JP5074312B2 - Optical head device - Google Patents

Description

Optical discs such as DVDs and CDs are widely used as optical recording media, and high-density information recording optical discs BD (Blu-ray Discs) have been commercialized. Information recording on optical discs and reproduction of recorded information (hereinafter referred to as “recording / reproduction”). ) Is an optical head device.
A schematic diagram of a configuration example of a conventional optical head device 300 is shown in FIG. A laser light source 1, a polarizing beam splitter prism 2 a, a collimating lens 3, an objective lens 5, and a photodetector 6 are provided, and light emitted from the laser light source 1 passes through the polarizing beam splitter prism 2 a and is collimated by the collimating lens 3. The signal light is collimated and focused on the information recording surface of the optical disk 7 by the objective lens 5, and the signal light reflected by the information recording surface is transmitted again through the objective lens 5 to become parallel light. The signal light reflected by 2a is collected on the light receiving surface of the photodetector 6.

  Here, in order to efficiently collect the light emitted from the laser light source 1 on the information recording surface and efficiently demultiplex the signal light reflected by the information recording surface to the photodetector, two right isosceles sides A polarizing beam splitter prism 2 a in the form of a hexahedral glass block formed by bonding a multilayer film on one inclined surface of a triangular prism glass is used together with the quarter wavelength plate 4. The linearly polarized light emitted from the laser light source enters the polarizing beam splitter prism 2a as P-polarized light (first linearly polarized light) and is transmitted therethrough. Then, the linearly polarized light is condensed and reflected on the information recording surface, and is transmitted back and forth through the quarter wavelength plate 4. As a result, it becomes S-polarized light (second linearly polarized light) and is incident on the polarization beam splitter prism 2a and reflected.

In the polarizing beam splitter prism 2a used here, the refractive index ( _ng > 1) of the right isosceles triangular prism glass which is a prism is larger than the refractive index of air ( _ng = 1). Compared with a flat plate beam splitter that is obliquely incident on (where the refractive index of the multilayer film material is n), the refraction angle θ in the multilayer film with respect to the incident angle θ _{0 according} to the Snell refraction rule is θ = arcsin {( _ng / n) There is little decrease in sin θ ₀ }. As will be described later, the larger the refraction angle in the multilayer film is, the wider the polarization separation wavelength region for transmitting P-polarized light and reflecting S-polarized light is, so that a wide wavelength band that functions as a polarizing beam splitter can be secured. As a result, for example, in a wavelength range of 398 to 415 nm of a blue semiconductor laser light source used for recording / reproduction of a BD optical disc, polarization separation characteristics exhibiting a high S-polarized reflectance and a high P-polarized transmittance for divergent incident light. Thus, high light utilization efficiency can be realized in the forward path from the laser light source to the optical disk and in the backward path from the optical disk to the photodetector.

FIG. 12 shows a schematic diagram of another configuration example of the conventional optical head device. The optical head device 400 uses a flat plate beam splitter 2b in which a multilayer film is formed on one surface of a parallel flat light transmitting substrate instead of the polarizing beam splitter prism 2a shown in FIG. 11 (for example, Patent Document 1). And 2).
JP 2007-280460 A JP 2002-4915 A

  However, in the optical head device 300 shown in FIG. 11, it is necessary to process two right-angled isosceles triangular prism glasses with high precision, and then to form a multilayer film and adhere to a hexahedral glass block form. Compared with the beam splitter, the processing is complicated and the cost is increased. In addition, since an organic adhesive is used, there are problems in reliability such as long-term characteristic stability in a high temperature and high humidity environment and light resistance accompanying blue laser light irradiation.

  In the optical head device 400 shown in FIG. 12, since the refractive index difference between air and the multilayer film material is large in the flat plate beam splitter 2b, the refraction in the multilayer film is based on the Snell refraction rule as compared with the polarization beam splitter prism 2a. The corner is a small value. As a result, the difference in effective refractive index between P-polarized light and S-polarized light at oblique incidence is reduced. For example, high polarization separation characteristics can be realized with respect to divergent incident light in the wavelength range of 398 to 415 nm that is used by the blue semiconductor laser light source. It was difficult. As a result, there is a problem that the light utilization efficiency in the forward path and the backward path is reduced, a high-power blue semiconductor laser light source is required, and the SN ratio (signal to noise ratio) of the photodetector is lowered. In addition, by setting the central angle of incidence to a large value of 70 ° or more, the refraction angle in the multilayer film of the flat plate beam splitter 2b can be set to a large value and the polarization splitting characteristics can be improved. There is a problem that the splitter 2b is required and the size of the optical head device is increased.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an optical head device having a high light utilization efficiency equipped with a flat polarizing beam splitter having high polarization separation characteristics.

  The present invention converts a laser light source and first linearly polarized light emitted from the laser light source into circularly polarized light, and converts the circularly polarized light reflected from the optical recording medium into a second straight line orthogonal to the first linearly polarized light. A quarter-wave plate for converting to polarized light, an objective lens for condensing the circularly polarized light transmitted through the quarter-wave plate on the optical recording medium, and reflected by the optical recording medium and converted by the quarter-wave plate. The second linearly polarized light detector is disposed in the optical path between the laser light source and the quarter-wave plate, and transmits or reflects the first linearly polarized light emitted from the laser light source. And a polarizing beam splitter that reflects or transmits reflected light reflected from the optical recording medium and converted from circularly polarized light to second linearly polarized light by the quarter-wave plate. The polarizing beam splitter is transparent. A multilayer film made of at least two kinds of film materials having different refractive indexes is formed on the surface of the conductive substrate, and utilizing the difference in polarization transmission characteristics of the rising and falling of the reflection band according to the incident angle of the oblique incident light, Transmits and reflects polarized components that are orthogonal to each other, and has two reflection band multilayer films with a long wavelength and a short wavelength as the center wavelength on the separation surface. The rising side of the short wavelength reflection band and the long wavelength reflection band The effective wavelength range of polarized light separation is expanded by making the falling of the short wavelength side close to each other and multiplying the reflection and transmission polarization characteristics thereof.

  Further, in the optical head device, the incident angle dependence of the spectral characteristic generated due to the difference in the incident angle in a plane defined by the normal of the polarization beam splitter and the principal ray of the incident light is corrected. As described above, the optical film thickness of the multilayer film layer is adjusted.

  In the optical head device, the multilayer film is at least a short wavelength reflection band multilayer film layer and a long wavelength reflection band multilayer film layer.

  In the optical head device of the present invention, a flat polarizing beam splitter having a multilayer film formed on the surface of a light-transmitting substrate is used. Therefore, a multilayer film can be formed with high productivity using a low-cost and large-area glass substrate. This is possible, and the cost is reduced as compared with a conventional optical head device using a polarizing beam splitter prism.

  Further, in the optical head device of the present invention, high polarization separation characteristics can be realized in the use wavelength band of the laser light source, so that high light utilization efficiency can be obtained in the forward path and the return path. As a result, it is possible to detect an optical signal with a high S / N ratio using a laser light source with a relatively low output.

  Further, in the optical head device of the present invention, when the flat polarizing beam splitter is arranged in the divergent light, the wavelength dependency and the incident angle dependency of the polarization separation characteristic can be corrected. The intensity distribution of the light separated by the polarization beam splitter is stabilized. As a result, high light utilization efficiency can be realized in the forward path and the return path without increasing the size of the optical head device.

  In addition, since the flat polarizing beam splitter does not use an adhesive on the light transmitting surface and the light reflecting surface, high reliability is achieved and light resistance against blue laser light used in an optical head device for a high-density information recording optical disk BD is achieved. It is easy to secure sex.

  A schematic diagram of a configuration example of the optical head device 100 of the present invention is shown in FIG. Parts having the same functions as those in the configuration example of the conventional optical head device 400 shown in FIG.

  As the polarizing beam splitter, a flat polarizing beam splitter 10 in which a multilayer film is formed on one side of a parallel flat translucent substrate is used, and the principal ray of light emitted from the laser light source 1 is a polarizing reflecting surface of the flat polarizing beam splitter 10. Is incident at an incident angle θ. It should be noted that with respect to the plane defined by the normal of the multilayer film surface, which is the polarization reflection surface of the plate polarization beam splitter 10, and the principal ray of the incident light, the in-plane polarization of the polarization of the incident light is P-polarized and the surface is Vertically polarized light is referred to as S-polarized light.

  Here, when the linearly polarized light emitted from the laser light source 1 is incident on the flat polarizing beam splitter 10 as S-polarized light (first linearly polarized light), a high reflectance can be obtained. When the linearly polarized light emitted from the laser light source 1 is not S-polarized light, a half-wave plate is disposed in the optical path between the laser light source 1 and the flat-plate polarization beam splitter 10 and the transmitted polarization of the half-wave plate is rotated. To convert it to S-polarized light. Further, part of the light emitted from the laser light source 1 passes through the flat polarizing beam splitter 10 and the transmitted light is detected by a photodetector (not shown), so that the light reaching the information recording surface of the optical disc 7 is constant. Thus, when the emission intensity of the laser light source 1 is feedback controlled, the laser light source 1 is rotated about the principal ray as the rotation axis so that the P-polarized component is also incident on the flat-plate polarization beam splitter 10, or the half-wave plate is It is only necessary to adjust the angle of the slow axis and adjust the direction of the outgoing linearly polarized light to a desired angle.

  The S-polarized light reflected by the flat polarizing beam splitter 10 is reflected on the information recording surface of the optical disk 7 to become signal light, and is converted into P-polarized light (second linearly polarized light) by reciprocating the quarter-wave plate 4. The light enters the flat polarizing beam splitter 10. Since the flat polarizing beam splitter 10 exhibits a high polarization transmittance with respect to the P-polarized light, it is efficiently condensed on the light receiving surface of the photodetector 6 and converted into an electric signal.

The polarization separation function of the flat polarizing beam splitter 10 used in the optical head device 100 of the present invention will be described below with reference to FIG. 2 which is a sectional view showing the structure thereof.
In FIG. 1, a flat polarizing beam splitter 10 is arranged in the divergent light, and the optical head device is configured to transmit S-polarized light on the forward path and transmit P-polarized light on the return path. The optical head device may be used.

  However, when the flat polarizing beam splitter 10 is arranged at an oblique incidence in diverging light, astigmatism occurs in the transmitted light according to the incident angle of the principal ray, the angular width of the diverging light, and the thickness of the substrate. When the transmitted wavefront aberration in the forward path is large, the condensing spot shape of the information recording surface of the optical disk deteriorates and a recording / reproducing error occurs. Therefore, the optical head device of FIG. A configuration is preferred.

  FIG. 2 shows the structure of a flat polarizing beam splitter 10 used for the optical head device 100 according to the present invention. The short wavelength reflection band multilayer film 12S and the long wavelength reflection band multilayer are composed of two different center wavelengths. In this case, the film layer 12L is overlaid on the entire surface of the flat substrate 11. FIG. 3 shows the spectral transmission characteristic shapes of P and S polarized light when a light beam is incident. In the figure, solid lines (black circles ● and white circles ○) are rising portions on the long wavelength side of the short wavelength reflection band, the left line represents the P-polarized spectral transmission characteristics, and the right line represents the S-polarized spectral transmission characteristics. The broken lines (black squares ■ and white squares □) are the falling portions on the short wavelength side of the long wavelength reflection band, the left line represents the S-polarized spectral transmission characteristic, and the right line represents the P-polarized spectral transmission characteristic. . By superimposing these two spectral transmission characteristics, the P-polarized transmission wavelength region in the center of the figure is synthesized. Similarly, for the S-polarized light, the S-polarized reflection wavelength region over the entire region is synthesized by superimposing the S-polarized light transmission characteristics of the short wavelength reflection band and the long wavelength reflection band, as indicated by the white rhombus 形 in the figure. The short wavelength side falling part of the long wavelength reflection band and the long wavelength side rising part of the short wavelength reflection band alone are small in the conventional effective polarization separation wavelength region, but by combining both characteristics, as a result, As shown in the center of the figure, a wide effective polarization separation wavelength region is obtained.

The order of the short wavelength reflection band multilayer film layer 12S and the long wavelength reflection band multilayer film layer 12L formed on one surface of the flat substrate 11 is not limited, and may be the reverse of FIG.
Since the multilayer film layer hardly absorbs light in the used wavelength band, the sum of transmittance and reflectance is almost 100%.
In addition, on the other plane of the flat polarizing beam splitter 10, an arrangement of an incident angle at which reflection is reduced or an antireflection treatment such as an antireflection film is applied to at least P-polarized light in the use wavelength band of the laser light source 1. Is preferred.

The short-wavelength reflection band multilayer film layer 12S and the long-wavelength reflection band multilayer film layer 12L are composed of multilayer film layers in which transparent dielectric films having different refractive indexes are laminated, and have a high refractive index n _H dielectric film and a low refractive index. A structure in which n _L dielectric films are alternately stacked is common. TiO ₂ , Ta ₂ O ₅ , Nb ₂ O ₅ , ZrO ₂ or the like having a refractive index of 2 or more is used as a dielectric film having a high refractive index n _{H, and} a refractive index of 1.5 or less is used as a dielectric film having a low refractive index n _L. SiO ₂ , MgF ₂ , AlF ₃ , NaF, amorphous fluororesin, or the like is used.

As a constituent material of the multilayer film, a birefringent material having an ordinary light refractive index n _o and an extraordinary light refractive index n _e (n _e > n _o ) is used as a high refractive film, and the homogeneous refractive index is substantially equal to n _o. It may be a multilayer film layered alternately with the low refractive film. In this case, by matching the direction of the ordinary refractive index n _o of the birefringent material in the P polarization direction, a high transmittance is obtained for P-polarized light in a wide incident angle range and a wide wavelength band. The multilayer film is not limited to two layers including a short wavelength reflection band multilayer film layer and a long wavelength reflection band multilayer film layer, and may be three or more layers.

  As a multilayer film forming method, a dry method such as a vacuum deposition method, a sputtering method, or a CVD method, and a wet method such as a coating method or a sol-gel method are used.

Hereinafter, the principle of the flat polarizing beam splitter 10 used in the optical head device 100 of the present invention will be described.
When light is incident on the multilayer film in the atmosphere having a refractive index of 1 at an incident angle θ, effective refractive indices η _HP , η _HS and η _LP for P-polarized light and S-polarized light having a high refractive index n _H and a low refractive index n _L , η _LS is described by the formulas (1a) to (1d).
η _HP = n _H / cos θ _H = n _H / {1- (sin θ / n _H ) ² } ^0.5 (1a)
η _HS = n _H × cos θ _H = n _H × {1- (sin θ / n _H ) ² } ^0.5 (1b)
η _LP = n _L / cos θ _L = n _L / {1- (sin θ / n _L ) ² } ^0.5 (1c)
η _LS = n _L × cos θ _L = n _L × {1- (sin θ / n _L ) ² } ^0.5 (1d)

Therefore, when not perpendicularly incident, the effective refractive index differences Δη _P and Δη _S of the high-refractive index dielectric film and the low-refractive index dielectric film with respect to the P-polarized light and the S-polarized light are different and depend on the incident angle θ (2a ) And (2b).
Δη _P = η _HP −η _LP = (n _H −n _L ) × A _P × C (2a)
Δη _S = η _HS −η _LS = (n _H −n _L ) × A _S (2b)
Here, A _P , A _S and C are described by the formulas (3a) to (3c).
A _P = (n _H + n _L ) / (η _HP + η _LP ) (3a)
A _S = (n _H + n _L ) / (η _HS + η _LS ) (3b)
^{_{C = {(n H n L}} ) 2 - (n H 2 + n L 2) sin 2 θ} / (η HS η LS) 2 (3c)

Since sin θ increases as the incident angle θ increases, the effective refractive indices η _HS and η _LS of S-polarized light decrease from the equations (1b) and (1d). As a result, to increase from _{A S} is (3b) wherein the effective refractive index difference △ eta _S is increased from (2b) equation. Further, the effective refractive index difference Δη _S becomes larger as the dielectric film material has a larger refractive index difference (n _H −n _L ).

On the other hand, the effective refractive index eta _HP and eta _LP of P-polarized light (1a), increases with increasing angle of incidence θ from (1c) formula, (3a), (3c) A P and for C is decreased from the equation The effective refractive index difference Δη _P decreases from the equation (2a). In particular, in the formula (3c), since (n _H n _L ) ² / (n _H ² + n _L ² ) ≦ 0.5, depending on the setting of n _H , n _L and θ, the value is close to zero, and effective refraction the rate difference △ η _P can be reduced.

The optical film thicknesses of the dielectric film having a high refractive index n _H and a film thickness d _H and the low refractive index n _L and the dielectric film having a film thickness d _L are n _H × d _H and n _L × d _L. The optical path length differences L _H and L _L that contribute to interference are described by equations (4a) and (4b). For multilayer film in which the optical path length difference L _H and L _L are alternately stacked under the condition of lambda _0/4 and the film thickness d _H and d _L with respect to the reference wavelength lambda ₀ of incident light, the transmittance of 50% and The wavelength widths Δλ _P and Δλ _S of the reflection bands of P-polarized light and S-polarized light defined by the wavelength interval of the reflection bands are expressed by the equations (5a) and (5b).

L _H = n _H × d _H × cos θ _{H
= N H × d H × {} 1- (sinθ / n H) 2} 0.5 (4a)
L _L = n _L × d _L × cos θ _{L
= N L × d L × {} 1- (sinθ / n L) 2} 0.5 (4b)
Δλ _P / λ ₀ = (4 / π) × sin ⁻¹ {Δη _P / (η _HP + η _LP )} (5a)
Δλ _S / λ ₀ = (4 / π) × sin ⁻¹ {Δη _S / (η _HS + η _LS )} (5b)

FIG. 4 shows a calculation example of the spectral transmittances Tp and Ts for the P-polarized light and S-polarized light at this time.
As the number of stacked dielectric films having a high refractive index n _H and a low refractive index n _L is increased and the effective refractive index differences Δη _P and Δη _S are increased, the boundary wavelength region between the transmission band and the reflection band is increased. The spectral characteristics of the image become steep.

Therefore, the wavelength widths Δλ _P and Δλ _S of the reflection bands for incident light of P-polarized light and S-polarized light have a magnitude relationship of Δλ _P <Δλ _S , and the incident angle θ and the refractive index difference (n _H −n _L ). The difference between the wavelength widths Δλ _P and Δλ _S of the reflection band is increased with the increase of.

As a result, in the short wavelength region and the long wavelength region of the reflection wavelength band having the reference wavelength λ ₀ , the polarization separation wavelength band of the falling portion and the rising portion that transmits the P-polarized light and reflects the S-polarized light appears, and the plate polarized beam The function of a splitter is obtained.

In the flat polarizing beam splitter 10 of the present invention, by adjusting the film thicknesses d _H and d _L of the multilayer films constituting the short wavelength reflection band multilayer film layer 12S and the long wavelength reflection band multilayer film layer 12L, The reference wavelength λ ₀ corresponding to the substantially central wavelength of the wavelength width of each reflection band with respect to the center wavelength is set to the short wavelength side in the short wavelength reflection band multilayer film layer 12S and to the long wavelength side in the long wavelength reflection band multilayer film layer 12L. At the same time, the multilayer film layer is designed so that the S-polarized light rising portion of the short-wavelength reflective band multilayer film layer 12S and the S-polarized light falling portion of the long-wavelength reflective band multilayer film layer 12L are close to the center wavelength of the used wavelength region. As a result, a flat polarizing beam splitter 10 having a desired wide effective polarization separation wavelength region shown in FIG. 3 is obtained.

Incidentally, when the optical path length difference of each layer L _H and L _L is a simple multi-layer film of lambda _0/4, because the wavelength variation of the P-polarized light transmittance in the effective polarization separation wavelength region is increased, the film thickness d _H and d It is preferable to adjust the _L so that the P-polarized light transmittance is 95% or more.

As the refractive index difference (n _H −n _L ) between the high refractive index dielectric film and the low refractive index dielectric film is larger and the incident angle θ of the principal ray is larger, the shorter wavelength reflection band multilayer film layer 12S and the longer wavelength are increased. Since each polarization separation wavelength band of the reflection band multilayer film layer 12L is wide, a wide effective polarization separation wavelength region can be obtained also in the flat plate polarization beam splitter 10.

  For example, when a blue semiconductor laser mounted on an optical head device for recording / reproduction of a high information recording density optical disc BD is used as the laser light source 1, the variation in the oscillation wavelength of the laser light source and the variation in the oscillation wavelength due to the temperature change are considered. Then, the used wavelength band is about 398 to 415 nm, and the polarization separation function of S polarized light reflection and P polarized light transmission becomes important in this used wavelength band. By using the flat polarizing beam splitter 10 of the present invention as an optical head device with an incident angle θ = 45 °, a polarization separation function of high S-polarized reflection and P-polarized light transmission of 95% or more in the used wavelength band is realized. it can.

  On the other hand, when a conventional flat-plate polarization beam splitter in which only the short-wavelength reflection-band multilayer film layer 12S or the long-wavelength reflection-band multilayer film layer 12L is formed on one surface of the flat-plate substrate 11, a wavelength capable of obtaining an equivalent polarization separation function. Since the bandwidth remains in a narrow region of 10 nm or less, the signal light amount of the optical head device is not stabilized with the wavelength variation of the blue semiconductor laser, and a recording / reproducing error occurs, which becomes a problem.

As shown in the optical head device 100 of FIG. 1, when the flat polarizing beam splitter 10 is used in the incident light arrangement of diverging light and convergent light, the short wavelength reflection band multilayer film layer 12S and the long wavelength reflection band multilayer film layer are used. The optical path length differences L _H and L _L of the high refractive index n _H and low refractive index n _L constituting 12L change according to the incident angle θ as shown in the equations (4a) and (4b). . Compared with the incident angle θ of the principal ray, the polarization separation wavelength region shifts to the short wavelength side at a large incident angle, and the polarization separation wavelength region shifts to the long wavelength side at a small incident angle.

  As a result, for example, a high polarization separation characteristic can be realized in the used wavelength band for parallel light with an incident angle θ = 45 °, but the used wavelength band is covered for divergent light with an incident angle of 45 ° ± 8 °. However, there is a possibility that the high polarization separation characteristic cannot be realized. Specifically, in the case of the flat polarizing beam splitter 10 that exhibits a high polarization separation function with respect to a wavelength bandwidth of about 25 nm at an incident angle θ = 45 °, a high polarization separation characteristic for divergent light with an incident angle of 45 ° ± 8 ° is obtained. The resulting wavelength width is reduced to about 5 nm.

  In the optical head device 100, the light emitted from the laser light source 1 has an elliptical intensity distribution with a half-value intensity full-angle radiation angle of about 6 ° to 22 °. Condensed on the information recording surface. At this time, the taking-in angle by the collimating lens 3 with respect to the emitted divergent light of the laser light source 1 is set so as to obtain an intensity distribution that does not reduce the effective numerical aperture with respect to the aperture diameter of the objective lens. As a result, the flat polarizing beam splitter 10 disposed in the optical path between the laser light source 1 and the collimating lens 3 emits divergent light whose incident light is about ± 6 ° to ± 9 ° with respect to the incident angle θ of the principal ray. Arrangement.

  Two methods for realizing high polarization separation characteristics even in the divergent light in which the incident angle to the flat polarizing beam splitter is distributed with respect to the principal ray will be described below.

1. Multilayer film layer in which optical film thickness is spatially distributed The optical path length differences L _H and L _L of the dielectric film having a high refractive index n _H and a low refractive index n _L constituting the multilayer film layer of the flat polarizing beam splitter are (4a), Since there is the incident angle dependency shown in the equation (4b), if the change in optical path length difference due to the difference from the incident angle θ with respect to the principal ray is compensated by adjusting the film thicknesses d _H and d _L of each dielectric film, Good.

  FIG. 5 is a schematic diagram of a cross-sectional structure in the XZ plane defined by the normal line of the flat-plate polarizing beam splitter 20 having such a multilayer film structure and the principal ray of incident light. The actual total film thickness of the multilayer film is as small as about 0.5 μm to 10 μm, but the film thickness is enlarged and emphasized for the sake of explanation.

From the narrow angle side to the wide angle side, the thicknesses d _H and d _L of the dielectric films are thicker at the wide angle (θ + α) side and thinner at the narrow angle (θ−α) side than the incident angle θ of the principal ray. A multilayer film structure in which the thickness of each dielectric film is continuously spatially distributed is adopted.

In addition, in the Y-axis direction perpendicular to the paper surface of FIG. 5, the incident angle component of the principal ray with respect to the flat polarizing beam splitter 20 is 0 °. Therefore, in the divergent light of about ± 10 ° or less from the equations (4a) and (4b). Variations in the optical path length differences L _H and L _L with respect to the change in the incident angle are slight, and the possibility of adjusting the film thickness of each dielectric film in the Y-axis direction is low.

  The technology for correcting the incident angle dependence in the spectral characteristics of the multilayer film layer by adjusting the film thickness is a technique such as arranging a film thickness correction plate during film formation applied to products such as optical filters with wedges. It can be realized by using.

2. Arrangement in which the incident angle θ of the chief ray is large As described above, the refractive index difference (n _H −n _L ) between the high refractive index dielectric film and the low refractive index dielectric film constituting the multilayer film layer of the flat polarizing beam splitter Is larger, the larger the incident angle θ of the principal ray, the wider the individual polarization separation wavelength bands of the short wavelength reflection band multilayer film layer 12S and the long wavelength reflection band multilayer film layer 12L. The polarization separation wavelength region can be expanded.

The refractive index difference (n _H −n _L ) is limited by the practical transparent dielectric film material used. On the other hand, it is important to increase the light receiving area of the flat polarizing beam splitter 10 with respect to the incident light beam diameter as the incident angle θ of the chief ray increases. This is because problems such as an increase in the size and cost of the flat polarizing beam splitter 10 and an increase in the size of the optical head device 100 are likely to occur. Further, since the laser light source 1 and the photodetector 6 approach and interfere in FIG. 1, there is an upper limit on the incident angle θ that is allowed in the arrangement.

  In order to avoid such problems and achieve high polarization separation characteristics over the wavelength band of the laser light source in the divergent light arrangement, it is effective to set the incident angle θ of the chief ray in the range of 45 ° to 65 °. It is.

[Example 1]
A blue semiconductor laser used in an optical head device for recording / reproducing of a high-density information recording optical disc BD is a laser light source 1 having a used wavelength band of 398 to 415 nm for BD, and an incident angle θ of a principal ray is 45 °. FIG. 6 shows a schematic diagram of a configuration example of the optical head device 200 of the present invention in which the flat polarizing beam splitter 21 is arranged in parallel light. Components having the same functions as those in FIG. 1 showing a schematic diagram of a configuration example of the optical head device 100 are denoted by the same reference numerals, and description thereof is omitted.

  A difference from the optical head device 100 is that the divergent light of P-polarized light (first linearly polarized light) emitted from the laser light source 1 becomes parallel light by the collimator lens 3 and is incident on the flat polarizing beam splitter 21 at an incident angle θ = 45 °. Incident at. Further, the signal light reflected by the information recording surface of the optical disk 7 passes through the objective lens 5 again to become parallel light, and is converted into S-polarized light (second linearly polarized light) by reciprocating the quarter-wave plate 4. Then, the light incident on and reflected by the flat polarizing beam splitter 21 is reflected by the flat beam splitter 21b, passes through the flat beam splitter 21a, and is collected on the light receiving surface of the photodetector 6a by the collimator lens 3a.

  Further, the optical head device 200 integrates the DVD and CD optical head device 200a in order to realize recording and reproduction of DVD and CD optical disks. About 90% of the s-polarized divergent light emitted from the two-wavelength laser light source 1a that oscillates by switching between the light for the 660 nm wavelength band for DVD and the light for the 785 nm wavelength band for CD is reflected by the flat plate beam splitter 21a and collimated. The light is converted into parallel light by the lens 3a, is transmitted through the flat beam splitter 21b, and is condensed on the information recording surface of the optical disk 7a by the objective lens 5a for DVD and CD. The signal light reflected by the information recording surface of the optical disk 7a is transmitted again through the objective lens 5a to become parallel light, converted into P-polarized light by reciprocating through the quarter-wave plate 4a, and transmitted through the flat beam splitter 21b. A part of the light transmitted through the flat beam splitter 21a by the collimator lens 3a is condensed on the light receiving surface of the photodetector 6a.

  Here, the plate beam splitter 21b reflects at least S-polarized light in the 405 nm wavelength band for BD, and has a spectral characteristic that transmits P-polarized light and S-polarized light in the 660 nm wavelength band for DVD and the 785 nm wavelength band for CD. It is a splitter. The flat beam splitter 21a transmits at least S-polarized light in the 405 nm wavelength band for BD, reflects at least 90% of S-polarized light in the 660 nm wavelength band for DVD and 785 nm wavelength band for CD, and at least 10 P-polarized light. It is a flat plate beam splitter having a spectral characteristic of transmitting at least%. Since the plate beam splitters 21a and 21b do not necessarily have a polarization separation function, a desired spectral characteristic can be realized by a conventional multilayer film design.

  Although not shown in FIG. 6, in order to obtain a tracking servo signal in the optical path between the laser light source 1 and the collimating lens 3 or in the optical path between the laser light source 1a and the collimating lens 3a, A three-beam diffraction grating that generates ± first-order diffracted light may be arranged. Further, in order to obtain a focus servo signal in the optical path between the flat beam splitter 21a and the photodetector 6a, a cylindrical lens that generates astigmatism in transmitted light may be arranged.

Next, a specific configuration and function of the flat polarizing beam splitter 21 will be described below with reference to FIG. A Ta ₂ O ₅ dielectric film having a high refractive index n _H = 2.20 and a SiO ₂ having a low refractive index n _L = 1.47 are formed on one side of a flat glass substrate 11 made of translucent glass having a refractive index of 1.52. _{The two} dielectric films are alternately stacked with 30 layers each of the long-wavelength reflection band multilayer film layer 12L and the short-wavelength reflection band multilayer film layer 12S so as to form a total of 60 layers. This corresponds to a configuration in which the order of 12S and 12L in FIG. 2 is reversed.

When the reference wavelength λ ₀ = 405 nm of the blue semiconductor laser light source is used, the average value of the 60 layers of the optical path length difference defined by the equations (4a) and (4b) is 0.268λ ₀ , and the long wavelength reflection band multilayer film layer 12L the average value of the optical path length difference of each 30 layers of short wavelength reflection band multilayer film 12S is 0.305Ramuda ₀ and 0.231λ _0. In order to ensure a high P-polarized light transmittance for ± 3 ° divergence and convergent light at an incident angle θ = 45 ° of the principal ray in the used wavelength band 398 to 415 nm, a flat substrate in the long wavelength reflection band multilayer film layer 12L and 11 and short-wavelength reflection band multilayer film 12S and several layers of boundary, several layers of the boundary between the atmosphere in the short-wavelength reflection band multilayer film 12S is each average 0.305Ramuda ₀ of the optical path length difference 0.231λ _The optical path length difference is set to be different by about 0.4λ ₀ from ₀ at the maximum, and the film thicknesses d _H and d of each dielectric film are set within ± 0.06λ ₀ from each average value. _L is finely adjusted.

  Note that the divergence and convergent light of ± 3 ° with respect to the incident angle θ = 45 ° of the chief ray is used in the case of an optical disc for BD having an information recording surface of two layers according to the difference in optical disc cover thickness. In order to correct the spherical aberration of the condensed spot on the information recording surface that occurs, it is necessary to move the collimating lenses 3 and 3a in the direction of the optical axis and adjust them to divergent light or convergent light within a range of ± 3 ° or less. Because. To move the collimating lenses 3 and 3a in the optical axis direction, two collimating lenses are integrally fixed to a holder and driven using a stepping motor or the like.

  FIG. 7 shows the calculation result of the spectral transmittance wavelength dependency for the P-polarized light and the S-polarized light of the plate polarizing beam splitter 21 having such a configuration. (P-45) and (S-45) have an incident angle of 45 °, (P-42) and (S-42) have an incident angle of 42 °, and (P-48) and (S-48) have an incident angle of 48 °. The spectral transmittance of P-polarized light and S-polarized light is shown. That is, the plate-type polarizing beam splitter 21 exhibits a P-polarized light transmittance of 95% or more and an S-polarized light reflectance of 96% or more with respect to divergent light having an incident angle of 45 ° ± 3 ° in the used wavelength band 398 to 415 nm. . For parallel light having an incident angle θ = 45 °, a P-polarized light transmittance of 98% or more and an S-polarized light reflectance of 98% or more are shown in the wavelength band 395 to 417 nm.

  As Comparative Example 1, FIG. 8 shows the calculation result of the spectral transmittance wavelength dependence for P-polarized light and S-polarized light when the plate polarization beam splitter is formed of only the short wavelength reflection band multilayer film layer 12S as the prior art. The wavelength band showing P-polarized light transmittance of 90% or more and S-polarized light reflectance of 90% or more for parallel light with an incident angle θ = 45 ° is 400 to 412 nm, and divergent light with an incident angle of 45 ° ± 3 °. On the other hand, the wavelength band showing P-polarized light transmittance of 90% or more and S-polarized light reflectance of 90% or more is 405 to 409 nm, and it is difficult to obtain high polarization separation characteristics with respect to the used wavelength band.

  As Comparative Example 2, FIG. 9 shows the calculation results of the spectral transmittance wavelength dependence for P-polarized light and S-polarized light when a plate polarization beam splitter consisting only of the long-wavelength reflection band multilayer film 12L, which is the prior art, is used. . The wavelength band showing a P-polarized light transmittance of 90% or more and an S-polarized light reflectance of 90% or more for parallel light with an incident angle θ = 45 ° is 400 to 410 nm, and divergent light with an incident angle of 45 ° ± 3 °. On the other hand, the wavelength band showing P-polarized light transmittance of 90% or more and S-polarized light reflectance of 90% or more is 405 to 407 nm, and it is difficult to obtain high polarization separation characteristics with respect to the used wavelength band.

  According to the optical head device 200 of the present invention, high light utilization efficiency in the forward path and the backward path can be obtained in the recording / reproduction of the BD optical disc 7, and a blue semiconductor laser light source with relatively low power consumption can be used. Stable operation is realized because high optical signal detection is possible. In addition, since the plate polarization beam splitter 21 is used instead of the polarization beam splitter prism and the other optical multiplexing / demultiplexing elements are also composed of the plate beam splitter, the productivity is high and the cost can be reduced. Further, the optical detector 6a common to that for BD is used for recording and reproduction of the optical disk 7a for DVD and CD, which leads to miniaturization of the optical head device and cost reduction.

[Example 2]
A second embodiment of the present invention will be described with reference to FIG. 1, which is a schematic diagram of a configuration example of the optical head device 100. FIG.
The same blue semiconductor laser as that of the first embodiment is used as the laser light source 1, and the flat polarizing beam splitter 20 is arranged in the divergent light of ± 8 ° at the incident angle θ of the chief ray of 45 °. Since the configuration and operation of the optical head device 100 are described in the embodiment, description thereof is omitted.
A specific configuration and function of the flat polarizing beam splitter 20 will be described below with reference to FIG.

The multilayer film formed on one surface of the flat polarizing beam splitter 20 has the same film thickness d of each dielectric film as the flat polarizing beam splitter 10 of Example 1 at a position where the incident angle θ of the chief ray is θ = 45 °. and a multilayer film design structure of _H and d _L. Further, in the XZ plane of the plate polarizing beam splitter 10, each dielectric film is continuously formed from the narrow angle (45 ° -8 °) side to the wide angle (45 ° + 8 °) side with respect to 45 °. The multi-layer film with wedges is spatially distributed so that the film thickness of the multi-layer film becomes thick, and the incident angle dependence in the spectral characteristics of the multi-layer film layer is corrected.

  Specifically, when the incident angle is 37 ° (= 45 ° -8 °) and 53 ° (= 45 ° + 8 °), the effective polarization separation wavelength region is approximately +9 nm compared to the incident angle θ = 45 °. Further, since the wavelength shift is performed by approximately −9 nm, the thickness of each dielectric film is approximately −2.3% at the incident angles of 37 ° and 53 ° as a ratio to the film thickness at the incident angle θ = 45 °. Further, a continuous film thickness distribution of about + 2.3% is formed.

  As a result, in the used wavelength band of 398 to 415 nm, high polarization having a P-polarized light transmittance of 96% or more and an S-polarized light reflectance of 96% or more with respect to the divergent light of ± 8 ° at the incident angle θ = 45 ° of the principal ray A flat polarizing beam splitter 20 exhibiting separation characteristics can be obtained, and an optical head device capable of detecting an optical signal having a relatively high power consumption and a stable SN can be realized.

[Example 3]
The same blue semiconductor laser as in Examples 1 and 2 is used as the laser light source 1, and the flat polarizing beam splitter 10 is arranged in the divergent light of ± 8 ° at the incident angle θ = 60 ° of the principal ray. In the optical head device 100, a specific configuration and function of the flat polarizing beam splitter 10 shown in FIG. Since the configuration and operation of the optical head device 100 are described in the embodiment, description thereof is omitted.

A Ta ₂ O ₅ dielectric film having a high refractive index n _H = 2.20 and a SiO ₂ having a low refractive index n _L = 1.47 are formed on one side of a flat glass substrate 11 made of translucent glass having a refractive index of 1.52. _A short-wavelength reflection band multilayer film layer 12S and a long-wavelength reflection band multilayer film layer 12L having a configuration in which a total of 58 layers of _two dielectric films are alternately stacked are formed. When the reference wavelength λ ₀ = 405 nm of the blue semiconductor laser light source is used, the average value of 58 layers of the optical path length difference defined by the equations (4a) and (4b) is 0.263λ ₀ , a short wavelength reflection band consisting of 30 layers average value of the optical path length difference of the multilayer film layer 12S and a long wavelength reflection band multilayer film 12L formed of 28 layers is 0.223Ramuda ₀ and 0.304λ _0. In order to ensure a high P-polarized light transmittance with respect to divergent light having a used wavelength band of 398 to 415 nm and an incident angle of 60 ° ± 8 °, several layers at the boundary with the flat substrate 11 in the short wavelength reflection band multilayer film layer 12S In the long wavelength reflection band multilayer film layer 12L, the short wavelength reflection band multilayer film layer 12S and several layers at the boundary with the atmosphere have a maximum of 0.4λ with respect to the respective average values of the optical path length differences of 0.223λ ₀ and 0.304λ ₀ . _The optical path length differences differed by about _0, and the film thicknesses d _H and d _L of each dielectric film were finely adjusted so that the optical path length differences of the other layers were within ± 0.06λ ₀ from the respective average values. Multi-layer design.

  FIG. 10 shows the calculation result of the spectral transmittance wavelength dependency for the P-polarized light and the S-polarized light of the plate polarizing beam splitter 10 having such a configuration. (P-60) and (S-60) have an incident angle of 60 °, (P-52) and (S-52) have an incident angle of 52 °, and (P-68) and (S-68) have an incident angle of 68 °. The spectral transmittance of P-polarized light and S-polarized light is shown. That is, the plate-type polarizing beam splitter 10 exhibits a P-polarized light transmittance of 98% or more and an S-polarized light reflectance of 98% or more for divergent light having an incident angle of 60 ° ± 8 ° in the used wavelength band 398 to 415 nm. . For parallel light with an incident angle θ = 60 °, a P-polarized light transmittance of 93% or more and an S-polarized light reflectance of 99% or more are shown in the wavelength band 389 to 422 nm.

  As a comparative example, calculation of spectral transmittance wavelength dependence for P-polarized light and S-polarized light when using a conventional polarizing beam splitter composed of only the short-wavelength reflection band multilayer film layer 12S or the long-wavelength reflection band multilayer film layer 12L. From the results, the wavelength band showing the P-polarized light transmittance of 90% or more and the S-polarized light reflectance of 90% or more with respect to the parallel light with the incident angle θ = 60 ° is 400 to 412 nm, and the incident angle is 60 ° ± 8 °. A wavelength band of 405 to 409 nm showing 90% or higher P-polarized light transmittance and 90% or higher S-polarized light reflectance with respect to divergent light, and high polarization separation characteristics for the used wavelength band cannot be obtained.

  According to the optical head device 100 of the present invention, high light utilization efficiency in the forward path and the backward path can be obtained in the recording / reproduction of the BD optical disc 7, and a blue semiconductor laser light source with relatively low power consumption can be used. Stable operation is realized because high optical signal detection is possible. Further, since a flat polarizing beam splitter is used instead of the polarizing beam splitter prism, the productivity is high and the cost can be reduced.

In Examples 1 to 3, an example in which a blue semiconductor laser light source having a used wavelength band of 398 to 415 nm of a BD optical disk is used as the laser light source 1 is described. However, a red laser light source having a used wavelength band of 645 to 675 nm of a DVD optical disk, Even when a near-infrared laser light source having a wavelength band of 765 to 805 nm for a CD optical disk is used, the film thicknesses d _H and d of the dielectric films are varied according to the difference in the reference wavelength λ ₀ of the multilayer film layer of the plate polarization beam splitter. _This can be dealt with by changing to a multilayer film design configuration in which _L is adjusted.

  In addition, by using the same multilayer design method, a multi-layer film for BD wavelengths and a multilayer film layer for DVD wavelengths are stacked to achieve a high polarization separation function for the BD wavelength band and DVD wavelength band. Therefore, the optical head device can be further reduced in size and cost.

1 is a schematic diagram illustrating a configuration example of an optical head device according to an embodiment of the present invention. Sectional drawing which shows the structure of the flat plate polarization beam splitter mounted in the optical head apparatus of this invention. The graph which shows the example of the spectral transmission characteristic shape of P and S polarization | polarized-light of the flat polarizing beam splitter mounted in the optical head apparatus of this invention. Graph showing P, and spectral transmission characteristic shape example of S-polarized light with respect to oblique incident light of multilayer film optical path length difference are alternately stacked dielectric films of high and low refractive index of λ _0/4. Sectional drawing which shows the structure where the film thickness distribution was provided to the multilayer film layer in the flat polarizing beam splitter mounted in the optical head apparatus of this invention. 1 is a schematic diagram showing a configuration example of an optical head device according to a first embodiment of the present invention. 3 is a graph showing the spectral transmission characteristics of P and S polarized light of the flat polarizing beam splitter mounted on the optical head device according to the first embodiment of the present invention. As Comparative Example 1, a graph showing the spectral transmission characteristics of P and S polarized light of a conventional flat plate beam splitter. As Comparative Example 2, a graph showing the spectral transmission characteristics of P and S polarized light of a conventional flat plate beam splitter. 7 is a graph showing the spectral transmission characteristics of P and S polarized light of a flat-plate polarization beam splitter mounted on the optical head device according to the third embodiment of the present invention. FIG. 6 is a schematic diagram showing a configuration example of an optical head device equipped with a conventional polarizing beam splitter prism. The schematic diagram which shows the structural example of the optical head apparatus carrying the conventional flat plate beam splitter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1a: Laser light source 2a: Polarizing beam splitter prism 2b, 21a, 21b: Flat plate beam splitter 3, 3a: Collimating lens 4, 4a: 1/4 wavelength plate 5, 5a: Objective lens 6, 6a: Photo detector 7 7a: optical discs 10, 20, 21: flat plate polarization beam splitter 11: flat plate substrate 12S: short wavelength reflection band multilayer film layer 12L: long wavelength reflection band multilayer film layers 100, 200, 200a: optical head device

Claims (5)

A laser light source;
A quarter wavelength that converts the first linearly polarized light emitted from the laser light source into circularly polarized light and converts the circularly polarized light reflected from the optical recording medium into a second linearly polarized light orthogonal to the first linearly polarized light. The board,
An objective lens for condensing circularly polarized light transmitted through the quarter-wave plate on an optical recording medium;
A photodetector for detecting a second linearly polarized light reflected by the optical recording medium and converted by the quarter-wave plate;
Wherein arranged in the optical path of the divergent light between the laser light source the quarter-wave plate, and transmitting or reflecting the first linearly polarized light emitted from the laser light source and reflected from the optical recording medium A polarizing beam splitter that reflects or transmits reflected light converted from circularly polarized light to second linearly polarized light by the ¼ wavelength plate, and an optical head device comprising:
In the polarizing beam splitter, a multilayer film made of at least two kinds of film materials having different refractive indexes is formed on the surface of one surface of a parallel-plate translucent substrate,
The multilayer film uses the difference in polarization transmission characteristics between the rising and falling of the reflection band according to the incident angle of the obliquely incident light in the wavelength range of 398 to 415 nm that is the use wavelength band of the blue semiconductor laser light source, and the polarization components orthogonal to each other and to reflect, reflect, together long, consists of two reflective bands multilayer film centered wavelength shorter wavelength, it is brought close to the short wavelength side falling on the long wavelength side rise and long wavelength reflection band of the short wavelength reflection band An optical head device characterized in that the polarization separation effective wavelength region is expanded by adjusting the film thickness of each layer so that the transmittance of one polarization component is 95% or more .   2. The optical head device according to claim 1, wherein the polarization beam splitter has an incident angle of a principal ray of incident light in a range of 45 ° to 65 °.   3. The optical head device according to claim 1, wherein the polarization beam splitter is disposed in divergent light having an angle of ± 6 ° to ± 9 ° with respect to an incident angle of a principal ray of incident light. The optical film thickness of the high refractive index dielectric film and the low refractive index dielectric film constituting the multilayer film layer is thicker at the incident angle on the wide angle side and thinner at the incident angle on the narrow angle side than the incident angle of the principal ray. As described above, the multilayer film structure in which the thickness of each dielectric film is continuously spatially distributed from the narrow-angle side to the wide-angle side is defined by the normal line of the polarization beam splitter and the principal ray of the incident light. the optical head device according to any one of claims 1-3, characterized in a Turkey to correct the incident angle dependence of the spectral characteristics caused by the difference in the incident angle in a plane. A laser light source;
A quarter wavelength that converts the first linearly polarized light emitted from the laser light source into circularly polarized light and converts the circularly polarized light reflected from the optical recording medium into a second linearly polarized light orthogonal to the first linearly polarized light. The board,
An objective lens for condensing circularly polarized light transmitted through the quarter-wave plate on an optical recording medium;
A photodetector for detecting a second linearly polarized light reflected by the optical recording medium and converted by the quarter-wave plate;
Wherein arranged in the optical path of the divergent light between the laser light source the quarter-wave plate, and transmitting or reflecting the first linearly polarized light emitted from the laser light source and reflected from the optical recording medium A polarizing beam splitter that reflects or transmits reflected light converted from circularly polarized light to second linearly polarized light by the ¼ wavelength plate, and an optical head device comprising:
In the polarizing beam splitter, a multilayer film made of at least two kinds of film materials having different refractive indexes is formed on the surface of one side of a parallel-plate translucent substrate, and a reflection band rises with an incident angle of obliquely incident light. Utilizing the difference in falling polarization transmission characteristics, it transmits and reflects polarized components orthogonal to each other, and two reflective band multilayer films with a long wavelength and a short wavelength as the center wavelength are provided on the separation surface, It is close to the short wavelength side falling on the long wavelength side rise and long wavelength reflection band wavelength reflection band, each of the reflection, Rutotomoni expanding the polarization separating effective wavelength range by multiplying the transmitting polarization characteristics, the multilayer film structure The optical film thickness of the high-refractive index dielectric film and the low-refractive index dielectric film is narrower so that the incident angle on the wide-angle side is thicker than the incident angle of the principal ray, and is thinner at the narrower-angled incident angle. Dielectric from wide angle side to An optical head device which is characterized in that the multilayer film structure thickness of a film was continuously spatial distribution.

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