the thickness non-uniformity Δd expressed by equation (35) is given by heating with recording laser to initialize the medium. It is to be noted here that, ordinarily, thickness non-uniformity Δd is dispersed to positive and negative values while refractive index change Δn M created by heating is either positive or negative. Accordingly, to obtain the initial value (n M 'd')int satisfying the condition (4), which is given by the following equation (36), the value of n M d is previously shifted, during the manufacture process, to the minus or plus direction with respect to equation (36) according to plus/minus of the change in refractive index caused by heating. (n M 'd') int = m λ int 2 As is apparent from the above explanation, the present invention enables not only correction of thickness non-uniformity Δd but also correction of refractive index non-uniformity Δn M . For example, a recording medium in which the change in refractive index caused by heating is positive may be manufactured by using, as a target value, a value of n M d obtained by deducting a value equal to or greater than the total minus deviation of Δ(n M d) including dispersions of various conditions at the time of manufacture and the influence of changes with time. The variation due to Δ(n M d) is expressed by the following equation (37) if Δλ = 0 in equations (3) and (5). Δψ = 4π λ (n M Δd + dΔn M ) This is cancelled out by the phase change Dy' due to the change in refractive index, i.e., Dy' obtained by the following equation (38) Then, from Dy = -Dy', DnM' is obtained by the following equation (39). DnM' may be created by heating. Δψ = 4π λ d'Δn M ' ≅ 4π λ dΔn M ' Δn M ' = - 1 d (n N Δd + dΔn M ) The phase difference Δψ to be corrected can be detected with the reproduction system of the optical head shown in Fig. 3. Figs. 14A to 14C are time charts of the operation of detecting Δψ. Fig. 14A shows a waveform of sweeping the wavelength of the reproducing semiconductor laser 4. A solid line 67 indicates a waveform used for detection of Δψ to be corrected, and a broken line 68 indicates a waveform used for ordinary multiple-value information reproduction. Time t c shown in Fig. 14A is a time for reading each cell, and information items are successively read from cells C 1 to C 3 . At the time of ordinary reproduction, the saw-tooth wave for wavelength sweeping is started at wavelength λ 0 satisfying the condition (4), as indicated by the broken line 68. At the time of initialization, for detection of phase Δψ to be corrected, the saw-tooth wave covering wavelengths on both sides of λ 0 as indicated by the solid line 67 is used because Δψ may be positive or negative as mentioned above. That is, the waveform 68 is shifted in a direction of wavelength to effect wavelength sweeping as indicated by the solid line 16. Fig. 14B shows an example of the waveform of the intensity of reflected light from the etalon when information is read from each cell with a light beam wavelength-swept a shown in Fig. 14A. Changes in the intensity of reflected light are exhibited as pulses in a waveform 69. The output from the photodetector 10 corresponds to this waveform. With respect to the first cell C 1 , Δψ = 0 in equation (37), λ = λ 0 , and the condition (4) is satisfied. A pulse corresponding to that shown in Fig. 2B is generated at time t 0 at which λ = λ 0 . λ = λ 0 is also established at a falling portion of the saw-tooth wave and a pulse is thereby generated. In this case, this pulse is ignored and is not used. With respect to the second cell C 2 , Δψ ≠ 0 in equation (37), Δn M or Δd exists, and the condition (4) is not satisfied when λ = λ 0 but satisfied when λ = λ 0 - Δλ. A pulse is therefore generated at t = t 0 - Δt. Δλ is determined from Δt, Δψ is obtained, and Δn M ' used to correct it is calculated. The relationship therebetween is given by equations (6) and (39). Conversely, with respect to the third cell C 3 , the condition (4) is satisfied when λ = λ 0 + Δλ', and a pulse is generated at t = t 0 + Δt'. By this detection operation, necessary refractive index change Δn M ' for correcting phase difference non-uniformity is obtained for initialization. The corresponding cell is heated in accordance with this value by laser heating using the recording semiconductor laser 2, thereby removing the phase difference non-uniformity of the medium. Thus the medium can be uniformly initialized. More specifically, in the case of a disk-like medium, phase difference non-uniformities of the necessary regions (tracks or sectors) are detected with reproducing semiconductor laser 4 and are stored in a memory. Then, the regions are initialized with recording semiconductor laser 2. In a simplest case, the operation may be such that a phase difference non-uniformity of a track to be corrected is detected during one revolution of the disk and is stored in a buffer or the like, and this track is initialized during the second revolution. If necessary, the result of initialization is verified during the third revolution to improve the reliability. These steps are repeated to complete initialization of the whole surface. It is possible to effect initialization during one revolution by performing steps of spatially shifting the reproducing light spot and the recording light spot in the direction of tracks, detecting, by the leading reproducing spot, a phase difference to be corrected, and effecting initialization by the trailing recording spot. For ease of explanation, the method has been explained on the assumption that the phase difference non-uniformity Δψ expressed by equations (37) and (38) should be reduced to 0. In accordance with the present invention, however, it is essential to reduce the non-uniformity of the medium due to Δn M and Δd caused during manufacture or by changes with time, and it is not specifically necessary to establish Δψ = 0. The initial value may be set to a phase difference such that Δψ = - Δψ' = constant and such that equation (39) is satisfied irrespective of places. This is particularly effective in a situation where the direction of dispersions of thickness non-uniformity Δd or refractive index non-uniformity Δn M with respect to a target value is indefinite so that the phase difference non-uniformity Δψ in accordance with equation (37) is either positive or negative as mentioned above, while refractive index change Δn M caused by laser heating is either positive or negative. For example, if the problem of compatibility is considered, it is not always advantageous to make the initial value λ int expressed by equation (36) and the reading start wavelength λ 0 shown in Fig. 14A coincide with each other because of differences between lots of mediums, materials and recording/reproduction apparatuses. For example, this is true in a case where laser wavelength sweeping characteristics are particularly unstable at an initial rising stage. In such a case, the reference position of reproduction pulses may be shifted with respect to the rise of the sweeping waveform. The present invention, however, is not influenced by such a condition. Erasable mediums are included in the mediums listed above as refractive index changeable mediums. However, needless to say, any medium is applicable in accordance with the present invention irrespective of the erasable type and the post-writing types. In the case of the erasable type, the present invention may be applied to re-initialization of the medium after erasion of information recorded on the medium to improve the reliability of information.

Claims: An optical information reproducing method comprising: directing a light beam (41) onto an optical information recording medium (1) comprising first (12a, 12b; 42) and second (14a, 14b; 44) reflecting layers parallel to and at a predetermined distance from each other, and an information storage layer (13a, 13b; 43) formed between the first (11a, 11b; 42) and second (12a, 12b; 44) reflecting layers, and thereby reproducing the information recorded in said information storage layer (13a, 13b; 43); the method being characterized in that: said information storage layer (13a, 13b; 43) has a plurality of cells (Ci; i=1,2..), each of which has a refractive index of a different value dependent on multiple value information recorded on the medium (1), and the method includes the steps of: irradiating a cell of the recording medium (1) with a light beam (41) whose wavelength changes within a range including at least one wavelength at which multiple interference occurs between the first (12a, 12b; 42) and second (14a, 14b; 44) reflecting layers in the cell (Ci; i=1,2..) to be reproduced; and detecting the wavelength of the light at which multiple interference occurs, to reproduce thereby the information recorded in said cell (Ci; i=1,2..). A method according to Claim 1 including the step of: irradiating the same cell (Ci; i=1,2..) with a plurality of different wavelengths sequentially in order to detect multiple interference. A method according to claim 1, including the steps of: changing the wavelength of the light beam (41) at a predetermined rate; detecting the intensity of light reflected by or transmitted through said medium (1); determining the wavelength of the detected light at which the detected intensity of light shows a peak value so as to determine the wavelength at which multiple interference occurs; and measuring the time between a reference time and the time at which said peak value of the intensity is detected, to reproduce thereby the information. A method according to claim 3, wherein said reference time is the time at which the wavelength of the light beam (41) starts changing. A method according to claim 1, wherein said wavelength includes two wavelengths at which multiple interference occurs in each of the cells (Ci; i=1,2..), and the method includes the steps of: changing the wavelength of the light beam (41) at a predetermined rate, and detecting the intensity of light reflected by or transmitted through said medium (1), wherein the detected intensity of light shows a peak value at the time at which the wavelength of the detected light becomes equal to the wavelength at which multiple interference occurs, and the method also includes the step of: detecting the peak value twice at different times corresponding to two wavelengths at which multiple interference occurs; and measuring the time between the two times at which the peak value is detected, and thereby reproducing the information. A method according to claim 1-5, including the step of moving said light beam (41) and the recording medium (1) relative to each other in the direction of arrangement of said cells (Ci; i=1,2..), and reproducing the information in each cell (Ci; i=1,2..). A method according to claim 2, wherein said information storage layer includes, a plurality of reference cells (Ri; i=1,2..) having a refractive index corresponding to reference information and a plurality of data cells (Ci; i=1,2..) having different refractive indices according to the information the reference cells (Ri) and data cells (Ci) being alternately arranged along the layer in an array, said method comprising the further steps of: scanning the medium (1) with a light beam in the direction of the array of cells (Ri,Ci) at a predetermined speed; changing the wavelength of the light beam (41) within a predetermined first range at a predetermined first rate during the period of time when one of the' reference cells (Ri) is irradiated with the light beam, the first range including a wavelength at which multiple interference occurs between the first (12a, 12b; 42) and second (14a, 14b; 44) reflecting layers with respect to the reference cell (Ri); changing the wavelength of the light beam within a predetermined second range and at a predetermined second rate during the period of time when one of the data cells (Ci) is irradiated with the light beam (41), the second range including a wavelength at which multiple interference occurs between the first (12a, 12b; 42) and second (14a, 14b; 44) reflecting layers with respect to each data cell (Ci); detecting the intensity of light reflected by or transmitted through the medium (1), the intensity of light detected being peaked at the moment when the wavelength of the detected light becomes equal to the wavelength at which multiple interference occurs, the peak value of the intensity being detected during each of the period of time when the reference cell (Ri) is irradiated with the light beam and during the time when the data cell (Ci) is irradiated with the light beam (41); measuring the period between the time at which the peak value is detected with respect to the reference cell (Ri) and the moment at which the peak value is detected with respect to the data cell (Ci); and reproducing information recorded in the data cell (Ci) in accordance with the time measured. A method according to claims 1-7, wherein said information storage layer (13a, 13b; 43) of the recording medium (1) comprises a material whose refractive index changes by being irradiated with the light beam. A method according to claim 8, wherein said information storage layer (13a, 13b; 43) comprises a material selected from the group consisting of Te0 x , InSeTlCo, GeTeSbTl, GeTeSe, an anthraquinone dielectric, a dioxazine compound and a triphenodithiazine compound, where x is a positive integer. A method according to claim 8 or 9, wherein each of said cells (Ci) is formed by modulating the intensity of the light beam (41) according to the information; and irradiating the information storage layer (13a, 13b; 43) with the light beam (41). A method according to claims 1-10, wherein each of the first (12a, 12b; 42) and second (14a, 14b; 44) reflecting layers of said recording medium is formed of a dielectric multilayer film formed by alternately laminating a dielectric layer having a large refractive index and another dielectric film having a small refractive index. A method according to claim 11, wherein said dielectric layer comprises a material selected from the group consisting of Si0 2 , Si 3 N 4 , MgF 4 and Al 2 0 3 . A method according to claims 1-12, wherein the product of the refractive index of each of said cells (Ci; i=1,2..) and the thickness of the information storage layer (13a, 13b; 43) is almost equal to an integer multiple of half of either of said lights of different wavelengths. A method according to any one of the preceding claims in which said first (12a, 12b; 42) and second (14a, 14b; 44) reflecting layers are formed on a base (11a, 11b). An apparatus for reproducing information from an optical information recording medium (1) comprising first (12a, 12b; 42) and second (14a, 14b; 44) reflecting layers parallel to and at a predetermined distance from each other, and an information storage layer (13a, 13b; 43) which is formed between the first and second reflecting layers (12a, 12b; 42; 14a, 14b; 44), the apparatus comprising means (4) for irradiating the medium (1) with a light beam (41) and means (35) for detecting the intensity of light reflected by or transmitted through the medium (1), the apparatus being characterised in that said information storage layer (13a, 13b; 43) has a plurality of cells (Ci; i=1,2..), each of which has a refractive index of a different value dependent on multiple value information recorded on a medium (1), and said light beam (41) includes lights of a plurality of wavelengths including at least one wavelength at which multiple interference occurs between the first (12a, 12b; 42) and second (14a, 14b; 44) reflecting layers in each of the cells (Ci; i=1,2..) and the apparatus includes wavelength detecting means (33, 36, 38) for detecting the wavelength at which multiple interference has occurred out of the said plurality of wavelengths from the output of said detecting means (35) and thereby reproducing the information recorded in the cells (Ci; i=1,2..). An apparatus according to claim 15, wherein said light beam irradiation means (4, 34) is arranged to change the wavelength of the light beam (41) within a predetermined range and at a predetermined rate, and said wavelength detecting means (33, 36, 38) is arranged to measure the period between a reference time and the time at which the intensity of the light transmitted through or reflected by said medium (1) shows a peak value and thereby reproduce the information recorded in the cells (Ci; i=1,2..). An apparatus according to claim 16, further comprising means (33) for generating a clock signal (33), wherein said light beam irradiation means (4, 33, 34) is arranged to start changing the wavelength simultaneously with the input of the clock signal, and said wavelength detecting means (33,36,38) is arranged to start measuring the period simultaneously with the input of the clock signal. An apparatus according to claim 15, wherein said light beam irradiation means is arranged to change the wavelength of the light beam (41) through a range including two wavelengths at which multiple interference occurs between the first and second reflecting layers with respect to each cell (Ci; i=1,2..), and said wavelength detecting means (33, 36, 38) is arranged to measure the period between the two times at which the intensity of the light transmitted through or reflected by said medium (1) shows a peak value and to reproduce thereby the information recorded in the cells (Ci; i=1,2..). An apparatus according to claim 15, wherein said light beam irradiation means (4) is arranged to irradiate the same cell (Ci; i=1,2..) with light having a plurality of different wavelengths at the same time. An apparatus according to any one of claims 15-19, comprising an optical element (40) for introducing the reflected or transmitted light from the medium (11) to the detecting means (39) while dispersing the wavelength of the reflected or transmitted light, said detecting means (39) including a sensor array of a plurality of sensors arranged in the direction of the dispersion effected by said optical element (40). An apparatus according to claim 20, wherein said optical element comprises a prism (40). An apparatus according to claim 20, wherein said sensor array (39) includes a charge-coupled device. An apparatus according to any one of claims 15 to 22, wherein the first (12a, 12b; 42) and second (14a, 14b; 44) reflective layers are carried on a base (11a, 11b). An optical reproducing method according to Claim 1, further comprising the step of recording said value information on the recording medium (1) by irradiating said cell with a light beam so as to change the refractive index of said cell dependent on the intensity of the light beam. An apparatus according to Claim 15, including means for recording said multiple value information on the recording medium (1), comprising means for irradiating said cell with a light beam effective to change the refractive index of said cell dependent on the intensity of the light beam.

Inventors: HOSHI HIROAKI, MATSUMURA SUSUMU, MORISHIMA HIDEKI, YAMAGUCHI EIJI, YAMAMOTO MASAKUNI, HOSHI, HIROAKI, MATSUMURA, SUSUMU, MORISHIMA, HIDEKI, YAMAGUCHI, EIJI, YAMAMOTO, MASAKUNI, HOSHI, HIROAKI, C/O CANON KABUSHIKI KAISHA, MATSUMURA, SUSUMU, C/O CANON KABUSHIKI KAISHA, MORISHIMA, HIDEKI, C/O CANON KABUSHIKI KAISHA, YAMAGUCHI, EIJI, C/O CANON KABUSHIKI KAISHA, YAMAMOTO, MASAKUNI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: G11B7
IPC codes: G11B7