Abstract:
An optical disk unit, for reading signals stored on an optical disk medium, is provided with an optical pickup  23  selectively irradiating light of one wavelength, from several mutually different wavelengths, to an optical disk medium by means of a single objective lens and outputting signals based on light reflected by the optical disk medium, a drive section for causing relative movement of the objective lens of the optical pickup with respect to a surface of the optical disk medium, wherein a peak level of a fake signal generated in the received light signal based on surface reflection at the optical disk surface while the objective lens is being moved relative to the optical disk medium surface by the drive section is detected, and the received light signal is compared with a threshold value that is set based on the detected peak level, to detect reflected light of a data storage layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The priority application Numbers JP 2006-207181 and JP 2006-229816 upon which this patent application is based are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical disk unit, including a CD-ROM, CD-R or CD-RW drive, a DVD drive, or a Blu-Ray Disc (trademark) drive, a control method for such a disk unit, and a computer readable storage medium. 
         [0004]    2. Description of the Related Art 
         [0005]    Various types of optical disk medium have been developed in recent years, and they are appropriately used depending on the purpose. Generally, optical disk media have a structure with a plurality of layers arranged on top of one another. That is, regardless of the type of optical disc medium, protective layers are provided on both surfaces of the medium, and data storage layers for storing signals are formed enclosed by the protective layers. However, many parameters are different, such as thickness of the optical disk medium itself, distance from a surface of the protective layer to a surface of a data storage layer (signal surface), number of signal surfaces (for example, with a DVD, there is a maximum of two signal surfaces), and wavelength of a laser to be used in reading out information from the signal surface. 
         [0006]    For this reason, it is normal to use a dedicated drive for each type of optical disk medium. However, with the need to purchase and install a dedicated drive for each type of optical disk, the user must master the operations of each drive, and the financial burden is also significant. There has therefore been a demand for a drive (optical click drive) that can handle many types of optical disk media. 
         [0007]    For a drive to handle these many types of optical disk media, technology is being developed to use a different wavelength of light source (laser) used in reading out information for each type of optical disk medium, using an optical element having wavelength selection characteristics in a single objective lens, and changing a numerical aperture of the lens. 
         [0008]    An optical pickup  1  for such a drive, as exemplarily shown in  FIG. 8 , is constructed including a light-emitting element  11  for outputting laser light of a plurality of wavelengths, a beam splitter  12 , a photodetector  13  and an objective lens body  14 . Also, the objective lens body  14  is comprised of an objective lens  14 L and a hologram element  14 H including a diffraction grating. 
         [0009]    The light-emitting element  11  is a semiconductor laser element for outputting laser light of, for example, three mutually different wavelengths (a so-called three-wavelength laser). The three wavelengths here are controlled so that in the case of handling, for example, a Blu-ray disk, a DVD (Digital Versatile Disk) and a CD (Compact Disk), laser light is output having a wavelength of 405 nanometers for the Blu-ray disk, 650 nanometers for the DVD, and 780 nanometers for the CD. 
         [0010]    The beam splitter  12  guides light output by the light emitting element  11  to the objective lens body  14 . This beam splitter  12  also guides light that is input by being reflected by the optical disk body and passing through the objective lens body  14  to the photodetector  13 . The photodetector  13  is provided with a plurality of light detection elements arranged in a matrix of N×N, for example. This photodetector  13  is also provided with a cylindrical lens, for example for measuring beam diameter. Light that has been guided by the beam splitter  12  reaches the respective plurality of light receiving elements by way of this cylindrical lens. The photodetector  13  then respectively outputs signals for strength of light that has been respectively detected by the plurality of light receiving elements. 
         [0011]    The hologram element  14 H of the objective lens body  14  diffracts laser light that has been guided by means of the objective lens  14 L and reflected by the medium so as to become a predetermined numerical aperture (NA) for each wavelength of laser light, and guides the diffracted laser light to the beam splitter  12 . Also, the objective lens  14 L is an aspherical lens, and refracts and outputs light that has been guided from the light emitting elements through the beam splitter  12  and the hologram element  14 H so that focal points are focused at positions specified by focal distance F that is different for each wavelength. This objective lens  14 L also condenses laser light that has been reflected by the medium and guides it to the hologram element  14 H laser. 
         [0012]    A signal representing focus error of laser light on the storage surface of the optical disk medium (focus error signal; FE signal) and a signal equivalent to a sum of strengths of light that has reached the light receiving elements (pull-in signal; PI signal) are generated from a signal (RF signal) output by the photodetector  13 . It is also standard practice to generate a signal representing tracking error (TE signal) etc. from the signal output by the photodetector  13 , but detailed description thereof has been omitted here. 
         [0013]    Here, the PI (pull-in) signal is a signal shown as (a) in  FIG. 9 . Specifically, this PI signal has a peak at a positioned where focus is optimum. Also, the FE signal is shown as (b) in  FIG. 9 . Specifically, the FE (focus error signal) becomes substantially “0” when focus is achieved. Also, when the distance between the optical disk medium changed around a position where focus is achieved, the FE signal has positive and negative peaks respectively at points of certain distance from a position where focus is achieved, and the FE signal crosses zero (crosses a reference position) at a position where focus is achieved. 
         [0014]    In  FIG. 9 , an example is shown of each signal in the case where the objective lens  14 L of the optical pickup  1  is moved in a direction approaching the optical disk medium surface, starting at a position separated from the optical disk medium. When light that has been reflected by the optical disk medium surface arrives at the focal point in the optical pickup  1 , a peak occurs in the PI signal (S) due to the surface reflection, as shown in (a) of  FIG. 9 . If the objective lens  14 L of the optical pickup  1  is brought closer to the surface of the optical disk medium, the surface reflection light becomes stray light inside the optical pickup  1 , and this stray light is detected as a fake signal (Fake). This fake signal is not limited to one signal, and can also be a plurality of signals. If the optical pickup  1  is brought closer to the disk surface, reflected light (T) at the signal surface is detected. 
         [0015]    Similarly, for the FE signal which is shown as (b) in  FIG. 9 , at positions where the surface reflection (S), fake signal (Fake) and reflected light (T) at the signal surface are respectively obtained, a signal representing that an image has been formed is detected. 
         [0016]    With the optical disk unit, it is possible to readout signals corresponding to the plurality of optical disk media by controlling the distance between the objective lens body  14 L and the medium surface so that a distance from a planar section P of the objective lens  14  to the signal surface inside the medium becomes the focal distance F, that is, so that it is possible to achieve focus on the signal surface. Here, whether or not focus is achieved is determined using the FE signal and/or PI signal, and in focus is determined, for example, when an absolute value for the FE signal exceeds a first threshold level (FZC 1 ), but is less than a second threshold value (FZC 2 ) (approaches “0”). In focus is also determined when the PI signal exceeds a specified threshold value. 
         [0017]    An example of an optical disk unit that uses such an optical pickup is disclosed in Japanese patent application 2986587. 
         [0018]    Incidentally, depending on the optical disk medium, there are cases where the signal surface of the disk is inclined along a radial axis direction. In such a case, a distance from the objective lens to the signal surface is changed at the rotation cycle accompanying rotation of the optical disk medium. This phenomenon is called axial runout. 
         [0019]    If axial runout occurs, there are cases where a fake signal (Fake) appears repeatedly a plurality of times in the PI signals and FE signals, and a result (F 2 , F 3 ) of the fake signal (FAKE) constituting substantially the same peak level repeatedly appearing is that there are cases where discrimination between a reflected light (T) at the signal surface where focus should actually be achieved and a fake signal appearing due to axial runout is difficult ( FIG. 9 ). 
         [0020]    Therefore, attention has focused on the fact that the level of reflected light (T) at the signal surface is generally higher than the fake signal (Fake), and a method has been considered where a threshold value that is a higher level than the level of a fake signal (Fake) that would be expected due to the occurrence of axial runout is set experimentally. For example, the threshold value is set by writing to a read only memory (ROM) at the time of leaving the factory etc., and detection of a reflection at the signal surface (FOK, FZC 1  shown by the dotted line in  FIG. 9 ) when a signal exceeds this threshold value. 
         [0021]    However, as shown in  FIG. 10 , the level of a signal detected by the optical pickup becomes low overall due to variations in reflectance of the optical disk medium inserted, dust attached to the optical pickup, or environmental variations, such as temperature. When the level of a signal detected by the optical pickup becomes low, in the case of where the predetermined threshold values FOK, FZC 1  are set as described above and fixed values are always used, there may be cases where the level of reflected light at the signal surface does not reach this threshold value and it will be difficult to achieve focus at the signal surface. 
       SUMMARY OF THE INVENTION 
       [0022]    The present invention has been conceived in view of the above described situation, and an object of the invention is to provide an optical pickup unit that is capable of performing focus control on a signal surface, regardless of the state of the optical pickup. 
         [0023]    According to one aspect of the present invention, there is provided an optical disk unit, being an optical disk unit for reading signals stored on an optical disk medium, comprising an optical pickup selectively irradiating light of one wavelength, from several mutually different wavelengths, to an optical disk medium by means of a single objective lens and outputting signals based on light reflected by the optical disk medium, a drive section for causing relative movement of the objective lens of the optical pickup with respect to a surface of the optical disk medium, a signal output section for receiving light reflected by the optical disk medium and outputting received light signals based on the received light, and a control section for detecting peak level of a fake signal generated in the received light signal based on surface reflection at the optical disk surface while the objective lens is being moved relative to the optical disk medium surface by the drive section, comparing the received light signal with a threshold value that is set based on the detected peak level, and detecting reflected light of a data storage layer. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a block diagram showing a structural example of an optical disk unit of an embodiment of the present invention. 
           [0025]      FIG. 2  is a functional block diagram showing an example of an optical disk unit of an embodiment of the present invention. 
           [0026]      FIG. 3  is an explanatory drawing showing an example of signals detected by an optical pickup of the optical disk unit of an embodiment of the present invention. 
           [0027]      FIG. 4  is an explanatory drawing showing another example of signals detected by an optical pickup of the optical disk unit of an embodiment of the present invention. 
           [0028]      FIG. 5  is an explanatory drawing showing an example of signals relating to movement in focus processing of the optical disk unit of an embodiment of the present invention. 
           [0029]      FIG. 6  is a flowchart showing an example of movement in focus processing of the optical disk unit of an embodiment of the present invention. 
           [0030]      FIG. 7  is a flowchart showing another example of movement in focus processing of the optical disk unit of an embodiment of the present invention. 
           [0031]      FIG. 8  is an outline view showing a structural example an optical pickup also used in the optical disk unit of an embodiment of the present invention. 
           [0032]      FIG. 9  is an explanatory drawing showing one example of signals detected by an optical pickup of an optical disk unit. 
           [0033]      FIG. 10  is an explanatory drawing showing another example of signals detected by an optical pickup of an optical disk unit. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0034]    An embodiment of the present invention will now be described with reference to the drawings. As shown in  FIG. 1 , an optical disk unit of the embodiment of the present invention comprises a medium support section  21 , a spindle motor  22 , an optical pickup  23 , a biaxial actuator  24 , a feed motor  25   a,  a focus control actuator  25   b,  a driver amp  26 , an RF amp  27 , a servo signal processing section  28 , a servo processing control section  29 , a signal processing section  30  and a drive controller  31 . 
         [0035]    The medium support section  21  supports an optical disk medium in a rotatable state. Also, this medium support section  21  rotates the optical disk medium using power transmitted from the spindle motor  22 . The optical pickup  23  is the same as the optical pickup shown in  FIG. 8 , and here can be moved in two directions, namely a radial direction of the optical disk medium and a direction perpendicular to the surface of the optical disk medium, by the biaxial actuator  24 . 
         [0036]    The biaxial actuator  24  is moved along the radial direction of the optical disk medium by the feed motor  25   a.  Also, this biaxial actuator  24  is provided with an actuator (focus control actuator)  25   b  for moving the optical pickup  23  in a direction perpendicular to the surface of the optical disk medium. In this way, a distance between an objective lens body  14  contained in the optical pickup  23  and the surface of the optical disk medium is controlled. 
         [0037]    The drive amp  26  controls rotation amount of the feed motor  25   a.  Also, this drive motor  26  drives the focus control actuator  25   b  of the biaxial actuator  24  in accordance with signals input from the servo signal processing section  28 . 
         [0038]    The RF amp  27  receives respective output signals of a plurality of light detecting elements from the optical pickup  23 . The RF amp  27  generates and outputs at least one of an FE signal and a PI signal as received light signals based on these received output signals. 
         [0039]    The servo signal processing section  28  can be implemented using, for example, an A/D converter for converting servo signals to digital signals, and a DSP (digital signal processor) for subjecting the converted digital signals to digital signal processing. This servo signal processing section  28  detects a peak of the PI signal output by the RF amp  27 . 
         [0040]    Also, the servo signal processing section  28  detects whether or not this PI signal has exceeded a predetermined PI signal threshold value (FOK; Focus OK). Further, this servo signal processing section  28  performs specified processing using a predetermined FE signal threshold (FZC; Focus Zero Cross), based on the FE signal output by the RF amp  27 . This processing will be described in detail later. The servo signal processing section  28  outputs these detection results and results of specified processing to the servo processing control section  29 . 
         [0041]    Further, the servo signal processing section  28  outputs signals relating to drive of the focus control actuator  25   b  to the driver amp  26  in accordance with commands input from the servo processing control section  29 . 
         [0042]    The servo processing control section  29  is a microcomputer, for example, and includes execution modules and storage elements. Storage elements of this servo processing control section  29  are computer readable storage media, and store programs to be executed and parameters. It is also possible for this program to be provided in a state stored in another computer readable storage medium such as DVD-ROM, and be duplicated in the storage elements. Execution modules of the servo processing control section  29  carry out processing in accordance with programs stored in the storage elements. 
         [0043]    This servo processing control section  29  receives input of signals (signal relating to results of PI signal peak detection and processing result signals relating to the FE signal) etc. input from the servo signal processing section  28 . The servo control processing section  29  then executes processing (focus control processing) to set a distance between the optical pickup  23  and the optical disk medium at a position where focus is achieved at the signal surface based on these received signals. This focus control processing will be described in detail later. 
         [0044]    The signal processing section  30  demodulates signals stored in the optical disk medium based on signals output by the RF amp  27 . The signal processing section  30  then outputs the demodulated signals. The drive controller  31  is connected to a personal computer, or home game machine or video recorder, constituting a host, and drives the driver amp  26  in response to requests from the host to move the optical pickup  23  to a desired position on the optical disk medium. Demodulated signals output from the signal processing section  30  and stored in the optical disk medium are then output to the host. 
         [0045]    Focus control processing using the optical disk unit of this embodiment will now be described. With this embodiment, focus control processing is implemented in software by the servo processing control section  29 . Specifically, as shown in  FIG. 2 , the servo processing control section  29  is functionally constructed including an initialization section  41 , a fake signal detection section  42 , a peak level calculation section  43  and a signal layer detection section  44 . 
         [0046]    The initialization section  41  drives the spindle motor  22  to rotate and thus rotates the optical disk medium. Also, this initialization signal  41  sets a wavelength of light irradiated by the optical pickup  23  to a value established as a predetermined initial value. The initialization section  41  also drives the focus control actuator  25   b  so that the objective lens of the optical pickup  23  is moved to a position (initial position) that is furthest away from the surface of the optical disk medium. The initialization section  41  commences control to move the lens towards the surface of the optical disk medium at a specified speed once the objective lens of the optical pickup  23  has been moved to the initial position. 
         [0047]    The fake signal detection signal  42  detects surface reflection from the PI signal or the FE signal output by the servo signal processing section  28  and detects the first fake signal appearing after the surface reflection. As an example, using an FE signal after the initialization section  41  has commenced movement of the objective lens of the optical pickup  23 , this fake signal detection section  42  determines that surface reflection has been detected when the FE signal initially exceeds an upper peak and then crosses a reference position (zero cross). Then, when the FE signal next exceeds the upper peak and performs a zero cross, it is determined that an initial fake signal (Fake) has been detected. This fake signal detection section  42  is also provided with a peak hold circuit, and holds the immediately preceding peak value. The fake signal detection section  42  then outputs a peak value pbase held by the peak hold circuit to the peak level calculation section  43  when it has been determined to be focused on the initial fake signal (Fake). This peak value pbase is used as a value constituting a basis for a threshold value which is for detecting a signal layer. 
         [0048]    As another specific example of this fake signal detection section  42 , it is possible to use a PI signal after the initialization section  41  has commenced movement control of the objective lens of the optical pickup  23 , and establish a peak value pbase constituting a basis for a threshold value for signal layer detection. In this case, the fake signal detection section  42  determines that surface reflection has been detected when the value of the initial PI signal becomes the peak value. Then, when the PI signal next becomes the peak value, it is determined that an initial fake signal (Fake) has been detected. Incidentally, in the peak detection thus far, in order to eliminate the effects of noise, it is also possible to set a threshold value at which a noise level is exceeded (noise elimination threshold), set detection of surface reflection when an initial peak value exceeding the noise elimination threshold is detected, and detect a fake signal (Fake) when the next peak value is detected. This fake signal detection section  42  is also provided with a peak hold circuit, and holds the immediately preceding peak value. The fake signal detection section  42  then outputs a peak value held by the peak hold circuit to the peak level calculation section  43  as a peak value pbase constituting a basis for a threshold value for signal layer detection when it has been determined that the initial fake signal (Fake) has been focused. 
         [0049]    As a further example of this fake signal detection section  42 , it is possible to use both a PI signal and FE signal after the initialization section  41  has commenced movement of the objective lens of the optical pickup  23 , and establish a peak value pbase constituting a basis for a threshold value for signal layer detection. In this case, the fake signal detection section  42  determines that surface reflection has been detected when the value of the initial PI signal becomes the peak value. In this peak detection, in order to eliminate the effects of noise, it is also possible to set a threshold value at which a noise level is exceeded (noise elimination threshold), and detect surface reflection when an initial peak value exceeding the noise elimination threshold is detected. 
         [0050]    Then, after surface reflection has been detected using the PI signal, the fake signal detection section  42  determines that an initial fake signal (Fake) has been detected when the FE signal exceeds the upper peak and performs a zero cross. This fake signal detection section  42  is also provided with a peak hold circuit, and holds a peak value of the immediately preceding FE signal using this peak hold circuit. It is also possible for the fake signal detection section  42  to then output a peak value pFE held by the peak hold circuit to the peak level calculation section  43  as a peak value pbase constituting a basis for a threshold value for signal layer detection when it has been determined that the initial fake signal (Fake) has been focused on. 
         [0051]    Even in the event that a peak value pFE of the FE signal is detected, it is possible for the fake signal detection section  42  to also detect a peak value (peak value at the time of fake signal (Fake) detection) pPI of the next PI signal after surface reflection has been detected, and to output the peak value pFE of the FE signal as pbase, and also to output this pPI to the peak level calculation section  43 , which can be used in signal layer detection. 
         [0052]    The peak level calculating section  43  receives the peak value pbase input from the fake signal detection section  42 . The peak level calculation section  43  uses this received peak value pbase as a value constituting the basis of a threshold value for signal layer detection. Specifically, the peak level calculating section  43  establishes a threshold value based on this peak value base, and outputs the established threshold value to the signal layer detection section  44 . Here the threshold value can be the peak value pbase itself. Or, the threshold value can be set higher than by a level that is a fixed proportion (for example, between 5 and 10%) of the peak value pbase (pbase×1.05 to pbase×1.1) taking into consideration variations in measurement of the peak value in the fake signal detection section  42 . Incidentally, as has already been described, in the case of carrying out signal layer detection using the FE signal, using first and second threshold values FZC 1 , FZC 2  (|FZC 1 |&gt;|FZC 2 |) it is determined that focus has been achieved when the absolute value of the FE signal becomes a peak exceeding the first threshold value (FZC 1 ), and then the absolute value of the next FE signal drops below the second threshold value (FZC 2 ). As a result, in the event that the fake signal detection section  42  outputs a peak signal pFE of the FE signal as a value pbase constituting the basis of the signal layer detection threshold value, the peak level calculating section  43  sets a threshold value that is generated and output based on this pbase to the first threshold level FZC 1 , being a threshold value relating to the peak level. 
         [0053]    Also in the case where the fake signal detection section  42  outputs the peak value pPI of the PI signal as a value pbase constituting the basis for a signal layer detection threshold, a threshold value is established based on the peak value pbase (=pPI) output by the fake signal detection section  41 , and the established threshold value is output to the signal layer detection section  44 . Here the threshold value can be the peak value pbase, or can be set higher by a level that is a fixed proportion (for example, between 5 and 10%) of the peak value pbase (p×1.05 to p×1.1) taking into consideration variations in measurement of the peak value in the fake signal detection section  42 . In this case, the threshold value output by the peak level calculating section  43  is used as FOK relating to the PI signal. 
         [0054]    Even in the event that the fake signal detection section  42  outputs the peak value pFE of the FE signal as a value pbase constituting the basis for a signal layer detection threshold, if a peak value pPI of the PI signal at the time of detection of the initial fake signal (Fake) with the PI signal is also input, a threshold value FZC 1  is established based on pbase, and a threshold value FOK for the PI signal is calculated based on the peak value pPI. Here, FOK can also use the input peak value pPI directly, or can use pPI increased in level by a fixed proportion (for example, 5 to 10%). In this case, the peak level calculating section  43  also outputs FOK relating to the PI signal together with the first threshold value FZC 1  relating to the FE signal. 
         [0055]    Specifically, the signal layer detection section  44  detects a position of focus on the signal layer using a signal used by the fake signal detection section  42 , from the PI signal or the FE signal output by the servo signal processing section  28 . As an example, in the case the fake signal detection section  42  is using the FE signal, it is determined that the signal layer has been detected when the FE signal exceeds the threshold value FZC 1  established by the peak level calculating section  43 , and then drops below the predetermined second threshold level FZC 2  (FZC 2 &lt;FZC 1 ). 
         [0056]    Also, when the peak level calculating section  43  outputs a threshold value FOK for the PI signal and a threshold value FZC 1  for the FE signal, the signal layer detection section  44  performs a signal layer detection operation using the FE signal within a period when the input PI signal exceeds the threshold value FOK. Specifically, within the period, it is determined that the signal layer has been detected when the FE signal exceeds the threshold value FZC 1  established by the peak level calculating section  43 , and then drops below the predetermined second threshold level FZC 2  (FZC 2 &lt;FZC 1 ). 
         [0057]    Also, in the case of using only the PI signal, the signal layer detection section  44  receives the threshold value (FOK relating to the PI signal) output by the peak level calculating section  43 , and determines that the signal layer is in focus when a peak exceeding this threshold value FOK is detected from the PI signal. 
         [0058]    With these configurations, the optical disk unit of this embodiment operates as follows. Specifically, in a state where the level of a signal detected by the optical pickup  23  is not falling, the objective lens of the optical pickup  23  is brought close to the optical disk medium from a position separated from the optical disk medium, and as shown in  FIG. 3  image formation using surface reflection of the PI signal and the FE signal is detected (S). Then, a fake signal (Fake) is detected at a position where the objective lens is closer to the optical disk medium. The optical disk unit of this embodiment establishes a threshold value (FOK, FZC 1 ) of a level greater than or equal to the level of the fake signal (Fake) detected here. Incidentally, for the sake of explanation threshold values are shown in  FIG. 3  for both the PI signal and the FE signal, but it does not matter which one is used. 
         [0059]    Here, since the optical disk medium is rotating, there may be occasions when the distance between the surface of the optical disk medium and the optical pickup  23  periodically varies due to axial runout, and the fake signal (Fake) is measured a plurality of times (F 2 , F 3  . . . ). The fake signal (Fake) here is the original fake signal (Fake) appearing periodically, and so appears as peaks that do not exceed FOK or FZC 1 . 
         [0060]    Then, if the objective lens is brought even closer to the optical disk medium, a peak (T) that is higher than the threshold value (FOK or FZC 1 ) determined based on the level of the fake signal (Fake) is respectively detected in the PI signal and the FE signal. 
         [0061]    In the event that the optical disk unit executes focus processing to the signal surface using the PI signal, it is determined that the signal surface has been detected at a position (T) where the PI signal exceeds the established threshold value FOK, and processing transfers to focus servo control. Specifically, if focus servo for the objective lens is “ON”, an operation to move the objective lens to track upward and downward movement of the optical disk medium is commenced, and data reproduction is carried out. 
         [0062]    On the other hand, in the event that the optical disk unit executes focus processing to the signal surface using the FE signal, it is determined that the signal surface has been detected at a position (T) where the FE signal exceeds the established threshold value FZC 1  and further drops below the established threshold FZC 2 , and processing transfers to focus servo control. 
         [0063]    Next, for this optical disk unit, description will be given for the case where the level of a signal detected by the optical pickup  23  has fallen. In this case also, the objective lens of the optical pickup  23  is brought close to the optical disk medium from a position separated from the optical disk medium, and as shown in  FIG. 4  image formation using surface reflection of the PI signal and the FE signal is detected (S). Then, a fake signal (Fake) is detected at a position where the objective lens is even closer to the optical disk medium. The level of the fake signal (Fake) here falls more than the level shown in  FIG. 3  in accordance with the fact that the level of the signal detected by the optical pickup  23  is falling. 
         [0064]    The optical disk unit of this embodiment establishes a threshold value (FOK, FZC 1 ) of a level greater than or equal to the level of the fake signal (Fake) detected here. For the sake of explanation in  FIG. 4  also, threshold values are shown for both the PI signal and the FE signal, but it does not matter which one is used. The threshold value established here (either FOK or FZC 1 ) is a lowered level compared to that shown in  FIG. 3 , in accordance with the lowering of the level of the fake signal (Fake). 
         [0065]    Further, there may be occasions when the distance between the surface of the optical disk medium and the optical pickup  23  periodically varies due to rotation and axial runout of the optical disk medium, and the fake signal (Fake) is measured a plurality of times (F 2 , F 3  . . . ). The fake signal (Fake) here is the original fake signal (Fake) appearing periodically, and so appears as peaks that do not exceed FOK or FZC 1 . 
         [0066]    Then, if the objective lens is brought even closer to the optical disk medium, a peak that is higher than the threshold value (FOK or FZC 1 ) determined based on the level of the fake signal (fake) is respectively detected in the PI signal and the FE signal. 
         [0067]    In the event that the optical disk unit executes focus processing to the signal surface using the PI signal, it is determined that the signal surface has been detected at a position where the PI signal exceeds the established threshold value FOK, and processing transfers to focus servo control. 
         [0068]    On the other hand, in the event that the optical disk unit executes focus processing to the signal surface using the FE signal, it is determined that the signal surface has been detected at a position where the FE signal exceeds the established threshold value FZC 1  and further drops below the established threshold FZC 2 , and processing transfers to focus servo control. 
         [0069]    In this way, according to this embodiment, a fake signal (Fake) occurring within the optical pickup  23  is detected by surface reflected light, and reflected light at the signal layer is detected utilizing a signal level of the fake signal (Fake). Therefore, even if the level of the signal detected by the optical pickup  23  is lowered overall due to dirt attached to the optical pickup  23  or environmental variations such as temperature, since reflected light at the signal layer is detected at a threshold that has been corrected by the signal level of the fake signal (Fake), it is made possible to achieve focus at the signal layer regardless of the state of the optical pickup  23 . 
         [0070]    Incidentally, here, in the peak level calculation section  43 , description has been given of an example where the peak value pbase of the fake signal detection section  42  is output as a threshold value either as it is, or increased by a level that is a specified proportion of the peak value pbase, but this is not limiting and it is also possible to have the following. 
         [0071]    First, in the optical disk unit, a test disk medium for evaluating axial runout is used, and the level of the fake signal (Fake) was detected a plurality of times. Among the levels detected in this way, a value Ref, being a level 4 to 5 times higher than a distribution σ (4σ to 5σ) from an average value Ave was referred to, and a coefficient α was established as 
         [0000]      α= Ref/Ave    
         [0000]    and stored. 
         [0072]    Then in the peak level calculating section  43 , a peak value p detected by the fake signal detection section  42  and a threshold value using this coefficient α were calculated as 
         [0000]      p×α 
         [0073]    In this way, if a coefficient α is established for each optical disk unit it becomes possible to calculate a threshold value taking into consideration individual differences for every optical desk unit. 
         [0074]    Also, an example has been described here where a coefficient α is established using a test disk medium for evaluating axial runout, but there is no problem in using a general optical disk medium instead of the test disk medium for evaluating axial runout. 
         [0075]    It is also possible for this coefficient α to be determined for every type of machine (for every circuit used, DSP, type of microcomputer, or combination). 
         [0076]    Further, in the description thus far, the signal layer detection section  44  has been described as detecting the signal layer using either of the PI signal or the FE signal, but it is also possible to use both of them. Specifically, as shown in  FIG. 5 , the signal layer detection section  44  can carry out focus processing to the signal layer using the FE signal in a range (R) where a PI signal exceeding the threshold value FOK, established based on the level of the PI signal at the time a fake signal (Fake) occurs, is being detected. Even in this case, it is also possible to establish the threshold value FZC 1  relating to the FE signal based on a level of the FE signal at the time a fake signal (Fake) occurs. 
         [0077]    In this example also, FOK and FZC 1  can be the level of a signal corresponding to the time a fake signal (Fake) occurs, or can be made values resulting from multiplying the level of a signal corresponding to the time a fake signal (Fake) occurs by a specified coefficient. A coefficient for multiplying the PI signal when a fake signal (Fake) occurs for FOK calculation, and a coefficient for multiplying the FE signal at the time a fake signal (Fake) occurs for FZC 1  calculation, can also be different from each other. 
         [0078]    Specifically, the servo processing control section  29  of this embodiment operates as shown in  FIG. 6 . That is, the servo processing control section  29  executes initialization processing (S 1 ), drives the spindle motor  22  to rotate so as to rotate the optical disk medium, and sets a wavelength of light irradiated by the optical pickup  23  to a value established as a predetermined initial value. Also, the focus control actuator  25   b  is driven so that the objective lens of the optical pickup  23  is moved to a position (initial position) that is furthest away from the surface of the optical disk medium. 
         [0079]    The servo processing control section  29  then commences movement of the objective lens of the optical pickup  23  towards the surface of the optical disk medium at a specified speed (S 2 ). 
         [0080]    The servo processing control section  29  then stands by until an initial peak (namely surface reflection) exceeding a threshold value (noise elimination threshold value) that exceeds a noise level, in order to eliminate noise, is detected, using the PI signal after commencement of movement of the objective lens of the optical pickup  23  (S 3 ). 
         [0081]    Then, if surface reflection is detected, the servo processing control section  29  commences hold of peak values of the PI signal and the FE signal after detection of surface reflection (S 4 ), and then stands by until the FE signal passes an upper peak 
         [0000]    initially appearing after detection of surface reflection, and crosses a reference value (zero cross) (S 5 ). If the FE signal exceeds the initially appearing upper peak and further performs a zero cross, the servo processing control section  29  determines that an initial fake signal (Fake) has been detected. The servo processing control section  29  then establishes a threshold value FOK for the PI signal and a threshold value FZC 1  (FZC 1 &gt;0) for the FE signal in order to detect the signal surface, based on peak values (respectively expressed as pPI and pFE, here pFE&gt;0) of (absolute values of) the PI signal and the FE signal that are being held (S 6 ). 
         [0082]    For example, with this processing S 6 , it is possible to make 
         [0000]        FOK=pPI×β,  and 
         [0000]        FZC 1= pFE×γ   
         [0083]    Here β and γ are values greater than “1”, for example, “1” or values between “1.05” and “1.1”. β and γ can have the same value, or different values. 
         [0084]    The servo processing control section  29  stands by until the PI signal becomes greater than or equal to FOK determined in processing S 6 , and the FE signal exceeds FZC 1  (S 7 ). Then, if the PI signal becomes greater than or equal to FOK determined in processing S 6 , and the absolute value of the FE signal exceeds FZC 1 , there is again a standby state until the PI signal reaches FOK determined in processing S 6  and the absolute value of the FE signal becomes smaller than a predetermined FZC 2  (0&lt;FZC 2 &lt;FZC 1 ) (S 8 ) Then, if the PI signal becomes greater than or equal to FOK determined in processing S 6  and the absolute value of the FE signal becomes smaller than a predetermined FZC 2  (0&lt;FZC 2 &lt;FZC 1 ), the servo processing control section  29  determines that the signal layer has been detected, and the focus control actuator  25   b  is controlled so that there is a transfer from operation to move the objective lens of the optical pickup  23  towards the surface of the optical disk medium to focus servo control where a focus servo is turned on (S 9 ). 
         [0085]    Also, with a different example, the servo processing control section  29  of this embodiment operates as shown in  FIG. 7 . Parts that are the same as the operation of  FIG. 6  have the same reference numerals attached, and detailed description thereof is omitted. 
         [0086]    Specifically, the servo processing control section  29  executes initialization processing (S 1 ) then commences movement of the objective lens of the optical pickup  23  towards the surface of the optical disk medium at a specified speed (S 2 ). 
         [0087]    The servo processing control section  29  then stands by until an initial peak (namely surface irradiation) exceeding a threshold value (noise elimination threshold value) exceeding a noise level, in order to eliminate noise, is detected, using the PI signal after commencement of movement of the objective lens of the optical pickup  23  (S 3 ). 
         [0088]    If surface reflection is detected, the servo processing control section  29  then commences holding of a peak value of the PI signal (S 14 ) and then stands by until a peak initially appearing in the PI signal is detected (S 15 ). If a peak of the PI signal is detected, the servo processing control section  29  determines that an initial fake signal (Fake) has been detected, and establishes a threshold value FOK for the PI signal in order to detect the signal surface, based on peak values (expressed as pPI) of the PI signal that are being held (S 16 ). 
         [0089]    For example, with this processing S 16 , it is possible to make FOK=pPI×β. 
       Here β is a value values greater than “1”, for example, “1” or, values between “1.05” and “1.1”. 
       [0090]    The servo processing control section  29  stands by until the PI signal becomes greater than or equal to FOK established in processing S 16  (S 17 ). Then, if the PI signal becomes greater than or equal to FOK determined in processing S 16  it is determined that the signal layer has been detected, and the focus control actuator  25   b  is controlled so that from operation to move the objective lens of the optical pickup  23  towards the surface of the optical disk medium, a focus servo is turned on and there is a transfer to focus servo control (S 19 ). 
         [0091]    With the examples of  FIG. 6  and  FIG. 7 , surface reflection is detected using the PI signal in both cases, but it is possible to carry out detection using the FE signal. 
         [0092]    Also, description has been given here of an example where after the signal surface has been detected a focus servo is turned on and a signal reproduced, but after signal surface detection, in the event that reflectance of the signal layer is measured, it is also possible to suspend the operation of the objective lens in the processing S 9  and the processing S 19  in  FIG. 6  and  FIG. 7  described above, and measure reflectance of the signal layer. 
         [0093]    While the present invention is described in terms of preferred or exemplary embodiments, it is not limited thereto.