FORM 2

The Patents Act,; 1970 (39 of 1970)
&
The Patent Rules, 2003 COMPLETE SPECIFICATION (See section 10 and rule 13)
'FOCUS SERVO DEVICE"
Pioneer Corporation, a company incorporated in Japan having its Registered Office at 4-1, Meguro 1-chome, Meguro-ku, Tokyo 153-8654 Japan.
The following specification particularly describes the invention and the manner in which it is to be performed


DESCRIPTION FOCUS SERVO DEVICE
Technical Field
The present invention relates to a focus servo device in an optical disc recording / reproducing apparatus, and more particularly to the focus servo device provided with a capture range based on an analysis of a lead-in range of focus servo.
Background Art
A focus-error detection system of the focus servo in a conventional optical disc recording / reproducing apparatus is basically designed to increase the sensitivity of the focus-error detection and thereby to extend a range of the focus-error detection. However, in this design, there are the restrictions of the capture range (which is referred to as the "CR: Capture Range" in this application as occasion demands) of the focus servo as described below.
Firstly, it is necessary for the focus servo to detect the deviation of the focus position caused by an external disturbance and keep the deviation of the focus position within the depth of focus by using an actuator.
Regarding to this, according to the standard of a DVD (Digital Versatile Disc), the focus servo is defined such that the amount of surface-runout or surface-wobble of the disc is at most plus or minus


(±) 0.3 mm and such that the focus position is within a range of ± 0.23 µm, which is the depth of focus to allow the obtaining of a good reproduction signal. Therefore, the focus-error detection range needs to be at least —0.23 µm or less, or +0.23 µm or more; otherwise there is a possibility that the focus will be off. Generally, the focus-error detection range is about 1/2 of the CR in a linear region of a focus-error signal (FES) of S-curve.
Therefore, the lower limit of the CR is as follows:
CR lower limit DVD > 0.23 X 2 X 2 = 0.92[µm].
Moreover, such a method has been taken that the CR of the focus servo in considering a multi-layered disc is 1/4 of an interlayer thickness or less, as disclosed in Japanese Patent Application Laying Open NO. Hei.8-315370, for example. Applying this method to a DVD whose Numerical Aperture (NA) is 0.6, for example, since a thickness between two layers is defined to be about 55 p,m in the DVD standard, the following expression applies:
CR upper limit DVD <55 / 4 = 13.75 [|µm].
Taking a Blu-ray Disc (BD) for an example, which has a further increased recording capacity, since a focus residual of ±0.23 µm defined by the DVD is converted into ±0.07 µm using a ratio of the depth of focus ά (λ / NA2), the CR needs to be 0.07 X 2 X 2 = 0.28 µm or more. Considering the multi-layered disc, since the interlayer thickness of the DVD is converted into about 17 µm using the ratio of the depth of focus ά(X / NA2), the CR needs to be 17 / 4 = 4.25 µm or less. Thus, the CR range in the focus-error detection system is as follows'

3 0.28 [µm] ≤ CRBD ≤ 4.25 [µm].
Disclosure of Invention
In the DVD, for example, the defocus margin, the focus close position of the focus servo, or the like have never been considered because the tilt tolerance has more effect on the reproduced signal than the defocus tolerance does.
Shown in FIG. 1 is the comparison of a wavefront aberration RMS (Root Means Square) value [λ] generated by the defocus in the DVD with that of the Blu-ray Disc (BD). FIG. 1 teaches that the BD has a larger effect of the defocus on the aberration value than the DVD does. With respect to a wavefront aberration RMS value [λ] generated by the tilt, the DVD and the BD have the substantially same aberration value, as shown in FIG. 2.
Therefore, if the numerical aperture NA of an object lens is increased more than that of the conventional DVD and a light source wavelength λ is shorten in order to make the recording capacity as large as that of the BD, the defocus has an increased effect on the reproduction signal, so that the defocus margin becomes important in determining system margin.
In this case, as is conventionally done, if the CR of the focus servo, i.e. the focus close position, is not considered, the focus servo may close at a position deviating widely from the above-described optimum focus position. On that account, the defocus margin may decrease, which causes a possibility that the system does not function.

4
It is therefore an object of the present invention to provide a focus servo device for an optical disc reproducing apparatus, having a capture range of focus servo determined for establishing a stable system in an optical disc, in which the numerical aperture NA of an object lens is increased and a light source wavelength λ, is shorten so as to increase the recording capacity of the optical disc.
The above object of the present invention can be achieved by a focus servo device for focusing a light irradiated from a light source onto a recording layer of a recording medium via an objective lens, wherein a capture range to lead in focus is determined from: a lower limit determined on the basis of an average run length period; and an upper limit determined on the basis of a track pitch.
According to the focus servo device of the present'invention, with respect to the capture range to lead-in focus, its upper limit is determined by the condition of the track pitch, while its lower limit is determined by the condition of the average run length period. By assuming that a range determined by the conditions is the capture range, it is possible to close the focus in a range in which enough defocus margin can be obtained with respect to an optical recording medium of high-recording.density and high-recording capacity, which uses a pickup optical system having a high NA and a light source giving short-wavelength light.
In one aspect of the focus servo device of the present invention, assuming that a wavelength of the light is X, a numerical aperture of the objective lens is NA, the average run length period of the recording medium normalized by λ / NA is RL, the track pitch

5
normalized by λ / NA is TP, and the capture range of the focus servo
device is CR [µam], the following equation applies'
0.114/(RL-0.72) + 0.84 ≤ CR ≤ 18000 (TP - 0.69)5 + 3.13.
According to this aspect, it is shown that the capture range is specifically a range between 0.114 / (RL - 0.72) + 0.84 and 18000 (TP -0.69)5+ 3.13.
In another aspect of the focus servo device of the present invention, the following equation applies for the average run length period RL:
RL = 2 X {integer part of [(average length of data inversion / channel bit length) + 0.5] } X channel bit length X NA / λ.
According to this aspect, it is shown that the average run length period RL is 2 X {integer part of [(average length of data inversion / channel bit length) + 0.5] } X channel bit length X NA / λ. This run length period is one factor in determining the specific range of the capture range. Incidentally, this aspect means to include not only that the left side in the above equation, "the average run length period RL", completely corresponds to the right side, "2 X {integer part of [(average length of data inversion / channel bit length) + 0.5] } X channel bit length X NA / V , but also that the left side substantially corresponds to the right side under a condition that the enough focus margin can be obtained.
In another aspect of the focus servo device of the present invention, the wavelength λ of the light ranges from 0.395 urn to 0.415 µm and the numerical aperture NA of the objective lens ranges from 0.80 to 0.90. Moreover, if the normalized track pitch TP is 0.6

6
or more and the average run length period RL is 0.86 or more, the following equation applies: 1.65[µm] ≤ CR[µm] ≤ 3.02[|am].
According to these aspects, it is possible to determine the capture range of the pickup with respect to an optical recording medium of high-recording-density and high-recording capacity, which uses a light source giving a short-wavelength light and which has a high NA.
These functions and other advantages of the present invention will be apparent from the following description of embodiments.
Brief Description of Drawings
FIG. 1 is a graph showing the relationship between defocus and generated aberration values;
FIG. 2 is a graph showing the relationship between tilt and generated aberration values;
FIG. 3 is a schematic diagram showing the relationship between a focus-error signal and a capture range;
FIG. 4 is a schematic diagram showing a calculation method of the focus-error signal using an astigmatism method;
FIG. 5 is a graph showing the capture range dependency of focus servo close positions;
FIG. 6 is a graph showing threshold values of the evaluation function of crosstalk;
FIG. 7 is a graph showing threshold values of the evaluation function of normalized average signal amplitude;
FIG. 8 is a schematic diagram showing the relationship

7
between the crosstalk and the deterioration of the signal amplitude;
FIG. 9 is a graph showing the normalized track pitch dependency of positive (+) side focus positions which will be reproduction limits;
FIG. 10 is a graph showing the normalized average run length period dependency of negative (•) side focus positions which will be reproduction limits;
FIG. 11 is a graph showing the upper limit values of the capture range;
FIG. 12 is a graph showing the lower limit values of the capture range; and
FIG. 13 is a schematic diagram showing one example of an optical disc recording / reproducing apparatus.
Best Mode for Carrying Out the Invention
The best mode for carrying out the present invention will be hereinafter explained in order, for each embodiment with reference to the drawings.
If a high NA objective lens and a short-wavelength laser diode (LD) are used as with the BD, for example, the aberration value obtained by the defocus increases more than that of the conventional DVD or the like, as described above. In this case, the defocus margin is dominant in the stability of a BD apparatus. Moreover, it has been found that the phenomena described below in (l) and (2) occur if there is a spherical aberration, so that it is necessary to define the capture range of the focus servo for ensuring reproduction

8
performance of the BD apparatus.
(1) If there is the spherical aberration, the focus close position
(a zero point in the S-curve) has a capture range dependency.
Therefore, the focus close position can be represented as a function of
the CR as in (A).
focus close position (CR)
••••(A)
(2) If there is the spherical aberration, the shape of a
reproduction beam on the disc varies depending on the defocus
direction, so that there are different main factors in the deterioration
of the reproduction signal as described below.
Firstly, in the positive direction (in which the disc and the objective lens move away from each other), the increase of the crosstalk is a main factor in the deterioration of the reproduction signal. Therefore, the crosstalk can be represented as a function of the track pitch (TP) as in (B).
crosstalk (TP)
••••(B)
Secondly, in the negative direction (in which the disc and the objective lens approach), the deterioration of signal amplitude is a main factor in the deterioration of the reproduction signal. Therefore, the signal amplitude can be represented as a function of the normalized average run length period or cycle (RL) as in (C).
signal amplitude (RL)
••••(C)
From these functions (A), (B), and (C), it is possible to set a range of

9
the capture range in which the stable reproduction signal can be obtained. The normalized average run length period will be explained later with reference to FIG. 10. (Embodiment)
An embodiment will be explained with reference to FIG. 3 to FIG. 12. FIG. 3 is a schematic diagram showing the relationship between a focus-error signal and a capture range. FIG. 4 is a schematic diagram showing a calculation method of the focus-error signal using an astigmatism method. FIG. 5 is a graph showing the capture range dependency of the focus servo close position. FIG. 6 is a graph showing the threshold value of the evaluation function of crosstalk. FIG. 7 is a graph showing the threshold value of the evaluation function of normalized average signal amplitude. FIG. 8 is a schematic diagram showing the relationship between the crosstalk and the deterioration of the signal amplitude. FIG. 9 is a graph showing the normalized track pitch dependency of the positive side focus position which will be reproduction limits. FIG. 10 is a graph showing the normalized average run length period dependency of the negative side focus position which will be reproduction limits. FIG. 11 is a graph showing upper limit values of the capture range. FIG. 12 is a graph showing lower limit values of the capture range. (Study about Focus Servo Close Position)
Generally1 the focus error signal (FES) can be obtained by integrating a beam spot intensity distribution I (u, v) on a detector. Here, I (u, v) is a reproduction beam spot intensity distribution on the detector and can be represented by an equation (l) on the basis of

10
wave optics. Incidentally, u and v are coordinates on the detector, and x and y are coordinates on a pupil of an objective lens.

••••(!) Here,
T (x, y): an aberration-free incident light
••••(2), win (x, y): a wavefront on an exit pupil
(3), and
Wout (x, y): a wavefront generated between the exit pupil and the detector.
••••(4).
A wavefront aberration w (x, y) can be represented by an equation (5) on the basis of geometrical optics using a Zernike polynomial Unm, an aberration coefficient Anm, and a wavefront analysis equation of the defocus Wdef (x, y).

••••(5)
Here, n and m are integers which are not negative, n≥m, n — m is an even number, and Anm is a constant. Moreover, the aberration characteristics depend on the values of n and m. For example, the following applies: (n, m) = (2, l): defocus
(2, 2): astigmatism (three-dimensional)

11
(2, 0): astigmatism (three-dimensional) (3, 2): coma aberration (three-dimensional) (3, l): coma aberration (three-dimensional) (4, 2): spherical aberration (three-dimensional)
Taking it as an example to explain a Blu-ray Disc (BD) with the recording capacity 27GB, which is described in NIKKEI ELECTRONICS No. 817 and the like, the track pitch TP is 0.32 µm and the shortest recording mark length is 0.138 (am in this disc.
Incidentally, this disc system has the storage capacity about 5.7 times as large as that of a disc having a storage capacity of 4.7 GB per one layer on one side of the DVD, for example, so as to record a two-hour or longer digital high vision broadcast. In order to realize this large recording capacity, the numerical aperture NA of the objective lens constituting a pickup is set higher and the light source wavelength X is set shorter.
On that account, even in the same defocus amount, the defocus gives an effect on the deterioration of the reproducing signal in the BD more severely than in the DVD, resulting that the defocus margin gets in a more difficult condition (severe condition). Therefore, the focus position at which the focus servo closes (i.e., the focus close position) becomes important for the stability of the disc system. (Study about How to Obtain Focus Close Position)
The focus close position is a focus position at which the FES is zero. The concept of the S-curve of the FES is shown in FIG. 3. As shown in FIG. 3, the capture range is determined to be a distance of the focus positions corresponding to the peak to peak (P.P) of the

12
S-curve amplitude. Generally, when the spherical aberration is generated, its S-curve's shape is different from that in the aberration-free case, so that the focus close position measured from the best focus point is different from that in the aberration.free case, which is shown in FIG. 3.
Now, the focus close position at which there is the spherical aberration in the positive (+) polarity (in such a direction that the thickness of a transmission layer increases) is studied.
As described above, the focus close position is a focus position at which the FES obtained by calculating the reproduction beam spot intensity distribution in the equation (l) is zero. If there is the aberration, win (x, y) and wout (x, y) in the equation (l) will have values other than zero.
If there is a thickness error of the transmission layer as the spherical aberration, the aberration coefficient Anm, win (x, y) in the equation (3) will have terms of A21U21 + A42U42 + A63U63 + A84U84+''*** by using the above-described Zernike polynomial Unm and under the defocus condition, win (x, y) will have a term of Wdef (x, y). Therefore, the equation (3) can be represented by an equation (6).

Incidentally, Anm and Unm corresponding to the spherical aberration in the above equation can be represented by the following equations (7) and (8) if n=8 and m=4 are even considered.

13

•••(8)
In the equation (7), NA is the numerical aperture of the objective lens, n is the refractive index of the transmission layer, λ. is the light source wavelength [µn], and AT [µm] is the thickness error of the transmission layer. With respect to the polarity of the 15 thickness error, a direction in which the thickness increases is regarded as positive. Moreover, t in the equation (8) satisfies t = x2
+ y2.
Wdef (x, y) corresponding to the defocus can be represented by equations (9) and (10).

14

•••(9)
defocuso = the amount of focus shift moved in order to minimize the RMS value [µm]
•••(10)
Here, the defocus in the equation (9) is the amount of defocus [µm] measured from the best focus point in air, and with respect to the polarity of the defocus, a direction in which the objective lens and a recording surface move away from each other is regarded as positive.
In a focusing optical system until reaching to the detector, the spherical aberration having the same amount of the above-described Win (x, y) is given. Moreover, if the astigmatism method is used for the focus error signal detection, for example, the astigmatism in the ±45 degree direction is generated and a corresponding term Was (x, y, CR) is given, so that wout(x, y) can be represented by an equation (11).



Here, Was (x, y, CR) can be represented by an equation (12).
•••(12)
As learned from the equations (ll) and (12), wout (x, y) can be represented as a function of CR as shown in an equation (13).

15
••-(13)
Therefore, the equation (l) can be represented by an equation (14), and thus I (u, v) is found to be a function depending on CR.

••(14)
With respect to the FES in the astigmatism method which uses a four-division detector 10, as shown in FIG. 4, the FES can be obtained by calculating (S1 + S3) - (S2 + S4) at a subtracter 13 from (S1 + S3) calculated at an adder 11 and (S2 + S4) calculated at an adder 12 using values (S1 to S4) detected at relative detection elements lOa to lOd for detecting I (u, v, CR). The focus position at which the FES obtained in the above manner is zero will be the close position of the focus servo, and thus it is found that the focus servo close position has a CR dependency.
As the amount of spherical aberration assumed in the BD, the thickness dispersion of a transmission layer in the disc surface is expected to be about ± 3 µm in an optical disc with the 0.1 mm transmission layer (a cover layer), which is as in the BD, according to this literature Jpn. J. Phys. Vol. 39 (2000) pp 775-778. In addition, the RMS value of residual spherical aberration of the objective lens and other optical systems is about 20 λ, and this spherical aberration amount is converted into a thickness error of about ± 2 µm. Thus, as the worst condition, it is assumed that the spherical aberration is 5 µm in the equivalent of the thickness error.
Therefore, if AT = +5 [µm] in the equation (7), a function

16
indicating the CR dependency of the focus servo close position can be obtained by calculating the equation (14), and can be represented by the following approximation function in an equation (15).
fclose(CR)=-0.29/(CR+0.5)2-0.038 [µm]
••(15)
FIG. 5 shows an expansion of the equation (15) to an illustration, wherein the vertical axis is the focus servo close position [µm] and the horizontal axis is the capture range [µm]. The zero point in FIG. 5 is the best focus point.
(Study about Evaluation Functions of Factors in the Deterioration of Reproduction signal)
Next, the factors in the deterioration of the reproduction signal are studied in order to obtain the focus position which will be the reproduction limit. As the factors in the deterioration of the reproduction signal, there are assumed the increase of the crosstalk and the deterioration of the signal amplitude. The evaluation functions representing the relative characteristics are as follows, (i) Crosstalk Evaluation Function
The crosstalk is generated by a signal from an adjacent track leaking into a signal of a reproduction track. Here, the crosstalk amount varies along with the track pitch TP and can be represented by an equation (16).
Crosstalk (TP)
= light intensity on the adjacent track / light intensity on the
reproduction track [dB]
•••(16)

17
The above equation is specifically a function represented by an equation (17).

Here, u' and v' are coordinates on the disc normalized by λ / NA, where v' is the one in the radial direction and u' is the one in the tangential direction. In the equation (17), the integration range in the numerator (Adjacent_Track) is a radial-direction range corresponding to the adjacent track, and the integration range in the denominator (Main_Track) is a radial-direction range corresponding to the reproduction track. Moreover, f (x, y) is a complex amplitude distribution on the exit pupil and can be represented by an equation (18) using the equations (2) and (3).

•••(18) (ii) Signal Amplitude Evaluation Function
What influences the deterioration of the signal amplitude is the deterioration of a MTF (Modulation Transfer Function). Here, the normalized average run length period RL is defined as an equation (19) and the MTF at a frequency corresponding to the period is used to represent the evaluation function of the signal amplitude. Incidentally, [x] means a maximum integer not greater than x. An average length of data inversion is an average of distances from 1 to

18
0 or from 0 to 1 after the reproduction signal is binarized..
normalized average run length period RL
= 2 X {integer part of [(average length of data inversion /
channel bit length) + 0.5] } X channel bit length X NA / λ.
***(19)
Assuming that a normalized frequency corresponding to the normalized average run length period RL is represented by an equation (20), the MTF can be represented by an equation (21). Here, f* is a complex conjugate function.
s = 1 / normalized average run length period
• • • (20)

•••(21)
Using the above equation, a function of the signal amplitude can be represented by an equation (22).
normalized average signal amplitude (s)
= fmtf (s) in considering the aberration / fmtf (s) when free of the aberration [dB]
•••(22) (Specific Example)
A specific example in which these objective functions of the crosstalk and the signal amplitude are applied to an optical disc of

19
the Blu-ray Disc having a 27GB recording capacity will be explained.
A threshold value obtained when the value of each objective function becoming the reproduction limit is basically taken as a value at which jitter is 15% when a scalar diffraction simulation is performed with using parameters of the DVD.
Since the crosstalk is mainly generated by the radial tilt (i.e., the tilt in the radial direction), it is studied with increasing the radial tilt. As a result, as shown in FIG. 6, the jitter is 15% when the radial tilt is 0.75 degrees, and the crosstalk objective function at this time is -16 dB.
The signal amplitude is studied with the defocus. The reason of using the defocus is that the use of tangential tilt (i.e., the tilt in the tangential direction) may influence the phase shift in the reproduction signal and thus the deterioration of the signal amplitude cannot accurately represent the deterioration of the reproduction signal. If the system has defocus, the crosstalk also occurs, but the evaluation is performed in the condition that its track pitch is broader than that of the DVD, thereby reducing the influence of the crosstalk. In this case, the jitter is 15% when the defocus is 0.9 µn as shown in FIG. 7. At this time, the signal amplitude objective function is — 3 dB.
Thus, by obtaining the focus positions when the crosstalk objective function and the signal amplitude evaluation function take the threshold values as shown in FIG. 6 and FIG. 7, respectively, the focus positions which will be the reproduction limits can be obtained.
In the optical disc of the Blu-ray Disc having a 27 GB

20
recording capacity, the track pitch TP is 0.32 µm and the shortest recording mark length Tmin is 0.138 µm. In this case, a normalized TP is 0.672 and the normalized average run length period RL is 0.868 because it is based on 17 PP as an encoding method. In this case, changing the focus position and evaluating the crosstalk and the signal amplitude with the equations (17) and (22) give the result shown in FIG. 8. FIG. 8 teaches that the focus positions which will be the reproduction limits can be shown in equations (23) and (24).
reproduction limit focus position in the positive (+) direction = +0.05 µm
••(23 In this case, the crosstalk increase is a dominant factor.
reproduction limit focus position in the negative (-) direction = -0.21 µm
••-(24)
In this case, the deterioration of the signal amplitude is a dominant factor.
This result teaches that the factor in determining the focus position of the reproduction limit varies along with the focus direction.
Using the focus position which will be the reproduction limit as obtained above, it is possible to obtain the stably reproducible optimum focus servo close position. Therefore, if using the focus servo having a CR which enables the focus to close at this optimum focus servo close position, it is possible to obtain the stable reproduction signal and prevent the deviation of the focus servo.
Next, on the basis of the above-described condition, the

21
specific value of the capture range will be obtained. About 0.11 µm defocus margin is required between the focus position of the reproduction limit and the optimum focus servo close position. The details are a servo residual 0.04 (µm, an offset of an electric system 0.02 µm, and a pickup adjustment error 0.05 µm, and thereby the total is 0.11 um.
The defocus margin estimated from the reproduction limit focus position in the equation (23) is —0.06 (µm. Substituting this into the equation (15) gives a CR value of 3.13 um corresponding to the reproduction limit in the position (+) direction. This value is the upper limit of the CR. In the same manner, the defocus margin estimated from the reproduction limit focus position in the equation (24) is —0.10 (µm. Substituting this into the equation (15) gives a CR value of 1.66 um corresponding to the reproduction limit in the negative (•) direction. This value is the lower limit of the CR. Therefore, the CR range can be represented by an equation (25).

•-•(25)
Designing the focus servo to be in a range in which the CR satisfies the equation (25) makes it possible to close the focus servo at the focus position at which the stable reproduction signal can be obtained, thereby allowing a stable recording / reproducing system of the optical disc.
So far, the CR in the optical disc of the Blu-ray Disc having a 27 GB recording capacity has been explained. This CR can be represented by a function of the track pitch or the normalized

26
On the basis of these error signals, the position of an objective lens of the pickup 22 is controlled by a drive controller 24 in a pickup control circuit 25 with respect to the focus servo and a tracking servo, and the rotation of a spindle motor 27 is controlled by a motor driving control circuit 26. Moreover, there is a positioning servo for determining the position of the pickup 22 in the radial direction of the optical disc by using a slider, and the position is controlled using position information of the pickup 22, a track address signal of the optical disc, and the like.
The focus servo device associated with the present invention is provided with the above-described capture range, and by applying this device for the optical disc of the Blu-ray Disc having a 27 GB recording capacity, it is possible to realize the focus servo and its stable lead-in.
Obviously, the optical disc recording / reproducing apparatus 20 to which the focus servo device associated with the present invention is applied is provided not only with the focus servo system, the tracking servo system, and the spindle servo system, but also with other mechanisms required for the optical disc recording / reproducing apparatus, such as a recording / reproducing signal processing system and a signal input / output system.
As explained above, according to the focus servo device of the present invention, it is possible to optimize the capture range to close the focus servo at the focus position where the required defocus margin can be ensured. Therefore, it is possible to perform the recording / reproduction with respect to the optical recording

27
medium.
The present invention is not limited to the above*described embodiments, and changes may be made if desired without departing from the scope or spirit of the invention which can be read from the claims and the entire specification. A focus servo device that accompanies such changes is also intended to be within the technical scope of the present invention.
Industrial Applicability
A focus servo device associated with the present invention can be applied to a high*density optical disc for consumer or industrial use, such as a DVD, on which various information can be recorded at high density and further can be applied to a DVD player, a DVD recorder, and the like. Moreover, the focus servo device can be applied to an optical recording medium of high-recording-density and high-recording capacity, which uses a pickup optical system having a high NA and a light source giving short-wavelength light.

28 CLAIMS
1. A focus servo device for focusing a light irradiated from a light
source onto a recording layer of a recording medium via an objective
lens, wherein a capture range to lead in focus is determined from-
a lower limit determined on the basis of an average run length period; and
an upper limit determined on the basis of a track pitch.
2. The focus servo device according to claim 1, wherein,
assuming that a wavelength of the light is λ, a numerical aperture of
the objective lens is NA, the average run length period of the
recording medium normalized by λ / NA is RL, the track pitch
normalized by λ I NA is TP, and the capture range of said focus servo
device is CR [[µm], the following equation applies:
0.114/(RL-0.72) + 0.84
≤ CR ≤ 18000 (TP - 0.69)5 + 3.13.
3. The focus servo device according to claim 2, wherein the
following equation applies for the average run length period RL:
RL = 2 X {integer part of [(average length of data inversion / channel bit length) + 0.5] } X channel bit length X NA / X.
4. The focus servo device according to claim 3, wherein the
wavelength X of the light ranges from 0.395 µm to 0.415 µm and the

29 numerical aperture NA of the objective lens ranges from 0.80 to 0.90.
5. The focus servo device according to claim 4, wherein if the
normalized track pitch TP is 0.6 or more and the average run length 5 period RL is 0.86 or more, the following equation applies:



To,
The Controller of Patents, The Patent Office at Mumbai.