Abstract:
An integrated optical head records and reproduces information to and from optical discs of different types, including CD and DVD, with use of a single information recording/reproducing apparatus. The optical head has a plurality of semiconductor lasers of different wavelengths integrated by index alignment onto a substrate formed with an optoelectric integrated circuit, photodetection patterns, and a reflecting mirror.

Description:
This is a continuation application of U.S. Ser. No. 09/512,822, filed Feb. 25, 2000, now abandoned. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a laser module or an optical head. For example, the invention relates to a laser module or an optical head in which a semiconductor laser beam modulated by an electric signal is applied to an optical information recording medium such as an optical disc to record information on the medium or reproduce a recorded information from the medium. In particular, the invention is concerned with a laser module or an optical head each using a plurality of light sources. 
   In an optical head mounted on an optical disc recording and read-out apparatus, light sources and photodetectors are separated from each other. Consequently, it is impossible to attain a high fabrication integration density of light sources and photodetectors and hence it has so far impossible to attain the reduction in size and thickness of the entire optical disc apparatus. In an effort to solve this problem, a hybrid integration of photodetectors and semiconductor lasers in a reproducing head of an optical disc has been tried heretofore as is disclosed in Japanese Patent Laid-open No. Hei 1-150244. 
   SUMMARY OF THE INVENTION 
   Recently there has been developed an optical disc apparatus capable of playing various optical discs of CD, CD-ROM, CD-R, and CD-Rewritable specifications having a wavelength of 780 nm and DVD, DVD-ROM, and DVD-RAM specifications having a wavelength of 65.0 nm, in which, however, light sources and photodetectors are separated for semiconductor lasers of different wavelengths. Further, lasers of blue or purple color or even shorter wavelengths, which are more improved in recording density, are going to be used in future. Thus, an increase in the number of components in the optical head will be unavoidable. Under the circumstances, a further reduction in size and thickness of the entire apparatus such as an optical disc recording and read-out apparatus is desired. 
   It is an object of the present invention to improve the above-mentioned problems. More particularly, it is an object of the invention to provide a breakthrough for reducing the size and thickness of the whole of a driver capable of recording and reproducing information for various optical discs. 
   According to the first means adopted in the present invention, various semiconductor lasers of different wavelengths generated and photodetectors corresponding to those different wavelengths are subjected to alignment with a masking accuracy and then the plural semiconductor lasers are integrated as hybrid integration to reduce the number of components to a level equal to that of a monolithic configuration. According to the first means, moreover, although a plurality of optical paths have been used in the conventional optical head, a single optical path corresponds thereto. 
   According to the second means adopted in the present invention, index marks for alignment are affixed onto both a silicon substrate with photodetectors formed thereon and semiconductor lasers, their images are formed on a photoelectric conversion surface such as CCD and are inputted into a computer, followed by calculation of centroids of the marks and alignment. The centroid calculation permits ensuring a submicron order of alignment accuracy. 
   According to the third means adopted in the present invention, a reflecting mirror is formed on a silicon substrate with photodetectors formed thereon. More specifically, an off-axis substrate of 9.7 degrees or so is provided, then a reflecting mirror of 45 degrees or so is formed thereon by an anisotropic etching of silicon, and a beam emitted from a semiconductor laser is reflected by the mirror and is bent in a direction nearly perpendicular to the silicon substrate surface. 
   According to the fourth means adopted in the present invention, the width of the reflecting mirror is defined relative to a beam spread angle of a semiconductor laser. More particularly, the beam emitted from a semiconductor laser has a spread width approximated by Gaussian distribution. If this spread is intercepted near a light spot of the semiconductor laser, there occurs a Fresnel diffraction phenomenon, which changes into aberration when a spot is formed by an objective lens located just before an optical disc, with consequent decrease in central intensity of the spot. As a result, the power for resolving pits on the optical disc decreases and there occurs an error in a reproduced signal. To avoid this inconvenience, the width of a reflecting mirror is set so as to become wider than the full width at half maximum of the spread of the semiconductor laser beam at the position of the reflecting mirror. 
   According to the fifth means adopted in the present invention, an amplifier for electrically amplifying light currents generated by photodetectors is formed monolithically on a silicon substrate with the photodetectors formed thereon, and a tilted mirror alignment index mark is affixed onto the silicon substrate. 
   According to the six means adopted in the present invention, the above second and fifth means are combined together and a plurality of semiconductor lasers and a monolithically integrated silicon are integrated as hybrid integration with a higher alignment accuracy than that of the index mark. 
   According to the seventh means adopted in the present invention, an amplifier for electrically amplifying light currents generated by photodetectors is formed monolithically on a silicon substrate with the photodetectors formed thereon, and when semiconductor lasers are soldered onto the silicon substrate with a tilted mirror formed thereon and with an alignment index mark affixed thereto, a material having a high thermal conductivity is interposed between the semiconductor lasers and the silicon substrate for the purpose of widely diffusing the heat generated from the semiconductor lasers. 
   According to the eighth means adopted in the present invention, an amplifier for electrically amplifying light currents generated by photodetectors is formed monolithically on a silicon substrate with photodetectors formed thereon, and when semiconductor lasers are soldered onto the silicon substrate with a tilted mirror formed thereon and an alignment index mark affixed thereto, a material having a stress relaxation effect is interposed between the semiconductor lasers and the silicon substrate for the purpose of relaxing a stress induced by a difference in thermal expansion coefficient between the two. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing an optical head of a single optical path carrying an integrated light source module according to an embodiment of the present invention; 
       FIG. 2  is a diagram showing a beam splitting combined element; 
       FIGS. 3A and 3B  are structural diagrams of an integrated light source used in the embodiment; 
       FIGS. 4A ,  4 B and  4 C are diagrams for explaining the width of a mirror used in the embodiment; 
       FIGS. 5A and 5B  are diagrams showing a package form of the integrated light source; 
       FIGS. 6A ,  6 B and  6 C are diagrams showing the integrated light source mounted on a horizontal flat package; 
       FIG. 7  is a diagram showing an integrating substrate for the integrated light source, as well as alignment index, solder and electrode patterns; 
       FIG. 8  is a diagram showing alignment index patterns affixed to semiconductor lasers in the embodiment; 
       FIG. 9  is a diagram showing a method for alignment between indexed semiconductor laser light sources and an integrating substrate with corresponding index patterns affixed thereto; 
       FIG. 10  is a sectional view taken on line A-A′ in  FIG. 3A ; 
       FIG. 11  is a sectional view of an integrating substrate having a layer for promoting the radiation of heat from semiconductor laser light sources; 
       FIG. 12  is a diagram showing three types of semiconductor laser light sources mounted on an integrating substrate used in the embodiment; and 
       FIG. 13  is a diagram showing an OEIC (Optoelectric Integrated Circuit) including an amplifier and photodetectors is integrated monolithically on the integrating substrate. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates the configuration of an optical head according to the present invention. An integration module  100  comprises a semiconductor substrate  1 , semiconductor laser chips  4   a  and  4   b , a reflecting mirror  5 , and photodetectors  7 ,  8  and  9 . Laser beams, indicated at  6   a  and  6   b , from the integration module  100  are collimated by a collimator lens  10 , then pass through a mirror  11  and a grating plate  12 , and reach an objective lens  13 , whereby the beams are formed as spots  15  and  16  on a surface of an optical disc  14 . The objective lens  13  comprises plural such lenses according to wavelengths of the semiconductor lasers or a single lens capable of focusing beams of different wavelengths. The objective lens is focused onto a recording surface of the optical disc in accordance with a rotational movement of the same disc by means of an actuator  17  and performs tracking, that is, follows a recording track  18  formed on the disk surface. Thus, in accordance with ON or OFF of the semiconductor lasers, signals are recorded as a train of pits on the optical disc or already recorded pits are read out to reproduce signals. By thus integrating a plurality of semiconductor lasers in the integration module  100 , the number of collimator lens  10 , that of objective lens  13  and that of mirror  11  each become one and it is possible to singularize the optical path in the optical head. Using this optical head, for example CD, CD-R having a thickness of 1.2 mm can be subjected to recording and reproduction using the semiconductor laser  4   a  of 780 nm in wavelength, while DVD, DVD-RAM having a thickness of 0.6 mm can be subjected to recording and reproduction using the semiconductor laser  4   b  of 650 nm in wavelength. 
     FIG. 2  explains the grating plate  12 . This grating plate is a combined element obtained by integrally laminating a polarizable grating  23  divided into four parts and a quarter wave plate  24  to each other. It is disposed so that the grating  23  faces the semiconductor laser chip side. The polarizable, quartered grating  23  is constituted by a birefringent optical crystal or liquid crystal plate, through which an incident light passes without refraction if it is an ordinary ray or which functions as a diffraction grating if the incident light is an extraordinary ray. When linearly polarized beams  6   a  and  6   b  emitted from the semiconductor lasers  4   a  and  4   b  are incident on the combined element  12  comprising the polarizable, quartered grating and the quarter wave plate, if they are incident as ordinary rays, pass through the polarizable grating portion and are then made into circularly polarized beams by the quarter wave plate in the combined element  12 . The laser beams  6   a  and  6   b  after reflected by the optical disc are made into extraordinary rays by the quarter wave plate in the combined element and are then diffracted by the polarizable, quartered grating. The combined element  12  shown in  FIG. 2  is divided into four regions by boundary lines  21  and  22 . In the same figure, a circle  20  stands for the laser beam  6   a  or  6   b , which is separated into four +1st order diffracted beams and four −1st order diffracted beams by the quartered grating. The beams thus separated reach the photodetector  7  or  8  on the semiconductor substrate  1  and are thereby subjected to photoelectric conversion into autofocus signal, tracking signal, and information signal. This point will be described below in detail. 
     FIG. 3A  shows a surface of the semiconductor substrate  1  as seen from the collimator lens  10  side. In the same figure, eight black-painted quarter circles indicated at  32   a  represent laser beams of wavelength λa separated by the grating  23 , while eight quarter circles not painted out and indicated at  32   b  represent laser beams of wavelength λb separated by the grating. The photodetector  7 , which is for obtaining an out-of-focus detection signal, comprises eight strip-like photodetector elements  7   a  for receiving the laser beams  32   a  of wavelength λa and eight strip-like photodetector elements  7   b  for receiving the laser beams  32   b  of wavelength λb. As an out-of-focus detecting method there is adopted a knife edge method (Foucault method), in which wiring is made using an electrically conductive thin film  33  such as aluminum film as shown in  FIG. 3A , whereby differential signals are obtained from terminals A and B of wire bonding pads  34 . The photodetectors  8  are four photodetectors used for obtaining a tracking error detection signal and an information reproduction signal. Output signals provided from the four photodetectors  8  pass through an amplifier  35  formed on the semiconductor substrate and are outputted from terminals D, E, F and G of pads  34 . The photodetector  9  is for monitoring the quantities of light beams emitted from the semiconductor laser chips  4   a  and  4   b . An output signal provided from the photodetector  9  is outputted from the terminal C of pad  34 . Spots  31   a  and  31   b  represent reflected positions on a surface of the reflecting mirror  5  of the laser beams  6   a  and  6   b  emitted from the semiconductor chips  4   a  and  4   b , respectively. For example, assuming that the grating pitches P of the four regions shown in  FIG. 2  are equal to one another, that grating directions are +α, −α, +3α, and −3α degrees with respect to a vertical lien  21 , and that a focal distance of the collimator lens is fc, the laser beams  32   a  of wavelength λa separated by the grating are focused on a circumference of radius Ra=fc*λ a/P centered at spot  31   a  and at positions spaced  2   a  degree from the center. Likewise, the laser beams  32   b  of wavelength λb separated by the grating are focused on a circumference of radius Rb=fc*λb/P centered at spot  31   b  and at positions spaced  2 α degree from the center. If the spacing D between the light spots of the semiconductor laser chips  4   a  and  4   b , which is the spacing between the spots  31   a  and  31   b , is D□fc*(λb−λa)/P, the focused positions of the laser beams of wavelength λa and the focused positions of the laser beams of wavelength λb can be made substantially coincident with each other. Consequently, as in this embodiment, the photodetectors and amplifier can be used in common to beams of different wavelengths, whereby not only the surface of the semiconductor substrate  1  can be saved but also the number of wire bonding pads and output lines can be decreased, with consequent reduction in size of a package which houses the semiconductor substrate therein. 
     FIG. 3B  shows a sectional structure of the semiconductor substrate  1  at the position of dotted line A-A′ in  FIG. 3A . Preferably, the reflecting mirror  5  is formed at an angle of 45 degrees relative to a laser chip mounting surface  2 . For example, the processing for forming a mirror surface on the silicon substrate is based on an anisotropic etching such that if the silicon (100) plane is etched using an aqueous solution of potassium oxide, there is formed a pyramid-shaped concave portion using the flat (111) plane as a slant surface because the etching speed of the (111) plane relative to the (100) plane is lower by approximately two digits. In this case, the angle of the (111) plane relative to the (100) plane is approximately 54.7°, so for forming a reflecting mirror of 45° it is necessary to use a silicon substrate with an off angle of about 9.7° in which a crystallographic axis is inclined relative to the surface. However, it is necessary that the off angle be determined taking also into account the adaptability of the semiconductor process for the formation of photodetectors and electronic circuits. The reflecting mirror  5  may be displaced from 45° or the direction of emission of the laser beam  6   a  or  6   b  may be displaced from the direction perpendicular to the semiconductor substrate  1 . 
     FIGS. 4A ,  4 B and  4 C are diagrams explaining at what value the width of the reflecting mirror should be set. Generally, as shown in  FIG. 4B , the beam emitted from a semiconductor laser spreads at a certain angle and the intensity distribution relative to the spread angle is approximated by Gaussian distribution. As in the configuration according to the invention shown in  FIG. 4A , such a beam is partially reflected by the reflecting mirror  5  in the vicinity of the semiconductor laser  4   a  or  4   b . If a portion of the beam is truncated, there occurs a so-called Fresnel diffraction phenomenon and the phase of wave surface is distorted, as shown in  FIG. 4C . If such beams in a distorted state of the wave surface phase reach the objective lens  13 , there occurs aberration in the spots  15  and  16  formed on the optical disc. If this point is considered geometrical-opticswise, such a phenomenon does not occur, but this phenomenon can be explained in terms of a wave-optic model. Since the amount of aberration generated depends on the truncation of beam, it is necessary that the width of the reflecting mirror be taken sufficiently large. In the present invention, the width of the reflecting mirror is set so that the full width at half maximum or more of the semiconductor laser intensity distribution is reflected, as shown in  FIG. 4B . 
     FIGS. 5A and 5B  show a package for housing the semiconductor substrate  1  therein. The package comprises a package substrate  200  having conductor pins  201 , and a silicon substrate  202 .  FIG. 5B  is a sectional view taken on line A-A in  FIG. 5A , in which there are used a cap  203  and a package sealing window  204  as components of the package. The window  204  of the package can also serve as the combined element  12  shown in  FIG. 1 . 
     FIGS. 6A ,  6 B and  6 C show another example of a package with the semiconductor substrate  1  housed therein, of which  FIG. 6A  shows the structure of the package,  FIG. 6B  is a sectional view taken on broken line A-A′, and  FIG. 6C  is a sectional view taken on broken line B-B′. Numeral  42  denotes a lead wire, which is connected to the semiconductor substrate  1  through bonding wires of pads  34 . A surface of a pedestal  43  for mounting the semiconductor substrate  1  thereon is inclined so that the laser beams  6   a  and  6   b  are emitted in a direction perpendicular to the package. Numeral  44  denotes a glass cover for sealing the semiconductor substrate  1 . On an inner side of the glass cover  44  is provided a reflecting surface  45  for reflecting outer peripheral portions of the laser beams  6   a  and  6   b . Beams reflected by the reflecting surface  45  are received by the photodetector  9  on the semiconductor substrate  1 , which in turn afford signals for monitoring the quantity of light emitted from each of the semiconductor laser chips  4   a  and  4   b.    
   Now, with reference to  FIGS. 7 ,  8 ,  9 , and  10 , the following description is provided about a method for mounting a plurality of semiconductor lasers onto a silicon semiconductor substrate. In  FIG. 7 , index patterns  400  are affixed to the silicon substrate  1  according to the present invention. Numeral  401  denotes a solder pattern, onto which a semiconductor laser is soldered. An electrode pattern  402  is formed and connected to the solder pattern  401 . On the other hand,  FIG. 8  shows a solder pattern  501  formed on rear sides of the corresponding semiconductor lasers  4   a  and  4   b  and index patterns  502  for alignment.  FIG. 9  explains a method for alignment between the index patterns  400  formed on the substrate  1  and the index patterns  502  formed on the rear sides of the semiconductor lasers  4   a  and  4   b . In the same figure, the substrate  1  and the semiconductor lasers  4   a ,  4   b  are irradiated with an infrared light  600  from the surface side or the back side, then reflected or transmitted beam is received by a microscope  601 , and index patterns are enlarged and projected on a video monitor  602 . Further, center positions of the index patterns  400  and  502  are calculated by means of a computer  603  and the substrate  1  or the semiconductor lasers are inched until a positional deviation between two centers becomes zero. Completion of the alignment is followed by tact bonding and subsequent soldering in a solder reflow oven. 
     FIG. 10  is a sectional view showing a state in which the semiconductor lasers  4   a  and  4   b  have been soldered onto the substrate  1  with mirror, which corresponds to a section taken along line A-A′ in  FIG. 3A . On the rear sides of the semiconductor lasers are formed electrodes  701  and index patterns  502  for alignment, and the semiconductor lasers are soldered onto the substrate  1  with electrode  701  and solder  702  formed thereon. Alignment of the semiconductor lasers and the substrate is performed between index patterns  502  and  400 . The beam from the semiconductor laser  4   a  or  4   b  forms a light spot  704  and is reflected by the mirror  5 , then passes the beam splitter and objective lens and reaches the optical disc. In order that the beam from the light spot  704  should not be truncated by the bottom of the substrate, a pedestal  705  is formed on the substrate. 
     FIG. 11  shows an example in which, for improving the radiation of heat, a layer of a material  800  having a high thermal conductivity is disposed just under the semiconductor lasers. The heat generated in an active layer of each semiconductor laser is diffused just thereunder, allowing heat conduction to take place over a wider area to decrease the thermal resistance up to a heat sink. The material layer  800  can be endowed with a function of relaxing a stress induced by a difference in thermal expansion coefficient between the semiconductor lasers and the substrate. 
     FIG. 13  shows an example in which three semiconductor lasers are arranged in a multi-wavelength module according to the present invention, which lasers are, successively from the right-hand side, a bluish purple color semiconductor laser  810  having a wavelength of approximately 410 nm, a red color laser  306  having a wavelength of approximately 660 nm, and an infrared laser  307  having a wavelength of approximately 780 nm. Three sets of corresponding photodetectors  304 ,  303  and  811  are formed for tracking, showing an example in which one set of a photodetector serves for both tracking and reproduction signal. These three kinds of wavelengths are associated with recording/reproducing optical discs, including super DVD, DVD, and CD, which are being standardized. 
     FIG. 13  illustrates an integration module further embodying the present invention. An amplifier  900  for amplifying light currents provided from photodetectors  303 ,  304  and  302  is formed monolithically on a silicon or GaN substrate  102 , whereby the number of components used is reduced and it is thereby possible to improve the degree of integration. 
   According to the present invention, as set forth above, it is possible to effect the reduction in size and integration of an optical head which carries a plurality of semiconductor lasers and hence possible to attain the reduction in size and thickness of the whole of an optical disc apparatus for both recording and reproduction such as CD, DVD, and an optical disc capable of carrying bluish purple color lasers.