JPH0482033A - Optical information recording medium - Google Patents

Abstract

PURPOSE:To allow the recording of information at a high degree of many values by setting the product of the thickness and refractive index of a medium layer approximately at integer times the half wavelength of the wavelength to generate multiple interferences by the luminous flux with which the medium layer is irradiated. CONSTITUTION:Reflecting layers 12a, 12b, 14a, 14b consisting of multiple layers are formed by alternately stacking layers having the high refractive index and layers having the low refractive index at a specified optical path length thickness. Dielectrics, such as SiO2, Si3N4, MgF4, and Al2O3, are used as the material of the respective reflecting layers. The product of the thickness of the medium layer and the refractive index of the medium layer is about integer times the half wavelength of the wavelength to generate the multiple interferences with the luminous flux with which the medium layer is irradiated. The high SNR and the high degree of the many values are obtd. and the efficient reproduction is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical information recording medium that records information using optical multiple interference effects.

[Background Art] In recent years, optical information recording media called optical memories, such as optical disks, optical cards, and optical tapes, and recording and reproducing devices using them have made remarkable progress. Such a recording/reproducing device has the advantage that the cost per bit is lower than that of a magnetic disk, etc., and a high transfer rate can be obtained because the optical memory has the feature of high-density recording.

Therefore, research and development is actively being conducted to further increase the density of optical memory.

In order to increase the density of optical memories, various methods have been proposed, such as miniaturization of the minimum bit size, multi-level recording of information, and multiple recording of information. Of these, most of the conventionally known multilevel techniques have been proposed using continuous reflectance changes of dye media or phase change media. These proposals basically involve further subdividing the intermediate value between two different reflectance levels with good contrast, which corresponded to conventional binary information. Therefore, in principle, multileveling is performed at the expense of SNR, which leads to deterioration of error rate.

In the method using this change in reflectance, the simplest method is:
Assuming that the complex refractive index of the substrate is N3 and the complex refractive index of the medium is N, the reflectance R is expressed by the following equation.

However, n + a (m = 1.2) is the refractive index, K, (m
= 1.2) is the damping coefficient Nff1・n m −I K
It is m. For example, in the case of a Tea-based phase change medium, N2=3.5-0.8i in the amorphous state and N2:3.9-1.3i in the crystalline state, so N,
=1.6, the change in reflectance ΔR is as small as 6%. Further, this ΔR is divided into smaller parts, but since the change in ΔR with respect to the optical energy required for recording is non-linear, it is difficult to divide it into equal parts, and it is also not easy to control the optical energy required for recording.

On the other hand, S
As a method to increase the NR, as shown in Japanese Patent Publication No. 63-26463, a metal mirror with high reflectance is provided on the back surface of the medium, and the difference in reflectance between the two states is widened by the multiple interference effect of the thin film. Methods have been proposed to improve contrast. This Japanese Patent Publication No. 63-26463 proposes an increase in reflectance change due to a change in absorption coefficient, but this method solves the problems of multileveling, nonlinearity, and difficulty in controlling light energy. It's not something that can be solved. In fact, if it is applied to multi-leveling, there is a strong possibility that the nonlinearity of the change in reflectance will increase due to the combination of changes in the absorption spectrum and multiple interference effects. In other words, the difference between the two reflectances corresponding to such binary values is increased, the contrast is improved, and the SNR as binary data is increased.
Even if the characteristics can be improved, it does not necessarily mean that characteristics suitable for multilevel processing can be obtained.

In a digital memory, if the number of multivalue levels at the medium level is L, the number M of recording bits of information when recording binary data is expressed by the following equation.

M=log, L For example, in the case of multilevel conversion using reflectance changes, even if the medium achieves 8 levels of multilevel (L=8), the amount of information that can be recorded is 3 bits (M=3), so the recording density will only increase three times. When considering the signal processing of devices such as computers that receive data from digital memory, and the processing systems such as modulation and demodulation within the memory, the effect of multilevel conversion becomes noticeable from 8-bit multilevel conversion. , it is necessary to ensure 256 levels of multi-value for the medium.

When performing multi-value conversion using reflectance for such light intensity, even if the reflectance is divided equally between 0 and 100% in a linear manner, ideally, 256 levels of multi-value conversion will be performed. This results in a change of 0.391% per rubel, and it can be seen that it is extremely difficult to put it into practical use when considering medium manufacturing technology, light source intensity stabilization technology, detection technology, etc.

Furthermore, a multiplex recording system using a photochemical hole burning (hereinafter referred to as PHB) effect using a light absorption spectrum has also been proposed, as in Japanese Patent Publication No. 63-26463. In principle, this multiplex recording method should enable multilevel recording by multiplex recording. However, PHB
At present, experiments to confirm the principle of this method at extremely low temperatures have been completed, and it is technically difficult to stably realize a medium with many narrow-band absorption spectra at room temperature. Furthermore, when performing a series of basic operations as a memory such as recording, reproducing, and erasing information, the light source needs to have a spectral width sufficiently narrower than each of the narrow absorption spectral widths. In addition, there are currently many issues that need to be resolved in addition to the instability of the medium itself, such as the need to simultaneously sweep the wavelength and the need to control the absolute value of the wavelength.

[Problems to be solved by the invention] As mentioned above, many problems remain with the conventional multilevel technology, especially problems such as low SNR and low degree of multilevel conversion. was there. In addition, although proposals have been made to increase the degree of multi-leveling, the reality is that many technical issues remain.

The present invention was made in view of the above circumstances, and its purpose is to provide an optical information recording medium that can realize a high SNR and a high degree of multi-value, and that can perform reproduction efficiently. .

[Means for Solving the Problem] In order to achieve the above object, first and second reflective layers are arranged parallel to each other at a predetermined interval so that light reflected by each layer causes multiple interference; It consists of a medium layer that is provided between the reflective layers and records information as a change in its refractive index, and the product of the thickness and refractive index of the medium layer is an abbreviation of the wavelength at which multiple interference occurs due to the irradiated light beam. An optical information recording medium characterized in that the wavelength is an integral multiple of a half wavelength is provided.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view showing an embodiment of the optical information recording medium of the present invention.

In FIG. 1, a recording medium 1 is a double-sided medium that allows recording and reproduction on both the front and back sides. 118.degree. 11b is a transparent polycarbonate substrate, and reflective layers and medium layers are alternately formed on each substrate.

First, first reflective layers 12a and 12b are formed on substrates 11a and llb, respectively, and medium layers 13a and 13b whose refractive index changes with heat are formed on the surfaces thereof. Further, a second reflective layer 14a is provided on each medium layer 13a, 13b,
14b are formed, and these are adhered by an adhesive layer 15. The layers are formed parallel to each other at predetermined intervals, and are designed to cause multiple interference as will be described later.

As the medium layers 13a and 13b, DingO,, In5eT
A Co.

Inorganic media such as GeTe5bTj2 and GeTeSe, and organic media such as anthraquinone dielectrics, cyokikedine compounds, and triphuedithiazine compounds are suitable.

In addition, multilayer reflective layers 12a, 12b and 14a, 1
4b is formed by alternately stacking layers with a high refractive index and layers with a low refractive index with an optical path length and thickness corresponding to 1/4 of the wavelength. The material of each reflective layer is S10□, 5
iJ4. MgF<, Al. A dielectric material such as 03 is used.

To create such a recording medium 1, first, a first reflective layer, a medium layer, and a second reflective layer are sequentially formed on each of the substrates 11a and llb on both sides by sputtering or coating, respectively. Then, a recording medium for double-sided recording is completed by bonding each substrate 11a, llb with an adhesive (adhesive layer 15) with the second reflective layers facing each other.

Further, in the recording medium 1 of this embodiment, the first reflective layer and the second reflective layer
The structure of a medium layer sandwiched between reflective layers realizes a so-called Fapley bellows etalon. That is, as will be described later, the incident light is repeatedly reflected between the two reflective layers, resulting in a multiple interference effect. The present invention effectively utilizes this multiple interference effect to perform multilevel recording of information as will be described in detail later.

Next, the principle of recording and reproducing information using the recording medium 1 described above will be explained. First, in order to reproduce information, the recording medium 1 is irradiated with a laser beam focused to a minute spot using a pickup lens in the same way as a normal optical head, and information is reproduced from the reflected light from the etalon section. To simplify the explanation, assuming that both sides of the two reflective layers 12a and 14a are air and both reflectances are R, the reflectance R9 from the etalon is expressed by equation (1).

ψ 5in2 Rt" ... (1) ψ 1+Fsin" Here, F corresponds to so-called finesse and represents the sharpness of interference fringes, and is obtained by equation (2).

Further, ψ is a phase difference expressed by the following equation (3).

However, nt+ is the refractive index of the medium layer 13a, and is the wavelength of light.

Here, if the reflectance R5 is expressed as a function of the phase difference ψ, then the second
The result will be as shown in the figure. In Figure 2, m is an integer and (4
) sharp and dark interference fringes appear where the equation is satisfied.

ψ=2πm (4) Changes in the phase difference ψ are generally expressed by equation (5) rather than equation (3).

This new multi-level recording method converts information corresponding to multiple values into a refractive index change Δn2 and records it on a medium as a phase difference change ΔA. In reproduction, by sweeping the wavelength of the light, the condition of equation (4) is restored at the wavelength corresponding to Δλ, which provides a phase difference that exactly cancels the phase difference ΔA caused by ΔnM. That is, when the wavelength is swept over time, sharp interference fringes appear at the wavelength that matches the phase difference corresponding to the value of the recorded information, and a pulse corresponding to FIG. 2 is obtained as a time waveform. The position Δt of this pulse on the time axis provides multi-value information. The correspondence between refraction angle, wavelength, and time is expressed by equation (6) when the wavelength sweep is linear.

To perform recording, change in phase difference Δ expressed by equation (5) is required.
ψ is given by controlling ΔnII produced by laser heating. For reproduction, light with a wavelength change that cancels out the change in phase difference is incident, and sharp dark fringes are detected. That is, if the wavelength is swept as described above and a pulse appearing in the intensity of reflected light from the medium is detected, the wavelength at that time is the wavelength corresponding to the phase difference.

At this time, since the condition for the appearance of the pulse is Equation (4), the corresponding wavelength is given by Equation (3), but naturally the wavelengths that satisfy the condition also become discrete, corresponding to FIG. 2.

nyd =... (7) Therefore, it is desirable that the change in phase difference ΔK is a change from ψ which satisfies equation (4) as an initial value. In this case, the initial value is ψ
If it deviates significantly from 1, it not only cancels Δψ, but also ψ.

Since the shift in the initial value from . From equation (7), the most extreme case is that the refractive index nii of the medium layer of the recording medium 1 is small and the thickness d is also small (if the order m of interference fringes is 1, that is, (7)
From the formula, when the round trip optical path length 2 n M d is 1λ1, m=
To produce interference fringes of 2, it is as follows. Therefore, it is necessary to sweep the wavelength λ1 to a half wavelength 1/2, and if the initial value of the phase is between A2 and A, the wavelength must be swept by about 50%. If n2=5 and d=0.6 μm, then m
= 6, = 0.8, d = 1.0 μm, m = 13,
=0.76 μm, it is possible to match the wavelength range that can be realistically swept with the optical path length nvd of the medium as shown below. In other words, it is sufficient to set the initial value ψINT of the phase difference to satisfy the following equations (3) and (4), and (lO) equation % equation % (1
1) It is sufficient to create a medium in the vicinity of (nvd) that satisfies the following. The meaning of neighborhood here is exactly as follows. In general, the refractive index change due to heating is in one direction, either positive or negative, depending on the medium, so depending on the possible wavelength sweep direction and range, the refractive index change after recording multilevel information is This means that the initial value (nM, d) + sa is shifted.

Next, a specific example of an information recording/reproducing apparatus using the optical information recording medium as described above will be described with reference to FIG.

In FIG. 3, 1 is a recording medium for recording the above-mentioned multivalued information, 2 is a semiconductor laser which is the first light source for recording information,
3 is a collimator lens that converts the laser beam of the semiconductor laser 2 into parallel light. 4 is a semiconductor laser which is a second light source for information reproduction, and 5 is a collimator lens that converts the laser beam of the semiconductor laser 4 into parallel light. Further, 6.7 is a beam splitter, 8 is a pickup lens, 9 is a sensor lens, and 10 is a light detector.

The light converted into parallel light by each collimator lens 3, 5 is
They are combined by a beam splitter 6. The medium 1 is driven by a motor (not shown), and is configured to be scanned by the light beam emitted from the light source.

The recording semiconductor laser 2 is a relatively high output laser, and is driven by a recording laser drive circuit 31. From the terminal 30, words expressed as binary signals of a large number of bits (for example, 8 bits) are input one after another. The input word is converted into a corresponding signal level in the modulation circuit 32 according to a predetermined table. for example,
If one word consists of 8 bits, there are 256 words, and 2 words corresponding to each word.
A recording signal modulated into 56 levels is output from the modulation circuit. This recording signal is transmitted to the recording laser drive circuit 31
The semiconductor laser 2 emits a recording light beam having an intensity corresponding to the signal level.

The recording light beam is collimated by a collimator lens 3,
The light passes through the beam splitters 6 and 7 and is imaged as a minute spot on the medium layer of the medium 1 by the pickup lens 8. The medium layer is heated by the irradiation of the recording light flux, and signals are recorded as a series of minute recording areas with varying refractive indexes. Here, each minute recording area exhibits a refractive index corresponding to the intensity of the irradiated light beam. That is, one word is recorded in one recording area. The signal output from the modulation circuit 32 is performed at regular intervals based on the clock signal output from the clock signal generation circuit 33. Therefore, minute recording areas are formed on the recording medium l at a constant pitch in the scanning direction of the light beam.

On the other hand, as the semiconductor laser 4 for reproduction, a relatively low output laser is used. This semiconductor laser 4 is driven by a reproduction laser drive circuit 34. The reproducing laser drive circuit 34 sweeps the wavelength of the reproducing light flux emitted from the semiconductor laser 4 at a constant cycle in synchronization with the clock signal input from the clock signal generating circuit 33. Such wavelength sweeping can be performed, for example, by using a normal DH laser and changing the amount of current injected into the laser. Further, if a tandem electrode type wavelength tunable semiconductor laser proposed in Japanese Patent Application Laid-Open No. 63-32985 and the like is used, it is possible to sweep an even wider range and at higher speed. The clock signal generation circuit 33 synchronizes the minute recording area on the medium with the clock signal by, for example, detecting the pressure of a synchronization mark recorded on the medium in advance. Based on this clock signal, the semiconductor laser 4 is controlled to sweep the wavelength at least once within the minute recording area.

The reproducing light beam emitted from the semiconductor laser 4 is reflected by the beam splitter 6, transmitted through the beam splitter 7, and imaged as a minute spot on the medium layer of the medium 1 by the pickup lens 8. The light reflected by the medium 1 and modulated according to the recorded information is separated from the incident light from the laser by the beam splitter 7, focused by the sensor lens 9, and received by the optical detector 10.

The output of the optical detector 10 is input to a peak detection circuit 35, where the peak of the pressure detection signal is detected. Based on the clock signal inputted from the clock signal generation circuit 33, the timer circuit 36 counts the time from the start of the wavelength sweep of the reproducing light beam to the time when the peak is detected. As will be described later, the present invention detects information recorded as a change in the refractive index of the medium layer of the medium 1 by converting it into a change in the wavelength at which interference occurs using optical multiple interference effects. . In the apparatus shown in FIG. 3, since the wavelength is swept at a constant rate, the change in wavelength appears as the time from the start of the sweep. Therefore, the information reproducing circuit 37,
The information recorded on the medium 1 is reproduced from the time measured by the clock circuit 36 and output from the terminal 38.

Although not shown in the drawings, the focal position of the light spot of the recording light beam and reproduction light beam is three-dimensionally controlled by autofocus and autotracking control. Further, in both of the semiconductor lasers described above, it is also possible to convert the optical output having an elliptical distribution into one having an approximately perfect circular distribution using a beam shaping prism or the like, if necessary.

In the recording/reproducing apparatus, multi-value information is recorded by heating the medium with light from the recording semiconductor laser 2 and changing its refractive index nii. Therefore, unlike the conventional method in which the principle of recording is a change in reflectance due to a change in refractive index, the present invention records a change in refractive index in correspondence with a change in phase difference of an etalon. On the other hand, during reproduction, the wavelength of the reproducing semiconductor laser 4 is swept, and the optical detector 10 detects a change in wavelength corresponding to a change in the recorded phase difference as a time change in the optical pulse.

FIG. 4 shows a time chart of the information reproducing operation by this wavelength sweep. FIG. 4(a) is a diagram showing the arrangement of minute recording areas (hereinafter referred to as cells) in the track direction of the recording medium 1.

In this example, the cells Ci (i:1, 2, . . . ) are arranged in a line in the track direction, and are arranged at equal pitches in space or time. The cell referred to here corresponds to a bit in a compact disc or a domain in a magneto-optical disc, and is fundamentally different from conventional mark-to-mark recording or mark-length recording. That is, multivalued information can be recorded at the same right position, and the interval and length of each cell are unrelated to the information to be recorded.

FIG. 4(b) shows the wavelength sweep waveform 16 of the reproducing semiconductor laser 4, where the horizontal axis is time and the vertical axis is wavelength. The wavelength sweep waveform 16 is a sawtooth wave as shown in the figure, and has a linear relationship between time and wavelength. Therefore, since the wavelength can be detected by measuring time, the reproducing system can be easily configured. FIG. 4(c) shows the reflected light intensity when the information of each cell in FIG. 4(a) is read using the wavelength sweep waveform 16 in FIG. 4(b), that is, the output waveform 17 of the optical detector 10. The output pulse shown in (c) of the same figure is the recorded phase difference △
This pulse is output at a wavelength that cancels ψ, and this pulse corresponds to the pulse shown in FIG. The time at which the pulse is output is te, (i=1.2.3>, and these times indirectly represent the wavelength. On the other hand, the time at which the wavelength λ5 corresponds to the initial value is t, (i=1 .2.3
) is obtained, and by determining Δt=1 c, -1, a wavelength that satisfies the interference fringe condition (6) for each cell can be obtained. In addition, (3), (The recorded phase difference and refractive index change can be obtained from Equation 61, and the recorded multilevel information can be reproduced. The pulse waveform 18 is a pulse waveform 18 in which each pulse width Δt represents multilevel information of each cell.

In addition, in FIG. 4(b), the effective range of wavelength sweep is defined as ≦λ≦λ. That is, since the wavelength changes near the peaks and valleys of the wavelength sweep waveform 16 tend to be steep and unstable, both sides of the sweep wavelength are not used, and a certain margin is provided. In this example, a recording medium whose refractive index decreases by heating is used, and the initial value of the refractive index n M
The wavelength that satisfies the interference fringe condition (1) corresponding to l and a is , and n MlNTd : m 'e, /2
-(12) holds true. The change in phase difference due to heating is
(3).

From equation (5), for λ1, the following is true. Therefore, the phase difference after heating is given by equations (10) and (13). The wavelength at which this change in phase difference is canceled and the peak of the interference fringe appears again is (3).

From equation (4), it becomes. From equations (11) and (15), the change in wavelength is as follows, and as can be seen from equation (7), it has a different sign with respect to Δn2. Also, the wavelength range for the maximum 1△nvl.

bawl. In other words, in this case, the initial value is nM+Nt,
By setting ci, h, it becomes possible to cancel the maximum 1Δr1M that decreases due to recording due to heating at λ8.

In this way, for the wavelength sweep range λ8≦λ≦3, that is, the refractive index change range n, NT≦08≦n□, considering the direction of the change, the initial value of the phase difference is set to ψ1N and 2πm. ni, n 1. A recording medium having a refractive index and thickness that satisfies the conditions of lI NT d = m 'λ5/2 eliminates the need for extra wavelength sweeping, making it possible to reproduce optimal multilevel information. In the above embodiments, a reflective type recording medium is shown, but a transmissive type is also applicable.

[Effects of the Invention] As explained above, the optical information recording medium of the present invention not only provides a high SNR, but also has the effect of recording information with a high multi-level degree. Further, when reproducing information, it is sufficient to sweep the wavelength of the reproducing light beam with the minimum necessary width, and there is an advantage that multilevel information can be efficiently reproduced without unnecessary wavelength sweeping.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the optical information recording medium of the present invention, FIG. 2 is a characteristic diagram showing the optical multiple interference effect of the recording medium, and FIG. 3 is information recording using the recording medium. FIG. 4 is a block diagram showing an example of a reproducing apparatus, and a time chart showing an information reproducing operation of the recording/reproducing apparatus. 1: Recording medium 2. Semiconductor laser for recording 3.5:
Collimator lens 4: Semiconductor laser for reproduction 6.7: Beam splitter 8: Big up lens 9: Sensor lens 10: Optical disk 11a, llb
: Substrate 12a, 12b: First reflective layer 13a,
13b: Medium layer 14a, 14b: Second reflection M
15: Adhesive layer 31: Recording laser drive circuit 32
: Modulation circuit 33: Crop signal generation circuit 34: Reproduction laser drive circuit 35, peak detection circuit 36/clock circuit 37, information reproduction circuit Agent Patent attorney Seki Yamashita Figure 1 Figure 2/"''"''4 1粍ゑr

Claims (1)

[Claims] first and second reflective layers arranged parallel to each other at a predetermined interval so that the light reflected by each layer causes multiple interference;
It consists of a medium layer that is provided between these reflective layers and records information as a change in its refractive index, and the product of the thickness and refractive index of the medium layer is an abbreviation of the wavelength at which multiple interference occurs due to the irradiated light beam. An optical information recording medium characterized by an integral multiple of a half wavelength.