JP2008217866A - Optical information recording/reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical information recording/reproducing device capable of surely preventing collisions between a SIL and an optical disk, even when external vibrations are applied. <P>SOLUTION: This device is provided with a condenser optical system constituted of an objective lens 10 and a SIL 11; an actuator for driving the condenser optical system, in a direction vertical to the surface of the optical disk; a gap error generating circuit for detecting a gap error signal; a gap servo circuit for controlling the space between the condenser optical system and the optical disk based on the gap error signal; and a vibration detecting circuit for detecting vibrations. When the vibration detecting circuit detects vibrations of a level equal to a predetermined level or higher, the gap servo is turned OFF, and the condenser optical system is made to retreat from the optical disk. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical information recording / reproducing apparatus such as an optical disc apparatus, and more particularly to an optical information recording / reproducing apparatus that records or reproduces information using a solid immersion lens (hereinafter abbreviated as SIL).

  In order to improve the recording density of the optical disc, it is required to shorten the wavelength of light used for recording and reproduction, increase the numerical aperture (NA) of the objective lens, and reduce the light spot diameter on the optical disc recording surface. . Conventionally, there has been an attempt to construct a so-called SIL by bringing the front lens of the objective lens close to a fraction of the recording wavelength (for example, ½) or less on the recording surface, and to set NA to 1 or more even in the air. It has been made. For example, it is described in detail in Non-Patent Document 1, Non-Patent Document 2, and the like.

  A conventional technique will be described with reference to FIGS. The configuration of the optical pickup for near-field recording described in Non-Patent Document 1 will be described with reference to FIG. A light beam emitted from the semiconductor laser 1 having a wavelength of 405 nm is converted into a parallel light beam by the collimator lens 2 and is incident on the beam shaping prism 3 to form an isotropic light amount distribution.

  The light beam from the beam shaping prism 3 passes through the non-polarization beam splitter (NBS) 4 and passes through the polarization beam splitter (PBS) 7 and further passes through the quarter wave plate (QWP) 8 to convert from linearly polarized light to circularly polarized light. Is done. A light detector (LPC-PD) 6 for receiving the light beam reflected by the non-polarizing beam splitter (NBS) 4 through the lens 5 and controlling the emission power of the semiconductor laser 1 is provided.

  The light beam that has passed through the quarter-wave plate (QWP) 8 enters the expander lens 9. The expander lens 9 is a lens for correcting spherical aberration generated in an objective lens and SIL described later. The expander lens 9 is configured to be able to control the lens interval between the two lenses according to the spherical aberration.

  The light beam from the expander lens 9 enters the rear lens 10 of the objective lens. The objective lens includes a rear lens 10 and a SIL (front lens) 11. These condensing optical systems are mounted on a biaxial actuator (not shown) that integrally drives two lenses in a focus direction and a tracking direction. In the present specification, the rear lens is called the objective lens 10 and the front lens is called SIL11.

  FIG. 11 shows how the light beam focused by the objective lens 10 is condensed on the bottom surface of the SIL 11 of the hemispherical lens. The light beam is incident on the spherical surface of the hemispherical lens perpendicularly, and is condensed on the bottom surface through the same optical path as when there is no hemispherical lens. There is an effect to.

  That is, assuming that the refractive index of the hemispherical lens is N and the numerical aperture of the objective lens 10 is NA, a light spot equivalent to N × NA is obtained on the recording surface of the optical disk 12. For example, if the objective lens 10 with NA = 0.7 is combined with the SIL 11 of hemispherical lens with N = 2, the effective NA is NAeff and NAeff = 1.4 is reached. Since the thickness error of the hemispherical lens 11 can be allowed to be about 10 μm, mass production is easy.

  Only when the distance between the SIL bottom surface and the optical disk 12 is a short distance of a fraction of the wavelength of the light source 405 nm, for example, 100 nm or less, acts on the recording surface as evanescent light from the SIL bottom surface, depending on the NAeff light spot diameter. Recording / reproduction is possible. In order to maintain this distance, a gap servo described later is used.

  Returning to FIG. 10, the return optical system will be described. The light beam reflected by the optical disk 12 becomes reverse circularly polarized light, enters the SIL 11 and the objective lens 10 and is converted again into a parallel light beam. The parallel light beam passes through the expander lens 9 and the quarter wave plate (QWP) 8 and is converted into linearly polarized light in a direction orthogonal to the forward path. This light beam is reflected by a polarization beam splitter (PBS) 7 and enters a half-wave plate (HWP) 13.

  Of the light beam whose polarization plane has been rotated by 45 ° by the half-wave plate (HWP) 13, the S-polarized light component is reflected by the polarization beam splitter (PBS) 14 and passes through the lens 15 on the photodetector (PD 1) 16. It is focused on. An RF output 17 that is information on the optical disk 12 is obtained from the detection signal of the photodetector (PD1) 16.

  On the other hand, the P-polarized component of the light beam whose polarization plane is rotated by 45 ° by the half-wave plate (HWP) 13 passes through the polarization beam splitter (PBS) 14 and is reflected by the non-polarization beam splitter (NBS) 18. . The reflected light beam is condensed on the two-divided photodetector (PD2) 20 via the lens 19. A tracking error 21 is obtained from the light reception signal of the two-divided photodetector (PD2) 20 by a known method.

  In addition, among the light beams reflected by the bottom surface of the SIL 11, a light beam of NAeff <1 that does not totally reflect is reflected as circularly polarized light that is reverse to the incident, similarly to the light reflected from the optical disk 12. For a light flux of NAefff ≧ 1 that causes total reflection, a phase difference δ shown in the following equation is generated between the P-polarized component and the S-polarized component, and is shifted from circularly polarized light to become elliptically polarized light.

tan (δ / 2)
= Cos θi × √ (N ² × sin ² θi−1) / (N × sin ² θi) (1) Therefore, when passing through the quarter wave plate (QWP) 8, the polarization component in the same direction as the forward path is included. It will be. This polarization component passes through the polarization beam splitter (PBS) 7, is reflected by the non-polarization beam splitter (NBS) 4, and is condensed on the photodetector (PD 3) 27 via the lens 26. Since the amount of the light beam monotonously decreases as the distance between the bottom surface of the SIL and the optical disk approaches in the near-field region, it can be used as the gap error signal 28.

  If the target threshold value is determined in advance, the distance between the bottom surface of the SIL and the optical disk can be kept at a desired distance of 100 nm or less by performing gap servo. The gap servo is detailed in the paper of Non-Patent Document 1 described above. Further, since this light beam is not modulated by the recording information on the optical disc 12, a stable gap error signal can be obtained regardless of the presence or absence of the recording information.

  As described above, the distance between the optical disk and the SIL when performing gap servo is as small as 100 nm or less. For this reason, if the SIL is shaken even slightly, the SIL collides with the optical disk due to the influence of a disturbance due to scratches or dirt on the optical disk, which may cause damage to the SIL or the optical disk.

  A method for solving this problem is proposed in, for example, Japanese Patent Application Laid-Open No. 2001-76358 (Patent Document 1). FIG. 12 shows the configuration of the publication. When normal gap servo is performed, first, a gap error signal that is an output of the gap error generation circuit 104 is input to the phase compensation circuit 105.

The actuator driver circuit 106 drives the SIL 11 in the pickup 102 with a signal corresponding to the output of the phase compensation circuit 105. During this normal gap servo, the gap error signal is input to the comparator 107. The threshold value of the comparator 107 will be described with reference to FIG. The gap error signal level at which the target value during normal gap servo corresponds to a gap length of 30 nm is GE _ref . In this case, the threshold value of the comparator 107 is set to GE _th1 corresponding to a larger gap length of 50 nm and GE _th2 corresponding to a smaller gap length of 10 nm.

The output of the comparator 107 switches from Low to High when the gap error signal exceeds GE _th1 or GE _th2 . When the output of the comparator 107 is switched from Low to High, some disturbance is applied in the gap control loop, and there is a possibility of collision between the SIL and the optical disc 101. In that case, the switch 108 is switched from the phase compensation circuit 105 side to the adder 111 side.

  The phase compensation circuit 105 interrupts the phase compensation process. The adder 111 performs addition processing of outputs from the drive voltage hold circuit 109 and the phase compensation circuit 110. The drive voltage hold circuit 109 holds the output voltage of the phase compensation circuit 105 when the output of the comparator 107 is switched. The phase compensation circuit 110 performs a phase compensation process for realizing a control band narrower than the control band realized by the phase compensation circuit 105.

  That is, when control driving is performed by the output of the adder 111, the SIL position when the output of the comparator 107 is switched by the drive voltage hold circuit 109 is maintained, and the response of disturbance is reduced. By doing so, the SIL is not unnecessarily driven, and the collision between the SIL and the optical disc 101 is prevented.

After that, when the output of the comparator 107 changes from High to Low, it is assumed that the disturbance to the control loop is settled, and the switch 108 is switched to the phase compensation circuit 105 side and returns to the normal gap servo. Thus, the collision between the SIL and the optical disk is prevented by detecting the disturbance from the change in the gap error signal level.
Japan Journal NA1 Applied Physics Vol. 44 (2005) P. 3564-3567, “Near Field Recording on First-Surface Write-Once Media with Iol. Optical Data Storage 2004, Proceedings of SPIE 5380 (2004) “Near Field read-out of first-surface disk with NA-1.9 and a proposal for a coer JP 2001-76358 A

  As in the above-described prior art, the control loop bandwidth is lowered to make it impossible to respond to scratches and dirt on the optical disk, and unnecessary driving of the SIL is eliminated, thereby preventing collision between the SIL and the optical disk. However, when external vibration is applied to the drive device, the drive voltage is held as in the prior art, and the SIL collides with the optical disc when the position of the SIL is fixed. Unlike scratches and dirt, the optical disk is actually moving during vibration, so the SIL needs to move accordingly.

  An object of the present invention is to provide an optical information recording / reproducing apparatus capable of reliably preventing a collision between an SIL and an optical disk even when external vibration is applied.

  The present invention relates to a condensing optical system comprising an objective lens and a solid immersion lens (SIL) disposed between the objective lens and an optical disc for condensing a light beam from a light source, and the condensing optical system on the surface of the optical disc. An actuator that drives in a vertical direction with respect to the gap, a gap error generation circuit that detects a gap error signal indicating the distance between the focusing optical system and the optical disc, and servo-controls the actuator based on the detected gap error signal A gap servo circuit for controlling the distance between the condensing optical system and the optical disc, and a vibration detection circuit for detecting vibration. When the vibration detection circuit detects a vibration exceeding a preset level, the gap servo is turned off and the actuator is driven to retract the condensing optical system from the optical disk.

  According to the present invention, since external vibration is detected from the gap error signal level and the SIL is retracted from the optical disk, collision between the SIL and the optical disk can be reliably prevented, and damage to the SIL and the optical disk can be prevented. Also, when external vibration is detected, the SIL condensing control is switched from the gap servo to the focus servo, and the collision between the SIL and the optical disc is prevented while continuing the condensing control, so that the recovery time after external vibration can be shortened. .

  Next, the best mode for carrying out the invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram showing a first embodiment of an optical information recording / reproducing apparatus according to the present invention. In the figure, blocks having the same functions as those in FIG. The optical information recording / reproducing apparatus of the present invention collects a light beam from a light source (semiconductor laser) and records or reproduces information on an optical disk.

  FIG. 2 is a timing chart for explaining the operation of the present embodiment. This embodiment will be described in detail with reference to FIGS. In FIG. 1, a circuit for recording information on an optical disk, a circuit for reproducing recorded information, a tracking servo circuit, a focus servo circuit, a circuit and a mechanism for rotating the optical disk, and the like are omitted.

  Also in this embodiment, in the gap servo operation during recording or reproduction, the phase compensation circuit 105 performs phase compensation processing on the gap error signal that is the output of the gap error generation circuit 104. Further, the actuator driver circuit 106 drives the SIL in the pickup 102 according to the output.

  For example, the pickup 102 has the same configuration as the optical pickup shown in FIG. That is, a condensing optical system comprising an objective lens 10 and a solid immersion lens (SIL) 11 disposed between the objective lens 10 and the optical disk 12 that collects a light beam from the semiconductor laser 1 as a light source. I have. As described above, the condensing optical system is mounted on a biaxial actuator (not shown) that integrally drives two lenses in the focus direction and the tracking direction.

  That is, an actuator for driving the condensing optical system in a direction perpendicular to the surface of the optical disc is provided. In the present invention, a light beam from a light source (semiconductor laser) is condensed by the condensing optical system, and information is recorded or reproduced on an optical disk.

  The gap error generation circuit 104 detects a gap error signal indicating the distance between the condensing optical system and the optical disc from the output signal of the photodetector (PD3) 27 in FIG. Control loops such as the gap error generation circuit 104, the phase compensation circuit 105, and the actuator driver circuit 106 constitute a gap servo circuit. That is, a gap servo circuit that controls the distance between the condensing optical system and the optical disc by servo-controlling the actuator based on the detected gap error signal is provided.

During normal gap servo, a gap error signal is input to the comparator 107. As shown in FIG. 2, the threshold value of the comparator 107 is set to a signal level GE _th1 that is larger than the gap error signal level GE _ref corresponding to the target value of the gap servo and a signal level GE _th2 that is smaller. The comparator 107 detects vibration applied to the apparatus from the gap error signal level, and includes a vibration detection circuit that detects vibration.

When the gap error signal level exceeds GE _th1 or GE _th2 , the output of the comparator 107 changes from Low to High (at time t0 in FIG. 2). When the output of the comparator 107 is switched, external vibration is applied to the apparatus, and the SIL 11 and the optical disc 101 may collide. The controller 113 stops the phase compensation processing of the phase compensation circuit 105 to interrupt the gap servo operation. .

  The band may be limited by LPF processing for the gap error signal input to the comparator 107 so that the comparator 107 can accurately detect and detect scratches, dirt on the optical disk and external vibration. The cutoff frequency of the LPF may be about 500 Hz.

  This is because the disturbance component applied to the gap servo control loop due to scratches, dirt on the optical disk, etc. is a relatively high frequency (several KHz), whereas the disturbance frequency due to external vibration is as low as several hundred Hz. is there.

  After the output of the comparator 107 is switched and the gap servo is interrupted, the controller 113 causes the save voltage generation circuit 112 to output a drive voltage for saving the SIL from the optical disc 101. This signal is a pulse-like signal that drives the SIL in a direction away from the optical disc 101 as shown in FIG.

  In the present embodiment, when the vibration detection circuit detects a vibration of a preset level or higher, the gap servo is turned off and the actuator is driven to retract the condensing optical system from the optical disk.

  Thus, the gap servo control is immediately interrupted when vibration is detected, and the SIL is retracted from the optical disk, so that collision between the SIL and the optical disk 101 can be prevented even when vibration is applied. Further, by using the gap error signal for vibration detection, the SIL retract timing can be detected accurately. After detecting the vibration and retracting the SIL, it is assumed that the vibration disappears when a predetermined time elapses, and the controller 113 starts the gap servo control again. When gap servo control is successfully pulled in, recording and playback are resumed.

  As described above, in the present embodiment, external vibration is detected based on the gap error signal level during gap servo, and when the external vibration is detected, the gap servo is interrupted and the SIL is retracted from the optical disk. By performing this process, the collision between the SIL and the optical disk can be reliably prevented in the optical information recording / reproducing apparatus using the SIL.

(Second Embodiment)
FIG. 3 is a block diagram showing a second embodiment of the present invention. In the figure, blocks having the same functions as those in FIG. 1 and FIG. In FIG. 3, a circuit for recording information on an optical disk, a circuit for reproducing recorded information, a tracking servo circuit, a circuit and a mechanism for rotating the optical disk, and the like are omitted.

  This embodiment will be described with reference to FIGS. First, the configuration of the optical system of the present embodiment is shown in FIG. In FIG. 4, the same parts as those in FIG. 10 of the prior art are denoted by the same reference numerals and description thereof is omitted, but FIG. 4 is different from FIG. 10 in that a part surrounded by a broken line is added. The feature of this optical system is a portion surrounded by a dotted line in FIG. 4, and its configuration will be described below. Other configurations are the same as those in FIG.

  The light beam that has passed through the non-polarizing beam splitter 18 passes through the opening 22, the outer periphery of the light beam is shielded, and is condensed on the photodetector (PD 4) 24 via the sensor lens 23. The output of the photodetector PD4 (24) is input to the focus error generation circuit 114 in FIG. 3, and a focus error signal is generated.

  A portion surrounded by a broken line will be described in detail with reference to FIGS. In FIG. 5, the reflected light beam from the optical disk is NA = 1.4 (NA> 1) at the periphery of the pupil diameter. The opening 22 transmits a light beam with NA <1 at the center, for example, NA = 0.85, and shields a light beam with NA> 1 at the outer periphery. The reason why the transmitted light flux is made about 10% smaller than NA = 1 is that when the objective lens 10 and the SIL 11 are moved in the radial direction of the optical disk due to the eccentricity of the optical disk, the light flux with NA> 1 in the outer peripheral portion is not mixed. It is.

  As a method for generating the focus error signal, when the sensor lens 23 is, for example, a toric lens and the photodetector PD4 is, for example, a quadrant sensor, it can be detected by a known astigmatism method. . A light beam with NA <1 or less contains a lot of reflected light from the recording layer of the optical disk, and a focus error signal can be detected with high accuracy.

Next, the operation of the present embodiment will be described in detail with reference to FIGS. First, the gap error signal is input to the comparator 107 during gap servo operation during recording and reproduction as in the above-described embodiment. Similarly, the threshold value of the comparator 107 is set to a value GE _th1 larger and a value GE _th2 smaller than the signal level corresponding to the target value during normal gap servo. The comparator 107 is a vibration detection circuit that similarly detects vibration.

  When external vibration is detected due to a change in the gap error signal level, the output of the comparator 107 is switched from Low to High (Step 1 in FIG. 7). When the controller 113 detects a change in the output of the comparator 107, the SIL condensing control is switched from the normal gap servo to the focus servo based on the focus error signal generated from the light flux of NA <1 described above (step 2).

  That is, when the vibration detection circuit detects a vibration of a predetermined level or higher, the gap servo based on the gap error signal is switched to the focus servo based on the focus error signal. In this case, the focus servo circuit performs focus servo by servo-controlling the above-described actuator based on the focus error signal. As described above, the focus error signal is generated from a light flux whose effective numerical aperture by SIL is less than 1 out of the light flux reflected from the optical disk.

  Specifically, the switch 116 is switched from the phase compensation circuit 105 side to the phase compensation circuit 115 side. Here, the focal point of the gap error signal and the focus error signal generated from the light flux of NA <1 during recording or reproduction has the relationship shown in FIG. That is, the SIL-optical disc distance, which is the target value of the gap servo, is 30 nm, and the focus error signal is in focus.

  After detecting external vibration during the gap servo and the controller 113 switches from the gap servo to the focus servo, the control target position is changed from the control target position A shown in FIG. At this time, the SIL-optical disk distance at the control target positions A and B is substantially the same.

  After switching the control target position, the controller 113 controls the expander drive circuit 117 to drive the expander lens 9 in the pickup 102. The expander lens 9 is driven by a voice coil motor 29 as shown in FIG. 8, and the distance between the focal point of the light spot and the SIL can be changed depending on the position thereof.

  That is, the focus position of the focus error signal is changed. After detecting the external vibration and switching the SIL focusing control from the gap servo to the focus servo, the expander lens 9 is driven while the focus servo is being performed to change the focus position of the focus error signal (step 3). .

  Thus, when switching to the focus servo, the focusing position of the light beam is moved to widen the distance between the focusing point of the light beam and the SIL.

  After the in-focus position is changed, the in-focus position of the focus error signal is a position (control target position C in FIG. 8B) where the distance between the SIL and the optical disc is about 1 μm as shown in FIG. 8B. Become. By moving the in-focus position in a direction in which the distance between the SIL and the optical disk increases, even if the optical disk moves due to external vibration, the collision between the SIL and the optical disk can be prevented. The processing after switching the SIL light collection control will be described in detail with reference to FIG.

After changing the focus position while performing the focus servo, the focus error signal is input to the comparator 118. Similar to the comparator 107, the threshold value of the comparator 118 is set to ± FE _th having a predetermined range with the focus servo target value FE _ref as the center, as shown in FIG.

  The output of the comparator 118 is switched if the focus error signal level is within a predetermined range for a predetermined time or more. Since the influence of the external vibration remains at the time of executing Step 3 in FIG. 7 immediately after switching the focusing control of the SIL and moving the in-focus position, the output of the comparator 118 remains High.

  After moving the focus position, the controller 113 continues to monitor the output of the comparator 118 until the focus error signal level is within a predetermined range for a predetermined time or more (step 4). If the focus error signal level falls within a predetermined range in Step 4 for a predetermined time or more, the output of the comparator 118 is switched from High to Low, and the controller 113 determines that the external vibration detected during the gap servo has subsided.

  If it is determined that the external vibration has subsided, the SIL light collection control is returned to the gap servo, and control to resume recording and reproduction is performed. That is, the controller 113 controls the expander lens driving circuit 117 to drive the expander lens 9 to move the focus position of the focus error signal (step 5). That is, the expander lens 9 is driven while performing the focus servo, and the focus position is moved from the control target position C to the control target position B in FIG.

  When the movement of the in-focus position is completed, the SIL condensing control is switched from the focus servo to the gap servo (step 6). When switching to the gap servo is completed, recording and playback are resumed.

  As described above, the focusing position of the focus error signal is moved in the direction in which the distance between the SIL and the optical disk is increased after switching the focusing control of the SIL from the gap servo to the focus servo when external vibration is detected.

  By doing so, the margin between the SIL and the optical disk is increased during external vibration, so that it is possible to prevent a collision between the SIL and the optical disk even during external vibration. In addition, since the focusing control of the SIL is not interrupted during external vibration and is switched to the focus servo, the gap servo can be restarted only by moving the expander lens 9 after the external vibration has subsided, so the recovery time is short. I'll do it.

1 is a block diagram showing a first embodiment of an optical information recording / reproducing apparatus according to the present invention. It is a timing chart which shows operation | movement of 1st Embodiment. It is a block diagram which shows the 2nd Embodiment of this invention. It is a block diagram which shows the pick-up optical system of 2nd Embodiment. It is explanatory drawing of the optical system which detects the focus error signal of 2nd Embodiment. It is explanatory drawing of the light beam which permeate | transmits the opening of the optical system which detects the focus error signal of 2nd Embodiment. It is a flowchart explaining operation | movement of 2nd Embodiment. It is a figure explaining the change method of the control target position of 2nd Embodiment. It is a timing chart which shows operation of a 2nd embodiment. It is a figure which shows the conventional optical pick-up for near field recording. It is a figure explaining hemisphere SIL. It is a block diagram which shows the conventional optical information recording / reproducing apparatus using SIL. It is a figure explaining the setting of the threshold value with respect to the gap error signal for detecting a disturbance in a prior art.

Explanation of symbols

1 Semiconductor laser (light source)
2 Collimator lens 3 Beam shaping prism 4, 18 Non-polarizing beam splitter (NBS)
5, 15, 19, 23, 26 Lens 6 LPC-PD
7, 14 Polarizing beam splitter (PBS)
8 1/4 wave plate (QWP)
9 Expander lens 10 Objective lens (rear lens)
11 SIL (tip lens)
12, 101 Optical disc (recording medium)
13 1/2 wavelength plate (HWP)
16, 20, 24, 27 Photodetector 17 RF output 21 Tracking error 22 Aperture 28 Gap error 29 Voice coil motor 102 Pickup 103 Spindle motor 104 Gap error generation circuit 105 Phase compensation circuit 106 Actuator driver circuit 107, 118 Comparator 108, 116 Switch 109 Drive voltage hold circuit 110, 115 Phase compensation circuit 111 Addition circuit 112 Saved voltage generation circuit 113 Controller 114 Focus error generation circuit 117 Expander lens drive circuit

Claims (4)

In an optical information recording / reproducing apparatus for condensing a light beam from a light source on an optical disk and recording or reproducing information,
A condensing optical system that condenses the luminous flux from the light source and includes an objective lens and a solid immersion lens (SIL) disposed between the objective lens and the optical disc;
An actuator for driving the condensing optical system in a direction perpendicular to the surface of the optical disc;
A gap error generating circuit for detecting a gap error signal indicating a distance between the condensing optical system and the optical disc;
A gap servo circuit for controlling the distance between the condensing optical system and the optical disc by servo-controlling the actuator based on the detected gap error signal;
A vibration detection circuit for detecting vibration,
Optical information characterized by turning off the gap servo and driving the actuator to retract the condensing optical system from the optical disc when the vibration detection circuit detects a vibration exceeding a preset level. Recording / playback device. 2. The optical information recording according to claim 1, wherein the vibration detection circuit switches from a gap servo based on the gap error signal to a focus servo based on a focus error signal when a vibration exceeding a preset level is detected by the vibration detection circuit. Playback device. 3. The optical information recording / reproducing apparatus according to claim 2, wherein the focus error signal is generated from a light flux whose effective numerical aperture by the SIL is less than 1 out of a light flux reflected from the optical disc. 4. The optical information recording / reproducing apparatus according to claim 2, wherein when the focus servo is switched to, the focusing position of the light beam is moved to widen the distance between the focusing point of the light beam and the SIL.