Apparatus and method for recording and reproducing multi-level information

An information recording apparatus includes a record signal analog processor which converts information to be recorded into an analog signal having a plurality of levels, a laser beam source for emitting a laser beam whose energy and radiating time are modulated at multiple levels in accordance with the analog signal, and an optical system for directing the laser beam on an information recording medium. The information recording medium has a recording layer. As a result of the modulation of the laser energy, a plurality of states of recording marks are obtained. As a result of the modulation of the laser radiating time, a plurality of lengths of recording marks are obtained in the radiated portion. Thus, multi-level information is recorded in the data recording medium. The recorded information can be reproduced by detecting the optical characteristics of these recording marks.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an information recording apparatus and 
method capable of radiating a light beam onto an information recording 
medium to record multi-level data in a portion of the recording medium. 
2. Description of the Related Art. 
In order to increase the capacity and processing speed of computers in the 
future, an increase in the density and capability of memories is an 
important technical object. In currently available optical memories, such 
as optical disks and cards, only a one-level signal (i.e., a signal 
corresponding to the presence or absence of a data bit record) can be 
written in a single record spot. Mark-position recording and mark-edge 
recording are currently utilized to record information onto such optical 
memories. In mark-position recording, the center of the recording mark is 
positioned at the location of a data bit, whereas in mark-edge recording, 
the edge of recording mark is positioned at the location of a data bit. 
Generally, recording density is much higher in mark-edge recording than in 
mark-position recording. In both recording methods, however, only a 
one-level signal (i.e., a signal corresponding to the presence or absence 
of a data bit) can be written in a single record spot. Thus, if mark-edge 
recording is utilized, the recording density achieved is not sufficiently 
high. In order to overcome this limited memory density, a multi-value 
signal recording system for writing a multi-value signal in a single 
record spot is necessary. An ultra-high density optical memory using a 
photochemical hole burning (PHB) technique is an example of a memory 
employing such a multi-value recording system. See, for example, U.Itoh et 
al. Page 147 through 150 in Topical Meeting on Optical Data Storage, Mar. 
11-13, 1987, Stateline, Nev. The PHB material has a host made of 
transparent materials and guest material dispersed in the host. Only the 
guest material absorbs light and a photochemical reaction occurs in the 
guest at a temperature as low as several degrees to several tens of 
degrees K. When a light beam having wavelength is radiated onto PHB 
material, the guest material absorbs the light beam so as to generate a 
hole of an absorption spectrum at wavelength .lambda.. By modulating 
wavelengths of light into n-levels and directing the light onto the guest 
material of PHB material, the guest material has n different absorption 
spectra, and n-value data can be recorded. When the recorded data is 
reproduced, the reflectivity or transmissivity of the recording medium is 
detected at the n-level wavelength. In such an n-value recording system 
using PHB, however, the PHB memory film must be kept at a very low 
temperature. Many problems concomitant with its use as an optical memory 
device remain unsolved. Hence, to date no practical applications of a 
high-density optical recording system have been realized. 
SUMMARY OF THE INVENTION 
The present invention solves the above-described problems, and has as its 
object to provide an information recording apparatus and method capable of 
realizing practical high-density recording. 
According to the present invention, there is provided an information 
recording apparatus. The apparatus comprises signal processing means for 
converting an information signal to be recorded into a signal of not less 
than three levels. Light beam output means are provided for outputting a 
light beam whose energy and radiating time is modulated at multiple levels 
in accordance with the processed signal. Optical means direct the light 
beam onto the information recording medium, wherein the multi-level 
information is recorded in the information recording medium by changing a 
portion of the information recording medium in at least one of a plurality 
of states. 
According to the present invention, there is provided an information 
recording method utilizing a mark-edge recording process comprising the 
steps of converting information to be recorded into a signal having not 
less than three levels. A light beam whose energy and radiating time are 
modulated at multiple levels in accordance with the signal is radiated 
onto a recording medium, wherein the multi-level signal is recorded onto 
the medium by changing of a portion of the medium in at least one of a 
plurality of states, and, preferably, not less than three states, thereby 
recording the multi-level information in the recording medium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will be described in detail with 
reference to the accompanying drawings. FIG. 1 is a schmatic view showing 
an arrangement of an information recording apparatus in accordance with 
the present invention. 
A record signal processor 10 converts an input data signal into a signal 
having not less than three levels which is then outputted to a 
semiconductor laser 11. The laser 11 radiates a recording laser beam in 
response to the signal from the signal processor 10 having not less than 
three energy levels. When a laser beam having such multi-level optical 
characteristics is directed onto a recording medium (to be described 
later) of an optical disk 20 through an optical system 30, the state of a 
radiated portion of the optical disk 20 is changed in accordance with the 
energy of the laser beam to thereby record multi-level data. For example, 
the transmissivity or reflectivity of the optical disk 20 can be changed 
to a number of descrete levels of transmissivity or reflectivity, 
respectively. Each of these discrete levels corresponds to a level of 
information stored on the optical disk 20. Preferably, the number of 
discrete levels of transmissivity or reflectivity is not less than three. 
In this way, not less than three bits of information can be stored at one 
record spot and not less than three data bits can be read from the record 
spot. 
When data is to be reproduced from the optical disk 20, the semiconductor 
laser 11 outputs a constant laser beam having an intensity lower than that 
of a recording laser beam. The reproducing laser beam output from the 
laser 11 is directed to the optical disk 20 through the optical system 30 
and a reflected beam is incident on an opto-electric conversion element 12 
through the optical system 30. A signal converted by the opto-electric 
conversion element 12 is supplied to a reproduction signal processor 40 
through a processing circuit 14. As a result, reproduction signals 
corresponding to multi-level data are output, as will be described later. 
It can be understood that where the transmissivity of the optical disk 20 
is to be detected, an optical system, opto-electric conversion element, 
and processing circuit can be disposed on the side of optical disk 20 
opposite from lens 36. These elements may correspond in structure and 
function to optical system 30, opto-electric conversion element 12, and 
processing circuit 14 described above and shown in FIG. 1. 
In the optical system 30, a divergent laser beam output from the 
semiconductor laser 11 is collimated by a collimator lens 31, and is 
incident on a beam splitter 32. A beam reflected by the beam splitter 32 
is incident to the optical disk 20 through a focusing lens 36. When a 
reproducing laser beam is output from the laser 11, the laser beam is 
reflected by the optical disk 20 and passes through the beam splitter 32 
is incident to a half mirror 33. The beam passing through the half mirror 
33 is directed to the opto-electric conversion element 13 through a lens 
35, and the beam reflected by the half mirror 33 is directed to the 
photoelectoric conversion element 12 through a lens 34. 
As described above, the signal output from the opto-electric conversion 
element 12 is output to the reproduction signal processor 40 through 
processing circuit 14. Another signal from processing circuit 14 is guided 
to a tracking controller 19 through an amplifier 16 so as to adjust the 
position of the beam. In addition, the signal output from opto-electric 
conversion element 13 is supplied to a focusing controller 18 through a 
processing circuit 15 and an amplifier 17, thereby performing focusing 
control. 
The record signal processor 10 will be described in detail below. FIG. 2 is 
a block diagram showing a schmatic arrangement of record signal processor 
10 comprising an n-value circuit 101, a D/A converter 102, a 
radiating-time modulating circuit 103, a pulse-power modulating circuit 
104 and a semiconductor laser driver 105. Input data is input into n-value 
circuit 101 in binary form. The data input into n-value circuit 101 
corresponds to the degree of multiplexing of the data to be stored on 
optical disk 20. The n-value data is then converted into an analog signal 
by the D/A converter 102 and is input into the semiconductor laser driver 
105 after its radiating-time and power are modulated in accordance with 
the processed signal by a radiating-time modulating circuit 103 and a 
pulse-power modulating circuit 104. 
The recorded information is reproduced by radiating a continuous and 
low-energy laser beam onto a recording portion and detecting changes in 
reflectivity of the laser beam reflected by the recording layer. 
FIG. 3 is a block diagram showing a schematic arrangement of the 
reproduction signal processor. As shown in FIG. 3, processor 40 comprises 
a mark length detecting circuit 401, a reflectivity detecting circuit 402, 
and A/D converter 403, an n-value circuit 404, and a binary circuit 405. 
Reflectivity is detected by recording layer circuit 401 and mark length is 
detected by mark length detecting circuit 402. The detected analog value 
is digitized by A/D converter 403. The digitized signal is then converted 
into an n-value signal by n-value circuit 404 by comparing it with a 
predetermined reference voltage. Subsequently, the n-value signal is 
converted into an binary signal by binary circuit 405. 
The information recording medium 20 will now be described in detail below. 
FIG. 4 is a sectional view of phase-transformation type information 
recording medium for use with the present invention. A substrate 21 is a 
transparent material whose quality does not change significantly over 
time, e.g., an acrylic resin such as polymethylmethacrylate, a 
polycarbonate resin, an epoxy resin, a styrene resin, or a glass material. 
A protective layer 22, a recording layer 23, a protective layer 24, and a 
surface protective layer 25 are formed on the substrate 21 in the order 
named. 
Recording layer 23 is formed of a material whose state is changed upon 
radiation of a laser beam of a certain power on it. A phase transformation 
type material may be used as one example such a material. In a recording 
layer of the transformation type, phase transformation occurs between, 
e.g., a crystalline phase and an amorphous phase depending on the 
radiating conditions of the laser beam. In this case, when the radiation 
energy of a laser beam is modulated between one or more levels, one or 
more intermediate phases with differences in optical characteristics such 
as reflectivity and transmittance are formed in the recording medium. 
Thus, multi-value data can be recorded and reproduced by utilizing this 
phase transformation. 
Examples of phase transformation type materials include chalcogenide 
amorphous semiconductor materials, e.g., GeTe, TeSe, GeSbSe, TeOx, InSe, 
and InSbTe. Recording layer 23 can be formed by vacuum evaporation, 
sputtering, or the like. The thickness of recording layer 23 preferably 
falls within the range from several nm to several .mu.m in terms of 
practical applications. Protective layers 22 and 24 are arranged to 
sandwich recording layer 23, thus preventing disintegration of recording 
layer 23 while permitting formation of holes therein upon radiation of a 
laser beam. Protective layers 22 and 24 can be made of SiO2, SiO, AlN, 
Al203, ZrO2, ZrO, TiO2, Ta203, ZnS, Si, or Ge formed by vacuum evaporation 
or sputtering. The thickness of each of protective layers 22 and 24 
preferably falls within the range from several nm to several um in terms 
of practical applications. Protective layer 25 is formed to prevent 
damage, dust contamination, and the like during use, and normally is made 
of an ultraviolet-curing resin or the like. Protective layer 25 can 
preferably be formed by coating, e.g., an ultraviolet-curing resin on 
protective layer 24. Protective layer 25 preferably falls within the range 
from several nm to several hundreds of .mu.m in thickness. 
The above description illustrates a one-sided information recording medium. 
However, the present invention can be used with a two-sided data recording 
medium, as would be formed by bonding two one-side information recording 
media to each other with a recording layer 23 located inside each. 
Data recording can be performed by radiating a laser beam from 
semiconductor laser 11 onto the recording layer of the recording medium 
through optical system 30 to form a recording mark. In this case, a binary 
data signal is converted into n-value data(n&gt;2) by n-value circuit 101 of 
record signal processor 10. The energy of the laser beam output from laser 
11 is modulated at n levels and the radiation time of the laser beam is 
modulated in accordance with the signal. 
An example in which a phase-transformation type recording medium is 
employed will be described below. FIG. 5 shows the state of a 
beam-radiated portion of a recording layer corresponding to various power 
and pulse width settings of the recording laser beam. Referring to FIG. 5, 
when a laser beam having a pulse width T and power P1 is radiated on 
medium 200, radiated portion is crystallized upon annealing. When a laser 
beam having pulse width T and power P2 is radiated the beam-radiated 
portion becomes amorphous after melting. When a laser beam having pulse 
width T and power P3 is radiated, the beam-radiated portion is 
crystallized after melting. That is, if the power level of the radiated 
laser beam exceeds the level at which the recording medium becomes 
amorphous after melting, the radiated portion cannot be quickly cooled, 
and an amorphous state is difficult to obtain. If the power of the laser 
beam falls within the range at which melting/amorphous formation occurs, 
the degree of amorphous formation in the recording medium is increased 
with changes in the power of the laser beam, and variations in the optical 
characteristics, e.g., reflectivity and transmittance, of the record spot 
can be achieved. 
The length of the recording mark in which melting/amorphous region 
formation occurs is increased in proportion to the increase in radiating 
time of the laser beam onto the recording medium. 
As described above, various states and lengths of the recording mark can be 
formed by converting data into signals having n levels and modulating the 
energy and radiating time of a laser beam in accordance with the signals. 
The radiated area (hereafter called the record spot) will be either an 
anneal-crystallization region, a melting/amorphous formation region, or a 
melting/crystallization region. 
If the intensity and pulse width of the laser beam are respectively set to 
be P and T, energy E of the laser beam can be given by E=P.times.T. 
Therefore, in order to change the energy E, one or both of the intensity 
and pulse width of the laser beam may be changed. Note that as the energy 
of a radiated beam is changed, the record spot is changed. The recording 
operation will be described in greater detail by illustrating a recording 
operation using 6-bit data. In this example, the amorphous state of 
recording layer 23 is set to be an initial state. Crystallinity of the 
recording layer is taken into consideration first. Assume that the laser 
beam energy required for complete crystallization of a radiated portion is 
given as E5, and energies corresponding to intermediate levels are 
respectively given as E1 to E4 in increasing order of energy. When a laser 
beam having energy levels from E1 and E4 is radiated, different portions 
of the illuminated portions of the record medium are crystallized, and the 
remaining portions become amorphous; thus, intermediate states can be 
obtained in which crystalline and amophous phases are present at the same 
time. As the energy is increased from E1 to E4, the crystallinity of the 
radiated portion is increased. 
The reflectivity is increased or decreased in accordance with changes in 
the crystallinity. Thus, it is possible to create regions with five 
different reflectivities. In this case, if the number of possible bits is 
selected to correspond to no laser beam energy, and a radiated laser beam 
having energies E1 to E5, 6-bit data can be recorded in a single record 
spot. For example, the reflectivity of a non-radiated portion is R0, and 
reflectivities corresponding to laser beam energies E1 to E5 are R1 to R5, 
respectively, a relationship as shown in the following Table 1 is 
established between laser beam energy, reflectivity, and the number of 
bits. 
TABLE 1 
______________________________________ 
Laser beam energy 
Reflectivity 
Data bit 
______________________________________ 
.sup. .sup. 0 R0 0 
E1 R1 1 
E2 R2 2 
E3 R3 3 
E4 R4 4 
E5 R5 5 
______________________________________ 
FIG. 6 shows the relationship between the energy of a laser beam and 
reflectivity given in Table 1. 
Such multi-value recording can also be performed by changing the wavelength 
of a laser beam output from semiconductor laser 11 in a multi-level manner 
in accordance with the data signal. More specifically, in the present 
invention, the above-described semiconductor laser, an HeNe laser or the 
like, can be suitably used as a light source. The laser beam emitted from 
such a light source has an emission pattern based on the TEM00 fundamental 
mode, and its sectional intensity distribution exhibits a Gaussian 
distribution. When such a beam is converged by a lens, it can be converged 
only to a limited radius due to diffraction. If the minimum beam radius is 
D (beam waist), the following equation can be established: 
EQU D=k.lambda./NA, 
where K is a constant(K=0.41), .lambda. is the wavelegth of a light beam, 
and NA is the numerical aperture of a lens. 
As is apparent from this equation, as the wavelength of a laser beam is 
decreased, the converged spot diameter is decreased. That is, when only 
the wavelength of the laser beam is changed, with the intensity and pulse 
width kept constant, the diameter of a beam-radiated portion can be 
changed according to the equation described above. In addition, since the 
spot size itself is changed, the reflectivity is changed. 
REPRODUCTION 
Since the reflectivity and transmittance of a record spot change in 
accordance with information recorded in the spot, data can be reproduced 
by radiating a reproducing laser beam (to be described later) and 
detecting the reflectivity or transmittance of the record spot. 
Data reproduced by multi-value recording is converted into an electrical 
signal by optical-electric conversion element 12 in accordance with the 
reflectivity of a laser beam-radiated portion e.g., a record spot. This 
signal is then processed by reproducing signal processor 40. Data can also 
be reproduced by detecting transmissivity of the recording medium, instead 
of the reflectivity. 
As shown in Table 2, a 1:1 correspondence may be established between values 
of reflectivity and data bits by detecting the reflectivity of a 
laser-radiated portion subjected to multi-value recording. In addition, an 
approximate value of each reflectivity may be set to correspond to each 
data bit, as shown in Table 1, or stepwise correspondence may be 
established, as shown in Table 3. 
TABLE 2 
______________________________________ 
Reflectivity 
Data bit 
______________________________________ 
R0 0 
R1 1 
R2 2 
R3 3 
R4 4 
R5 5 
______________________________________ 
TABLE 3 
______________________________________ 
Reflectivity 
Data bit 
______________________________________ 
-R0 0 
R0-R1 1 
R1-R2 2 
R2-R3 3 
R3-R4 4 
R4-R5 5 
______________________________________ 
ERASURE 
If the recording layer is a phase transformation type, phase transformation 
of the layer can be reversed under certain radiating conditions of a laser 
beam. In this embodiment, since part or all of a multi value-recorded 
portion is crystallized, a laser beam may be radiated on this portion and 
the radiated potion will be melted and then quickly cooled to cause phase 
transformation from a crystalline state to an amorphous state, thereby 
erasing the data stored therein and new information is recorded in said 
recording layer by said pulse beam. 
EXAMPLE 1 
A phase-transformation type optical disk was used as the information 
recording medium. The optical disk was obtained by sequentially forming a 
1000-A thick SiO protective layer, a 1000-A thick (In.sub.48 
Sb.sub.52).sub.98 Te.sub.2 recording layer, a 1000-A thick SiO protective 
layer, and an ultraviolet-curing resin layer on a polycarbonate substrate. 
The recording layer exihibited an amorphous state as its initial state, 
and its phase transformation was caused upon radiation by a laser beam. 
Upon radiation of a laser beam having a power from W1 to W3, 
crystallization of the radiated portion progressed halfway, e.g. to an 
intermediate stage. The reflectivity of the radiated portion became 
respectivly R1 to R3. Radiation time was also modulated into L1 to L5 
shown in FIG. 7. The relationship between reflectivity and radiation time 
and information bit is shown in Table 4. 
TABLE 4 
______________________________________ 
non 
radiated 
R1 R2 R3 
______________________________________ 
non 0 -- -- -- 
radiated 
L1 -- 1 2 3 
L2 -- 4 5 6 
L3 -- 7 8 9 
L4 -- 10 11 12 
L5 -- 13 14 15 
______________________________________ 
Including the nonradiated portion, sixteen bits of information can be 
recorded in this example. Data recorded in this manner was reproduced as 
follows. A 0.5 mW continuous beam emitted from a semiconductor laser was 
radiated on the recording layer of a recording medium. A beam reflected by 
the recording layer was input to a processing circuit which output an 
analog signal having sixteen bits of information. Thus, in this example, 
it was proven possible to record a data bit having one of up to 16 
different data at a single spot on the recording medium.