Patent Application
20080198728
1. Field of the Invention
The present invention relates to an optical information recording-reproducing apparatus which employs a solid immersion lens (hereinafter referred to as a “SIL”) for recording or reproducing information on a recording medium. In particular, the present invention relates to servo-control of a gap or a focus.
2. Description of the Related Art
For increasing the recording density of an optical disk, generally the light spot diameter on the recording face of the optical disk is made smaller by shortening the wavelength of light for recording and reproduction and increasing the numerical aperture (NA) of the objective lens. Conventionally, a so-called SIL is employed and a front lens of an objective lens system is brought close to the recording face to a distance of a fraction (e.g., ½ or less) of the recording light wavelength to increase the NA to be higher than 1 in the air.
A method for this is disclosed in detail, for example, in a document: Japan Journal Applied Physics vol. 44, (2005) pp. 3564-3567, “Near Field Recording on First-Surface Write-Once Media with a NA=1.9 Solid Immersion Lens” (Reference Document 1).
This method is also described in detail in a document: Optical Data Storage 2004, Proceedings of SPIE vol. 5380 (2004), “Near Field Read-Out of a 50 GB First-Surface Disk with NA1=1.9 and a Proposal for a Cover-Layer Incident, Dual-Layer Near Field System”; and so forth.
The prior art techniques are described below with reference to
The light beam is further allowed to pass through non-polarized light beam splitter (NBS) 4, polarized beam splitter (PBS) 7, and quarter-wave plate (QWP) 8 successively. The light beam which is linearly polarized is converted by the quarter-wave plate to a circularly polarized light beam. A part of the light beam reflected by non-polarized beam splitter (NBS) 4 is introduced to photodetector (LPC-PD) 6 for control of the emission power of semiconductor laser 1.
The light beam, having passed through quarter-wave plate 8, is introduced to expander lens system 9. Expander lens system 9 serves to correct spherical aberration caused in the objective lens or SIL as mentioned later, and is constituted of two lenses: the distance between the two lenses is adjustable to correct the spherical aberration. The light beam from expander lens system 9 is introduced to rear objective lens 10 of the objecting lens system.
The objective lens system is constituted of rear objective lens 10 and SIL (front objective lens) 11, and is mounted on a two-axis actuator (not shown in the drawing) to drive the two-lens system in the focusing and tracking directions. Hereinafter, rear objective lens 10 is simply referred to as an objective lens, and the front objective lens is referred to as SIL 11.
Specifically, the light spot on the recording face of optical disk 12 corresponds to N×NA, where N denotes the refractive index of the hemisphere lens and NA denotes the numerical aperture of objective lens 10. For example, a combination of objective lens 10 of NA=0.7 with hemispherical lens SIL of N=2 gives effective NA (NAeff) of 1.4. The tolerable thickness error of SIL 11 is about 10 μm. Therefore the device can be mass-produced readily.
The recording and reproduction with the light spot of a diameter corresponding to NAeff can be conducted by action of evanescent light from the bottom face of the SIL onto the recording face, only when the distance between the SIL bottom face and optical disk 12 is not longer than a fraction of the wavelength 405 nm of the light source, for example in a short distance of 100 nm or less. To keep this distance, the gap is servo-controlled as described later.
The optical system for the return path is described with reference to
The light beam reflected by PBS 7 is allowed to pass through halfwave plate (HWP) 13 to turn the polarization plane by 45°. The s-polarized light component of the light beam is reflected by polarized light beam splitter 14 to pass lens 15 and focused on photodetector-1 (PD-1) 16. From the optical signal detected by photodetector-1 (PD-1) 16, the information on optical disk 12 is reproduced as an RF output 17 by an information reproduction circuit not shown in the drawing.
On the other hand, the p-polarized light component of the light beam which has the polarization plane turned by 45° by halfwave plate (HWP) 13 is allowed to pass through polarized light beam splitter (PBS) 14 and is reflected non-polarized light beam splitter 18. The reflected light beam is allowed to pass lens 19 and is focused on two-division photodetector-2 (PD2) 20. From the signal detected by the two-division photodetector-2 (PD2) 20, tracking error signal 21 is generated by a tracking error-detecting circuit not shown in the drawing. According to tracking error signal 21, the tracking is servo-controlled by a tracking servo circuit not shown in the drawing.
Of the light beam reflected by the bottom of SIL 11, the portion not totally reflected corresponding to NAeff<1 is also reflected as circularly polarized light rotating in a direction reverse to the incident light beam similarly as the reflected light from optical disk 12. In the totally reflected light beam corresponding to NAeff≧1, a phase difference δ shown by the equation below is caused between the p-polarized light component and the s-polarized component and the circularly polarized light beam is deformed to an elliptically polarized light beam.
tan(δ/2)=cos θi×√{square root over ( )}(N2×sin2θi−1)/(N×sin2θi) (1)
Therefore, the reflected light beam after passing through quarter-wave plate contains a polarized light component of the same polarization direction as that of the incident light beam. This polarized light component is allowed to pass through PBS 7, reflected by NBS 4, allowed to pass through lens 26, and focused on photodetector-3 (PD3) 27. The light quantity of this light beam decreases monotonically with the decrease of the distance between the SIL bottom face and the optical disk in the near-field region. Therefore, gap error signal 28 can be obtained from the signal detected by photodetector-3 (PD3) 27.
The distance between the SIL bottom face and the optical disk can be kept at an intended distance of not more than 100 nm by servo-control of the gap within a prescribed threshold. The servo-control of the gap is described in detail in the above-mentioned Reference Document 1. Since this light beam is not modulated by the information recorded on the optical disk, the gap error signal can be obtained stably regardless of the presence or absence of the recorded information.
In the servo-control of the gap, the overshoot should be less than the above-mentioned 100 nm. The overshoot of more than 100 nm will cause collision of the SIL against the optical disk to damage the SIL and the optical disk. For preventing the overshoot, as one method, the speed of approach of the SIL to the optical disk in the servo-control may be lowered. However, the lower approaching speed requires a longer time for the servo-control, and is not desirable practically.
To solve this problem, Japanese Patent Application Laid-Open No. 2005-209246, for example, discloses an apparatus.
During approach of SIL 11 to optical disk 101, a gap error signal generated by gap error-generating circuit 104 is input to comparator 107. Comparator 107 outputs a signal LOW to meet a gap error signal higher than a prescribed level Vth, and a signal HIGH to meet a gap error signal lower than the prescribed level Vth to switch 109.
When the SIL is detected to have approached close to the optical disk in a near field state, comparator 107 outputs a signal HIGH to turn on switch 109. Thereby servo-control of the gap is started. In this process, the gap error signal is transmitted through phase compensation circuit 105, switch 109, and adder 130 to actuator driver circuit 106.
During the transition from the far field state to the near field state, approach speed-generating circuit 108 generates a signal in a waveform 142 shown in
In the method of the aforementioned Japanese Patent Application Laid-Open No. 2005-209246, the approach voltage is set to control the speed of the SIL to be nearly zero at the start of the servo-control of the gap. However, in the servo-control with optical disk 101 rotated by spindle 103, it is difficult to keep the SIL speed to be nearly zero at the start of the servo-control, in consideration of swing of the face of optical disk 101 and spindle 103.
With a disk face swinging, for example, the gap can come to a near field state before the time t1, or before the approach voltage comes to a prescribed constant voltage. In such a case, the non-constant output voltage of approach voltage-generating circuit 108 can start the servo-control of the gap in a state of the approaching speed of not zero or in a state of a high speed of the SIL relative to the optical disk face.
In the upper graph in
In the upper graph of
In the servo-control method of Japanese Patent Application Laid-Open No. 2005-209246, the gradient of the ramped actuator-driving signal is constant before the gap error signal comes lower than a prescribed level, which is affected by the face swing. The SIL is allowed to approach the optical disk at a constant speed according to a ramp-shaped function, and the approach movement is stopped and the servo-control of the gap is started at the time when the gap error signal comes to a prescribed level. According to this method, the gap size can become zero or less immediately after start of the servo-control. This signifies collision of the SIL against the optical disk. To avoid the collision in this method, the speed of the approach should be controlled to be sufficiently low. Further, the gap size for the prescribed level Vth of the gap error signal is as small as about 100 nm or less. Therefore, overshoot of 100 nm or more will cause collision of the SIL against the optical disk, disadvantageously.
The present invention intends to provide an optical recording-reproduction apparatus, which enables a high speed approach of the SIL to the optical recording medium without collision of the SIL against the optical recording medium by stable servo-control of the gap in a short time.
The present invention is directed to an optical information recording-reproduction apparatus comprising: a light source; an objective lens and a SIL (solid immersion lens) for focusing a light beam from the light source on an optical recording medium; an aperture element for collecting a part of the light beam reflected by an optical recording medium, corresponding to the effective aperture number of the objective lens and SIL of less than 1; a detecting element for detecting the part of the light beam from the aperture element corresponding to the effective aperture number of less than 1; a speed-generating circuit for reducing the speed of approach of the SIL to the recording medium in accordance with the level of the signal detected by the detecting element; and a driver circuit for driving the objective lens and the SIL in accordance with output from the speed-generating circuit.
In the optical information recording-reproduction apparatus, servo-control of the gap can be started after the driver circuit starts reduction of the speed of approach in accordance with a gap error signal derived from the reflected light beam from the recoding medium.
In the optical information recording-reproduction apparatus, servo-control of the focus can be started after the driver circuit starts reduction of the speed of approach in accordance with a focus error signal derived from the output of the detecting element. In the optical information recording-reproduction, servo-control of the gap can be started, after the servo-control of the focus is completed, in accordance with a gap error signal derived from the reflected light beam from the recoding medium.
According to the present invention, for starting servo-control of the gap, a signal is employed which is generated by a light beam corresponding to effective aperture number of less than 1 of SIL. Thereby the timing can be obtained exactly regarding separation of the SIL from the optical recording medium before the start of the servo-control. Therefore, the speed of the approach of the SIL to the optical disk can be decreased at that timing, and the servo-control can be started stably in a short time.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
The best mode for conducting the present invention is described below with reference to drawings.
The optical recording medium (optical disk 12) in
Firstly, the constitution of the optical system is described with reference to
The portion surrounded by the broken line of the optical system in
The light beam of NA<1 gives sum signal 44 which is a reflected light beam from the recording layer of optical disk 101. Gap error signal 43 is generated by gap error-generating circuit 104 in
The gap error signal will come to decrease when the distance 45 of SIL 11 from the optical disk surface decreases to about 100 nm or less as illustrated in
The servo-control of the gap employing the sum signal of the present invention is described below in detail with reference to
During the approach, the sum signal derived from the light beam of NA<1 is input to comparator 112. The output 54 of comparator 112 is switched from LOW to HIGH when the signal level of sum signal 52 becomes higher than a prescribed level SUMth as shown in
With the driving signal in a form of a ramp function, the approach speed is lowered by decreasing the gradient. The approach movement is further continued at the lowered approach speed. During the approach movement, gap error signal 51 is input to comparator 107. At the time when gap error signal 51 becomes lower than a prescribed level GEth2, output level 55 from comparator 107 is switched from LOW to HIGH (time t2 in
At the time t2 in
Approach speed-generating circuit 108 and actuator driver circuit 106 constitute the driving means for bringing SIL 11 near to optical disk 101. Approach speed-generating circuit 108 constitutes a means for lowering the speed of the SIL at the time when the detection signal from the detection means becomes higher than a prescribed level in the SIL driving.
The effect of the servo-control is described with reference to
Specifically, the overshoot at the start of the servo-control can be reduced to 10 nm or less to avoid surely collision of the SIL against the optical disk. Moreover, the SIL can be brought close to the optical disk at a high speed by the time t1 to shorten the time for start the servo-control.
In the present invention as described above, the servo-control of the gap is conducted by use of a sum signal derived from a light beam of NA<1. That is, at the start of the servo-control of the gap, use of the sum signal derived from the light beam of NA<1, whereby the timing of lowering the speed of approach of the SIL can be decided to start servo-control of the gap stably in a short time.
Focus error-generating circuit 113 detects a focus error signal from the output from photodetector (PD4) 24. The focus error signal can be generated, for example, by use of a toric lens as sensor lens 23, and a four-division sensor as photodetector (PD4) 24 according to a conventional astigmatism method.
The light beam of NA<1 contains much reflected light from the recording layer of optical disk 101, enabling generation of the focus error signal precisely even if the optical disk is multi-layered. The focus control is conducted by the focus error signal.
This focus control enables precise follow to slight irregularity in the thickness of the cover layer and interlayer for recording and reproduction of information. Further, sum signal generated by the light beam of NA<1 enables the focus control stably in a short time.
The focus control in this Embodiment is described below in detail with reference to
In a near field state, the focusing error signal controls the focusing at a position of the SIL at a distance of about 1 μm from the optical disk, where the gap error signal level begins to lower (100 nm or less in
In the state illustrated in
When the level of sum signal 1002 becomes higher than the prescribed level SUMth as shown in
With the driving signal in a form of a ramp function, the approach speed can be lowered by decreasing the gradient. The approach movement is further continued at the lowered approach speed. During the approach movement, focus error signal 1001 is input steadily from focus error-generating circuit 113 to comparator 114. At the time when the zero-cross point of the focus error signal is detected, output level 1004 from comparator 114 is switched from LOW to HIGH (time t2 in
On detection of the zero-cross point of the focus error signal, output signal 1005 from approach speed-generating circuit 108 is fixed at the level at the time t2, and simultaneously switch 116 is turned to select phase compensation circuit 115 to start the servo-control of the focus.
In this state, the focus error signal is transmitted through phase compensation circuit 115, switch 116, and adder 110 to actuator driver 106. The use of the sum signal derived from the light beam of NA<1 enables high-speed approach before the time t1 as shown in
The decrease of the approach speed after the time t1 prevents collision of the objective lens and SIL against the optical disk during the focus control. Further, the focusing is conducted at a distance of more than 1 μm between the SIL and the optical disk by expander lens system 9 to lower further the probability of collision of the SIL against the optical disk.
As described above, in the focus control also, the sum signal derived from the light beam of NA<1 is utilized for stable focusing in a short time. Controller 117, having detected normal completion of the focus control, switches the SIL focusing from focus control to gap servo-control by gap error signal.
Specifically, when the sum signal has been kept at a level higher than a prescribed level for a prescribed time or longer after the time t2 in
After the drive of expander lens system 9 is completed and the gap error signal comes to be below a prescribed level, controller 117 turns switch 116 to select phase compensation circuit 105 to change the focusing control to gap servo-control. In this state, information is recorded on optical disk 101 or reproduced therefrom.
As described above, in servo-control of the focus with the focus error signal generated by the light beam of NA<1, the focusing to the recording layer of the optical disk can be controlled stably by utilizing the sum signal derived from the light beam of NA<1.
As described above, according to the present invention, servo-control is conducted stably in a short time without collision of SIL against the optical recording medium by utilizing a sum signal derived from the light beam corresponding to effective aperture number of less than 1.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2007-040603, filed Feb. 21, 2007, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-040603 | Feb 2007 | JP | national |
