Patent Publication Number: US-2011051587-A1

Title: Structure For Optical Pickup Head

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a structure for optical pickup head, and more specifically to a structure for optical pickup head having a 10%-30% low reflectivity beam-splitter and moreover an optical signal detector with a 3° to 15° tilt to optical axis to reduce the laser feedback light. 
     BACKGROUND OF THE INVENTION 
     The compact disc (CD), video compact disc (VCD) and digital versatile disc (DVD), High Density-DVD (HD-DVD) and BD-DVD are widely used in offices and households for information and/or entertainment. Therefore, the industry has been researching to develop more efficient optical pickup technology to improve the quality of the recording media, such as CD, VCD, DVD, HD-DVD and BD-DVD. 
     An optical pickup head reads or writes data on the recording media. In general, an optical pickup head employs a laser diode (LD) to emit an optical beam through an optical element set to focus on the surface of the recording media. An optical signal detector (PDIC) is used for receiving the optical signal and detecting the strength of the optical signal. 
       FIG. 1  shows a schematic view of a conventional optical pickup head. As shown in  FIG. 1 , an optical pickup head includes a recording medium  101 , an optical signal detector  102 , a laser diode set  103 , a beam-splitter  104 , a collimator lens  105  and an objective lens  106 . The laser light of CD or DVD is emitted by laser diode set  103 , passes a grating  104   a,  is reflected by beam-splitter  104 , and then passes through collimator lens  105  and objective lens  106  to focus on the surface of recording medium  101  for reading or writing data. The laser light is then reflected by the surface of recording medium  101 , passes through objective lens  106  and collimator lens  105 , and is divided by beam-splitter  104  into two parts. One part passes through beam-splitter  104  to optical signal detector  102  for detecting the signal strength. The other part is reflected by beam-splitter  104 , passes through grating  104 a and feeds back to laser diode set  103 , which causes laser diode set  103  relative intensity noise problem. The beam-splitter  104  of the optical pickup head usually has a higher reflectivity ranging from 35% to 85% to achieve higher objective lens output power from the beam-splitter first time reflection to write data on the disc, and to achieve better optical efficiency for the laser light on PDIC which is the beam-splitter reflection and transmission combination. However, the high reflectivity beam-splitter  104  will greatly increase the reflected light from recording medium  101  feed back to laser diode set  103 , shown as line  42  of  FIG. 3 . Therefore, the laser will have strong noise problem. This is laser noise source  1  for laser diode set  103 . 
     Similarly, reflected light  105   c,    105   d  which are reflected by the surface of optical signal detector  102  may also pass through beam-splitter  104 , collimator lens  105  and objective lens  106  to focus on the surface of recording medium  101  again, and the reflected light then passes through objective lens  106 , collimator lens  105 , is reflected again by beam-splitter  104  and feeds back to laser diode set  103  to cause laser noise problem. This is laser noise source  2  for laser diode set  103 . 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to reduce or eliminate the noise problem for the laser diode caused by the feedback light of the recording medium. 
     To achieve the above object, the present invention provides an optical pickup head having a low reflectivity beam-splitter with an average reflectivity ranging from 10% to 30%. The optical pickup head includes a laser diode set, an optical element set and an optical signal detector. The laser light is emitted by the laser diode set, passes through and is reflected by the optical element set to focus on the surface of recording medium for data reading and writing. Then, the laser light is reflected by the surface of the recording medium, and is guided by the optical element set to the optical signal detector for detecting the signal strength. Because the optical signal detector has a 3° to 15° ranging θ tilt to the optical axis, the optical signal detector reflects the incident laser light to a direction out of optics axis so that the reflected light will reduce the feedback light to the optical element set and the light is reflected back again by the recording medium to the optical element set and the laser diode set. 
     The optical element set includes a low reflectivity beam-splitter, and an objective lens. The low reflectivity beam-splitter has the capability of both reflecting and transmitting the laser light. The objective lens is for focusing the laser light on the recording medium. The optical element set further includes a grating to split a light beam into three light beams. The optical element set also further includes a coupling lens to adjust the collimation of the laser beam and improve the optical efficiency. The low reflectivity beam-splitter of the present invention has the average reflectivity ranging from 10% to 30%, and reflects the laser light twice to reduce the aforementioned noise source  1  for laser diode set  103 . Moreover, with the optical signal detector having an 3° to 15° ranging θ tilt to the optical axis to reduce the aforementioned noise source  2  for the laser diode set, the optical pickup head of the present invention can achieve the object of reducing or eliminating the noise problem of the laser diode caused by the feedback light from the recording medium. 
     The low reflectivity beam-splitter of the present invention is cost effective, and the optical signal detector having an 3° to 15° ranging θ tilt to the optical axis shows technological improvement. With one or the combination of the two features, the present invention can effectively reduce or eliminate the feedback light and solve the existing noise problem for the laser diode. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be understood in more detail by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein: 
         FIG. 1  shows a schematic view of a conventional optical pickup head; 
         FIG. 2  shows a schematic view of an optical pickup head according to the present invention; and 
         FIG. 3  shows a schematic view of the optical efficiency of the low reflectivity beam-splitter of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows a schematic view of an optical pickup head according to the present invention. As shown in  FIG. 2 , an optical pickup head includes a recording medium  201 , an optical signal detector  202 , a laser diode set  203 , and an optical element set  204 . Laser diode set  203  is a laser light source with single, two or three different wavelengths. Optical element set  204  includes a low reflectivity beam-splitter  204   b  and an objective lens  204   d.  Optical element set  204  may further include a grating  204   a  and a coupling lens  204   c.  Grating  204   a  is to split a light beam into three light beams. Coupling lens  204   c  is to adjust the collimation of the laser beam and improve the optical efficiency. Grating  204   a  and Coupling lens  204  can be added or removed as necessary. 
     Laser diode set  203  emits a laser light source λ 1 . Laser light source λ 1  passes through grating  204   a,  and is reflected by low reflectivity beam-splitter  204   b.  The first-time reflected light  205   a  passes coupling lens  204   c  and objective lens  204   d  to focus on recording medium  201  for data reading and writing. The laser light reflected from recording medium  201  passes through objective lens  204   d  and coupling lens  204   c  is divided by low reflectivity beam-splitter  204   b  into reflection light and transmission light. The second-time reflected light  205   b  reflected by low reflectivity beam-splitter  204   b  feeds back to laser diode set  203 . Because laser light source λ 1  is reflected by low reflectivity beam-splitter  204   b  twice, low reflectivity second-time reflected light  205   b  will be much less than the light from a high reflectivity beam-splitter. Hence, when light  205   b  feeds back to laser diode, light  205   b  will not cause the laser noise problem. The same situation applies to laser light sources λ 2  and λ 3 . 
     When recording medium  201  uses laser light source λ 1  emitted by laser diode set  203  for data reading or writing, optical signal detector  202  receives reflected laser light  205   c  reflected by recording medium  201 , and detects the signal strength of laser light  205   c.  Optical signal detector  202  is placed with a 3° to 15° ranging θ tilt to optical axis  30 . Therefore, when laser light  205   c  arrives at optical signal detector  202 , laser light  205   c  will be reflected by optical signal detector  202  with a specific angle into a reflected light  205   d.  Similarly, laser lights  205   c   1 ,  205   c   2  will be reflected by optical signal detector  202  into reflected lights  205   d   1 ,  205   d   2 . With a tilt angle θ ranging from 3° to 15°, the tilt angle makes reflected lights  205   d,    205   d   1 ,  205   d   2  deviated from the direction of the laser light source to effectively avoid the feedback of reflected lights  205   d,    205   d   1 ,  205   d   2  to the laser light source and solve the laser feedback noise problem. 
       FIG. 3  shows a schematic view of the optical efficiency of the low reflectivity beam-splitter of the present invention. The X-axis of  FIG. 3  shows the reflectivity (R) of the beam-splitter, which affects the light output power from the objective lens. Line  41  shows the total optical efficiency (R*T) of one-time reflection (R) and one-time transmission (T), which affects the light power on the optical signal detector. When the beam-splitter reflectivity is 50%, the total optical efficiency (R*T) will be maximum and reach 25%. Line  42  shows the optical efficiency of twice reflection (R*R), which affects the amount of disk feedback light to laser diode set. The efficiency of twice reflection (R*R) can reach 100% when the reflectivity (R) is 100%. At lower reflectivity, a much lower twice reflection can be obtained, that means less disk feedback light to laser diode set. 
     Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.