Abstract:
To have laser light sources with different wavelengths as one body, to guide the optical axes of the laser light sources through a common optics as far as possible, and to realize compactness, manufacturing easiness, low cost and high performance and reliability, when the wavelengths of laser beams from first, second and third light sources  21, 22 , and  23  are different, a laser beam with a shortest wavelength is output from the first light source, a laser beam with a intermediate wavelength is output from the second light source, and a laser beam with a longest wavelength is output from the third light source. In this arrangement, a laser beam from the light source with a shortest wavelength is reflected by the least number of times.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2005-129855, filed Apr. 27, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND  
       [0002]     1. Field  
         [0003]     One embodiment of the invention relates to an optical head and an optical disc apparatus, which are devised to be able to read information from or write it to in any type of optical disc. In the present circumstances many kinds of optical disc have been developed.  
         [0004]     2. Description of the Related Art  
         [0005]     A digital versatile video disc (DVD) with a density higher than a conventional compact disc (CD) has been developed and is popular at present. A high density DVD (HD DVD) having higher density than a DVD will be developed in the future.  
         [0006]     For reproducing information recorded on a CD, a red laser beam with a wavelength of approximately 785 nm is used to read the information. For reproducing information recorded on a DVD, a laser beam with a wavelength of approximately 655 nm is used to read the information. For reproducing information recorded in HD DVD, a laser beam with a wavelength of approximately 405 nm will be used to read the information.  
         [0007]     As a laser beam wavelength is different according to a type of optical disc, it is necessary to prepare two or more laser beam sources (for wavelengths of 785, 655 and 405) for reproducing the above three types of optical disc in an information reproduce apparatus. It is also necessary to contrive the configuration of an optical head.  
         [0008]     Examples of Japanese Patent Application Publication (KOKAI) Nos. 2004-14008, 2000-268397 and 11-339307 are known as an optical head having two or more light sources with their optical axes arranged on a common optical system.  
         [0009]     However, in any of the Publications, light sources for laser beams with three wavelengths (785 nm, 655 nm and 405 nm) are not formed as a single unit. An idea of using light sources for laser beams with three wavelengths is not indicated in any publication. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0010]     A general architecture that implements the various feature of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention.  
         [0011]      FIG. 1  is an exemplary diagram showing an example of an optical head apparatus in accordance with an embodiment of the invention;  
         [0012]      FIG. 2  is an exemplary diagram showing an example of an essential part of the optical head shown in  FIG. 1 ;  
         [0013]      FIG. 3  is an exemplary diagram showing an example of another essential part of the optical head shown in  FIG. 1 ;  
         [0014]      FIGS. 4A and 4B  are graphs each explaining an exemplary film characteristic inverting band (wavelength characteristic) of a wavelength selection film used for an optical head (PUH) of the optical disc apparatus shown in FIGS.  1  to  3 ; and  
         [0015]      FIG. 5  is an exemplary diagram showing an example of an optical disc apparatus including an optical head (PUH) shown in  FIG. 1 , according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, an optical head including: a first light source which outputs a laser beam with a first wavelength; an object lens which condenses an input laser beam, emits the light to an optical disc, and receives a return laser beam reflected by the optical disc; a second light source which emits a laser beam with a second wavelength longer than the first wavelength; a first optical coupling prism which is placed on an optical axis between the first light source and object lens, and guides the laser beam with the second wavelength from the second light source to the object lens; a third light source which emits a laser beam with a third wavelength longer than the second wavelength; and a second optical coupling prism which is placed on an optical axis between the first optical coupling prism and object lens, and guides the laser beam with the third wavelength from the third light source to the object lens.  
         [0017]     According to an embodiment,  FIG. 1  shows an example of an optical head, to which the embodiments of the invention are applicable.  
         [0018]     An optical disc apparatus  1  shown in  FIG. 5  can record or reproduce information on/from an optical disc  11 , by condensing a laser beam of predetermined wavelength explained hereinafter from an optical head  51  (including a PUH actuator  52 ) shown in  FIG. 1 , on an information recording layer of an optical disc  11  corresponding to an optional kind (standard) explained hereafter. The optical disc  11  is a disc of the CD or DVD standard, or HD (high density) DVD disc with the recording density increased to higher than the CD and DVD standards.  
         [0019]     The PUH  52  can output any one of optical beams with first wavelength (405 nm), second wavelength (655 nm) and third wavelength (785 nm), according to the kind of a mounted optical disc  11 , as explained in a later paragraph with reference to  FIG. 1 . The PUH  52  also detects a reflected laser beam reflected on a not-shown information-recording surface of the optical disk  11 , and outputs an output signal usable for reproducing information already recorded.  
         [0020]     The PUH  52  includes a first light source  21  that is a semiconductor laser element, for example. The wavelength of an optical beam emitted from the first light source  21  is 400 to 410 nm, preferably 405 nm. The PUH  52  also includes a second light source  22  that is a semiconductor laser element, for example. The wavelength of an optical beam emitted from the second light source  22  is preferably 655 nm.  
         [0021]     The PUH  52  also includes a third light source  23  that is a semiconductor laser element, for example. The wavelength of an optical beam emitted from the third light source  23  is preferably 785 nm.  
         [0022]     The laser beams from the first and second light sources are overlaid on the optical axis S 1  by a polarization plane  31   a  of a first coupling prism  31 . The laser beams emitted from the first and second light sources and traveling on the optical axis S 1  are further overlaid by a half-mirror plane of a second optical coupling prism  32 .  
         [0023]     At a predetermined position of the PUH  52  opposite to the optical disc  11 , an object lens  12  is provided. The object lens condenses the laser beam emitted from one of the first to third light sources  21  to  23  according to the kind of the optical disc  11 , on a not-shown recording surface of the optical disc  11 , and captures the reflected laser beam reflected on the recording surface.  
         [0024]     The object lens  12  is a lens applicable to three wavelengths and capable of providing a predetermined numerical aperture (NA) for each laser beam output from the first to second laser elements  21  and  23 . The object lens  12  is made of plastic, and has a numerical aperture NA of 0.65 for a laser beam with a wavelength of 405 nm, and 0.6 for a laser beam with a wavelength of 655 nm, for example.  
         [0025]     The object lens  12  condenses a laser beam entered through the optical axis S 2  of a light source, emits it to the optical disc  11 , and receives a return laser beam reflected by the optical disc  11 .  
         [0026]     In the laser beam incident side of the object lens  12 , a diffraction element and a λ/4 plate  13  are placed. A laser beam emitted from the first light source  21  is transmitted along the optical axis S 1 , reflected and changed in the traveling direction by a rising mirror  14 , transmitted through a collimator lens  15  on the optical axis S 2 , transmitted through the diffraction element and λ/4 plate  13 , and entered the object lens  12 .  
         [0027]     The optical axes S 1  and S 2  may be arranged on a straight line. In this case, the rising mirror  14  is unnecessary. The optical axis S 2  is shown as extending parallel to the third laser beam source  23  in  FIG. 1 , but actually it extends vertically to the surface of paper.  
         [0028]     A second laser beam emitted from the second light source  22  enters the first optical coupling prism  31  placed on the optical axis S 1  between the first laser beam source  21  and the rising mirror  14 . The first optical coupling prism  31  reflects the second laser beam on the surface of a wavelength selection film  31   a , aligns with the optical axis S 1 , and advances to the rising mirror  14  (object lens  12 ).  
         [0029]     A third laser beam emitted from the third light source  23  enters the second optical coupling prism  32  placed on the optical axis S 1  between the first optical coupling prism  31  and the rising mirror  14 . The second optical coupling prism  32  reflects the third laser beam on the surface of a wavelength selection film (half-mirror)  32   a , aligns with the optical axis S 1 , and advances to the rising mirror  14  (object lens  12 ).  
         [0030]     The return laser beam reflected by the optical disc  11  is returned through the object lens  12 , diffraction element and λ/4 plate  13 , collimator lens  15 , and rising mirror  14 .  
         [0031]     If the third laser beam source  23  is used, the return laser beam is sent from the rising mirror  14  to the second optical coupling prism  32 , reflected on the surface of the wavelength selection film  32  of the second optical coupling prism  32 , and sent to and received by a light-receiving unit  23   a  provided integrally with the third laser beam source  23 .  
         [0032]     The collimator lens  15  controls a spread angle, so that the laser beams from the laser beam sources  21 ,  22  and  23  are stably input to the object lens  12 .  
         [0033]     In the diffraction element and λ/4 plate  13 , the λ/4 plate  13  polarizes a traveling laser beam circularly. Further, in the diffraction element and λ/4 plate  13 , the λ/4 plate  13  changes the polarization direction of the return laser beam based on the first and second laser beam sources  21  and  22 , to S-polarized. After the polarization direction of the plane of polarization is changed to S-polarized, the reflected laser beam is divided in its area.  
         [0034]     Explanation will now be given on the return laser beam when the second laser beam source  22  is used. The return laser beam reflected by the optical disc  11  is input to a beam splitter  40  through the object lens  12 , diffraction element and λ/4 plate  13 , collimator lens  15 , rising mirror  14  and second optical coupling prism  32 . The return laser beam entered the beam splitter  40  is reflected by the wavelength selection film  40   a , and input to a main light-receiving unit (photodetector)  42 . The main light-receiving unit  42  receives the return laser beam at the center of a 4-divided photodiode, for example. The output of the photodiode is amplified, and synthesized as a high frequency reproducing signal H. After being amplified, the output of the photodiode is input to a signal processing unit in which subtraction and addition processing are combined. The signal processing unit can detect a tracking error signal and a focus error signal.  
         [0035]     Now, explanation will be given on the return laser beam when the third laser beam source  23  is used. The return laser beam is led to a main light-receiving unit  42 , as when the second laser beam source  22  is used.  
         [0036]     The beam splitter  40  can lead a part of a traveling laser beam (a laser beam from the first optical coupling prism  31  to the second optical coupling prism  32 ) to a light-receiving unit  43  for automatic power control, as well as leading the return laser beam to the main light-receiving unit  42  as described above. The wavelength selection film  40   a  of the beam splitter  40  is used also as a mirror  40   b  for dividing a traveling laser beam at a predetermined ratio. Namely, the laser beams from the first and second laser beam sources  21  and  22  are partially reflected on the surface of the wavelength selection film  40   b  of the beam splitter  40 , and input to the light-receiving unit  43  for automatic power control. A change in the intensity of the laser beam detected by the light-receiving unit  43  is selectively input to a gain control circuit of the first and second laser beam sources  21  and  22 , to stabilize the laser beam output to a preset power.  
         [0037]      FIG. 2  shows the extracted characteristic part of the invention applied to the optical head (PUH) of  FIG. 1 . The same components as in  FIG. 1  are given the same reference number.  
         [0038]     The PUH  51  shown in  FIG. 2  is characterized by the arrangement that the number of reflections is decreased to the least for a laser beam emitted from the first laser beam source  21  which outputs a laser beam with a short wavelength. A laser beam from the first laser beam source  21  is reflected only once by the rising mirror  14  until reaching the optical disc  11 . Laser beams from the second and third laser beam sources  22  and  23  are reflected twice until reaching the optical disc  11 .  
         [0039]     This also means that the first laser beam source  21  corresponding to a larger number of numerical aperture (NA) of lens is preferentially arranged, and designed to reach a disc with less number of reflections. Because, when a wavelength is short and a numerical aperture is many, strict design is requested.  
         [0040]     Namely, as a sequence of synthesizing a laser beam on the optical axis S 1  of the first laser beam (wavelength of 405 nm), a dichroic prism is used to synthesize the second laser beam (wavelength of 655 nm). The unit is designed so that the third laser beam (wavelength of 785 nm) is further synthesized with respect to the optical axis S 1  after a synthesizer.  
         [0041]      FIG. 3  shows an example of the third laser beam source  23  arranged on an extension line of the optical axis S 2 . The same components as in  FIG. 2  are given the same reference numerals. In the arrangement of the third laser beam source  23  shown in  FIG. 3 , the rising mirror  33  is given only a function as a half-mirror compared with the rising mirror ( 14 ) shown in  FIGS. 1 and 2 , and can transmit a laser beam with an wavelength of 785 nm emitted from the third laser beam source  23 .  
         [0042]     In the arrangement of the laser element of PUH  151  shown in  FIG. 3 , the second optical coupling prism  32  does not exist in the optical paths of the laser beams from the first and second laser beam sources  21  and  22 , and the light use efficiency can be increased.  
         [0043]      FIGS. 4A and 4B  show examples of film characteristic inverting characteristics demanded for a film characteristic inverting wavelength band of the wavelength selection films of the first and second optical coupling prisms  31  and  32 .  
         [0044]      FIG. 4A  shows an example of the characteristic of the wavelength selection film  31   a  of the first optical coupling prism  31  (dichroic prism).  FIG. 4B  shows an example of the characteristic of the wavelength selection film  32   a  of the second optical coupling prism  32  (trichroic prism). In  FIGS. 4A and 4B , the vertical axis indicates a reflectivity (%), and the horizontal axis indicates a wavelength. Therefore, when calculating a transmissivity (%), follow the equation, transmissivity (%)=(100−reflectivity (%)). The film inverting wavelength mentioned here means a wavelength band to invert the characteristic of reflection.  
         [0045]     In  FIG. 4A , the film characteristic inverting wavelength band  1  is set preferably to 405 to 655 nm, as explained with reference to FIGS.  1  to  3 . As shown in  FIG. 4B , the wavelength characteristic of the film characteristic inverting wavelength band  2  is defined to 655 to 785 nm, as explained with reference to FIGS.  1  to  3 .  
         [0046]     It is known that a wavelength of a laser beam output from a laser element is usually fluctuated by 10 nm/5° C., for example, by fluctuations in the temperature of a laser element and ambient temperature. A central wavelength of an output laser beam is different by individuals. Of course, a wavelength of a laser beam output from a laser element to output a laser beam with a wavelength of 785 nm is also fluctuated by fluctuations in the temperature of a laser element and ambient temperature. A central wavelength of an output laser beam is different by individuals. Therefore, actually, a wavelength area of film characteristic inverting wavelength band is of course defined including the influence of the temperature fluctuations.  
         [0047]      FIG. 5  is a block diagram of the configuration of the optical disc unit according to the invention. A laser beam emitted from the optical head (PUH actuator)  52  is condensed on the information recording layer of the optical disc  11 , information is recorded on the optical disc  11 , and the recorded information can be reproduced from the optical disc  11 . The block enclosed by a broken line corresponds to the optical head explained in  FIG. 1 .  
         [0048]     A laser beam reflected by the optical disc  11  is detected as an electric signal by a photodetector (PD)  53  of PUH  52  (the photodetector  42  in  FIG. 1 ). The output signal of the PD  53  is amplified by the amplifier  54 , and output to a servo circuit (lens position control unit)  501 , a RF signal processing circuit (output signal processing circuit)  502  and an address signal processing circuit  504 , which are connected to the controller  500  (lens position control amount setting unit (main controller)).  
         [0049]     The servo circuit  501  generates a focus servo signal (to control the difference in the distance between a recording layer of the optical disc  11  and an object lens, with respect to the focal position of an object lens) for an object lens ( 12 ) of the PUH  52 , and a tracking servo signal (to control the position of an object lens in the direction of crossing the track of the optical disc  11 ). These signals are output to a not-shown focus actuator and tracking actuator (lens position control mechanism), respectively.  
         [0050]     The RF signal processing circuit  502  takes out user data and management information from a signal detected and reproduced by the PD  53 . The address signal processing circuit  503  takes out address information, that is, information indicating a track or sector of the optical disc  11  opposed now to the object lens ( 12 ) of the PUH  52 . The taken-out information is output to the controller  500 .  
         [0051]     The controller  500  executes data processing to read data such as user data at a desired position, or to record user data and management information at a desired position, based on the address information. The controller also generates a control signal to control the position of PUH  52 .  
         [0052]     The controller  500  instructs an optical intensity of a laser beam to be output from first to third laser elements  21  to  23  when recording or reproducing information on/from the optical disc  11 . According to the instruction of the controller  500 , the data recorded at an address of a desired position (track or sector) can be erased.  
         [0053]     When recording information on the optical disc, (under the control of the controller  500 ) a recording signal processing circuit  504  supplies the laser driving circuit (LDD)  505  with a recording data, or a recording signal modulated to a recording waveform signal suitable for recording on the optical disc. Therefore, the laser element of the PUH  52  emits a laser beam with the intensity changed according to recording information, corresponding to a laser driving signal output from the LDD (laser driving circuit)  121 . Information is recorded on the optical disc  11  by this.  
         [0054]     As explained hereinbefore, according to an embodiment of the optical head of the invention, a high grade apparatus can be easily designed by arranging a laser beam source for emitting a laser beam with a short wavelength to decrease the number of reflections to the least. Concretely, in this example, a light source with a shortest wavelength is arranged at a position farthest from an object lens.  
         [0055]     While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.