Optoelectronic device

An optoelectronic device that provides an excellent reproduction signal and a stable servo control while preventing deterioration of S/N ratio caused by stray light. The optoelectronic device has a semiconductor laser (101) that emits light beams on an information-recording medium (105), a hologram optical element (102) having a diffraction grating region (108) and located between the semiconductor laser (101) and the information-recording medium (105), and a photodetector (106) for receiving light that is diffracted at the diffraction grating region (108) of the hologram optical element (102), among returning light beams from the information-recording medium (105), and the optoelectronic device has also a diffraction grating region (107) in the vicinity of the diffraction grating region (108) of the hologram optical element (102) so as to prevent stray light other than diffracted light from the diffraction grating region (108) from entering the photodetector (106).

TECHNICAL FIELD

The present invention relates to an optoelectronic device for an optical head unit used as a main part of an optical information processor to apply light for recording, reproducing and erasing information. Specifically, the present invention relates to an optoelectronic device that divides a beam of light emitted from a light source into plural light beams using a light-beam-dividing means, and detects reproduction signals and various servo signals by means of the divided light beams.

BACKGROUND ART

Structures and operations of a conventional optoelectronic device are described below referring toFIGS. 17,18and19.

FIG. 17shows a cross section of an optical system and an information-recording medium of a conventional optoelectronic device. InFIG. 17, a broken line indicates how a light beam emitted from a semiconductor laser1and a light beam reflected at an information-recording medium5spread. InFIGS. 17 and 19, cross sections are not hatched for specifically indicating the light paths.

The light beam emitted from the semiconductor laser1as a light source enters a hologram optical element2having a diffraction grating region8as shown inFIG. 18. The 0th-order diffracted light is converged on the information-recording medium5by a collimator lens3and an objective lens4, and reflected. The light beam reflected on the information-recording medium5is guided by the objective lens4and the collimator lens3to the diffraction grating region8of the hologram optical element2. The reflected light is diffracted again there so that the ±first-order diffracted light enters photodetectors6. The reflected light beam guided to the photodetector6is detected and calculated so as to detect a reproduction signal and various servo signals.

InFIG. 17, an alternate long and short dash line indicates an optical axis10of the light emitted from the semiconductor laser1.FIG. 18shows an effective light beam9of the emitted light passing through the diffraction grating region8.

In the above-identified conventional optoelectronic device, various signals are detected by selecting among light beams emitted from the semiconductor laser1, exclusively 0th-order diffracted light that passes through the hologram optical element2.

However, a part of the emitted light entering the hologram optical element2is subject to ±first-order diffraction and mixed with the 0th-order diffracted light. That is, as shown inFIG. 19, a part of the ±first-order diffracted light is converged on the information-recording medium5and reflected to return to a region where the diffraction grating region8is not formed on the hologram optical element2, and it passes through the hologram optical element2so as to enter the photodetectors6. For clarification,FIG. 19shows only +first-order diffracted light received by the photodetector6, among +first-order light beams that are generated at the diffraction grating region8.

In other words, since a light beam other than the normal signal light (the 0th-order diffracted light of a light beam emitted from the semiconductor laser1) becomes stray light20to enter the photodetector6, a signal/noise ratio (S/N ratio) deteriorates during reproduction of the information-recording medium5, resulting in degradation in the reproduced signal and instability of the various servo controls.

In view of the above problems, the object of the present invention is to provide an optoelectronic device that obtains an excellent reproduction signal while allowing a stable servo control by providing a means for preventing deterioration of the S/N ratio caused by stray light.

DISCLOSURE OF INVENTION

For achieving the above-noted object, the present invention provides an optoelectronic device comprising a light emitter for emitting light beams on a reflection medium, a light-beam-dividing element having a diffraction grating region and located between the light emitter and the reflection medium, and a photodetector for receiving light that is reflected at the reflection medium and diffracted at the diffraction grating region of the light-beam-dividing element, wherein the light-beam-dividing element comprises a stray-light-removing region in the vicinity of the diffraction grating region, and the stray-light-removing region prevents stray light from entering the photodetector, where the stray light is other than diffracted light from the diffraction grating region, among returning light beams reflected of the reflection medium.

In this structure, a stray-light-removing region is provided in the vicinity of the diffraction grating region of a light-beam-dividing element. Accordingly, ±first-order diffracted light (stray light) that is generated during emitted light passing through the diffraction grating region is prevented from entering the photodetector via a region other than the diffraction grating region of the light-beam-dividing element. This can prevent deterioration of the S/N ratio and serve to provide an optoelectronic device that can provide a reproduction signal of improved quality and a stable servo control.

It is preferable in the structure that the stray-light-removing region of the light-beam-dividing element is a shielding region.

Accordingly, stray light entering a region other than the diffraction grating region of the light-beam-dividing element will be shielded by this shielding region in order not to enter the photodetector.

It is further preferable in the structure that the shielding region is formed of a material that absorbs the returning light beam.

Or it is preferable in the structure that the shielding region is formed of a material that reflects the returning light beam.

It is further preferable that the material of the shielding region is a metal.

It is preferable in the structure that the stray-light-removing region is formed with a diffraction grating having a 0th-order diffraction efficiency of the reflected light beam that is 5% or less.

Accordingly, substantially all the stray light entering a region other than the diffraction grating region of the light-beam-dividing element is diffracted without passing through the diffraction grating at the stray-light-removing region so as not to enter the photodetector.

It is preferable in the structure that the diffraction grating that forms the stray-light-removing region has a convexity and a concavity that are different from each other in the optical path length by m/2 times a wavelength of the reflected light, where m denotes an odd number.

Accordingly, the 0th-order diffraction efficiency of the diffraction grating at the stray-light-removing region can be suppressed to a remarkably low level.

It is preferable in the structure that the optoelectronic device comprises a plurality of photodetectors, each of which is located between a spot of mth-order diffracted light and a spot of (m+1)th-order diffracted light of the returning light beam provided by the diffraction grating of the stray-light-removing region located in an optical axis direction of the returning light beam with respect to the other photodetector, where m denotes an integer.

Accordingly, a higher-order diffracted light (diffracted light other than 0th-order diffracted light) provided by the diffraction grating of the stray-light-removing region is prevented from entering the photodetectors, and thus, deterioration of S/N ratio can be suppressed more effectively.

It is preferable in the structure that the light-beam-dividing element has a lens at a side opposite to the light emitter, and the stray-light-removing region is provided so as to stisfy the following formula:
r>d·tan(sin−1(NA))
where ‘d’ denotes an air conversion distance from a light-emitting point of the light emitter to a face of the light-beam-dividing element where the diffraction grating region and the stray-light-removing region are formed, NA denotes a numerical aperture of one side of the lens facing the light-beam-dividing element, and ‘r’ denotes a distance to an arbitrary point P on the stray-light-removing region from an intersection of an optical axis of the light beam provided by the light emitter and a face of the light-beam-dividing element at the stray-light-removing region side.

Accordingly, since a light beam that is emitted from the light emitter and enters the lens will not be shielded with the stray-light-removing region, a light beam from the light emitter can be used efficiently, and the light beam can be irradiated efficiently on a reflection medium.

It is preferable in the structure that at least one part of the photodetector is present in a region obtained by projecting the stray-light-removing region along with the optical axis of the returning light beam.

It is also preferable in the structure that the diffraction grating region and the stray-light-removing region are located adjacent to each other with no spacing.

Accordingly, stray light that may enter the photodetector from any regions other than the diffraction grating region of a light-beam-dividing element can be removed efficiently.

It is preferable in the structure that a three-beam-generating diffraction grating is provided in an optical path between the light emitter and the light-beam-dividing element.

Accordingly, a tracking servo signal can be detected by a three-beam method, whereby an optoelectronic device with a stable servo control can be provided.

It is preferable in the structure that the light emitter, the photodetector and the light-beam-dividing element are provided within one single package.

Accordingly, reduction in size and thickness of the optoelectronic device as well as reduction in cost can be achieved.

It is preferable in the structure that the light emitter and the photodetector are integrated on one substrate, the substrate is located inside the package, and the package is sealed with a member provided with the light-beam-dividing element.

Accordingly, the light emitter and the photodetector can be protected from any external influences such as changes in temperature and humidity, and dust pollution.

It is preferable in the structure that the light emitter is an end-face light emitter, the substrate has a concavity with a bottom on which the light emitter is located while the side face of the concavity comprises a mirror inclined about 45° with respect to the bottom so as to reflect light beams emitted from the light emitter.

Accordingly, an end-face light emitter, which is less expensive than a face light emitter, can be used for cost reduction.

It is preferable in the structure that the optoelectronic device comprises a monitor element for receiving light beams emitted from the light emitter toward a side opposite to the mirror and adjusting the output of the light emitter.

Accordingly, the output of the light emitter can be adjusted, whereby power consumption is reduced.

It is preferable in the structure that an integrated circuit for processing an electric signal from the photodetector is mounted on the substrate.

Accordingly, external noises caused by wire-routing can be reduced when compared to a case of providing a circuit for processing electric signals outside the substrate and connecting by wires. As a result, the obtained optoelectronic device will have a further improved S/N ratio.

It is preferable in the structure that the three-beam-generating diffraction grating and the light-beam-dividing element are integrated in one optical member.

Accordingly, reduction in size and thickness of the optoelectronic device as well as reduction in cost can be achieved.

It is preferable in the structure that the optoelectronic device comprises a polarized-light-beam-dividing means for dividing a part of light reflected by the reflection medium, a reflector for reflecting light divided by the polarized-light-beam-dividing means, a polarized-light-separating means for separating light reflected by the reflector, and a polarization-signal-detecting light-receiving means for detecting light separated by the polarized-light-separating means.

Accordingly, the optoelectronic device can be used for detecting photo-magnetic signals.

It is preferable in the structure that the polarized-light-beam-dividing means, the reflector and the polarized-light-separating means are formed integrally as one optical member.

Accordingly, reduction in size and thickness of the optoelectronic device as well as reduction in cost can be achieved.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described below referring to the attached drawings. In the cross-sectional views referred to in the description, hatching is omitted for specifying the optical paths.

First Embodiment

An optoelectronic device in a first embodiment of the present invention comprises, as shown inFIGS. 1 and 2, a semiconductor laser101as a light source, a hologram optical element102having diffraction grating regions107and a diffraction grating region108, a collimator lens103, an objective lens104, and photodetectors106.

A light beam emitted from the semiconductor laser101enters the diffraction grating region108of the hologram optical element102, providing 0th-order diffracted light that is converged by the collimator lens103and the objective lens104as a converging means on an information-recording medium105. Thereby, the reflected light is guided back by the converging means to the diffraction grating region108of the hologram optical element102.

Here, an enlarged view of a cross section of the hologram optical element102is shown inFIG. 3. The diffraction grating regions107and108are formed with diffraction gratings different from each other in the depths and/or pitches.

The diffraction grating region108is composed of a single or plural diffraction grating pattern(s) providing a lens effect. Reflected light from the information-recording medium105is divided at the diffraction grating region108so as to provide ±first-order diffracted light that is then converged at the photodetector106. A reflected light beam guided by the photodetector106is calculated and detected for detecting a reproduction signal and various servo signals.

The diffraction grating region107has a diffraction grating with a depth determined to be 1/{2(n−1)} times the wavelength of a laser beam emitted by the semiconductor laser101, so that a light beam entering the diffraction grating regions107will not pass through substantially. Here, ‘n’ denotes a refractive index of the hologram optical element. In fact, the 0th-order diffraction efficiency of the diffraction grating regions107was not more than 5%.

InFIG. 2, numeral109denotes a spot of an effective light beam passing through the diffraction grating region108. The effective light beam is included in light emitted from the semiconductor laser101and it is obtained as light reflected at the information-recording medium105and it is obtained from the 0th-order diffracted light passing through the hologram optical element102.

As described above, the diffraction grating regions107are provided in the vicinity of the diffraction grating regions108where the effective light beam enters. The diffraction grating regions107function as stray-light-removing regions for removing stray-light other than the above-identified effective light beam, among the reflected light beams from the information-recording medium105.

The diffraction grating regions107are formed to meet the following Formula 1:
r>d·tan(sin−1(NA))  Formula 1
where ‘d’ denotes an air conversion distance from a light-emitting point of the semiconductor laser101to a face of the hologram optical element102where the diffraction grating regions107/108are formed along an optical axis110connecting the light-emitting point of the semiconductor laser101and a converging spot on the information-recording medium105, NA denotes a numerical aperture of the collimator lens103at a side facing the hologram optical element102, and ‘r’ denotes a distance between an arbitrary point P on the diffraction grating regions107and an intersection between the optical axis110and the face of the hologram optical element102formed with the diffraction grating regions107/108.

When a light beam passes through a medium having a refractive index of ‘n’ and a thickness ‘t’, an air conversion distance ‘d’ that the light beam progresses is represented as follows:
d=t/n.

The diffraction grating regions107are located as shown inFIG. 4so that at least one (preferably all) of the photodetectors106is included in a projection region when the diffraction grating region107is projected along the optical axis110into a face including the photodetectors106. The numeral120inFIG. 4denotes a light-receiving element.

Accordingly, as the diffraction grating regions107as stray-light-removing regions are formed in the vicinity of the diffraction grating region108, ±first-order diffracted light that is obtained from the emitted light at the diffraction grating region108is prevented from being mixed in 0th-order diffracted light and from turning into stray light that will enter the photodetectors106. This enables further suppression of deterioration of the S/N ratio during reproduction of the information-recording medium105, and thus, provides an optoelectronic device that can provide an excellent reproduction signal and a stable servo-control.

When a plurality of photodetectors106are provided as shown in this embodiment, it is preferable that any higher-order diffracted light (diffracted light other than 0th-order diffracted light) from the diffraction grating regions107positioned above the photodetectors106is prevented from entering any other photodetectors106. For this purpose, as shown inFIG. 6, directions and pitches of diffraction gratings of the respective diffraction grating regions107are designed so that the higher-order diffracted light is prevented from entering a photodetector106located below another diffraction grating region107. In other words, the direction and pitches of the diffraction grating of each diffraction grating region107are determined so that each photodetector106is positioned between a spot of mth-order diffracted light and a spot of (m+1)th-order diffracted light with respect to reflected light from the information-recording medium105.

Thereby, higher-order diffracted light generated at the diffraction grating regions107can be prevented as well from turning into stray light and entering photodetectors106. As a result, deterioration of the S/N ratio can be suppressed further during reproduction of the information-recording medium105, providing an optoelectronic device that can provide an excellent reproduction signal and a stable servo-control.

Particularly, since the photodetector106is located below the diffraction grating region107so as to face the semiconductor laser101, the reflected light can be prevented from partially turning into stray light and entering the photodetector106.

Since at least a part of the photodetectors106is shielded with a region formed by projecting the diffraction grating region107along with the optical axis of the emitted light, the reflected light is prevented from partially turning into stray light and entering the photodetector106.

Moreover, since among the light beams from the semiconductor laser101, a light beam entering the collimator lens103will not be shielded with the diffraction grating region107. Therefore, light beams from the semiconductor laser101can be used efficiently and the information-recording medium105can be irradiated with light beams efficiently.

Furthermore, the diffraction efficiency has a relationship as shown inFIG. 6with a difference in the optical path lengths between a convexity and concavity of a diffraction grating. The difference in the optical path lengths in the convexity and concavity of diffraction gratings of the diffraction grating region107is preferably to be m/2 times a wavelength of the light beam, where m denotes an odd number. Accordingly, a transmittance of 0th-order light at the diffraction grating region107can be decreased to 5% or less, and this can prevent the reflected light from partially turning into stray light and entering a photodetector106.

Description concerning the first embodiment refers to an infinite optical system comprising both a collimator lens103and an objective lens104. Similar effects are obtainable by using a finite optical system using an objective lens104alone.

A stray-light-removing region of the present invention is not limited to the above-described diffraction grating region107, but it can be a shielding region formed of a shielding material. For example, the diffraction grating regions107can be replaced by shielding regions122as shown inFIG. 7that are formed of a reflective material such as a metal. Alternatively, as shown inFIG. 8, shielding regions123can be formed of a light-absorbing material.

The diffraction grating regions107/108can be shaped arbitrarily when viewed in the optical axis direction of the effective light beam109as long as the diffraction grating region108is located to include the effective light beam109while the diffraction grating regions107are located in the vicinity of the diffraction grating region108or adjacent to the diffraction grating region108with no spacing as shown inFIGS. 9 and 10.

Second Embodiment

An optoelectronic device in a second embodiment is distinguished from the optoelectronic device described in the first embodiment in that a three-beam-generating diffraction grating element111is located in an optical path between a semiconductor laser101and a hologram optical element102as shown inFIG. 11, and one photodetector106is used for detecting a tracking servo signal by a three-beam method.

Since the optoelectronic device in the second embodiment comprises a single photodetector106as shown inFIG. 12, the diffraction grating region107of the hologram optical element102is provided adjacent to only one side of the diffraction grating region108. The photodetector106is located to be included at least partially (or entirely in a preferred example) in a region provided by projecting the diffraction grating region107along the optical axis110.

Accordingly, similar to the first embodiment, deterioration of the S/N ratio can be suppressed during reproduction at the information-recording medium105and the optoelectronic device in this embodiment can have a servo control more stable than that of the optoelectronic device in the first embodiment due to the three-beam method.

Similar to the first embodiment, the stray-light-removing region of the hologram optical element102is not limited to the diffraction grating region107, but it can be a shielding region122of a reflective material like a metal or a shielding region123of a light-absorbing material as shown inFIGS. 7 and 8.

Third Embodiment

An optoelectronic device in a third embodiment of the present invention has a structure as shown inFIGS. 13 and 14. Specifically, a semiconductor laser101and photodetectors106are mounted integrally on the same substrate (substrate113) and located in a package112, and the package112is sealed with an optical part124prepared by integrating a three-beam-generating diffraction grating element111and a hologram optical element102.

The substrate113is dented by means of a semiconductor fine processing technology. The semiconductor laser101is chip-bonded to the bottom of the concavity. In a typical example where an end-face light emitter is used for the semiconductor laser101, light emitted from the end face of the semiconductor laser101should be directed to the information-recording medium105. For this purpose, the concavity has a side face inclined by about 45° with respect to the bottom, and a metal, dielectric film or the like is deposited on the side face so as to form a micro-mirror114.

When the semiconductor laser101is an end-face light-emitting type, preferably it comprises a monitoring photodetector115for receiving light emitted toward a side opposite to the micro-mirror114so as to adjust the output of the semiconductor laser101. This structure serves to adjust the optical output of the semiconductor laser101to keep the optimum condition, and to suppress excessive power consumption caused by excessive optical output.

Similar to the first and second embodiments, the diffraction grating regions107/108, the three-beam-generating diffraction grating element111, and the photodetectors106are located to prevent ±first-order diffracted light generated out of the emitted light at the diffraction grating region108from being mixed in 0th-order diffracted light and turning into stray light that will enter the photodetectors106.

As a result, deterioration of the S/N ratio can be suppressed during reproduction, providing an optoelectronic device that can provide an excellent reproduction signal and a stable servo-control.

Furthermore, since the semiconductor laser101and the photodetectors106are located together in a package112and the package112is sealed with an optical part124formed by monolithically integrating the three-beam-generating diffraction grating element111and the hologram optical element102, the reliability of the optoelectronic device can be improved considerably.

Particularly, since the semiconductor laser101and the photodetectors106are monolithically integrated on the same substrate113, a small and thin optoelectronic device can be provided.

As shown inFIG. 14, integrated circuits121can be integrated on the substrate113so as to convert or calculate the current and voltage of the electric signals from the photodetectors106by using the semiconductor fine processing technology.

Accordingly, any external noises caused by wire-routing can be reduced when compared to a case of providing a circuit for processing electric signals outside the substrate113and connecting the circuit and the substrate113using wires. As a result, the obtained optoelectronic device will have a further improved S/N ratio.

This integration can be performed in a hybrid manner, i.e., by forming all photodetectors106on a silicon substrate by the semiconductor fine processing technology before chip-bonding the semiconductor laser101. Alternatively, a semiconductor hetero-epitaxial technique can be used to form a compound semiconductor layer monolithically, and the semiconductor laser101and all photodetectors106can be formed on either a silicon substrate or a compound semiconductor layer. Alternatively, no silicon substrates are used but only a compound semiconductor layer is used for monolithically integrating the semiconductor laser101and all the photodetectors106.

When the semiconductor laser101and the photodetectors106are integrated in a hybrid manner, the semiconductor laser101can be a face light-emitting type for emitting light from the upper surface. In this case, the micro-mirror114can be omitted as long as the semiconductor laser101is chip-bonded with its light-emitting face directed upwards.

In this structure, the stray-light-removing region of the hologram optical element102is not limited to the diffraction grating regions107. It can comprise shielding regions122formed of a reflective material such as a metal, or shielding regions123formed of a light-absorbing material as shown inFIG. 7or8regarding the first embodiment.

Fourth Embodiment

An optoelectronic device according to a fourth embodiment of the present invention has a structure as in the third embodiment, and it further comprises a polarized-light-beam-dividing means116, a reflector,117, a polarized-light-separating means118and a polarization-signal-detecting photodetector119, which are shown inFIGS. 15 and 16respectively.

The polarized-light-beam-dividing means116, the reflector117and the polarized-light-separating means118are integrally formed as an optical member126. The polarized-light-beam-dividing means116can be a polarized-light-beam splitter or the like, the reflector117can be a mirror or the like, and the polarized-light-separating means118can be a Wollaston prism or the like.

The semiconductor laser101, the photodetector106and the polarization-signal-detecting photodetector119are integrated on a substrate113. The substrate113is located inside a package112. The package112is sealed with an optical part125prepared by integrating a three-beam-generating diffraction grating111and a hologram optical element102. The optical member126, which is prepared by integrally assembling the above-described polarized-light-beam-dividing means116and the like, is mounted on the optical part125.

Since a polarized-light-beam-dividing means116, a reflector117, a polarized-light-separating means118and a polarization-signal-detecting photodetector119are arranged in this manner, photo-magnetic signals can be detected.

The optoelectronic device can be reduced in the size and thickness and also the production cost can be reduced by 1) integrating on one substrate113a semiconductor laser101, a photodetector106and a polarization-signal-detecting photodetector119; 2) integrating in one optical part125a three-beam-generating diffraction grating element111and a hologram optical element102; and 3) monolithically integrating as an optical member126a polarized-light-beam-dividing means116, a reflector117, a polarized-light-separating means118.

Furthermore, since the optical part125seals the package112having the substrate113in the interior of the package112, precision parts such as the semiconductor laser101and the photodetectors106can be protected from changes in humidity and temperature and from dust pollution. Accordingly, reliability of the optoelectronic device can be improved considerably.

Photo-magnetic signals can be detected using this optoelectronic device. Specifically, a reflected light beam from the information-recording medium105enters the polarized-light-beam-dividing means116so as to be divided and directed to the hologram optical element102and to the reflector117.

As mentioned above, the reflected light beam divided in the direction to the hologram optical element102is diffracted and converged at the photodetector106by the diffraction grating region108of the hologram optical element102so that the servo-signals are calculated and detected. On the other hand, a part of returning light divided by the polarized-light-beam-dividing means116is directed to the reflector117. The light beam is reflected at the reflector117and further polarized at the polarized-light-separating means118into P polarized light and S polarized light to be guided to the polarization-signal-detecting photodetector119where the reproduction signal is calculated and detected.

As shown inFIG. 16, an integrated circuit121can be integrated on the substrate113by using a semiconductor fine processing technology, and the integrated circuit121is used for current/voltage-converting electric signals provided by the photodetector106.

In this structure, the stray-light-removing region of the hologram optical element102is not limited to the diffraction grating regions107. It can comprise shielding regions122formed of a reflective material such as a metal, or shielding regions123formed of a light-absorbing material as shown inFIG. 7or8regarding the first embodiment.

The semiconductor laser101exemplified in this embodiment is an end-face light-emitting semiconductor laser. This can be replaced by a face light-emitting semiconductor laser as described in the third embodiment. In this case, the micro-mirror114can be omitted as long as the semiconductor laser101is chip-bonded with its light-emitting face directed upwards.

The respective embodiments relate to a use for mainly detecting optical signals from an information-recording medium. However, the present invention can be applied for optical communications and other optical information processing systems as well.

INDUSTRIAL APPLICABILITY

As mentioned above, the present invention can prevent deterioration of S/N ratio caused by stray light, and thus an optoelectronic device according to the present invention can provide improved quality of a reproduction signal and a stable servo control.