JP2013093082A - Optical pickup - Google Patents

Optical pickup Download PDF

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Publication number
JP2013093082A
JP2013093082A JP2011235538A JP2011235538A JP2013093082A JP 2013093082 A JP2013093082 A JP 2013093082A JP 2011235538 A JP2011235538 A JP 2011235538A JP 2011235538 A JP2011235538 A JP 2011235538A JP 2013093082 A JP2013093082 A JP 2013093082A
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Prior art keywords
laser light
phase difference
reflection
optical pickup
mirror
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JP2011235538A
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Japanese (ja)
Inventor
Yoshihiro Konuma
順弘 小沼
Katsuhiko Izumi
克彦 泉
Naoki Ito
直紀 伊藤
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Hitachi Media Electoronics Co Ltd
株式会社日立メディアエレクトロニクス
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Abstract

An optical pickup that eliminates the need for a quarter-wave plate and realizes its phase difference function with another optical element without imposing a burden on a multilayer film design is provided.
A first reflection mirror 5 that reflects laser light emitted from a laser light source 1 (2, 3) and generates a first phase difference (B) with respect to the laser light, and a first enhancement mirror A second reflection mirror 6 that reflects the laser light from the reflection mirror 5 and generates a second phase difference (C) with respect to the laser light, and is generated in the optical path from the laser light source 1 to the optical disk. The sum of the phase differences was configured to be approximately ¼ wavelength (± 90 °).
[Selection] Figure 1

Description

  The present invention relates to an optical pickup that reproduces a signal recorded on an optical disk by irradiating the optical disk with laser light.

  The optical pickup is configured to guide laser light from a laser light source to an optical disc (CD, DVD, BD, etc.) and guide laser light reflected by the optical disc to a photodetector. Linearly polarized laser light emitted from a laser light source such as a semiconductor laser is applied to the optical disk via a beam splitter. A part of the laser light reflected by the optical disk is guided to the photodetector by the beam splitter and reproduced, but a part of the laser light also returns to the laser light source. When the same polarization direction component as the linearly polarized light emitted from the laser light source returns to the laser light source, laser noise is generated. As a countermeasure, an optical element that generates a phase difference of ¼ wavelength is disposed between the laser light source and the optical disk, the laser light applied to the optical disk is circularly polarized, and the linear polarization direction of the laser light returning to the laser light source is 90%. Rotate. As a result, the return light having the same polarization direction component as the linearly polarized light emitted from the laser light source can be reduced, and generation of laser noise can be suppressed and a good reproduction signal can be obtained. As an optical element for generating a quarter-wave phase difference, a quarter-wave plate using a birefringent material such as quartz or polycarbonate is generally used.

  When using a quarter wave plate, there are the following problems. An inorganic birefringent material such as quartz is excellent in blue light resistance, and is therefore suitable for BD using a laser beam with a short wavelength (405 nm). However, inorganic birefringent materials are expensive. On the other hand, organic birefringent materials such as polycarbonate are suitable for CDs and DVDs because they are less expensive than inorganic birefringent materials such as quartz. However, since the blue light resistance is inferior, it is not suitable for BD.

  The quarter-wave plate is used by cutting a large birefringent material into small pieces. If the cutting angle shifts when cutting into small pieces, a shift occurs in the optical axis azimuth with respect to the outer shape of the small pieces. Further, if the angle is shifted when the small quarter-wave plate is attached to the optical pickup, the optical axis azimuth is shifted. If a deviation occurs in the azimuth angle of the optical axis in this way, a desired phase difference of ¼ wavelength cannot be generated, and the reproduction performance deteriorates.

  In view of this, a configuration has been proposed in which a quarter-wave plate is eliminated and the function of the reflection mirror is provided, and a predetermined phase difference is generated by the reflection mirror (see Patent Documents 1 to 4). In that case, a quarter-wave plate is not required, and therefore cost reduction can be expected. In addition, since the phase difference generated in the reflection mirror is based on the reflection phase difference between the P-polarized light and the S-polarized light of the multilayer film, the azimuth angle of the optical axis is always determined by the tilting posture of the mirror and is fixed in the P-polarization direction or the S-polarization direction. Is done. Therefore, in the case of a reflection mirror with a phase difference function, unlike the quarter wave plate, there is a merit that no deviation occurs in the optical axis azimuth even if there is a deviation in the cutting angle or attachment angle of the small piece. .

JP 2003-098350 A JP 2008-251112 A JP 2008-305525 A JP 2010-238274 A

  In the above-mentioned Patent Document 1, a newly introduced optical element is described as a configuration in which a retardation film that imparts a quarter-wave phase difference and a total reflection film that reflects incident light are laminated. According to this, a conventional quarter wave plate is not required, but a multi-layered organic film such as polycarbonate is used as a retardation film, and a birefringent material used for a conventional quarter wave plate. The problem is not solved.

  In the above-mentioned Patent Document 2, the reflecting film of the rising mirror has a phase difference of π (2n + 1) / 2 (n = 0, 1, 2, 3,... When the first and second laser beams are reflected.・ ”Is set to be”. In this case, since the phase difference generated in the reflecting film of the rising mirror is used, the birefringent material used for the conventional quarter-wave plate is not necessary. However, it is expected that it is technically difficult to provide a quarter-wave phase difference only with the reflective multilayer film of a single rising mirror, that is, to achieve both a high reflectivity and a quarter-wave phase difference. Is done.

  In Patent Document 3, it is described that “the optical element for imparting phase difference is composed of a phase difference mirror or a quarter wavelength plate”. When the optical element is composed of a phase difference mirror, a quarter wavelength plate is not required. However, as in the above-mentioned Patent Document 2, both a high reflectance and a phase difference of a quarter wavelength are compatible in the phase difference mirror. Will be difficult.

  Patent Document 4 describes a configuration in which a phase difference is generated by a reflection film of a beam splitter, a phase difference is generated by a reflection film of a reflection mirror, and a phase difference of approximately ¼ wavelength is generated by the phase difference between the two. Has been. In this case, a quarter-wave plate is not necessary, and the phase difference generated in the reflection film of the beam splitter can be used, so that the reflection film of the reflection mirror can be easily designed. However, it is difficult to give a predetermined phase difference to the reflection film of the beam splitter. This is because a beam splitter is required to have a predetermined spectral transmission reflectance characteristic (for example, both transmittance and reflectance are approximately 50%), and in general, the number of reflection mirror films (approximately 20 to 40 layers) for which high reflectance is required. In comparison, the number of films is reduced. For example, a half mirror having a transmittance of about 50% can be designed with a film number of 10 layers or less. It is difficult to generate a predetermined phase difference using such a multilayer film having a small number of films. The reason is that when the optimum design is performed by a computer using the film thickness as a parameter, an optimal solution cannot be found if the number of parameters is small.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pickup that eliminates the need for a quarter-wave plate and realizes the phase difference function with another optical element without placing a burden on multilayer film design.

  In order to solve the above problems, an optical pickup of the present invention reflects a laser light source that emits laser light, a laser light emitted from the laser light source, and generates a first phase difference (B) with respect to the laser light. A first increasing reflection mirror, a second increasing reflection mirror that reflects the laser light from the first increasing reflection mirror and generates a second phase difference (C) with respect to the laser light, and a second An objective lens for irradiating the optical disk with the laser beam from the reflection mirror is provided, and the total phase difference generated in the optical path from the laser light source to the optical disk is set to approximately ¼ wavelength (± 90 °).

  Furthermore, a beam splitter that reflects the laser light with a predetermined spectral transmission / reflection characteristic is provided between the laser light source and the first reflection mirror, and a phase difference (A) with respect to the laser light reflected by the beam splitter. Is generated, the first and second phase differences (B, C) generated in the first and second reflection-increasing mirrors are set to be B + C = ± 90 ° -A.

  According to the present invention, a quarter wavelength plate is not required, and an optical element that imparts the phase difference function can be easily realized. Therefore, an inexpensive optical pickup having stable reproduction performance can be provided.

1 is a plan view showing an example of an optical system of an optical pickup according to the present invention (Example 1). FIG. 10 is a diagram illustrating an example of distribution of phase differences in Embodiment 2. FIG. 6 is a graph of spectral reflectance characteristics of a second increasing reflection mirror 6 in Example 2. 6 is a graph of spectral reflection phase difference characteristics of a second increasing reflection mirror 6 in Example 2. FIG. 10 is a diagram illustrating an example of distribution of phase differences in the third embodiment.

  Hereinafter, embodiments of an optical pickup according to the present invention will be described with reference to the drawings.

  FIG. 1 is a plan view showing an embodiment of an optical system of an optical pickup according to the present invention. The illustrated optical pickup is configured to support playback of CDs and DVDs.

  The semiconductor laser 1 is a monolithic type including a first laser light source 2 that emits a laser beam having a wavelength of 785 nm for CD and a laser light source 3 that emits a laser beam having a wavelength of 660 nm for DVD. The semiconductor laser 1 is attached by rotating by 45 ° around its optical axis. The laser light emitted from each of the laser light sources 2 and 3 is linearly polarized light, and the polarization direction is 45 ° by rotating the semiconductor laser 1 by 45 °.

  The laser beams emitted from the first and second laser light sources 2 and 3 enter the beam splitter 4 and are reflected by the reflection film 4a. The reflection film 4a has a so-called half-mirror spectral transmission reflection characteristic in which both the S-polarized light and the P-polarized light have a reflectance of about 50% and a transmittance of about 50%. Here, since the polarization direction of the incident laser light is 45 °, the S-polarized component and the P-polarized component of the incident light are equal. And since the reflecting film 4a is a half mirror, the emitted S-polarized component and P-polarized component are also equal.

  The laser light reflected by the reflection film 4 a of the beam splitter 4 enters the first increasing reflection mirror 5. Note that the term “increased reflection mirror” here means a reflection mirror having a structure in which a dielectric multilayer film is formed on the reflection surface to increase the reflectivity, thereby clearly defining the reflection mirror of the beam splitter. They are distinguished. Further, the reflection film used for this will be referred to as an “increased reflection film”. The multi-reflection film 5a is designed as a multilayer film so as to exhibit spectral transmission / reflection characteristics with a reflectance of approximately 100% for both S-polarized light and P-polarized light.

  The laser light reflected by the increased reflection film 5 a of the first increased reflection mirror 5 is incident on the second increased reflection mirror 6. The second reflective mirror 6 is disposed at an angle of 45 ° with respect to the paper surface. Further, the increasing reflection film 6a of the second increasing reflection mirror 6 is designed to be a multilayer film so as to exhibit a spectral transmission / reflection characteristic with a reflectance of approximately 100% for both S-polarized light and P-polarized light.

  The laser light reflected by the increased reflection film 6 a of the second increased reflection mirror 6 travels in the direction perpendicular to the paper surface and enters the collimator lens 7. In the collimating lens 7, the laser light becomes substantially parallel light and enters the objective lens 8. The objective lens 8 is designed to be compatible with a CD and a DVD, and a laser beam is applied to an optical disk (CD or DVD) (not shown).

  The laser beam reflected by the optical disk is incident on the beam splitter 4 through the objective lens 8, the collimator lens 7, the second increasing reflection mirror 6, and the first increasing reflection mirror 5. The laser light transmitted through the reflective film 4 a of the beam splitter 4 enters the photodetector 9. On the other hand, the laser light reflected by the reflection film 4 a of the beam splitter 4 returns to the laser light sources 2 and 3 of the semiconductor laser 1. However, since the linear polarization direction of the return laser light to the laser light sources 2 and 3 is rotated by 90 °, it does not interfere with the laser light emitted from the laser light source to generate laser noise.

  The configuration of this embodiment is characterized in that the first increasing reflection mirror 5 and the second increasing reflection mirror 6 are provided as optical elements (phase difference generating elements) for generating a phase difference in the laser light. Although the phase difference is not intentionally generated in the beam splitter 4, the reflected light has a certain value as a result of the film design of the reflection film 4a giving priority to the spectral transmission / reflection characteristics of the half mirror. A phase difference occurs.

  The phase difference of each element is defined as follows. A reflection phase difference of the CD laser light generated at the time of reflection by the beam splitter 4 (reflection film 4a) is A1, and a reflection phase difference of the DVD laser light is A2. The reflection phase difference of the CD laser light generated at the time of reflection by the first increasing reflection mirror 5 (the increased reflection film 5a) is B1, and the reflection phase difference of the DVD laser light is B2. The reflection phase difference of the CD laser light generated at the time of reflection by the second increasing reflection mirror 6 (enhanced reflection film 6a) is C1, and the reflection phase difference of the DVD laser light is C2. Both are phase differences that occur during reflection, and the “reflection phase difference” is hereinafter simply referred to as “phase difference”. The unit of phase difference includes “wavelength”, “radian”, and “degree (°)”, and there is a relationship of ¼ wavelength = π / 2 radians = 90 °. In the following notation, the unit “degree (°)” is used and converted from −180 ° to + 180 ° and displayed.

In the present embodiment, the phase difference generated in the entire phase difference generating element is set to be approximately ¼ wavelength (= 90 °). That is,
A1 + B1 + C1 = ± 90 ° (1)
A2 + B2 + C2 = ± 90 ° (2)
In order to satisfy the above, a multilayer film design of the first increasing reflection mirror 5 (the increasing reflection film 5a) and the second increasing reflection mirror 6 (the increasing reflection film 6a) is performed.

Among them, the beam splitter 4 (reflective film 4a) prioritizes the spectral transmission / reflection characteristics, and as a result, the phase differences A1 and A2 are determined separately. Therefore, regarding the phase difference between the first increasing reflection mirror 5 (the increasing reflection film 5a) and the second increasing reflection mirror 6 (the increasing reflection film 6a),
B1 + C1 = ± 90 ° -A1 (3)
B2 + C2 = ± 90 ° -A2 (4)
The phase difference is distributed so as to satisfy
Next, specific examples of phase difference distribution will be described in the second and third embodiments.

The second embodiment is a case where no phase difference is generated in the beam splitter 4 (reflection film 4a).
FIG. 2 is a diagram illustrating an example of the phase difference distribution in the second embodiment, and illustrates the value of the phase difference generated in each of the beam splitter 4, the first increasing reflection mirror 5, and the second increasing reflection mirror 6.

Since there is no phase difference in the beam splitter 4, A1 = A2 = 0 ° (or ± 180 °). At this time, from the equations (3) and (4), the phase differences B1 and B2 of the first reflective mirror 5 and the phase differences C1 and C2 of the second reflective mirror 6 are
B1 + C1 = ± 90 °
B2 + C2 = ± 90 °
What is necessary is just to distribute a phase difference so that it may become.

In particular, when the phase difference between the first reflection mirror 5 and the second reflection mirror 6 is equally distributed,
B1 = C1 = ± 45 ° (or ± 135 °)
B2 = C2 = ± 45 ° (or ± 135 °)
What should I do?

Here, an example of an actual multilayer film design will be described. Here, since the purpose is to show the possibility of designing the multilayer film in the present embodiment, a multilayer film design example of the reflective film 6b of the second reflective mirror 6 will be described as a representative. The phase difference generated in the reflective film 6b of the second increasing reflection mirror 6 is
C1 = + 135 °
C2 = + 135 °
Table 1 shows the structure of the film design. The incident angle at the design center was 45 °, and a white plate was used as the glass. Between the air and the glass, TiO 2 and SiO 2 as dielectric deposition materials were alternately deposited. The total number of membranes is 33 layers, which are numbered 1, 2, 3,..., 33 in order from the air side. Table 1 shows the number of each layer, dielectric deposition material, refractive index, and film thickness.

  FIG. 3 is a graph of spectral reflectance characteristics at an incident angle of 45 °. The horizontal axis is wavelength and the vertical axis is reflectance. P-polarized reflectance and S-polarized reflectance are described. From FIG. 3, it can be seen that the reflectance is approximately 100% for both P-polarized light and S-polarized light at a wavelength near 660 nm and a wavelength near 785 nm.

  FIG. 4 is a graph of reflection phase difference characteristics at an incident angle of 45 °. The reflection phase difference was calculated by (P polarization phase−S polarization phase). FIG. 4 shows that the reflection phase difference is + 135 ° near 660 nm and the reflection phase difference is + 135 ° near 785 nm.

According to the present embodiment, the phase difference shared by the first increasing reflection mirror 5 (the increasing reflection film 5a) and the second increasing reflection mirror 6 (the increasing reflection film 6a) is designed with a single increasing reflection film. Can be reduced from 1/4 wavelength (± 90 °) to ± 45 °. Thereby, in the film design of the increased reflection films 5a and 6a, a predetermined phase difference (± 45 °) can be easily generated while maintaining a high reflectance. In particular, by setting the phase difference between the increasing reflection films 5a and 6a used in the first increasing reflection mirror 5 and the second increasing reflection mirror 6 to be equal, the film design can be shared.
Further, since it is not necessary to generate a phase difference with respect to the beam splitter 4, the spectral transmission / reflection characteristics of the beam splitter 4 are not impaired.

The third embodiment is a case where the beam splitter 4 and the first increasing reflection mirror 5 are designed with priority given to the respective spectral transmission / reflection characteristics, and a predetermined phase difference is set with respect to the second increasing reflection mirror 6. .
FIG. 5 is a diagram illustrating an example of distribution of phase differences in the third embodiment, and illustrates values of phase differences generated in the beam splitter 4, the first reflection mirror 5, and the second reflection mirror 6.

  The beam splitter 4 gives priority to the spectral transmission / reflection characteristics (half mirror characteristics), and as a result of designing the reflection film 4a, the phase difference value is A1 = + 175 ° at a wavelength of 785 nm (for CD) and a wavelength of 660 nm. (For DVD) A2 = −155 ° (= + 205 °), and the first reflective mirror 5 is a film of the reflective film 5a giving priority to its spectral transmission and reflection characteristics (high reflectance characteristics). As a result of designing, the phase difference values were B1 = + 175 ° at a wavelength of 785 nm and −175 ° (= + 185 °) at a wavelength of 660 nm. At this time, the phase differences C1 and C2 of the second reflecting mirror 6 may be determined so as to satisfy the expressions (1) and (2). As a result, C1 = + 100 ° was set at a wavelength of 785 nm, and C2 = −120 ° was set at a wavelength of 660 nm, so that the reflective reflection film 6a was designed. As a result, the overall phase difference was + 90 ° at the wavelength of 785 nm and −90 ° at the wavelength of 660 nm, and in both cases, a phase difference of ¼ wavelength could be realized.

  According to the present embodiment, the phase difference generated by the beam splitter 4 and the first increasing reflection mirror 5 is used, and the phase difference shared by the second increasing reflection mirror 6 (the increasing reflection film 6a) is changed to a single value. It is possible to shift from ¼ wavelength (± 90 °) to + 100 ° or −120 °, which is difficult to design with an increased reflection film. Thereby, in the film design of the increased reflection film 6a, a predetermined phase difference (± 45 °) can be easily generated while maintaining a high reflectance.

  In the present embodiment, the film design is performed so as to generate a predetermined phase difference for the second reflective mirror 6, but the present invention is not limited to this. It is also possible to design the film so as to generate a predetermined phase difference for the first reflective mirror 5 and to design the film for the second reflective mirror 6 giving priority to high reflectance characteristics.

  According to the optical pickups of the respective embodiments described above, the conventional quarter wavelength plate for providing a quarter wavelength phase difference becomes unnecessary. As a result, various problems due to the use of the quarter-wave plate are solved, and an inexpensive and stable optical pickup is realized.

  In each of the above-described embodiments, the phase difference function of the quarter wavelength plate is realized by sharing the two reflection mirrors, and the phase difference of the quarter wavelength is given as a whole. Therefore, there is no deviation from the quarter wavelength at any wavelength for CD and DVD. At this time, since the phase difference shared by the two reflection mirrors is deviated from the quarter wavelength, it is possible to easily realize a reflection-increasing film that generates a predetermined phase difference while maintaining a high reflectance. In addition, since it is not necessary to intentionally add a phase difference to the beam splitter, a predetermined spectral transmission / reflection characteristic (half mirror characteristic) is not impaired.

  In the above embodiment, the optical pickup corresponding to both the CD and the DVD has been described. However, the present invention is not limited to this. The present invention can also be applied to a case of supporting a CD, DVD, or BD alone, or a case of supporting any combination thereof. In that case, it is only necessary to share such that the total of the phase differences generated by the respective phase difference generating elements becomes approximately ¼ wavelength at each wavelength to be handled.

  In the above embodiment, the total of the phase differences generated by the phase difference generating element is set to about ¼ wavelength. However, this may be changed slightly depending on the application. For example, in order to improve the reproduction performance of a recordable DVD, the present invention can also be applied to a case where phase difference design is performed so that the major axis direction is a predetermined elliptical polarization on the disc.

1 Semiconductor laser,
2 ... 1st laser light source (for CD),
3. Second laser light source (for DVD),
4 ... Beam splitter,
4a ... reflective film,
5 ... 1st increase reflection mirror,
5a ... increased reflection film,
6 ... Second reflection mirror,
6a: Intensifying reflection film,
7 ... Collimating lens,
8 ... Objective lens,
9: Photodetector,
A1, A2, B1, B2, C1, C2... Reflection phase difference.

Claims (5)

  1. In an optical pickup that reproduces a signal from an optical disk by irradiating the optical disk with a laser beam,
    A laser light source for emitting the laser light;
    A first reflection mirror that reflects the laser light emitted from the laser light source and generates a first phase difference (B) for the laser light;
    A second intensifying mirror that reflects the laser light from the first intensifying mirror and generates a second phase difference (C) for the laser light;
    An objective lens for irradiating the optical disk with laser light from the second reflection mirror;
    An optical pickup characterized in that a total of phase differences generated in an optical path from the laser light source to the optical disk is approximately ¼ wavelength (± 90 °).
  2. The optical pickup according to claim 1,
    A beam splitter for reflecting laser light with a predetermined spectral transmission reflection characteristic between the laser light source and the first increased reflection mirror;
    When a phase difference (A) is generated for the laser light reflected by the beam splitter,
    The first and second phase differences (B, C) generated by the first and second reflection mirrors are
    B + C = ± 90 ° -A
    An optical pickup that is set to be
  3. The optical pickup according to claim 2,
    The phase difference generated in the beam splitter is zero (A = 0),
    An optical pickup characterized in that the first and second phase differences generated by the first and second reflection-increasing mirrors are set to satisfy B = C = ± 45 °.
  4. The optical pickup according to claim 2,
    One of the first and second increasing reflection mirrors reflects a laser beam with a predetermined high reflectance characteristic,
    An optical pickup characterized in that a phase difference generated in the other reflection mirror is set to be shifted from ± 90 °.
  5. The optical pickup according to any one of claims 1 to 4,
    The laser light source can emit a plurality of types of laser light having different wavelengths,
    The first and second phase differences (B, C) generated in the first and second reflection-increasing mirrors are approximately ¼ wavelength regardless of the total phase difference generated in the optical path. An optical pickup characterized by being configured to be (± 90 °).
JP2011235538A 2011-10-27 2011-10-27 Optical pickup Pending JP2013093082A (en)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009181689A (en) * 2008-01-30 2009-08-13 Jds Uniphase Corp Optical pickup unit having two-mirror phase shifter
JP2010238274A (en) * 2009-03-30 2010-10-21 Sanyo Electric Co Ltd Optical pickup apparatus
WO2011013484A1 (en) * 2009-07-27 2011-02-03 コニカミノルタオプト株式会社 Optical pickup device

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009181689A (en) * 2008-01-30 2009-08-13 Jds Uniphase Corp Optical pickup unit having two-mirror phase shifter
JP2010238274A (en) * 2009-03-30 2010-10-21 Sanyo Electric Co Ltd Optical pickup apparatus
WO2011013484A1 (en) * 2009-07-27 2011-02-03 コニカミノルタオプト株式会社 Optical pickup device

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