DE4330794C2 - Optical head device for use in an optical recording and writing / reading device - Google Patents

Description

Background of the Invention 1. Field of the Invention

The present invention relates generally to a Technique of controlling a light emitting unit including light emitting devices which is constructed on an optical information beam to send out a target object, and more specifically to one optical information recording device including a light source unit for providing information performing writing beam on an optical Information storage medium. The invention relates also on a laser control system for use in a  optical recording and writing / reading device, which have a removable, round, plate-shaped, photosensitive Record carrier body used.

2. Description of the related art

The document DE 30 22 299 A1 discloses a device in which reflected light passes through the light source and is detected by a photodetector. In particular, the device according to this document detects the light output in accordance with a change in the coupling state of the light reflected from the plate and the laser diode serving as the light source, whereby a focus error signal is detected. The device according to DE 30 22 299 A1 has in particular the following disadvantages:

• a) The output of the emitted by the light source Light strongly depends on the temperature. So it is difficult to determine precisely whether that of the Change in light output from the photodetector Change the plate level (or the plate position) or results from the temperature change. So it is difficult to precisely focus the error signal itself determine;
• b) in the device according to document DE 30 22 299 A1 almost all of the reflected light is sent to the laser diode or returned to the light source. Like that Well known to those of ordinary skill in the art, this increases light reflected the noise of the laser diode or light emitted by the light source. So here it is possible for the noise to interfere with the focus error signal, and focus control cannot be performed precisely;
• c) the light returned to the laser diode acts also on the light emitted by the laser diode. Consequently the emitted light can have an uneven distribution exhibit.

Document EP 0 369 510 A1 shows a device for optical scanning of a beam reflecting Information level, with a laser diode Scans information area. To align one of the Area of reflected beam to a detector is one Beam splitting device provided. Here are the Detection of reflected rays also several Detectors used. However, is from publication EP 0 369 510 A1 an arrangement that a Beam shaping device, a biographical device and provides a polarizing device, not known.

Furthermore, from "Applied Optics", Vol. 27, No. 4, 15. February 1988, pages 668-671, the use of a holographic device for fault detection at a known optical system. A specially built one Photodetector device, as well as the division of a holographic device into several holograms and the Division of the photodetector device into a plurality areas, however, this publication is not closed remove.

With the recent advances in technology from Semiconductor lasers as light-emitting Solid state elements find optical Information recording / reproducing devices including an optical head device which Output laser light as a writing beam or reading beam used, increasingly application. Typically one closes optical head device a semiconductor laser as theirs Light source. The optical head device closes also a driver circuit for matching one Bias current of the laser, so that the output light of the laser is at a power level that is in A mode of operation is selected, and an optical system for projecting the output Laser light onto a target recording medium in one focused state. The ones described above constituent elements are high density in one small housing.

As one of optical Information recording / reproducing devices are digital audio information- Commercial recording / playback devices important. A device is expected to do this Type also as a peripheral device such as an external one Storage unit of a computer system in the near  Future is used. As digital Audio information recording / reproducing apparatus a recording / reproducing device known is used appropriately, information on an optical accessible, previously recorded Record or store information storage medium reproduced. It is typically considered as the above Recording medium a round, plate-shaped Record carrier body rotatable on the optical Reading device set. An optical head device is movable in the radial direction of the rotating Plate carrier body arranged and focused one power-controlled write or read beam on a optically detectable, radiation-sensitive layer of the Plate carrier body. The device closes an optical forward path (light transmission system) to guide a beam output from the laser the disc carrier, and an optical reverse path (Light receiving system) for guiding that Plate carrier reflected light to a photodetector. Obviously, each of these is light transmission and Light receiving systems from independent and separate optical components formed. The photodetector leads a photoelectric conversion through to a to generate an electrical reproduction signal, which the represents information read.

Because the demand for a decrease in the size of a optical recording / reproducing device stronger , an optical must be integrated into it Head device can be reduced. To the size of the to reduce optical head device, it is particularly important, the number of required  Reduce components, especially those of optical light transmission and Light receiving system components. To meet this demand Attempts have been made to match optical components to use both as a light transmission as well Light receiving components are used, or optical To share components between the two systems.

Unfortunately, the sharing of optical components between the light transmission and light receiving systems Compromise. If the transmission efficiency (which the Effective lighting ratio is influenced), and the Image magnification is reduced, as is inherent in the Light transmission system is required, the Function of the light transmission system can be improved. In this case, however, that for the Light receiving system required Signal detection stability deteriorated, and the Function of the light receiving system, which is inherently a must have large image enlargement is worse. With in other words, there is a crucial difference in the between the light transmission and Light receiving systems need magnification. That is, the former must be a small one Have magnification, but the latter requires a large image enlargement. This in conflict related conditions pose a serious one Obstacle to sharing optical components for one Size reduction. Therefore, if a conventional, compact optical head device designed exclusively for Playback, used, is simply constructed, also used as an optical recording device to become, the light usage ratio is reduced. Around  To compensate for this, a laser with higher Performance inevitably used. This can undesirable measures for increased power consumption result in larger dimensions and higher ones Manufacturing costs.

Summary of the invention

It is an object of the present invention known optical head device to improve the size of the optical head device is reduced and the most accurate control possible Head device is enabled.  

The task is accomplished by a device according to Claims 1, 7, 14 and 17 solved.

Advantageous refinements of the invention result itself from the subclaims.

The previous task, features and Advantages of the invention will be apparent from the following Description of preferred embodiments of the Invention as in the accompanying drawings shown, more clearly.

Brief description of the drawings

Fig. 1 is a diagram schematically showing the overall configuration of an optical head apparatus in accordance with a preferred embodiment of the invention schematically;

Figs. 2A to 2C are diagrams showing models of an optical system including a collimating lens and a Strahlformerprisma of FIG. 1;

Fig. 3 shows the top view of an arrangement including a laser light source and photodetectors of Fig. 1; and

Fig. 4 is a side view of the arrangement of Fig. 3;

Fig. 5 illustrates, in plan views of the photodetectors, the main steps of a focal length control process of these photodetectors side by side;

Fig. 6 shows a modification of the main optical system of the embodiment shown in Fig. 1;

FIGS. 7 to 9 and 12 show other embodiments of the invention, and

Fig. 10 and 11 show two possible detailed prism structures that can be used preferably reasonably shown as Strahlformerprismaeinheit as shown in Fig. 9;

Fig. 13 shows an optical head device which is also an embodiment of the invention; and

Fig. 14 explains the main part of an optical system of Fig. 13;

FIG. 15A and 15B show an equivalent optical system of the embodiment of Fig 13 and that of a corresponding standard optical system.

Fig. 16 and 17 are graphical representations of wavefront aberration along different directions in the optical system of Fig. 13;

Fig. 18 shows an optical head device which is also an embodiment of the invention; and

Fig. 19 explains the main part of an optical system of Fig. 18;

Fig. 20 to 22 are graphs showing the optical characteristic of a hologram device used in the apparatus of Fig. 18;

Figs. 23A to 23C show an effectively diffracted light beam pattern and some of scattered light on the photographic device of Figure 19 under different conditions relating to their positioning.

Fig. 24 illustrates an optical head device which is also an embodiment of the invention, which allows the selective use of a plurality of optical disks which differ from each other in the substrate thickness;

Fig. 25 is a graphical representation of the optical characteristic of the embodiment of Fig. 24;

Fig. 26 is a block diagram showing the overall configuration of the embodiment;

FIGS. 27 and 28 show optical head devices in accordance with further embodiments of the invention.

Detailed description of the preferred Embodiments

Referring to FIG. 1, an optical head device in accordance with a preferred embodiment of the present invention is generally indicated at 30 . The optical head device 30 selectively supplies a write beam or read beam to a round, disc-shaped record carrier body 32 for use in an optical record and read / write device, which differ from each other in performance. The record carrier 32 is hereinafter referred to as "optical disk", which is removably and rotatably inserted in a known manner in a disk holding mechanism (not shown) and rotated by means of a known spindle (not shown) which is operated by a rotating motor (not shown) is driven.

The optical disk 32 can be an optical disk with a base layer and a radiation-sensitive layer placed thereon, on which digital audio information can be recorded and read by means of optical devices. As shown in FIG. 1, an optically detectable, spiral beam guiding track 34 , which covers the entire surface of the plate 32 , is formed on its radiation layer. The guide track 34 may alternatively be a plurality of concentric tracks.

The optical head device 30 includes a semiconductor laser device 36 as a read / write light source. Laser 36 emits a controlled light beam in accordance with a bias current provided by a laser driver associated with laser 36 . This output laser light approaches the radiation sensitive layer of the optical disk 32 along an optical path that extends through a holographic optical element (HOE) 38 , a collimation lens 40 , a beam shaping prism 42 , a mirror 44 and an objective lens 46 . This optical path serves both as an optical forward path (light transmission path) and an optical backward path (light reception path) of laser light. A pair of solid state photodetector devices 48 and 50 are arranged on a vertical plane including the laser 36 . These photodetectors are optically coupled to HOE 38 so that the photodetectors receive light reflected from the plate 32 and each produce electrical detection signals Sd, each of which indicates light input to the photodetectors.

The photodetectors 48 , 50 are electrically connected to a known amplifier circuit 52 . The amplifier 52 is coupled to an operation processing circuit 54 . The processor unit 54 has three outputs: a focus error signal output Sf, a tracking error signal output St and an output Si for a reproduced information signal. The processor 54 is connected to a driver circuit 56 which causes the focus error signal to be fed thereto. The driver 56 is connected to an inductive element 58 , such as a known coil, which functions as a lens actuation that is responsive to a control signal Sdrv provided by the driver 56 . The actuator 58 adjusts the position of the lens 46 by moving it so that the light beam is just focused on the target surface of the optical disk 32 . Driver 56 is called the "lens actuation driver" or "actuation driver" hereinafter.

The semiconductor laser 36 emits a light beam with an anisotropic beam profile, such as an elliptical beam shape. This laser beam (that is, reading light) enters the collimation lens 40 . The collimation lens 40 collimates the laser beam into a parallel light beam. The resulting light is reshaped by the beamformer 42 to have an isotropic beam profile. The light beam is then reflected vertically upward from the mirror 44 onto the objective lens 46 (indicated by "X" in FIG. 1). The shape-matched laser beam is then focused by the lens 46 onto the recording surface of the radiation sensitive layer of the rotating optical disk 32 , thereby forming a fine beam spot thereon.

The light reflected from the recording surface of the optical disk 32 propagates along the same optical path as the forward optical path of the optical head device 30 in the opposite direction and strikes the pair of photodetectors 48 , 50 . More specifically, the reflected light (reproducing light) on HOE 38 indicating reproduced information is guided through the objective lens 46 , the mirror 44 , the beam shaping prism 42, and the collimation lens 40 . HOE 38 prevents the playback light falling on it. The diffracted display light is deflected in a direction other than that of the incident light beam. As a result, the reproduction light is split into first and second diffracted light components. These first and second light components are deflected light in the positive (+) direction and deflected light in the negative (-) direction, respectively. The deflected light in the positive (+) direction and the deflected light in the negative (-) direction are each guided to photodetectors 48 , 50 , which then respectively generate corresponding electrical detection signals Sd (+), Sd (-).

The detection signals Sd (+), Sd (-) are supplied to the amplifier circuit 52 . The amplifier 52 amplifies each of the detection signals Sd to a predetermined suitable level. The amplified detection signals are then input to processor circuit 54 which, with respect to these signals, performs known predetermined processing to produce a focus error signal Sf, a tracking error signal St and a reproduction information signal output Si at its three outputs.

The focus error signal Sf is supplied to the lens actuation driver circuit 56 . On the focus error signal Sf toward the driver 56 is similar to a drive current to the focus drive coil 58 decreases so that the coil 58 causes the objective lens 46 to move in the optical axis direction (the X-direction perpendicular to the rotating surface of the plate 32) to provide a To compensate for focus errors, if any.

The significant feature of the optical head device 30 is as follows: Light reflected from the recording surface of the optical disk 32 propagates in the reverse direction along the same optical path as the optical path of the optical system of the device and is guided to fall on the collimating lens 40 . While a laser beam is being transmitted, the beam shape prism 42 serves to expand incident light only in one axial direction. The optical characteristics of collimation lens 40 and beamform prism 40 are shown in FIGS. 2A to 2C.

First, consider FIG. 2A, which is a model of the optical system, along with the collimating lens 40 and the beamform prism 42 in one plane (YZ plane) parallel to an active layer 37 formed in the semiconductor laser 36 . Anisotropic (elliptical) light beams are emitted by the active layer 37 of the laser 36 . The divergence angle of the output beams from the laser 36 is represented by theta 11 . "FcL" and "M" are the focal length of the collimation lens 40 and the beam expansion ratio, respectively. As shown in FIG. 2A, the laser 36 and the lens 40 are arranged on the same optical axis (Z direction). The output light from the collimation lens 40 has parallel rays. The beamform prism 42 is arranged to be inclined at a predetermined angle with respect to the optical axis.

An optical system equivalent to the optical system of FIG. 2A is shown in FIG. 2B, which replaces the collimation lens 40 and the beam shaping prism 42 of FIG. 2A with an equivalent characteristic lens 59 . This lens 59 has a by means of the reference symbol "fcL * M." designated focal length. FIG. 2C shows an optical system on another plane (that is, the XZ plane perpendicular to the YZ plane) perpendicular to the active layer 37 of the laser 36 in FIG. 2A. At the XZ level, laser light output is completely free of the effect of the beam shape prism 42 . Therefore, as shown in Fig. 2C, the state of the optical system is equivalent to the state in which there is only one collimating lens 40 . It is noted that when the detection of a focus error on the optical disk 32 is performed by detecting a beam size along the Y-axis direction parallel to the active layer 37 , it becomes equivalent to a condition that a lens with the focal length fcL * M as the detection lens is used.

The laser light source 36 and the photodetectors 48 , 50 of FIG. 1 are arranged on a holding plate 60 , as shown in FIGS. 3 and 4. Photodetectors 48 , 50 are separated from one another to define a predetermined gap therebetween. The laser 36 is arranged in the middle of the center line of photodetectors 48 , 50 . A three-part light sensor type photodetector is used as the photodetector 48 , 50 : the photodetector 48 has three photosensitive areas 48 a, 48 b, 48 c, while the photodetector 50 has three photosensitive areas 50 a to 50 c. The active layer 37 of the laser 36 is perpendicular to the surface of the holding plate 60 (that is, the XY plane); layer 37 is also perpendicular to the center line for connecting photodetectors 48 , 50 together. When the laser active layer 37 is parallel to the Y axis, the pair of photodetectors 48 , 50 are aligned along the X axis. The photosensitive area of each photodetector 48 , 50 is divided into three sub-areas by means of dividing lines parallel to the X axis. With the embodiment, it becomes possible to detect any variations in the size (cross-sectional shape) of a laser beam due to focus errors by using a detection lens of focal length fcL * M.

The three subdivided photosensitive regions 48 a to 48 c of the photodetector 48 of FIG. 3 generate electrical detection signals PDA1, PDA2, PDA3, which each display the detection results. The three subdivided photosensitive regions 50 a to 50 c of the photodetector 50 generate electrical detection signals PDB1, PDB2, PDB3, which each display the detection results. In this case, the focus error signal Sf, the tracking error signal St and the reproduction information signal Si of FIG. 1 are represented by the following equations:

Sf = (PDA2 - (PDA1 + PDA3)) - (PDB2 - (PDB1 + PDB3)) (1),

St = (PDA1 - PDA3) - (PDB1 - PDB3) (2),

Si = (PDA1 + PDA2 + PDA3) + (PDB1 + PDB2 + PDB3) (3).

The processor unit 54 of FIG. 1 calculates equations 1 to 3 to generate the respective signals Sf, St, Si.

The holographic optical element (HOE) 38 adjusts the azimuth of a diffraction grating formed on its surface so that the diffractions are provided in a direction perpendicular to the active layer transition plane of the light source laser 36 . HOE 38 shows a somewhat light-focusing function with respect to the diffracted light for the purpose of focus error detection. With the light focusing function, a light beam focused in front of the detection plane falls on the photodetector 48 to detect diffracted light in the positive (+) direction; a light beam focused at a position slightly behind the detection plane falls on the other photodetector 50 .

FIG. 5 shows side by side the projected patterns of incident light rays that are applied to the first and second photodetectors 48 , 50 in different focused states. Numeral 62 denotes a focused state, in which light beam patterns on the two photodetectors 48 , 50 are almost the same, in that the incident light is radiated onto the three divided photosensitive areas 48 a to 48 c ( 50 a to 50 c) from each photodetector .

When the objective lens 46 of FIG. 1 comes closer to the recording surface of the disk 32 , as shown by an arrow 64 in FIG. 5, the photodetectors 48 , 50 become different from each other in their projected beam patterns. In the photodetector 48 , a light beam incident on the upper and lower photosensitive regions 48 a, 48 c grows in magnitude in comparison with the case of the focused state 62 . When the objective lens 46 moves away from the recording surface of the disc 32 , the light beam patterns on the photodectors 48 , 50 change inversely to that described above. More specifically, when the lens 46 is moved far from the recording surface of the disk 32 , as indicated by an arrow 66 in Fig. 5, an incident on the upper and lower photosensitive areas 50 a, 50 c of the photodetector 50 increases in amount, compared with the case of the focused state 62 . In the first step, the incident beam pattern is reduced in the photodetector 48 and light only falls on the middle photosensitive region 48 b. Then, when the lens 46 moves further from the plate 32 , a light beam on the three-divided areas 48 a to 48 c grows again in terms of amount.

In other words, the characteristics of the formation patterns of the incident rays of the photodetectors 48 , 50 are reversed in relation to the distance between the objective lens 46 and the plate 32 . In this embodiment, when the lens 46 passes the position of a focused state to come too close to the plate 32 , the light pattern incident on the photodetector 48 increases in area as that of the photodetector 50 decreases. When the lens 46 is far from the focused state position and from the plate 32 , the area of an incident light pattern on the photodetector 48 decreases as that of the photodetector 50 increases. It is noted that the incident beam pattern on the three photosensitive areas 48 a C (50 a to 50 c) is different of each of the photodetectors 48, 50 depending to 48, if the pattern is viewed from the direction in which these photo-sensitive Areas are aligned, or from a direction perpendicular to it. As previously described with reference to Figs. 2A to 2C, such a difference is based on the fact that the detection of light is performed using lenses that are equivalently different from each other in focal length.

The significant of embodiment 30 is that when reflected light (information reading light) from optical disc 32 reversely travels along the same optical path as the forward optical path, the reflected light is forced to pass through beamform prism 42 and into photodetectors 48 , 50 . Specifically, the light reflected from the optical disk 32 travels along the same optical path in the direction reverse to the forward optical path and passes through the objective lens 46 and the beamform prism 42 . Thereafter, the light propagates in an optical path other than the forward optical path to be incident on the photodetectors 48 , 50 . Because the light reflected from the optical disk 32 light is detected by the beam shaping prism 42, even when the focal length (fc * M) of the detection lens is reduced to an extent that the beam shape magnification of the prism may correspond to 42, a focus error on the plate 32 at almost the same magnification as that of a conventional long focal length lens. That is, the size of the optical system required can be reduced accordingly. Therefore, it is possible to achieve an improved stability in signal detection that is required for the light receiving system, while maintaining an excellent function (light utilization ratio and the like) of the light transmission system with an optical signal path. This can greatly benefit from the size reduction or miniaturization of an optical recording / reproducing device.

More specifically, the prism 42 acting as the beam-forming functional element shapes or modifies the profile of an incident light beam emitted from the light source laser 36 to provide an isotropic beam shape by enlarging it in the direction along which the angle of divergence is relatively small. For focus / tracking error detection, the photodetectors 48 , 50 are arranged to detect variations in the beam shape of a reflected light from the optical disc 32 in the same direction. In order to obtain a further improved convergence ratio (i.e., a higher light utilization ratio as defined with the minimum light output and the maximum temperature rise on the recording surface of an optical disc) of a light source laser beam on the recording layer of an optical disc as required in optical head devices with a recording function, the collimation lens 40 and the beam shaping prism 42 are arranged in the light transmission system. With such an optical system, in the event that the reflected light from the plate 32 travels along the same path in the backward direction to the laser 36 , the beamform prism 42 performs a reducing function in the backward optical path when it has a magnifying function in the forward optical path causes; therefore, with respect to this beam shape direction, the apparent length of the collimation lens 40 increases by a certain magnification, which may correspond to the beam shape magnification (the diameter ratio of an incident light to an output light).

As previously described, the beamforming direction coincides with a specific direction along which the angle of divergence of the light output from the light source laser 36 is relatively small; in the event that the focal length is detected by detecting variations in the beam diameter of a plate reflected light using the photodetectors 48 , 50 , the results may be equivalent to a situation where focal length detection is performed using an optical lens which is larger Has the focal length as the collimation lens 40 . Therefore, even if a collimation lens of shorter focal length is used than to serve the converging lens in the detection system, it becomes possible to achieve an equivalent magnification magnification (which is a square of the focal length ratio of the converging lens in the detection system and the objective lens 46 ). A stable optical detection system can thus be achieved. Most optical components can be common between the light transmission system and the light reception system in the same optical system, thereby successfully reducing the total number of components required. Thus, it is possible to obtain a very reliable, small optical head device.

In practicing the present invention, the optical head device 30 described above can be variously modified. For example, a relatively long focal length lens can be used as the collimation lens 40 . In the beamform prism 42 , a parallel light beam output from this collimating lens is subjected to a method of reducing a light beam component perpendicular to the laser active layer 37 , whereby collimated light having a small angle of divergence which runs in a direction parallel to the active layer 37 of the light source laser 36 , can be properly adjusted to the aperture of the objective lens 46 . In this case, a focus error is also detected by detecting a change in the diameter of a light beam parallel to the active layer 37 .

An optical head device 36 a shown in FIG. 6 is similar to that of FIG. 1, with two beam splitter devices 70 , 72 being added, which causes an output light of one beam splitter 70 to reach the other beam splitter 72 by means of a detection lens 74 . More specifically, the first beam splitter 70 is arranged between the collimation lens 40 and the beam shaping prism 42 . The beam splitter 70 receives light reflected from the plate 32 of FIG. 1 and guides it to the detection lens 74 . Lens 74 is equivalent to collimation lens 40 in optical properties, such as focal length. A light beam bundled or converged by means of the lens 74 enters the other beam splitter 72 . The beam splitter 72 divides the incident beam into two beams, which are then projected onto the photodetectors 48 and 50 , respectively. With the embodiment, the same technical advantages as those described above can be obtained.

Another, in Fig. 7 shown optical head device 30 b is similar to that of Fig. 1, wherein (1) the beam shaping prism is modified 42, of a plurality of beam shaping prism 42 a to exist 42 b, and (2) the holographic optical Element (HOE) 38 is replaced by a HOE 38 a, which is spaced from the collimation lens 40 .

An optical head device shown in FIG. 8 c 30 similar to that of Fig. 1, wherein a laser output control loop 76 is additionally provided (1) to be coupled 42 and the light source laser 36 to the output of the beam shaping prism. More specifically, the mirror 44 of FIG. 1 is replaced by a beam splitter 78 which is assigned to a photodetector 80 . The beam splitter 78 splits the output light of the beamform prism 42 into two light beams, one of which (that is, light reflected by this beam splitter) is guided to enter the objective lens 46 and the other one (that is, light transmitted through the beam splitter) the photodetector 80 is to be given. The added control loop 76 includes an amplifier circuit 82 coupled to an output detection signal Swd of the photodetector 80 and an optical output controller 84 connected to the amplifier 82 . The controller 84 has an input for receiving an electrical recording signal Sr and an output connected to the laser 36 .

The optical output control 84 is an electrical circuit for proper matching of the optical output of the laser light source 36 in accordance with an operating state (i.e., recording mode or reproducing mode) c of the optical head apparatus 30th During a playback operation, controller 84 performs an operation that compensates for laser emission variation to obtain the actual optical output of laser 36 , which tends to vary at a predetermined standard level in accordance with changes in laser operating conditions, such as ambient temperature. During a recording operation, the controller 84 compensates for undesirable variations in the amount of light in response to the detection signal Swd output from the photodetector 80 , which provides information about a laser write beam actually projected onto the optical disk 32 , and thus adjusts the actual amount of light emitted by the laser 36 .

With such an arrangement, even if a light beam reflected from the optical disc 32 returns to the light source laser 36 to undesirably disturb the output laser light, such disturbance can be suppressed or suppressed, so that the output laser light is precisely controlled.

A is d in Fig. 9 optical head device 30 shown similar to that of Fig. 1, wherein (1) the HOE 38 and the photodetectors 48, 50 to 90 ° to the main optical axis are rotated with respect to, and (2) the mirror 44 of Figure is replaced. 1 by a beam splitter 86 form, which both has the beam shaping function and the beam reflection function.

The beam shape splitter 86 of FIG. 9 has three main optical surfaces 87 to 89 as shown in FIG. 10. The first surface 87 has a beam shaping function and is specially designed so that the angle of incidence becomes equal to the Brewster angle. The beam shape enlargement of the first surface 87 is equal to the refractive index of a glass material. The second surface 88 opposite the incident surface 87 is a reflective surface that is parallel to the surface 87 . The third surface 89 is an exit surface that lies obliquely opposite the surface 88 . A light beam reflected from surface 88 falls vertically onto surface 89 and is (forever) transmitted.

With such an arrangement, by only performing a simple process on an originally flat glass plate with two opposing, parallel surfaces so that the third surface 89 has a proper angle, a beamform splitter structure 86 can be produced with a desired optical characteristic precision. This leads to a reduction in the manufacturing cost of the optical head device 30 d. In addition, because the beamform splitter 86 has both the beamforming function and the beam reflecting function, the number of optical element components required for an optical head device can be further reduced. Therefore, a further reduction in the size of an optical head device can be achieved.

The prism structure 86 of FIG. 10 can be modified as shown in FIG. 11, in which a beam shape prism 86 a has a first surface (beam incidence surface) 87 a, which is defined by the thickness of an original prism plate part with two opposite, parallel main surfaces, a second surface (reflecting surface) 88 a, which is perpendicular to the surface 87 a and a third surface (exit surface) 89 a, which is cut obliquely with respect to the main surface. Of the reflecting surface 88 a reflected light vertically enters the exit surface 89 a is, in a manner similar to the prism structure 86, as previously described.

A further optical head device 30 e shown in FIG. 12 is in principle similar to the combination of the exemplary embodiments 30c, 30d of FIGS. 8 to 9. More specifically, the prism structure 86 of FIG. 9 is used and is arranged between the collimation lens 40 and the objective lens 46 , whereby the laser output control loop 76 is coupled to FIG. 8 with the prism 86th With such an arrangement, a reduction in size of an optical head device can be achieved according to the advantages of the embodiments of FIGS. 8 to 9.

An optical head device 100 , which is also an exemplary embodiment of the invention, is shown in FIG. 13, in which an optical unit 102 is fastened above a base plate 106 with holding parts 104 . The optical unit 102 is held by a suspension unit 108 which is movable in the direction of the optical axis and in the radial direction of an optical disk 32 a. The suspension 108 includes two lens actuation coils 110 , 112 which control the movement of the optical unit 102 , together with the electromagnetic action of a well-known magnetic circuit (not shown).

The optical unit 102 includes the light source laser 36 of FIG. 1. An output light beam from the laser 36 spreads onto a mirror 44 a through a holographic optical element (HOE) 38 b. The laser beam is reflected by the mirror 44 a and focused by an objective lens 46 a to be projected to form a small dot on a recording surface 33 of the rotating optical disk 32 a, which recording surface is formed on a transparent substrate 35 . The light reflected by the optical plate 32 a propagates in the same optical path in the opposite direction. That is, the reflected light enters HOE 38 b through the objective lens 46 a and the mirror 44 a. HOE 38 b causes the incident light beam to fall onto a photodetector device 114 arranged near the laser 36 . A detection signal Sd from the photodetector 114 is supplied to a processing circuit 54 including an arithmetic unit via an amplifier 52 . The processor 54 performs signal processing operations to generate the above-mentioned focus error signal Sf, the tracking error signal St and the reproduction information signal Si. The focus error signal Sf and the tracking error signal St are input into an actuation driver circuit 56 a, which is connected to the processor 54 . The driver 56 a is connected to the coils 110 , 112 and supplies them with driver signals Sdr1 and Sdr2.

As shown in FIG. 14, the HOE 38 b is spatially opposite the photodetector 114 . The photodetector 114 has a matrix of first to fourth photosensitive regions 115 a to 115 d, which are obtained by dividing a surface of the photodetector 114 vertically and laterally into four regions. HOE 38 b has two subdivided holographic areas 116 a, 116 b, which are divided by an area division line 116 c. This line 116 c is parallel to the track direction, indicated by an arrow 118 .

Holographic areas 116 a, 116 b have holographic patterns, each of which is formed by a large number of lattice elements that extend in a direction perpendicular to the track direction 118 . A "pincushion-like" hologram is formed in one of the areas, while a "barrel-like" hologram is formed in the other area. The regions 116 a, 116 b differ in their lattice sections. The formation of these holograms cause a change in the beam profile, which is necessary for the detection of a focus error, more specifically when light beams reflected by the plate 32 a run in the same optical path as the optical forward path of the optical unit 102 in the opposite direction, to HOE 38 b to fall, the light beams from the different holographic regions 116 a, 116 b are diffracted. As a result, the light beams are emitted with different exit angles in different directions (indicated by dashed arrows 120 a, 120 b). The photodetector 114 is in these directions 120 a, 120 b so that the upper two photosensitive areas 115 a, 115 b of the photodetector 114 receive light emitted by the first holographic area 116 a, and the lower two photosensitive areas 115 c, 115 d leaked light received from the holographic region 116b . The detection signals from the four photosensitive regions 115 a to 115 d are amplified by the amplifier 52 of FIG. 13 and are supplied to the processor 54 .

The optical properties of the objective lens 46a will be described below with reference to Figs. 15A and 15B, in which Fig. 15A shows a main optical system of the optical head device 100 in Fig. 13, and Fig. 15B shows an optical system according to a comparative example of a standard structure. Both the objective lens 46 a and the standard objective lens are optical lenses with a finite conjugate. The mirror 44 a is omitted in FIGS. 15A to 15B only for explanatory purposes.

Referring to Figure 15B, the objective lens generally has a 1/4 magnification. The lens aperture corresponds approximately to 1.25 times that of an optical lens of an infinite conjugate. For example, in a digital audio disc system (DAD) known as a compact disc (CD) player, the apparent numerical aperture is 0.56 while the numerical aperture is 0.45. In an objective lens of the infinite conjugate, a numerical aperture of approximately 0.55 is considered practical. Therefore, when the numerical aperture is small, an objective lens of the finite conjugate which has a relatively larger image magnification can be manufactured. However, in order to reproduce information from optical disks with a high information recording density, it is necessary to focus a laser beam to project a small beam spot onto an optical disk. For this, it is effective to reduce the wavelength of the light source laser or to increase the numerical aperture of the objective lens. For example, when an objective lens with a numerical aperture of 0.55 is formed by a finite conjugate lens, the apparent numerical aperture becomes as large as 0.69 with a normal magnification. Such an arrangement is very difficult to implement in practice.

In contrast, according to the embodiment shown in Fig. 15A, the magnification is made small to prevent the apparent numerical aperture from growing, and the numerical aperture is pressed to a level that allows the manufacture of a lens by means of previously known techniques, thereby making a finally conjugate lens is realized by using an objective lens with a suitable numerical aperture. It is assumed that the magnification is one seventh. In this case, even if an objective lens 46 with a numerical aperture of 0.55 is used, the apparent numerical aperture can be suppressed to 0.63, which is approximately 10% larger than a normal numerical aperture.

If, in addition, the focal length of the objective lens 46 a is reduced, the aperture of the lens is reduced in comparison with an objective lens with the same numerical aperture. This makes the manufacture of the lens easier. In an optical disk device, however, the working distance of the objective lens 46 a must be set to a predetermined value in order to avoid contact of the optical disk 32 a with the objective lens 46 a, even if in the position control in the direction of the optical axis of the lens 46 a with respect to one Surface deflection of the optical plate 32 a error is caused. That is, the focal length of the objective lens cannot be reduced infinitely. In addition, an information recording layer is formed on the lower surface of a transparent substrate ( 122 of Fig. 15A) of the disc to prevent written information from being destroyed after dust is attached to the surface of the disc or scratches are formed thereon. As the substrate increases in thickness (to ensure a predetermined working distance or more), the objective lens is required to have a longer focal length.

Taking this into account, the optical disk 32a of FIG. 13 is particularly arranged to have a smaller thickness than currently available optical disks. As is known to those skilled in the art, optical disks generally have a thickness of 1.2 mm. If the required working distance is 1 mm, the focal length of an objective lens is 3 mm. If the substrate ( 122 in Fig. 15A) of the optical disc has a thickness of 0.6 mm, the focal length can be allowed to decrease to about 2.3 mm, although the same working distance is ensured. In addition, the operating distance of an infinitely conjugated objective lens tends to increase even if it has the same focal length as that of a finely conjugated objective lens. That is, if only a certain required working distance needs to be ensured, the focal length can be further reduced. As described above, the focal length can be reduced to about 2 mm if the image magnification is set to 1/7. Compared to the comparison case shown in Fig. 15B, the focal length is allowed to decrease to 2/3. Therefore, the physical length of the optical unit 102 can be reduced to 2/3. This contributes significantly to a decrease in the size of an optical head device.

Generally, an objective lens with a large numerical aperture carries a large amount of wavefront aberrations with respect to a shift from the installation position of an object point (that is, a light source). When the image magnification is reduced, the allowable range of position shifts in the direction of the optical axis is expanded. If the object point position is slightly shifted from the optimal object point position of a finite objective lens in the direction of the optical axis, the permissible range of position shifts of the object in a plane perpendicular to the optical axis can be expanded. This results from the graphs in FIGS. 16 to 17.

Fig. 16 is a graph showing the distribution of wave front aberrations lambda RMS in the optical axis direction (Z-axis). Fig. 17 is a graphical representation of the distribution of wavefront aberrations Lambda RMS in a direction parallel to one of orthogonal axes (X or Y axes) on a plane perpendicular to the optical axis.

As shown in FIGS. 16 to 17 clearly, in the case that the Objektpunkt- (light sources) position, the optimum position (Z = 0) while the wave-front aberration lambda RMS remains less when the direction of the optical axis by X = Y = 0, the waveform aberration along one of the X and Y axis directions is large. However, when the object point is moved to a position far from the objective lens 46a along the Z axis, the wavefront aberration becomes smaller in each of the X and Y axis directions than in the case of Z = 0.

According to the optical head device 100 , by reducing the image magnification and using a finite system objective lens with a focal length required to ensure the minimum working distance, an optical system with a large numerical aperture can be obtained. At the same time, the optical system can be simplified, and thus a decrease in the size of the optical head device can be realized. By reducing the thickness of the associated with the optical head device optical disk, the focal length of the finite conjugate objective lens 46 may be a further reduced in addition. This further promotes a decrease in the size of the optical head device. Improved reliability can also be realized. In the optical unit 102, including the finite system objective lens 46 a, by arranging the light source laser 36 at a position slightly shifted from the position of the optimal object point, the permissible installation precision value of the object point (light source point) can be set on a plane perpendicular to the optical axis (Z axis ) can be enlarged. This facilitates the component assembly process in the manufacture of an optical head device.

An optical head device 114 shown in FIG. 18 includes an optical unit 142 which includes the light source laser 36 of FIG. 1. An output beam of the laser 36 extends to a mirror 44 b by a holographic optical element (HOE) 38 c from. The light reflected by the mirror 44 b is focused by an objective lens 46 b and is projected onto the recording surface of an optical disc 32 a to form a small dot on it. The reflected beam (that is, the read information indicating reflected beam) from the optical disk 32 a travels along the same optical path in the opposite direction to c in the HOE 38 through the objective lens 46 b and b enter the mirror 44th

The HOE 38 c is a multi-function diffraction type optical element with a beam splitting function, a beam splitter function and / or other special lens functions. The above term of HOE 38 c is used to distinguish it from an optical element of the diffraction type, which is formed by means of a known, simple diffraction grating with grating elements arranged at the same distance. An incident beam (plate-reflected beam) on HOE 38 c is optically branched by this element in order to spread onto a photodetector 144 , as in the exemplary embodiment shown in FIG. 13. An electrical control system associated with photodetector 144 is the same as that in the embodiment shown in FIG. 13.

A main optical system of the optical unit 142 can be developed, with the optical disc 32 a being the center of the system, as shown in FIG. 19. Fig. 19 explains forward and backward optical paths, which are actually the same optical path, separately (symmetrically) to be easily visually understood for explanatory purposes.

In the equivalent optical system shown in FIG. 19, a light beam P1 emitted by the laser 36 falls on HOE 38 c. A beam of light P0, which is a reproduction signal detecting beam is c of this element 38 thereby emitted as a transmitted light. The output light from HOE 38 c can also include undesired light beams P + 1, P - 1, which propagate at a predetermined diffraction angle. These rays of light are scattered light. Light rays P + 1, P0 and P - 1 are focused by the objective lens 46 b in order to be projected onto the optical plate 32 a. This occurs from the optical disk 32 a reflected light to the objective lens (indicated by 46 ') and propagates on a holographic optical element (HOE) 38 c' to off. A light beam PD of the light beam P0, which is diffracted by HOE 38 c, approaches the photodetector 144 as the reproduction signal detection beam. There is also an optical beam component PL, which is simply transmitted through HOE 38 c ', which light component is so-called return light, which propagates along the optical axis and returns to a laser 36 '.

The relationship between the reproduction signal detection beam PD and the return light PL will next be described with reference to the graph of Fig. 20, which shows the distribution (light amount distribution) of diffraction efficiency of the zero-order, first-order and third-order diffractions with respect to the optical phase difference of a holographic shows optical element with a binary phase grating. When the phase difference is Pi, the amount of light transmitted becomes substantially zero, as indicated by the zero order curves. The first order diffracted light is diffracted in the positive and negative directions by a maximum of 40% (diffraction efficiency is 0.4). When the transmittance of the zero order light is 50%, that of the first order light is 20%. If the transmittance of the zero order light is 28%, that of the first order light is 30%.

Fig. 21 shows the distribution (a change in the amount of light) of a diffraction efficiency of each output light beam HOE 38 c, that is, the reproduced signal detection beam PD and the return light PL with respect to the optical phase difference. In order to compare and analyze the influence of the diffraction efficiency of HOE 38 c, this characteristic diagram assumes that in the optical system in Fig. 19, no optical loss is caused in optical elements other than this HOE. The same applies to the characteristic diagram of FIG. 22.

The characteristic diagram of FIG. 21 shows a case in which the HOE 38 c has the optical properties shown in FIG. 20. The amount of the reproduced signal detection beam PD becomes maximum when the optical phase difference is Pi / 2. When the transmittance increases or decreases, the amount of light decreases. The amount of the return light PL decreases as the phase difference approaches the value Pi. In the prior art, under the conditions that maximize the amount of the reproduced signal detection beam PD, the amount of light that actually reaches the photodetector is 10% and the amount of return light to the light source is 25%. In contrast, the transmittance and diffraction efficiency of HOE 38 c of embodiment 140 are specifically chosen to satisfy the following inequality:

PL ≦ PD (4).

When the transmittance is 28%, the diffraction efficiency is typically about 30%. With this setting, the amount of return light to the light source laser 36 was 7.8%, and the amount of the reproduced signal detection beam PD that actually reached the photodetector 144 was 8.4%. If the amounts of light distributed to the optical elements 36 , 144 of the embodiment 140 are compared with those in the prior art, it can be seen that the amount of undesired return light PL to the light source 36 is reduced to almost a third in the present invention and at the same time one Decrease in the amount of light actually received by the photodetector 144 can be suppressed to approximately 20%. As described above, by increasing the diffraction efficiency of HOE 38 c and reducing its transmittance, the amount of return light PL can be greatly reduced. This leads to a large signal / noise ratio.

The characteristic diagram of Fig. 22 shows a case in which a diffraction type optical element having a sawtooth cross-sectional grating pattern, which is designed to cause diffraction light to concentrate on positive diffraction orders, is used as HOE 38 c. In this case, the transmittance and diffraction efficiency of the HOE are chosen to satisfy the relationship ( 4 ) described above. For example, if the transmittance is 42%, the diffraction efficiency is 46%. In this case, the amount of the return light PL to the light source 36 was 18%, and the amount of the reproduced signal detection beam PD which reached the photodetector 144 was 20%.

In the optical system of FIG. 19, the ratio of the scattered light beams P + 1, P - 1, which fall on the photodetector 144 , increases when the HOE 38 c inserted between the laser 36 and the objective lens 46 b is separated from the laser 36 . In particular, unwanted light components, such as the scattered rays P + 1, P - 1, which are diffracted by HOE 38 c in the forward optical path, are reflected by the optical plate 32 a, pass through the same holographic optical element ( 38 c '), and reach the photodetector 144 , deterioration in operational reliability.

The effective diffracted light P0 and the scattered light P - 1 emerging from HOE 38 c form the following optical patterns on the surface of the HOE 38 c '. FIGS. 23A to 23C show spot diagrams, which can be observed when the insertion position of HOE 38 c in the optical path in three positions (i.e., zhuh = 6.5, 7.5 and 8.5) is changed. Consider scattered light P - 1. When HOE 38 c is placed after the light source laser 36 , the resulting diffraction angle is large, and the amount of light incident on the aperture of the objective lens 46 b is small. Accordingly, the amount of light that is reflected by the optical plate 32 a and incident on HOE 38 c is small. See Figure 23A in this connection. It is noted that under this condition, it is difficult to produce HOE 38 c with high precision because the lattice spacing of HOE 38 c becomes small. When the insertion position of the light source laser 36 is separated from the HOE 38 c, the expansion of the scatter plot of the scattered light component P-1 increases. In the case of Fig. 23C, the scattered light P-1 grows in area to overlap the effective diffracted light P0. If HOE 38 c is located far from laser 36 , the resulting diffraction angle is reduced. Therefore, the amount of light that is reflected from the optical plate 32 a and returns to the HOE 38 c increases as a whole. When the insertion position of HOE 38 c is varied by the light source laser 36 , the amount of the stray light component overlapping the reproduction signal detection beam PD increases. Overlapping between effective light and stray light makes it more difficult to extract the stray light, which results in difficulty in achieving a larger signal-to-noise ratio and improved reliability.

In order to solve the problems described above, in this embodiment, the position at which the HOE 38 c is inserted in the optical path between the light source laser 36 and the objective lens 46 is specifically determined as follows. Provided that a transmission aperture that allows the incidence of a light beam with a diameter required for the aperture of the objective lens 46 is provided for HOE 38 c, the insertion position of HOE 38 c is determined ( 1 ) at a position which is in a Position range falls, which meets a condition that the stray light components P + 1, P - 1 are blocked to prevent their incidence on the photodetector 144 when the stray light components are reflected by the optical plate 32 a and by a certain from the center pass position shifted from HOE 38 c, and ( 2 ) at a specific optical axis position furthest from laser 36 in this position range. In this embodiment, the adverse effects of unwanted stray light components are eliminated to increase the signal-to-noise ratio of a reproduction signal while achieving a reduction in the size of the optical head device 140 .

Another optical head device 150 shown in FIG. 24 is similar to that of FIG. 18 with another set of a light source laser 152 , a photodetector 154 and a holographic optical element (HOE) 156 added in ( 1 ), and ( 2 ) the mirror 44 b of FIG. 18 is replaced by a beam splitter structure 158 in an optical unit 142 a. Optical elements 36 , 38 c, 144 form a first optical system OS1, while additional optical elements 152 , 154 , 156 form a second optical system OS2. The beam splitter 158 is optically connected at a first level to the first system OS1 and to the second system OS2 at a second level thereof. The main optical axes of these optical systems are transverse to one another. The second optical system OS2 differs from the first system OS1 in that an aperture limiting plate 160 is inserted between the holographic device 156 and the second plane of the beam splitter 158 , as shown in FIG. 24. The second system OS2 also differs from the first system OS1 in that the laser 152 and the holographic device 156 are shifted in position towards the beam splitter 158 , while the laser 152 and the photodetector 154 are similar to the laser 36 and the photodetector 154 in optical terms Properties and arrangement are: each of the first and second optical systems OS1, OS2 can be arranged as shown in FIG. 14.

The optical unit 142 a with a pair of optical systems OS1, OS2 can achieve an optimized read (reproduction) operation by successfully cutting off any scattered light in the event that any of the optical disks 32 a, 32 b with different substrate thicknesses are used selectively, while allowing the entire optical system to be made smaller. This applies because the first optical system OS1 is specifically arranged so that the reading operation for a thin optical disk 32 a is optimal, and the second system OS2 is arranged so that the reading operation for the thick optical disk 32 b is achieved as commercially available in optical disk technology. First and second optical systems OS1, OS2 are controlled to work selectively in accordance with the thickness of an optical disk 32 a, 32 b currently used. It is noted that the objective lens 46 b is constructed so that it has a desired image magnification when the thin optical plate 32 a is used.

An explanation will now be given of how the optical disks 32 a, 32 b of different thicknesses can be reproduced or reproduced by using the single objective lens 46 b, with reference to the characteristic diagram of Fig. 25. Fig. 25 is a graph showing the wavefront aberration as a function of the distance (shift amount) Delta d is along the direction of the optical axis, the distance between the light source and the objective lens is used as a reference value with respect to the optical disks 32 a, 32 b of Fig. 24. This graph shows the measurement results in a case that the objective lens 46 b uses an optical lens with a focal length of 2 mm. This optical lens is constructed under the following conditions: the wavelength of the light source laser is 680 nm, the numerical aperture is 0.6, the substrate thickness of the optical disc is 0.6 mm, and the image magnification is 0.14.

From Fig. 25 shows that in the case of optimal for the thin optical disk 32 a constructed objective lens, the wavefront aberration RMS Lambda = 0.5 occurs when the light source position is shifted by 1 mm; the intensity at the center of a point formed on the optical disk decreases, with the result that it is equivalent or 90% of the central intensity at an optimal position. The main reason affected in this case is spherical aberration. The aberration of the objective lens 46 b, which occurred with respect to an optical disk with a thickness different from the intended thickness, also results from the spherical aberration. This leads to the following conclusion: a single objective lens can be commonly used, have to effectively optical disks having different thicknesses, play, by canceling the wavefront aberration which b in the optical discs 32a, 32b occurs with different thicknesses, with the spherical aberration which takes place due to variations in the light source position. For example, assume that the thick optical plate 32 b, a substrate having 1.2 mm in thickness (t = 1.2 mm). If it is positionally shifted in this case, the laser light source, by about 5 mm of the objective lens 46 b to come closer, the resultant wave-front aberration can be suppressed to the extent that practically no problems.

Returning to Fig. 24. If the numerical aperture NA2 with respect to the light source laser 152 in the second optical system OS2 is smaller than the numerical aperture NA1 of the light source laser 36 in the first optical system OS1, the aperture restriction plate 160 is positioned in front of the second plane of the beam splitter 158 thereby the diameter limit of the objective lens 46 b the incident light beam to a reduced preset value. The embodiment is arranged on an assumption that the optical head device is applied to an external read-only memory device (such as "CD-ROM") of a small computing device called "personal computer"; accordingly, it is preferred that the aperture limiter 160 be designed to have a specific aperture limiting function that limits the numerical number to 0.45.

An electrical control circuit system associated with the optical head device 150 of FIG. 24 is shown in FIG. 26. The configuration of an electrical detection circuit connected to the photodetector 144 in the first optical system OS1 is similar to that of the embodiment of FIG. 13; a redundant explanation of it is omitted. The photodetector 154 of the second optical system OS2 is connected to an amplifier circuit 162 and an operational processing circuit 164 , each of which may be similar in arrangement to a corresponding one of the components 52 , 54 . Processors 54 , 164 generate first and second information playback signals Si1 and Si2, respectively.

As shown in FIG. 26, the outputs Sf, St from two operation processing circuits 54 , 164 are connected to the first and second inputs of a signal selection circuit 166 , which includes a switch circuit. The signal detector 166 has a with actuator coils 110, 112 by means of the lens actuator driver 56 a of FIG. 13 output connected. The first and second light sources 36 , 152 are each connected to control circuits 168 , 170 , each of which electrically supplies a corresponding one of the lasers 36 , 152 . Selector 166 and laser controllers 168 , 170 are connected to a main controller 172 , which may include a microprocessor unit (MPU) and an interface unit. The main controller 172 instructs laser controllers 168 , 170 so that each laser 36 , 152 performs a required laser oscillation when selected. The main controller 172 also controls the selector 166 so that a set of processor outputs Sf, St passes through the selector 166 in accordance with the selective operation of the first and second optical systems OS1, OS2.

An optical head device 150 a shown in FIG. 27 is similar to that of FIG. 24, the beam splitter 158 being replaced by a beam splitter structure 174 with two parallel light incidence paths. The first and second optical systems OS1, OS2 are arranged so that their optical axes are parallel to each other, causing the entire space to decrease. This can lead to further miniaturization of the optical head device.

An optical head device 150 b shown in FIG. 28 is similar to that of FIG. 24, with ( 1 ) the lasers 36 , 152 being replaced by semiconductor lasers 36 a, 152 a, which have different wavelengths, and ( 2 ) the beam splitter 158 of FIG a beam splitter structure 176 is replaced with two reflection planes 178 a, 178 b, so as to allow laser beams to be optically synthesized with lasers 36 a, 152 a with different wavelengths.

More specifically, the first and second optical systems OS1, OS2 are arranged in a plane parallel to the plane of rotation of a thin or thick optical plate 32 a, 32 b. The optical axes of these optical systems OS1, OS2 are transverse to one another. The "synthetic" beam splitter 176 has a cubic structure, which has first and second light incidence planes, which HOE 38 c, 156 each face at predetermined distances. The beam splitter 176 also has an upper plane of the objective lens 46 b in a direction substantially to the plane of rotation of the plates 32 a, 32 b rectangular direction facing.

The first reflection plane 178 a is inclined with respect to the main optical axis of the first optical system OS1, which causes the output beam of the laser 36 a thereof to be reflected onto the lens 46 b. The second reflection plane 178 b is inclined with respect to the main optical axis of the second optical system OS2, which allows the output beam of the laser 152 a thereof to be reflected onto the lens 46 b. The reflective plane 178 a is covered with a special reflection film, which shows a higher reflectivity for a light beam emitted by the first laser 36 a, and an increased transmittance with respect to a light beam from the second laser 152 a. The optical nature of the reflective film is reversed as far as the second reflective plane 178 b is concerned: the reflective film 178 b uses a different film, which shows an increased reflectivity for the light beam from the second laser 152 a, and an increased transmittance with respect to the light beam from the first laser 36 a. With the embodiment, the optical head apparatus 150 can be reduced in height b, which causes the optical disk reproducing device is thinner in shape.

The present invention is not based on the above described specific embodiments limited and can be done in other ways be without the spirit and essential character of it to leave.

The determination of the combination in the thickness of the optical disks of various substrate thicknesses and the optical characteristics of the objective lens are not only limited to those previously described in the embodiments of FIGS. 13 to 29. Any other combination of different thicknesses may be possible when practicing the invention.

Claims (20)

1. Optical head device which is suitable for optical coupling to an optical disc ( 32 ), the device comprising a light source ( 36 ) for emitting a light beam, a collimation device ( 40 ) for collimating the light beam, an objective lens ( 46 ) for converging the Light beam onto the optical plate ( 32 ) and for receiving a light beam reflected from the optical plate ( 32 ), a beam shaping device ( 42 ) which is arranged between the collimation device ( 40 ) and the objective lens ( 46 ), and a photodetector ( 48 , 50 ) for detecting some signals from the reflected light beam, characterized in that the photodetector ( 48 , 50 ) comprises a plurality of photosensitive areas for detecting a change in a diameter of the reflected light beam, and the reflected light beam passing through the objective lens ( 46 ) and the beam shaping device ( 40 ) passes through without the Lic htquelle ( 36 ) to pass through, receives. 2. Apparatus according to claim 1, characterized in that the light source ( 36 ) comprises a semiconductor laser having an active layer, and the photodetector ( 48 , 50 ) a change in a diameter of the reflected light beam in a direction parallel to the active layer of the Semiconductor laser detected. 3. Device according to claim 2, characterized in that the variety of photosensitive areas in the Direction perpendicular to the active layer of the Semiconductor laser are divided. 4. The device according to claim 1, characterized in that the beam shaping device ( 42 ) has at least one function for shaping the light beam into an isotropic radiation profile by reducing the light beam in a direction in which the angle of divergence of the light beam emitted by the light source ( 36 ) is relative is great. 5. The device according to claim 1, characterized in that the beam shaping device ( 42 ) has at least one function for shaping the light beam into an isotropic radiation profile by enlarging the light beam in a direction in which the angle of divergence of the light beam emitted by the light source ( 36 ) is relative is small. 6. The device according to claim 1, further characterized by
a further photodetector for detecting an optical output of the light beam, and
a beam splitting device ( 38 ) for splitting the light beam into a first light beam which points to the objective lens ( 46 ) and a second light beam which points to the further photodetector. 7. Optical head device, which is suitable for optical coupling with an optical disc ( 32 ), the device comprising a light source for emitting a light beam, a collimation device ( 40 ) for collimating the light beam, an objective lens ( 46 a, 46 b) for converging of the light beam to the optical plate ( 32 ) and for receiving a light beam reflected from the optical plate ( 32 ), a beam shaping device ( 42 ) which is arranged between the collimation device ( 40 ) and the objective lens ( 46 a, 46 b), a Beam splitting means ( 38 ) for splitting the reflected light beam and a photodetector for detecting some signals from the reflected light beam, characterized in that the beam splitting means ( 38 ) reflected through the objective lens ( 46 a, 46 b) and the beam shaping means ( 42 ) Beam of light into a first reflected beam of light that hits the Lichtque llle ( 36 ), and divided into a second reflected light beam, which points to the photodetector ( 114 , 144 ), wherein the photodetector ( 114 , 144 ) a plurality of divided photosensitive areas for detecting a change in a diameter of the second reflected light beam Has. 8. The device according to claim 7, characterized in that the light source ( 36 ) comprises a semiconductor laser having an active layer, and the photodetector detects a change in a diameter of the reflected light beam in a direction parallel to the active layer of the semiconductor laser. 9. The device according to claim 8, characterized in that the variety of photosensitive areas in the Direction perpendicular to the active layer of the Semiconductor laser are divided. 10. The device according to claim 7, characterized in that the beam shaping device ( 42 ) has at least one function for shaping the light beam into an isotropic radiation profile by reducing the light beam in a direction in which the angle of divergence of the light beam emitted by the light source ( 36 ) is relative is great. 11. The device according to claim 7, characterized in that the beam shaping device ( 42 ) has at least one function for shaping the light beam into an isotropic radiation profile by enlarging the light beam in a direction in which the angle of divergence of the light beam emitted by the light source ( 36 ) is relative is small. 12. The apparatus according to claim 7, characterized in that the beam splitting device ( 38 ) comprises a holographic element ( 38 b) for deflecting the reflected beam. 13. The apparatus according to claim 1, characterized by a further photodetector ( 154 ) for detecting an optical output of the light beam, and a further beam splitting device ( 158 ) for splitting the light beam into a first light beam pointing to the objective lens and a second light beam, which points to the further photodetector ( 154 ). 14. Optical head device, which is suitable for optical coupling to an optical disk ( 32 ), the device being a light source ( 36 ) for emitting a light beam, an objective lens ( 46 a) for converging the light beam onto the optical disk ( 32 ) and for receiving a light beam reflected from the optical plate ( 32 ) and a photodetector for detecting some signals from the reflected light beam, characterized in that the objective lens ( 46 a) has a specific magnification of 1.6 or less and a numerical aperture of 0 , 55 or larger. 15. The apparatus according to claim 14, characterized in that the light source ( 36 ), the objective lens ( 46 a, 46 b) and the photodetector ( 114 , 144 ) are designed as an optical unit. 16. The apparatus of claim 15, characterized in that the optical unit in at least one focusing Direction is movable. 17. Optical head device, which is suitable for optical coupling with an optical disk ( 32 ), the device comprising a light source ( 36 ) for emitting a light beam, an objective lens ( 46 b) for converging the light beam onto the optical disk ( 32 ) and a reflected from the optical disk (32) light beam for receiving, a photodetector (144) for detecting some signals of the reflected light beam and an optical element diffraction type (38 c), which the light passing through the objective lens (465) reflected light beam into a first reflected light beam, which has the light source (36), and bows in a second reflected light beam, which has the photodetector (144), characterized in that the optical element from the diffraction type (38 c) has a predetermined diffraction efficiency and a has predetermined transmissivity, so that an amount of light of the first reflected light beam is smaller as an amount of light of the second reflected light beam. 18. The apparatus according to claim 17, characterized in that the light source ( 36 ), the objective lens ( 46 b), the photodetector ( 144 ) and the optical element of the diffraction type ( 38 c) are designed as an optical unit. 19. The apparatus of claim 18, characterized in that the optical unit in at least one focusing Direction is movable. 20. The apparatus according to claim 17, characterized in that the optical element of the diffraction type ( 38 c) has an aperture which limits the diameter of the light beam when the light beam from the light source ( 36 ) strikes the objective lens ( 46 b).