Patent Publication Number: US-6335809-B1

Title: Optical pickup and hologram device

Description:
TECHNICAL FIELD 
     This invention relates to an optical pickup and hologram unit and, more particularly, to an optical pickup and hologram unit for diffracting reflection light from an optical disk (hereinafter, referred to merely as “disk”), such as a CD or DVD, and converging it onto a light-receiving device. 
     PRIOR ART 
     The optical pickup for reading information out of a disk requires a function to reproduce recorded information as well as a function to detect focus error and tracking error. Conventionally, focus error has been detected by the well-known Foucault or astigmatism method while tracking error by the push-pull or three-beam method. Where using the Foucault and three-beam methods in combination, the light receiving elements  2   a - 2   c  for receiving a main beam and the light receiving elements  3   a  and  3   b  for receiving sub-beams have been employed as shown, for example, in FIG.  9 . Focus error signals are determined from the difference of output signals between the light receiving elements  2   a  and  2   b  (S 2   a −S 2   b ). Tracking error signals are determined from the difference of output signals between the light receiving elements  3   a  and  3   b  (S 3   a −S 3   b ). 
     Because the sub-beam light reflected upon the disk passes obliquely through a lens, division is not equally two by the hologram unit  4  (FIG. 9) and hence the two sub-beams divided are not same in spot size. Consequently, the sub-beam diffracted as greater spot size (hereinafter, referred to as “aperture side”) after division and the sub-beam diffracted as smaller spot size (hereinafter, referred to as “shade side”) are not in symmetry on a light receiving surface of the photodetector  1 . On the light-receiving surface, the spot size of the shade-side sub-beam B is greater than the spot size of the aperture-side sub-beam A. 
     Meanwhile, in the conventional pattern design for a hologram unit  4 , it has been emphasized that wavefront aberration be reduced for a main beam. However, large wavefront aberration remains left for sub-beams. That is, as shown in FIG. 10, conventionally a first pattern  6  and a second pattern  7  have been designed which are to be expressed as an even function Ax4+Bx2+C with respect to a distance x from a division line  5 . These patterns have been divided into two and then joined together, thereby obtaining a whole pattern. In the pattern design, however, wavefront aberration has not been taken into consideration for sub-beams. 
     Due to this, there is a fear that the shade-side sub-beam B be possibly out of the light receiving elements  3   a  and  3   b , as shown in FIG.  9 . There has been a problem that the tracking error balance and jitter might vary significantly depending upon temperature change, resulting in unstable optical pickup characteristics. 
     SUMMARY OF THE INVENTION 
     Therefore, it is a primary object of this invention to provide an optical pickup capable of stabilizing the characteristics of the optical pickup. 
     A first invention is an optical pickup, comprising: a diffraction element for dividing a laser beam from a laser device into one main beam and two sub-beams; a lens for converging the main beam and the sub-beams onto a disk; a hologram unit divided into two of a first pattern and a second pattern to respectively diffract reflection light from the disk; and a photodetector including a first portion to receive the main beam and a second portion to receive the sub-beams respectively diffracted by the first pattern and the second pattern; wherein the first pattern and the second pattern in the hologram unit is made to minimize the spot of the sub-beam converging onto the second portion based on an optical length and a wavelength when the sub-beams are taken as light sources. 
     A second invention is a hologram unit, comprising: a hologram pattern for diffracting and converging onto a light receiving element two sub-beams reflected by a disk, the hologram pattern being made to minimize a spot of the sub-beams converging onto the second portion based on an optical path length and a wavelength when the sub-beams are taken as light sources. 
     Because the pattern on the hologram unit is designed to minimize a spot size of a sub-beam converging onto the light receiving device based on an optical path length and a wavelength when the sub-beam is taken as a light source, there is no fear that the sub-beam spot goes out of a light receiving surface of the light receiving device. 
     According to the invention, the optical pickup characteristics can be stabilized because the sub-beam spot can be prevented from going out of the light receiving surface. 
     The above described objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustrative view showing an optical pickup of one embodiment of this invention. 
     FIG. 2 is an illustrative view showing a photodetector used in the FIG. 1 embodiment. 
     FIG. 3 is an illustrative view showing a hologram unit used in the FIG. 1 embodiment. 
     FIG. 4 is an illustrative view showing a design method for a hologram unit. 
     FIG. 5 is a graph illustrating a first pattern. 
     FIG. 6 is a graph illustrating a second pattern. 
     FIG. 7 is a graph illustrating a hologram pattern in combination of the first pattern and the second pattern. 
     FIG. 8 is a graph illustrating a relationship between a distance from a sub-beam spot and a light intensity. 
     FIG. 9 is an illustrative view showing a prior art. 
     FIG. 10 is an illustrative view showing a conventional hologram pattern. 
    
    
     BEST FORM FOR PRACTICING THE INVENTION 
     An optical pickup  10  of this embodiment shown in FIG. 1 is to read information out of a disk  14 , such as a CD or DVD, rotated by a disk motor  12 , and includes a semiconductor laser device  16  as a light source to emit a predetermined wavelength of laser light. The semiconductor laser device  16  is connected with an APC (Automatic Power Control)  18  so that the output of the semiconductor laser device  16  is under control of the APC  18 . Also, a hologram unit  20  as a diffraction unit and an objective lens  22  are arranged between the semiconductor laser device  16  and the disk  14 . A photodetector  24  (FIG. 2) is arranged obliquely below the hologram unit  20 . 
     The hologram unit  20  includes a substrate  26  formed of quartz glass or the like. The substrate  26  has a grating pattern  28  formed in an underside to cause one main beam and two sub-beams. The substrate  26  has a hologram pattern  30  for polarization formed in a top surface thereof. The hologram pattern  30  is divided by a division line A 0  extending radially of the disk  14 , as shown in FIG. 2, to have a first pattern  30   a  and a second pattern  30   b  that are different in pitch length from each other. 
     The objective lens  22  is fixed on a not-shown actuator coil constituting a focussing actuator so as to be vertically displaced by energizing the actuator coil. 
     The photodetector  24  is divided into five light receiving elements  32   a ,  32   b ,  32   c ,  32   e  and  32   f , as shown in FIG. 2. A division line B 0  separating the light receiving elements  32   a  and  32   b  is formed at a slight angle with respect to a direction of diffraction by the hologram pattern  30  (FIG.  2 ), in order to prevent focus offset. 
     Generally the diameter d of a spot (aeri-disk), restricted to a beam wavelength λ by an objective lens having a numerical aperture NA, is to be expressed as d=1.22 λ/NA. Provided that the NA of the objective lens  22  on a side of the semiconductor laser  16  is NAL and the NA of the main beam (0-order light) after division by the hologram pattern  30  is NAO, expression is given as NAO=NAL/2 because the main beam (0-order light) is equally divided by the hologram pattern  30 . Consequently, the spot diameter d 0  of the main beam (0-order light) is expressed as d 0 =2.44 λ/NAL. On the other hand, the sub-beam is divided in a position deviated from the center. Accordingly, as shown in FIG. 2, the spot of an aperture-side sub-beam is rendered small on the light receiving elements  32   e  and  32   f  whereas the spot of a shade-side sub-beam B is in a highly blurred state on the light receiving elements  32   e  and  32   f . Consequently, there is a need to set a width of the light receiving element  32   e  and  32   f  wider than the size of the shade-side spot. However, if this width is set excessively wide, the chip size increases. Accordingly, in this embodiment, a hologram pattern  30  is designed so that the spot size of a sub-beam on the light receiving element  32   e  and  32   f  can be minimized in size based on an optical path length and wavelength where the sub-beam is taken as a light source. 
     That is, as shown in FIG. 3, a first pattern  30   a  is designed on an entire circular region so that the sub-beam spot is minimized in size on the light receiving element based on an optical path length and waveform when +1-order sub-beam is taken as a light source. A second pattern  30   b  is designed on the entire circular region so that the sub-beam spot is minimized on the light receiving element based on an optical path length and wavelength when −1-order sub-beam is taken as a light source. These are divided into two and then joined together thereby obtaining a hologram pattern  30 . Explaining in greater detail, when designing a first pattern  30   a , the hologram unit  20  and the photodetector  24  are arranged in predetermined locations, as shown in FIG.  4 (A). When it is assumed that a +1-order sub-beam has a virtual light source point of P 1  and a convergence point on the light receiving element  32   f  of S 1 , a path of a point O 1  on the hologram unit  20  is determined where the difference in the +1-order sub-beam optical path length (optical distance) between O 1 P 1  and O 1 S 1  (O 1 P 1 −O 1 S 1 ) is n λ (n=0, ±integer, λ: wavelength). This is taken as a first pattern  30   a . On the other hand, a second pattern  30   b  is determined by a similar method based on a virtual light source point P 2  and convergence point S of −1-order sub-beam, as shown in FIG.  4 (B). Then, the first pattern  30   a  and the second pattern  30   b  are divided into two and then joined together. Incidentally, for an optical path length extending in the air and substrate  26 , respective optical path lengths are determined by so-called ray-tracking calculation and then added together. 
     For example, laser light is divided into three by using 16 μm pitch length of a grating pattern  28  to form a sub-beam spot on the disk  14  in a position spaced by ±13 μm from a main beam spot. In the case of converging the reflection light of this sub-beam to a position spaced by ±66 μm from a main-beam optical axis through the objective lens  22  and hologram pattern  30 , the respective virtual light source points P 1  and P 2  of the sub-beams will deviate by ±70 μm from an actual light-emission point P 0 . Under this condition, a first pattern  30   a  shown in the graph of FIG. 5 and a second pattern  30   b  shown by a graph of FIG. 6 are obtained according to the above design method. Based on these patterns  30   a  and  30   b , a hologram pattern  30  as shown in FIG. 7 is obtained. The first pattern  30   a  and the second pattern  30   b  can be expressed by a polynominal expression on the basis of Ax4+Dx3+Bx2+Ex+C. That is, the first pattern  30   a  and the second pattern  30   b  in the hologram pattern  30  are configured by a pattern having odd-order dependency with respect to a distance x from a boundary line between them. 
     In operation, when a switch to the optical pickup  10  is turned on, the disk  14  is rotated by the disk motor  12  and the semiconductor laser device  16  is caused to emit light. Thereupon, the laser beam from the semiconductor device  16  is diffracted through the grating pattern  28  into a main beam and two sub-beams. The three beams divided by the grating pattern  28  pass through the hologram pattern  30  and then converge onto the disk  14  due to the objective lens  22 . The light reflected by the disk  14  is passed through the objective lens  22  and then diffracted by the hologram pattern  30 . Thus, the main beam converges on the light receiving elements  32   a-   32   c  of the photodetector  24  while the sub-beams are on the light receiving elements  32   e  and  32   f . A focus error signal is determined from a difference of output signals between the light receiving elements  32   a  and  32   b  (S 32   a− S 32   b ) while a tracking error signal is determined from a difference of output signals between the light receiving elements  32   e  and  32   f  (S 32   e− S 32   f ). 
     According to this embodiment, the hologram pattern  30  is designed such that the spot size of a sub-beam is minimized based on an optical path length and wavelength when the sub-beam is taken as a light source. Accordingly, there is no fear that a shade-side sub-beam goes out of the light-receiving surface. It is therefore possible to prevent against tracking-error balance or jitter due to temperature change thereby stabilizing the characteristics of the optical pickup  10 . It is also possible to broaden an allowable range of attaching accuracy of the hologram unit  20 , photodetector  24 , etc. Incidentally, FIG. 8 is a graph illustrating a relationship between a distance from a center of a sub-beam and a light intensity when using the hologram pattern  30  shown in FIG.  7 . In this graph, the sub-beam spot at a foot is greatly reduced in right intensity as compared to that of the prior art. It is to be understood from this that the spot size of the sub-beam is substantially decreased. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.