1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly, to an optical pickup device provided with a collision prevention member preventing collision between an objective lens focusing light from a light source on an information recording surface of an optical recording medium and the optical recording medium.
2. Description of Related Art
Optical recording media such as a compact disc (hereinafter, referred to as CD) and a digital versatile disc (hereinafter, referred to as DVD) are widely spread. Moreover, recently, a blue-ray disc (hereinafter, referred to as BD) on which a large amount of information can be recorded has been put into practical use. When information is read from and written in the optical recording medium as described above, an optical pickup device is used. The optical pickup device is provided with an objective lens focusing light emitted from a light source on an information recording surface of an optical recording medium, and a light receiving portion receiving return light that is reflected on the information recording surface.
In order for the optical pickup device to read and write information, it is necessary to control a focal position of the objective lens so that, regardless of wobbling of an optical recording medium or the like, light emitted from the light source focuses on the information recording surface of the optical recording medium at all times. Accordingly, conventionally, control is performed whereby the objective lens is held by a lens holder, and the lens holder is moved in a direction close to and away from the optical recording medium by an actuator, so that a positional relationship between the objective lens and the optical recording medium is constant at all times. Hereinafter, this control is referred to as focus servo control.
On the optical recording medium, like the aforementioned BD, on which a large amount of information can be recorded, information is recorded with high density. Accordingly, in order for the optical pickup device to perform reading of information and the like, it is necessary to decrease the size of a light spot formed on the optical recording medium. Conventionally, as a method of decreasing the size of a light spot, a wavelength of light emitted from a light source is decreased, and a numerical aperture of the objective lens is increased.
As the numerical aperture of the objective lens is increased, a distance from the tip end of the objective lens to the optical recording medium (working distance; WD) tends to be decreased when the optical pickup device is operated. With the optical pickup device compatible with the BD in particular, the WD tends to be decreased. With the WD so decreased, for example, in cases where an optical recording medium whose surface greatly wobbles is used, and where the focus servo control cannot be performed owing to a flaw on an optical recording medium, vibration from the outside or the like, there is an extremely strong possibility that the objective lens and the recording medium may collide with each other.
A condition leading to a decreased WD is not only encountered by the optical pickup device compatible with the BD but may also be encountered by the optical pickup device compatible with the optical recording media such as CD and DVD. Specifically, in a notebook personal computer and the like that need to be slim, an objective lens provided in the optical pickup device has to be small in diameter. Even in a case, like this, where the objective lens having a small diameter is used, the WD is decreased, yielding an extremely strong possibility that the objective lens and the optical recording medium may collide with each other.
Thus, conventionally, the optical pickup device is provided with a collision prevention member for preventing collision between the objective lens and the optical recording medium (e.g., see Patent Documents 1 and 2). This helps prevent situations, owing to collision between the objective lens and the optical recording medium, where no information recorded on the optical recording medium can be used, and where the optical pickup device cannot be used with its objective lens damaged.
A friction force produced by the collision prevention member when making contact with the optical recording medium needs to be small. This is because the collision prevention member should not damage the optical recording medium when colliding with it. For that, conventionally, the collision prevention member is processed such that a tip end thereof is formed into a spherical surface, helping reduce an area of the contact with the optical recording medium (e.g., see Patent Document 1). As another method of reducing a friction force produced by the contact, a protruding member protruding from the objective lens, at a tip end thereof, is coated with a low friction material such as fluoroplastic resin, polyimide material and the like (e.g., see Patent Document 1 and 2).
However, formation of a coating film that helps reduce a friction force produced by the collision necessitates preparation of a protruding member and a material with a composition specialized for coating. This disadvantageously leads to an increase in cost necessary for preventing the collision between the objective lens and the optical recording medium.
Moreover, the collision prevention member needs to have wear resistance as its property for coping with the collision with the optical recording medium. However, by selecting a hard-to-wear material (material having wear resistance) as a material of the collision prevention member, difficulty is increased in processing a spherical surface like that mentioned above. Accordingly, so long as a molded article as disclosed, for example, by Patent Document 1 is used as the collision prevention member, it is impossible to secure wear resistance satisfactorily, causing the collision prevention member to wear out when colliding with the optical recording medium with a possibility that the optical recording medium may be stained. Moreover, when the collision prevention member wears out, and the recording medium is stained accordingly, quality of information reading or the like is disadvantageously degraded.    [Patent Document 1] JP-A-2001-209956    [Patent Document 2] JP-A-2001-297478
USPTO Backgrounds
USPTO Backgrounds