Optical Recording Disk

1. Field of the Invention

This invention relates to an optical recording disk.

2. Prior Art

An additionally or fully recordable optical recording disk was proposed in accordance with the compact disk (generally abbreviated as CD) standard. See Nikkei Electronics, Jan. 23, 1989, No. 465, page 107; the Functional Dye Department of the Kinki Chemical Society, Mar. 3, 1989, Osaka Science & Technology Center; and SPIE, Vol. 1078, Optical Data Storage Topical Meeting, 80, 1989. This disk has a dye layer, a reflective Au layer, and a protective layer disposed on a transparent resin substrate in this order. That is, the reflective layer is in close contact with the dye layer.

The dye layer as a light absorbing layer is preferably formed by spin coating. Also preferably, grooves for tracking servo control are formed on the surface of the substrate on which the light absorbing layer is formed. In applying a dye layer on a grooved substrate by spin coating or other coating techniques, the dye layer presents a nearly flat surface independent of whether or not the substrate has a groove, that is, the light absorbing layer in the groove has a thickness equal to its thickness on the land plus the groove depth.

Tracking control on such optical recording disks is preferably carried out by the push-pull method. The push-pull method uses a two-divided photodetector (2D-PD) having two light-receiving sections disposed symmetrical with respect to the track center, which produces a differential output upon receipt on the two light-receiving sections of the light that is reflected and diffracted by the groove on the disk, thereby detecting a tracking error.

However, we have found that the tracking servo control by the push-pull method is impossible on disks in which the thickness of the dye layer in the groove is approximate to the sum of the thickness of the dye layer on the land and the groove depth so that the dye layer has a relatively flat surface, because the tracking error signal produced by the push-pull method becomes zero (0).

A primary object of the present invention is to provide an optical recording disk having a dye layer and a reflective layer stacked thereon which is designed so as to develop normal and accurate push-pull tracking error signals.

To achieve this and other objects, the present invention is directed to an optical recording disk comprising at least a light absorbing layer and a reflective layer stacked on a substrate having grooves formed between lands wherein the grooves form recording tracks for detecting tracking errors by the push-pull method. The disk meets .DELTA.D>.DELTA.D.sub.0 wherein

.DELTA.D is given by the formula:

wherein D.sub.G is the thickness of the light absorbing layer on the groove, D.sub.L is the thickness of the light absorbing layer on the land, and d.sub.G is the depth of the groove, and .DELTA.D.sub.0 is a value of .DELTA.D associated with a tracking error signal of zero.

In a preferred embodiment, the groove has a depth d.sub.G of 500 .ANG.<d.sub.G .ltoreq.2,500 .ANG., a width W.sub.G of 0.35 to 0.6 .mu.m and a pitch P of 1.5 to 1.7 .mu.m. The light absorbing layer preferably has a refractive index of 2.0 to 2.9 and an extinction coefficient of 0.02 to 0.08. Also preferably, D.sub.G >D.sub.L and D.sub.L .ltoreq.2,200 .ANG.. Further preferably, the light absorbing layer is a film obtained by coating a solution containing a dye, and a topcoat is stacked on the reflective layer.

The optical recording disk having a light absorbing layer and a reflective layer stacked thereon in accordance with the present invention offers normal tracking error signals when tracking servo control is carried out by the push-pull method.

FIG. 1 is a fragmentary radial cross section of an optical recording disk of the present invention.

FIG. 2 is a view schematically illustrating optical constants associated with the optical recording disk of the invention.

FIG. 3 is a schematic view of a system for producing a tracking error signal by the push-pull method according to the invention.

FIGS. 4 to 8 are graphs illustrating tracking error signals relative to the thickness of a dye layer as a light absorbing layer in a groove.

One embodiment of the present invention is now described.

Referring to FIG. 1, there is illustrated one embodiment of the present invention. The optical recording disk 1 shown in FIG. 1 includes a substrate 3 and a dye layer 2 thereon as a light absorbing layer. The substrate 3 preferably has a transmittance of at least 85% relative to recording and reproducing light (of the order of 600 to 900 nm, especially 700 to 800 nm, typically 780 nm) and is formed of substantially transparent resin, glass or similar material.

One surface of the substrate 3 where a recording layer is to be formed is provided with tracking grooves 31 in a spiral pattern (or in a concentric pattern as the case may be). The grooves 31 each have a depth d.sub.G of 500 to 2,500 .ANG. and a width W.sub.G of 0.35 to 0.6 .mu.m and are arranged at a pitch P of 1.5 to 1.7 .mu.m alternately with lands 35 having a width W.sub.L in accordance with the CD standard. Reflectivity would become low with d.sub.G >2,500 .ANG. whereas modulation factor would be reduced with d.sub.G <500 .ANG.. For better modulation factor, a depth d.sub.G of 1,300 to 2,500 .ANG., especially 1,500 to 2,500 .ANG. is preferred. Substrate molding would become difficult and modulation factor would be reduced with W.sub.G <0.35 .mu.m whereas waveform distortion and crosstalk increase with W.sub.G >0.6 .mu.m. The grooves 31 and the lands 35 may be either directly formed in the substrate 3 as shown in the figure or formed in a resin layer on the substrate 3.

The dye layer 2 as a light absorbing layer is a coated film of a dye or dye composition containing at least one member of known light absorbing dyes such as cyanine dyes. The light absorbing layer may be formed from another material as long as it is a coated film.

On the dye layer 2 is formed a reflective layer 4 of Au or the like. The reflective layer 4 preferably has a reflectivity of at least 90% by itself. On the reflective layer 4 is formed a topcoat 5 of various resins as a protective layer. Understandably, an intermediate layer may be formed between the substrate 3 and the dye layer 2 and between the dye layer 2 and the reflective layer 4.

With this construction, recording and reproducing operations are carried out using the groove 31 as a recording track. The surface of the dye layer 2 has depressed and raised portions corresponding to the grooves 31 and lands 35 in the substrate 3.

As a result, a relationship D.sub.L +d.sub.G >D.sub.G exists provided that D.sub.G is the thickness of the dye film on the groove 31 (strictly stated, at the groove center) and D.sub.L is the thickness of the dye film on the land 35 (strictly stated, at the land center). If difference in film height .DELTA.D=D.sub.L +d.sub.G -D.sub.G, then .DELTA.D>0. This .DELTA.D is set to be larger than a predetermined .DELTA.D.sub.0 which is equal to a difference in film height .DELTA.D=D.sub.L +d.sub.G -D.sub.G associated with a push-pull tracking error signal of zero.

It is described how to calculate .DELTA.D.sub.0. Assume that .lambda. is the wavelength of a laser beam used as recording and reproducing light, the respective layers have a thickness D.sub.1 (dye layer), D.sub.2 (Au), and D.sub.3 (topcoat), and the refractive index n and extinction coefficient k of each layer are (ni, ki) as shown in FIG. 2. Reflectivity R is expressed as follows.

Then tracking error signals are calculated in accordance with FIG. 3. In FIG. 3, the groove 31 is a recording track. An incident beam is directed to the dye layer 2 on the disk through a lens 7 having a focal length F. The beam reflected then passes to receiving surfaces W1 and W2 of photodiodes 61 and 62 of a push-pull type detector through the lens 7 Assume that the disk groove bottom surface is (.xi., .eta.) plane, the amplitude distribution B(.xi., .eta.) in (.xi., .eta.) plane of a laser beam is represented by the following formula. ##EQU1##

A (x, y)=exp(=2x.sup.2 /B.sup.2 +-2y.sup.2 /B.sup.2)

.SIGMA.: lens upper region

B: the beam diameter of a beam pattern having a value of at least 1/e.sup.2 of its peak value.

Assume the receiving surfaces W1 and W2 of the detector are (u, v) plane, the reflected beam has a power U(u, v) on (u, v) plane, the reflected beam powers in the W1 and W2 regions are U1 and U2, and the incident beam has a power I.sub.0, then the push-pull tracking error signal TE is given by the following formula.

Therefore, actual TE can be calculated by substituting actual Di, (ni, ki) and other numerical values and varying .DELTA. to determine the maximum tracking error signal TE.

Results of one exemplary calculation of relative values of the maximum tracking error signal are shown in FIGS. 4 and 5 as relative tracking signals. This calculation was based on the following data.

substrate 3 (n0, k0)=(1.58, 0),

dye layer 2 (n1, k1)=(2.4, 0.04),

reflective layer 4 (n2, k2)=(4.7, 0.159),

reflective layer 4 D.sub.2 =1,000 .ANG.,

topcoat (n3, k3)=(1.5, 0),

topcoat D.sub.3 =5 .mu.m,

.lambda.=780 nm,

groove W.sub.G =0.4 .mu.m,

d.sub.G =1,600 .ANG.,

P=1.6 .mu.m,

lens 7 NA=0.47, F=4.3 mm, B=5 mm.

The thickness D.sub.L of the dye layer 2 on the land 35 was 1,200 .ANG. in FIG. 4 and 1,400 .ANG. in FIG. 5 while the thickness D.sub.G of the dye layer 2 in the groove 31 was varied.

Results of another calculation of relative values of the maximum tracking error signal are shown in FIGS. 6 and 7 as relative tracking signals. This calculation was based on the following data.

substrate 3 (n0, k0)=(1.58, 0),

dye layer 2 (n1, k1)=(2.4, 0.04),

reflective layer 4 (n2, k2)=(4.7, 0.159),

reflective layer 4 D.sub.2 =1,000 .ANG.,

topcoat (n3, k3)=(1.5, 0),

topcoat D.sub.3 =5 .mu.m,

.lambda.=780 nm,

groove W.sub.G =0.4 .mu.m,

d.sub.G =1,600 .ANG.,

P=1.6 .mu.m,

lens 7 NA=0.47, F=4.3 mm, B=5 mm.

The thickness D.sub.L of the dye layer 2 on the land 35 was 1,200 .ANG. in FIG. 4 and 1,400 .ANG. in FIG. 5 while the thickness D.sub.G of the dye layer 2 in the groove 31 was varied.

FIG. 8 was obtained under the same conditions as above except for

dye layer 2 (n1, k1)=(2.1, 0.02),

W.sub.G =0.45 .mu.m,

d.sub.G =2,100 .ANG.,

D.sub.L =1,800 .ANG.,

while the thickness D.sub.G of the dye layer 2 in the groove 31 was varied.

As is evident from these figures, the tracking error signal becomes zero (0) at .DELTA.D.sub.0 =D.sub.L +d.sub.G =1200+8300-1910=120 .ANG. in FIG. 4, .DELTA.D.sub.0 =1400+830-2010=220 .ANG. in FIG. 5, .DELTA.D.sub.0 =1200+1600-2270=530 .ANG. in FIG. 6, .DELTA.D.sub.0 =1400+1600-2360=640 .ANG. in FIG. 7, and .DELTA.D.sub.0 =1800+2100-3300=600 .ANG. in FIG. 8. If .DELTA.D is smaller in magnitude than .DELTA.D.sub.0, the tracking error signal has a negative value. Upon receipt of negative tracking error signals, a conventional device cannot conduct groove tracking because the polarity is reverse to that commonly used in the optical pickup control. Therefore, the present invention requires .DELTA.D>.DELTA.D.sub.0 in order to produce accurate tracking error signals.

Better results are obtained when .DELTA.D is at least 1.1.DELTA.D.sub.0, especially from 1.1.DELTA.D.sub.0 to 1.4.DELTA.D.sub.0. With D.sub.G >D.sub.L as described above, better results are obtained when D.sub.L is up to 2,200 .ANG., more preferably from 1,000 to 2,000 .ANG., most preferably from 1,300 to 2,000 .ANG.. A too smaller D.sub.L value would be concurrently accompanied by a smaller D.sub.G value and a lower modulation factor therewith. A too larger D.sub.L value would be concurrently accompanied by a larger D.sub.G value so that .DELTA.D approaches to .DELTA.D.sub.0 and the tracking error signal becomes weaker.

Moreover, D.sub.G is preferably from 1,800 to 3,200 .ANG., more preferably from 2,100 to 3,200 .ANG.. A too smaller D.sub.G would result in a lower modulation factor whereas a too larger D.sub.G would result in a lower reflectivity. From the standpoints of sensitivity, modulation factor, reproducing S/N and other factors, the dye layer 2 should preferably have a refractive index n1 of from 2.0 to 2.9 and an extinction coefficient k1 of from 0.02 to 0.08.

Push-pull tracking error signals were measured in examples in which cyanine dyes having (n1, k1)=(2,4 0.04) and (2.1, 0.02) were spin coated under varying conditions to D.sub.G and D.sub.L as used in FIGS. 4 to 8, finding substantially identical results to those of FIGS. 4 to 8.