JP4328302B2 - Optical recording apparatus and data writing method - Google Patents

Description

  The present invention relates to an optical recording apparatus and a data writing method for writing data on a multilayer recording medium having a plurality of stacked recording layers using light.

  Conventionally, in order to increase the recording capacity, an optical recording medium (multilayer recording medium) in which a plurality of recording layers are stacked has been proposed. Since the plurality of recording layers of the multilayer recording medium are at different depths from the surface of the multilayer recording medium, in order to write data on such a multilayer recording medium, the optical recording apparatus is located at positions having different depths. It is desirable to have an optical recording device (optical head) capable of arranging a light spot.

  In a multilayer recording medium, as an optical head capable of arranging a light spot on one desired recording layer, there is a method using a lens position control in the optical axis direction (lens shift method). Hereinafter, an optical head that controls the position of the objective lens and an optical head that controls the position of the relay lens will be described. Here, a conventional lens shift type optical head capable of changing the position of the light spot will be described with reference to FIG. FIG. 11 is a schematic diagram showing the configuration of a conventional lens shift type optical head and the light propagating through the optical head optical system. FIG. 11A shows the configuration of the optical head for controlling the position of the objective lens in the optical axis direction. FIG. 2B is a schematic diagram illustrating a configuration of an optical head that controls the position of the relay lens in the optical axis direction.

  First, an optical head 100 that can change the position of the light spot 104 by controlling the position of the objective lens 103 in the optical axis direction will be described with reference to FIG. The optical head 100 includes a light source 101, a collimator lens 102, and an objective lens 103. The light emitted from the light source 101 first enters the collimator lens 102 and is emitted as parallel light. The parallel light enters the objective lens 103, and the light emitted from the objective lens 103 is condensed at a position on the optical axis that is a predetermined distance (focal length) away from the objective lens 103. Therefore, the light can be condensed on a desired recording layer 105 by adjusting the position of the objective lens 103 on the optical axis with respect to the light source 101 and the collimator lens 102.

  Next, an optical head 110 that can change the position of the light spot 116 by controlling the position of the relay lens 113 in the optical axis direction will be described with reference to FIG. The optical head 110 includes a light source 111, a first collimator lens 112, a relay lens 113, a second collimator lens 114, and an objective lens 115. The optical head 110 is separated into two parts: an optical system on the light source side (first collimator lens 112 and relay lens 113) and an optical system on the recording medium side (second collimator lens 114 and objective lens 115). can do.

  The light emitted from the light source 111 first enters the first collimator lens 112 and is emitted as parallel light. The parallel light is incident on the relay lens 113, and the light emitted from the relay lens 113 is incident on a second collimator lens 114 disposed on the optical axis at a predetermined distance from the relay lens 113. . Then, it becomes parallel light again and is emitted from the second collimator lens 114. Finally, the parallel light enters the objective lens 115 and is condensed at a position away from the objective lens 115 on the optical axis by a focal length. Therefore, by adjusting the position of the relay lens 113 on the optical axis with respect to the light source 111 and the first collimator lens 112, the light emitted from the objective lens 115 can be recorded as desired according to Newton's imaging formula. The light can be condensed on the layer 117.

  Further, as an optical head having another configuration capable of arranging a light spot on a desired recording layer of a multi-layer recording medium, there is one using a liquid crystal shutter method or an axicon prism method (for example, see Patent Document 1). The liquid crystal shutter type optical head includes a liquid crystal shutter panel in which two sets of annular transparent electrodes are attached to the front surface and the back surface. A plurality of sets of transparent electrodes are attached to the liquid crystal shutter panel concentrically. When a voltage is applied to a pair of transparent electrodes, an electric field is generated in the portion sandwiched between the two transparent electrodes, and the light distribution of the liquid crystal element existing in that region changes, so that only that portion is made transparent. Can do.

  Therefore, when parallel light is incident on the liquid crystal shutter panel, the parallel light passes only through the transparent part of the liquid crystal element and is emitted as annular parallel light. The annular parallel light is collected by the objective lens. The surface of the objective lens is provided with protrusions having different shapes and periods depending on the distance from the center of the lens, and depending on the shape (radius and width) of the annular parallel light incident on the objective lens due to light interference Thus, light spots can be formed at different positions on the optical axis. That is, the shape (radius and width) of the annular parallel light emitted from the liquid crystal shutter panel can be changed by selecting a set to which the voltage of the concentric transparent electrode on the liquid crystal shutter panel is applied. Then, by entering this parallel light into the objective lens described above, it is possible to form light spots at different depths in the multilayer recording medium.

  The axicon lens type optical head includes two axicons (conical prisms) that convert light from the light source into annular parallel light (first axicon and second axicon). When parallel light from the light source enters the first axicon, it is emitted as annular divergent light. This annular divergent light is incident on the second axicon and emitted as annular parallel light. At this time, since the light emitted from the first axicon is diverging light, the size (radius) of the annular light is expanded according to the distance between the first axicon and the second axicon. Therefore, by adjusting the distance between the first axicon and the second axicon, the size (radius) of the annular parallel light emitted from the second axicon can be changed. Then, the parallel light is incident on the same objective lens as that of the liquid crystal shutter system described above, whereby light spots can be formed at different depths in the multilayer recording medium.

  Furthermore, there is a wavelength multiplexing system as another type of optical head. The optical head includes a plurality of light sources that emit light having different wavelengths, a collimator lens that multiplexes these lights, and a conical prism (objective lens) that collects the parallel light multiplexed by the collimator lens. Prepare. The plurality of light sources are arranged at positions away from the collimator lens by a focal length, and light emitted from each light source is converted into parallel light by the collimator lens. The parallel light enters the conical prism, and the light collected by the conical prism can form light spots at different depths according to the wavelength due to axial chromatic aberration generated by the conical prism. .

  In addition, in order to reduce the influence of crosstalk (optical and thermal interference) generated between adjacent recording layers, the interlayer spacing of the recording layers is compared with the focal depth of light collected by the objective lens. Must be large enough. In the end, in order to increase the number of recording layers as much as possible, it is necessary to make the interval between the recording layers as narrow as possible within a range in which the crosstalk generated in the recording layer is not increased by the beam condensed on the adjacent recording layer. .

Here, the interlayer spacing and the crosstalk on the recording medium are estimated, and the minimum value of the interlayer spacing is estimated. It is assumed that a Gaussian beam whose spot size (radius) at the focal point (beam waist) is W ₀ [m] propagates in the optical axis direction. Then, assuming that the beam focused on the recording layer is almost a plane wave near the optical axis and propagates while gradually changing the curvature with respect to the optical axis, approximating the slowly changing envelope, the beam waist extends to the optical axis direction. The beam size W (z) [m] at the position propagated by z [m] is expressed by the following equation (1). Here, k is a wave number of 2π / λ, and λ is a wavelength.

Therefore, when the interlayer spacing is d [m], the beam size (radius) of the Gaussian beam focused on the adjacent recording layer in the recording layer is W (d) [m]. That is, the beam shape is expanded by a magnification [W (d) / W ₀ ]. Since the crosstalk is inversely proportional to the energy density ratio, that is, the beam area ratio, the crosstalk P (d) in the adjacent recording layer is expressed by the following equation (2).
P (d) = 20 log [(W ₀ / W (d)) ² ] = 40 log (W ₀ / W (d)) (2)

When the Gaussian beam is collected by an objective lens (convex lens) having a numerical aperture NA, the spot size (radius) W ₀ is expressed by the following equation (3).
W ₀ = 0.46λ / NA (3)

Here, with reference to FIG. 12, the crosstalk when data is recorded on the recording medium by the Blu-ray disc apparatus will be described. FIG. 12 is a graph showing the relationship between the interlayer spacing of the recording layers and the crosstalk on the recording medium for a multilayer optical disk recording apparatus equipped with a blue-violet (multi-beam) optical head. Here, the numerical aperture NA of the objective lens of the optical disk apparatus was 0.85, the wavelength λ of the blue-violet laser used was 0.405 μm, and the refractive index n of the recording layer (recording medium) was 1.5. The allowable value of crosstalk from the beam focused on the adjacent recording layer depends on material parameters such as absorption rate, heat transfer rate and specific heat of the recording layer and the layer configuration. If the talk P (d) is about −50 dB or less, there is often no problem. Therefore, as shown in FIG. 12, when the crosstalk is taken into consideration, it can be estimated that the interlayer distance d is required to be about 10 μm or more.
JP-A-11-259895

  However, the lens shift method cannot simultaneously form light spots on a plurality of recording layers. Further, when recording on another recording layer after recording on a certain recording layer, it is necessary to move the objective lens in the optical axis direction, and it takes time until focusing is stabilized. Furthermore, in this lens shift method, when recording on another recording layer, an ID signal for identifying the recording layer forming the light spot is sent to each recording layer in order to move the position of the light spot. Need to embed.

  Further, in the liquid crystal shutter system, when annular parallel light is generated from a Gaussian beam, light near the central portion having a high energy density is blocked, so that light use efficiency is lowered. Further, when a plurality of concentric transparent electrodes are mounted on the liquid crystal shutter panel, a plurality of annular parallel lights are generated. The light utilization efficiency varies depending on each annular parallel light, and the light utilization efficiency decreases as the parallel light is more outer. For this reason, when a general optical device such as a semiconductor laser diode or a surface emitting laser is used as the light source, the intensity of the beam necessary for recording data on the recording medium cannot be obtained, which is difficult to realize. .

  Furthermore, since the speed of change in the light distribution of the liquid crystal element is slower than the speed of light modulation during recording on the recording medium, this type of optical head uses the light generated by turning on / off the liquid crystal shutter panel. Cannot be modulated, and the light must be modulated by a drive circuit for driving the light source. Therefore, even though multiple light spots can be formed at different depths on the optical axis at the same time, the recording pattern (modulation waveform) is the same, so different data can be recorded for each recording layer. There wasn't.

  Further, in the axicon lens method, only one annular parallel light can be generated at a time, and therefore, a light spot cannot be formed on a plurality of recording layers at the same time. When changing the recording layer, it is necessary to change the distance between the two axicon lenses, and it takes time to move the axicon lens.

  In the wavelength multiplexing method, an axicon is used as an objective lens. Therefore, light (Gaussian beam) from the light source is converted into parallel light by a collimator lens, and further converted into light having a Bessel distribution by this axicon and emitted. Since the depth of focus of light having this Bessel distribution is deep, adjustment of focusing is unnecessary, and the mechanism can be omitted. On the other hand, when the depth of focus is deep, the size of the light spot increases, so that the recording density (recording capacity) decreases, and the recording / reproducing speed decreases. Furthermore, since the crosstalk generated between adjacent recording layers becomes large, the quality (shape and size) of the light spot formed on the recording medium is deteriorated during recording, and the light spot is reproduced during reproduction. The quality of the light spot formed on the detector is deteriorated.

  The present invention was made to solve the problems of the prior art, and can simultaneously form data spots on a plurality of recording layers, write data in parallel, has good light utilization efficiency, and An object of the present invention is to provide an optical recording apparatus and a data writing method that can reduce the size of the light spot (shallow depth of focus).

In order to solve the above problems, an optical recording apparatus according to claim 1 is an optical recording apparatus that emits light for writing data on a multilayer recording medium including a plurality of recording layers, each having a different wavelength. A plurality of light sources for emitting light, and beam converting means for converting the light emitted from the plurality of light sources into an annular beam having the same optical axis and a different radius for each wavelength ; and Condensing means for condensing the annular beam converted by the beam converting means for each wavelength on recording layers of different depths of the multilayer recording medium.

  As a result, the optical recording apparatus condenses the annular beam by the condensing means, and forms a light spot at a depth position corresponding to the radius and wavelength of each beam. Therefore, a light spot can be formed by condensing the beam of each wavelength on each of the stacked recording layers.

  In addition, since the optical recording apparatus collects an annular beam to form a light spot, the size of the light spot can be reduced as compared with a case where a Gaussian beam is condensed. To explain qualitatively, the energy density distribution of the Gaussian beam becomes higher near the optical axis. On the other hand, the energy density distribution of the annular beam is zero near the optical axis and becomes maximum at a predetermined distance from the optical axis. In other words, energy can be concentrated on the area outside the lens (condensing means) (the portion with a large numerical aperture), so the larger the size of the annular beam, the more the light spot collected by the condensing means. The size can be reduced.

  The light source emits light such as laser light used for writing data to the multilayer recording medium. This optical recording apparatus includes a plurality of light sources, and each light source is excited with a different wavelength. Then, by utilizing axial chromatic aberration generated in the optical system, light from each light source forms a light spot on a different recording layer. As a result, by modulating the light for each light source, different data can be recorded simultaneously for each light spot.

  The optical recording apparatus according to claim 2 is the optical recording apparatus according to claim 1, wherein the beam converting unit multiplexes the light emitted from the light source into the light having the same optical axis. The optical multiplexing unit and the multiple beam converting unit that converts the light multiplexed by the optical multiplexing unit into an annular beam having a different radius for each wavelength are provided.

  As a result, the optical recording apparatus generates an annular beam having a different radius for each wavelength after multiplexing the light having different wavelengths. Then, each annular beam can be condensed to form a light spot at a depth corresponding to the radius and wavelength.

  Furthermore, the optical recording apparatus according to claim 3 is an optical recording apparatus that emits light for writing data on a multilayer recording medium having a plurality of recording layers, each of which emits light of different wavelengths. And the light emitted from the plurality of light sources has an energy density peak on an annular ring having the same optical axis and a different length for each wavelength, and the light. A rotationally symmetric beam converting means for converting into a beam having an energy density distribution rotationally symmetric with respect to the axis, and a beam converted by the rotationally symmetric beam converting means on recording layers of different depths of the multilayer recording medium, Condensing means for condensing light for each wavelength.

  As a result, the optical recording apparatus collects a beam having an energy density peak on an annulus having a radius having a different length for each wavelength by the condensing means, and utilizes axial chromatic aberration generated in the optical system. A light spot is formed at a depth corresponding to the radius and the wavelength. Then, by matching the interval between the recording layers and the distance between the light spots, the light spots can be formed by condensing the beams of the respective wavelengths on the stacked recording layers.

  In addition, the optical recording apparatus collects a beam having an energy density peak on a ring having the same optical axis and a different radius and having a rotationally symmetric energy density distribution with respect to the optical axis. Since the light spot is formed by light, the size of the light spot can be reduced as compared with the case where the Gaussian beam is condensed. In other words, the energy density distribution of the Gaussian beam becomes higher in the vicinity of the optical axis of the condensing means such as a convex lens or a conical prism. On the other hand, the energy density distribution of a beam having a rotationally symmetric energy density distribution is low in the vicinity of the optical axis and is maximized at a predetermined distance from the optical axis. Therefore, as with the annular beam, the larger the distance, the smaller the size of the light spot. Examples of the beam having a rotationally symmetric energy density distribution include a Laguerre-Gaussian beam, a Hermit-Gaussian beam, an Ins-Gaussian beam, and the like. It is done.

  Further, the optical recording apparatus includes a plurality of light sources, and light from each light source forms a light spot on a different recording layer. Therefore, by modulating the light for each light source, different data can be simultaneously transmitted in each light spot. Can be recorded.

  The optical recording apparatus according to claim 4 is the optical recording apparatus according to claim 3, wherein the rotationally symmetric beam converting means multiplexes the light emitted from the light source into the light having the same optical axis. Optical multiplexing means, and light multiplexed by this optical multiplexing means has an energy density peak on an annulus having a different radius for each wavelength and rotational symmetry with respect to the optical axis. And a multiple rotationally symmetric beam converting means for converting into a beam having a uniform energy density distribution.

  As a result, the optical recording apparatus has the energy density peak on the circular ring having the different radii for each wavelength after multiplexing the lights having different wavelengths, and the rotationally symmetric energy density with respect to the axis. A beam having a distribution is generated. Then, each beam can be condensed and a light spot can be formed at a position corresponding to the radius and the wavelength in the same manner as the annular beam.

  The optical recording apparatus according to claim 5 is the optical recording apparatus according to claim 4, wherein the multiple rotationally symmetric beam converting means is composed of a photonic crystal device. As a result, the optical recording device uses the photonic crystal device to guide the light incident from the incident end face to a predetermined position on the circumference of the predetermined radius on the output end face, and multiplex it by the optical multiplexing means. The converted light can be converted into a beam having a rotationally symmetric energy distribution with a radius corresponding to the wavelength of the light.

  In addition, a line defect or a point defect is artificially introduced into the photonic crystal device from a position where light is incident on the incident end face to a predetermined position on a circle having a predetermined radius on the outgoing end face, and the defect is caused by the light confinement effect. Light can be guided along a region (waveguide) and converted to a beam having an energy density peak on an annulus having a different radius for each wavelength. A beam having a rotationally symmetric energy distribution can be obtained by providing the waveguide structure at predetermined intervals on a circle having a predetermined radius on the emission end face.

Furthermore, the data writing method according to claim 6 is a data writing method for writing data to a multilayer recording medium having a plurality of recording layers, and a light emitting step for emitting light of different wavelengths from a plurality of light sources. A beam conversion step for converting the light emitted from the plurality of light sources by the light emission step into an annular beam having the same optical axis and a radius having a different length for each wavelength ; and the beam conversion And a condensing step of condensing the annular beam converted by the steps for each wavelength on recording layers stacked at different depths of the multilayer recording medium.

  As a result, the data writing method condenses the annular beam by the condensing step, and forms a light spot at a depth corresponding to the radius and wavelength of each beam. Therefore, a light spot can be formed by condensing the beam of each wavelength on each of the stacked recording layers.

  Furthermore, the data writing method according to claim 7 is a data writing method for writing data to a multilayer recording medium having a plurality of recording layers, and a light emitting step for emitting light of different wavelengths from a plurality of light sources. And the light emitted from the plurality of light sources by this light emitting step has an energy density peak on an annular ring having the same optical axis and having a different radius for each wavelength. A rotationally symmetric beam converting step for converting into a beam having an energy density distribution rotationally symmetric with respect to the optical axis, and recording the beams converted by the rotationally symmetric beam converting step at different depths on the multilayer recording medium. The layer includes a condensing step for condensing each wavelength.

  As a result, the data writing method collects light by the condensing step, condenses a beam having an energy density peak on an annulus having a radius with a different length for each wavelength, A light spot is formed at a position corresponding to the depth. Therefore, it is possible to collect light of each wavelength on each of the stacked recording layers to form a light spot.

  The optical recording apparatus and data writing method according to the present invention have the following excellent effects. According to the invention of claim 1, claim 3, claim 6 or claim 7, a plurality of lights having different wavelengths are simultaneously condensed to form light spots on different recording layers in the multilayer recording medium. Therefore, data can be written to a plurality of recording layers at a time. Therefore, the writing speed can be improved according to the number of light sources.

  In addition, since a smaller light spot can be formed as compared with the case of condensing a Gaussian beam, an adjacent recording layer is smaller than the recording layer of the conventional multilayer recording medium. Data can be written while preventing crosstalk (thermal interference). Therefore, the number of recording layers of a multilayer recording medium for writing data, which should be within a predetermined range in the thickness direction, can be increased compared to a conventional multilayer recording medium, and data that can be recorded on one multilayer recording medium It becomes possible to increase the amount.

  According to the invention described in claim 2 or claim 4, in order to multiplex light having different wavelengths, the light is converted into an annular or rotationally symmetric energy density distribution beam having a different length for each wavelength. Light from each light source can be collectively converted. In other words, a single optical device can be converted into an annular or rotationally symmetric energy density distribution beam, which facilitates alignment when connecting the devices and reduces light connection loss (axis misalignment loss). Can do.

  According to the fifth aspect of the present invention, light in which a plurality of wavelengths is multiplexed can be converted by one device, and the beam shape is gradually and continuously changed using a photonic crystal device. By doing so, conversion can be performed with low loss.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Configuration of Optical Head (Optical Recording Device) (First Embodiment)]
With reference to FIG. 1, the structure of the optical head 1 which is 1st Embodiment in this invention is demonstrated. FIG. 1 is a block diagram showing the configuration of the optical head. The optical head (optical recording apparatus) 1 writes data on a multilayer recording medium 7 in which a plurality of recording layers 7a to 7e are laminated. The optical head 1 includes a light source 2, a beam converting means 6, and an objective lens 5. In FIG. 1, the light source 2, the beam converting means 6, the objective lens 5, and the wavelength multiplexing circuit 3 and the multiple beam converting circuit 4 described later are shown separated from each other by a predetermined interval. The optical head 1 may be formed by optically integrating all components, or may be formed at a predetermined interval. Furthermore, the emitted light, such as a waveguide circuit that guides light, is passed to the next circuit or lens (wavelength multiplexing circuit 3, multiple beam conversion circuit 4, objective lens 5) in the gap separated by a predetermined interval. An incident circuit, an optical system, or the like may be provided.

The light source 2 emits a laser beam for writing data on the multilayer recording medium 7. Here, the light source 2 has its light emission pattern (modulation of light emission intensity) controlled by light source control means (not shown). Therefore, here, the optical head 1 is provided with five light sources 2 (2a to 2e). Further, the light sources 2 (2a to 2e) are semiconductor laser diode arrays that emit laser beams having wavelengths λ _{1 to} λ ₅ (λ ₁ <λ ₂ <λ ₃ <λ ₄ <λ ₅ ), respectively. The light source 2 used here may be a device that emits laser light or the like, and may be, for example, a surface emitting laser or the like.

  The beam converting means 6 converts each laser beam emitted from the light source 2 (2a to 2e) into an annular beam having a predetermined radius and the same optical axis. Here, the beam conversion means 6 includes a wavelength multiplexing circuit 3 and a multiple beam conversion circuit 4.

  The wavelength multiplexing circuit (optical multiplexing means) 3 multiplexes the laser beams emitted from the light sources 2a to 2e. Here, the wavelength multiplexing circuit 3 is internally combined with a waveguide that guides each of a plurality of lights to form one waveguide, thereby multiplexing (combining) light having different wavelengths to be guided. Thus, it is configured by an embedded waveguide that emits light having the same optical axis. Here, the structure of the wavelength multiplexing circuit 3 will be described with reference to FIG. FIG. 2 is a perspective view showing the structure of the wavelength multiplexing circuit of the optical head.

As shown in FIG. 2, the wavelength multiplexing circuit 3 includes a core 3a and a clad 3b. The core 3a has a refractive index higher than that of the cladding 3b, and the laser light from the light source 2 can be confined and guided in the core 3a by totally reflecting the laser light at the interface between the core 3a and the cladding 3b. The core 3a has five waveguides on the incident end face 3c side. In the wavelength multiplexing circuit 3, the five waveguides on the incident end face 3c side are coupled to one waveguide, and on the output end face 3d side. One waveguide is used. Then, a spot size converter (not shown) that enlarges the diameter of the laser light from the light source 2 to the diameter of the opening surface of the core 3a is provided on each opening surface of the five waveguides on the incident end surface 3c side. The light sources 2a to 2e are attached. As a result, the five laser beams having different oscillation wavelengths input from the light sources 2a to 2e on the incident end face 3c side are multiplexed inside the core 3a and have a distribution of _five peak wavelengths (λ _{1 to} λ ₅ ). The laser beam is emitted from the emission end face 3d as a single laser beam.

  Here, it is necessary to propagate the light in the waveguide in a single mode, and the propagation loss (attenuation of the intensity of the laser light) is reduced by reducing the relative refractive index between the core 3a and the clad 3b. be able to. That is, since the size of the core 3a of the waveguide through which visible light propagates in a single mode is small, the influence of manufacturing irregularities such as the structural undulation of the core 3a becomes relatively large. For example, the propagation loss increases due to Rayleigh scattering at the boundary between the core 3a and the clad 3b, but the magnitude is proportional to the 2.5th power of the relative refractive index difference between the core 3a and the clad 3b. The propagation loss can be reduced as the rate difference is reduced.

  At this time, the energy density of the propagating laser beam has a distribution close to a Gaussian distribution, and the radiation angle of the laser beam becomes small. Therefore, by reducing the relative refractive index difference between the core 3a and the clad 3b, it is possible to efficiently connect to an external optical device (multiple beam conversion circuit 4).

  Such a core 3a and a clad 3b can be realized by the following, for example. That is, when laser light having a wavelength of 400 to 440 nm is multiplexed in a single mode, a perfluoroalkenyl vinyl ether cyclized polymer (CYTOP (registered trademark)) and an organosilicon compound are mixed as a core material. Then, the core 3a having a square end face with a side of 2 μm and embedded in the clad 3b is formed using a material whose refractive index is adjusted to 1.3484. Further, the cladding 3b is formed using CYTOP (registered trademark) having a refractive index of 1.3450. According to such a wavelength multiplexing circuit 3, a propagation efficiency of about 70% can be obtained.

  The wavelength multiplexing circuit 3 may be, for example, a waveguide device, or a beam splitter or a dichroic mirror that optically synthesizes light having different wavelengths. In the conventional method in which the light sources 2 excited at different wavelengths are arranged in the direction perpendicular to the optical axis, a plurality of light spots cannot be simultaneously arranged along the optical axis direction. By controlling the beam shape (profile), a plurality of light spots formed by the objective lens 5 can be simultaneously arranged on the optical axis.

Returning to FIG. 1, the description will be continued. The multiple beam conversion circuit (multiple beam conversion means) 4 converts the laser light incident from the wavelength multiplexing circuit 3 into an annular beam (hereinafter referred to as an annular beam). Here, the structure of the multiple beam conversion circuit 4 that converts a Gaussian beam into an annular beam will be described with reference to FIGS. FIG. 3 is a schematic diagram showing the structure of the multiple beam conversion circuit of the optical head and the structure of the end face and the internal cross section thereof. FIG. 4 is a schematic diagram schematically showing an annular beam emitted from the multiple beam conversion circuit of the optical head, and FIG. 4A is a diagram illustrating conversion of a Gaussian beam having wavelengths λ _{1 to} λ ₅ by the multiple beam conversion circuit of the optical head. FIG. 5B is a schematic diagram schematically showing the light intensity in the radial direction of the annular beam and the position of the peak.

  First, the structure of the multiple beam conversion circuit 4 will be described with reference to FIG. Here, the multiple beam conversion circuit 4 is constituted by a conical inner clad 4c and a conical shell shaped core 4a by an outer clad 4b so that the entire shape is a cylinder. And like the optical fiber, it produced by drawing a preform | base_material. The multiple beam conversion circuit 4 may be, for example, a diffraction grating or a hologram element that generates an annular interference fringe (light having only zero-order high-order diffracted light without zero-order light).

  The multiple beam conversion circuit 4 forms a circular core 4a at the center of the incident end face 4d, branches the waveguide into a conical shape inside the multiple beam conversion circuit 4, and has a circular shape with a predetermined radius at the output end face 4e. An annular core 4a is formed on the circumference. Therefore, the core 4a inside the multiple beam conversion circuit 4 has a larger radius in a cross section perpendicular to the incident direction of light as it goes from the incident end face 4d of the multiple beam conversion circuit 4 toward the exit end face 4e. The laser light incident from the incident end face 4d is confined inside the core 4a of the multiple beam conversion circuit 4, and the radius increases as it propagates toward the exit end face 4e, and is converted into an annular beam having a predetermined radius. And is emitted from the emission end face 4e.

  The multiple beam conversion circuit 4 includes a core 4a, an outer cladding 4b, and an inner cladding 4c having different Abbe numbers. The outer cladding 4b has the smallest Abbe number, and the Abbe number increases in the order of the core 4a and the inner cladding 4c. . The Abbe number is a numerical value for evaluating the wavelength dispersion characteristic of the refractive index of the optical material. A medium having a larger Abbe number has a smaller change in the refractive index for different wavelengths.

  Therefore, as compared with light having a certain wavelength (reference wavelength), the light having a wavelength shorter than the reference wavelength has a slightly higher refractive index of the inner cladding 4c having a large Abbe number, and the refractive index of the core 4a having an intermediate Abbe number. And the refractive index of the outer cladding 4b having a small Abbe number becomes very large. Therefore, the refractive index difference between the core 4a and the inner cladding 4c is increased, and the refractive index difference between the core 4a and the outer cladding 4b is decreased. As a result, the position of the peak of light having a wavelength shorter than the reference wavelength is shifted to the inner cladding 4c side, and the radius of the annular beam emitted from the emission end face 4e is reduced. On the other hand, the peak position of light having a wavelength longer than the reference wavelength is shifted to the outer cladding 4b side, and the radius of the annular beam emitted from the emission end face 4e is increased. Thus, by adjusting the Abbe number of the medium constituting the core 4a, the outer cladding 4b, and the inner cladding 4c, the radius of the annular beam converted by the multiple beam conversion circuit 4 can be controlled by the wavelength. .

Therefore, when a laser beam obtained by multiplexing laser beams having wavelengths λ _{1 to} λ ₅ (λ ₁ <λ ₂ <λ ₃ <λ ₄ <λ ₅ ) is incident on the multiple beam conversion circuit 4, FIG. As shown, the multiple beam conversion circuit 4 emits five annular beams b1 to b5 having peak radii r (λ ₁ ) to r (λ ₅ ), respectively. The five annular beams b1 to b5 have concentric peaks on the plane C. Then, as shown in FIG. 4B, the peak radius r (λ ₅ ) of the light with the wavelength λ ₅ is the largest, and the peak radius r (λ ₁ ) of the light with the wavelength λ ₁ is the smallest. A plane C in FIG. 4A indicates a plane orthogonal to the propagation direction (optical axis direction) of the annular multiple beam composed of the five annular beams b1 to b5 emitted from the multiple beam conversion circuit 4. The arrow on C (r-axis) indicates the radial direction from the center of the annular multiple beam. Furthermore, the r-axis on the plane C coincides with the r-axis of the graph of FIG. 4B, and below this graph, the core 4a from the center to the radial direction on the exit end face 4e of the multiple beam conversion circuit 4 is shown. The arrangement of the outer cladding 4b and the inner cladding 4c is schematically shown.

Returning to FIG. 1, the description will be continued. The objective lens (condensing means) 5 condenses the annular multiple beam emitted from the multiple beam conversion circuit 4. Here, the objective lens 5 has a wavelength dispersion characteristic having a refractive index that varies depending on the wavelength of incident light. Due to axial chromatic aberration caused by this wavelength dispersion characteristic, the light of each wavelength λ _{1 to} λ ₅ is converted into an optical axis. The light is condensed at a position where the distance from the objective lens 5 is different in the direction. Here, the objective lens 5 is a conical prism. The objective lens 5 may be any lens as long as it can condense an annular multiple beam and has wavelength dispersion characteristics. For example, it may be a convex lens such as a biconvex lens. And when the width | variety of an annular | circular shaped beam is narrow, a convex lens functions similarly to a conical prism.

  Here, with reference to FIG. 5, the light spot of the annular multiple beam in the case where the objective lens 5 is a conical prism made of an optical material whose refractive index decreases as the wavelength becomes longer will be described. FIG. 5 is an explanatory diagram for explaining the light spot of the annular multiple beam incident on the objective lens of the optical head.

As shown in FIG. 5, since the refractive index of the objective lens 5 decreases as the wavelength increases, _five laser beams having wavelengths λ _{1 to} λ ₅ (λ ₁ <λ ₂ <λ ₃ <λ ₄ <λ ₅ ) are obtained. Is incident, the refractive index of the annular beam b _{1 having} the shortest wavelength λ ₁ becomes the largest, and the first layer recording closest to the laser light irradiation surface of the multilayer recording medium 7 is performed. A light spot c1 is formed on the layer 7a. Then, then increase the refractive index of the wavelength lambda ₂ of the annular beam b2 forms a light spot c2 in the second recording layer 7b. Similarly, the light spot c3 is formed on the recording layer 7c of the annular beam b3 wavelength lambda ₃ is the third layer, a light spot c4 form annular beam b4 of the wavelength lambda ₄ is the recording layer 7d of the fourth layer Te, the annular beam b5 of the longest wavelength lambda _5, to form a light spot c5 the farthest lowermost recording layer 7e from the objective lens 5. As a result, the optical head 1 can form light spots having the wavelengths λ _{1 to} λ _{5 on} the recording layers 7 a to 7 e and write data to the multilayer recording medium 7.

Since the annular multiple beam is composed of five annular beams centered on the z axis, by aligning the z axis with the optical axis of the objective lens 5, the objective lens 5 can be Light spots having wavelengths λ _{1 to} λ ₅ can be arranged on the optical axis.

  Further, since the annular multiple beam obtained by multiplexing the annular beams b1 to b5 is condensed, the light spots c1 to c5 can be simultaneously formed on each of the recording layers 7a to 7e. Independent data can be simultaneously written for each of the recording layers 7a to 7e by controlling the light emission pattern of each light source 2 (2a to 2e) by a light source control means (not shown). Therefore, writing speed can be improved according to the number of the light sources 2 by writing data in the several recording layers 7a-7e at once.

Further, an annular beam of each wavelength lambda ₁ to [lambda] _5, the energy density is maximized at the position separated by the radius from the z-axis, respectively _{r (λ 1) ~r (λ} 5). When the annular beam is condensed, the size of the light spot can be reduced as compared with the case where the light whose energy density distribution is increased near the optical axis of the objective lens, such as a Gaussian beam. .

  Therefore, even if the interval between the respective light spots is shortened, crosstalk (thermal interference) between the respective light spots can be prevented if the light condensing angle is the same. Thereby, the interlayer interval of the multilayer recording medium 7 can be narrowed, and the number of recording layers that can be stacked in a certain range can be increased. That is, the amount of data that can be recorded on the multilayer recording medium 7 can be increased.

  Next, with reference to FIG. 6, the space | interval between each light spot c (c1-c5) is demonstrated. FIG. 6 is a schematic diagram schematically showing the optical path of the annular beam incident on the objective lens. Here, the peak radius of an annular beam having a certain wavelength λ is r (λ), and an annular beam b having a half-value width sufficiently small with respect to r (λ) is present in the atmosphere (with a refractive index of A case where the light enters the objective lens 5 (conical prism) from 1) will be described.

  Here, the radius of the bottom surface of the objective lens 5 (conical prism) is y, the opening angle of the entrance-side cone is 2θ ′ degrees, the opening angle of the exit-side cone is 2θ degrees, and the refractive index of the objective lens 5 at the wavelength λ is Assuming n (λ), the apex P ′ of the cone on the incident side and the distance z to the light spot c are expressed by the following equation (4).

In Expression (4), l (λ) is the propagation length of the annular beam b having the wavelength λ in the objective lens 5, and α (λ) is the annular lens b having the wavelength λ from the atmosphere. Is the refraction angle when the annular beam b having the wavelength λ is emitted from the objective lens 5 to the atmosphere. α (λ) and β (λ) are expressed by the following formula (5) according to Snell's law.

Next, the distance between each light spot will be described with a specific example. Here, the wavelengths of the laser beams emitted from the light sources 2a to 2e are λ ₁ = 400 [nm], λ ₂ = 410 [nm], λ ₃ = 420 [nm], λ ₄ = 430 [nm], λ _5. = 440 [nm], the peak radius of the annular multiple beam is the peak radius r (400) of the annular beam with the wavelength λ ₁ = 1800 [μm], and the peak radius r (410) of the annular beam with the wavelength λ ₂ = 1802 [μm], peak radius r (420) of an annular beam with wavelength λ ₃ = 1804 [μm], peak radius r (430) of an annular beam with wavelength λ ₄ = 1806 [μm], wavelength λ ₅ The peak radius r (440) of the annular beam is set to 1808 [μm]. The material of the objective lens 5 (conical prism) is a heavy flint glass having a refractive index n (400) = 1.7 when the Abbe number V is 30 and the wavelength is 400 nm, θ = 90 [°], θ ′ = 29. 4 [°] and the radius of the bottom surface y = 2000 [μm].

  The Abbe number V is a numerical value for evaluating the wavelength dispersion characteristic of the refractive index of the optical material, and is defined by the following formula (6). Here, Nd, Nf, and Nc are refractive indexes with respect to light having a material wavelength of 589.2 nm, 486.1 nm, and 656.3 nm, respectively.

  Furthermore, in the wavelength band of 400 to 440 nm, the calculation was performed assuming that the refractive index changes in proportion to the wavelength. As a result, from the above formulas (4) and (5), the annular multiple beam is condensed at a position of about 4.1 mm from the vertex P ′ on the light source side of the objective lens 5 (conical prism). The interval between the spots c1 to c5 was 8.3 μm. And if each light spot is spaced apart by this interval (8.3 μm), the crosstalk can be sufficiently reduced. The wavelength dispersion characteristic of the objective lens 5 depends on the Abbe number of the optical material constituting the objective lens 5. The distance between the light spots can be increased by forming the objective lens 5 with an optical material having a small Abbe number and a small refractive index. As described above, in the optical head 1, the interval between the light spots can be adjusted to a desired length by positively utilizing the axial chromatic aberration of the optical system constituting the light condensing means.

  Here, the case where the multiple beam conversion circuit 4 has a positive wavelength dispersion characteristic has been described. However, the constituent materials of the outer cladding 4b and the inner cladding 4c are exchanged, and the outer cladding 4b has the largest Abbe number, and the core 4a, By setting the Abbe number to be smaller in the order of the inner cladding 4c, the multiple beam conversion circuit 4 having negative chromatic dispersion characteristics can be obtained. For example, when r (400) = 1800, r (410) = 1798, r (420) = 1799, r (430) = 1794, r (440) = 1789, each light spot (not shown) The interval is 3.1 μm. At this time, axial chromatic aberration due to the optical system (objective lens 5 and beam converting means 6) is reduced, but the effect of separating the light spot is not impaired.

[Operation of Optical Head (Optical Recording Device) (First Embodiment)]
Next, referring to FIG. 1 (refer to FIG. 2 and FIG. 3 as appropriate), in order to write data on the multilayer recording medium 7, the optical head 1 in the present invention applies a light spot of laser light to the recording layers 7a to 7e. The operation to be formed will be described.

(Light emission step)
The light sources 2 (2a to 2e) attached to the five opening surfaces on the incident end surface 3c side of the core 3a (see FIG. 2) of the wavelength multiplexing circuit 3 via spot size converters (not shown) are lasers having different wavelengths. Emits light. In this light source 2 (2a to 2e), the light emission pattern of the laser light is controlled by a light source control means (not shown), and the laser light emitted from the light source 2 (2a to 2e) passes through a spot size converter (not shown). The light enters the core 3 a of the wavelength multiplexing circuit 3.

(Beam conversion step)
The wavelength multiplexing circuit 3 guides the incident laser light along the core 3a. The core 3a is formed by combining five waveguides that guide laser beams of respective wavelengths into one waveguide inside the wavelength multiplexing circuit 3, and one opening on the emission end face 3d (see FIG. 2) side. Since it has a surface, the incident laser beams of different wavelengths are multiplexed and guided to the exit end surface 3d.

  The multiplexed laser light is incident from the central core 4a (see FIG. 3) of the incident end face 4d (see FIG. 3) of the multiple beam conversion circuit 4. The multiple beam conversion circuit 4 guides the incident laser light along the core 4 a inside the multiple beam conversion circuit 4. Since the core 4a branches in a conical shape inside the multiple beam conversion circuit 4 and is formed up to a position on a circle having a predetermined radius on the exit end face 4e (see FIG. 3), the core 4a enters from the entrance end face 4d side. The laser beam thus converted is converted into an annular beam and emitted from the emission end face 4e.

  Here, the laser light incident on the multiple beam conversion circuit 4 depends on the wavelength dependency (the Abbe number of the medium) of the core 4a, the outer cladding 4b, and the inner cladding 4c constituting the multiple beam conversion circuit 4 for each wavelength. Each of the annular beams having different peak radii is converted into a multiplexed annular beam.

(Condensing step)
Then, the annular multiple beam is incident on the objective lens 5. Since the refractive index of the objective lens 5 varies depending on the wavelength of the incident light, the light of each wavelength is stacked at different depths in the optical axis direction inside the multilayer recording medium 7 due to the wavelength dispersion characteristics (axial chromatic aberration). The light is condensed on each of the recording layers (7a to 7e).

  Through the above operation, the optical head 1 focuses the laser beams emitted from the light sources 2a to 2e on the recording layers 7a to 7e, respectively, to form light spots. Therefore, the light source control means (not shown) that controls the light emission of the light source 2 controls the light emission patterns of the light sources 2 (2a to 2e) corresponding to the respective recording layers (7a to 7e), whereby data is stored in the multilayer recording medium 7. Can be written.

[Configuration of Optical Head (Optical Recording Device) (Second Embodiment)]
Next, the configuration of the optical head 1A according to the second embodiment of the present invention will be described with reference to FIG. FIG. 7 is a block diagram showing the configuration of the optical head. The optical head (optical recording apparatus) 1A writes data on a multilayer recording medium 7 in which a plurality of recording layers 7a to 7e are stacked. The optical head 1A includes a light source 2, a rotationally symmetric beam converting means 6A, and an objective lens 5A. In FIG. 1, the light source 2, the rotationally symmetric beam converting means 6A, the objective lens 5A, and the wavelength multiplexing circuit 3 and the multiple rotationally symmetric beam converting circuit 4A described later are separated from each other by a predetermined interval. Although shown, the optical head 1A may be formed by integrating all components. Further, the emitted light such as a waveguide circuit that guides light into the gap separated by a predetermined distance is sent to the next circuit or lens (wavelength multiplexing circuit 3, multiple rotationally symmetric beam conversion circuit 4A, objective lens 5A). It is good also as providing the circuit and optical system, etc. which enter into this.

  The optical head 1A includes a rotationally symmetric beam converting unit 6A instead of the beam converting unit 6 of the optical head 1 (see FIG. 1), and an objective lens 5A instead of the objective lens 5. Since the configuration other than the rotationally symmetric beam conversion means 6A and the objective lens 5A in the optical head 1A is the same as that shown in FIG. 1, the same reference numerals are given and description thereof is omitted.

  The rotationally symmetric beam converting means 6A energizes each of the laser beams emitted from the light sources 2 (2a to 2e) on an annular ring having the same optical axis and having a different radius for each wavelength. The beam is converted into a beam having a density peak and having an energy density distribution rotationally symmetric with respect to the optical axis. Here, the rotationally symmetric beam converting means 6A includes a wavelength multiplexing circuit 3 and a multiple rotationally symmetric beam converting circuit 4A. The wavelength multiplexing circuit 3 is the same as that shown in FIG.

  The multiple rotationally symmetric beam conversion circuit (multiple rotationally symmetric beam conversion means) 4A is a circle having the same optical axis and a radius having a different length for each wavelength. The beam is converted into a beam having an energy density peak on the ring and a rotationally symmetric energy density distribution with respect to the optical axis. Here, the rotationally symmetric beam converting means 6A converts the beam into an mth order Laguerre Gaussian beam (hereinafter referred to as an LG beam) having 2m energy density peaks at equal intervals on a ring with a predetermined radius. And

Here, the LG beam having 2m energy density peaks and the structure of the multiple rotationally symmetric beam conversion circuit 4A for converting the Gaussian beam into the LG beam will be described with reference to FIGS. FIG. 8 is a schematic diagram showing the energy density distribution of the LG beam, FIG. 8A is a schematic diagram schematically showing the energy density of the LG beam in a plane orthogonal to the traveling direction of the LG beam, and FIG. It is a graph which shows the energy profile in the radial direction of a beam. FIG. 9 is a schematic diagram showing the structure of the multiple rotationally symmetric beam conversion circuit of the optical head, (a) is a schematic diagram showing the multiple rotationally symmetric beam conversion circuit, its end face, and the internal structure, and (b) is ( It is a schematic diagram which shows the structure in the AA cross section of the multiple rotation symmetry beam conversion circuit in a). FIG. 10 is a schematic diagram showing a peak position of an LG beam obtained by converting a Gaussian beam with wavelengths λ _{1 to} λ ₅ by a multiple rotationally symmetric beam conversion circuit of the optical head.

First, the LG beam will be described. The LG beam is rotationally symmetric (2 m peaks having a high energy density are spaced at an interval of 180 / m degrees, as expressed by the following formula (7) in the circular coordinate system (r, θ). Distribution). The radius r _{0, m} at which this peak appears is expressed by the following formula (8). U _{0, m} represents the energy density of the m-th order LG beam. M is a natural number of 2 or more, and U ₀ and ω ₀ are constants.

For example, when m = 2, as shown in FIG. 8A, the energy density of the LG beam in the plane C has four peaks appearing at 90 ° intervals on the plane CA, and FIG. As shown in b), the energy density is zero at the center (near the z-axis). Note that a plane CA in FIG. 8A indicates a plane (rθ plane) orthogonal to the z-axis of the cylindrical coordinate system, and the traveling direction of light coincides with the z-axis direction (the direction penetrating the paper surface). Here, when ω ₀ has wavelength dependence, the peak radius r _{0, m} varies depending on the wavelength. When ω ₀ has a positive wavelength dependency, the peak radius r _{0, m} increases as the wavelength increases.

  Next, the structure of the multiple rotationally symmetric beam conversion circuit 4A will be described with reference to FIG. Here, the multiple rotationally symmetric beam conversion circuit 4A is configured by a photonic crystal device. A photonic crystal device is an optical device in which two or more kinds of substances having different refractive indexes are periodically arranged with a size of about the wavelength of light, and does not propagate light in a certain frequency region (here, laser light). Have. As shown in FIG. 9 (a), here, two kinds of substances having different refractive indexes are used as the medium constituting the multiple rotationally symmetric beam conversion circuit 4A and the air in the holes 4Aa, 4Aa,. In the medium, holes 4Aa, 4Aa,... Having a diameter approximately equal to the wavelength of light are periodically arranged. Then, a part of the periodically arranged holes 4Aa, 4Aa,... Is artificially lost to form an artificial defect 4Ab. As shown in FIG. 9B, by introducing a three-dimensional artificial defect 4Ab (see FIG. 9A) from the incident end face 4Ac side to the outgoing end face 4Ad, the multiple rotationally symmetric beam conversion circuit 4A Light can be guided along the artificial defect 4Ab.

Then, as shown in FIG. 9A, the multiple rotation symmetric beam conversion circuit 4A forms one waveguide at the center of the incident end face 4Ac, and this waveguide is 2 m inside the multiple rotation symmetric beam conversion circuit 4A. The laser beam has a rotationally symmetric energy density distribution, ie, an m-order LG beam (2 in FIG. 9). Next LG beam). Then, by introducing the artificial defect 4Ab three-dimensionally, it propagates in the device while increasing the peak radius r _{0, m} of the m-th order LG beam. FIG. 9 shows a multiple rotationally symmetric beam conversion circuit 4A that divides the waveguide into four from the center of the incident end face 4Ac and converts the laser light incident from the incident end face 4Ac into a secondary LG beam. Yes. The order m may be a natural number equal to or greater than 2, and the higher the order m, the higher the order diffracted light (side lobe) of the LG beam is suppressed, and the shape becomes closer to a ring.

In addition, the constants U ₀ and ω ₀ in the above equation (7) are controlled by the structure of the photonic crystal device that constitutes the multiple rotationally symmetric beam conversion circuit 4A, such as the arrangement period of the artificial defects 4Ab and the holes 4Aa. be able to. When the wavelength dependence of the photonic crystal device constituting the multiple rotationally symmetric beam conversion circuit 4A is expressed by a linear function of the refractive index, the difference in wavelength of the laser light emitted from the light sources 2a to 2e is If they are equally spaced, as shown in FIG. 10, LG multiplexed beams in which LG beams b ′ (b′1 to b′5) with equally spaced peak radii are multiplexed can be generated. Here, the multiple rotationally symmetric beam conversion circuit 4A receives a laser beam obtained by multiplexing five laser beams having wavelengths λ _{1 to} λ ₅ (λ ₁ <λ ₂ <λ ₃ <λ ₄ <λ ₅ ). An LG multiple beam with one peak radius r _0,2 is emitted. Then, the LG multiple beams on a plane CA, has five peaks of LG beams of each wavelength lambda ₁ to [lambda] ₅ concentrically, on a circle of the inner peak wavelength lambda ₁ of the light is the most The peak of light of wavelength λ ₅ is on the outermost circle. The plane CA corresponds to the plane CA in FIG. 8 and indicates a plane (rθ plane) orthogonal to the z-axis of the cylindrical coordinate system.

  Further, by configuring the multiple rotationally symmetric beam conversion circuit 4A with a photonic crystal device, it is possible to reduce the propagation loss of the laser light due to the optical confinement effect. Furthermore, by integrally processing the wavelength division multiplexing circuit 3 and the multiple rotation symmetric beam conversion circuit 4A, it is possible to greatly reduce the loss that occurs when the wavelength division multiplexing circuit 3 and the multiple rotation symmetry beam conversion circuit 4A are connected. it can. 8 to 10, the second-order LG beam has been described as an example. However, the order m may be any natural number equal to or greater than 2, and the multiple rotationally symmetric beam conversion circuit 4A may include, for example, incident light. May be converted into a third-order LG beam, or may be converted into a fourth-order or higher LG beam. The higher the order m, the smaller the higher-order diffracted light. However, from the viewpoint of design and production, it is preferable to set the order m to a natural number from 5 to 8.

Returning to FIG. 7, the description will be continued. The objective lens (condensing means) 5A condenses the LG multiple beam emitted from the multiple rotationally symmetric beam conversion circuit 4A. Here, the objective lens 5A has a wavelength dispersion characteristic in which the refractive index changes depending on the wavelength of incident light, and axial chromatic aberration is generated by this wavelength dispersion characteristic. Thereby, the light of each wavelength (lambda) _1- (lambda) ₅ is condensed to the position where the distance from the objective lens 5 differs in an optical axis direction. The objective lens 5A has the same function as the objective lens 5 (see FIG. 1) except that the incident light is changed from an annular multiple beam to an LG multiple beam. Here, the objective lens 5A is a conical prism. However, the objective lens 5A may be any lens as long as it can collect the LG multiple beam and has wavelength dispersion characteristics. For example, the objective lens 5A is a convex lens such as a biconvex lens. Also good. When the width of the LG multiple beam is sufficiently narrow, the convex lens functions similarly to a conical prism.

[Operation of Optical Head (Optical Recording Device) (Second Embodiment)]
Next, referring to FIG. 7 (refer to FIG. 2 and FIG. 9 as appropriate), in order to write data on the multilayer recording medium 7, the optical head 1A in the present invention applies a light spot of laser light to the recording layers 7a to 7e. The operation to be formed will be described.

(Light emission step)
The light sources 2 (2a to 2e) emit laser beams having different wavelengths. In this light source 2 (2a to 2e), the light emission pattern of the laser light is controlled by a light source control means (not shown), and the laser light emitted from the light source 2 (2a to 2e) passes through a spot size converter (not shown). The light enters the core 3a (see FIG. 2) of the wavelength multiplexing circuit 3.

(Rotationally symmetric beam conversion step)
The wavelength multiplexing circuit 3 guides the incident laser light along the core 3a, and multiplexes and emits the incident laser beams having different wavelengths. Then, the multiplexed laser light is transmitted from the artificial defect 4Ab (see FIG. 9A) at the center of the incident end face 4Ac (see FIG. 9A) of the multiple rotationally symmetric beam conversion circuit 4A made of a photonic crystal device. Incident.

  The multiple rotationally symmetric beam conversion circuit 4A guides incident laser light along a region (waveguide) where the artificial defect 4Ab inside the multiple rotationally symmetric beam conversion circuit 4A is continuous. This waveguide has an m-th order LG beam having a distribution of rotationally symmetric energy density in which 2 m peaks are branched into 2 m inside the multiple rotationally symmetric beam conversion circuit 4A and 2m peaks appear at intervals of 180 / m degrees. Is converted to The LG beam gradually increases in peak radius as it propagates inside the multiple rotationally symmetric beam conversion circuit 4A, and is emitted from the emission end face 4Ad.

  Here, the laser light incident on the multiple rotationally symmetric beam conversion circuit 4A has LG beams having different peak radii for each wavelength due to the wavelength dependence of the photonic crystal device constituting the multiple rotationally symmetric beam conversion circuit 4A. It is converted into a multiplexed LG multiple beam.

(Condensing step)
The LG multiple beam is incident on the objective lens 5A. Since the refractive index of the objective lens 5A varies depending on the wavelength of the incident light, the light of each wavelength is stacked at different depths in the optical axis direction inside the multilayer recording medium 7 due to the wavelength dispersion characteristics (axial chromatic aberration). The light is condensed on each of the recording layers (7a to 7e).

  Through the above operation, the optical head 1A converts the laser light emitted from each of the light sources 2a to 2e into an LG beam, and then focuses the light on the recording layers 7a to 7e to form a light spot. Therefore, the light source control means (not shown) that controls the light emission of the light source 2 controls the light emission patterns of the light sources 2 (2a to 2e) corresponding to the respective recording layers (7a to 7e), whereby data is stored in the multilayer recording medium 7. Can be written.

  Here, the optical head 1A can convert the laser light emitted from each of the light sources 2a to 2e into an LG beam by a multiple rotationally symmetric beam conversion circuit 4A made of a photonic crystal device. Since the wavelength dispersion can be increased by using the photonic crystal device, the peak radius of the LG beam can be largely controlled by the oscillation wavelength of each of the light sources 2a to 2e. As a result, the optical head 1A can set a sufficiently wide interval between the light spots collected on the multilayer recording medium 7 by using the axial chromatic aberration of the objective lens 5A. Further, as the order m of the LG beam converted by the multiple rotationally symmetric beam converting circuit 4A is larger, the higher-order diffracted light (side lobe) is suppressed and the shape approaches a ring shape. It is possible to form a light spot having a shape close to.

1 is a block diagram illustrating a configuration of an optical head according to a first embodiment of the present invention. It is a perspective view which shows the structure of the wavelength division multiplexing circuit of the optical head which is 1st embodiment in this invention. It is a schematic diagram showing the structure of the multiple beam conversion circuit of the optical head according to the first embodiment of the present invention, and the structure of its end face and the internal cross section. Schematic diagram showing schematically an annular beam emitted from the multi-beam conversion circuit of the optical head is a first embodiment of the present invention, (a) is, by multi-beam conversion circuit of the optical head, the wavelength lambda ₁ ~ A schematic diagram schematically showing an annular beam obtained by converting a Gaussian beam of λ ₅ , and (b) is a schematic diagram schematically showing the light intensity in the radial direction of the annular beam and the peak position. It is explanatory drawing for demonstrating a mode that a light spot is formed from the annular | circular shaped multiple beam which injected into the objective lens of the optical head which is 1st embodiment in this invention. It is the schematic diagram which showed typically the optical path of the annular beam which injected into the objective lens of the optical head which is 1st embodiment in this invention. It is the block diagram which showed the structure of the optical head which is 2nd embodiment in this invention. The schematic diagram which shows distribution of the energy density of the secondary LG beam of 2nd embodiment in this invention, (a) shows typically the energy density of LG beam in the plane orthogonal to the advancing direction of LG beam. Schematic (b) is a graph showing an energy profile in the radial direction of the LG beam. FIG. 2 is a schematic diagram showing a structure of a multiple rotation symmetric beam conversion circuit of an optical head according to a second embodiment of the present invention. FIG. 4A is a schematic diagram showing a multiple rotation symmetric beam conversion circuit, its end face, and an internal structure. (B) is a schematic diagram which shows the structure in the AA cross section of the multiple rotation symmetry beam conversion circuit in (a). It is a schematic diagram showing the position of the peak of the LG beam obtained by converting the Gaussian beams of wavelengths λ _{1 to} λ ₅ by the multiple rotationally symmetric beam converting circuit of the optical head according to the second embodiment of the present invention. . A schematic diagram showing a configuration of a conventional lens shift type optical head and a state of propagation through the optical head optical system, (a) is a schematic diagram showing a configuration of an optical head that controls the position of the objective lens in the optical axis direction. FIG. 2B is a schematic diagram showing the configuration of an optical head that controls the position of the relay lens in the optical axis direction. 6 is a graph showing the relationship between the interlayer spacing of recording layers and crosstalk on a recording medium for a multilayer optical disk recording apparatus equipped with a blue-violet (multi-beam) optical head.

Explanation of symbols

1, 1A optical head (optical recording device)
2 Light source 3 Wavelength multiplexing circuit (optical multiplexing means)
4 Multiple beam conversion circuit (Multi beam conversion means)
4A Multiple rotation symmetry beam conversion circuit (Multi rotation symmetry beam conversion means)
5, 5A Objective lens (condensing means)
6 Beam conversion means 6A Rotationally symmetric beam conversion means 7 Multilayer recording medium

Claims (7)

An optical recording apparatus that emits light for writing data on a multilayer recording medium having a plurality of recording layers,
A plurality of light sources each emitting light of a different wavelength;
Beam converting means for converting the light emitted from the plurality of light sources into an annular beam having the same optical axis and a different length for each wavelength ; and
An optical recording apparatus comprising: condensing means for condensing the annular beam converted by the beam converting means for each wavelength on recording layers of different depths of the multilayer recording medium. The beam converting means is
An optical multiplexing means for multiplexing the light emitted from the light source into the light having the same optical axis;
2. The optical recording according to claim 1, further comprising multiple beam conversion means for converting the light multiplexed by the optical multiplexing means into an annular beam having a different radius for each wavelength. apparatus. An optical recording apparatus that emits light for writing data on a multilayer recording medium having a plurality of recording layers,
A plurality of light sources each emitting light of a different wavelength;
The light emitted from the plurality of light sources has the same optical axis, and has an energy density peak on an annular ring having a different radius for each wavelength. Rotationally symmetric beam converting means for converting into a beam having a rotationally symmetric energy density distribution;
An optical recording apparatus comprising: a condensing unit that condenses the beam converted by the rotationally symmetric beam converting unit for each wavelength on recording layers of different depths of the multilayer recording medium. The rotationally symmetric beam converting means is
An optical multiplexing means for multiplexing the light emitted from the light source into the light having the same optical axis;
The beam multiplexed by the optical multiplexing means has an energy density peak on an annulus having a different radius for each wavelength, and a beam having an energy density distribution rotationally symmetric with respect to the optical axis. 4. The optical recording apparatus according to claim 3 , further comprising a multiple rotationally symmetric beam converting means for converting into an optical recording medium. 5. The optical recording apparatus according to claim 4 , wherein the multiple rotationally symmetric beam converting means is a photonic crystal device. A data writing method for writing data to a multilayer recording medium having a plurality of recording layers,
A light emitting step for emitting light of different wavelengths from a plurality of light sources;
A beam converting step of converting the light emitted from the plurality of light sources by the light emitting step into an annular beam having the same optical axis and a different radius for each wavelength ;
A data writing method comprising: a condensing step of condensing the annular beam converted by the beam converting step for each wavelength on recording layers of different depths of the multilayer recording medium. A data writing method for writing data to a multilayer recording medium having a plurality of recording layers,
A light emitting step for emitting light of different wavelengths from a plurality of light sources;
The light emitted from the plurality of light sources by this light emitting step has an energy density peak on an annular ring having the same optical axis and having a different radius for each wavelength, and A rotationally symmetric beam converting step for converting into a beam having an energy density distribution rotationally symmetric with respect to the optical axis;
A data writing method comprising: a condensing step for condensing the beam converted by the rotationally symmetric beam converting step for each wavelength on recording layers of different depths of the multilayer recording medium.

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