JP2005004964A - Optical pickup, multi-wavelength semiconductor laser unit, and optical information recording / reproducing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve compatibility between various types of optical disks with a simple configuration and to be able to stably detect a signal even when a polarization detection optical system is used. Provide optical pickup.
A GaN blue semiconductor laser chip 45, an AlGaInP red semiconductor laser chip 46, and an AlGaAs near infrared semiconductor laser chip 47 are mounted on a Si submount 45-4. The blue semiconductor laser chip 45 (wavelength 405 nm), which is the shortest wavelength, is mounted in the center, and the red semiconductor laser chip 46 (wavelength 650 nm) and the near-infrared semiconductor laser chip 47 (wavelength 790 nm) are connected to the blue semiconductor laser chip 45 (wavelength). 405 nm) on both sides. The light emitted from each of the laser chips 45 to 47 is reflected by the etched mirror 45-3 to change the direction of the optical path, and then becomes parallel light by a collimating lens having a focal length of 25 mm.
[Selection] Figure 5

Description

  The present invention relates to an optical pickup including a plurality of light sources (for example, semiconductor lasers) that emit light beams having different wavelengths, a multi-wavelength semiconductor laser unit, and an optical information recording / reproducing method.

  In 1996, a DVD (digital versatile disk) system having a recording capacity of 4.7 GB was developed using an AlGaInP red semiconductor laser (wavelength near 650 nm). In a conventional CD (compact disk) system, an AlGaAs-based near infrared semiconductor laser (wavelength near 780 nm) is used, and its recording capacity is 650 MB.

  There are many differences between the CD system and the DVD system. One of them is that the thickness of the base material of the optical disk to be used is different. Specifically, the thickness of the base material of the optical disk is 1.2 mm in the CD system, whereas the thickness of the base material of the optical disk is 0.6 mm in the DVD system. Therefore, in the DVD system, various methods have been proposed in order to obtain compatibility with the CD system.

  One of them is a configuration using a bifocal lens (for example, see Non-Patent Document 1).

  In the bifocal lens, concentric hologram elements are formed on an objective lens having a numerical aperture NA of 0.6 designed for a wavelength of 650 nm. If this bifocal lens is used, light collected without aberration on a CD having a substrate thickness of 1.2 mm using the + 1st order light of the hologram element and a normal objective lens (the 0th order of the hologram element). Light), separation between light collected without aberration on a DVD having a substrate thickness of 0.6 mm is possible, and compatibility between CD and DVD can be realized.

  However, in an optical system using a bifocal lens, compatibility with CD is realized, but compatibility with CD-R is not obtained. This is because CD-R has a remarkably small reflection characteristic with respect to the red region, so that a sufficient reproduction signal cannot be obtained. Therefore, an optical pickup having two integrated units 125 and 126 (wavelengths of 650 nm and 780 nm) as shown in FIG. 21 has been proposed.

In the configuration of FIG. 21, the laser beam having a wavelength of 650 nm emitted from the DVD integrated unit 125 is transmitted through the wavelength separation prism 127, and then a diffraction grating is formed on the polarization hologram 128 (LiNbO ₃ substrate by proton exchange). ) And a wave plate 129 ((5/4) λ plate with respect to a wavelength of 650 nm), and is focused on an optical disk (DVD-ROM) 131 by an objective lens 132. The reflected light from the optical disk 131 is rotated 90 degrees in the forward direction by the wave plate 129, diffracted by the polarization hologram 128, and imaged on the photodetector (PD) in the DVD integrated unit 125. . In the detection optical system, the focus direction is controlled by an SSD (spot size detection) method, and the tracking direction is controlled by a phase difference detection method.

  On the other hand, the laser beam having a wavelength of 780 nm emitted from the CD integrated unit 126 is reflected by the wavelength separation prism 127 after passing through the plastic hologram element 126b having a narrow pitch. Then, similarly to the laser beam having a wavelength of 650 nm from the DVD integrated unit 125, the light passes through the polarization hologram 128 and the wave plate 129 and is condensed on the optical disk (CD or CD-R) 130 by the objective lens 132. . The reflected light from the optical disk 130 passes through the wave plate 129 and the polarization hologram 128 again. At this time, since the wave plate 129 acts as a λ plate for the wavelength of 780 nm, the polarization direction is maintained, and the polarization hologram 128 does not receive the diffraction action. The light diffracted by the plastic hologram element 126 b after being reflected by the wavelength separation prism 127 is imaged on a photodetector (PD) in the CD integrated unit 126. In the detection optical system, the focus direction is controlled by the SSD method, and the tracking direction is controlled by the three-beam method.

  In the objective lens 132, the aberration of light having a wavelength of 780 nm is smaller than that of an optical disk (CD or CD-R) 130 having a base material thickness of 1.2 mm, and light having a wavelength of 650 nm is an optical disk (DVD-) having a base material thickness of 0.6 mm. ROM) 131 is designed to reduce aberrations.

Using the optical pickup having the above-described configuration, an optical disk (CD or CD-R) 130 having a substrate thickness of 1.2 mm is reproduced by laser light having a wavelength of 780 nm emitted from the CD integration unit 126, and integrated for DVD. By reproducing the optical disk (DVD-ROM) 131 having a base material thickness of 0.6 mm by the laser beam having a wavelength of 650 nm emitted from the unit 125, good reproduction characteristics can be obtained.
Optical Review, Vol.1, No.1, pp.27-29 (1997)

  The recording capacity of the current DVD is 4.7 GB, and NTSC (National Television System Commitee standard) broadcast data can be recorded for about 2 hours. However, in order to develop media for image data of high-vision and high-difinition (hereinafter generally referred to as “HD”), it is essential to further improve the recording density of the optical disc.

  As means for improving the recording density of the optical disk, there are conceivable means such as (1) shortening the wavelength of the light source, and (2) increasing the numerical aperture NA of the objective lens. However, it is difficult to make the numerical aperture NA of the objective lens larger than the current 0.6 from the viewpoint of system margin and the like, and it is also difficult from the viewpoint of compatibility with CD and DVD.

  On the other hand, as means for shortening the wavelength of the light source, a method using a second harmonic generation (SHG) technique of a near-infrared semiconductor laser, a method using a GaN-based semiconductor laser, or the like can be considered. By using blue light having a wavelength of about 400 nm, the recording density of the current DVD can be improved by about 2.3 times. Hereinafter, the DVD thus obtained is referred to as “HD-DVD”.

  Even in the age of HD DVD using blue light, it is important to obtain compatibility with DVD and CD. Dye-based DVD-Rs are currently being developed in the same way as CD-Rs, but CD-Rs and DVD-Rs have degraded reflection characteristics in the blue region. Therefore, in order to realize compatibility, an optical pickup equipped with a coherent light source that emits light in three wavelength regions of the blue region, the red region, and the infrared region is required.

However, since an optical pickup configured using a multi-wavelength coherent light source requires a large number of optical components, it is difficult to design an optical pickup that can be mass-produced at a practical level. Specifically, for example,
(1) Since the number of optical components increases, the accuracy of aberration of each optical component becomes strict and integration becomes difficult, and the design of a small (thin) optical pickup becomes strict.
(2) Since it is necessary to simultaneously realize a detection optical system for each of the multi-wavelength light, if a polarizing hologram element or a polarization separation element is used for the detection optical system, the configuration of the quarter wavelength plate is complicated. become,
Such problems occur.

  The present invention has been made to solve the above-described problems in the prior art, and can realize compatibility between various types of optical disks with a simple configuration, and uses a polarizing detection optical system. Even so, an object of the present invention is to provide a small optical pickup, a multi-wavelength semiconductor laser unit, and an optical information recording / reproducing method capable of stably detecting a signal.

In order to achieve the above object, the configuration of the optical pickup according to the present invention is mounted on the same submount and emits a plurality of coherent beams that emit light beams having different wavelengths (λ ₁ <λ ₂ <...). A light source and a collimating lens that converts all of the light beams having different wavelengths into parallel light are provided. According to this configuration of the optical pickup, since there is one collimating lens that converts the light beam emitted from the light source into parallel light, the adjustment of the collimating lens can be simplified.

Further, in the configuration of the optical pickup of the present invention, the collimating lens is arranged such that the light beam emitted from the coherent light source that emits the light beam having the shortest wavelength λ ₁ is positioned on the optical axis of the collimating lens. Is preferably arranged. According to this preferable example, it is possible to realize an optical pickup having a small aberration with respect to light of all wavelengths.

In the configuration of the optical pickup of the present invention, the plurality of coherent light sources are three coherent light sources that respectively emit light beams having different wavelengths (λ ₁ <λ ₂ <λ ₃ ), and the three coherent light sources Are preferably in the ranges of 370 nm <λ ₁ <430 nm, 635 nm <λ ₂ <690 nm, and 760 nm <λ ₃ <810 nm, respectively.

  In the configuration of the optical pickup of the present invention, the variable phase plate is divided into concentric regions and inserted in the optical path between the coherent light source and the optical disc, and the coherent light source emits the light. It is preferable that an objective lens for condensing the light beam of each wavelength on the optical disc is further provided, and that the spherical aberration generated in the objective lens is compensated by the variable phase plate.

The first configuration of the multi-wavelength semiconductor laser unit according to the present invention is mounted on the same submount and emits a plurality of light beams having different wavelengths (λ ₁ <λ ₂ <...). A semiconductor laser chip is provided, and the plurality of semiconductor laser chips are arranged such that their emission end face positions are different in the optical axis direction.

The second configuration of the multi-wavelength semiconductor laser unit according to the present invention is mounted on the same submount and emits light beams having different wavelengths (λ ₁ <λ ₂ <λ ₃ ). The semiconductor laser chip that includes a laser chip and emits the light beam having the shortest wavelength λ ₁ is located at the center of the three semiconductor laser chips.

The optical information recording / reproducing method according to the present invention is mounted on the same submount and emits the light from a plurality of coherent light sources that respectively emit light beams having different wavelengths (λ ₁ <λ ₂ <...). A step of emitting a beam; and a step of converting the light beam into parallel light by a collimating lens, wherein the collimating lens emits a light beam emitted from the coherent light source that emits the light beam having the shortest wavelength λ _1. It arrange | positions so that it may be located on the optical axis of the said collimating lens.

  According to the present invention, compatibility between various types of optical disks can be realized with a simple configuration, and even when a polarizing detection optical system is used, stable signal detection can be performed. An optical pickup, a multi-wavelength semiconductor laser unit, and an optical information recording / reproducing method can be provided.

  Hereinafter, the present invention will be described more specifically using embodiments.

<First Embodiment>
In the present embodiment, an optical pickup that includes a coherent light source that emits light in two wavelength regions of a blue region and a red region and includes a detection optical system that uses a polarizing hologram element and a wavelength plate will be described.

  FIG. 1 is a schematic configuration diagram showing an optical pickup according to the first embodiment of the present invention.

  In the present embodiment, the SHG blue laser unit 1 using the second harmonic generation (SHG) technique is used as a coherent light source in the blue region. FIG. 2 shows the configuration of the SHG blue laser unit 1. The SHG blue laser unit 1 is composed of a near-infrared semiconductor laser and an optical waveguide type wavelength conversion element.

  In the configuration of FIG. 2, a distributed Bragg reflection (DBR) semiconductor laser 11 capable of changing the wavelength is used as a near-infrared semiconductor laser. The DBR semiconductor laser 11 includes an active region and a DBR region, and has independent electrodes. The oscillation wavelength of the DBR semiconductor laser 11 is 810 nm, and the output is 50 mW.

On the other hand, as an optical waveguide type wavelength conversion element, a periodically domain-inverted region 17 and a proton exchange optical waveguide (hereinafter referred to as “optical waveguide”) 16 are formed on a LiNbO ₃ substrate 15 doped with 5 mol% of Mg. A quasi phase matching (QPM) polarization inversion type optical waveguide device 14 is used.

  The laser light emitted from the DBR semiconductor laser 11 is coupled into the optical waveguide 16 by the collimating lens 12 and the focus lens 13, and blue light of 3 mW as output light from the optical waveguide 16 with respect to the output 50 mW of the DBR semiconductor laser 11. Light (wavelength 405 nm) is obtained. The beam divergence angle is 13.2 degrees in the vertical direction and 5.7 degrees in the horizontal direction.

  In the present embodiment, an AlGaInP red semiconductor laser unit 2 is used as a red region coherent light source. The oscillation wavelength of the red semiconductor laser unit 2 is 670 nm, and the operating current at the threshold values of 76 mA and 5 mW is 90 mA.

  Hereinafter, the configuration of the optical pickup according to the present embodiment will be described more specifically with reference to FIG.

  The laser beams emitted from the SHG blue laser unit 1 and the red semiconductor laser unit 2 each having a detection photodetector are converted into parallel beams by the collimating lenses 3 and 4, respectively, and the same light is generated by the dielectric multilayer mirror 5 as optical multiplexing means. Combined to propagate on the axis. As described above, in this embodiment, the dielectric multilayer mirror 5 is used as the optical multiplexing means, but it is also possible to propagate on the same optical axis using a diffraction grating or the like. However, in consideration of light utilization efficiency, it is desirable to use the dielectric multilayer mirror 5. The dielectric multilayer mirror 5 is configured to transmit light having a wavelength of 500 nm or less and reflect light having a wavelength of 500 nm or more for both the P wave and the S wave. The light transmitted and reflected through the dielectric multilayer mirror 5 is transmitted through the polarizing hologram 6 and the wave plate 7 and then bent in the direction perpendicular to the paper surface by the rising mirror 8 (however, in FIG. The light is condensed on the optical disk 10 by the objective lens 9.

The polarizing hologram 6 has a configuration in which a grating is formed on a LiNbO ₃ substrate by proton exchange (δn = 0.04) and etching (depth 100 μm). Here, the pitch of the grating is set to 8 μm (= Λ). Since the refractive index of extraordinary light increases and the refractive index of ordinary light decreases due to proton exchange, the refractive index change of extraordinary light is compensated for by reducing the thickness by etching the proton exchange region, and is sensitive only to ordinary light. A grating is formed. Therefore, the polarization hologram 6 is arranged so that the forward polarization direction is parallel to the abnormal light direction (C-axis direction or crystal main axis). The polarization direction is shown in FIG.

The wave plate 7 is designed so that phase modulation (delay amount) of λ can be obtained with respect to light having a wavelength of 504 nm (3/4 · λ ₂ <λ <5/4 · λ ₁ ). Therefore, the wavelength plate 7 acts as a 5/4 wavelength plate (506 nm) for blue light (wavelength λ ₁ ) and as a 3/4 wavelength plate (502 nm) for red light (wavelength λ ₂ ). To do. Therefore, the blue light and the red light are both converted into circularly polarized light by the wave plate 7 and then condensed on the optical disk 10 by the objective lens 9.

That is, the delay amount lambda wave plate 7, it is substantially an odd multiple of lambda _1/4 with respect to blue light (wavelength lambda _1), also lambda _2/4 substantially an odd multiple for red light (wavelength lambda ₂₎ It is. Accordingly, the wave plate 7 acts as a quarter wave plate for light of either wavelength. The objective lens 9 is designed so as to reduce aberration with respect to blue light and red light.

  Reflected light from the optical disk 10 passes through the objective lens 9 and the rising mirror 8 again, and is converted into linearly polarized light by the wave plate 7. Since the polarization direction of the reflected light is rotated by 90 degrees with respect to the polarization direction of the forward path, the reflected light is incident on the polarization hologram 6 in parallel with the normal light direction, and θ (= Diffraction in the λ / Λ direction. The wave plate 7 reciprocally acts as a 10/4 wave plate for blue light, as a 6/4 wave plate for red light, and both as a half wave plate, so that the polarization direction is 90 degrees. Rotate. The diffraction efficiency of the polarizing hologram 6 was 60% for blue light and 50% for red light.

  The blue light and red light transmitted and reflected through the dielectric multilayer mirror 5 are transmitted through the collimating lenses 3 and 4, respectively, and then collected on the detection photodetector integrated in the laser units 1 and 2. Then, a focus signal, a tracking servo signal, and an RF signal are detected by the detection photodetector. In this case, the focus servo is performed by a spot size detection method (hereinafter referred to as “SSD method”) using a three-division photo detector, and the tracking servo is performed by a phase difference method using a four-division photo detector. In the present embodiment, 50% or more of the reflected light from the optical disk 10 reaches the photodetector, and a stable servo operation and reproduction signal are obtained.

  The wave plate 7 of the present embodiment can act as a quarter wave plate for red light and blue light used in DVDs and HD-DVDs by setting the thickness to a predetermined value. it can. By providing such a wave plate 7, it is possible to stably perform servo operation and detection of a reproduction signal using a detection optical system configured using a polarization hologram 6 which is a polarization separation means. it can. Since the wave plate 7 can be easily manufactured using quartz or the like as in the prior art, its practical effect is great.

  In the present embodiment, the SHG blue laser unit 1 is used as a coherent light source in the blue region, but the same effect can be obtained by using a GaN blue semiconductor laser.

Here, a method for manufacturing a GaN-based blue semiconductor laser will be described. First, n-type GaN is grown on a SiC substrate, an n-type AlGaN / GaN superlattice cladding layer and an n-type GaN light guide layer are formed, an InGaN multiple quantum well active layer is formed, and then a p-type AlGaN A p-type GaN light guide layer, a p-type AlGaN / GaN superlattice clad layer, a p-type GaN layer, and a p-type electrode. Since the SiC substrate is conductive, an n-type electrode is formed on the back surface of the substrate. As a result, 5 mW of blue light (wavelength 405 nm) is obtained for the operating current I _op = 100 mA.

  In the optical pickup of the present embodiment, a blue light source and a red light source are mounted, and each has a polarizing detection optical system, so that a large amount of light is guided to the detection photodetector. Can do. Therefore, by using the optical pickup of the present embodiment, it is possible to stably reproduce a DVD-R with red light and an HD-DVD-R with blue light.

  Next, a configuration in which the detection photodetector is separated from the coherent light source and a polarization separation element (PBS) is used instead of the polarization hologram will be described with reference to FIG.

  In the configuration of FIG. 3, the light emitted from the SHG blue laser 18 and the red semiconductor laser 19 passes through the collimating lenses 20 and 20-1 and the PBSs 21 and 22, respectively, and then is a dielectric multilayer mirror 23 as optical multiplexing means. Are combined so as to propagate on the same optical axis. The combined light beam passes through the wave plate 24 that acts in the same manner as the wave plate 7 used in the configuration of FIG. 1, and is then bent in the direction perpendicular to the paper surface by the rising mirror 25-1 (however, In FIG. 3, the light is focused on the optical disk 26 by the objective lens 25.

  Reflected light from the optical disk 26 passes through the objective lens 25, the rising mirror 25-1, and the wave plate 24 again, and then is transmitted and reflected by the dielectric multilayer mirror 23, and PBSs 21 and 22 arranged in the respective optical paths. Led to. Since the polarization directions of the blue light and the red light are rotated by 90 degrees with respect to the original polarization direction by the wave plate 24, the optical paths are bent by 90 degrees by the PBSs 21 and 22, respectively. After passing through the cylindrical lenses 29, 30, the light is condensed on the photodetectors 31, 32.

  In the configuration as described above, a good reproduction signal was obtained by performing the servo operation in the focus direction by the astigmatism method and performing the servo operation in the tracking direction by the phase difference method.

In the configuration of FIG. 3 as well, the wave plate 24 is designed so that phase modulation (delay amount) of λ can be obtained with respect to light having a wavelength of 504 nm (3/4 · λ ₂ <λ <5/4 · λ ₁ ). Therefore, it acts as a 5/4 wavelength plate (506 nm) for blue light (wavelength λ ₁ ) and as a 3/4 wavelength plate (502 nm) for red light (wavelength λ ₂ ). That is, the delay amount lambda wave plate 24, is substantially an odd multiple of lambda _1/4 with respect to blue light (wavelength lambda _1), a substantially odd even lambda _2/4 with respect to red light (wavelength lambda ₂₎ Is double. Accordingly, the wave plate 24 acts as a quarter wave plate for light of either wavelength. Therefore, even in a detection optical system using a polarization separation element (PBS) as shown in FIG. 3, a sufficient amount of light can be guided onto the detection photodetector, so that stable servo operation and signal detection are possible. Become. In particular, unlike SHG blue lasers, GaN-based and AlGaInP-based semiconductor lasers have a large variation in oscillation wavelength between samples, so that the effect obtained by the configuration of FIG. 3 is great.

  In addition, since the wave plate of the present embodiment is designed to function as a quarter wave plate, the allowable wavelength range is large and the practical effect is great.

  Furthermore, even in an optical pickup using an AlGaInP semiconductor laser having a wavelength of 650 nm and a GaN semiconductor laser having a wavelength of 395 nm, an optical pickup having the same effect as described above can be realized by designing a wavelength plate for a wavelength of 490 nm. Can do.

  As described above, the wave plate of the present embodiment can be easily manufactured using quartz or the like, but can also be manufactured using a material having a large birefringence wavelength dispersion. For example, an aromatic polysulfane-based resin has a birefringence at a blue wavelength (380 nm) of 1.65 times larger than a birefringence at a red wavelength (690 nm). Therefore, when this resin is used, it is possible to produce a wave plate that acts as a quarter-wave plate for light with a wavelength of 690 nm and acts as a quarter-wave plate for light with a wavelength of 380 nm. Hereinafter, a wave plate produced using a material having a large birefringence wavelength dispersion will be described.

Here, the birefringence for light having a wavelength of 690nm in the aromatic polysulfane resins n _a, the thickness of the wavelength plate produced by using an aromatic polysulphanes resin as d. In this case, the birefringence of wavelength 380nm to light becomes 1.65n _a. In this case, since the phase difference given to the light having a wavelength of 690 nm is n _a × d, the phase difference is ¼ of 690 nm, that is, the relationship of n _a × d = 690 × 1/4 is established. Select the thickness d.

If the thickness of the wave plate produced by using an aromatic polysulphanes resin is d, the phase difference given to the wavelength 380nm light 1.65n _a × d, and the above _{n a × d = 690 × 1} / If the relationship of 4 is used, 1.65 × 690 × 1/4 = 285 nm. This phase difference corresponds to 3/4 of 380 nm. Therefore, if an aromatic polysulfane resin is used, linearly polarized light can be converted into circularly polarized light with respect to red (wavelength 690 nm) light, and linearly polarized light can be converted with respect to blue light (wavelength 380 nm). A wave plate that can be converted into circularly polarized light can be produced.

  Here, an aromatic polysulfane-based resin is used as the resin, but a polyester having a unit containing a biphenyl skeleton, a naphthalene skeleton or a stilbene skeleton in a bond forming a main chain as a structural unit (Japanese Patent Laid-Open No. 7-233249) There is no problem as long as it is a material having a large birefringence wavelength dispersion, such as a publication.

  Furthermore, by using a material having a large birefringence, the same as described above can be realized by using blue light having a wavelength of around 400 nm.

When using a wavelength plate produced by using a material having a large wavelength dispersion of birefringence as described above, the delay amount lambda of the wave plate, the wavelength lambda ₁ of lambda _1/4 to light (2n + 3) at double There, the wavelength λ ₂ (> λ ₁₎ of the lambda _2/4 for light of (2n + 1) times (where, n = 0, 1, 2, · · ·) and it is desirable.

<Second Embodiment>
In the present embodiment, an optical pickup using a laser unit in which a plurality of coherent light sources that emit light of different wavelengths are mounted on the same substrate (submount) will be described.

  Specifically, in this embodiment, a GaN-based blue semiconductor laser chip (wavelength 405 nm), an AlGaInP-based red semiconductor laser chip (wavelength 650 nm), and an AlGaAs-based near infrared semiconductor laser chip (wavelength 790 nm) are Si sub-layers. The laser unit mounted on the mount will be described. FIG. 4 is a schematic configuration diagram showing an optical pickup according to the present embodiment, and FIG. 5 is a schematic diagram showing a laser unit used therefor.

  As shown in FIG. 5, a GaN blue semiconductor laser chip 45, an AlGaInP red semiconductor laser chip 46, and an AlGaAs near infrared semiconductor laser chip 47 are mounted on the Si submount 45-4. Here, the blue semiconductor laser chip 45 (wavelength 405 nm) which is the shortest wavelength is mounted in the center, and the red semiconductor laser chip 46 (wavelength 650 nm) and the near infrared semiconductor laser chip 47 (wavelength 790 nm) are blue semiconductors. The laser chip 45 (wavelength 405 nm) is mounted on both sides with a separation of 200 μm.

  The light emitted from each of the laser chips 45 to 47 is reflected by the etched mirror 45-3 to change the direction of the optical path, and then becomes parallel light by the collimating lens 34 having a focal length of 25 mm (see FIG. 4). . In the present embodiment, the collimating lens 34 is disposed so that the light emitted from the blue semiconductor laser chip 45 (wavelength 405 nm) is located at the center of the lens (that is, on the optical axis). The positions of the laser chips 45 to 47 are adjusted to optimum focal positions so that parallel light can be obtained by the collimating lens 34.

  As shown in FIG. 4, the light that has become parallel light by the collimating lens 34 passes through the polarizing hologram 35 and the dielectric multilayer mirror 36, and then is bent in a direction perpendicular to the paper surface by the rising mirror 37 (however, In FIG. 4, it is drawn as if it is bent leftward on the paper surface). Then, after passing through the variable phase plate 44 and the wave plate 38, it is condensed on the optical disk 40 by the objective lens 39. Here, the dielectric multilayer mirror 36 is designed to have a reflectivity of 50% with respect to light having a wavelength of 780 nm and to transmit 100% of light in the blue region and the red region. This operates in the same manner as the wave plate used in the embodiment.

  As the wave plate 38, the same wave plate as the wave plate of the first embodiment designed to obtain λ phase modulation (delay amount) with respect to light having a wavelength of 504 nm is used. The reflected light from the optical disk 40 was diffracted by the polarization hologram 35, and a high value of 50% or more was obtained as the diffraction efficiency.

  A variable phase plate 44 installed between the rising mirror 37 and the wave plate 38 is for correcting spherical aberration generated in the objective lens 39. 6A and 6B show the configuration of the variable phase plate 44. FIG.

As shown in FIG. 6B, the variable phase plate 44 includes a liquid crystal 48-3, an alignment film (polyimide-based) 48-2 and an ITO (InSnO _x ) electrode 48- disposed so as to sandwich the liquid crystal 48-3 from both sides. 1 and a glass substrate 49. The thickness of the liquid crystal 48-3 is 2 μm, and the thickness of each alignment film 48-2 is 80 nm. The alignment film 48-2 is rubbed so that the long axis (arrow) of the liquid crystal 48-3 is aligned in the direction of the C axis shown in FIG.

  As the liquid crystal 48-3, a liquid crystal having positive dielectric anisotropy (for example, ZLI-4792 manufactured by Merck) is used. The liquid crystal 48-3 is gradually inclined in the Z-axis direction by applying a voltage (˜60 Hz), and the refractive index is lowered with respect to the polarization direction of the C-axis. Therefore, by adjusting the applied voltage, it is possible to give the liquid crystal 48-3 a refractive index change in the C-axis direction, that is, a phase change. FIG. 6A shows the state of the liquid crystal 48-3 when the applied voltage is 0V.

  The ITO electrode 48-1 is divided into concentric regions every radius of 500 μm. However, in FIG. 6A, only four ITO electrodes 48-1 are depicted conceptually.

  The objective lens 39 is an aspherical lens designed to minimize aberration with respect to blue light. FIG. 7A shows the distribution of spherical aberration that occurs when red light is incident on the objective lens 39. In this case, the spherical aberration shown in FIG. 7A can be compensated by applying a voltage having a distribution as shown in FIG. 7B to the variable phase plate 44.

Specifically, when the objective lens 39 is an aspherical lens having an effective beam diameter of 4 mm, spherical aberration of φ ₁ = 100 mλ at maximum occurs with respect to red light. In this case, by applying a voltage of V ₁ = 5V, a refractive index distribution as shown in FIG. 7C can be generated, and the spherical aberration of the aspherical lens (objective lens 39) for red light is compensated. We were able to. As a result, the linearly polarized red light incident in the C-axis direction of the variable phase plate 44 is transmitted through the wave plate 38 and then condensed by the objective lens 39 onto the optical disk 40 having a substrate thickness of 0.6 mm. Condensing characteristics were obtained. The same applies to near-infrared light, and good condensing characteristics can be obtained by applying a voltage that compensates for spherical aberration generated in the objective lens 39.

  In the present embodiment, the case where spherical aberration caused by chromatic aberration is compensated is described as an example. However, spherical aberration caused by an error in substrate thickness can also be compensated. In particular, for blue light, since the spherical aberration generated by the error in the thickness of the base material becomes large, the practical effect is great.

  In the present embodiment, a liquid crystal having a positive dielectric anisotropy is used as the liquid crystal material, but a liquid crystal having a negative dielectric anisotropy (for example, MLC-6608 manufactured by Merck) is used. The same effect can be obtained. In the case where a liquid crystal having a negative dielectric anisotropy is used, it is desirable to perform the alignment treatment so that the major axis of the liquid crystal is slightly inclined from the Z-axis direction to the C-axis direction.

  As shown in FIGS. 4 and 5, the reflected light from the optical disk 40 is guided to a detection photo detector 43 and detection photo detectors 45-1 and 45-2 integrated in the laser unit 33, respectively.

  Near-infrared light having a wavelength of 790 nm is reflected by the dielectric multilayer mirror 36, passes through the detection lens 41 and the cylindrical lens 42, and then is guided to the four-divided photodetector 43. In this case, the focus servo is performed by the astigmatism method, and the tracking servo is performed by the push-pull method.

  On the other hand, the blue light having a wavelength of 405 nm and the red light having a wavelength of 650 nm are diffracted by the polarization hologram 35 and transmitted through the collimating lens 34, and then divided into three-divided photo detectors 45-1 and 6-divided photo detectors provided on the laser unit 33. 45-2. In this case, the focus servo is performed by the SSD method using the three-divided photo detector 45-1, and the tracking servo is performed by the phase difference method using the six-divided photo detector 45-2. In particular, as shown in FIG. 5, the photodetector 45 is configured such that blue light is detected by the upper four photodetectors of the six-divided photodetector 45-2 and red light is detected by the lower four photodetectors. -2 can be reduced from 8 to 6.

  4 and 5, the blue semiconductor laser chip 45 having the shortest wavelength is adjusted to the center of the collimating lens 34 (that is, on the submount 45-4, corresponding to the optical axis center of the collimating lens 34). In addition, since the aberration of the objective lens 39 can be corrected by the liquid crystal lens (variable phase plate 44), the total transmitted wavefront aberration is reduced to a small value of 50 mλ or less for light of all wavelengths. Can be suppressed. Thereby, good reproduction characteristics can be obtained.

  In the optical pickup of this embodiment, each light source of blue light, red light, and near-infrared light is mounted and each has a detection optical system. Therefore, the HD-DVD-R is converted into red by blue light. DVD-R can be stably reproduced by light, and CD-R can be stably reproduced by near-infrared light.

  4 and 5, a laser unit in which light sources of three wavelengths (GaN-based blue semiconductor laser chip 45, AlGaInP-based red semiconductor laser chip 46, and AlGaAs-based near-infrared semiconductor laser chip 47) are mounted. 33 has been described as an example, but the same effect can be obtained even when a laser unit in which a semiconductor laser chip having two wavelengths, a GaN blue semiconductor laser chip and an AlGaInP red semiconductor laser chip, is mounted. can get. FIG. 8 shows a schematic configuration diagram in that case.

  In the configuration of FIG. 8, the laser unit 50 is mounted with two semiconductor laser chips, a GaN-based blue semiconductor laser chip and an AlGaInP-based red semiconductor laser chip. A six-divided photodetector is mounted (however, these are not shown in FIG. 8). The collimating lens 51 is arranged so that light emitted from a GaN blue semiconductor laser chip having a shorter wavelength is located at the center of the lens (that is, on the optical axis). The light after passing through the polarization hologram 52 is bent in a direction perpendicular to the paper surface by the rising mirror 53 (however, in FIG. 8, it is depicted as being bent in the left direction of the paper surface). After passing through the wave plate 54 that acts in the same manner as the wave plate used in the first embodiment, it is condensed on the optical disk 56 by the objective lens 55.

  The reflected light from the optical disk 56 passes through the objective lens 55, the wave plate 54, and the rising mirror 53 again, is diffracted by the polarizing hologram 52, and is guided to the detection photodetector integrated in the laser unit 50. Servo operation and RF signal detection are performed.

  Also in the configuration of FIG. 8, the blue semiconductor laser chip having the shortest wavelength is adjusted to the center of the collimating lens 51 (that is, mounted on the submount at a position corresponding to the center of the optical axis of the collimating lens 51). ) As a result, the total transmitted wavefront aberration can be suppressed to a small value of 50 mλ or less with respect to the light of all wavelengths, so that excellent reproduction characteristics can be obtained. Also, the optical pickup is equipped with a blue light source and a red light source, and each has a detection optical system, so that the HD-DVD-R is stable with blue light and the DVD-R is stable with red light. Can be played.

  If a plurality of semiconductor laser chips are mounted on the same submount and converted into parallel light by one collimating lens as in this embodiment, a small optical pickup can be realized, and an optical disc The thickness of the drive can be significantly reduced. Also, a small SHG blue laser by direct coupling has been proposed, and the same effect can be obtained even if an SHG blue laser is mounted instead of a GaN-based semiconductor laser.

  In addition, when an aspheric lens is used as the objective lens, spherical light is generated due to chromatic aberration when light of different wavelengths is incident. However, a variable phase composed of liquid crystal divided into concentric areas as in this embodiment. By using the plate, spherical aberration generated in the objective lens can be compensated. In particular, in an optical pickup equipped with a multiwavelength coherent light source, its practical effect is great.

<Third Embodiment>
In the present embodiment, an optical pickup capable of combining light emitted from a plurality of coherent light sources with a dielectric multilayer mirror as an optical combining means and condensing it on an optical disk with one objective lens will be described. To do. Although light transmitted and reflected by a plurality of optical components is affected by the aberration of the optical components, the configuration of the present embodiment is effective in solving such problems.

  FIG. 9 shows a schematic configuration diagram of the optical pickup in the present embodiment. As shown in FIG. 8, the optical pickup of the present embodiment includes a GaN blue semiconductor laser unit 57 (wavelength 405 nm), an AlGaInP red semiconductor laser unit 58 (wavelength 650 nm), each of which is integrated with a photodetector for detection. And an AlGaAs near-infrared semiconductor laser unit 59 (wavelength 790 nm). And the light radiate | emitted from each of these laser units 57-59 is multiplexed by the two dielectric multilayer mirrors M1 and M2 as an optical multiplexing means.

  The light emitted from each of the semiconductor laser units 57 to 59 is converted into parallel light by the collimating lenses 60 to 62, and then passes through the polarizing holograms 63 to 65 and the wave plates 66 to 68. The wave plates 66 to 68 are designed so that phase modulation (delay amount) of λ / 4 can be obtained with respect to light of each wavelength, and the light of each wavelength is circularly polarized by the wave plates 66 to 68. Is converted to

  10A and 10B show the transmission characteristics of the dielectric multilayer mirrors M1 and M2. As shown in FIG. 10 (a), the dielectric multilayer mirror M1 transmits 95% or more of light having a wavelength of 500 nm or less for both P wave and S wave, and 95% of light having a wavelength of 500 nm or more for both P wave and S wave. Reflected above. On the other hand, as shown in FIG. 10B, the dielectric multilayer mirror M2 transmits 95% or more of light with a wavelength of 700 nm or less for both P wave and S wave, and transmits light with a wavelength of 700 nm or more for both P wave and S wave. Reflects 95% or more.

These dielectric multilayer mirrors M1 and M2 are composed of multilayer films (total of 20 layers or more) of SiO ₂ and TiO ₂ . Of the two dielectric multilayer mirrors M1 and M2, the dielectric multilayer mirror M2 is configured such that the wavelength difference between the rise of the P wave and the S wave is 50 nm or less by controlling the film thickness with high accuracy. Have been made.

  Blue light with a wavelength of 405 nm emitted from the blue semiconductor laser unit 57 is transmitted through the dielectric multilayer mirrors M1 and M2. The red light having a wavelength of 650 nm emitted from the red semiconductor laser unit 58 is reflected by the dielectric multilayer mirror M1, and then passes through the dielectric multilayer mirror M2. Further, near infrared light having a wavelength of 790 nm emitted from the near infrared semiconductor laser unit 59 is reflected by the dielectric multilayer mirror M2. Thereby, the three lights are multiplexed so as to propagate on the same optical axis. The combined light is bent in a direction perpendicular to the paper surface by the rising mirror 69 (however, in FIG. 9, it is drawn as if it is bent above the paper surface), and the optical disk 71 by the objective lens 70. Focused on top. In the configuration of FIG. 8, a combined lens having a numerical aperture (NA) of 0.6 is used as the objective lens 70 in order to reduce the aberration with respect to light of three wavelengths.

  Reflected light from the optical disc 71 passes through the same optical path as the forward path, but the polarization direction is rotated 90 degrees with respect to the forward path by the wave plates 66-68. Thereafter, the reflected light from the optical disc 71 is diffracted by the polarization holograms 63 to 65 and guided to the respective detection photodetectors integrated in the laser units 57 to 59. Similar to the second embodiment, the photodetector is composed of a three-divided photodetector and a six-divided photodetector. In this case, the focus servo is performed by an SSD method using a three-divided photo detector, and the tracking servo is performed by a phase difference method using a six-divided photo detector.

  In general, an optical component has smaller transmitted wavefront aberration than reflected wavefront aberration. When light passes through the optical component, aberrations cancel out on the front surface and the back surface, so that only the aberration that occurs in the surface state needs to be considered. On the other hand, for light reflected by the optical component, distortion of the optical component directly affects the wavefront aberration. Therefore, it is desirable to use the optical component as a transmissive component because aberrations are reduced. In addition, optical components having the same wavefront aberration affect the light having a shorter wavelength as a larger aberration.

  Considering the above points, in the optical pickup of the present embodiment shown in FIG. 9, light emitted from the GaN-based blue semiconductor laser unit 57 having the shortest wavelength is transmitted through the dielectric multilayer mirrors M1 and M2. It is configured as follows.

  In the configuration of FIG. 9, the transmitted wavefront aberration after passing through the objective lens with respect to each wavelength of light can be suppressed to 50 mλ or less, and a good result in reproduction characteristics can be obtained. Further, in the optical pickup having the configuration shown in FIG. 9, each light source of blue light, red light and near infrared light is mounted and each has a detection optical system. DVD-R can be reproduced stably by red light, and CD-R can be reproduced stably by near-infrared light.

  FIG. 11 shows a schematic configuration diagram of another optical pickup according to the present embodiment. In the optical pickup shown in FIG. 11, a dielectric multilayer mirror M3 having different transmission characteristics for the P wave and the S wave is used instead of the dielectric multilayer mirror M2.

  In order to obtain the same transmission characteristics for both the S wave and the P wave in the dielectric multilayer mirror, it is necessary to precisely control the deposition. In consideration of the yield at the time of mass production, the configuration including the dielectric multilayer mirrors M1 and M3 having the transmission characteristics as shown in FIGS. 12A and 12B is more advantageous. Specifically, as shown in FIG. 12B, the dielectric multilayer mirror M3 has a P-wave transmittance of 95% or more and a S-wave reflectance of 95% or more with respect to light having a wavelength of 790 nm. Designed to be On the other hand, as shown in FIG. 12A, the dielectric multilayer mirror M1 transmits 95% or more of light with a wavelength of 500 nm or less for both the P wave and the S wave, as shown in FIG. Reflects light having a wavelength of 500 nm or more for both P wave and S wave by 95% or more.

In the configuration of FIG. 11, the light emitted from the GaN blue semiconductor laser unit 72 (wavelength 405 nm) and the AlGaInP red semiconductor laser unit 73 (wavelength 650 nm) is converted into parallel light by the collimating lenses 75 and 76, respectively. The signals are combined so as to propagate on the same optical axis by the dielectric multilayer mirror M1. The two combined lights are transmitted through the polarization hologram 78 and the wave plate 80, and each light is converted into circularly polarized light. Here, the wave plate 80 operates in the same manner as the wave plate used in the first embodiment. That is, the delay amount lambda wave plate 80, is substantially an odd multiple of lambda _1/4 with respect to blue light (wavelength lambda _1), a substantially odd even lambda _2/4 with respect to red light (wavelength lambda ₂₎ Is double. Therefore, the wave plate 80 acts as a quarter wave plate for light of either wavelength.

  On the other hand, near-infrared light having a wavelength of 790 nm emitted from the AlGaAs-based near-infrared semiconductor laser unit 74 is converted into parallel light by the collimator lens 77 and transmitted through the non-polarizing glass hologram 79, and then the dielectric multilayer film mirror M3. Reflect on. As a result, the three lights are finally combined to propagate on the same optical axis.

  The combined three lights are bent in a direction perpendicular to the paper surface by the rising mirror 81 (however, in FIG. 11, they are drawn to be bent above the paper surface), and by the objective lens 82. It is condensed on the optical disk 83. In the configuration of FIG. 11 as well, a combined lens having a numerical aperture (NA) of 0.6 is used as the objective lens 82 in order to reduce aberration with respect to light of three wavelengths.

  The reflected light from the optical disk 83 is guided to the respective detection photodetectors integrated in the laser units 72 to 74 through the same optical path as the forward path.

  That is, the blue light (wavelength 405 nm) and the red light (wavelength 650 nm) are diffracted by the polarization hologram 78 after the polarization direction is rotated by 90 degrees with respect to the original polarization direction by the wave plate 80, and the laser unit. 72 and 73 are guided to the respective photodetectors for detection.

  On the other hand, for near infrared light (wavelength 790 nm), the transmission characteristics of the dielectric multilayer mirror M3 are different between the P wave and the S wave, so that the detection optics provided corresponding to the AlGaAs near infrared semiconductor laser unit 74 is provided. A non-polarizing glass hologram 79 is used as an optical component constituting the system. The near-infrared light (wavelength 790 nm) reflected by the optical disk 83 is reflected by the dielectric multilayer mirror M3, passes through the glass hologram 79 and the collimator lens 77, and is then integrated in the laser unit 74. Led up. Compared with the configuration of FIG. 9, a glass hologram 79 is used, and the amount of near-infrared light guided onto the detection photodetector is reduced, so that it is necessary to adjust the gain of the servo system, etc. Such deterioration was not observed.

  Also in the configuration of FIG. 11, the light emitted from the GaN-based blue semiconductor laser unit 72 having the shortest wavelength is configured to pass through the dielectric multilayer mirrors M1 and M3. The transmitted wavefront aberration after passing through the lens can be suppressed to 50 mλ or less, and good results can be obtained in the reproduction characteristics. Further, in the optical pickup having the configuration shown in FIG. 11, each light source of blue light, red light and near infrared light is mounted and each has a detection optical system. DVD-R can be reproduced stably by red light, and CD-R can be reproduced stably by near-infrared light.

  In the present embodiment, a GaN-based semiconductor laser is used as the blue semiconductor laser. However, the same effect can be obtained by using an SHG blue laser instead of the GaN-based semiconductor laser.

  In the present embodiment, a combined lens is used as an objective lens in order to reduce the aberration with respect to light of three wavelengths. However, as in the second embodiment, as an objective lens, The same effect can be obtained even when the aspheric lens is used and the aberration of the aspheric lens (objective lens) is corrected by the liquid crystal lens (variable phase plate).

<Fourth embodiment>
In this embodiment, an optical pickup and a light that have a plurality of coherent light sources and a detection optical system using a polarization hologram and can perform stable servo operation and reproduction signal detection with a single wavelength plate. An information recording / reproducing apparatus will be described.

  In order to realize such an optical pickup and an optical information recording / reproducing apparatus, in this embodiment, a phase variable wave plate made of a liquid crystal material is used. As a coherent light source, a GaN blue semiconductor laser (wavelength 405 nm), an AlGaInP red semiconductor laser (wavelength 650 nm), and an AlGaAs near infrared semiconductor laser (wavelength 790 nm) are used.

  FIG. 13A is a schematic configuration diagram showing the optical pickup according to the present embodiment, and FIG. 13B is a polarization hologram or a birefringent crystal main axis of an optical disc and a polarization of a coherent light source in the configuration of FIG. It is a figure which shows the relationship with a direction. Also in this configuration, the dielectric multilayer mirrors M1 and M2 similar to the configuration shown in FIG. 9 are used, and these dielectric multilayer mirrors M1 and M2 have the same transmission characteristics for both the P wave and the S wave at each wavelength. Indicates.

  In the configuration of FIG. 13, the light emitted from the GaN-based blue semiconductor laser unit 84 (wavelength 405 nm) and the AlGaInP-based red semiconductor laser unit 85 (wavelength 650 nm) is converted into parallel light by the collimator lenses 87 and 88, respectively. The signals are combined so as to propagate on the same optical axis by the dielectric multilayer mirror M1. On the other hand, near-infrared light having a wavelength of 790 nm emitted from the AlGaAs-based near-infrared semiconductor laser unit 86 is collimated by the collimator lens 89 and then reflected by the dielectric multilayer mirror M2. Thereby, the three lights are multiplexed so as to propagate on the same optical axis. The combined three light beams are transmitted through the polarization hologram 90 and then bent in a direction perpendicular to the paper surface by the rising mirror 91 (however, in FIG. 13A, they are bent above the paper surface). Is drawn). Then, after passing through the phase variable wave plate R 1, it is condensed on the optical disk 94 by the objective lens 93. In addition, as the objective lens 93, a combined lens is used.

  In this embodiment, since the transmission characteristics of the dielectric multilayer mirrors M1 and M2 are the same for the P wave and the S wave, the detection optical system uses the polarization hologram 90 for light of all wavelengths. It is configured. Therefore, it is necessary to adjust the phase variable wave plate R1 so as to act as a quarter wave plate for light of all wavelengths. Therefore, in the present embodiment, a phase variable wavelength plate made of a liquid crystal material as shown in FIGS. 14A to 14C is used as the phase variable wavelength plate R1.

Here, the configuration of the phase variable wave plate R1 will be described. As shown in FIG. 14B, the phase-variable wave plate R1 is arranged so as to sandwich the liquid crystal 98 and the liquid crystal 98 from both sides, similarly to the variable phase plate 44 described in the second embodiment. An alignment film (polyimide) 97, an ITO (InSnO _x ) electrode 96, and a glass substrate 95 are included. The thickness of the liquid crystal 98 is 3 μm, and the thickness of the alignment film 97 is 80 nm. The alignment film 97 is rubbed so that the long axis (arrow) of the liquid crystal 98 is aligned in the direction of the C axis shown in FIG.

  As the liquid crystal 98, a liquid crystal having a positive dielectric anisotropy (for example, ZLI-4792 manufactured by Merck) is used. By applying a voltage (˜60 Hz), the liquid crystal 98 is gradually tilted in the Z-axis direction, and the refractive index is lowered with respect to the polarization direction of the C-axis. Therefore, by adjusting the applied voltage, it is possible to give the liquid crystal 98 a refractive index change in the C-axis direction, that is, a phase change. FIG. 14B shows the inclination of the liquid crystal 98 when the applied voltage V = 0, and FIG. 14C shows the inclination of the liquid crystal 98 when the applied voltage V = V1.

  The phase modulation φ (rad) due to the tilt of the liquid crystal 98 is approximately φ (rad) ˜2π × Δn where Δn is the birefringence amount of the liquid crystal, d is the thickness of the liquid crystal, and θ is the tilt of the liquid crystal. Xd × (cos 2θ) / λ. Therefore, when θ = 0 (applied voltage V = 0) with respect to λ = 780 nm and Δn = 0.1, 0.385λ is obtained as the phase modulation. Therefore, by applying a voltage of 1.5 V and tilting the liquid crystal 98 to have an angle θ as shown in FIG. 13C, the phase modulation is 0.25λ (with respect to light having a wavelength λ = 780 nm. The phase variable wave plate R1 was adjusted so that λ / 4).

  As described above, the variable phase type wave plate R1 used in the present embodiment can arbitrarily change the phase modulation by changing the applied voltage. Therefore, even for red light and blue light, λ / 4 phase modulations can be obtained. Specifically, in the present embodiment, by applying a voltage of 2V for red light and 5V for blue light to the phase variable wave plate R1, the phase variable wave plate R1 is It can act as a quarter wavelength plate for light of each wavelength.

  The phase variable wave plate R1 is installed with its crystal main axis (C axis: extraordinary light direction) inclined by 45 degrees with respect to the polarization direction of the combined light.

  The reflected light from the optical disk 94 is rotated by 90 degrees with respect to the original polarization direction by the phase variable wave plate R1, diffracted by the polarization hologram 90, and then each of the semiconductor laser units 84 to 86. The light is guided onto a photo detector for detection integrated in the sensor.

  Since the refractive index in the C-axis direction is changed by adjusting the applied voltage, the phase variable wave plate R1 can change the obtained phase modulation. Further, since the polarization direction of the combined light is inclined by 45 degrees with respect to the crystal main axis (C axis) of the phase variable wave plate R1, it is λ with respect to the wavelength λ of the light collected on the optical disc 94. By adjusting the voltage applied to the phase variable wave plate R1 so that / 4 phase modulation can be obtained, each combined light can be converted into circularly polarized light. The reflected light from the optical disk 94 is further converted to linearly polarized light whose polarization direction is rotated by 90 degrees with respect to the forward path because phase modulation of λ / 4 is obtained. By providing such a phase-variable wave plate R1, even in an optical pickup having a light source of a plurality of wavelengths, a servo can be stably servoed using a detection optical system configured using polarization separation means. Since the operation and the reproduction signal can be detected, the practical effect is great.

  In the configuration of FIG. 14, a liquid crystal material is used alone, but a more practical device can be obtained by using a phase variable wave plate R1b composed of two liquid crystal materials having different refractive index wavelength dispersion relationships. Can be realized. The configuration will be described below with reference to FIGS. 15 (a) and 15 (b).

Specifically, the phase variable wavelength plate R1b is composed of a liquid crystal 124 including a tolan series and a liquid crystal 123 having a phenylcyclohexane series as a main component. As shown in FIGS. 15A and 15B, the respective orientation directions (shown by arrows) are orthogonal to each other (when applied voltage V = 0). Note that the liquid crystal 124 including a tolan series and the liquid crystal 123 having a phenylcyclohexane series as a main component are sandwiched between an alignment film (polyimide series) 122, an ITO (InSnO _x ) electrode 121, and a glass substrate 120, respectively.

  FIGS. 16A and 16B show the relationship between the wavelength and the amount of phase modulation for each of the liquid crystal 124 including a tolan group and the liquid crystal 123 having a phenylcyclohexane group as a main component. As shown in FIG. 16A, the liquid crystal 124 including a tolan series is a material having a large wavelength dispersion. On the other hand, as shown in FIG. 16B, the liquid crystal 123 mainly composed of phenylcyclohexane is a material having a small wavelength dispersion. Note that the thickness of the liquid crystal 123 mainly composed of phenylcyclohexane in FIG. 16B is twice the thickness of the liquid crystal 124 including the tolan system in FIG.

  By configuring the slow optical axes (C-axis) of the two liquid crystals 123 and 124 to be orthogonal, the C-axis direction in which birefringence occurs is also an orthogonal direction. As a result, the phase modulation amount obtained by the combination of the two liquid crystals 123 and 124 is a difference in birefringence amount generated in both the liquid crystals 123 and 124, respectively. Accordingly, the wavelength dispersion characteristic shown in FIG. 16C can be obtained by setting the thickness of the liquid crystal 123 mainly composed of phenylcyclohexane to twice the thickness of the liquid crystal 124 including the tolan system. This corresponds to the characteristic obtained by subtracting the characteristic shown in FIG. 16A from the characteristic shown in FIG.

  In this embodiment, by setting the thickness of the liquid crystal 124 including a tolan series to 2 μm and the thickness of the liquid crystal 123 having a phenylcyclohexane series as a main component to 4 μm, a phase difference of 200 nm with respect to light having a wavelength of 780 nm, A phase difference of 160 nm can be obtained for light having a wavelength of 650 nm, and a phase difference of 90 nm can be obtained for light having a wavelength of 405 nm, and can function as a quarter-wave plate for light of each wavelength. A phase-variable wave plate R1b could be realized. In addition, since the characteristics can be finely adjusted by adjusting the voltage applied to the liquid crystal 123 having phenylcyclohexane as a main component, even when the environmental temperature changes, the quarter wavelength is stably obtained. Can act as a plate.

  The phase variable wavelength plate R1b in FIG. 15 is composed of only a liquid crystal material, but as shown in FIGS. 17A and 17B, a combination of a liquid crystal and a film having different refractive index wavelength dispersion relations. A more practical optical pickup can be realized by using the phase variable type wave plate R1c constituted by the combination.

This phase variable wave plate R1c is configured by combining a liquid crystal 232 and a polyvinyl alcohol film 133. The liquid crystal 232 is sandwiched between an alignment film (polyimide type) 134, an ITO (InSnO _x ) electrode 135, and a glass substrate 136. The polyvinyl alcohol film 133 is formed on the glass substrate 136.

  As the liquid crystal 232, a material having a large wavelength dispersion (for example, a liquid crystal containing a tolan series) is used. On the other hand, the polyvinyl alcohol film 133 is a material having a small wavelength dispersion. The slow axis (C axis) of the liquid crystal 232 and the polyvinyl alcohol film 133 are orthogonal to each other as shown by arrows in FIGS. 17A and 17B (when applied voltage V = 0). ), The C-axis direction in which birefringence occurs is also an orthogonal direction. Therefore, also in the phase variable wavelength plate R1c, the difference in birefringence generated in the liquid crystal 232 and the polyvinyl alcohol film 133 is the actual phase modulation amount.

  Specifically, when the thickness of the liquid crystal 232 was set to 2 μm, a phase modulation amount of 200 nm was obtained for light with a wavelength of 780 nm, and a wavelength dispersion relationship as shown in FIG. On the other hand, when the thickness of the polyvinyl alcohol film 133 was set to 100 μm, a phase change amount of 400 nm was obtained with respect to light having a wavelength of 780 nm, and a wavelength dispersion relationship as shown in FIG. The resulting phase modulation amount has a chromatic dispersion characteristic as shown in FIG. As a result, a phase difference of 200 nm can be obtained for light having a wavelength of 780 nm, a phase difference of 160 nm can be obtained for light having a wavelength of 650 nm, and a phase difference of 90 nm can be obtained for light having a wavelength of 405 nm. In contrast, it was possible to realize a phase variable wavelength plate R1c that can function as a quarter wavelength plate.

  In the phase-variable wavelength plate R1c, the characteristics can be finely adjusted by adjusting the voltage applied to the liquid crystal 232. Therefore, even when the environmental temperature changes, the quarter wavelength is stably obtained. Can act as a plate.

  In a conventional optical pickup provided with coherent light sources having different wavelengths, when one of the detection optical systems is configured using polarization separation means, the quarter-wave plate must be switched for each wavelength of light. Stable servo operation and signal detection cannot be performed. On the other hand, if the phase modulation type wave plate made of a liquid crystal material is inserted into the optical pickup and the phase modulation amount is controlled by adjusting the applied voltage as in the present embodiment, one The phase variable wave plate can act as a quarter wave plate for light of all wavelengths, so that signals can be detected stably and compatibility with various media can be realized. Can do. Therefore, the practical effect is great. In addition, a combination with a liquid crystal or a film having a different refractive index wavelength dispersion relationship can reduce the voltage applied to the phase-variable wavelength plate, thereby realizing a more practical device.

<Fifth embodiment>
In the fourth embodiment, the optical pickup and the optical information recording / reproducing apparatus using the phase variable wave plate R1, R1b, or R1c acting as a quarter wave plate for light of a plurality of wavelengths have been described. . In the present embodiment, an optical pickup and an optical information recording / reproducing apparatus that use a phase variable wavelength plate for correcting the amount of birefringence generated in an optical disk will be described.

  Polycarbonate used as a substrate for optical discs has a refractive index anisotropy during molding and birefringence occurs. FIG. 18 shows an example of birefringence with respect to a wavelength of 632.8 nm of the polycarbonate substrate. From FIG. 18, it can be seen that a large birefringence is exhibited in the inner circumferential direction of the optical disc, and a phase difference of about 100 nm (a phase difference of about ¼ wavelength with respect to light having a wavelength of 405 nm) occurs in the reciprocating path.

  In a configuration using a polarization hologram or PBS (polarization separation element) in the detection optical system, a quarter-wave plate exists between the polarization hologram or PBS (polarization separation element) and the optical disk. As described above, when birefringence occurs in the optical disc, the amount of light guided onto the detection photodetector changes. On the other hand, in the present embodiment, the amount of light guided onto the detection photodetector can be kept constant by using the phase variable wavelength plate.

  FIG. 19A is a schematic configuration diagram showing an optical pickup according to the present embodiment, and FIG. 19B is a polarization hologram or a birefringent crystal main axis of an optical disk and a polarization of a coherent light source in the configuration of FIG. It is a figure which shows the relationship with a direction.

  In the configuration of FIG. 19, the light emitted from the GaN-based blue semiconductor laser unit 110 (wavelength 405 nm) and the AlGaInP-based red semiconductor laser unit 111 (wavelength 650 nm) is converted into parallel light by the collimating lenses 113 and 114, respectively. The signals are combined so as to propagate on the same optical axis by the dielectric multilayer mirror M1. On the other hand, near-infrared light having a wavelength of 790 nm emitted from the AlGaAs-based near-infrared semiconductor laser unit 112 is collimated by the collimator lens 115 and then reflected by the dielectric multilayer mirror M2. Thereby, the three lights are multiplexed so as to propagate on the same optical axis. The combined three light beams are transmitted through the polarization hologram 116 and then bent in a direction perpendicular to the paper surface by the rising mirror 117 (however, in FIG. 19A, they are bent above the paper surface). Is drawn). Then, after passing through the phase variable wave plates R 1 and R 2, the light is condensed on the optical disk 119 by the objective lens 118.

  The phase-variable wave plate R1 is installed in a state where its C-axis direction is inclined 45 degrees with respect to the polarization direction of the combined light, and acts as a quarter-wave plate for light of all wavelengths. Has been adjusted to. The reflected light from the optical disk 119 is rotated by 90 degrees with respect to the original polarization direction by the phase variable wave plate R1 and diffracted by the polarization hologram 116, and then each of the semiconductor laser units 110 to 112. The light is guided onto a photo detector for detection integrated in the sensor.

  In the configuration shown in FIGS. 19A and 19B, the phase variable wavelength plate R2 is installed so that the C-axis direction thereof is parallel to the radial direction of the optical disk 119. By using this, the amount of birefringence generated in the optical disk 119 can be corrected. With respect to the amount of birefringence generated in the optical disk 119, a crystal axis is formed in the radial direction of the optical disk 119 and in a direction perpendicular thereto, so that the crystal axis and the crystal axis of the phase variable wavelength plate R2 are positioned in parallel. The amount of birefringence generated in the optical disk 119 can be corrected. As a result, even if the amount of birefringence generated in the optical disk 119 is large, the polarization direction of the reflected light can be rotated by 90 degrees with respect to the forward path by the phase variable wave plate R2, so that the laser units 110 to 112 are integrated. The amount of light guided onto the detected detector for detection can be kept constant. In other words, the phase variable wave plate R2 acts as a quarter wave plate with respect to the wavelength of the incident light, which is the sum of the delay amounts generated by the phase wavelength variable wave plate R1 and the optical disk 119. The phase difference is adjusted so that the amount of light guided onto the detection photodetector is maximized.

  For example, when reproducing using blue light, the voltage applied to the phase variable wave plate R1 is adjusted so that the phase variable wave plate R1 acts as a quarter wave plate for light having a wavelength of 405 nm. The applied voltage to the phase variable wavelength plate R2 is adjusted so that the amount of light guided onto the detection photodetector is constant. Thereby, even if the amount of birefringence varies between the optical disks 119, the servo operation and reproduction can be performed stably.

  In particular, in the blue region, the amount of birefringence increases due to the wavelength dispersion relationship. Further, since the wavelength is shortened, the amount of phase modulation is further increased. Therefore, as in the present embodiment, the practical effect of the optical pickup on which the phase variable wavelength plate R2 for compensating the birefringence amount generated in the optical disk 119 is mounted is great. In addition, since the amount of phase modulation caused by the birefringence of the optical disk 119 varies depending on the wavelength, the effect is particularly great in an optical pickup including coherent light sources having different wavelengths.

<Sixth embodiment>
FIG. 20A is a schematic configuration diagram showing an optical pickup in the present embodiment in which phase variable wavelength plates are combined into one, and FIG. 20B is a diagram of a polarization hologram or an optical disc in the configuration of FIG. It is a figure which shows the relationship between the birefringent crystal principal axis and the polarization direction of a coherent light source.

  In this configuration, as shown in FIG. 20B, the optical pickup and the optical disc 109 are installed in a state where they are inclined by 45 degrees. For this reason, the crystal principal axis (C axis) of the phase variable wavelength plate R3 and the radial direction of the substrate of the optical disk 109 are parallel to each other, so that one phase variable wavelength plate R3 is used as a quarter wavelength plate. The function and the function of compensating the amount of birefringence generated in the optical disc 109 can be combined. By providing such a phase variable wave plate R3, even in an optical pickup having a coherent light source having a different wavelength, the detection optical system configured using the polarization separation means can be used for stable servo control. Operation and reproduction signal detection can be performed. In addition, since the birefringence of the optical disk can be compensated, and the optical pickup can be simplified and the power consumption can be reduced, the practical effect is great. The same effect can be obtained even if the crystal principal axis (C axis) of the phase variable wave plate R3 and the radial direction of the substrate of the optical disk 109 are perpendicular to each other.

  When the optical disk 109 has birefringence, the total of the phase modulation of the phase variable wave plate R3 and the phase modulation of the optical disk 109 is the total phase modulation. For this reason, when the phase modulation of the optical disk 109 is as large as λ / 4, a total phase difference of λ / 2 occurs. This is a phase difference of λ in the reciprocation, so that when the polarizing detection optical system is used, the amount of light guided onto the detection photodetector becomes zero. On the other hand, in the present embodiment, one phase variable wavelength plate R3 has both a function as a quarter wavelength plate and a function for compensating the birefringence amount generated in the optical disk 109. Therefore, even when a polarizing detection optical system is used, the amount of light guided onto the detection photodetector does not become zero.

  Also in the configuration of the present embodiment, dielectric multilayer mirrors M1 and M2 are used as in FIGS. 9, 13, and 19, and these dielectric multilayer mirrors M1 and M2 are used at each wavelength. Both P wave and S wave show the same transmission characteristics.

  In the configuration of FIG. 20, light emitted from the GaN blue semiconductor laser unit 99 (wavelength 405 nm) and the AlGaInP red semiconductor laser unit 100 (wavelength 650 nm) is converted into parallel light by the collimating lenses 102 and 103, respectively. The signals are combined so as to propagate on the same optical axis by the dielectric multilayer mirror M1. On the other hand, near-infrared light having a wavelength of 790 nm emitted from the AlGaAs-based near-infrared semiconductor laser unit 101 is collimated by the collimator lens 104 and then reflected by the dielectric multilayer mirror M2. Thereby, the three lights are multiplexed so as to propagate on the same optical axis. The combined three light beams are transmitted through the polarization hologram 105 and then bent in the direction perpendicular to the paper surface by the rising mirror 106 (however, in FIG. 20A, they are bent above the paper surface). Is drawn). Then, after passing through the phase variable wave plate R 3, it is condensed on the optical disk 109 by the objective lens 108.

  The reflected light from the optical disk 109 passes through the objective lens 108, the phase variable wavelength plate R3, and the rising mirror 106 again, and is diffracted by the polarization hologram 105 and integrated in the respective semiconductor laser units 99 to 101. On the detected photodetector.

  In the configuration of the optical pickup of the present embodiment, the polarization direction of the combined light is inclined 45 degrees with respect to the radial direction of the optical disc 109, and the crystal main axis (C axis) of the phase variable wave plate R3. And the radial direction of the optical disc 109 are parallel to each other. Therefore, the polarization direction of the light emitted as linearly polarized light makes an angle of 45 degrees with respect to the crystal main axis (C axis) of the phase variable wave plate R3, so that the phase variable wave plate R3 is a quarter wave plate. Can act as Further, the amount of birefringence generated in the optical disk 109 is generated in the radial direction, and the direction is parallel to the crystal main axis (C axis) of the phase variable wavelength plate R3. Therefore, in the optical disk 109 using the phase variable wavelength plate R3. It is also possible to compensate for the amount of birefringence that occurs.

  In the present embodiment, a detection optical system is configured using the polarization hologram 105 for light of all wavelengths. For this reason, it is necessary to adjust the phase variable wave plate R3 so as to act as a quarter wave plate for light of all wavelengths. The light emitted from the coherent light source is incident on the polarizing hologram 105 in the abnormal light direction as linearly polarized light. Since the polarization hologram 105 does not feel a change in refractive index with respect to light in the extraordinary light direction, the light is not diffracted. The sum of the phase modulation of the phase variable wave plate R3 and the one-way phase modulation generated in the optical disc 109 is nπ / 4 (nπ / 2 in the round trip) (where n = 1, 3, 5, 7,...). If controlled in such a manner, the reflected light from the optical disc 109 is converted into linearly polarized light whose polarization direction is rotated by 90 degrees with respect to the original polarization direction after passing through the phase variable wave plate R3. That is, phase modulation is controlled so that the amount of light guided onto the detection photodetector integrated in each of the laser units 99 to 101 is maximized, and stable servo operation and signal detection are possible. .

  In the present embodiment, an optical pickup provided with light sources that respectively emit light in three wavelength regions has been described. However, the same effect can be obtained also in an optical pickup provided with light sources that respectively emit light in two wavelength regions. It is done. In particular, since the amount of birefringence of the optical disk increases in the short wavelength region, the effect is particularly great in an optical pickup equipped with a short wavelength light source.

  Also in this embodiment, a more practical device is realized by using the phase-variable wave plate composed of two liquid crystal materials having different refractive index wavelength dispersion relationships described in the fourth embodiment. be able to. In this case, since the characteristics can be finely adjusted by adjusting the voltage applied to each of the liquid crystal containing a tolan series and the liquid crystal having a phenylcyclohexane series as a main component, the environmental temperature is changed. In addition, the phase variable wave plate can be stably operated as a quarter wave plate.

1 is a schematic configuration diagram showing an optical pickup according to a first embodiment of the present invention. 1 is a schematic configuration diagram showing an SHG blue laser unit used for an optical pickup according to a first embodiment of the present invention. Schematic configuration diagram showing another optical pickup in the first embodiment of the present invention Schematic configuration diagram showing an optical pickup in a second embodiment of the present invention Schematic diagram showing a laser unit used in an optical pickup according to a second embodiment of the present invention. The schematic block diagram which shows the variable wavelength plate used for the optical pick-up in the 2nd Embodiment of this invention The figure for demonstrating the concept which correct | amends spherical aberration using the variable phase plate in the 2nd Embodiment of this invention. Schematic configuration diagram showing another optical pickup in the second embodiment of the present invention Schematic configuration diagram showing an optical pickup in a third embodiment of the present invention The figure which shows the transmission characteristic of two dielectric multilayer mirrors used for the optical pick-up in the 3rd Embodiment of this invention Schematic configuration diagram showing another optical pickup in the third embodiment of the present invention The figure which shows the transmission characteristic of the two dielectric multilayer mirrors used for the other optical pick-up in the 3rd Embodiment of this invention (A) is a schematic block diagram which shows the optical pick-up in the 4th Embodiment of this invention, (b) is the polarization | polarized-light hologram in the structure of (a), the birefringent crystal principal axis of an optical disk, and the polarization direction of a coherent light source. Diagram showing the relationship The block diagram which shows the variable wavelength plate used for the optical pick-up in the 4th Embodiment of this invention Schematic configuration diagram showing a phase-variable wave plate used for an optical pickup in a fourth embodiment of the present invention The figure for demonstrating the operation principle of the phase variable wavelength plate used for the optical pick-up in the 4th Embodiment of this invention. Schematic configuration diagram showing another phase variable wave plate used in the optical pickup in the fourth embodiment of the present invention Diagram showing birefringence of optical disc (A) is a schematic block diagram which shows the optical pick-up in the 5th Embodiment of this invention, (b) is the polarization | polarized-light hologram in the structure of (a), the birefringent crystal principal axis of an optical disk, and the polarization direction of a coherent light source. Diagram showing the relationship (A) is a schematic block diagram which shows the optical pick-up in the 6th Embodiment of this invention, (b) is the polarization | polarized-light hologram in the structure of (a), the birefringent crystal principal axis of an optical disk, and the polarization direction of a coherent light source. Diagram showing the relationship Schematic configuration diagram showing a conventional optical pickup provided with a two-wavelength semiconductor laser

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 SHG blue laser unit 2 Red semiconductor laser unit 3 Collimating lens 4 Collimating lens 5 Dielectric multilayer mirror 6 Polarizing hologram 7 Wavelength plate 8 Rising mirror 9 Objective lens 10 Optical disk 11 DBR semiconductor laser 12 Collimating lens 13 Focus lens 14 Polarization Inverted optical waveguide device 15 Mg-doped LiNbO ₃ substrate 16 Optical waveguide 17 Polarization inversion region 18 SHG blue laser 19 Red semiconductor laser 20 Collimating lens 20-1 Collimating lens 21 PBS
22 PBS
23 Dielectric multilayer mirror 24 Wave plate 25 Objective lens 26 Optical disk 27 Detection lens 28 Detection lens 29 Cylindrical lens 30 Cylindrical lens 31 Photo detector 32 Photo detector 33 Laser unit 34 Collimating lens 35 Polarizing hologram 36 Dielectric multilayer mirror 37 Rising mirror 38 Wave plate 39 Objective lens 40 Optical disk 41 Detection lens 42 Cylindrical lens 43 Photo detector 44 Variable phase plate 45-1 3 split photo detector 45-2 6 split photo detector 45-3 Etched mirror 45 Blue semiconductor laser chip 46 Red semiconductor laser chip 47 Near Infrared semiconductor laser chip 48-1 ITO electrode 48-2 Alignment film 48-3 Liquid crystal 49 Glass substrate 50 Laser unit 51 Collimating lens 5 2 Polarizing hologram 53 Raising mirror 54 Wave plate 55 Objective lens 56 Optical disk 57 Blue semiconductor laser unit 58 Red semiconductor laser unit 59 Near infrared semiconductor laser unit 60 Collimating lens 61 Collimating lens 62 Collimating lens 63 Polarizing hologram 64 Polarizing hologram 65 Polarizing hologram 66 Wave plate 67 Wave plate 68 Wave plate 69 Rising mirror 70 Objective lens 71 Optical disk M1 Dielectric multilayer mirror M2 Dielectric multilayer mirror 72 Blue semiconductor laser unit 73 Red semiconductor laser unit 74 Near infrared semiconductor laser Unit 75 Collimating lens 76 Collimating lens 77 Collimating lens 78 Polarizing hologram 79 Glass hologram 80 Wave plate 81 Raising mirror 82 Objective lens 83 Optical disk M 3 Dielectric multilayer mirror 84 Blue semiconductor laser unit 85 Red semiconductor laser unit 86 Near infrared semiconductor laser unit 87 Collimating lens 88 Collimating lens 89 Collimating lens 90 Polarizing hologram 91 Rising mirror R1, R1b, R1c Phase variable wavelength plate 93 Objective lens 94 Optical disk 95 Glass substrate 96 ITO electrode 97 Alignment film 98 Liquid crystal 99 Blue semiconductor laser unit 100 Red semiconductor laser unit 101 Near infrared semiconductor laser unit 102 Collimator lens 103 Collimator lens 104 Collimator lens 105 Polarizing hologram 106 Rise mirror R3 Phase variable wavelength plate 108 Objective lens 109 Optical disk 110 Blue semiconductor laser unit 111 Red semiconductor laser unit 112 Near infrared semiconductor laser Unit 113 Collimating lens 114 Collimating lens 115 Collimating lens 116 Polarizing hologram 117 Rising mirror R2 Phase variable wave plate 118 Objective lens 119 Optical disk 120 Glass substrate 121 ITO electrode 122 Alignment film 123 Liquid crystal 124 Liquid crystal 125 Integrated unit 126 for DVD Integrated unit 127 Wavelength separation prism 128 Polarization hologram 129 Wave plate 130 CD (CD-R)
131 DVD-ROM phase change recording / reproducing disk 132 objective lens 133 film 134 alignment film 135 ITO electrode 136 glass substrate 232 liquid crystal

Claims (7)

A plurality of coherent light sources that are mounted on the same submount and emit light beams having different wavelengths (λ ₁ <λ ₂ <...), Respectively, and all the light beams having different wavelengths are converted into parallel light. An optical pickup equipped with a collimating lens. 2. The optical pickup according to claim 1, wherein the collimating lens is disposed such that a light beam emitted from the coherent light source that emits the light beam having the shortest wavelength λ ₁ is positioned on an optical axis of the collimating lens. . The plurality of coherent light sources are three coherent light sources that respectively emit light beams having different wavelengths (λ ₁ <λ ₂ <λ ₃ ), and the wavelengths of the three coherent light sources are 370 nm <λ ₁ <430 nm and 635 nm, respectively. The optical pickup according to claim 1, wherein <λ ₂ <690 nm and 760 nm <λ ₃ <810 nm. A variable phase plate divided into concentric circular regions and inserted in an optical path between the coherent light source and the optical disc, and the light beam of each wavelength emitted from each of the coherent light sources is condensed on the optical disc. The optical pickup according to claim 1, further comprising: an objective lens configured to compensate for spherical aberration generated in the objective lens by the variable phase plate. A plurality of semiconductor laser chips that are mounted on the same submount and emit light beams having different wavelengths (λ ₁ <λ ₂ <...), Respectively,
The multi-wavelength semiconductor laser unit, wherein the plurality of semiconductor laser chips are arranged such that their emission end face positions are different in the optical axis direction. Three semiconductor laser chips mounted on the same submount and emitting light beams having different wavelengths (λ ₁ <λ ₂ <λ ₃ ), respectively,
The multi-wavelength semiconductor laser unit, wherein the semiconductor laser chip that emits the light beam having the shortest wavelength λ ₁ is positioned at the center of the three semiconductor laser chips. Emitting the light beams from a plurality of coherent light sources mounted on the same submount and emitting light beams having different wavelengths (λ ₁ <λ ₂ <...), Respectively, and the light beams by a collimating lens. Converting into parallel light,
The collimating lens is arranged such that a light beam emitted from the coherent light source that emits the light beam having the shortest wavelength λ ₁ is positioned on an optical axis of the collimating lens. Playback method.

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