JPWO1997033276A1 - Focusing device and optical disc device using same - Google Patents

Abstract

(57) [Abstract] An optical disk (100) is used as a recording medium, which is made by bonding together two substrates, each having a thickness h and at least one of which has a signal mark or a reflective layer that can become a signal mark on its back surface. A laser beam (210) is irradiated onto this optical disk (100) through an objective lens (203), and the light reflected from the reflective layer is detected by a photodetector (213). The output of this photodetector (213) is supplied to a signal reader (215) on the one hand, and to a focusing device (216) on the other hand, which detects a focus error of the objective lens (203) and controls the position of the objective lens (203) relative to the optical disk (100). In this optical disc device, when the central wavelength of the laser light source (206) is _λ0 , the half-width of the laser light source is Δλ, the numerical aperture of the objective lens is NA, the focal length of the objective lens is f, the refractive index of the substrate material _{is n2} , and the maximum diameter of the laser spot light irradiated onto the signal pit is dmax, and when the general function formula for finding the maximum half-width Δλ determined based on _λ0 , NA, f, and dmax is g, the half-width Δλ of the laser light source satisfies the following formula:

Description

[Detailed Description of the Invention] Focusing Device and Optical Disc Drive Using the Same [Technical Field] The present invention relates to a focusing device that is highly effective when recording and playing back signals on a Digital Versatile Disc (hereinafter abbreviated as DVD), and to an optical disc drive for DVDs that uses the same.

[Background Art] In recent years, there have been dramatic advances in digital signal processing, optical disc manufacturing, and optical disc recording technologies. These advances have led to the emergence of optical discs with recording capacities several times larger than conventional compact discs (CDs), despite being the same size.

This new type of optical disc, commonly referred to as a DVD, records and plays back information such as audio, images, programs, and computer data using digital signals. A global standard for DVDs (hereafter referred to as the DVD standard) has already been established, and the disc is expected to become a versatile recording medium suited to the multimedia era.

According to the DVD standard, DVDs have the same diameter as CDs (120 mm) and thickness (1.2 mm). However, to increase storage capacity, DVDs use two 0.6 mm discs bonded together, allowing for double-sided recording and playback. The laser beam used for recording and reading has a central wavelength of 650/635 nm, the objective lens used for recording and reading has a numerical aperture (NA) of 0.6, the track pitch is 0.74 μm, the minimum pit length is 0.4 μm, and the maximum pit length is 2.13 μm. These specifications allow for a single-sided storage capacity of over 4 gigabytes.

As can be seen from the above specifications, the depth of the recording layer (i.e., the depth from the substrate surface to the reflective recording layer) of a DVD is approximately half that of a conventional CD, and the minimum pit length and track pitch are also approximately half. Furthermore, the center wavelength of the laser beam used for recording and reading is significantly shorter than the 780 nm used for CDs. For this reason, it is often difficult to obtain a focus error signal using techniques similar to those used with conventional CD players.

In other words, with this type of optical disc, signals are recorded in the form of pits on the reflective recording layer. To read these signals from the optical disc, a laser beam is typically projected onto the rotating optical disc through an objective lens, and the recorded signal is obtained by detecting the light reflected from the reflective recording layer formed within the optical disc. At the same time, a portion of the reflected light is used to detect a focus error, and this error signal is used to control the objective lens's focusing so that the reflective recording layer is positioned at the focal point of the objective lens. The quality of this focusing control depends on the optical conditions from the objective lens to the reflective recording layer and the characteristics of the laser beam used.

Previously known DVD players have used a focusing control method similar to that used in CD players. However, this method has not been able to achieve high-precision control due to the optical conditions described above, which has led to problems such as a decrease in the signal-to-noise ratio of the playback signal and increased jitter.

[DISCLOSURE OF THE INVENTION] The present invention aims to provide a focusing device that can perform highly accurate focusing control without being affected by the optical conditions of the optical disc, and an optical disc device for DVDs that can use this focusing device to perform highly accurate recording and playback.

An optical disk device according to one embodiment of the present invention uses, as a recording medium, an optical disk formed by bonding two substrates, each having a thickness h, with a signal mark or a reflective layer capable of becoming a signal mark on the back surface of at least one of the substrates. A photodetector is provided for irradiating the optical disk with a laser beam through an objective lens and detecting light reflected from the reflective layer. The output of the photodetector is supplied to a signal reader on one side and to a focusing device that detects a focus error of the objective lens and controls the position of the objective lens relative to the optical disk on the other side.

Here, this optical disc device is characterized in that, when the central wavelength of the laser beam is _λ0 , its half-width is Δλ, the numerical aperture of the objective lens is NA, and the refractive index of the substrate material that makes up the optical disc is _n2 , the half-width Δλ of the laser beam satisfies the following formula (1): In other words, a laser light source that outputs a laser beam that satisfies formula (1) is used.

Δλ≧ _λ02 /{ ^2h ( _n22 ^{- NA2} ) ^0.5 } (1)Note that Δλ naturally has an upper limit. That is, it is limited by the condition that gives the maximum allowable laser spot beam diameter dmax, taking into account the chromatic aberration of the objective lens based on Δλ. Specifically, when the focal length of the objective lens is f and the general function for finding the maximum half-width Δλ determined based on the _λ0 , NA, and dmax is g, the upper limit of the half-width Δλ is Δλ≦g( _λ0 , NA, f, dmax).

In this optical disc device, a laser beam emitted from a laser light source passes through an objective lens and is then irradiated onto an optical disc. A portion of this irradiated light is reflected by the surface of the optical disc. The reflected light reflected by the surface of this optical disc is designated _U1 . Meanwhile, the remaining light is incident on a transparent material layer (substrate) that forms the surface portion of the optical disc, and is then reflected by a reflective layer that forms signal marks, passes through the transparent material layer again, and is emitted from the surface of the optical disc. This emitted light is designated _U2 .

Here, if the maximum angle of the incident light beam is _θ1 (where _sinθ1 = NA), the refractive index of air is _n1 , and the refraction angle in the transparent material layer is _θ2 , the optical path difference ΔL between _U1 and _U2 can be calculated using equation (2).

ΔL=2·n ₂ ·h·cos θ ₂ (2) Furthermore, according to Snell's law, the following holds: n ₁ ·sin θ ₁ =n ₂ ·sin θ ₂ (3)

On the other hand, the coherence length Lc of the laser beam is expressed as follows: Lc=λ ₀ ² /Δλ (4)

Therefore, if the optical path difference ΔL is shorter than the coherence length Lc, optical wave interference occurs between the surface of the optical disc and its internal reflective layer. Under these conditions, if the optical disc has thickness variations in the circumferential or radial direction, or if there is variation in the thickness of optical discs from different manufacturers, the above-mentioned interference will cause the focus error signal to fluctuate in synchronization with the rotation of the optical disc. As a result, high-precision focusing control will be impossible, resulting in a decrease in the S/N ratio and increased jitter in the playback signal.

The effect of such fluctuations in interference intensity on the focus error signal will now be described in more detail.

When light waves are multiple-reflected and interfere between the surface and the internal reflective layer of an optical disk, the intensity Ir of the total luminous flux emitted from the substrate with refractive index _n2 to the air side with refractive index _n1 is expressed by the following equation:

Ir = {4R ^sin2 δ/2 Ii} / {(1-R)2 + 4R ^sin2 δ/2} (5) In equation (5), Ii is the incident light intensity, and R is the reflectance at the interface between air (refractive index _n1 ) and the substrate (refractive index _n2 ). δ is related to the phase difference between reflected light _U1 at the interface and reflected light _U2 from the reflective layer, which is the information recording surface, and is given by the following equation:

δ=(2π/λ ₀ )·ΔL=(4πn ₂ /λ ₀ )h·cos θ ₂ (6) From equation (2), equation (6) becomes as follows.

δ＝（４π/ _λ0 ）h・( _n22 − ⁿ¹² _・ ^sin2θ1 ) ^0.5・・^・(7) From equation ( ₅ ), we can see that when the phase difference δ changes from 0 to π, the interference intensity takes on values ranging from 0 (dark) to maximum (bright). Therefore, from equation (7), the period Δh of the light and dark interference fringes is given by Δh＝ _λ0 /{４( _n22 − _n12 ^{・ sin2θ1} ) _0.5 ^}・・^・(8)

Substituting _n1 = ₁ (air), _n2 = _1.5 (substrate material), and _θ1 = 0 to 36.9 deg (the DVD standard specifies NA = 0.6 = _sinθ1 , from which the maximum angle of incidence can be determined) into equation (8) for n1, _n2 , and θ1, the minimum and maximum values of the period Δh of the light and dark interference fringes become Δhmin = _λ0 /6 and Δhmax = _λ0 /5.5, respectively.

According to the DVD standard, the central wavelength _λ0 of the laser beam used is _λ0 = 650/635 (nm). Therefore, when a laser beam with a central wavelength _λ0 = 635 (nm) is used, Δhmin = 0.105 (μm) and Δhmax = 0.115 (μm). Thus, the difference between Δhmin and Δhmax is only 0.01 (μm). Therefore, for light beams within a numerical aperture NA = 0.6, their interference intensity fluctuates for approximately the same Δh (i.e., uneven substrate thickness). Therefore, when the substrate thickness fluctuates as the optical disc rotates, the interference fringes change from bright to dark at their maximum width, resulting in a fluctuation in the focus error signal.

However, this invention uses a laser beam whose half-width Δλ satisfies equation (1). This allows the coherence length Lc to be shorter than the optical path difference ΔL, thereby preventing multiple interference of light waves between the surface and the internal reflective layer of the optical disc. Therefore, this invention allows for accurate detection of focus errors, enabling highly accurate focusing control and preventing a decrease in the S/N ratio and jitter in the playback signal.

[Brief Description of the Drawings] Figure 1 is a block diagram of an optical disc device according to the present invention.

FIG. 2 is a perspective view of a DVD used as a recording medium in the device shown in FIG.

3 is a partial cross-sectional view of the DVD shown in FIG. 2. FIG.

FIG. 4 is a diagram showing an optical system of the apparatus shown in FIG. 1 .

FIG. 5 is a front view of an optical sensor incorporated in the optical system shown in FIG. 4 to acquire a focus error signal.

Figure 6 shows the power spectrum of a laser diode incorporated into the optical system shown in Figure 4 and used for signal recording/readout and focusing control.

FIG. 7 is a diagram showing a focus error signal detected in the apparatus shown in FIG.

FIG. 8 shows the power spectrum of a laser diode that cannot be used in the optical disk device according to the present invention.

FIG. 9 shows a focus error signal obtained when using a laser diode having the characteristics shown in FIG.

FIG. 10 is a diagram showing the power spectrum of another laser diode that can be used in the optical disk device according to the present invention.

FIG. 11 is a diagram showing the power spectrum of yet another laser diode that can be used in the optical disc device according to the present invention.

FIG. 12 is a schematic diagram of a DVD master manufacturing apparatus incorporating the focusing device of the present invention.

FIG. 13 shows the main components of the focusing device incorporated in the device shown in FIG.

Figure 14 shows the power spectrum of a laser diode incorporated into the focusing device shown in Figure 13 and used for focusing control. Figure 15 is a schematic diagram of an exposure device incorporating a focusing device according to another embodiment of the present invention.

[Best Mode for Carrying Out the Invention] Figure 1 shows an optical disc device, in this case a DVD player, according to the present invention.

Reference numeral 100 denotes a DVD as an optical disc. This DVD 100 is manufactured in accordance with the DVD standard and is specifically formed as shown in Figures 2 and 3.

That is, DVD 100 is formed by bonding first and second information recording members 101, 102, each 0.6 mm thick, formed into a disc shape, to a total thickness of approximately 1.2 mm. First and second information recording members 101, 102 each include a light-transmitting substrate 103, 104, respectively. Reflective layers 105, 106, which have pits corresponding to recorded information such as compressed motion picture information and which reflect light, are applied to the back surfaces of these light-transmitting substrates 103, 104. Protective layers 107, 108, respectively, are provided on these reflective layers 105, 106 to prevent oxidation of the reflective layers. The first and second information recording members 101, 102 are bonded together by adhesive layer 109, which is made of a thermosetting adhesive and is sandwiched between protective layers 107, 108. A clamping hole 110 for clamping is provided in the center of the DVD 100, and a clamping zone 111 is provided around it.

Regarding the materials of each part constituting the DVD 100, the light-transmitting substrates 103 and 104 are made of polycarbonate or a resin containing polycarbonate or PMMA (polymethyl methacrylate). The reflective layers 105 and 106 are formed of a thin aluminum film. The protective layers 107 and 108 are formed of a photocurable resin (ultraviolet curable resin). The adhesive layer 109 is made of a hot-melt adhesive (thermoplastic resin adhesive), such as a polyvinyl ether paraffin-based material: [ _-CH2 -CH=CH- _CH2 ]n-[ _CH2 -CH(OCH3)-] _n '.

In the optical disc device shown in FIG. 1, a DVD 100 is chucked to a tapered cone 200 and rotated by a spindle motor 201 at a rotation speed of, for example, 1350 rpm. The spindle motor 201 is driven by a spindle motor drive circuit 202. Meanwhile, the recording/playback optical system is configured as follows: An objective lens 203 is disposed facing one side of the DVD 100. Movement of the objective lens 203 is controlled in the optical axis direction by a focus coil 204 and in the track width direction by a tracking coil 205. Opposite the objective lens 203 is a semiconductor laser diode (hereinafter abbreviated as LD) 206, which is disposed so as to be movable integrally with the objective lens 203. The LD 206 is energized by an LD driver 207.

The laser beam emitted from the LD 206 is converted into a parallel beam by a collimating lens 208 and then enters a polarizing beam splitter 209. Since the laser beam emitted from the LD 206 generally has an elliptical far-field pattern, if a circular pattern is required, a beam shaping prism (not shown) can be placed after the collimating lens 208.

The laser beam passing through the polarizing beam splitter 209 is focused by the objective lens 203 and, as shown in FIGS. 3 and 4, is incident on the light-transmitting substrate 103 or 104 of the DVD 100. In this example, the numerical aperture NA of the objective lens 203 is 0.6, in accordance with the DVD standard.

Here, we will explain the light emission characteristics of the LD 206, that is, the power spectrum of the LD 206. That is, as shown in Fig. 4, when the thickness of the light-transmitting substrate 104 (103) is h, the maximum angle of the incident light beam 210 incident on the light-transmitting substrate 104 (103) _{is θ1} (where sin _θ1 = NA), the refractive index of air _{is n1} , the refraction angle in the light-transmitting substrate 104 (103) _is θ2, the center wavelength of the laser beam emitted from the LD 206, that is, the incident light beam 210, is _λ0 , and its half-width is Δλ, the LD 206 used satisfies the condition Δλ≧ _λ02 /{ ^2h ( _n22 ^{- NA2} ) ^0.5 }.

Specifically, as shown in FIG. 6, for example, an LD having a central wavelength λ ₀ =635 (nm) and a half-width Δλ=0.5 (nm) is used.

During playback, a laser beam incident on the light-transmitting substrate 104 (103) of the DVD 100 via the objective lens 203 is focused as a tiny beam spot on the reflective layer 106 (105). The reflected light from the reflective layer 106 (105) passes through the objective lens 203 in the opposite direction to the incident light, is reflected by the polarizing beam splitter 209, and passes through a detection optical system including a condenser lens 211 and a cylindrical lens 212 before entering a photodetector 213.

As shown in Figure 5, photodetector 213 is composed of four photodetector elements 214a-214d arranged on the same plane. The four detection outputs of photodetector 213 are input to amplifier array 215, which includes amplifiers and adder-subtractors, and generates a focus error signal F, a tracking error signal T, and a playback signal S.

The tracking error signal T is obtained by a known technique called the push-pull method. The focus error signal F is obtained by a known astigmatism method. In this astigmatism method, the output Ia of the diagonally opposite photodetector element 214a (shown in FIG. 5) and the output Ib of the photodetector element 214b are added together, and a signal obtained by adding the output Ic of the photodetector element 214c and the output Id of the photodetector element 214d is subtracted from this sum signal, and the focus error signal is obtained from this subtraction signal.

The focus error signal F and the tracking error signal T are supplied to the focus coil 204 and the tracking coil 205, respectively, via the servo controller 216. This controls the movement of the objective lens 203 in the optical axis direction and the track width direction, thereby focusing the light beam on the reflective layer 106 (105), which is the recording surface of the DVD 100, and tracking the light beam on the target track.

Meanwhile, the playback signal S from the amplifier array 215 is input to the signal processing circuit 217, where it undergoes waveform equalization and binarization. During binarization, the waveform-equalized playback signal is fed to a PLL (phase-locked loop) and a data discrimination circuit, where the PLL extracts the channel clock, which is the basic clock used when recording information on DVD 100, from the playback signal. The playback signal is then discriminated between "0" and "1" based on the channel clock, thereby identifying the data recorded on DVD 100 and obtaining a data pulse. Specifically, data discrimination is performed by comparing the waveform-equalized playback signal with an appropriate threshold value within a predetermined time period (called the detection window width or window width) based on the timing of the rising or falling edge of the channel clock.

The data pulses detected by signal processing circuit 217 are input to disc controller 218, where they undergo format decoding, error correction, and other processes before being input to MPEG2 decoder/controller 219 as a bit stream of video information. On DVD 100, video information compressed and encoded according to the MPEG2 standard is recorded as a pit pattern on reflective layers 105 and 106. MPEG2 decoder/controller 219 then decodes (decompresses) the input bit stream to reproduce the original video information.

The reproduced moving image information is input to a video signal generating circuit 220, where a blanking signal and other signals are added to produce a video signal in a predetermined television format, such as NTSC, and the video signal is displayed on a display (not shown).

As explained above, this DVD optical disc device uses an LD 206 with light emission characteristics that satisfy the condition of equation (1), so the coherence length Lc expressed by equation (4) can be shortened relative to the optical path difference ΔL expressed by equation (2). As a result, optical wave interference between the surface of the DVD 100 and the internal reflective layer 105 (106) can be prevented.

To explain this numerically, for example, the LD 206 emits a laser beam with a center wavelength λ ₀ = 635 (nm) and a half-width Δλ = 0.5 (nm). Therefore, the coherence length Lc of this laser beam is calculated as Lc = 0.8 (mm) from equation (4). Meanwhile, the DVD 100 is made by bonding together information recording media 101 and 102 with a thickness h = 0.6 (mm) in accordance with the DVD standard, and the objective lens 203 is also set to a numerical aperture NA = 0.6 in accordance with the DVD standard. Therefore, if the light-transmitting substrates 103 and 104 constituting the information recording media 101 and 102 are made of polycarbonate with a refractive index n ₂ = 1.5, the optical path difference ΔL is calculated as ΔL = 1.650 (mm) from equation (2).

In this way, a laser beam whose half-width Δλ satisfies equation (1) is used. This allows the coherence length Lc to be shorter than the optical path difference ΔL, thereby preventing multiple interference of light waves between the surface of the DVD 100 and the internal reflective layer. Therefore, as shown in FIG. 7, a focus error signal that changes linearly with the focus direction displacement ΔZ of the objective lens 203 can be obtained. This enables highly accurate focusing control, preventing a decrease in the S/N ratio and jitter degradation of the playback signal.

If a laser beam whose half-width Δλ does not satisfy equation (1), for example, a laser beam with a center wavelength _λ0 = 685 (nm) and a half-width Δλ = 0.2 (nm) as shown in Figure 8, is used, the following occurs: In this case, the coherence length Lc is Lc = 2.34 (mm), which is longer than the optical path difference ΔL = 1.650 (mm). Therefore, interference between _U1 and _U2 (see Figure 5) cannot be prevented. As a result, only a focus error signal that changes pulsatingly with respect to the focus direction displacement ΔZ of the objective lens can be obtained, as shown in Figure 9.

As can be seen from the above explanation, in order to achieve more accurate recording and playback in optical disk devices that use DVDs as recording media, and in optical disk devices that handle high-definition DVDs known as High Definition DVDs that are expected to appear in the future, it is necessary to avoid the interference between _U1 and _U2 mentioned above. To do this, it is necessary for the optical path difference ΔL to always be longer than the coherence length Lc, even if the DVD has uneven thickness, for example. To shorten the coherence length Lc, it is desirable to use an LD with a large half-width Δλ.

The power spectrum of one such LD is shown in Figure 10. As can be seen from this figure, this LD has a central wavelength λ ₀ =650 (nm) and a half-width Δλ=0.3 (nm).

The coherence length Lc of this LD is calculated to be Lc = 1.408 (mm), which is shorter than the optical path difference ΔL = 1.65 (mm). Therefore, interference between _U1 and _U2 can be prevented.

In addition, in an optical disc device that handles High Definition DVDs, as shown in FIG. 11, the center wavelength λ₀Therefore, it is necessary to use a laser beam of about λ = 417 (nm).₀=417(nm), h=0.6(mm), n₂= 1.5 Substituting this, we obtain the required half-width Δλ, which is Δλ ≥ 0.105 (nm). In other words, for optical disc devices that handle High Definition DVDs, the center wavelength λ₀ The wavelength must be between 410 nm and 420 nm, with a half-width Δλ of 0.105 nm or greater. In addition, even for optical disc devices with more advanced features than High Definition DVD, U₁and U₂and interference can be prevented.

In the above embodiment, a DVD player is shown as an example of an optical disc, but the present invention can also be applied to ROM (DVD-ROM) and RAM (DVD-RAM) for recording and playing programs, computer data, etc.

FIG. 12 shows an optical disc master recording apparatus equipped with a focusing device according to the present invention. This master recording apparatus is used in the initial stage of master production, in the process of recording information using a laser beam on a resist master disk coated with photoresist.

Reference numeral 300 denotes a light source. This light source 300 is composed of, for example, a Kr ⁺ laser tube (wavelength 351 nm). A laser beam 301 emitted from the light source 300 is shaped into a parallel beam by a lens group 302, reflected by mirrors 303 to 308, and an electro-optical modulator 309 removes AC noise from the optical power. The laser beam then passes through a wave plate 310, is reflected by a mirror 311, the DC component of the optical power is stabilized by an acousto-optical modulator 312, is reflected by mirrors 313 and 314, and is then modulated by an acousto-optical modulator 315 in accordance with the information to be recorded. Note that the control system for controlling the acousto-optical modulator 315 is omitted from this diagram.

The recording light is reflected by mirror 316, has its beam diameter expanded by magnifying optical system 317, is reflected by mirrors 318 and 319, passes through polarizing beam splitter 320 and λ/4 plate 321, is reflected by dichroic mirror 322, and is focused by objective lens 324 in objective lens unit 323 to form a recording spot on the photosensitive agent surface of optical disk master 325.

This recording spot forms a recording area on the photosensitive surface of the optical disk master 325. This recording area is formed on a spiral trajectory by the rotation of the optical disk master 325 and the radial movement of the slider 326 supporting the objective lens unit 323. During this time, the objective lens 324 is controlled by a focusing device 327 so that its focal point is always located on the photosensitive surface of the master 325. The optical disk master 325 with recorded information is developed so that pits appear in the recording area, and is then silver-plated and used as a disk master.

The focusing device 327 comprises a coarse adjustment system 328 that roughly adjusts the position of the objective lens 324 before the start of recording, and a fine adjustment system 329 that finely adjusts the position of the objective lens 324 using the recording light during recording.

As shown in FIG. 13 , the coarse adjustment system 328 includes an objective lens actuator 330 that adjusts the position of the objective lens 324 in the optical axis direction, a focus error detection optical system 331 that optically detects focus errors of the objective lens 324, and a control unit 332 that controls the objective lens actuator 330 based on a signal obtained by the detection optical system 331.

The focus error detection optical system 331 is an optical system that uses the astigmatism method and includes a light source 341, a collimator lens 342, a cylindrical lens 343, relay lenses 344a and 344b, a polarizing beam splitter 345, a λ/4 plate 346, a mirror 347, a spherical lens 348, a cylindrical lens 349, and a four-segment light-receiving element 350.

Light source 341 emits a divergent elliptical light beam. This light beam is converted into a slightly convergent light beam by collimating lens 342 and then shaped into a circular light beam by cylindrical lens 343. The light beam then passes through relay lenses 344a and 344b and enters polarizing beam splitter 345. The laser light emitted from light source 341 is linearly polarized and set to p-wave. Therefore, the light beam entering polarizing beam splitter 345 passes through it, is converted into clockwise circularly polarized light by λ/4 plate 346, is reflected by mirror 347, passes through dichroic mirror 322, and is focused onto master 325 by objective lens 324.

The reflected beam from the master 325 is circularly polarized in the reverse direction, i.e., left-handed, and enters the objective lens 324, passes through the dichroic mirror 322, is reflected by the mirror 347, and is converted into an S-wave by the λ/4 plate 346. It is then reflected by the polarizing beam splitter 345, converted into a convergent beam by the spherical lens 348, passes through the cylindrical lens 349, and enters the four-segment light-receiving element 359. The light-receiving portions 351a, 351b, 351c, and 351d of the four-segment light-receiving element 350 output signals Ia, Ib, Ic, and Id corresponding to the amount of light received, respectively.

The control unit 332 receives the signals Ia-Id from the four-segment light-receiving element 350 and performs the calculation (Ia + Ib) - (Ic + Id) based on the astigmatism method. This calculation produces a focus error signal indicating the amount of defocus of the objective lens 324. The control unit 332 feedback-controls the objective lens actuator 330 to maintain the focus error signal at zero. This controls the position of the objective lens 324 so that its focal point always coincides with the photosensitive layer surface of the master 325.

The master recording device shown in Figure 12 is intended to produce a master with a track pitch of, for example, 0.74 μm. For this purpose, the objective lens 324 has a numerical aperture N = 0.9 and a focal length f = 4.002 mm (wavelength 351 nm). The working distance d (the distance from a protective, light-transmitting parallel plate attached to the bottom surface of the objective lens 324 to the surface of the master 325) is 350 μm.

The coarse adjustment system 328 of the focusing device 327 uses an SLD (superluminescent diode, manufactured by Anritsu Corporation) as the light source 341, whose power spectrum is shown in Fig. 14. As shown in Fig. 14, the power spectrum of this SLD has a central wavelength _λ0 = 670 (nm) and a half-width Δλ = 20 (nm). Therefore, the coherence length Lc of the light emitted from the SLD is Lc = 22 (µm) according to equation (4).

As described above, the coarse adjustment system 328 uses a light source 341 with a power spectrum that can make the coherence length Lc sufficiently small compared to the working distance d of the objective lens 324. In other words, a light source that satisfies approximately _λ02 ^/ Δλ≦2d is used. This prevents multiple interference of light waves between the transparent protective plate provided in front of the objective lens 324 and the master 325. As a result, as in the previous embodiment, the control unit 332 can obtain a focus error signal that changes linearly with the focus direction displacement ΔZ of the objective lens 324, as shown in FIG. 7.

Therefore, highly accurate focusing control is possible.

As described above, the focusing device of this embodiment can obtain a focus error signal with little noise, even when applied to a master recording device with a short working distance. As a result, focusing control can be performed with high precision, contributing to the realization of high-quality master disc production. Note that the fine adjustment system 329 uses the recording light during recording, and also performs focusing control using the astigmatism method. Also, in FIG. 12, 360 denotes a photodetector for checking the light intensity.

FIG. 15 shows an exposure apparatus equipped with a focusing device according to the present invention.

In this exposure apparatus, a laser beam 401 emitted from an LD 400 is irradiated obliquely onto the surface of a sample 404, such as a wafer, via lenses 402 and 403. Reflected light 405 from the surface of the sample 404 is then guided via lenses 406 and 407 to an optically conductive position detector 408, which detects the surface position of the sample 404. The detected surface position information is then provided to a position control system for the exposure apparatus main body 409 or the sample holder, and a focusing control system is configured to constantly match the focal position of the exposure apparatus main body 409 with the surface position of the sample 404.

This exposure apparatus also uses an LD 400 with a sufficiently large full width at half maximum Δλ, like the SLD described above. That is, the LD 400 has a power spectrum whose central wavelength _λ0 and full width at half maximum Δλ satisfy the following relationship: _λ02 /Δλ≦2t( _n22 − ^sin2θ ) ^{0.5 ,} where t is the thickness of ^an optical medium that may cover the sample 404.

where _n2 represents the refractive index of the optical medium, and θ represents the angle of incidence of light onto the optical medium. Therefore, focusing control can be performed without being affected by the optical medium covering the sample 404.

The present invention is not limited to the above-described embodiments, but can be practiced in various modifications.

[Industrial Applicability] The present invention contributes to highly accurate focus error detection, thereby contributing to the realization of highly accurate focusing control. Therefore, when applied to optical disc devices, for example, it can reduce the S/N ratio of the playback signal and jitter degradation.

─────────────────────────────────────────────────────── [Summary Continued] (Note) This publication is based on the gazette published internationally by the International Bureau of Patents (WIPO). The effect of the international publication of the Japanese patent application (Japanese utility model registration application) related to this publication arises pursuant to Article 184-10, Paragraph 1 of the Patent Act (Article 48-13, Paragraph 2 of the Utility Model Act), and is unrelated to this publication.

Claims (22)

[Claims] 1. An optical disc device configured to irradiate an optical disc having a structure in which two substrates of thickness h are stacked on top of each other, at least one of which has a reflective layer on which signal pits are formed at a distance of approximately h from the surface, with light emitted from a laser light source through an objective lens, to record or reproduce information on the optical disc by this irradiation, and to detect a focus error of the objective lens based on a signal obtained by detecting a portion of the laser light reflected from the reflective layer, wherein _the half-width Δλ of the laser light source satisfies the following formula: _λ02 ^/ { ^2h ( _n22 ^-NA2 ₎ ^0.5 }≦Δλ 2. The optical disk device according to claim 1, wherein the optical disk is formed by bonding together two substrates each having a thickness of 0.6 mm and a diameter of 120 mm. 3. The optical disk device according to claim 1, wherein the laser light source is a semiconductor laser diode. 4. The optical disk device according to claim 1 or 3, characterized in that the laser light source has a power spectrum with a central wavelength λ ₀ in the range of approximately 635 to 650 nm and a half-width Δλ of 0.3 nm or more. 5. The optical disk device according to any one of claims 1 and 3, characterized in that the laser light source has a power spectrum with a central wavelength _λ0 in the range of 410 (nm) to 420 (nm) and a half-value width Δλ of approximately 0.105 (nm) or more. 6. In an optical disc device configured to irradiate an optical disc having a structure in which two substrates each having a thickness of h are stacked on top of one another, with a reflective layer on which signal pits are formed at a distance of approximately h from the surface, with light emitted from a laser light source through an objective lens, to record or reproduce information on the optical disc by this irradiation, and to detect a focus error of the objective lens based on a signal obtained by detecting a portion of the laser light reflected from the reflective layer, wherein the center wavelength of the laser light source is λ₀, the half-width of the laser light source is Δλ, the numerical aperture of the objective lens is NA, the focal length of the objective lens is f, and the refractive index of the substrate material is n₂, the maximum diameter of the laser spot light irradiated onto the signal pit is dmax, and λ₀ The optical disc device is characterized in that the half-width Δλ of the laser light source satisfies the following formula, where g is a general function for determining the maximum half-width Δλ determined based on NA, f, and dmax. λ₀ ²/{2h(n₂ ²−NA²)^0.5}≦Δλ≦g(λ₀, NA, f, dmax) 7. The optical disk device according to claim 6, wherein the optical disk is formed by bonding together two substrates each having a thickness of 0.6 mm and a diameter of 120 mm. 8. The optical disk device according to claim 6, wherein the laser light source is a semiconductor laser diode. 9. The optical disk device according to claim 6 or 8, characterized in that the laser light source has a power spectrum with a central wavelength λ ₀ in the vicinity of 635 to 650 (nm) and a half-width Δλ of 0.3 (nm) or more. 10. The optical disk device according to any one of claims 6 and 8, characterized in that the laser light source has a power spectrum with a central wavelength _λ0 in the range of 410 (nm) to 420 (nm) and a half-value width Δλ of approximately 0.105 (nm) or more. 11. An optical disk device configured to irradiate an optical disk having a structure in which two stacked substrates of thickness h are provided, at least one of which has a reflective layer on which signal pits are formed at a distance of approximately h from the surface, with light emitted from a laser light source through an objective lens, to record or reproduce information on the optical disk using this irradiation, and to detect focus errors of the objective lens based on a signal obtained by detecting a portion of the laser light reflected from the reflective layer, wherein the half-width is set so that the coherence length, determined by the center wavelength and half-width of the light power spectrum emitted by the light source, is smaller than the optical path difference between light directly reflected from the substrate surface and light that enters the substrate, reflects off the signal pits, and then exits the substrate again. 12. The optical disk device according to claim 11, wherein the optical disk is constructed by bonding together two substrates each having a thickness of 0.6 mm and a diameter of 120 mm. 13. The optical disk device according to claim 11, wherein the laser light source is a semiconductor laser diode. 14. The optical disk device according to any one of claims 11 and 13, characterized in that the laser light source has a power spectrum with a central wavelength λ ₀ in the vicinity of 635 to 650 (nm) and a half-width Δλ of 0.3 (nm) or more. 15. The optical disk device according to any one of claims 11 and 13, characterized in that the laser light source has a power spectrum with a central wavelength _λ0 in the range of 410 (nm) to 420 (nm) and a half-value width Δλ of approximately 0.105 (nm) or more. 16. An optical disk device configured to irradiate an optical disk having a structure in which two stacked substrates of thickness h are provided, at least one of which has a reflective layer on which signal pits are formed at a distance of approximately h from the surface, with light emitted from a laser light source through an objective lens, thereby recording or reproducing information on the optical disk and detecting a focus error of the objective lens based on a signal obtained by detecting a portion of the laser light reflected from the reflective layer; wherein the coherence length determined by the center wavelength and half-width of the light power spectrum emitted from the light source is smaller than the optical path difference between light directly reflected from the substrate surface and light entering the substrate, reflecting off the signal pits, and then exiting the substrate again, and the half-width is set so that the radius of the laser spot, determined by the numerical aperture and focal length of the objective lens, the center wavelength and half-width of the light source, is smaller than the track pitch of adjacent signal pits. 17. The optical disk device according to claim 16, wherein the optical disk is constructed by bonding together two substrates each having a thickness of 0.6 mm and a diameter of 120 mm. 18. The optical disk device according to claim 16, wherein the laser light source is a semiconductor laser diode. 19. The optical disk device according to any one of claims 16 and 18, characterized in that the laser light source has a power spectrum with a central wavelength λ ₀ in the range of approximately 635 to 650 (nm) and a half-width Δλ of 0.3 (nm) or more. 20. The optical disk device according to any one of claims 16 and 18, characterized in that the laser light source has a power spectrum with a central wavelength λ ₀ in the range of 410 (nm) to 420 (nm) and a half-value width Δλ of approximately 0.105 (nm) or more. 21. A focusing device for adjusting the focal position of an objective lens on an object surface, comprising: an actuator for moving the objective lens in the direction of its optical axis; error detection means including a light source for optically detecting a focus error of the objective lens; and means for controlling the actuator based on a signal obtained by the error detection means, wherein the light source included in the error detection means emits light whose power spectrum has a center wavelength λ ₀ and a half-width Δλ that approximately satisfies λ ₀ ² /Δλ≦2d, where d is the working distance of the objective lens. 22. A focusing device for adjusting the focal position of an objective lens on an object surface that may be covered by an optical medium, comprising: an actuator for moving the object surface in the optical axis direction; error detection means including a light source for optically detecting focus errors of the objective lens; and means for controlling the actuator based on a signal obtained by the error detection means, wherein the light source included in the error detection means emits light whose center wavelength _λ0 and half-width Δλ of the power spectrum satisfy approximately _λ02 /Δλ≦2t( _n22 − ^sin2θ ) ^0.5 , where t is the thickness of the optical medium covering the object surface, _n2 is the refractive index of the optical medium, ^and θ is the angle of incidence of the light with respect to ^the optical medium.